How to properly design a VTOL/ADAV helicopter-plane?

[Remundo]

what interests us is to extract the ratio V1 / V0 of this last formula, here is:

eq9.png (8.23 KiB) Viewed 950 times

And this expression injected into the propulsion output RP results in

eq10.png (6.31 KiB) Viewed 950 times

Propulsion efficiency is only good if the wing force (M/A) is minimal.

Which corresponds well to what we were saying just before: you have to mix a large surface.

To be more precise, here are the calculations published on interaction.free.fr

charge_alaire_Rp.gif (248.67 KiB) Viewed 950 times

on this same page you can read about the inadequacy of turbojet engines on many small planes. If these turbojets certainly have a strong thrust, their wing loading is enormous, which considerably degrades performance and makes them fuel sinks.

Even big planes benefit from keeping big propellers.. what understood the Tupolev TU 95, a turboprop, drives counter-rotating propellers 5,6 m in diameter at a speed of 750 rpm, and provides them with a power of 11185 kW (i.e. 15207 hp).

However, turbojets are more suitable at high altitude.

And the propellers cannot be enlarged to infinity, otherwise there will be supersonic flows at the tip of the blades., and even strong turbulence in subsonic.

Here the flight envelope of an ADAV/VTOL is rather at moderate speeds and altitudes (X00 km/h, X000 m) with integer X not too large.

We will therefore retain from all this that we must not be "violent" for our light aeronautical applications.

[Christophe]

Why do you think there are 2 propellers on a Dragonfly? https://dragonfly-paramotor.fr/

To precisely increase the brewed surface and therefore the propulsive efficiency with the same ground clearance... in addition to improving their efficiency (arrival of laminar air) and having a 0 torque machine. These 3 points are the first conclusion of the preliminary studies of the 2017 project...

But there are still few people who understand that...hard to educate people accustomed to flying with aeronautical horrors (errors?)!

Otherwise the theory of the flow of the propellers, it is the theory of Froude not Bernouilli... even if it is derived from it...

If you are interested in propellers and wings, Remundo, I have a Heliciel license https://heliciel.com/ and 2 or 3 free software which will be useful to you... which I must find at the bottom of my pécé!

There is already AirfoilTools: http://airfoiltools.com/

[Remundo]

I was sure you would like it;

There is a convergence between your paramotor Dragonfly and my multirotor ADAV ideas.

It's light propeller aeronautics and we want good energy efficiency.

so for your large propellers, I validate 100%, but you had done the job before me I believe

[Christophe]

Yes I believe too

There are many convergences in the good aeronautical studies you know... but between the study and the practice years ago... you will see if you launch yourself concretely into this project...

I could have put more propellers (I stopped at 8 in the studies ) but it's bigger, more space on the ground, more risk of failure, more complex regulation, more wiring...etc etc...so 2 is the best compromise... according to me...

Then you mustn't disturb the pilots' brains too much...

[Remundo]

The question of THE rotor, and also OF the rotors

The single-rotor helicopter has long had great difficulty in flying.

This large rotor poses mechanical problems:
M1) the counter-torque on the frame
M2) the gyroscopic couple (which will be discussed in the next message)

and aerodynamic!
A) differential lift between the blades attacking the wind and the blades fleeing the wind.


The M1 problem by virtue of the action/reaction, causes the helicopter to spin on itself, it is possible to create a counter-torque by placing a tail rotor, as explained by our national Jamy:

This tail rotor is usefully put to good use in controlling the yaw of the helicopter and perhaps interesting for maneuvering the device more easily even if there is no counter-torque to compensate.


The aerodynamic problem A was a big thorn to solve: indeed when the helicopter moves forward in the fluid, the blade attacking the air forwards generates much more lift than the blade fleeing the air (towards the rear). If the blades are set to the same angle of attack, this generates a roll torque (the helicopter tends to roll over).

A solution, expensive but essential, was to create very complex rotors with variable incidence blades controlled by a swashplate

The swashplate can:
* to vary the pitch of the blades identically [Collectively], which allows up/down movements of the helicopter depending on the balance between the lift of the blades and the weight of the device

* to differentially [cyclically] vary the pitch of the blades : which allows the helicopter, in horizontal movement, to be more stable by not generating any roll torque.

A popular article on this subject


But since you can't tilt/adjust the blades infinitely, it limits the horizontal speed of the helicopter. 300 km/h is the usual limit.

