Polymath A (mostly) technical weblog for Archivale.com

November 8, 2013

Oblique Convergence – the Two Roots of Modern Unmanned Aircraft

Filed under: Aeronautics,Engineering,Propulsion — piolenc @ 5:40 am

Unlike most high-tech industries, which start with a single core idea and a small coterie of self-educated practitioners before branching out into diverse applications, ours has at least two quite distinct roots  and two very different seed groups. One root is the traditional target drone – reconnaissance drone – armed drone – UAV progression based on aeronautical experts qualified in aerodynamics, structures, propulsion and control systems, starting with primitive analog automation and progressing to digital systems of ever-growing sophistication. It traces its origin to the experiments of Sperry and others as early as the 1920s.

The other root of modern UAVs is embodied in multicopters – ugly, crude, primitive-looking things designed mostly by electronics hobbyists with only the vaguest connection to aeronautics.

The former group build competent, elegant air vehicles – fixed-wing or rotorcraft – and then try to endow them with the control systems that they need to perform their missions without a human being on board. For them, the object is the aircraft and its mission; the control hardware and software are extensions of the aircraft that allow it some autonomy.

The latter group are hardware and software hackers (in the original sense of the word – meaning an expert, not a criminal) who are looking for the cheapest, simplest platform that can pick up their micro-controller board and its code and fly it around. For them, the object is the code, and the ‘copter is merely an extension of the hardware platform on which that code runs – basically a peripheral that allows them to have fun with computers outdoors and burn off all those Twinkies and potato chips.

Neither group has much regard for the other, or much interest in the other’s preoccupations, but a funny thing is happening: as the traditional UAVs tend toward cheapness and ubiquity, the digital eggbeaters of the multicopter hobbyists are becoming more capable: more payload, longer endurance, and very advanced software (because that’s their strength, don’t you know).

Pretty soon the two groups will either be collaborating on or competing for missions that are within the reach of both of them, though they’ll be approaching those tasks from very different perspectives.

What missions, for instance? Well, deliveries for one. We’ve already seen the pizza delivery demo, but I’m thinking of more sensitive deliveries – the kind you probably wouldn’t entrust to a high-school student on a motorbike under any circumstances. The kind where you lose a lot more than twenty dollars plus tips if something goes wrong.

Here’s one:

We think of the mining industry as being concentrated into huge, self-contained operations, from which a very crude, low-value raw material – the ore – is sent to be refined remotely into a valuable and compact commodity. But the reality can be different. Highly valuable resources – we might as well say gold or diamonds because that’s what we’re talking about – often occur, not in a concentrated vein, but in pockets spread over a large area. We speak of gold or diamond fields. Another characteristic of these resources is that they don’t need refining – at least not in the same sense as, say, copper ore – only extraction from a matrix. Gold occurs as the pure metal rather than as an oxide or sulfide like some other metals, and diamond is…diamond. So these resources are essentially finished products at the mining site, and are already very valuable and, because of their portability, very vulnerable.

In a typical gold field operation, small quantities are collected from various small mining sites (in the case of panned gold, there could be hundreds of pans or small dredges strung out along a river) into a more or less central location within the gold field, then transported to a permanent installation for assay, refining to .999 purity, and casting into bullion or minting into coins. On the way there, however, it is subject to theft and to hijacking.

Here’s where UAVs come in. Autonomous UAVs have the capability of taking off from a secure base, flying to a remote location under GPS guidance, and landing within a few meters of their target. There, a payload can be loaded aboard, and the machine refueled or recharged for the return journey. It then takes off and returns to base with the payload. Autonomy means that it can’t be electronically hijacked by interference with the communication link with the ground. Typically, there would be no such thing, which also carries an economic benefit, namely elimination of the ground station and its operators. Arriving at its base, the craft lands inside the secure perimeter and is safely unloaded, then serviced for the next pickup.

Obviously, we are talking about vertical takeoff and landing here. The VTOL field has many different types of craft within it, but helicopters have the most attractive characteristics for this mission because their low-disc-loading rotors allow a given load to be lifted at the lowest cost. Speed is not an issue – only security – so a helicopter’s relatively modest top speed won’t be a problem.

The avoided loss in securely delivering one typical gold shipment – twenty kilos – is nearly a million of the green pieces of paper we laughingly call “dollars.” On the other hand, the direct operating cost of shipping by autonomous UAV, with no pilot or ground crew salaries to pay, comes down to amortization of the initial cost of the vehicle and its support equipment. It follows that a significant investment in this technology can be justified. Imagining a first cost of one million dollars, that investment is almost fully recovered in one avoided hijacking. Actually, payback is quicker because conventional shipping methods involve substantial personnel costs, mostly connected with guarding the shipment, even if nothing at all goes wrong.

Converging on this opportunity are two very different technologies. The drone crowd will offer an autonomous helicopter – essentially a scaled-down version of a manned helicopter design equipped with a combustion engine and a simplified version of conventional flight-control hardware like the rotor head. From the other side will come proposals for a multicopter that will be a scaled-up version of the ones we see buzzing around on YouTube, equipped with one electric motor per rotor and a battery pack.

There will already be some technological convergence: both machines will likely be controlled by programmable micro-controller boards of purely civil, hobbyist origin running private-origin code. This will be for reasons of cost, but also because the control system that would normally have equipped the “drone” machine for, say, a military mission are not legally exportable from their countries of origin. We might see this as the drone people learning from the multicopter hobbyists.

In the present state of battery development, however, it will probably be necessary for the multicopter crowd to adopt technology from the drones, namely combustion engines. This is because storable liquid fuels have much higher specific energy storage capacity than the very best batteries. The easiest way to incorporate a combustion engine into a multicopter will be to have it drive an alternator to recharge the on-board battery through a rectifier/filter in the usual way. The battery would then drive the motors as if the combustion engine weren’t there. In essence, the multicopter would take its recharging station with it, and the payload penalty that carries with it would be partly compensated by having a much lighter battery. One operational advantage of this arrangement is that the machine can be refueled at the remote site and be instantly ready for flight. Hooking it up to a generator to gradually recharge the internal battery won’t be needed.

What else can the multicopters learn from the drone people? Well, a lot. Multicopters are pretty straightforward to control when hovering or moving slowly, but they run into trouble when trying to build up a significant cruise speed. This is the result of the trailing rotors operating in the downwash of the ones ahead. In a conventional tandem-rotor helicopter, this is compensated by increasing the collective pitch of the rear rotor, but no such option exists in a multicopter – the trailing rotors have to turn faster. This works up to a point, where the limit of speed control is reached and the multicopter pitches up abruptly, braking its forward motion. Judging from what’s on YouTube, the speed limit for multicopters looks to be about 70 km/h at present. This may actually be adequate for the mission under consideration, but some means needs to be found for improving it without sacrificing the essential mechanical simplicity of the multicopter. Ideally, that means should not involve additional control channels. That solution, whatever it may be, will likely come from people with conventional helicopter experience.

