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.

Powered by WordPress