Explanation from 14 minutes:

Note that an experimental helicopter has reached 472 km/h, but it is starting to look like... an airplane, with small wings and traction propellers to compensate for the aerodynamic problems of the rotor...

It is Eurocopter X3

Eurocopter_X3.png (285.06 KiB) Viewed 784 times

An originality of this VTOL, more helicopter than plane, is that it has no tail rotor. The counter-torque of the rotor is compensated by a differential thrust between the 2 left-right propellers.

The machine does not need an anti-torque tail rotor, this function being ensured by a difference in thrust between the two propulsion propellers. Combining the speed of an airplane and the agility of a helicopter, particularly on take-off, the X3 made its first flight on September 6, 20101 from the facilities of the French Armament General Directorate at Air Base 125 Istres -The Tube. On May 12, 2011, the X3 reached 430 km/h for several minutes4.

Your Webmaster Christophe will appreciate, because his Dragonfly paramotor also uses this "vector thrust" aspect to improve the turns this time.

[Christophe]

What not to do:

But hey, this kind of useless shit, it makes you hard on social networks...

ps: there are still interesting things to see in the video... on the passages in slow motion... I say no more I'll let you look...

[Remundo]

The gyroscopic couple, its origins, how to cancel it?

in general, a heavy object launched at high speed does not like to change its trajectory. If one has the audacity to try, for any acceleration imposed on the object, this one opposes a force F = mass x acceleration.

So what about rotating objects?

rotating objects carry in a circular motion a multitude of small masses. If one decides to want to change the axis of rotation of the object, this amounts to disturbing the circle where each small mass evolves.

So the object will react and apply a gyroscopic torque which opposes the rotation imposed from the outside

The rotating object rotates relative to a frame, which itself can rotate relative to the ground.

So we have two rotation vectors:

eq11.png (3.48 KiB) Viewed 762 times

By calling I the moment of inertia of the rotor, these 2 rotations combined create the gyroscopic torque:

eq12.png (6.2 KiB) Viewed 762 times

Concretely, it is the vector product which intervenes. The helicopter in yaw will not induce a gyroscopic torque, but the roll and the pitch will induce new torques rotating around axes at 90° to the plane formed by the 2 rotation vectors.

These torques can destabilize the device, or minima be in the way, forcing it to be operated more gently.

To cancel the gyroscopic effects, you need several rotors, at least 2 counter-rotating and in the same direction

From a mathematical point of view, if we have a multitude of rotors 1, 2, 3....

eq13.png (10.82 KiB) Viewed 744 times

To cancel the gyroscopic torque, it is necessary to cancel the quantity in parentheses, that is:

eq14.png (7.48 KiB) Viewed 744 times

In general, the rotors are in pairs, let's think about 2 rotors:

eq15.png (3.38 KiB) Viewed 744 times

And very often for reasons of symmetry, the rotors are also dimensioned, their moment of inertia is the same, there comes the very simple equation, for each pair of rotors:

eq16.png (2.78 KiB) Viewed 744 times

translated into simple language: they rotate at the same speed, but in opposite directions.


Several technical possibilities:
* contra-rotating propellers on the same axis (but mechanically complicated)

helices_contrarotatives_train_planetaire.gif (11.94 KiB) Viewed 757 times

it's even worse for helicopters with counter-rotating coaxial rotors...

* implanted contra-rotating propellers on different parallel axes (more simple !)
as the CH47 Chinook

* an approximate solution: the counter-rotating meshing bi-rotor in V

like Kaman k-max

multirotor drones are particularly suited to counter-rotating propellers with parallel and distinct axes.

Note on Christophe's latest video: the Chinese XPeng prototype intended to carry a car chose a "quadcopter" approach, with 4 counter-rotating double-propeller units.

Christophe's Dragonfly has 2 contra-rotating propellers which improve the maneuverability of the paraglider.

[Christophe]

Gyro couple? kezako? Know more!

ps: look at this new page https://dragonfly-paramotor.fr/comparat ... lectrique/

[Remundo]

On Christophe's Dragonfly website
a paramotor accident because of the gyroscopic effect...

The 2 propellers of a Dragonfly are counter-rotating and completely cancel the effects of engine torque.

Engine torque can cause serious problems on a paramotor, as shown in the following video:

[Remundo]

and to conclude this aerial Sunday...

A little "top 10" video of pretty light planes, pretty well designed overall.

typical consumption: 15L/h
typical speed 200 km/h
typical range: 1500 km.