Effect of prerotation on takeoff distance

Greg Vos

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With an electric rotator drawing its power from a battery.
For a given rotator, leaving it engaged a few seconds after the brakes are released will affect additional energy for rotation. Rrpm is reached faster and after a shorter run. Less A.o.A of the disc is then required to reach the required forward speed and therefore less rotor drag. Thus the total distance is reduced (Only little bit, since 3000 Watts during 5 seconds is about little before 1000 Newtons during 100 meters)
Caution, the rotation torque applied to the rotor tends to make the gyro turn .
Caution the rotation torque applied to the rotor tends to make the gyro turn.... Yaw is what you mean?
stroke of genius right there in your comments and why a helicopter has a tail rotor referred to as the anti torque ..


pre rotate to 170 - 180 Release the pre rotator ..take power get nose off the ground, wheel balance on the mains gently fwd on the stick without front wheel touching the ground and fly it off what could be more simple ?
 

Greg Vos

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you can insult the French as long as you want, as we say in french " cela me touche une couille sans faire bouger l'autre" google it if you want
That was not an insult 😳😳😳I stated fact .... when I insult some one they will know about it 😁
 
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Greg Vos

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for me french arrogance is an insult , but you can go on I don't mind at all
😂😂😂that’s what I’m talking about

I could list a few more Darwin French moments if you like 😁

hey bud lighten up I’m just taking the piss ....but we can’t deny the facts ..... I was in a hotel in Joburg a few months back before Covid as I climbed into elevator (11 th floor) I was surrounded by the entire Air France crew, I said - going down 😁 to say the least that we’re not enamored with my sense of humor it was after a recent spate serious airline accidents all Air France at the time .
 

Greg Vos

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for me french arrogance is an insult , but you can go on I don't mind at all
The French can still take lessons from the Germans in arrogance,..so please don’t think the French are leading the pack 😁 sorry for the thread drift
 

XXavier

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If the cross wing is high take off into it ....use it 😳

Difficult, with a single decent runway. We have two others, at an angle, but they are dirt strips with many potholes...

I'm now just back from the airfield. Let's see what you wrote in the previous post...

To get get lift from blades one would need pitch, hence a heli can change the pitch of the blade, if what you say has any substance then you need to consider sitting in a heli, rotor at speed say (300 Rrpm normal range for a heli - or say a Robbie say 504 odd) and you tilt the disk back - pulling aft stick .... the heli sure as hell don’t lift its nose 😉..... in a strong head wind then yes the heli will lift its nose, We assume a wind free day? it will be a lot easier to explain this in a class

To witness this in real time do it with a model helicopter tilt the rotor disk in any angle ( while at speed) prior to pulling collective ( we call it pulling power - raise the collective) the thing will not lift, provided it is not subjected to any prevailing wind

with a gyro think of a controlled parachute not a heli blade, I’m sure the more exp instructors here will have time for a long scribe?


Some comments:

To get lift from a rotating blade you need pitch. It's the case with helicopters, and it's also the case with our gyros while in pre-rotation. During pre-rotation, the gyro's rotor works like a helicopter rotor, accelerating air downwards. At a given moment [1], you disconnect the pre-rotation drive, and the blades, that have now a very considerable amount of energy, continue turning and accelerating air downwards. Of course, the blades lose gradually their energy, in the uninterrupted process of accelerating air downwards, and revs drop.

But very soon after disconnecting the drive, you tilt the rotor axis back [2]. The flow of accelerated air is also tilted, and the reaction is not only upwards, as before, but in part upwards and in part backwards. Now, you release the brakes and the prop's thrust starts [3] moving the gyro forward. The 'backwards reaction' of the rotor would be felt as a sort of drag, reducing the acceleration of the gyro.

After a short time, the interaction between the rotor and the relative wind start to change, [4] and the rotor reduces gradually the 'helicopter-like' downdraft, and starts to work in autorotation, getting energy from the incoming relative wind and gradually increasing its revs. With the rotor disk moving edgewise at an angle, keeping its revs by autorotation, we have now a sort of wing, that –as any wing– changes the direction of the relative wind generating a downwash. That's the gyro's source of lift. The gyro takes off [5] as soon as the lift surpasses its weight...

