Effect of prerotation on takeoff distance

Greg Vos

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I agree with Xavier.
My calculation says that the rotor of an ELA would have to be rotated at more than 500 rpm to lift 450 kg in ground effect with no forward speed, and this would require 70 hp shaft power.
And 300 rpm still gives it a lift of 170 kg. This, due to the Aerodynamic pitch setting of 3 degrees.
Tilting this 20° backwards, then, it pulls the gyro 55 kg backwards.

My calculation also says that you need a collective pitch of 7.2° for the rotor of a Rob R22 turning at 540 rpm to lift 620 kg in an OGE hover. 104 Hp on the shaft seems then necessary.
How does a Robbie ever get 540 Rrpm? There is a very interesting calculation for every percent above 100% for Rrpm before destruction and I fear that 540 will be higher than what the spherical bearings could withstand?
in fact I will go on record and say that once it’s seen that it will need to go in for inspection ?

however we digress and we still cant seem to agree that winding up the rotor will aid take off distance and I’m still not in agreement with that? It would be great if you could do some calc to expand on take off distance using standard airfoil gyro blades and various Rrpm’s prior to actually rolling (and point of rotation) because my thinking and training is that the gyro disk is in fact like a big drag Shute and is not in anyway like a helicopter
if we were all debating this in a class with the aid of a white board I would gladly hash out my thinking and support it with drawings?


please explain the comment about pulling the gyro backwards at 55kg as this ties in with what I’m saying, you need this force and the thrust from the prop to enable flight? And things sort themselves out with thrust and aft stick at say 7 degrees? For level flight?
 

WaspAir

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cant believe you guys think a high pre rotate and level disk will make the gyro lift off ....if you do believ this I suggest you stop flying never mind instruction?
This is impossible, but if I am reading your comments properly, not for the reason you are suggesting.

Typical teetering fixed collective gyro rotor blades have a built-in, non-zero collective pitch, and pre-spinning them while sitting still on the ground will cause flow from above the disc to below, much as ordinarily expected for a helicopter with small positive collective pitch. If one had enough power available, and did an outrageously high pre-spin, one might expect to generate enough lift to levitate, but you won't ever get to that state for other reasons. As lift produced by the rotor increases, the load on the wheels is reduced. Friction of the tires on the pavement is all the anti-torque you can get in a gyro, and that friction drops as the normal (to the pavement) force is reduced by rotor lift. As the gyro gets lighter on the wheels, the friction of the tires will become insufficient to resist the torque, and the airframe would start to skitter around, rotating in the opposite direction of the rotor rotation. Lacking a tail rotor, there is no way to control this, and you'll be sliding around in a rather scary fashion on the pavement before you could get it to lift off.

My question to you would be: What sort of airflow do you expect to produce with a fixed-collective rotor disc while sitting still and pre-spinning?
 

Jean Claude

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OK, Greg for 530 rpm maxi . Collective pitch calculated at 530 rpm (104%) becomes 8.3° indeed 8.1° at 4 m/s vertically, and need 122 Hp indeed 124 hp. This seems science fiction to you?
 
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XXavier

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(...)


please explain the comment about pulling the gyro backwards at 55kg as this ties in with what I’m saying, you need this force and the thrust from the prop to enable flight? And things sort themselves out with thrust and aft stick at say 7 degrees? For level flight?

You still don't understand that the gyro rotor 'helicoptering' on the runway is working in a very different way as when the same rotor is in flight, behaving like a wing...
 

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If you stop, or roll very slowly after touching, the rotor will switch to 'helicoptering' mode, and the take off run will be longer. But if you don't stop or roll too slowly, the rotor will keep the 'autorotating wing' mode and your takeoff run will be very short.
Okay, now we're getting somewhere (JM, please continue contributing).
It has been stated many, many, many times on this forum that there is an "optimum" pre-rotation RPM.
I usually see 200-220 RRPM as that number.
It also seems like some/many of us would agree (per my touch & go observation) that take-off with a faster spinning rotor is quicker/shorter.
I don't recall looking at the airspeed indicator, but I feel like the take-off airspeed is also less when taking off (yes, Xavier, immediately) with a higher rotor RPM.
So, it does seem possible to shorten the take-off run with a different technique.
Also, it appears to me (just observation, no data) that a more powerful gyroplane that can accelerate (forward motion) quicker, also has a shorter ground roll distance and shorter time required to take-off.
Now, how to spin the rotor faster or accelerate the rotor more quickly (JM's question of engaged pre-rotator) and/or accelerate (forward motion) more quickly...
Brian
 

Greg Vos

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You still don't understand that the gyro rotor 'helicoptering' on the runway is working in a very different way as when the same rotor is in flight, behaving like a wing...