Another rotor-related problem is the vibration that occurs in a rigid rotor (propeller) in crossflow. You can hear this in the fluttery hum that multis make when moving in translation. This represents a loss of efficiency, and in the long run might lead to unpredictable rotor failure. Again, the conventional aero backgrounds of the drone people will help, with a bolt-on solution in the form of a teetering, flapping or even feathering rotor being the most likely result.

Net result – a much upgraded multicopter and/or a more economical, exportable helicopter drone…and happy miners.

February 13, 2010

Book Review: Leichter als Luft

Filed under: Aeronautics,Engineering,Lighter than Air,Propulsion,Structures — piolenc @ 5:37 pm

Leichter als Luft

Transport- und Traegersysteme
Ballone, Luftschiffe, Plattformen

by Juergen K. Bock and Berthold Knauer

reviewed for Aerostation by F. Marc de Piolenc

Hildburghausen: Verlag Frankenschwelle KG, 2003; ISBN 3-86180-139-6, price: 39.80 Euros. 21.5 x 24 cm, 504 pages, single color, many line illustrations and halftone photographs, technical term index, symbol table, figure credits, catalog of LTA transport and lifting systems.

Summary of Contents

1. General fundamentals of lighter-than-air transport and lifting systems
2. Physical fundamentals
3. Design of airships and balloons
4. Reference information for construction
5. LTA structural mechanics
6. Flight guidance
7. Ground installations
8. Economic indicators
9. Prospects

Appendices:

A. Time chart
B. Selective type tables of operating lighter than air flight systems
C. Development concepts of recent decades
D. Systems under development or under test
E. Author index
F. Table of abbreviations
G. Symbol table
H. Illustration credits
I. List of technical terms
J. Brief [author] biographies

In LTA, which has seen only two book-length general works appear since Burgess’ Airship Design (1927), comparisons are inevitable despite a language barrier. It is therefore quite pleasing to note that the authors of this book have consciously set themselves a task that complements the work embodied in Khoury and Gillett’s Airship Technology1. While Khoury’s work is a review of the current state of the art, the present book provides

“…a scientific, technical and economic basis for a methodical, consistent procedure in developing new lighter than air flight systems as well as a catalog and appraisal of prior solutions and achievements.”

as stated in the preface by Dr.-Ing Joachim Szodruch of the DGLR. This is amplified in the authors’ Foreword:

“The observations contained herein are future-oriented and encompass without euphoria the current state of science and technology.”

This is in contrast to Khoury and Gillett’s introduction to Airship Technology, which reads in part:

“This book is intended as a technical guide to those interested in designing, building and flying the airship of today.”

The body of the book is completely consistent with its stated purpose, looking always toward the future and emphasizing how things should be done rather than how they have been done. Where examples of actual hardware and operations are needed, they are drawn from the most recent available, and meticulously documented.

Considering the authors’ long association with the LTA Technical Branch of the DGLR, it is not surprising to find that much of the material, and many of the collaborating authors listed in the Foreword, are drawn from the many Airship Colloquia held by that Branch over the years. Yet the style is seamless; there is nothing to suggest to this admittedly non-native reader where one contribution ends and another begins; style is consistent from paragraph to paragraph, and across chapter boundaries. What is more, the authors seem to have made a conscious effort to make the text accessible to non-Germans by keeping sentence structure simple and straightforward. The three-column-inch sentences, gravid with nested subordinate clauses, so beloved of the Frankfurter Allgemeine Zeitung, for example, are not to be found here, much to this reviewer’s relief.

It is compulsory to say something about the thoroughness of the book’s coverage. It is, however, difficult to formulate a “completeness” criterion for LTA, which is now more than ever an open-ended field, in which-as the authors correctly point out-the possible types are still far from exhausted, despite the antiquity of aerostatic flight. It is to the book’s credit that its presentation, too, is open-ended; that is, the authors have avoided presenting the usual narrow typology of LTA craft and their almost equally narrow applications. Instead, and in keeping with modern practice, they take a systems approach to LTA, situating it within the field of aeronautics and providing the tools that the reader needs to translate his own requirements into appropriate technology.

The only omission that might be considered significant concerns tethered aerostats: the authors appear to have neglected both tethered-body dynamics and cable dynamics in their technical and mathematical treatments. Tethered balloons as a type are mentioned, but that seems to be all the coverage that they get. Admittedly, long-tether applications have poor prospects because of potential operational and safety problems, but short-tether dynamics have caused problems in some applications that are relevant, including balloon logging, so coverage of that end of the scale would have been welcome. Tethers also play a role in some existing and proposed stratospheric balloon systems, including the exotic NASA Trajectory Control System or TCS.

This, however, is the only flaw in an otherwise comprehensive LTA design/analysis toolkit.

One especially notable and praiseworthy inclusion is subchapter 1.4 regarding regulation and certification. This topic, though a concomitant of any aeronautical project, is one that most techically oriented authors would prefer to avoid or to give only summary treatment, but Bock and Knauer dive into it fearlessly, setting forth in considerable detail, and with the help of flowcharts, German, Dutch, British and American certification categories and procedures, with reference to the governing documents. Not surprisingly, there is more detail about the German process, with which both authors have considerable experience. They also review the history and evolution of the European Joint Airworthiness Regulations (JAR), which are keyed to—-and sometimes based on—-corresponding Parts of the US Federal Aeronautical Regulations (FAR).

They do not flinch even from discussing certification costs and fees. Although they admit that the general policy of regulatory authorities is to require payments to government from the applicant that offset the costs incurred in administering and examining a certification application, they conclude that, compared with the cost of development of an airship, the regulatory fees charged are of only minor importance. It is not clear whether they consider here the costs incurred by idling the works while some bureaucrat makes up his mind! Perhaps it hasn’t happened to them…

Typography, binding and book design

The basic layout is in two columns, with generous leading and gutters, making the somewhat smaller than usual typeface easy to follow and to read. Equations are set in a slightly larger, bolder font and occupy the full width of the page, avoiding a common legibility problem with two-column layouts. There are no drop-outs to be found anywhere. The eggshell-white paper is thin enough to keep the book’s 500-plus pages within a thickness of less than an inch (2.5 cm), yet the paper is completely opaque, without bleedthrough and with perfect reproduction of fine-screen halftones. A color section is mentioned in the table of contents, but all pages in the review copy are single-color. The cover is paper, rather than cloth covered, printed front, spine and back in white on a dark blue background (reproduced in reverse for this review). This type of cover is less durable than the traditional cloth, but is in widespread use for textbooks and technical works despite this.

Second (English) Edition

Work is now in progress on a second edition, which will be published in English by Atlantis Productions. Note that this will not simply be a translation of the first, German edition but a new work, composed ab initio and including whatever revisions might seem appropriate considering response to the first edition. Both of the authors have a very strong command of English, so there is no reason to fear the damage that some excellent German technical works have suffered at the hands of translators (Eck’s treatise on Fans comes to mind).

A “must have” in either language.

1 While a more thorough and detailed comparison of the two books would have been desirable, it is unfortunately not possible, as Aerostation never received a review copy of Airship Technology. Such comparisons as can be made here are based on brief access to that book during a consulting stint.

