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

[Remundo]

this time for distraction...a few half-nuts who like to get laid.

[Remundo]

Redundancy (in aeronautics)

The word redundancy has a literary interpretation, meaning "repetitions" or "restatements" which can have a pejorative connotation (heaviness of speech) or on the contrary informative (insistence / precision)

In the aeronautical field, redundancy is a purely technical matter that can be summarized as follows:
multiply the systems to avoid a global failure.

We can try to make systems redundant, whether they are computer, hydraulic or mechanical.

Nevertheless, in particular for non-lifting devices, it is necessary to avoid dead weight in flying devices.

I am interested today in the redundancy of thrusters.

For light aircraft
* the single engine has no redundancy: if its single propeller no longer works, it continues on its way while gliding, the pilot's only means of action is to manipulate the control surfaces (elevator and rudder) as well as the ailerons on wings. The aircraft irreversibly loses altitude, especially if its smoothness is mediocre.
* the twin engine, and a fortiori multi-engines are redundant: one thruster may fail, but the other thrusters maintain sufficient control of the aircraft (even if it is necessary to compensate for thrust-pull imbalances with the deflector devices on the wings).

For helicopters
* the single-rotor has no redundancy, and tail rotor failure frequently leads to yaw accidents. However, a single-rotor helicopter can land in disaster by letting its rotor "hover" while managing the descent in altitude (technique called autorotation).

* the multirotor (often 2) seem redundant, but in reality, the failure of a rotor leads to a huge imbalance, a priori non-compensable

For ADAV/VTOL
* One could imagine some redundancy by switching from airplane mode to helicopter mode if one of the modes fails, but the other is sustainable

I want to address more specifically in the case of a multirotor ADAV in hovering helicopter mode.

In this case, the aerodynamic drag and lift forces are almost zero (apart from those experienced by the propeller blades). The balance of the aircraft results from the forces F1, F2, F3... of the various rotors and from the weight mg of the aircraft.

Here is a diagram that represents the situation with 5 forces.

sustentation_multirotor.png (11.07 KiB) Viewed 1249 times

The device is suspended from a "support polygon" (which is not necessarily flat), the vertices of this polygon being the points where the pulls/thrusts of the propellers are exerted.

The balance of forces gives:

eq17.png (5.88 KiB) Viewed 1269 times

this equation guarantees the immobility of the center of gravity of the device



The balance of the moments of the forces brought back in G the center of gravity gives:

eq18.png (10.21 KiB) Viewed 1269 times

this equation guarantees the non-rotation of the device (which could turn it over and make it even more uncontrollable)

The smallest solution seems to be a counter-rotating twin-rotor (to avoid counter-torque and gyroscopic effects) below which the center of gravity hangs (this is basically what twin-rotor helicopters do).

Un quad-rotor drone whoever loses a rotor can possibly catch up with a fairly complex algorithm managing the last 3 propellers available (3 thrusts) guaranteeing both the balance of forces + the balance of moments + counter-torque compensation.

The more propellers and possible orientations there are, the more playable it seems (redundancy) to guarantee control of the aircraft.

I think that with 3 propellers, if the center of gravity is directly above the polygon (let's say that if the vertical passing through G pierces the polygon), we can recover the device.

with only 2 propellers, the aircraft risks tipping over (G will want to line up with the lift line), unless G is already directly above this line by design.

However, this is based on fairly complex calculations and above all on very responsive and efficient control algorithms.

Let me know what you think about it sometime.

[Christophe]

I have seen some videos of faulty drones…a quad that loses an engine is lost and crashes, yet there are tri drones but the configuration of a quad is made for 4 lifts…

I don't see how such a complex algo could manage the torque and the balance wheel on the failing axis: the remaining propeller would have to push…upwards to compensate for the weight of the HS engine! It's only possible with a variable-pitch propeller that would work in retro propulsion…and which should be super responsive…in my opinion it's impossible in this way.

As a reminder: a propeller that turns upside down works very poorly...trying to get a plane to take off in reverse...it's the same thing

Stopping him would still be the best thing to do?

Although the couple is not too serious it would just yaw ... enough to land safely.

Hexes on 5 engines can get by…

Do a youtube search.

That is why human drones on a quad base without a carrier wing…are inherently dangerous…even with a 4*2 configuration…although it adds extra security…

There will soon be deaths and they will be banned…yes, there are pigeons in town! (double meaning)

[Remundo]

I think this is a topic worth exploring...

If we look at a trirotor

sustentation_trirotor.png (8.18 KiB) Viewed 1218 times

The failure of a rotor does not seem to be recoverable. Indeed the 2 remaining rotors exert a moment of rotation which tilts the device.

For example if F3=0, G will try to pass directly above line 1-2

Although if we imagine tilting rotors, the situation is not hopeless. The device will be tilted, but stationary.

If we look at a quadrotor

sustentation_quadrotor.png (8.41 KiB) Viewed 1218 times

The failure of a rotor, if the 3 others remain at equal force, tilts the device.

BUT there is a (at least mathematical) solution, even assuming the non-tilting rotors.

It is necessary to turn off the rotor diametrically opposite to the faulty rotor. Point G remains suspended at the lift line defined by the diagonal of the 2 rotors still in motion.

For example if F4=0, it is ALSO necessary F2 = 0, and G remains directly above the middle of the diagonal 1-3.

[Christophe]

That's what I just said: switching off is still the best solution...

Except that in real life turning off won't work on a quad because everything in the air is moving…and the slightest disturbance will overturn the machine…like that of the stopped propellers which will not be set in the same way in relation to the relative wind…in short, it's dead!

It's like keeping anything balanced on a perfect pivot: one breath and it topples over…this is worse because the breath is automatic!

Otherwise, emergency retro-propulsion…

With a power wing it's different…human quads are crap…de…they have millions but it's crap

[Remundo]

if you place G below the rotors, it works fine.

It is the twin-rotor helicopter.

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

Indeed if G is above the rotors, it switches. And upside-down rotors will accelerate the device downward...in addition to weight.

Volocopter, the multirotor taxi, places G well below the support polygon

If we push the multirotor to the extreme, there is a taxicopter project with many propellers, called "VOLOCOPTERE"

and with all these rotors, must be able to maintain acceptable stability even if 1 or 2 fail.

[Christophe]

Place 2 unbalanced 750 kg 10m from the cabin and whose drag varies with the speed...it will work much less well! Place in one, it will never take off…or rather if and turn around directly…

The volocopter is actually more secure than quads, I was talking about it here in 2016… electric-transports/l-ehang-184-first-drone-for-man-t14437-10.html

[Remundo]

placing unbalance, as you say, amounts to violently displacing G outside the polygon/lift line, thus clearly destabilizing the aircraft.

Do not do anything eh

I try in this thread to bring together all the right material so as not to make design errors.

Aeronautics is not my specialty, but I have good will, and I take care of myself, especially when I find good doctors or very instructive documentation.

[Macro]

or very instructive documentation.

I'm going to mess around

I made planes thanks to the documentation... Well, rather gliders... Who crossed the classrooms annoying my teachers... It caused me some problems... But at least I don't worry. am not crashed....

I also stuck flies on confetti... It flies badly and not for long... A track to improve

[Remundo]

ah and did your grades "take off"? Were they stable?