Effect of prerotation on takeoff distance

XXavier

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You don't do that (keep pre-rotator in during ground run) - Magni is effective for these reason IMO:-

1) the pre-rotation process is to bring the stick fully aft from 120rrpm until the end of the PR phase at (say) 200-220. (so nobody sets off with the stick in the "wrong" place). Being both fully back and easily identifiable as centred.
2) the keel doesn't allow over rotation easily
3) fully aft stick in a Magni gives a rotor angle of around (off my head) around 17-18deg whilst AG is more like 21 (I think).

All these things make for the lurch into the air at low airspeed less likely.

Some people do that. The dealer for Magni in central Spain does participate in competitions, and he does that... A French rival he had (who won) not only did the same, but he had modified the pre-rotation transmission of his Magni installing a triple pulley in place of the standard double pulley.

I use to fly a Magni 24, but not (yet) dare to keep the pre-rotator engaged during the take-off run... The M24 is tricky, and I don't want (nor need) to introduce an additional complication... But I've been told that it's easily done in the M16/22.
 

Jean Claude

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One other thing I read a reply from JC I think giving a tipped ratio of 0.2 - that is great colour thanks for that. If you take the Magni example however and the full aft stick (i,e max flapping) at 120rrpm then that gives a max wind speed of 24mph (120 * 0.2) and i think that is slightly low so I think a ratio of 0.3 might be more realistic? What do you think?
If 20 degrees, maximum ratio before divergence is about 0.2 (*)
18 degrees allows slightly more 0.2


(*) when the blade pitch setting is usually 3° more zero lift
 

Philbennett

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Yeah I’m sure you can do it but I’m not sure it’s promoted widely.

In the tipspeed ratio how do we set off with wind at 30knts-40knts in a Magni?
 

Jean Claude

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With more 24 mph, and 120 rpm, the blades hits the stops if the stick is full pulled !
With more 48 mph, and 240 rpm
 
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Jean Claude

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It is because to keep your balance on the main gear, you don't have the stick on the back stop any more. So, disk A.oA is much less 18 degrees
In my opinion, pre-launched at only 150 rpm with only one light pilot on board and full throttle from the 115 hp engine, the flapping stops can be hit
 
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Philbennett

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well hang on though.... we have pulled the stick fully back at 120-130rrpm. What if we do that into a 30knt head wind? I agree it is and unlikely event but the aircraft limitation is 40knts and 24mph is broadly 20knots. That is not so unusual. You suggesting everyone pre-rotating in a Magni is at the limit?
 

anthom

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Cierva discovered, or perhaps re-discovered the flapping hinge, used in an all Cierva working gyros. The first three prototypes, with rigid rotors, did never fly...

The procedure that JC mentions was developed for the C.30, an advanced autogyro built in (relatively) large numbers.

From 'The Autogiro and how to fly it', by Reginald Brie, 1935:

View attachment 1148314
Look Xxavier, I get all that.

We are NOT flying Cierva type gyroplanes these days. Most have teetering rotor blades connected by a hub bar. So IMHO, it is not relevant to advocate a Cierva procedure, which seems to deviate from the established take off procedure of stick back after pre-rotation, just to shorten the take off distance.

Pushing the stick forward can, depending on the pre-rotated RPM and acceleration, cause a blade flap.

I believe if one needs to shorten the take off roll, it is best to have a powerful prerotator. This is quite evident from the videos referenced.
 

wolfy

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Thinking back (it's been a while since I have flown my gyro) in absolutely nil wind which is quite rare really, pre rotating to the maximum was as you say JC of no real benefit but in only the slightest of breeze the high rrpm certainly helped shorten the take off. JC do you think the airspeed required to reverse the airflow from helicoptering is actually less than the 20mph you were saying? Going from memory I think probably only 5 knots on the nose was enough to gain a benefit from a high pre spin.

wolfy
 

XXavier

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Look Xxavier, I get all that.

