Effect of prerotation on takeoff distance

fara

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Here is a takeoff into 6 gusting to 10 headwind of an AR-1 at Sun N Fun a couple of years back pre-rotating to 200 or so rotor RPM.
Not 300 just 200. You can play these with any gyro all day one up on a 2 seater machine. It's not the spec. Its fake news.

 

Tyger

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Could it be that the Magnis are so successful in that because they keep their pre-rotators engaged during the run, disconnecting them just before becoming unstuck...?
I have a Magni, and I ALWAYS release the prerotator before commencing the roll, which I also understand to be official Magni procedure.
 

XXavier

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W have a look to the windsock : it is vertical there is few wind , and the magni lifts i'st 3 wheels in 10m ( maybe less) and climbs like a jet ...


- first remark, the sport copter has nothing more then a magni
- second remark it is not a take off we are seeing it is a stupid game of death-dodger .. because if the engine stops the gyro will fall like a piece of crap on the ground

A friend who knows him has confirmed me that the Magni of Eric Changeur is the one having the 'enhanced' three-pulley transmission that allows him to perform these surprising take-offs by keeping the pre-rotator engaged during the initial part of the run...
 

XXavier

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Here is a takeoff into 6 gusting to 10 headwind of an AR-1 at Sun N Fun a couple of years back pre-rotating to 200 or so rotor RPM.
Not 300 just 200. You can play these with any gyro all day one up on a 2 seater machine. It's not the spec. Its fake news.



Just 200 RRPM, but with the headwind having already inverted the flow while the gyro was still stopped, so the rotor was in autorotating régime, working as a wing, and hence that very short take-off was possible...
 

Jean Claude

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Eric Changer's takeoff is irrelevant, because it is only possible by very low load and exceptionally high thrust-to-weight ratio (0.7).
In these exceptional conditions, takeoff at high rpm (330) becomes possible because the gyro can take a lot of support on the tail wheel and lift the main wheels until giving the disc an A.o.A of 30 degrees.
The vertical component of the propulsion then lightens the rotor load to the point that 25 mph and 300 rpm are enough to take off.
No mystery. Spectacular demonstration, not recommended for novices.
 

wolfy

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A friend who knows him has confirmed me that the Magni of Eric Changeur is the one having the 'enhanced' three-pulley transmission that allows him to perform these surprising take-offs by keeping the pre-rotator engaged during the initial part of the run...
Do you have anymore details about his "three pulley transmission"?

wolfy
 

XXavier

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Eric Changer's takeoff is irrelevant, because it is only possible by very low load and exceptionally high thrust-to-weight ratio (0.7).
In these exceptional conditions, takeoff at high rpm (330) becomes possible because the gyro can take a lot of support on the tail wheel and lift the main wheels until giving the disc an A.o.A of 30 degrees.
The vertical component of the propulsion then lightens the rotor load to the point that 25 mph and 300 rpm are enough to take off.
No mystery. Spectacular demonstration, not recommended for novices.

But that disk, at 30º is probably not in anti-rotation state, but already in autorotation régime, working as a wing. Perhaps that early switch to autorotation flow régime has to do with a pre-rotation prolonged for a few seconds at the takeoff 'run'...
 

XXavier

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Do you have anymore details about his "three pulley transmission"?

wolfy


No.. Just what my friend told me in an e-mail. An excerpt from the original follows:

(...) en la prueba más crítica prelanzaba hasta las 4500 rpm de motor independientemente de las vueltas del rotor, de forma que soltaba el freno y después de avanzar un poco soltaba el prelanzador.
Yo me fijé que el francés hacía lo mismo, pero él llevaba instaladas tres correas en vez de dos.
Lo que tengo claro es que cuantas más vueltas de rotor y sobre todo de motor mejor. (...)
 

ckurz7000

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Hi Chris,
this is me who dug out your thread making some researsh to help me decide my electric pre-rotation rppm
my question was : what rrpm at which rotor angle ?
I did not find any awnser as for the angle and I think the angle is also something important,
could you precise me at what rotor angle had you done your tests please ?

