Effect of prerotation on takeoff distance

[Vance]

I have stayed out of this thread because it seems there is an almost religious fever around what the rotor is doing backed up with what I feel is a poor interpretation of how an airfoil works.

In my opinion an airfoil stalls when the critical angle of attack is exceeded and stall is unrelated to airspeed.

In my opinion it is best to follow the Pilot’s Operating Handbook procedure for takeoff.

If the POH doesn’t have guidance for a short field takeoff then optimize each part of the takeoff like the Rotorcraft Flying Handbook says or find a better place to takeoff.

In my experience flying at an unfamiliar airport my takeoff performance may be very different from what it is at my home airport for a number of reasons.

Even at my home airport the takeoff and climb out performance may vary from takeoff to takeoff.

I was just watching the video of my maintenance flight of three patterns at SMX my climb out varied between four hundred and eight hundred feet per minute at fifty knots.

In my experience if you have a 50 foot obstacle to climb over in windy conditions there will be sink on the lee side further degrading climb out performance.

The only time I do a maximum performance takeoff (short field) is when I am giving instruction because that is one of the practical test standards.

In my opinion the reason short field takeoff procedures are taught is to show people a gyroplane in no wind conditions is not a short takeoff aircraft.

 

[Tyger]

You mean the nosewheel on the ground...? Or skimming the ground...?

ON the ground. I am not saying that this is my standard takeoff technique, but for short-field it works very well for me. It might be very different with a side-by-side.

 

[Heron]

There is a video somewhere that shows a gyro in heavy winds, taking off and flying backwards and repeting it, you should watch it.
The rotor needs only rpm to drag the gyro back, the gyro needs some wind over the controls surfaces to be maneuverable, you can prerotate until the wheels loose grip, no more!
The gliders tied to a rope with enough wind will fly, from where it sits.
So what was your question? You loose acceleration? Hell yes remember Uncle Ken Brock, they are draggy . . .
Does enough prerotation will shorten your take off roll? Yes if you have power enough!
If, lets say, a Magni is ahead of a Golden, both prerotate equaly and take off, the Golden will be atop of the Magni so it can't go up! Thats power, not prerotation!
Balance of power and disc load is a good thing!

 

[WaspAir]

That's not possible with our fixed-pitch gyros...

It's not possible with the A&S18A either. You can have either a gyro that lets you tilt the disc aft, or one that lets you increase blade pitch, but there's no gyro out there that would let you do both separately. On jump gyros, you will LOSE rpm with the rotor drive disengaged while airspeed increases, because you still have flat pitch in the blades. This paragraph is utter garbage that the FAA included in the handbook that doesn't apply to any aircraft ever put into manufacture, and was obviously written by some clueless soul who never flew a jump gyro or read the manuals for them.

If the prerotator is capable of spinning the rotor in
excess of normal flight r.p.m., the stored energy may be
used to enhance short-field performance. Once maximum
rotor r.p.m. is attained, disengage the rotor drive,
release the brakes, and apply power. As airspeed and
rotor r.p.m. increase, apply additional power until full
power is achieved. While remaining on the ground,
accelerate the gyroplane to a speed just prior to VX. At
that point, tilt the disk aft and increase the blade pitch
to the normal in-flight setting. The climb should be at a
speed just under VX until rotor r.p.m. has dropped to
normal flight r.p.m. or the obstruction has been cleared.
When the obstruction is no longer a factor, increase the
airspeed to VY.

 

[Burrengyro]

Does higher pre-rotation rpm capability in combination with the more powerful Rotax 915 and 916 engines help shorten takeoff distance to overfly a 50ft obstacle while adhering to the standard POH takeoff technique and by how much? Wondering what has been the experience of pilots who fly gyros with the new 915 and 916 Rotax engines versus the Rotax 914 engine?

 

[Abid]

It's not possible with the A&S18A either. You can have either a gyro that lets you tilt the disc aft, or one that lets you increase blade pitch, but there's no gyro out there that would let you do both separately. On jump gyros, you will LOSE rpm with the rotor drive disengaged while airspeed increases, because you still have flat pitch in the blades. This paragraph is utter garbage that the FAA included in the handbook that doesn't apply to any aircraft ever put into manufacture, and was obviously written by some clueless soul who never flew a jump gyro or read the manuals for them.