The merit of Urbani's method is to minimize the distance covered by the gyro between the instant [3] and somewhere between [4] and [5], that 'somewhere instant' (let's name it [4']), being the moment the rotor revs recover 220 rpm (in his gyro), that was the RRPM reached at [1]. From [4'] to [5] the RRPMs increase continuously until reaching the angular speed for flight.

Hope things are clear now...
 

Greg Vos

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Difficult, with a single decent runway. We have two others, at an angle, but they are dirt strips with many potholes...

I'm now just back from the airfield. Let's see what you wrote in the previous post...

To get get lift from blades one would need pitch, hence a heli can change the pitch of the blade, if what you say has any substance then you need to consider sitting in a heli, rotor at speed say (300 Rrpm normal range for a heli - or say a Robbie say 504 odd) and you tilt the disk back - pulling aft stick .... the heli sure as hell don’t lift its nose 😉..... in a strong head wind then yes the heli will lift its nose, We assume a wind free day? it will be a lot easier to explain this in a class

To witness this in real time do it with a model helicopter tilt the rotor disk in any angle ( while at speed) prior to pulling collective ( we call it pulling power - raise the collective) the thing will not lift, provided it is not subjected to any prevailing wind

with a gyro think of a controlled parachute not a heli blade, I’m sure the more exp instructors here will have time for a long scribe?



Some comments:

To get lift from a rotating blade you need pitch. It's the case with helicopters, and it's also the case with our gyros while in pre-rotation. During pre-rotation, the gyro's rotor works like a helicopter rotor, accelerating air downwards. At a given moment [1], you disconnect the pre-rotation drive, and the blades, that have now a very considerable amount of energy, continue turning and accelerating air downwards. Of course, the blades lose gradually their energy, in the uninterrupted process of accelerating air downwards, and revs drop.

But very soon after disconnecting the drive, you tilt the rotor axis back [2]. The flow of accelerated air is also tilted, and the reaction is not only upwards, as before, but in part upwards and in part backwards. Now, you release the brakes and the prop's thrust starts [3] moving the gyro forward. The 'backwards reaction' of the rotor would be felt as a sort of drag, reducing the acceleration of the gyro.

After a short time, the interaction between the rotor and the relative wind start to change, [4] and the rotor reduces gradually the 'helicopter-like' downdraft, and starts to work in autorotation, getting energy from the incoming relative wind and gradually increasing its revs. With the rotor disk moving edgewise at an angle, keeping its revs by autorotation, we have now a sort of wing, that –as any wing– changes the direction of the relative wind generating a downwash. That's the gyro's source of lift. The gyro takes off [5] as soon as the lift surpasses its weight...

The merit of Urbani's method is to minimize the distance covered by the gyro between the instant [3] and somewhere between [4] and [5], that 'somewhere instant' (let's name it [4']), being the moment the rotor revs recover 220 rpm (in his gyro), that was the RRPM reached at [1]. From [4'] to [5] the RRPMs increase continuously until reaching the angular speed for flight.

Hope things are clear now...
How does the spinning blade ( while in prerotatoin) work like a helicopter? The blades are set in the hub bar and have little pitch ? Please visit my example of the model helicopter it’s like this in full size helicopter too, so no your explanation is not clear if anything it raises more questions

SIR A GYRO BLADE DOES NOT PUSH AIR DOWNWARDS LIKE A HELICOPTER😳😳😳😳IF WE ACCEPT YOUR EXPLINATION WE WOULD EXPECT THE SAME FROM A PARACHUTE 😳
If I were to even remotely accept your theory then it would be fair to say a gyro with a blade rotation of 350+ Rrpm would be able to hover ? In a no wind zero airspeed condition if it were possible to keep the blade spinning and the fwd thrust at zero ( ie: motor off)
The gyro will simply not have sufficient pitch in the rotor blade to achieve this..
No I disagree and would suggest you consult Phil Harwood’s books chapter gyro technical

o dear ...
 