This review originally appeared in Aerostation, Volume 27, No. 3, Fall 2004

January 15, 2010

Research Resources: Lighter Than (LTA) Air Flight

Return to ABAC Page

LTA Research Resources

compiled by F. Marc de Piolenc

To suggest resources not listed here, or to correct errors, please leave a comment below.


Libraries & Special Collections
Name/Collection Address/Telephone Description
Embry-Riddle University Library Daytona Beach, FL 32014
(904) 239-6931
Northrop University Library—
Pacific Aeronautical Collection
5800 W. Arbor Vitae St.
Los Angeles, CA 90045
(213) 641-3470
Documentation on West Coast aeronautical activity, including LTA. Photographs.
National Air and Space Museum Library Smithsonian Institution-A157203
Washington, DC 20560
In addition to its collection of books and documents, NASM also has an extensive graphic archive, much of it digitized.
University of Akron
Arnstein Collection
The University of Akron
University Libraries
Polsky Building
225 South Main Street, Room LL10
Akron, OH 44325-1702
Tel: (330) 972-7670
Fax: (330) 972-6170
email: jvmiller@uakron.edu
http://www.uakron.edu/archival/arnstein/arnstein.htm
Papers of the late Dr. Karl Arnstein of Goodyear-Zeppelin Corp. Papers have been listed; the lists and some photographs are available on the University’s Web site. See Internet Resources for on-line access and use information.
University of Texas

Charles E. Rosendahl Collection

Douglas H. Robinson Collection

The University of Texas at Dallas
Special Collections Department
P.O. Box 830643
Richardson, Texas 75083-0643
Phone: 972-883-2570
Dr. Erik D. Carlson, Department Head for Special Collections (carlson@utdallas.edu)
Papers of the late VAdm Charles Rosendahl and the late author Douglas M. Robinson were donated to UT.
Zeppelin Archive

(Luftschiffbau Zeppelin GmbH)

c/o Zeppelin Museum
Seestraße 22
D-88045 Friedrichsafen
Germany
Contact: Barbara Weibel (waibel@zeppelin-museum.de)
Phone: 0049 7541 3801 70
Fax: 0049 7541 3786 249
Housed in the same building as the Zeppelin museum, this is a Zeppelin/LTA archive with about 500 linear meters of papers, 7,000 plan sheets and about 17,000 photographs. Another large collection, of books, is housed with the Zeppelin Company archives. Hours are Tuesday to Thursday 9-12 am and 1-5 pm, but an appointment is required.

Museums
Name Address Description
Zeppelin Museum Friedrichshafen Seestraße 2288045 Friedrichshafen

Germany

Tel: +49 / 7541 / 3801-0

Fax: +49 / 7541 / 3801-81

e-mail:zeppelin@zeppelin-museum.de

The Zeppelin museum. Open May-October Tuesday-Sunday, 10 am
to 6 pm; November-April Tuesday-Sunday, 10 am to 5 pm
Aeronauticum, Nordholz

Deutsches Luftschiff- und Marinefliegermuseum

Peter-Strasser-Platz 3, 27637 Nordholz

Postfach 68, 27633 Nordholz

Telephone: 04741-941074

0700-AERONAUTICUM
Telefax 04741-941090

Email: info@aeronauticum.de,

aeronauticum@t-online.de

Located at the site of a former military airship base; collaborator
of the Heinz Urban museum at Meersburg mentioned elsewhere in these pages.
Has custody of the archives of the now-defunct Marine Luftschiffer Kameradschaft.


1 March-30 June and 1 Sept-31 October: M-Sat: 1300-1700

Sun and holidays: 1000-1800

1 July-31 Aug: daily 1000-1800

26 Dec-10 Jan: daily 1100-1700

Other times: open for groups by appointment

New England Air Museum Bradley International Airport

Hartford, Connecticut

USA

…a true gem and a little treasure of LTA stuff. They have displays
and materials on the Hindenburg, various balloons, a CM-5 engine
nacelle (French WWI airship used by US), a large model of the R-100, a
Packard engine designed for the Shenandoah, and the K-28 control car undergoing
restoration. [Airship-List]
Point Sur Lighthouse Big Sur, California

USA

Lighthouse has a nice display of Macon material, model, diagrams
of where it lies, a short video and overall is worth the trip.
Maritime Museum of Monterey
Stanton Center
Monterey, California

USA

..has a good little area on the Macon, including some recovered
artifacts, models, and multiple videos which include interviews with Gordon
Wiley, son of CDR Wiley. Well worth a visit if you are in the area. [Airship-List]
Moffett Field Moffett Field

(near Sunnyvale, California)

USA

The hangar looks great. You can sometimes gain entrance through the
small museum. This museum is a real treasure. Carol Henderson and her docents
have assembled the most impressive museum I have ever seen. It truly rivals
any professionally run museum such as Smithsonian ones. [Airship-List]
Deutsches Museum Museuminsel 1

D-80538 München

Tel: (089) 2 17 91
Fax: (089) 2 17 93 24

Answering machine: (089) 2 17 94 33

Covers all fields of technology, but reported by Siegfried Geist to
have “a worthwhile section devoted to LTA.” Open daily (except holidays)
from 9:00 am to 5:00 pm.
Stadt Gersthofen Ballonmuseum Bahnhofstraße 10

86368 Gersthofen

Tel: (0821)2491 135 or 101

Five floors of ballooning history, technology and artifacts. Videos
of current aerostatics activity, and a special exhibit on balloons as a
decorative theme. Open Weds 2-6 pm; San, Sun and holidays 10 am to 6 pm.
Zeppelin-Museum

Meersburg am Bodensee

Schloßplatz 8

D-88709 Meersburg am Bodensee

Germany

Tel: 07532 7909

After hours: 07532 41042

Small private museum run by Heinz Urban, specializing in technical
Zeppelin artifacts. Collection includes a spark transmitter from a naval
Zeppelin, the complete bomb-release panel of LZ6 and many other technical
items. Open March through mid-November daily, 10 am to 6 pm. Guided tours
by appointment.
Albert-Sammt-Zeppelin-Museum Hauptstraße

D-97996 Niederstetten

Germany

Small museum honoring a commercial Zeppelin officer of local birth
who rose from helmsman in 1924 to command of LZ130. Multimedia presentation
on Zeps.
Zeppelin-Museum

Zeppelinheim (near Frankfurt/Main)

Zeppelin-Museum Zeppelinheim
Kapitän-Lehmann-Straße 2

63263 Zeppelinheim

Germany

A small Zeppelin museum housed in a municipal building in a Frankfurt
suburb, near the airport. When I was there in ’80, the Curator was an old
Zeppelin-Reederei Maschinist.
Tønder
Zeppelin Museum
Manfred Petersen

Museerne iTønder

Kongevej 55,

DK-6270 Tønder
Denmark

Tel:. (0045) 74 72 26 57 * (0045) 40 59 62 41

This is the old “Tondern” Zeppelin base.
Central Museum of Aviation & Cosmonautics Krasnoarmeyskaya 14

Moscow

Russia

NAS Richmond Museum

c/o Ford U. Ross

11020 SW 15th Manor

Davie, FL 33324

USA

Display commemorating Navy blimp ASW activity in World War II
Soukup &
Thomas Balloon Museum
700 N. Main St.