If the prerotator is capable of spinning the rotor in
excess of normal flight r.p.m., the stored energy may be
used to enhance short-field performance. Once maximum
rotor r.p.m. is attained, disengage the rotor drive,
release the brakes, and apply power. As airspeed and
rotor r.p.m. increase, apply additional power until full
power is achieved. While remaining on the ground,
accelerate the gyroplane to a speed just prior to VX. At
that point, tilt the disk aft and increase the blade pitch
to the normal in-flight setting. The climb should be at a
speed just under VX until rotor r.p.m. has dropped to
normal flight r.p.m. or the obstruction has been cleared.
When the obstruction is no longer a factor, increase the
airspeed to VY.

Wow. That paragraph was written by a very ignorant author as it relates to gyroplanes.

 

[Greg Vos]

Does higher pre-rotation rpm capability in combination with the more powerful Rotax 915 and 916 engines help shorten takeoff distance to overfly a 50ft obstacle while adhering to the standard POH takeoff technique and by how much? Wondering what has been the experience of pilots who fly gyros with the new 915 and 916 Rotax engines versus the Rotax 914 engine?

I cannot speak for the more powerfull 916 prime mover, I will simply add that pre rotation to a higher Rrpm has little purchase in ganging height compared to a standard or even a performance take off in a Gyroplane
There are techniques used to gain height, and pre rotation to higher values has proven to be less effective - topic well debated on this forum
Saying this I will add the extra Hp we see in our 240hp and 320hp equipped with 5/blade prop gyro’s (way more powerful than the rotax) has the ability to carry more payload, so in my experience more power means more weight carrying capability obviously we have a different rotor head and a bespoke blade set up not found in recreational gyroplanes that permits a higher payload …we a also don’t read the Rrpm as a rotation speed rather a percentage but I don’t want to contaminate the thread with the tech on our commercial machines

 

[PJB]

Does higher pre-rotation rpm capability in combination with the more powerful Rotax 915 and 916 engines help shorten takeoff distance to overfly a 50ft obstacle while adhering to the standard POH takeoff technique and by how much? Wondering what has been the experience of pilots who fly gyros with the new 915 and 916 Rotax engines versus the Rotax 914 engine?

The answer is yes. I have not flown with 916 power but have quite a bit of time across Cavalon and Sport with 915 power and you are airborne to clear 50ft sooner. The “standard” POH technique with higher RRPM pre rotation speeds is different to POH technique without. I haven’t spent time doing what the makers should do in making measurements.

 

[Burrengyro]

Hi Greg and PJB and all,
Many thanks for the replies. I have re-read the posts in this topic.

The introduction of the more powerful Rotax engines and those used by Greg would seem to have changed some of the parameters defining how pre-rotation and takeoff process techniques effects takeoff distance since the topic was first posted by Chris in 2014.

Chris commented:
"What is surprising is that takeoff distance does not seem to depend at prerotation rrpm at all -- over the range between 200 and 300 rrpm. Why? Well, apparently accelerating the gyro to takeoff speed happens at a slower time scale than the acceleration of the rotor, at least over the investigated range of rotor rpm."

My understanding is that the engine used for Chris' test was a Rotax 914 turbo. It would be interesting to have a similar test done using a Rotax 915 or 916 engine. If the standard POH pre-rotate, Stick fully back and roll on the power is used, how close is the ground acceleration available to a 915 or 916 engine to out pacing the rotor acceleration?

Interested to hear pilots experiences of how they have optimized their pre-rotation / takeoff techniques and what pre-rotator RRPM's were used to make maximum use of the power available with 915 and 916 Rotax engines to safely achieve very short takeoffs on short runways / rough fields. Many thanks, John H.

 

[Abid]

Just read the POH for AutoGyro with 915. It gives you the takeoff distance at gross weight to clear 50 foot obstacle on a standard day at sea level. That is the real tested number.

