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
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,
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
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'
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
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.
November 9 2004