Mitchell, SD 57301

Tel: (605) 996-2311

Fax: (605) 996-2218

Museum Director, Becky Pope : beckyp@btigate.com

Museum of Flight East Fortune Airfield

North Berwick

East Lothian. EH39 5LF

Scotland

Tel: 062 088308 or

0131 225 7534

Models of the R100 and R34, plus the Lion Rampant Standard which adorned
the front of the R34.  There is also a plaque commemorating R34’s
[transatlantic] flight  to be seen [East Lothian was the point of
departure]. Several other LTA items are featured, including film excerpts,
handouts and bits of Zeppelin frame. [Ian Paterson]

Organizations
Name Address Description
Association
of Balloon & Airship Constructors
(ABAC)
P.O. Box 3841

City of Industry, California 91744

email: abac@archivale.com

Publishes quarterly Aerostation (now part of LTAI’s Airshipworld

journal)

Airship Heritage Trust c/o Shuttleworth College

Old Warden Park

Biggleswade, Bedfordshire SG 18 9EA

UK

Tel: +44 (0)1767 627195

Charitable organisation with a large collection of airship artefacts
and photographs relating to the

British Airship Programme from its early days at
the turn of the century to the Skyships of the

1980’s.

The Airship Association
[UK]
The SecretaryThe Airship Association

6 Kings Road,

Cheriton Folkestone, Kent CT20 3LG England.

Email: info@airship-association.org

Premier UK-based LTA association. Publishes the quarterly magazine
Airship.
Balloon Federation of America Box 400

Indianola, IA 50125

Tel: (515) 961-8809

Fax: (515) 961-3537

Publishes bimonthly Balloon Life
The Bombard Society 6727 Currant Street

McLean, VA 22101

Association of upmarket hot-air ballon operators.
Experimental Balloon
and Airship Association
Brian Boland

PO Box 51

Post Mills Airport

Post Mills, VT 05058

Free membership for anyone interested in experimental balloons or airships
Fédération Française de l’Aérostation 3 bis, square Antoine Arnauld
75016 Paris

France

LTA Society Box 6191

Johannesburg

2000 Republic of South Africa

Japan Bouyant Flight Association

Kyoritsu Kenkyru

402 Hitotsumatsu Bldg 1

2-3-14 Shiba Daimon, Minato-ku

Tokyo

Japan

33-433-2541

The Lighter Than Air Society 1436 Triplett Blvd

Akron, OH 44306

Tel: (847) 384-0215 (Robert Hunter)

fax: (330) 668-1105 (Attn: E. Brothers)

Publishes Buoyant Flight
National Balloon Racing Association Rt 11, Box 97

Statesville, NC 28677

(740) 876-1237

Naval Airship Association 901 Pillow Drive
Virginia Beach, VA 23454

(757) 481-1563

Publishes newsletter The Noon Balloon
Scandinavian LTA Society Drevkarlsstigen 2-4

Sollentuna

S-191 53 Sweden

Zeppelin Kameradschaft Kapitän-Lehmann Str. 2

Zeppelinheim 6078

Germany

Internet Resources
World Wide Web (WWW) Sites
Name Description
Association
of Balloon and Airship Constructors
Direct access to the 1600+ item Library List of LTA technical documents
available as reprints. LL can also be downloaded in ASCII or PDF format.
Links to other LTA organizations.
AIRSHIP –
The
Home Page for Lighter-Than-Air Craft
Hosted at the University of Colorado’s Web server by John Dziadecki,
this is truly the central reference for LTA on the Web.
The Airship Association
(UK)
Announces AA meetings and other LTA activities, esp. in Britain, plus
membership and subscription information. It has many links to other LTA
resources.
Airship & Blimp
Resources
Maintained by a young Swiss studying in the USA, it has many links
to other LTA resources, including photo archives.
Balloon Technology Database NASA-funded database of balloon technology, with 2300 documents indexed
as of 1997. Check the “Balloon Technology” box before beginning your search.
Promotions Dirigeables Web site of Paris-based LTA organization. Pages are bilingual (English/French).
Lighter-Than-Air
Technical Committee
,

American Institute of Aeronautics and Astronautics

Announces LTA TC activities. Note that permission may be required for
attendance by other than TC members; email first.
Lighter-Than-Air
Society [USA]
LTA organization with a primary emphasis on LTA history. Web page has
membership information, announcements and an email link.
Naval Airship Association Organization of former US Navy airshipmen dedicated to preserving the
memory of USN airship anti-submarine activity in WW II. Helps maintain
the LTA exhibits at the Naval Aviation Museum, Pensacola, Florida. Page
has announcements and membership information.
University of Akron
Archival Services
Information on how to use the University’s archival services. U. of
Akron is the custodian of the Karl Arnstein Papers.
Alan Gross (Airship Al) Independent consultant and lighter-than-air archivist.
Email Lists
Airship-List-Server

listproc@lists.Colorado.edu

World-wide discussion group about airships sponsored by the [UK] Airship
Association. To subscribe, send email to the address at left with the words
subscribe
airship-list
in the message body.
LTA-builder

lta-builder@launch.net

The emphasis in this list is on airships. To subscribe, send an email
message with the word

subscribe

in the subject line

Balloon Mailing List

majordomo@lboro.ac.uk

Hosts discussion of balloons, both gas and hot-air. To subscribe, send
a message to the address at left with

subscribe balloon [your email address]

in the body of the message.

AirshipList To subscribe, send a blank message to AirshipList-subscribe@yahoogroups.com

Indexes and Bibliographies
Source/Order Number Title & Description
Kent O’Grady

36 Martinglen Way NE

Calgary, Alberta T3J 3H9

Canada

email: kogrady@cadvision.com

Index of Buoyant Flight Bulletin – Lighter Than Air Society
260 pp. Cost:

$23.00 US for orders from the USA

$28.00 CDN for orders within Canada

$30.00 CDN for orders from any other country-surface

$45.00 CDN for orders from any other country-airmail

Index of Dirigible – Airship Heritage Trust

23 pp. Cost:

$4.50 US for orders from the USA
$6.00 CDN for orders within Canada

$8.00 CDN for orders from any other country-surface

$14.00 CDN for orders from any other country-airmail

ABAC – Acq. #126 Index of Daniel Guggenheim Airship Institute Report file. This is a different body of work from the papers that appeared in the DGAI’s three Publications. Now if we only knew where to get our hands on the reports themselves…
ABAC – Acq. #301 LTA Society Preliminary Inventory [this is a list of what LTAS donated to the University of Akron, which appears to have retained the Arnstein papers and donated the books to a county library]
ABAC – Acq. #439 Index of LTA Articles in Military Review
ABAC—Acq. #1427 Bibliography of LTA Articles in the US Naval Institute Proceedings 1912-60
ABAC – Acq. #463 David Taylor Model Basin tests of airship models
ABAC – Acq. #713 BuAer Technical Notes, 1916-1924. Another obscure report series.
ABAC – Acq. #802 Index of Aerostation through Volume 7 Number 3 [current volume is 22]. Kent O’Grady (see above) is preparing an up-to-date index.
ABAC – Acq. #946 Index of Airship #s 51-65 (Mar 81-Sep 84)
ABAC – Acq. #1409 Index of US Army Air Corps LTA Information Circulars

Return to ABAC Page

December 11, 2009

Putting Numbers to your Cooling System

Filed under: Engineering,Propulsion — piolenc @ 9:04 pm

[Originally published in Contact! magazine, Issue #62, pp 8 et seq.]


