The lifting skin

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The lifting skin

Basically this is yet another idea to build a skycar. I had to withdraw half my Web pages on the subject because they were nonsense so this time I made at least a basic experiment.

If you've read my other pages you know that the larger the surface a propeller sweeps over the stronger its pull force will be (for a same motor power). That's why a helicopter has such a huge propeller above it. Now that propeller is what everybody wants to avoid in a skycar, because it's expensive and it's dangerous.

How can we get the pull force of an helicopter propeller while using a cute little airplane propeller? The answer inside this page is to use a nozzle. At first a kingsize of a nozzle. Let's consider this helicopter with its big propeller:

Using a little propeller and a nozzle will yield the same lift force and the helicopter will be able to rise in the air just the same. Provided the input of the nozzle has the same diameter as the big propeller above:

Such a giant nozzle would be quite heavy and utterly sensitive to wind. No way a real helicopter ever be designed that way. Even though it would ease the gyration problem. The giant nozzle can only be used for toys or inside a balloon or dirigible.

In order to try out the nozzle system I build this little test bank with a little electric DC motor, cardboard, copper wire, toothpicks, hot glue and a little kitchen scale:

A basic formula that links motor power and propeller sweep surface is the one beneath. F is the pull force, S is the surface swept over, rho is the air density, P is the motor power.

The propeller has a sweep surface of 0.007 m². Without the nozzle it pulls with a force of 0.49 Newton. The nozzle input surface is 0.025 m². When the nozzle is latched to the propeller the pull force should become 0.75 Newton according to the formula. I measured 0.78 Newton, which is a good match. Thus the nozzle allows to generate a pull force equivalent to that of a propeller with the same sweep surface as the input surface of the nozze. (At first I'd forgotten to take the electric power consumption into account. But it's OK: without the nozzle the motor consumes 1.90 A. With the nozzle it consumes 1.88 A. (12 V DC))

How to reduce the weight and size of the nozzle? The linear nozzle I depict isn't optimal. Using an aerodynamically shaped and optimized nozzle for sure will help. Yet the difference won't be tremendous. A more serious answer is to use several little nozzles side by side. Together they present the same input surface as the big nozzle. Yet their global height and side surface is far reduced. Their outputs are ducted towards the little propeller above the helicopter:

Most obvious problem with such a layout are the losses due to friction and turbulences inside the ducts. I will assume those losses are acceptable but I've no proof thereof. (In the worst case it is possible to use no ducts, by using a little propeller beneath each nozzle.)

The reasoning has to be pushed further, towards a near flat array of nozzles; a sea of little nozzles on the upper surface of a fat discus:

That fat discus can be shaped in order to become an acceptable wing. The "helicopter" would rise in the air by sucking the air through the upper nozzle skin of the discus. The helicopter would accelerate forwards (using a second propeller, whatever...). The helicopter would pitch up a little, which would make the discus act as a lifting wing. Gradually the sucking action on the upper surface would be stopped and the helicopter would fly like an airplane. This isn't the best solution anyway its realistic (provided the nozzle array trick works).

For sure there is no best design to use a lifting skin system. It all depends on the purposes, the available technologies and the designers' skills.

If a flying device is meant to fly at high speed over long distance it is forced to have the shape of an airplane: a slim body and wide wings. Such aircraft has a powerful motorization. The drawing below shows an aircraft with its wings and upper front covered with arrays of nozzles. Inside the plane those nozzles are ducted towards a compressor linked to the front motor. When taking off or landing the front propeller blades can be at zero pitch and have no influence. All the power goes to the inside compressor. That makes the nozzle skin lift the aircraft in the air. Once in the air the power can go gradually to the front propeller and the compressor be disconnected. When landing the reverse happens: the power on the propeller decreases and goes gradually to the compressor.

Many questions arise with such a design:

How be the aircraft roll, pitch and yaw controlled when hovering? Should the compressor output be used like in a harrier aircraft, linked to output nozzles at the wing tips and aircraft body extremities? On the opposite should the skin nozzle suction be modulated?

What about security? What happens if the motor fails when hovering? When the motor fails on a plane it can glide. When it fails on a helicopter it can rely on its wide propeller to brake the fall. No such options with this design. At low altitude there is even no possibility to deploy a parachute. Besides a parachute capable to sustain the whole device would be heavy. The motorization and nozzle control should be made very reliable, thus expensive. What if a simple plastic bag covers part of the input nozzles? Should the nozzle surfaces be protected by pikes or wires?

What is the influence of the nozzle skin when the device is flying horizontally? Will the little nozzle inputs act like useful riblets? Or will they cause turbulences and brake? Can a light suction on the nozzles eat up the surface turbulences and contribute to the device's aerodynamics? Or would it be too energy consuming? Should part of the nozzles inputs close themselves once the device is in flight, say the onces near the leading edge?

There will be a strong vertical air flow downwards towards the nozzle skin surfaces. Much like the air flow around a helicopter. How well will that flow interact with the wing shape? What turbulences and instabilities will it cause?

For short distances and low speeds, maybe a more lentil-shaped device should be preferred. I like much the design below, which is closer to a domestic use. It has nozzle arrays all over its body. It would use them for all movements : lift off, pitch, yaw and roll control, fast forward movement and backwards. The left picture is a side view with the front right. The right picture is a view of the front, with a large front nozzles array for normal frontwards travel:

The supposed advantage of this design is it is insensible to wind. Whatever wind and turbulences occur, a simple computer system can tune the suction on the nozzle skin arrays to maintain the position and attitude. Much like a submarine does with its attitude control propellers all around the body. The pilot can drive it a simple way, without any knowledge of aerodynamics.

To get some security and comfort maybe the following can be done:

Rely on redundancy. The device can contain a set of compressors and their suction be shared amongst the air ducts. The failure of some compressors would cause no problem till their repair or replacement. Also I would subdivide each nozzle array in many little arrays. Each little array with its own little air power actuator and rudimentary inertial platform and attitude control processor. Such rudimentary little electronic inertial platform and processor can weight a few grams and consume little electric energy. There should be maybe a hundred such sets of inertial platform + processor + actuator + nozzle array. The failure of even many sets would be harmless. The processors are linked through a cross-meshed highly redundant network and share their informations. A few more accurate and refined inertial platforms and navigation control computers are spread amongst the device and help the processors take fine-tuned decisions. Ultimately the whole obeys to the pilot's wishes.

The device should be rather lightweight and its structure be designed to crunch during a crash. So a fall from some height would destroy the device yet spare the passengers. Some additional control surfaces and a little parachute may help tune the fall.

A little foam can be used inside the air ducts to decrease the noise. Especially the compressors' outputs can be guided through flat ducts with foamed inner surfaces.

What other tricks can be used?

Inflatable, foldable or shovable nozzle arrays? This would increase the nozzle skin surface.

Inflatable, foldable or shovable wing surfaces? Popping a few little wings aside of the lentil shaped device would act like huge winggrids and offer a better flight efficiency. A Flying Flea wings shape would offer some advantages and better security.

Superposed lifting surfaces? Like a biplane aircraft, I expect superposed surfaces to add each other's lift force:

Is a nozzle better than a big propeller? Propellers have proven their efficacity. A propeller is a lightweight device that can adapt to very different speeds, from zero up to near the speed of sound. A nozzle on the contrary will act like a brake at high speeds. An adaptive nozzle can be conceived but it will be heavy and expensive.

Eric Brasseur - November 9 2004