Yes, you are correct. For a full investigation you'd need a sensor telling you the angle of the rotor disk. A simple stick position vs. rotorhead angle sensor isn't quite enough because it doesn't measure the flapping angle. Of course, you can always calculate the flapping angle. But when doing an experiment, I want as little as possible calculated and as much as possible measured.

The procedure I followed was this:

Prespin the rotor with the stick fully forward (flat disk). At the desired rrpm I release the breaks, CHECKE RRPM, and if rrpm was at least 200, bring the stick fully back and advanced the throttle, like in a normal takeoff. Except, At that point I would be already rolling along at something like 40-50 km/h. Mind you, my prerotator can do about 230 rrpm max. Therefore, at the higher rrpm, I would land, and wait for the rrpm to decline to the desired point instead of prespinning the rotor.

The interesting follow-up to this experiment would be to explore prerotation speeds between, say, 150 and 200 rrpm while bringing the stick back in a controlled way depending on rrpm and forward speed. Again, I had no way to tell the angle of the rotor disk that's why I stuck to a procedure which has the stick either fully forward or fully backward.

As several people already pointed out, the shortest takeoff distance would require a prerotator which remains engaged during the takeoff run. My hunch is that it wouldn't even need to be all that powerful. You have to be careful, though, not to lift the nose wheel prematurely, before the rudder is sufficiently effective to counteract the torque of the prerotator.

Greetings, -- Chris.
 

XXavier

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google :
(...) in the most critical test it would pre-launch up to 4500 engine rpm regardless of the rotor turns, so that it released the brake and after advancing a little it released the pre-launcher.
I noticed that the Frenchman did the same, but he had three straps installed instead of two.
What I have clear is that the more rotor turns and especially the engine, the better. (...)

Very, very good. Only that 'correas' should be translated as 'pulley belts' in this case... Besides, the 'prueba' was a contest, not a test...
 

ckurz7000

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It is one thing to un stick behind the power curve but to stay behind the power curve through out the climb out is asking for trouble for sure.

wolfy
Note that my original data quotes the standard take-off distance, which is take-off to clear a 50 ft obstacle. This is different from the ground roll.
 

XXavier

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yes and as far as I know he used a gyroperfo box allowing 120 hp to the rotax 914
mr Changeur is not tall (he probably weights 80 kg) , and with a 280 kg M16 and 15 kg of fuel total weight would be 375 kg , so the weight to power ratio is huge


Let's do a few numbers... According to 'Flugphysik der Tragschrauber', the maximum stationary thrust of a MTOsport (Rotax 100 hp) is 2000 N.
As the relation of thrust T and power P under stationary conditions is T^3 prptnl. to P^2, I get a thrust of 2260 N for Changeur's Magni. That's a lift/weight ratio of 2260/375 · 9,8 = 0,61 Really good...
 
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ckurz7000

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It is one thing to un stick behind the power curve but to stay behind the power curve through out the climb out is asking for trouble for sure.

wolfy
Note that my original data quotes the standard take-off distance, which is take-off to clear a 50 ft obstacle. This is different from the ground roll.

Also, no getting into the dead-man-zone of the H/V diagram. It was a very revealing exercise to chop the power at 50 ft for various IAS. I had to do this on the test flights for the H/V curve. You have to wait 0.5 seconds before initiating recovery (as per certification standards). It is really scary and you can bang your gyro real fast. Since that time I make a point of staying out of the red zone.

-- Chris.
 

Jean Claude

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But that disk, at 30º is probably not in anti-rotation state, but already in autorotation régime, working as a wing. Perhaps that early switch to autorotation flow régime has to do with a pre-rotation prolonged for a few seconds at the takeoff 'run'...