no And it’s is after completing over 1000 hours in Helicopter ....doing an in depth test pilot course in the East, and then passing it again in South Africa, and then giving actual in flight gyro lessons for over 900 hours and multiple ground school classes some under the eyes of very experienced flight instructors? No I don’t understand it after building a RAF from the ground up, rebuilding a M16 and three Xenon gyroplanes ....
no I don’t understand it after having formal technical training in an engineering background 😳
no I don’t understand it after flying as gyro pic for over 1000 hours in some of the most challenging environments and inclement weather .... it must be me
unless somehow our descriptions are being distorted with lack of explination.....but a gyro rotor blade does not offer any dynamics of a rotor that one can adjust the pitch ( fixed pitch gyroplane teeter set up) a gyro rotor in flight is similar to a controllable paraglider.
In a gyro we drag the rotor behind us ....
 

Greg Vos

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Okay, now we're getting somewhere (JM, please continue contributing).
It has been stated many, many, many times on this forum that there is an "optimum" pre-rotation RPM.
I usually see 200-220 RRPM as that number.
It also seems like some/many of us would agree (per my touch & go observation) that take-off with a faster spinning rotor is quicker/shorter.
I don't recall looking at the airspeed indicator, but I feel like the take-off airspeed is also less when taking off (yes, Xavier, immediately) with a higher rotor RPM.
So, it does seem possible to shorten the take-off run with a different technique.
Also, it appears to me (just observation, no data) that a more powerful gyroplane that can accelerate (forward motion) quicker, also has a shorter ground roll distance and shorter time required to take-off.
Now, how to spin the rotor faster or accelerate the rotor more quickly (JM's question of engaged pre-rotator) and/or accelerate (forward motion) more quickly...
Brian
Brian a gyro with say 200 Rrpm will take off like a helicopter vertically if it has a head wind component that imitates airspeed so you would not need any forward roll, ...but this is not what these experts are saying and I’m saying that it’s going to happen at 200 Rrpm and higher pre rotate is of no value
 

XXavier

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Okay, now we're getting somewhere (JM, please continue contributing).
It has been stated many, many, many times on this forum that there is an "optimum" pre-rotation RPM.
I usually see 200-220 RRPM as that number.
It also seems like some/many of us would agree (per my touch & go observation) that take-off with a faster spinning rotor is quicker/shorter.
I don't recall looking at the airspeed indicator, but I feel like the take-off airspeed is also less when taking off (yes, Xavier, immediately) with a higher rotor RPM.
So, it does seem possible to shorten the take-off run with a different technique.
Also, it appears to me (just observation, no data) that a more powerful gyroplane that can accelerate (forward motion) quicker, also has a shorter ground roll distance and shorter time required to take-off.
Now, how to spin the rotor faster or accelerate the rotor more quickly (JM's question of engaged pre-rotator) and/or accelerate (forward motion) more quickly...
Brian

Yes, keeping the rotor in its 'wing-like' condition is essential. Any lost time and lost speed after touching, and the rotor will revert to its 'helicoptering' condition, braking the gyro and making the takeoff run longer...

This subject that we are discussing is not only important in the theory, but can be vital in practice...
 

Greg Vos

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OK, Greg for 530 rpm maxi . Collective pitch calculated at 530 rpm (104%) becomes 8.3° indeed 8.1° at 4 m/s vertically, and need 122 Hp indeed 124 hp. This seems science fiction to you?
DA and QNH for your experiment?


This seems science fiction to you? YES!! 530Rrpm! Name a Helicopter that is commercially accepted that offers this Rrpm....you watch to much back to the future...
 

Greg Vos

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Yes, keeping the rotor in its 'wing-like' condition is essential. Any lost time and lost speed after touching, and the rotor will revert to its 'helicoptering' condition, braking the gyro and making the takeoff run longer...

This subject that we are discussing is not only important in the theory, but can be vital in practice...
WTF is helicoptering condition!?
 

Greg Vos

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Vance

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If I was trying to find out the optimum prerotation rotor rpm for short take offs on a no wind day I would land and come to a full stop on some measurable spot, watch the rotor rpm and add full power as I reached the rpm I wanted to test and see how much distance was used lifting off.

I would repeat this at various rotor rpms until I found the optimum rotor rpm for short takeoffs.
 

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WTF is helicoptering condition!?
When the flow through the disc is in the opposite direction of the flow during autorotation, for example, while stationary and pre-rotating with a positive pitch fixed collective rotor.
 

Greg Vos

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When the flow through the disc is in the opposite direction of the flow during autorotation, for example, while stationary and pre-rotating with a positive pitch fixed collective rotor.