 

[PJB]

If the standard POH pre-rotate, Stick fully back and roll on the power is used, how close is the ground acceleration available to a 915 or 916 engine to out pacing the rotor acceleration?

Interested to hear pilots experiences of how they have optimized their pre-rotation / takeoff techniques and what pre-rotator RRPM's were used to make maximum use of the power available with 915 and 916 Rotax engines to safely achieve very short takeoffs on short runways / rough fields. Many thanks, John H.

That isn't the POH technique with higher RRPM's. The full back stick isn't necessary as the tip speed ratio is well in limits [i.e. the rotor won't flap] and full back stick gives a lot of drag.

 

[Aerofoam]

That isn't the POH technique with higher RRPM's. The full back stick isn't necessary as the tip speed ratio is well in limits [i.e. the rotor won't flap] and full back stick gives a lot of drag.

The full back stick will give you max RRPM acceleration until the front wheel comes off, then you have to lower the disk angle.
The higher powered gyros with strong pre-rotators can take advantage of the faster early acceleration because they are well beyond the danger zone of flapping with the high initial pre-rotation so they can go to a higher power setting earlier.

 

[PJB]

The full back stick will give you max RRPM acceleration until the front wheel comes off, then you have to lower the disk angle.
The higher powered gyros with strong pre-rotators can take advantage of the faster early acceleration because they are well beyond the danger zone of flapping with the high initial pre-rotation so they can go to a higher power setting earlier.

You can go to 100% power in a 914/915 with full backstick if you only pre rotate to 220. But you don’t go to full back stick ever if you pre-rotate to 320 as there is no need to and if you do pre-rotate to 320 and apply full back stick you have enormous rotor drag and a high RRPM initial decay as the ground roll speed (and thus airflow over the rotors) enough to accelerate the rotors in this state.

 

[Aerofoam]

You can go to 100% power in a 914/915 with full backstick if you only pre rotate to 220. But you don’t go to full back stick ever if you pre-rotate to 320 as there is no need to and if you do pre-rotate to 320 and apply full back stick you have enormous rotor drag and a high RRPM initial decay as the ground roll speed (and thus airflow over the rotors) enough to accelerate the rotors in this state.

Maybe not going full aft tilt starts the reverse flow faster at those high prerotation rrpms, but the flow has to be established before the rotote increases speed, the full aft tilt may be slowing that transition, I can not speak from any experience with this aspect, increased drag could also mean the rotor is spinning up faster, but the shorter take off distance with higher powered gyros is due to being able to accelerate faster while aging RRPMs. There are people who can chime in with higher powered single seat Dominators that take off very short despite not having powerful
pre-rotators.
Can you start the forward movement with full aft stick at 200 and then continue to accelerate with the full aft stick and pre-rotator still engaged?

 

[PJB]

Maybe not going full aft tilt starts the reverse flow faster at those high prerotation rrpms, but the flow has to be established before the rotote increases speed, the full aft tilt may be slowing that transition, I can not speak from any experience with this aspect, increased drag could also mean the rotor is spinning up faster, but the shorter take off distance with higher powered gyros is due to being able to accelerate faster while aging RRPMs. There are people who can chime in with higher powered single seat Dominators that take off very short despite not having powerful
pre-rotators.
Can you start the forward movement with full aft stick at 200 and then continue to accelerate with the full aft stick and pre-rotator still engaged?

Hi - Not wishing to confuse things here nor throw spears but my recent post related to post No.425 and the context of my experience in higher pre-rotational speeds in AutoGyro product - both MTO2017 Sport and Cavalon models. So with that established I've flown all with Rotax 915, 914 and even 912 power.

My own experience with all is that full back stick is less effective than part and the reason is the drag to be over come. A 915 can cope but then the transition to unstick is aggressive. A 912 powered aircraft can not cope, struggling to accelerate [i.e. create a ground roll] and seeing RRPM decay [from 320] as a result [you can not have back stick and activate the pre-rotator as there is a micro switch to prevent - actually the intent of it is so that you can not start the pre-rotation accidentally with the stick aft - but the result is the same].