Radiator Installations


Putting Numbers to your Cooling System


Marc and his partner George E. Wright, Jr., formed Mass Flow in 1996 to pursue the design of ducted-fan-powered vehicles and to disseminate information concerning the design and integration of ducted propulsors. Their first book, Ducted Fan Design, Vol. 1, first appeared in 1997 and has just been extensively revised; Volume 2 is in preparation. MCM


INTRODUCTION


One of the difficulties that my co-author George Wright and I have encountered in preparing Volume 2 of our Ducted Fan Design series is in devising a practical procedure for designing an integral radiator installation. By “integral,” I mean only that the duct containing the radiator is a branch of the propulsive ducting—typically, an auxiliary inlet—allowing the propulsion fan to serve as the cooling fan as well. This is necessary (we think) to ensure cooling of a “buried” engine during runups and during long waits on the taxiway. Unfortunately, none of the more or less rule-of-thumb procedures we found in the light aircraft press would work for us, as the task we had set ourselves required a good match between radiator duct and propulsive duct flow where they met, and this required us to have some means of “putting numbers to it,” as Pazmany is fond of saying.

Not, I hasten to add, that this article will be useful only to those planning exotic airplanes. It is a fact, noted by Bruce Carmichael in his book Personal Aircraft Drag Reduction, that cooling drag in ordinary airplanes is often as much as ten times what theory says it must be; this is typical of the oversized, high drag installations that result when we really don’t know how much heat transfer we are getting and what the pressure drop across the radiator is likely to be under flight conditions or during ground runup.

We were therefore delighted when Contact! reprinted Prof. Miley’s AIAA paper,”Review of Liquid Cooled Aircraft Engine Installation Aerodynamics,” then made it available again in both volumes of Alternative Engines. This was a great service to firewall-forward innovators and reflects editor Myal’s recognition of the capital importance of efficient cooling in improving the overall efficiency of light aircraft.

The three key tools needed in designing a radiator installation are:

1. a mathematical model of heat transfer per unit frontal area as a function of air mass flow

2. a mathematical model of pressure drop as a function of air flow speed, and

3. valid data to plug into the two models, to obtain the parameters and get useful numeric results.

The first is needed to size the radiator for your engine, aircraft and flight regime. The second allows you to design the ducting and the cooling fan (if any), and to calculate the power loss (or drag increment) incurred by the cooling installation.

MODELING PRESSURE DROP

Miley’s paper seemed to offer a ready solution to the second requirement in the form of his equation #2,

w = a (σex Δp)b

where w represents the mass flow of air per unit time, σex is the ratio of the density of the air leaving the radiator to that approaching the radiator (which will typically be about 0.95 for most installations), Δp is the static pressure drop across the radiator (the pressure difference that you would measure with a manometer having one end upstream of the radiator and the other end just downstream), and a and b are constants to be determined from charted data for the radiator or core to be used. Solving for Δp in terms of mass flow gives:

Δp = (1/σex)(w/a)1/b

 

fig1.JPG


But when I attempted to apply it to data found in NACA Research Memorandum E7J013*, I ran into some trouble with the formula as published.

The first problem is fairly obvious with the equation in its original form, and that is that it is written in terms of mass flow, not mass flow per unit area. This makes it applicable to one radiator design, whereas what we need is something that characterizes a type of core and allows us to size the core to fit our needs. This problem is solved by rewriting in terms of mass flow per unit core frontal area w/A. Another benefit of this change is that, in the rearranged form of the equation, the quantity in brackets is more clearly related to the real independent variable, namely speed v, because mass flow divided by frontal area is air speed times air density (which for our purposes is practically constant), i.e. w/A = ρv. The original version was mathematically correct, but the constant a had to take care of the proportionality constants to convert mass flow to speed, as well as the parameter required to fit the data. The altered version makes the actual independent variable more obvious.The second problem also shows up when you try to use the equation in its original form, namely dimensions. As written, the quantity inside the brackets (that is raised to the power b) is dimensional, having the dimensions of pressure (F/L2). The chaos that this causes when it is raised to a fractional power is unbelievable. The dimensions can of course be made to come out right by adjusting the units of the constant a, but that gives units for a that have no clear physical connection to the problem, and are dependent on b. It makes sense to make the quantity inside the brackets non-dimensional. The easiest way is to substitute for Δp a ratio of pressures, Δp divided by some suitable pressure related to the problem. The quantity being normalized is a static pressure drop, so the logical normalizing quantity is the static pressure of the incoming air. With the quantity in brackets now non-dimensional, the dimensions of the constant a are M/L2T, or mass flow per unit area. Perfect! Our revised formula is then

w/A = a(σex Δp/p)b

where p is the static pressure of the air stream coming into the radiator, and A is the frontal area of the radiator core.

Rearranging this to solve for Δp gives:

Δp/p = (1/σex)(w/aA)1/b = (1/σex)(ρv/a)1/b

 

Incidentally, for the information of our non-mathematically-inclined readers, it’s worth mentioning at this point that it is very easy to determine whether a given dataset is a good fit to an equation of the form y = axb, provided that the data are plotted on a log-log chart, that is one with logarithmic scales for both the x and y axes. Such datasets give straight, or nearly straight lines on a log-log graph, and the slope of the line is the value of the exponent b! Finding the value of the exponent is as easy as selecting two extreme points (xmin, ymin) and (xmax, ymax) on the line and using the formula:

b = (log ymax – log ymin)/(log xmax – log xmin)

Another interesting property of log-log charts is that even if the axes are rescaled (values multiplied by a constant), the slope remains the same.

Once the slope is determined, only one data point (which can be one of those used to determine the slope) is needed to determine the remaining parameter.

Turning now to NACA Research Memorandum E7J01, we find there some very useful data provided by the Harrison Radiator Division of GMC in connection with the development of condensers for nuclear-powered aircraft with Rankine (steam) cycle power plants. Oh well—a core is a core! Using the slope formula above and the second form of our revised equation to fit the pressure drop chart there (Figure 2), we get an exponent 1/b of 1.79—essentially identical to the 1.8 that Hoerner4 says to expect in this kind of data.

fig2.JPG

For obtaining the remaining parameter we need to rearrange our equation to suit the way that the Harrison data are presented, namely as σΔp vs. mass flow. We assume that their σ and our σex are the same; they aren’t, but both will be close to 1 in our application, so the different definitions don’t matter much. We end up with

σΔp = p(w/aA)1/b

Solving for the parameter a gives

a = (σΔp/p)-b (w/A)

 

and plugging in the values σΔp = 30 inches of water, w/a = 600 pounds per minute per square foot from curve “A” of Figure 2, and assuming that the static pressure of the incoming air is sea-level pressure (p = 407.2 inches of water absolute) just for the sake of getting a numerical answer, we get 2555 pounds per minute per square foot. We can now generate numbers for pressure drop for this type of core for any core size for any flow rate within reason.


