Effect of prerotation on takeoff distance

[XXavier]

Caution, this formula is only when the rpm is steady.
It is false during when an autorotation rpm is accelerating.

Of course, I assumed a constant speed, and hence a constant RRPM. Anyway, and with the exception of takeoffs, landings and slight changes in the rotor load during flight, the RRPM are quite constant...

 

[XXavier]

220 RRPM at which rotor pitch angle please ?

I think they used the Averso Stella blades & hub bar...

 

[Greg Vos]

These last few posts seem totally contrary to my current training and limited experience. I prerotate to about 170 RRPM then move the stick back and accelerate. As the nose comes up I move the stick forward and gain speed as liftoff occurs, I apply full power and get airspeed up to about 65mph then climb out.

You sir have been trained correctly ! The others, well possibly they train from the comfort of a pc keyboard and possibly by instructors who themselves are not profiecent With the technique you quote.


I will with you 👍🍺

 

[Greg Vos]

Well there is rubbished above ....a rotor when level or some drivel about it blows like a helicopter ....realy🤣.
what is described is the Best take off technique I’m not sure what you mean the standard ?

 

[XXavier]

yes but My question had nothing to see witht the sort of rotor
I was not thinking of the blades pitch, but simply about the backward tilting angle of the rotor rotation plan with regards to the horizontal Xavier

Yes, of course... My mistake...

Searching the internet, I haven't been able to find that for the Arrowcopter.

In case it is of any interest, I've found in the British CAA web page, that the rotors of the certified gyros (Arrowcopter not included) have an angular amplitude in pitch of 19º For the MTOs, the Magnis M-16 and M-24, and of 24º for the Calidus & Cavalon.

In 'Flugphysik der Tragschrauber', the authors give an angular amplitude of 20º for the MTOSport that they analyze; 15º for the angle with the stick fully back, and –5º for the stick fully forward...

 

[XXavier]

Well there is rubbished above ....a rotor when level or some drivel about it blows like a helicopter ....realy🤣.
what is described is the Best take off technique I’m not sure what you mean the standard ?

It's right, I think. When you pre-rotate the rotor, bringing it to –say– 220 rpm, and then disconnect the drive, the disk is working like that of a helicopter, blowing air downwards. Hence, if you tilt it back, it will push the giro backwards, with a force analog to drag...

After a few meters of the takeoff run, the circulation 'normalizes' itself, the rotor starting to work as that of a gyro, and increasing its rpms in autorotation...

 

[JETLAG03]

@jm-urbani with my new electric pre-rotator which can rotate to 180rrpm static with flat blades, I now pre-rotate to 140rrpm static, release the brakes and increase power maintaining the pre-rotator until I reach 200rrpm, after which the blades accelerate very fast on their own.

I am still learning my machine, now 10 hours flying, but this pre-rotation system seems to work very well

phil

 

[XXavier]

merci jean claude
what if I keep the electric pre-spinner actif for 5 seconds ( for example) after brake realease during acceleration ?

May I dare to answer...?

I think that will be good. You'll lose less revs in the first stage of the run, and when the circulation 'normalizes', and the rotor starts working in autorotation, the starting point in rotor revs will be higher, so you'll reach takeoff revs in less time and space...

 

[All_In]

220 RRPM at which rotor pitch angle please ?

IIRC they were Averso Stella blades I do not know the pitch angle Abid probably does.

 

[All_In]

Also if IIRC at 220 RRPM he could apply full power without any danger of flapping the blades on his make and model. That was one of the big factors in taking off in the shortest distance.

 

[Greg Vos]

It's right, I think. When you pre-rotate the rotor, bringing it to –say– 220 rpm, and then disconnect the drive, the disk is working like that of a helicopter, blowing air downwards. Hence, if you tilt it back, it will push the giro backwards, with a force analog to drag...

After a few meters of the takeoff run, the circulation 'normalizes' itself, the rotor starting to work as that of a gyro, and increasing its rpms in autorotation...

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?

 

[All_In]

To me, autorotation is much more like a kid's pinwheel or parachute. It flies more like a kite because of the drag. That is why most FW's have a 10 to 1 glide ratio and we have only 4 to 1. We are dragging the pinwheel through the air.

 

[Greg Vos]

It seems to me many of you wish to overcomplicate a take of in a gyro? It really is a very simple excersise and one can decide on the runway condition, length and prevailing wind to determine what will be the optimum pre rotation speed, it goes with out saying that on a windy day with wind directly on the nose it will be fine to release the pre rotator as low as 120 Rrpm, as we know it will speed up fast and why labor or wear the pre rotator?

just be mindful not to rotate ( lift the nose) until you have an indicated Rrpm of 200 or more 😉

 

[Greg Vos]

in fact the thread is very practical , the guy who started the thread is asking why his take off distance does not significantly reduced when he pre- rotated over 200 rpm ? ( 210, 220, 260 etcetc)

then we were explained that it is because rotor fully tilted backward at 200 rpm the rotor can only loose RPM first before starting accelerating

I am then I was wondering what if I accelerate before my electric pre spinner switches off ( 5 seconds before for example but I can program 2 seconds or 20 ..)

a little help to the rotor during this rpm loss does not seems crazy ?

I am not complicating the process at all it is just a question I am asking me and that I won't test before log ( I need the test the bird itself first)

Well let’s add the thing some seem to overlook....?. a gyro ( unlike a helicopter) needs airspeed to fly.... you can pre rotate till Taylor Swift comes over to fetch me for a night of adult fun 😁 a gyro will need airspeed, as well as rotor speed above 200 Rrpm, The distance to take off over a 50ft object has been tested and high pre rotate speed has been proven to be of little value ....

where is Wasp, Vance ? Am I missing something?

 

[XXavier]

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?

I’ll reply later... Im now At the airfield, trying to test Urbanís nethod, bit thetas a hefty crosswind...

 

[Greg Vos]

I’ll reply later... Im now At the airfield, trying to test Urbanís nethod, bit thetas a hefty crosswind...

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

 

[Jean Claude]

What if I keep the electric pre-spinner actif for 5 seconds ( for example) after brake realease during acceleration ?

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 .

 

[Smack]

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

 

[Greg Vos]

Greg can U imagine that any gyro pilot (including me) can ignore that a gyro can't take off without airspeed ?

I had issues with a French instructor ( low hours he had 700 total time) since then I never underestimate the silly things the French can do....no offense but he did take a brand new gyro with a paying passenger through a fence a few days after I tried to explain some basic flight dynamics to him...his French arrogance omitted to take cognizance of my acvice? he landed downwind ...in case your wondering 😳😳 with a 3 mile runway 180 degrees behind him in clear Wx

He also fried to convince me that turning a torque wrench a few degrees past the “Click” was normal and I was too anal....so best I don’t answer your question until we meet in person and you demonstrate your flight knowledge 😁😉🍺

Steve May has his totally avoidable accident on record so add that to the stellar reputation of Air France I hope you understand 🤣

 

[Greg Vos]

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

Yes after landing is in the near 300 Rrpm range and if your fast to take off its in the high 260 - 280 work that out as a percentage over the minimum of 200