Effect of prerotation on takeoff distance

All_In

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John (all in)
Put me down for one.
Hope you get this project going.
This is this best thread I have read in a while.
Marc is well on his way.
He got most of his kit just yesterday for his Aviomania two place G2SA.

So Now I will devote more time to this project and help. But Mark as it about finished and I do not have to write the driver another employee and friend is writing it. If he cannot get it to work I will help re-write the code but I may not be needed at all other than to fund it for PRA and buy the parts in volume for us all.

He has 36 volts now and our first version is for 2 places heavy rotors much more powerful than anything on the market.

The way the gearing is designed we can add a second motor side by side to the gearing and make it even more powerful if we need to.

He has 3 regulators circuit charging the 3 batteries while in flight and a ground system charging it on the ground.

He is a master machinist and knows gearing and where to get them and the right ones. That is part of is business doing projects much like this!!!

When this is done we will all owe Marc !!! I have the BEST friends in the world. I may be one of the riches people in the world in what really matters FRIENDS and Marc and I think alike (His brain is broken too) so we are very close and talk as much as we can!!

PS:
Once the heavy weight one is finished it should be easy for anyone to just step down the motor or voltage or both and create a cheaper version for light weight gyroplanes.
However they are going to be more expensive and the only way to reduce the cost is buying in VOLUME. PRA is perfect for that part of this mission!
 
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Jean Claude

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The unability to decrease the distance with more rpm of pre-launching is still very mysterious to me. In my simulations including braking gyro by the RTV, the calculated distance always is shorter when the pre-rotation rpm increases...
But the reality is different, which shows me that a phenomenon is still omitted.

Now, my hypothesis is that at the start of taxiing, the induced velocity exceeds the general flow. Then the situation is reversed more or less rapidly when the rotor slows down and the taxiing accelerate.
So, between the two situations, it seems to me possible that a VRS isolates the disk during a remote part even longer than the pre-launch rpm is high.

Not sure my explanations are clear enough!
 
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birdy

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Your right on the money Jean.:)
Iv found that, for the shortest possable ground roll, you need to;
Prespin to max rrpm, useing ground breaks and RTV to arrest any forward roll
Simutainiously stick full forward and hit WOT.
Acceleration is rapid, and wen at min lift off speed, ease back, all with the prespinner engauged.

Startn the roll with a virtical RTV means liftoff speed is reached much earlier, and wen you do pull back, the rotor gets clean air.

NB, i dont think theres a VRS, but theres definatly a time lag between startn the roll and the rotor getn clean air if the roll starts with backstick, which costs ground distance.

NB.2, if you plan on usen this method, think about it, hard, coz theres sum unexpected feed back thats a real attention getter.
 

All_In

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Your right on the money Jean.:)
Iv found that, for the shortest possable ground roll, you need to;
Prespin to max rrpm, useing ground breaks and RTV to arrest any forward roll
Simutainiously stick full forward and hit WOT.
Acceleration is rapid, and wen at min lift off speed, ease back, all with the prespinner engauged.

Startn the roll with a virtical RTV means liftoff speed is reached much earlier, and wen you do pull back, the rotor gets clean air.

NB, i dont think theres a VRS, but theres definatly a time lag between startn the roll and the rotor getn clean air if the roll starts with backstick, which costs ground distance.

NB.2, if you plan on usen this method, think about it, hard, coz theres sum unexpected feed back thats a real attention getter.
Good morning,
This is good stuff but I'm have a senior moment (really only had 1.5 hours sleep and not awake yet) and cannot remember what RTV stands for and VRS could you define the terms for those of us who do not know?
 

JAL

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Jean I understand you factored in the RTV as an opposing force to the initial acceleration so you don't think that explains the difference between the theoretical and practical but think it may be something similar to VRS. I would have thought that VRS would do the opposite, that is shorten the distance by allowing the rotor to move through "less dense" air thereby reducing resistance to acceleration. Maybe it has something to do with the prop. Could it be that the prop becomes more efficient as airspeed increase until it reaches optimum angle of attack and anything that slows that results in a longer ground run. Using Birdy's method of pre rotating with flat disk then opening up the rotor to the airstream once the gyro has accelerated would allow the prop to get closer to its optimum thrust quicker because the RTV would not be opposing it . Would also mean that it is producing max thrust at the time when the drag increase significantly or in other words you hit max thrust in the shortest time.
 
