Effect of prerotation on takeoff distance

XXavier

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Just to try that on Tuesday, when I have a gyro class with an ELA. How many meters are those 3,4 seconds, so I can use a ground reference?

Assuming during those initial 3,4 s a constant acceleration of 2-3 m/s2 (an interval that seems plausible to me) that's 12-18 meters more or less...

I hope my instructor won't complain...
 

Resasi

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...only when his knees start getting hammered by the stick.
 

Vance

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I feel that is bad advice Jean Claude.

I feel that is bad advice Jean Claude.

This is precisely what I said, Vance: Shorten the distance by more acceleration in the beginning, and monitor μ to avoid too much airspeed and not enough rotor rpm.
I said that my remarks are theoretical. A difficulty is introduced due to the landing gear of the Bensen clones, which tilts the gyro inadvertently during the run. Also, the fear threshold is not entered in my simulation. The practice would require appropriate instrumentation.
However the principle is not new. This was the procedure recommended for Cierva C30, as you can read in this A.R.C report 1859


That is precisely what I was advising against Jean Claude; too much forward speed without enough rotor rpm.

My advice is to follow the takeoff procedure in the Pilot's Operating Handbook for the gyroplane you are flying if there is one.

I don’t fly in theoretical air and in my experience the air I fly in is not as predictable as your theory.

I feel that risking flapping the blades for the potential benefit of shortening the takeoff roll slightly is poor technique.

The takeoff in a gyroplane is complicated enough without adding additional steps and pitfalls.

I feel those who are learning to fly a gyroplane should stick to the POH for that gyroplane.

If you are flying a Cierva C30 then follow that procedure.

Enough said.
 

Jean Claude

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All your points are good, Vance:
I do not encourage beginners to experiment with procedures that are unsuited to their aircraft.
I just encourage manufacturers and instructors to understand the procedure defined by true aerodynamicists like La Cierva.
 
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Vance

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All your points are good, Vance:
I do not encourage beginners to experiment with procedures that are unsuited to their aircraft.
I just encourage manufacturers and instructors to understand the procedure defined by true aerodynamicists like La Cierva.

Thank you Jean Claude, I was concerned your message of caution might be lost on inexperienced gyroplane pilots.
 

XXavier

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(...)
(...)

what μ = 0.15 means ?

µ is the ratio forward airspeed/tip speed. For example, with a 8,5 m rotor turning at 250 rpm (26 rad/s), and moving at at an airspeed of 60 km/h (17 m/s), µ = 17/4,25 · 26 = 0,15
 

XXavier

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Thanks Xav

some more foolish questions from an un-educated man

1/ if I understand well 4.25x.26 is the tip speed , sorry I am bad at maths can you explain the 4.25 in the formula ?

2/ if I understand well this ratio is used to calculate the max air speed beyond which the rotor will flap when the pilot will pull the stick and tilt back it's rotor at 20° , does it means that we can calculates this max speed like this : max forward air speed= (4.25 x 26) x 0.15 m/s ?


1) the radius... Half of 8,5 m...

2) Well, I wrote down the example just to match the µ = 0,15 mentioned my JC... You can get the same µ = 0.15 juggling the variables conveniently...

Anyway, the maximum flapping amplitude is a function of µ. The formula is : Captura de pantalla 2020-09-02 a las 10.47.13.png

The angle is in radians, of course...
 
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XXavier

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the radius ... ha ok (((-: ... every time I see a formula I must take a pain killer and and a antidepressant tablet in front of my depth less ignorance...

as for the example thank you very much I know that it is not something to apply with blind eyes of course

I really don't know if I will someday try out this method, in any case I do someday I won't do it not before a few cents of flight hours experience

I was reading all I could about rrpm at take off because I am going to test my electric pre-rotator, and because I have choices to make as for my max backward rotor angle .. so I am trying to understand things better then I am actually do


JC has written a lot on the takeoff distance and RRPMs... A search in the forum can be very rewarding...
 

