Effect of prerotation on takeoff distance

XXavier

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Great explanation, Sportcopter!

Being from a helicopter background myself, I have always felt that the higher the RRPM during pre-rotation, the easier to manage the take off roll, because there will not be much of a take off roll. Both yours and Wolfy's videos show this concept quite clearly. IMHO, reducing the disc AOA after pre-rotation to build up forward speed is inviting a blade flap and not worth the risk for shortening the T/O distance. I prefer to build up the speed low to the ground after getting airborne.

Look forward to some more videos in calm winds.

And the barrel rolls and loops are incredible!

With gyros during the take-off run, things are different, because the flow, during pre-rotation and in the first stage of the run, is 'from above to below' (antirotating) and has to change to 'from below to above' (autorotating) so that the rotor may behave as a wing and make take-off possible... That flow inversion should take place as soon as possible, minimizing the length of 'lost runway'...
 

thomasant

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With gyros during the take-off run, things are different, because the flow, during pre-rotation and in the first stage of the run, is 'from above to below' (antirotating) and has to change to 'from below to above' (autorotating) so that the rotor may behave as a wing and make take-off possible... That flow inversion should take place as soon as possible, minimizing the length of 'lost runway'...
That "flow inversion" seems to be real fast in the case of the Sportcopter and Wolfy videos. What am I missing?
 

SportCopter

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Great explanation, Sportcopter!

Being from a helicopter background myself, I have always felt that the higher the RRPM during pre-rotation, the easier to manage the take off roll, because there will not be much of a take off roll. Both yours and Wolfy's videos show this concept quite clearly.

Look forward to some more videos in calm winds.

And the barrel rolls and loops are incredible!
Thank you, thomasant, and we'll post them as soon as available. Right now, our field is LIFR due to smokey haze, and we haven't flown in days.

IMHO, reducing the disc AOA after pre-rotation to build up forward speed is inviting a blade flap and not worth the risk for shortening the T/O distance. I prefer to build up the speed low to the ground after getting airborne.
I understand your concern, although do keep in mind we're speaking of a high RRPM prerotation only minimally decayed during the onset of the take-off roll, so blade sail/flap seems a very minor possibility if AoA were reduced merely from aft stick to take-off AoA by balancing on the mains. The disk never reaches a flat attitude during such a roll, thus RRPM will not decay.

In other gyros, for a newbie (esp. those from FW) it's easy to forget to add sufficient aft stick during the onset of take-off roll.
Dozens of rotor-strikes and tip-overs have resulted.

In a Sport Copter, however, our procedure is the reverse, so it's difficult to forget to push the stick forward during onset of take-off roll because of the excessive nose-high attitude. Simply bring the nose down on the roll and hover it a few inches off the runway. The gyro will do the rest without any drama. Our gyros' main gear is purposely placed in relation to the cg to automatically give the pilot a take-off attitude when balancing on the mains. Those who have flown M912s vs. other gyros can chime in on the difference.


That flow inversion should take place as soon as possible, minimizing the length of 'lost runway'...
That sounds logical, XXavier. On a related note, during full aft stick, the rotor is acting like a brake (or "reverse gear" even), and subsequently reducing the AoA reorients the rotor thrust line to facilitate the increase of AS. Such must be done smoothly yet quickly.
 

Jean Claude

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When the prelauncher is powerful (> 2/3 of the stabilized flight rpm), the minimum distance is obtained by the procedure described by La Cierva. See my post #118 With the standard procedure, the run is not reduced.
However when there is strong headwind, the difference in distance between the two procedures is negligible, because the take-off air speed is almost reached and therefore very little run lost by the reduced acceleration.
 
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SportCopter

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I guess we've just not experienced such markedly "reduced acceleration" as you've described.
With the M2, we'll experiment with different prerotation RRPMs and report our findings.
 

thomasant

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JC, according to the referenced post, Cierva advocates reducing the disc AOA during initial acceleration with stick forward. Were the Cierva rotors having flapping hinges? In that case, I suspect that blade flap will not be a concern, as opposed to the two blade teetering system that is mostly used today.
 

wolfy

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Great explanation, Sportcopter!

Being from a helicopter background myself, I have always felt that the higher the RRPM during pre-rotation, the easier to manage the take off roll, because there will not be much of a take off roll. Both yours and Wolfy's videos show this concept quite clearly. IMHO, reducing the disc AOA after pre-rotation to build up forward speed is inviting a blade flap and not worth the risk for shortening the T/O distance. I prefer to build up the speed low to the ground after getting airborne.

Look forward to some more videos in calm winds.

And the barrel rolls and loops are incredible!
With powerful pre spinners (around 300rrpm) and any sort of breeze if the cyclic is left full aft you will be on the mains before your ready and trying to maintain heading with substantial torque acting on the aircraft. For that reason you will find your self with the stick held considerably forward of the back stop and the disk at pretty much flight attitude, from there full throttle and then further forward stick will be needed so acceleration is brisk the disc is almost at flying speed so your off real quick. I have also flown an M912 and they are balanced on the gear much like my last gyro, it works very well, and with the right technique absolutely 300 rpm will get off much quicker than 200.

wolfy
 

XXavier

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JC, according to the referenced post, Cierva advocates reducing the disc AOA during initial acceleration with stick forward. Were the Cierva rotors having flapping hinges? In that case, I suspect that blade flap will not be a concern, as opposed to the two blade teetering system that is mostly used today.

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:

il12sj.jpg
 
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Jean Claude

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Were the Cierva rotors having flapping hinges? In that case, I suspect that blade flap will not be a concern, as opposed to the two blade teetering system that is mostly used today.
The divergence of the flapping angle is related to the stalling of the blades on the retreating side.
Why wouldn't an articulated rotor be affected?
With the "Cierva procedure" when the angle of attack of the disc is zero, the flapping angle is less 1 degree. Then, when the stick is pulled back fully, the still large Rrpm does not allow the flaping angle to diverge.
This procedure is totally safe, if the high recommended Rrpm is rigorously respected by the pilot.
 
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Philbennett

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Well each blade could flap without interference with the other unlike a 2 blade teetering system. So likely does affect but the pilot is able to manage the process without LoC
 

XXavier

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nosing up so stte


(...)
(...)



in the french autogiro championship the short distance take off is usually won by the magnis, pilots are pre spinning to 320 rrpm, they take off in 10 meter and then climb nose up ... but ok I don't call it a take off, I call it is pure madness ...

when you read the gyro maker datasheets, they all tell the take off distance is 15 meters , ok 15 metesr is the distance to lift the wheels , but not to properly take of.

(...)
(...)

(...)

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...?
 

Philbennett

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What is excellent about this thread is that it is discovering some facts that have always remained with gyroplanes but have been unspoken about because of various regimes that have exerted influence over the years. As with all things aviation there is more than one way to do something effectively.
 

Philbennett

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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...?
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.
 

Philbennett

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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 (or tip speed of 120mph) 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?

My rational is that Magni don't highlight in the POH that take off in windspeed of 24mph is limiting or to change the process of pre-rotation in higher wind speed. Their only reference to wind speed in the POH is this:

Flight in strong gusty winds or wind velocities of more than 40 kts is prohibited.
 

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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