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Jean Claude

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This is good stuff but I'm have a senior moment (really only had 1.5 hours sleep and not awake yet) and cannot remember what RTV stands for and VRS could you define the terms for those of us who do not know?
Yes John. Sorry

VRS: Vortex Ring State. When a helico descends vertically with power, a little slower than auto-rotation, then he is confined in its own wake. https://www.youtube.com/watch?v=HrsGM0PzQFo

RTV: Rotor Thrust Vector
 

All_In

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Yes John. Sorry

VRS: Vortex Ring State. When a helico descends vertically with power, a little slower than auto-rotation, then he is confined in its own wake. https://www.youtube.com/watch?v=HrsGM0PzQFo

RTV: Rotor Thrust Vector
Thank you so much not only the anwser but a video that shows us what it is!
Your another one of my heroes on here = U-Rock!
 

Jean Claude

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I would have thought that VRS would do the opposite, that is shorten the distance by allowing the rotor to move through "less dense" air thereby reducing resistance to acceleration.
Quickly obtain the good forward speed is not sufficient to shorten the distance. We must also quickly obtain the good rotor rpm. But while it is confined in VRS, the air flow does not produce torque autorotation before leaving the VRS by forward speed higher and (or) rpm diminished. The benefit will be erased.
 
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birdy

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Jean, if you prespin with a flat disc, the rotor will be in clean air as soon as its traveled its own diameter horisontaly, so wen you pull back, its clean, and you have AS, so you have torque, added to the prespinner torque.
 

Jean Claude

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Yes, Birdy. If my thesis is true then your strategy is good, launcher still engaged or no.
 

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This is great!
I'm saving your and David's post on how to do the shortest take-off and posting them as a sticky in PRA forum so it's easy to find.
Thank you so much for helping our community!
 

JAL

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Yes, Birdy. If my thesis is true then your strategy is good, launcher still engaged or no.
Yes that maybe true but in practice if the pre spinner is not engaged the flat disk angle will slow the rotor down and risk flapping when pulled back.

I disagree that good acceleration from a standing start is not the a major factor to shorter takeoffs. Having the rotor all the way back before the gyro starts rolling will slow the ground acceleration a great deal.

I would wager that having the rotor spinning and flat would halve the time it would take for the gyro to get 40kts then having it spinning > 200 rpm and all the way back before the gyro starts rolling.

I think airspeed is the primary control of takeoff distance not rotor rpm which is just a consequence of airspeed. If you were to take off in a 55kt headwind and prerotate to 200 rpm and go stick back you would take off vertically or in some gyros backwards. Anything that gets you to your required airspeed quicker will reduce your ground run.

That's why I think having prerotator that can spin up to >250 plus is a bit of overkill and mostly unnecessary unless it can be engaged throughout the takeoff in the way described by Birdy (flat disk). In fact it is counterproductive as it would create more drag initially and slow the acceleration of the gyro down a lot more than if they were rotating at say at 180 at the start.

The blades will be flying at the same speed at takeoff regardless how fast you pre rotated them, so they either come up to that speed or slow down to that speed. Its probably quicker in terms of ground acceleration from a standing start if they come up to that speed rather than come down to that just in term of the amount of drag (RTV opposing acceleration) encountered at the beginning of the takeoff run where it will have the maximum effect.

Just that alone would account for the difference. I also think that faster initial acceleration increases the efficiency of the prop faster (ie accelerating thrust force even though power remains the same) and therefore the prop reaches its maximum thrust quicker because the airspeed is increasing quicker.
 
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phantom

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My experience is different , I find that a high power prerotator makes a big difference to takeoff distance, if you look at my youtube video, high powered prerotator takeoff in dominator gyro you will see that with enough power to the rotor to lift the nose and start the takeoff run with the prerotator engaged makes for a very short run, the wind was only about five mph so with no wind I would use about twenty feet more runway, this is on skis so there are no brakes which is why at a low prop rpm the rotor will drag me backward, the first part of every takeoff is a balancing act between prop thrust, rotor thrust and rotor torque.
Norm.
 