Greg Vos

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Thinking a bit about a electric prerotator that’s going to offer a good solution.
A few years ago I dabbled with a large 3phase model aircraft motor (the 3 phase is obtained from the DC controller that converts DC to the required AC and hence a soft start speed controller as seen in quarter scale Rc aircraft)

With little enthusiasm from our local gyro community in SA the project stalled as all mods to an aircraft need to be approved by our local CAA, and this is a big stumbling block for us.
Now is it not a viable option? ( for a clever person) to build a electric prerotator much like an out runner RC motor ? This can be lightweight and actually form part of the rotor head assembly ( this is where I was going with it a few years back) unfortunately my skills are limited ..this electric Brushless motor out runner to be part of the main bearing shaft? Now to reduce weight it can then double up at the alternator because as soon as take of is archived the rotors will now be driving the out runner and it in turn will generate power that goes back via a rectifier to charge the gyro battery and it’s electrical systems?
So no need for an alternator on the gyro ?
then the other thorn I have with gyros is the rotor brake, this electric motor system if designed by a clever engineer could use its emf ( now back emf) to slow the blades like an electric brake as well?

just some thoughts ...but we do live in a world where cement can cure underwater and in our case prisoners who have been jailed for murder become presidents or decorated politicians so anything is possible. 😉
 

wolfy

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It has been done already exactly like you say, by that German gyro manufacturer that I can't remember the name of. :) Rotortech maybe?
They promoted it on one of there single seat models, but all models after that went back to conventional pre spinners (unless it was still an option?).

wolfy
 
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Resasi

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That looks very interesting Jm. Look like you’ve done a lot of work on that.
 

Greg Vos

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Sir your a brave man....when you been married as long as I have we prefer to touch some other mans wife....trust me on this one I’m CFI 😁
 

Smack

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this electric Brushless motor out runner to be part of the main bearing shaft? Now to reduce weight it can then double up at the alternator because as soon as take of is archived the rotors will now be driving the out runner and
Yes, Greg, yes. We are thinking alike. Unfortunately, time and resources prevent this experiment in my hangar... for now.
"No" on the alternator idea. Instead keep that outrunner motor electrically POWERED to improve efficiency.
Brian
 

wolfy

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Hi Greg this was the machine that used to use the outrunner motor directly on the rotor head, Unfortunately I can no longer find the video.

wolfy
 

Jean Claude

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Hi JC, please can you tell me how can the air go top down (du haut vers le bas) when we accelerate stick forward (horizontal rotor ) ?
When the gyroplane is still at low forward speed, the launched rotor blows downwards, like a helicopter.
If the control stick is backwards, then it creates a strong drag which is detrimental to forward acceleration. In addition, the rotor rpm decreases rapidly, due to the high angle of attack of the blades linked to the forward rpm and speed.
On the contrary, if the control stick is forward, the rotor thrust does not oppose the forward acceleration, and in addition the low angle of attack of the blades causes it to lose less rpm.
This "anti-rotation" phase is then less consuming of takeoff distance.
 

XXavier

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This is a very interesting subject... If I get it correctly, for every gyro and meteo conditions, there's an optimum procedure for takeoff, involving a first (brief) acceleration phase with the stick fully forward, then a transition phase with the stick being gradually (but not too slowly...) brought from full forward to full backward position, and a last phase of acceleration with the stick fully backward...

It's more or less what I do with my Magni M24, if it's also true that I do it (the first phase) only in order to keep the gyro aligned with the runway till the speed is enough for the rudder to have some authority...
 

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This was an excellent learning thread. If you read most of it you will learn that for an ArrowCopter the shortest takeoff roll was when you can pre-rotate to 220 RRPMs. More than that and the drag just drops the RRPM and is wasted. Less and the takeoff distance is longer.
 

Jean Claude

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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.
 
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Jean Claude

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Anyway, the maximum flapping amplitude is a function of µ. The formula is : View attachment 1148202
The angle is in radians, of course...
Caution, this formula is only when the rpm is steady.
It is false during when an autorotation rpm is accelerating.
 
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