We have debated this before but for the same reasons we talk of here there is a sweet spot of pre-rotational speeds verse stick position and ground roll. Full back stick suits because it creates an easy narrative and is repeatable but [AutoGyro and 28ft dia. rotors] it is never actually the most efficient way of getting airborne at any "normal" pre-rotational velocity. Here is a real experiment I did to prove the same. Aircraft is an old MT-03 with 2 PoB weight of the aircraft empty was from memory around 257kgs, and the two people are c. 85kg each and 30litres of fuel is just under 450kgs. Wind relatively light.

 

[wolfy]

Maybe not going full aft tilt starts the reverse flow faster at those high prerotation rrpms, but the flow has to be established before the rotote increases speed, the full aft tilt may be slowing that transition, I can not speak from any experience with this aspect, increased drag could also mean the rotor is spinning up faster, but the shorter take off distance with higher powered gyros is due to being able to accelerate faster while aging RRPMs. There are people who can chime in with higher powered single seat Dominators that take off very short despite not having powerful
pre-rotators.
Can you start the forward movement with full aft stick at 200 and then continue to accelerate with the full aft stick and pre-rotator still engaged?

It doesn't gain you anything, from 200rrpm if the spinner is left engaged during the roll it just bogs the engine down too much and any advantage of a fast aircraft acceleration is lost.
If a pre rotator is capable of about 250+ your better off flattening the disk to about flight angle and going full throttle to gain airspeed as fast as possible.

 

[XXavier]

It doesn't gain you anything, from 200rrpm if the spinner is left engaged during the roll it just bogs the engine down too much and any advantage of a fast aircraft acceleration is lost.
If a pre rotator is capable of about 250+ your better off flattening the disk to about flight angle and going full throttle to gain airspeed as fast as possible.

And with the RRPMs going up... But you have to feed the rotor with enough energy to reach the ~320 RRPM that you need for flight, or somewhat less while the gyro is in ground effect. That energy is harvested from the relative wind, and the harvest will be much faster at higher airspeeds, since it grows with V^2. But –and that's my question– does it really mean a shorter takeoff run...?

 

[Abid]

And with the RRPMs going up... But you have to feed the rotor with enough energy to reach the ~320 RRPM that you need for flight, or somewhat less while the gyro is in ground effect. That energy is harvested from the relative wind, and the harvest will be much faster at higher airspeeds, since it grows with V^2. But –and that's my question– does it really mean a shorter takeoff run...?

It means a shorter takeoff ground roll but not a shorter Takeoff distance (to clear 50 foot obstacle).
I trained someone in their MTO 2017 914powered gyroplane. First doing the 300+ RRPM pre-rotation is quite a thing. You end up over 5200 RPM on the engine, holding the brakes like a bear to get there. You do not pull the stick all the way back but somewhat back after releasing the pre-rotation engagement. I would say about as much back as a 55 mph climb attitude and you let the machine accelerate, keeping and eye out on RRPM gauge. It breaks ground quicker but I did not find improvement in clearing a 50 foot obstacle distance wise compared to a 200 RRPM pre-rotation on a 914 powered AR-1 and then a normal good technique short takeoff procedure.

 

[Jean Claude]

With the standard procedure, pre-launching higher than 70% of the steady flight Rrpm does not shorten the take-off run.
But it can be useful to avoid flapping divergence when propeller thrust is very high. (too much forward speed in relation to the Rrpm reached at a point of the run).

 

[Burrengyro]

Looking at Wolfie's and PJB's posts on stick position for a given RRPM value of say 220 RRPM, and factoring in Jean Claude's warning about not generating excessive forward speed in relation to RRPM, is the following idea of any merit?

Reduce movement of the stick aft to the back stop to say 80% (if that is the appropriate figure) of current aft stick movement to give a disk flight angle to achieve best climb airspeed as efficiently as possible without generating too much forward speed in relation to the RRPM to cause flapping.

A trim system to limit the aft stick movement to say 80% of normal movement during takeoff run might achieve this goal with consistency. Once best climb airspeed has been achieved, the 80% limit could be immediately cancelled and 100% aft movement of the stick is then restored and the pilot can fly and land as normal.