HEAT TRANSFER MODELING

Okay, now that we have one of our tools, it’s time to start developing the other—the heat transfer model. Looking now at heat transfer data from the same very useful NACA RM (Figure 3), we make a very pleasant discovery—the heat transfer coefficient-vs-air flow data are also a good fit to the same type of equation! The slope is .84, which is pretty close to the figure of .80 given by Hoerner as typical. Tentatively, we write our curve fit in the form:

K = k(w/A)r = k(ρv)r

where K is the heat transfer per unit time, per unit frontal area, per unit of temperature difference between the coolant and the air; k is a parameter to be determined from the data, and r is the slope that we’ve already found, namely 0.84.

Again, we want the quantity that is raised to a fractional power to be nondimensional, so we need some suitable normalizing factor. The important quantity in the brackets really is mass flow (per unit area) this time, early radiator analysis having shown that “heat transfer is a function of mass flow of air, regardless of density1” so we need a “reference” density and speed for normalization of the quantity in parentheses. The logical choice of normalizing speed is the speed of sound, and for density the density of the incoming air. The quantity inside the bracket then becomes the Mach number of the air approaching the core, multiplied by our old friend σex.

We now have:

K = k(σex v/c)r = k Mr

Where c = (1.4 p/ρ)1/2 ,the speed of sound at the pressure and density of the air approaching the radiator.

“What happened to air density ratio σex inside the brackets?” you ask. To be absolutely honest, we cheated a little. Because it is essentially constant and very close to 1, we moved it outside the brackets and incorporated it into the parameter k . We will simply divide the mass flow numbers by ρc before determining the value of k. At sea level standard conditions, c = 67,200 feet per minute, while ρ = .07382 pounds per cubic foot. As long as we’re rescaling, we divide the K values on the vertical scale by 100, to put them on the basis of heat transfer per degree Fahrenheit, rather than per 100 °F temperature difference between coolant and air.

We’re now ready to find the parameter k. Taking the point on the curve where mass flow is 500 lb/min and heat transfer coefficient K is 80 BTU/min/°F/ft2, we divide 500 by ρc to get a Mach number of .1008. Solving for k gives

k = K/Mr = 80/.1008.84 = 550 BTU/min/°F/ft2

fig3.JPG

REPLOTTING DATA

But what if the data you get are not plotted in log-log form? If the data are in the form of a table of numbers, great! If the table is a clean copy, you may be able to scan the numbers, OCR them and import them into a spreadsheet program, which will oblige you with a plot scaled any way you like. If the data are presented graphically to linear scale, then you will have to pick off data points as accurately as you can, then enter them in a spreadsheet and get your log-log plot that way.

GETTING DATA?

Now that you, the long-suffering reader, have plowed through the above, you are no doubt wondering how to use this information. Obviously, the specific numbers that we got for a, b, k and r are not useful in themselves, unless you are planning to order a half-century-old core type from Harrison (might be interesting to get their reaction to that inquiry!). What is useful is the procedure for testing data for conformity to Miley’s formula and its ilk, and for calculating the parameters that apply to each particular core that you are interested in. What you still need, in order to apply this procedure to your project, is data on the core or cores that you are thinking of using. Logically, you would expect to be able to obtain these data from the manufacturer of the core, but I honestly don’t know whether pressure drop has even been measured for any automobile radiator. Judging from the design of the auto radiator cores that I’ve seen, the manufacturers are primarily interested in ensuring adequate heat transfer under adverse conditions, with aerodynamic efficiency either completely neglected or given only secondary consideration. Mind you, all the cores I’ve seen come from older vehicles; perhaps with the trend toward smaller engines, lighter vehicles and higher efficiency in general somebody has been gathering the data we need. Maybe. Maybe we can even get it. Maybe.

If we can’t get the data, then we have to develop it ourselves (or limit ourselves to the use of heat exchangers for which the necessary data can be obtained, which might be expensive). How would we go about doing that? When I first thought about this problem some years ago I sketched out an elaborate installation that would use an immersion heater to heat water in a reservoir, a pump to circulate it though the test radiator, a kind of wind-tunnel arrangement to blow air through the radiator, and instruments to measure temperature, pressure drop and flow rates and provide a basis for controlling all this. Needless to say, the rig did not get built. Early NACA radiator test rigs anticipated my thinking by many decades, and actually made mine look simple. Later work by NACA2, however, revealed that you don’t need to have heat transfer taking place to get realistic numbers for pressure drop in service—there’s a procedure for correcting test results obtained without heat transfer so as to to take heat transfer into account. That simplifies the test rig enormously, provided of course that we don’t need to take heat transfer data (not so unrealistic, considering that in the air conditioning industry, for example, heat transfer data are often provided, while pressure drop data often are not). Also please note that if the dataset fits the model we’ve been using, we only need two reliable data points to get the two parameters. That means that the calibration of our test rig need only be accurate at two points, again making the job easier. Other, intermediate data points would have to be taken to verify conformity with the model—otherwise we would be guilty of the sin of assuming our conclusions—but “record” accuracy would only need to be achieved at two widely separated points. Worth looking into, I think.

As a last resort we might consider yet another alternative, that of reverse engineering. This would consist of taking the heat exchanger design methods of e.g. Kays and London5 and inverting them to derive heat-transfer and pressure drop characteristics from the shape and dimensions of the radiator’s core. This could get tricky, but used in conjunction with cooling systems analysis technique presented in Küchemann and Weber6 it might be doable.
FMP

REFERENCES & FURTHER READING

1. Results of Tests on Radiators for Aircraft Engines;
Part I.—Heat Dissipation and Other Properties of Radiators by H. C. Dickinson, W.B. James, and H.V. Kleinschmidt
Part II.—Water Flow through Radiator Cores by W.S James. NACA Report No. 63

2. Becker, John V. and Donald D. Baals: Simple Curves for Determining the Effects of Compressibility on Pressure Drop Through Radiators. NACA Advance Confidential Report L4I23, September 1944.

3. Humble, LeRoy V. and Ronald B. Doyle: Calculated Condenser Performance for a Steam-Turbine Power Plant for Aircraft. NACA Research Memorandum E7J01, May 20, 1948.

4. Hoerner, S.F.: Fluid-Dynamic Drag (published by the Author, 1967)

5. Kays, W.M. & A.L. London: Compact Heat Exchangers (McGraw-Hill, 1964)

6. Küchemann, Dietrich & Johanna Weber: Aerodynamics of Propulsion. (McGraw-Hill,1953)

December 5, 2009

Applications of Ducted Fans

Filed under: Propulsion — piolenc @ 12:32 pm


from Ducted Fan Design, Volume 1


Applications of Ducted Fans



Why a Duct?