XXavier

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How does the spinning blade ( while in prerotatoin) work like a helicopter? The blades are set in the hub bar and have little pitch ? Please visit my example of the model helicopter it’s like this in full size helicopter too, so no your explanation is not clear if anything it raises more questions

SIR A GYRO BLADE DOES NOT PUSH AIR DOWNWARDS LIKE A HELICOPTER😳😳😳😳IF WE ACCEPT YOUR EXPLINATION WE WOULD EXPECT THE SAME FROM A PARACHUTE 😳
If I were to even remotely accept your theory then it would be fair to say a gyro with a blade rotation of 350+ Rrpm would be able to hover ? In a no wind zero airspeed condition if it were possible to keep the blade spinning and the fwd thrust at zero ( ie: motor off)
The gyro will simply not have sufficient pitch in the rotor blade to achieve this..
No I disagree and would suggest you consult Phil Harwood’s books chapter gyro technical

o dear ...


The fixed pitch of our gyros is 3º more or less. Little, but enough to lift the gyro if the RRPMs were enough. Around 500, I believe...

But a gyro –in flight– works in another way. The rotor disk interacts with the relative wind as a if it were a circular flat-plate wing, albeit a porous one, but a working wing anyway, that produces lift by changing the direction of the relative wind and generating a downdraft, as all wings use to do...
 

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Wings generate lift by creating a low-pressure area above the wings.
 

Sv.grainne

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Bobby,
Don't change the usual procedure which is the safest.
My theoretical analysis just explains why this usual procedure no longer shortens the takeoff when the rpm of the pre-launch exceeds 2/3 of the steady flight rpm. This is also what observed by the systematic measurements.
So, it is wrong to believe that pre-launching almost at flight rpm would give the shortest takeoff. To be actually beneficial a different, less safe procedure would have to be applied
Thanks Jean Claude, was worried for awhile. Got in another 2.5 hours of training today. No confusion!😁
 

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JC can you give me some colour on tip speed ratio values at the onset of blade flapping? I guess to provide a constant we can assume maximum back stick and therefore alpha.
 

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Jean Claude

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JC can you give me some colour on tip speed ratio values at the onset of blade flapping? I guess to provide a constant we can assume maximum back stick and therefore alpha.
As you know, the divergence of the flapping angle is due to the stall of the retreating blade. Therefore the acceptable limit of tip speed ratio also depends on this angle.
Unfortunately, the angle of attack of each blade element changes periodically during rotation, which creates an hysteresis: Later stall and re-stall. The stall angles then become difficult to predict for me, and thus also the critical ratio of divergence.
But to give you an order of magnitude, I find that in a situation quite usual for our gyroplanes (A.o.A disk at about 20°, aerodynamic pitch of the blades at 3°, and assumed stall angle of 13°) the calculated divergence seems to appear to me as soon as the speed ratio of 0.2
 
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XXavier

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Fellas, this is a great discussion (again?).
Here is a case that I don't recall being discussed: a Touch & Go.
You land with 'flying rotor RPM', then shove the throttle up and take off.
My feeling is that the ground run (and time) is less than from a standing start (normal) take-off.
Anyone have data to say that the take-off is the same or different?
The T&G takeoff rotor RPM certainly is higher (if you don't take too much time).
Brian


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.
 

Jean Claude

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And if you put the stick all the way back as soon as you land, you can see that your gyroscope starts to back up, even when there is no wind at all.
This is proof that the rotor of a gyroscope doesn't need airspeed to pull when it turns fast.
It is amazing that an instructor like Greg has never done this simple experiment.
 

Greg Vos

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And if you put the stick all the way back as soon as you land, you can see that your gyroscope starts to back up, even when there is no wind at all.
This is proof that the rotor of a gyroscope doesn't need airspeed to pull when it turns fast.
It is amazing that an instructor like Greg has never done this simple experiment.
Really .... so as you land your using the entire disk as a large handbrake ....that airflow is what speeds it up momentarily before it drops off .😳😳😳

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?

notice a heli about to take off..rotor is at flying speed, all the pilot does is pull in pitch ....the aircraft will sit on ground all day until he does this and helicopters don’t ever go to - pitch with full down collective ( normal everyday ships)


what part of this can’t you grasp?
 