We are NOT flying Cierva type gyroplanes these days. Most have teetering rotor blades connected by a hub bar. So IMHO, it is not relevant to advocate a Cierva procedure, which seems to deviate from the established take off procedure of stick back after pre-rotation, just to shorten the take off distance.

Pushing the stick forward can, depending on the pre-rotated RPM and acceleration, cause a blade flap.

I believe if one needs to shorten the take off roll, it is best to have a powerful prerotator. This is quite evident from the videos referenced.


But a powerful pre-rotator could be of use only if higher RRPMs at the start of the run might be of any use. And both ckurz's observations (that started this long thread) as well as Jean Claude's theoretical reflections deny that...

If theory and practice are against the 'advantage' of high pre-rotation revs, what should I think...?

Captura de pantalla 2020-09-16 a las 15.10.31.png
 

Abid

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You don't do that (keep pre-rotator in during ground run) - Magni is effective for these reason IMO:-

1) the pre-rotation process is to bring the stick fully aft from 120rrpm until the end of the PR phase at (say) 200-220. (so nobody sets off with the stick in the "wrong" place). Being both fully back and easily identifiable as centred.
2) the keel doesn't allow over rotation easily
3) fully aft stick in a Magni gives a rotor angle of around (off my head) around 17-18deg whilst AG is more like 21 (I think).

All these things make for the lurch into the air at low airspeed less likely.

What is the definition of successful here?
It sounds like the definition of successful is Magni not having as many rotor strikes than say AutoGyro in takeoff phase?
If that is the criteria, the blame falls squarely on the pilot and more than likely the instructor and instruction given. Magni's can have a fairly long takeoff run on the ground because of straight keel. In a soft field that's a significant disadvantage. Their design philosophy is to protect against over-rotation from bad pilot technique but they pay for it in longer unstick (from the ground time). No free lunch.
 
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Abid

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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


Very risky and not safe. Not something you will find a manufacturer put in any POH which is written with average pilot skill point and risk which is acceptable.
 

Jean Claude

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I have already explained why the standard procedure not increases the benefit at more 2/3 of the stabilized flight rpm.
To benefit from a faster rpm, you have to adopt the "Cierva procedure". This is what Sportcopter does.
But the video of Sportcopter in the wind is not a proof in itself of the shortened takeoff, because the headwind is always favorable for a short take-off distance
 
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wolfy

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But a powerful pre-rotator could be of use only if higher RRPMs at the start of the run might be of any use. And both ckurz's observations (that started this long thread) as well as Jean Claude's theoretical reflections deny that...

If theory and practice are against the 'advantage' of high pre-rotation revs, what should I think...?

View attachment 1148316
mmm maybe I should have read the start of this thread to save repeating what others have already said haha.

wolfy
 

wolfy

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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
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
 

Jean Claude

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JC I did not see the sportcopter accerating with the rotor disc at minimum incidence
In the Sportcopter video, the forward acceleration phase has disappeared, due to the headwind.
Only the phase of takeoff with an angle of attack increasing, instead of standard takeoff with a angle of attack decreasing.
 

Greg Vos

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mmm maybe I should have read the start of this thread to save repeating what others have already said haha.

wolfy
Yes ....🙏😉🍺
 

SportCopter

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In the Sportcopter video, the forward acceleration phase has disappeared, due to the headwind.
Only the phase of takeoff with an angle of attack increasing, instead of standard takeoff with a angle of attack decreasing.
Hmmm, we'll have to disagree on that.
The headwind that day was about 6G10kts, not even 15 kts, and not even the rough field take-off AS to "disappear" the forward acceleration phase.

Our flexshaft prerotator can not only spin up to flight RRPM, but do so while ending at any amount of aft stick. In our experience with our machines, this can shorten take-off distance for a competent pilot, headwind or not. 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. In our opinion, this lengthens the take-off roll, and invites pilot error. Have you flown both types for yourself to compare?
 
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Jean Claude

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Our flexshaft prerotator can not only spin up to flight RRPM, In our experience with our machines, this can shorten take-off distance for a competent pilot, headwind or not.
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.
 
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