Jean Claude

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Jordan

Here, based on gyroplane 560 lbs, Rotor 23' x 7.5", pitched 3°, Rotax 65 hp à 2500 rpm, propeller two blades, dia 65" pitch 43"

Curves of propeller thrust (according JavaProp)


I suppose after prelaunch at 250 rpm
1) the rotor is put to A.o.A= 20°
2) the rolling drag between 0 and 15 m/s is 100 N
3) the average parasitic drag between 0 and 15 m/s is 100 N
3) The propeller thrust between 0 and 15 m/s is 1500 N (see curves)
4) Rotor thrust initial is 370 N, and 430N at 15 m/s hence average rotor drag = 400 N
In this case the acceleration is 3.5m/s2 and 15 m/s is reached within 4.3 s and 32 m


Now suppose
1) the rotor is horizontal instead 20°.
2) the drag is only 200N , because no rotor drag.
In this case the acceleration is 5.1m/s2 and 15 m/s is reached within 2.95 s and 22 m
Aerodynamic torque = 113 mN
Loss rrpm if 85 kgm2: 1.33 rd/s2 or 30 rpm after 5.1 s

With disk A.o.A = 20° at 220 rpm, and 15 m/s, the blade fllaping is steady: 2.6° .
 
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JAL

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Jordan

Here, based on gyroplane 560 lbs, Rotor 23' x 7.5", pitched 3°, Rotax 65 hp à 2500 rpm, propeller two blades, dia 65" pitch 43"

Curves of propeller thrust (according JavaProp)


I suppose after prelaunch at 250 rpm
1) the rotor is put to A.o.A= 20°
2) the rolling drag between 0 and 15 m/s is 100 N
3) the average parasitic drag between 0 and 15 m/s is 100 N
3) The propeller thrust between 0 and 15 m/s is 1500 N (see curves)
4) Rotor thrust initial is 370 N, and 430N at 15 m/s hence average rotor drag = 400 N
In this case the acceleration is 3.5m/s2 and 15 m/s is reached within 4.3 s and 32 m


Now suppose
1) the rotor is horizontal instead 20°.
2) the drag is only 200N , because no rotor drag.
In this case the acceleration is 5.1m/s2 and 15 m/s is reached within 2.95 s and 22 m
Aerodynamic torque = 113 mN
Loss rrpm if 85 kgm2: 1.33 rd/s2 or 30 rpm after 5.1 s

With disk A.o.A = 20° at 220 rpm, and 15 m/s, the blade flapping is steady: 2.6° .
Thanks for the calculations Jean but that is pretty much says what I said accept that I guessed the acceleration is double if the rotor disk is flat compared to fully back. It seems by your numbers that it is more like one third faster.

Faster the acceleration the shorter the ground run to get to the required liftoff airspeed.

I did think if your prop is pitched for maximum efficiency at cruise then the thrust to would increase incrementally until it reached cruise airspeed then decrease after that. If I am reading your graph right the prop thrust peaks much earlier (lower airspeed) than what I thought, maybe because it is at max revs instead of cruise revs. But still anything that can maximise taking advantage of the extra thrust produced by the prop because of induced airflow at the start of a roll would make a bigger difference a the end. So having the disk flat reducing drag would take advantage of that "natural" free energy to accelerate the gyro even more.
 
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Jean Claude

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Faster the acceleration the shorter the ground run to get to the required lift off airspeed.
Yes Jordan, but since the rotor rpm is not reached, the distance still continues to be eaten waiting the enough rrpm, despite the good forward speed.
The procedure "launch the rotor and then full throttle" is easy but does not give shortest rolling.
The shortest, except VRS thesis, is "launch the rotor, aft stick, and then full throttle until mu = 0.2 and then regulate the power for keep this mu "

Anyhow, a higher launch should give a shorter distance. Yet, reality not shows that.

I did think if your prop is pitched for maximum efficiency at cruise then the thrust to would increase incrementally until it reached cruise airspeed then decrease after that.
Thrust always decreases as the forward speed increases. High pitch pushes better as the other when the forward speed is high.

 

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so am I right in my understanding ... PLEASE CORRECT ME IF I AM WRONG

to get the very shortest take off
1. pre spin to a high RRPM with a flat disk (stick forward)
2. release brakes and accelerate, with prespin engaged (with disk flat)
acceleration will be faster than normal due to less rotor drag
3. At a certain point at wide open throttle, release the pre spin, ease the stick back and leap into the air,
4. Then hold it down to gain air speed

if that is the shotest distance take off method, it sounds like something for the "advanced course" I have no need for it but it could end in a very bad way with flapping and a real behind the curve take off, ok if you really know what you are doing , but I feel a definate hesitation before giving it a try, I dont want high speed flap

Is my understanding right ?

Peter
 

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Good post Peter.

I know I will not be trying this until I have many more hours.
I've not been taught this method and for you other low time pilots do not try this without an instructor.
If Vance has experience with this method I will ask him to let me try when I see him at the end of this month or at least before the 10th and his fly-in!
 
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