In recent years, there has been a steady increase in interest among both model and full-scale light aircraft builders in the use of ducted fans for propulsion. In the case of RC modelers, the reason for the interest is often esthetic—the builder wants to build a realistic scale model of a jet airplane, and nothing but a buried engine and ducted fan will do. This is also true of some amateur airplane builders who want a “jet-like” airplane that offers the illusion of flying a high-performance military job, but without the cost or the risk. Still others have specific design requirements, e.g. for high static and low speed thrust, or design constraints such as a ground clearance that limit propeller diameter.

These designer/builders have read somewhere that ducted fans offer a higher static thrust/horsepower ratio for a given diameter than open propellers. Elsewhere, they read that ducted fans are useful for allowing large-diameter fans to operate at high speeds, and penalize low-speed performance to achieve this! Unfortunately, when they search for coherent written design guidance, they find that published information falls into two categories. At one end of the spectrum are the theoretical texts and technical papers written for professional engineers. The best of these texts are either admittedly out of print or are dishonestly listed by their publishers as in print, but are always “out of stock.” (Kuechemann and Weber has been “out of stock” for at least eighteen years to our certain knowledge.) At the other end of the spectrum are plans and construction manuals for specific projects, usually model-airplane related, that offer little information on scaling laws, still less on fundamental design principles. The model airplane and experimental aircraft Press sometimes publishes articles on specific ducted-fan aircraft, more rarely on ducted fans in general, but again the writers appear to have no understanding of their subject matter, as they frequently accept uncritically the claims of equally unqualified designers. The effect has been to leave the reader with a vague notion that fans can be a very good thing for someone, somehow, without clarifying to whom or how.

This chapter will summarize the advantages and limitations of ducted fans and list some of the applications in which they can do a better job than open propellers. We won’t go into the details of flow physics here, nor into the limitations of the breed. Design parameters influencing the performance of a ducted fan will also be discussed later. This is just a very brief explanation of the reasons that some engineers have for being interested in this class of propulsion machinery.

There are, as we hinted above, two major fields of application for ducted fans, at the high and low ends of the subsonic speed range respectively. The field of application that we have all experienced is the one least useful to us as light aircraft designers, namely the high speed range exemplified by the turbofan engines that propel the airliners on which we have all flown at one time or other. In these machines, low speed thrust/horsepower is sacrificed in order to make a ducted fan operate with reasonable efficiency at speeds approaching that of sound. More rare in our experience are low speed applications, typically VTOL aircraft and air-cushion vehicles. Here we will concentrate on static and low-speed thrust, because that is where the advantage of a duct is greatest and also because we won’t need any derivations to make the effects clear.

We start with the familiar diagram of an open propeller immersed in a stream of fluid; this is how cruising flight looks from the airplane’s point of view. The flow has three regions: the inflow into the propeller, the slipstream behind it and the free stream which doesn’t go through the propeller. Propeller design calculations are all carried out in this regime. But before a ‘plane can cruise it has to take off, and its takeoff performance depends on the performance of the propeller at low speed and at rest.


1


Applications


With the propeller immersed in a static air mass, the picture we saw earlier changes radically. Now there are two regions. The slipstream behind the propeller is familiar, but there is no more free stream; everything that isn’t slipstream is in the inflow region. This means that the propeller is aspirating air from behind it as well as from in front. Consider now the plight of an intrepid air particle starting just outside the slipstream, behind the propeller. It has to go forward to the propeller disk, make a 180-degree turn, accelerate instantly and enter the slipstream. As real air particles have mass and therefore inertia, and as open propellers have to be lightly loaded at the tips, the result is that a region of reversed flow exists near the propeller tip. This diminishes the effective disk area of the propeller and restricts the amount of air able to flow through it, just when the highest mass flow is needed! Now take the same propeller and surround it with a close-fitting shroud having a nicely rounded leading edge, and see how the picture changes.


We still have only two flow regions, but now there is a solid boundary between them in the neighborhood of the propeller—the shroud or duct—and our air particle has a much easier time doing what is expected of it, because it need only flow around a duct lip or leading edge of finite radius. We can even provide the duct lip with a slot to help keep flow attached, as suggested and tested by Krüger. The duct also shields the propeller from the harsh realities of the outside world, and the propeller “sees” air flowing in only one direction—front to rear. In fact, from the propeller’s point of view it is not at rest at all, merely cruising at some fraction of its maximum speed. What is more, the endplate effect of the duct wall allows the propeller to carry a non-zero load right out to the tip. The effects of all of this are that:


  • Fewer design compromises are required; the ducted propeller operates nearer its ideal operating point throughout the aircraft’s speed range

  • The effective diameter of the ducted propeller is larger than its physical diameter. To understand why, look at the diagram of the open propeller at cruise and note that the slipstream contracts behind the propeller. Now look at the ducted propeller and note that the slipstream diameter is that of the duct exit. That larger diameter represents a smaller exit velocity and a larger mass flow—and by now we know that means higher thrust/horsepower.

  • While the propeller develops about the same thrust as before, there is now a second force acting on the duct. If the duct is shaped correctly, this force is additional thrust.

Another area in which ducts offer advantages is noise suppression. Ducts allow noise to be reduced three different ways:

  • Running the propeller under optimum flow and loading conditions eliminates the propeller-tip “buzz,” a substantial component of the noise of a propeller-driven airplane.

  • Enclosing the propeller in a duct allows various acoustic treatments to absorb noise before it can impinge on the ears of bystanders.

  • By offering improved static and low-speed thrust, ducted fans make possible steeper climb-out, which in turn reduces perceived noise at the airport boundary, an important public-relations advantage.

openpr~1.JPG
openpr~2.JPG


2


Ducted Fan Design

At cruise, the advantages are less pronounced and limitations more obvious, but we can still note that the effective area of the propulsor is approximately that of the duct inlet, which may be considerably larger than the swept area of the propeller itself.

We can note here a subsidiary advantage of the ducted propulsor, namely that it offers the possibilty of thrust vectoring and even thrust reversal.


Applications


The alert reader will already have chosen one or more applications based on the characteristics mentioned above. They include:


Autogyros


Although it can be argued that autogyros (rotorcraft whose lifting rotors are not powered) are not inherently high-induced-drag machines, nearly all the machines currently on the market or built from plans are designed for low takeoff and landing speeds rather than high cruise performance. As a result, they need a great deal of static and low-speed thrust from their powerplants. This already makes them good candidates for ducted fan propulsion, but another factor is now intruding, namely noise regulations. Autogyros are noisy! Ducted fans allow lower installed power, lower source noise and faster climb—all tending to reduced perceived noise.

ductprop.JPG
ductpro2.JPG


3


Applications


Compound Helicopters


Compound helicopters are so called because they are hybrids of helicopters, autogyros and fixed wing aircraft. In addition to having supplementary fixed wings to supplement rotor lift at high speeds, they are also provided with means of forward propulsion independent of the lifting rotor. Not all compounds use ducted fans, but their small diameter for a given thrust/power ratio makes them attractive in this application, as does the fairly modest top speed of application (compound helicotpers, though faster than conventional helos, are still limited by rotor tip speed and stability).