Only Magni and Auto Gyro pre rotate with stick fully fwd ...and we are talking in this dramatic thread about pre rotate speeds to reduce take off distance ..( as we see it’s of little benafit)
This is not possible with a xenon .... we always pull the stick back slightly during pre rotation a fully fwd condition places the rotor brake surface in contact with the brake disk So forgive my rather vague notice of this condition. In fact I wonder how perceptive it is because we Prerotate with Magni and within seconds (120 Rrpm) we start coming back .... I think some people just like to take things that work and over complicate them?

the French come to mind at the moment ...
 

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@jm-urbani your video in #219 (Philbennet) is exactly as my French instructor taught me, commence the roll then on sight of rotor speed buildup push the stick a little forward, there is a rapid increase in acceleration and unstick quickly follows.

p.s. I am now practicing this technique along with only pre-rotating to 140rrpm prior to the roll, keeping the electric pre-rotator on until I hit 200 rrpm in the roll (my pre-rotator (electric) will run up to 180rrpm but I think this technique lessens the wear on the motor and ring gear)


phil
 
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Greg Vos

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@jm-urbani your video in #219 (Philbennet) is exactly as my French instructor taught me, commence the roll then on sight of rotor speed buildup push the stick a little forward, there is a rapid increase in acceleration and unstick quickly follows.


phil
This is a good technique for the lower time pilots, as your experience grows you will hold the stick fully back as the Rrpm builds and lift the front wheel and then only “check fwd” at this point without the front wheel touching again during the take off, this wheel balance phase limits the possibility of possibly steering the ship off runway if you had a crosswind as you now have full rudder authority and no nose wheel to interfere, once you have mastered this there is no going back

I know there will be loads of wise asses who will go on to say otherwise but they dont get paid to fly a gyro 😉
 

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As Phil Bennet, JM, Chris Lange and myself are pointing out, is that if the full back stick procedure is used, (lots of gyros can pre- rotate with stick fully forward) the initial forward acceleration phase is compromised by that forward component of the discs lift.

As Chris and I have pointed out, the relaxation of back stick...or, the employment of less than full back stick at brake release results is a more rapid forward acceleration, less component of forward lift to oppose ie less drag... BUT... for a less experienced pilot, it could result in not enough disc angle being present, thus causing an insufficient RRPM build-up to match the increased airflow through the disc...

...a possibility then exists where any further back stick at this stage will result in too much airflow at insufficient RRPM to cope, at a much more critical stage in the take-off run.

It will now be at a point where the forces generated by blade flap/sail are very much greater, will occur more rapidly, and thus creating an out of control situation that will be almost impossible to react to given the speed at which it will occur.


The minor criticism I would make here, is that Phil’s technique that he demonstrates in the video, certainly does produce a better result...BUT carries with it a greater danger for the inexperienced pilot, which I think he does not stress enough.

Hopefully during your training as a gyro pilot you will have had blade sail/flap not only explained to you fully...but will also have had the onset of blade flap/sail actually demonstrated to you. If you have not, try and get it done. The stick forces generated and speed at which they progress, if not immediately dealt with, are sobering.

The exercise is best done in a single seat gyro. Fully briefed. Engine off. Stationary and chocked. With the instructor outside the gyro and beside you and the gyro facing into a reasonable breeze. With the stick full forward the blades are patted up, the instructor will then have you easing the stick back to gently accelerate the blades to as high an RRPM as possible for the wind speed available. It is then demonstrated how the disc angle of attack governed by the stick position can accelerate, or allow the blades to decelerate...and how if the disc angle is increased too rapidly the onset of blade sail/flap will begin to occur.

If this occurs at the start of a take off roll stick fully forward and come to a stop...no big deal and reasonably manageable. If however it occurs proceeding down the runway at a much higher speed it will occur, faster and with much higher stick forces and very much more dangerous. A very good reason why a student will be taught full back stick until rock back, as it is easier to remember, easier to do, and safer even if less efficient. Only when more experienced can the less than full back stick be used early in the take off roll, and even then with the full realisation of the possibility that if not enough back stick is held the situation described above can occur.
 
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Jean Claude

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that is only half the story and some very interesting data is missing anyway as the facts prove pre rotate as fast as you want it makes no significant impact on the take off distance
Yes, the facts prove it, and the theory predicts it.
DA and QNH for your experiment?
1.22 kg/m3: Standart température at sea level

530Rrpm! Name a Helicopter that is commercially accepted that offers this Rrpm...
You don't know Robinson R22? Amazing! https://robinsonheli.com/r22-pilots-operating-handbook/
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