Effect of prerotation on takeoff distance

WaspAir

Supreme Allied Gyro CFI
Coefficient of lift, a property of the airfoil that will vary with angle of attack. (Lift is computed by multiplying air density (usually a greek rho is used for that), the square of velocity, the area , C-sub-L - - ringing bells for your memory now?)

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All_In

Gold Supporter
Coefficient of lift, a property of the airfoil that will vary with angle of attack. (Lift is computed by multiplying air density (usually a greek rho is used for that), the square of velocity, the area , C-sub-L - - ringing bells for your memory now?)
Thank you Jon!!

Yaw Mon, I do indeed, get it now, I know the coefficient of lift very well.
Thank you all so much!!!
U-ROCK!!

ckurz7000

Senior Member
It seems to me many of you wish to overcomplicate a take of in a gyro? It really is a very simple excersise and one can decide on the runway condition, length and prevailing wind to determine what will be the optimum pre rotation speed, it goes with out saying that on a windy day with wind directly on the nose it will be fine to release the pre rotator as low as 120 Rrpm, as we know it will speed up fast and why labor or wear the pre rotator?

just be mindful not to rotate ( lift the nose) until you have an indicated Rrpm of 200 or more

Hi everybody! I started this thread way back when and then took a hiatus from the forum. Vance recently told me that this thread had become active again, so I logged on and checked back. Let me say a few things, because there seems to be a misunderstanding regarding my reasons to post this.

At the time I wrote the original post, it seemed to me that a lot of manufacturers were boasting about their powerful prerotators and how that would cut the take-off distance. I was very skeptical of that and therefore did a series of test flights with my gyro. It turned out, no surprise, that there is a point of diminishing returns beyond which the take-off distance does not get any shorter anymore. Anything beyond 200 rrpm doesn't make any difference to take-off distance at all.

The second objective was to work out a procedure which would yield the absolute shortest take-off. Turns out, it is best to prerotate to, say, 220 rrpm, then push the stick forward (flattening the rotor disk). Then accelerate to about 40-50 km/h, during which time the rrpm will drop by about 10-15 rrpm, and only then pull the stick back. The theory is pretty clear cut why this works best, and Jean Claude has a lot to say about that.

This, however, is NOT a take-off technique I would recommend at all! It comes with the big danger of flapping the blades. Therefore I don't advise anybody to follow this procedure. Just stick with the take-off procedure the POH tells you to use.

Greetings, -- Chris.

WaspAir

Supreme Allied Gyro CFI
Nice to hear from you again!

SportCopter

Active Member
As we all know, for a gyro to take-off or maintain level flight, a relevant respective pair of numbers must be concurrently achieved:
minimum RRPM and minimum airspeed

In our experience, with our machines, the excessive drag of a high AoA to help initially achieve flight-speed 275-300 RRPM is less of a time hindrance for take-off AS than it is to roll with a flatter disk/200 RRPM to achieve take-off AS and flight RRPM. I.e., we already have flight RRPM, and merely need AS. The drive-shafted prerotators need both flight RRPM and flight AS, as they launch with only 200 RRPM.

With our fully castering nosewheel design, the pilot can safely hold the naturally correct AoA by just balancing on the mains and lightly tapping the NW to confirm its inches height above the runway. This is also a safer technique for FW pilots in transition to gyros, i.e., from our high RRPM and balanced mains take-off roll there is no "rotation."

Many other gyro owners have gotten into trouble by forgetting aft stick at the beginning of their take-off roll with 200 RRPM, and then suddenly trying to rotate once they realize their AS is nearing Vx. Drama ensues, depending on the preceding amount of flat disk RRPM decay.

We believe it is safer and better performing to have the stick nearly full aft at the end stage of prerotation, and then push the stick forward to roll while balancing on the mains and tapping the NW as a "feeler." However, few gyro designs allow this.

With such a wind, the pre-launcher rpm doesn't matter: 50 rpm would do just as well. Just to put the stick back slowly enough to avoid the impact of the blades on the flapping stops.

Not only would this seem an unwise technique in the attempt to achieve anything close to flight RRPM, but unsuccessful as well. Even a full value 15 kts of direct headwind wouldn't be sufficient for the M912, according to its POH:

Rough field Take-Off needs 25 mph • 22 kts
Minimum In-flight Straight/Level needs 23 mph • 20 kts

Our field's 15 kt windsock was not then showing full value, but ~10 kts gusts.

Certainly, any headwind helps but winds that day were pretty common for gyro pilots everywhere, so, everybody else with a similar power:weight should be taking off in <20 feet. Are they?

When we have a 0 wind day, we'll film some M912 and M2-915 STOL take-offs and post to our YouTube channel with links here.

Meanwhile, we wish everyone safe flying!

anthom

Member
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!

XXavier

Member
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'...

anthom

Member
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

Active Member
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

Junior Member
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

Active Member
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.

anthom

Member
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

Gold Supporter
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

Member
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:

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Jean Claude

Junior Member
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

Junior Member
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

Member
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

Junior Member
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

Junior Member
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

Junior Member
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.

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