Seaplanes, Flying Boats and Amphibians


These require a great deal of thrust at low speeds in order to overcome wave drag and viscous drag while on the water. Once the hull (or the pontoon) is “up on step” and planing, drag drops considerably and the airplane can accelerate to takeoff speed.

The limited static thrust of open propellers has two unfortunate effects on the design of seaplanes and amphibians. Typically, engine horsepower is dictated by the “hump drag,” the drag that must be overcome to begin planing (to “get up on step,” in seaplane parlance), rather than by cruise requirements, so the airplane ends up carrying a lot of extra engine weight that is used only at the very beginning of each flight. This extra engine weight in turn cuts into payload.

Unable to exert much control over water drag, designers often try to keep total thrust required at low speed as low as possible by minimizing induced aerodynamic drag; this is accomplished through some combination of increased wing area and/or span and by eschewing the use of flaps for takeoff. Of course, the designs thus arrived at end up carrying a lot of extra wing at cruise speed, which imposes both a weight and a drag penalty.

piasecki.JPG


4


Ducted Fan Design


The only way to get better thrust per horsepower out of an open propeller is to increase propeller diameter. This of course creates clearance problems, requiring truly heroic solutions—very high wing positions or pylon-mounted engines or deep fuselages. One recent design mounts the engine on the top of the vertical tail! It is probably not necessary to point out the weight penalties these configurations impose, although in recent years materials with very high strength/weight and stiffness/weight ratios have become available and have made some wild designs at least feasible, though not necessarily practical. Another consequence of larger propellers is the necessity of geared engines to control tip speed—more weight!

Ducted fans can help. First of all, they can achieve a higher thrust per horsepower for a given diameter, mitigating clearance problems and allowing the use of direct-drive engines turning high rpm. By making high thrust available at low speeds, they allow the use of higher wing and span loadings and the use of high-lift devices for takeoff. Finally, thrust vectoring and thrust reversal make mooring and taxiing much easier, but at a lower cost (and sometimes at a lower weight) than a controllable-pitch propeller.


Short Takeoff Aircraft


STOL airplanes need high static and low speed thrust, both for overcoming inertia to accelerate to takeoff speed and to counter the high induced drag at takeoff of wings operating at very high lift coefficients. In addition, aft-mounted ducted fans can provide additional static longitudinal and yaw stability at low speeds.


Vertical Takeoff Aircraft


Several VTOL aircraft have employed ducted fans in various configurations and capacities. The Ryan “Hummingbird,” for example, used fixed vertical-axis fans—one in each wing panel—to provide lift during VTOL and STOL operation. “Barn door” covers closed over the fan intakes and exhausts when they were not in use. More familiar shrouded lift/cruise fans were developed for other VTOL configurations. One Piasecki design used fixed fans with more or less horizontal axes, vectoring thrust with airfoil cascades in the exits. Another design (designation unknown to the authors) had swiveling ducted fans mounted at the tips of tandem wings. In every case, the high mass flow (hence high thrust/horsepower) per unit frontal area was the main reason for choosing ducts despite additional cost, complexity and manufacturing problems. The alternative, exemplified by the likes of the “Convertiplane” and the recently abandoned Osprey, is a very large-diameter open rotor reminiscent of helicopters but used for both lift and propulsion.

marvel2.JPG


5


Applications


Airboats and Air-Cushion Vehicles


The arguments for seaplanes apply even more strongly to these vehicles. They have, by aircraft standards, a very narrow speed range, but need substantial thrust for overcoming surface drag and (in the case of the air-cushion vehicles) for driving uphill (e.g. up a boat ramp). Thrust vectoring is essential to air-cushion vehicles that must occasionally travel crosswind or traverse a slope, and can be achieved either by swiveling the duct or by employing a cascade of airfoils in the duct exit. A very special kind of ducted fan is that used to provide air-cushion vehicles with lift, although it is outside the scope of this book because it is not a propulsor.1

Wing-In-Ground Effect (WIG) Craft


Wing-in-Ground effect machines minimize induced aerodynamic drag by flying near the ground (or rather near the water, since they are less likely to encounter obstacles there). They are inherently low-speed craft, since induced drag loses its importance as speed increases and parasite drag becomes the dominant component. Their application—if they ever have one—will be carrying cargo and perhaps passengers over open water at speeds higher than those of surface craft but lower than those of conventional aircraft. WIG machines are ideal for ducted fans, because their cruise power requirements are even lower than those of conventional seaplanes, but their takeoff thrust requirements are as great or higher. Their narrow speed range completes the profile of an ideal fan application. Here considerable engine weight savings should be possible over WIG machines with open propellers.2 The arguments concerning propeller clearance, taxiing, etc., used in the seaplane discussion, are equally applicable here. There is an additional reason for using ducts in WIG machines, and that is thrust vectoring in a vertical plane. Certain WIG schemes (e.g. Bartini) require that the jets from canard-mounted ducted fans be directed under the wing for liftoff, then redirected straight aft for cruise.

1060p.JPG


6


Ducted Fan Design


Airships


Vectored thrust is the key to improving the economics of operating an airship, whether it’s the monstrous rigid airships of old or the modern nonrigids (blimps). The ability to vector thust in the vertical plane allows an airship to take off statically heavy and to land light. As much as 25% can be added to the airship’s useful load, which can be devoted to payload, extra fuel or both. Navy blimps routinely used rolling takeoffs to achieve the same thing, but vectored thrust allows vertical takeoffs and landing approaches, eliminating the need for a runway. Ducted fans, though not essential, offer the advantages of reducing the weight and size of the swiveling propulsor. Ducted fans can also be very quiet, as mentioned above; this is a big advantage to ships operating near the ground in populated areas, which they must do in order to be effective advertising media! The Skyship 500 and 600 non-rigids currently in service use dual car-mounted swiveling ducted fans driven by Porsche aircraft engines. They are very quiet, and watching one rise vertically in a level attitude from its mooring circle is a very different experience from the violent and noisy “up ship” maneuvers of conventional blimps.


…and Why Not


This discussion would not be complete without some mention of the drawbacks and disadvantages of ducted fans for aircraft. We have talked about their potential for high static and low-speed thrust, but we haven’t talked about the cost.

sk600_1.JPG


7


Applications


As we will see later, a ducted fan with fixed exit area is optimized for one speed. So long as the speed range of the vehicle is low, there is no problem, but as the speed range increases it becomes increasingly difficult to get a satisfactory design. As speed range widens, first the exit, then the inlet must be provided with variable area. The mechanisms needed to do this add weight and cost to the design.

In this book, we will assume a speed range of 0-200 knots roughly. This allows us to design for cruise—which we must do in order to avoid a stiff drag penalty—while still ending up with a fixed duct geometry that can achieve decent static thrust.

Other ducted-fan material

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