I agree with you, Xavier, Even without prerotation prolonged. I once simulated this take-off precisely and since the autorotation was already established
The key point is that the propeller thrust still allows a comfortable forward acceleration, since the rotor drag is reasonable despite the angle of attack.
To make it short:
Prop. lift = 2400N * Sin (30°-10°) = 800 N
Rotor drag = (3700 N - 800 N)* tan 30° = 1700 N
Parasit. drag = 50 N

Prop. thrust= 2400 N* Cos(30°- 10°) = 2250 N
Forw. acc. = (2250 -1750)/370kg = 1.3 m /s^2
 
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fara

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Just 200 RRPM, but with the headwind having already inverted the flow while the gyro was still stopped, so the rotor was in autorotating régime, working as a wing, and hence that very short take-off was possible...

Exactly. That's why I said its fake news. Not something that can be specified following the standards for takeoff distance specification to clear a 50 foot (15 meter) obstacle with standard atmosphere at gross weight on a prepared runway with no wind with average pilot technique. That's what you have to put in the POH. That's the standard definition of takeoff distance in aviation and the procedure has to be what you put in the POH and I do not know any standard that allows a manufacturer to put a procedure in that will not allow for safe recovery from initial climbout in case of an engine out.. The rest is marketing ploys and show off tricks. Promoting those to be official ways of takeoff procedure or takeoff distance specification is not responsible or safe. Given the state of affairs of take off accidents in gyroplanes with behind the power curve climbouts, its surprising to see that being promoted by anyone who cares about safety.
 
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SportCopter

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We don't disagree, and that's what I've been trying to explain for a long time: The required competency you're talking about is just to change the standard procedure by the Cierva procedure. On this condition alone, a pre-launch higher than 2/3 of the flight rpm becomes beneficial. That's just what you make.
Sorry, Jean Claude, but in these 20+ pages such provisos were missed. We'll test and film our M2 take-offs in 0-wind conditions with differing prerotation RRPMs.



not true

this pre spinner I can pre rotate at 260 ... after this the brakes don't let me go higher (simple cheap cable brakes) bub if I had better brakes I am sure I would go higher,

and this is only an amateur pre spinner built in a very short time ... so 300 rpm have nothing technically exceptionnal
jm-urbani. please note that I was referring to 200 RRPM as the generally recommended technique—not some universal limitation—

Gyros using a U-jointed driveshafts prerotator do not well lend themselves to that technique, because it's hard on the U-joint—thus, they spin to 200 RRPM with a flat disk and then launch while adding aft stick.
....although some U-jointed prerotators are indeed limited to well <300, such the Cavalon (with Rotorhead II) and its max prerotation of 220 RRPM.



Here is a takeoff into 6 gusting to 10 headwind of an AR-1 at Sun N Fun a couple of years back pre-rotating to 200 or so rotor RPM.

Not 300 just 200. You can play these with any gyro all day one up on a 2 seater machine. It's not the spec. Its fake news.
From a standing stop to unstick in about 10 seconds, suggests a stronger headwind. But, accepting your figures, wouldn't the take-off have been even shorter beginning with 300 RRPM? Rhetorically asked, can the U-jointed AR-1 be power-prerotated to 300 RRPM with significant back stick, and, if so would such reduce the service life of the prerotator components (the U-joint in particular)?

We prefer the flexshaft prerotator for reasons already stated, and we've the brakes to hold engine thrust for 300 RRPM. Other firms choose the U-jointed driveshafts. To each their own.


The rest is marketing ploys and show off tricks. Promoting those to be official ways of takeoff procedure or takeoff distance specification is not responsible or safe.
fara, in this thread I've read of no "promoting" these company test pilot steep climbs (yours included) as "official ways of takeoff procedure." For the typical owner, it is safest to stay in ground effect after take-off until at least Vx or Vy has been achieved. This completely avoids the H/V danger zone.

OK, back to work; we're wrapping up the M2's final assembly. Engine is "wet" and we're plumbing the fuel system.
Safe flying all!
 
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