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Having cleared up the point about fixed pitch gyro rotor blades and jump gyros that have variable pitch... a light diversion here as I found this example of Dick Degraw’s two machines taking off wonderful to watch as he and his wife depart in style:-

Been following the thread with great interest.

It is evident, as has been clearly explained by various members, that there is an optimum RRPM to be reached with the stick full forward and applying pre-rotation. The stick is then tilted back brakes released and throttle applied to begin the acceleration phase of both gyro and the rotor.

It has also been pointed out that with the disc tilted back and above a certain rrpm there is a component acting against acceleration of the gyro forward.

The training of a student is always with worst case in mind, hence the stick fully back to prevent perhaps the disc not being tilted far enough back so that a rapid acceleration will produce an airflow through the disc that is such that blade sail/flap can then begin.

That optimum rrpm is obviously not an easy one to calculate given the diversity of blade pitch and length in the various types of rotors. But if it were possible to obtain, then to achieve a minimal ground roll you would pre-rotate to that figure, then ease the stick back, release the brakes while at the same time bringing in the power.

At this point numerous variables all come into play.
What is the headwind component?
How far does the stick come back to achieve and optimum disc angle so the least component of lift opposes the forward acceleration of the gyro?
How rapidly is the throttle applied?

All will directly effect the length of the take off roll. Since it is extremely difficult to quantify and apply these with any great accuracy on any given take off we revert back to the worst case technique and accept the limitations it imposes in order to be safe.

It has been noted, and I think mentioned by Chris (Ckurz7000), that one can carefully practice both stick back point and throttle application to decrease take off distance, but great care should be exercised as suddenly experiencing blade sail/flap at quite a high forward speed can be extremely dangerous.

I have experimented with this on the Bensen and found that I was able, on a particular day, temp, particular runway and a fairly constant wind, by varying the stick back position, the amount of initial throttle application, and the rate of that application, to vary the take off distance appreciably.

In doing so I was also extremely mindful of how rapidly and violently the onset of blade flap/sail can be. Anyone who has flown in high wind situations should also be aware, as blade sail/flap can be experienced with the gyro at a complete standstill if the blades are at a fairly slow rpm and the critical disc angle is inadvertently exceeded.
 

XXavier

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Who sait that a high pre rotate will make the gyro lift off ?

I did. But it has to be a very high-rev pre-rotation. I guess that around 500 rpm for my ELA. It's just a guess, but I'm 100% sure that there is a rev number at which the rotor lift will surpass the weight of my gyro, since any rotor angular speed produces a rotor lift, as the blades have a positive pitch...
 

Greg Vos

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I did. But it has to be a very high-rev pre-rotation. I guess that around 500 rpm for my ELA. It's just a guess, but I'm 100% sure that there is a rev number at which the rotor lift will surpass the weight of my gyro, since any rotor angular speed produces a rotor lift, as the blades have a positive pitch...
Ok so you like science fiction.... go look what the OEM says about max rotor speed ? I managed once in a M16 to pull 525 Rrpm in a very tight high speed turn, manufacturers have upper limits to never exceed, obviously if your the designer builder your limits are at your discretion.
i will simply remind what I am saying if one looks at a helicopter with blades spinning at flying speed one can pull the stick back (aft stick ) and the heli will not lift ... it will have the disk tilted aft but not lift off ...


Regarding comments further back about settings of pitch range when I set up a gyro ( having built a few) I use full fwd stick as -2 mid stick position around 7 degrees and full aft 19 degrees with max left right at 8.

as a rule of thumb this is fine

consider a R44 the blade is governed at 504 Rrpm IIRC it will not lift with 3 degrees of collective pull...so I doubt your theory with 500 Rrpm is going to lift the ELA?
 

XXavier

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The max. loaded weight of the R44 is 2,5 times that of the ELA.

I was writing a comparison between the thrusts attained by the R44 and the ELA at different RRPMs, but I deem it no longer necessary, as I see, with some relief, that I've got the blessings of J.C. above...
 
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Jean Claude

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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 hover OGE. Then, 104 Hp on the shaft seems need.
And collective pitch of 8.1° seems required for a vertical climb of 4 m/s. 124 HP on the shaft seems then need to keep 540 t/mn
 
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