Effect of prerotation on takeoff distance

[Jean Claude]

This is an interesting thread on the optimal prerotation RRPM for shortest departure. the below videos show take-off rolls <20 feet.

With such a wind, the pre-launcher rpm doesn't matter: 50 rpm would do just as well. Just to put the stick back slowly enough to avoid the impact of the blades on the flapping stops.

 

[Greg Vos]

With such a wind, the pre-launcher rpm doesn't matter: 50 rpm would do just as well.

View attachment 1148294

very good observation 👍

 

[Resasi]

It was an great demo of STOL, but the wind in the mic sounded just a tad more that 8mph and JC’s eagle eye spot of the windsock is a clincher. The Sport Copter may not be a beauty but it cerainly is a rugged performer.

The second vid however was an eye-opener. Had heard about but never seen these done by a gyro. Now I have. I was very impressed.

I like doing aeros... in the appropriate equipment. Somehow I find some reassurance in the fact that if I do mess up I will live.

Mess up doing a loop in a gyro and you had better be wearing a parachute... or have a ballistic one, and right now, I haven’t heard of any gyro that has one fitted.

 

[wolfy]

My last gyro had a better power to weight and larger rotors than a M912 but used an M912 pre rotator, and mine wouldn't have got off that quick in 8mph, but I reckon probably 10-12 knots to get off that quick. In light breezes though a 300rrpm absolutely gives a shorter take off than 200rrpm.

That first take off was into about a 8 knot breeze, with close to 300rrpm.
Wish I had more video's of that machine.

wolfy

 

[XXavier]

No air sucked from above the disc, but definitely air pushed down by the rotor like any other wing would do in level flight (hoping that we are actually in agreement at last)?

Yes, air sucked from above the disk... All that air pushed down by the rotor has to come from somewhere, and it obviously comes from above the disk... That flow of air from above will, btw, slightly reduces the AoA of the blades...

Anyway, it's clear that there's a downdraft, and a strong one. It has even been proved by experiment, as mentioned above in several posts of this thread...

 

[Jean Claude]

In light breezes though a 300rrpm absolutely gives a shorter take off than 200rrpm.

Yes, only slightly shorter while the power required by the pre-launcher is three to four times as strong.

 

[wolfy]

I think to gain the benefit of a 300rrpm pre spin a high power to weight machine is needed, anything less than a light and powerful gyro will waist to much time gaining airspeed to further accelerate the rotor. To state the obvious.

wolfy

 

[XXavier]

I find the powered rotor at takeoff a very welcome improvement. Shortening the run is always desirable, and the mechanical complication is not great... The takeoff may require more attention, but doesn't seem too difficult, either...

 

[Resasi]

Wish I had more video's of that machine.

Wolfy so do we. Nice one mate.

Guess mustering is great training for tight manoeuvring close to the ground...
... especially if those cattle have that strong Aussie FU attitude.

Otherwise they simply wouldn’t move.

 

[Jean Claude]

Yes, air sucked from above the disk... All that air pushed down by the rotor has to come from somewhere, and it obviously comes from above the disk... That flow of air from above will, btw, slightly reduces the AoA of the blades...

During the autotation, the air commes from front and then deflected down by the rotor. But not enough to cross the disk from above to bottom.
Crossing from bottom to above is the prerequisite for autorotation. it increases the A.o.A of the blade.
See my sketch

 

[Resasi]

Yes, air sucked from above the disk... All that air pushed down by the rotor has to come from somewhere, and it obviously comes from above the disk... That flow of air from above will, btw, slightly reduces the AoA of the blades...

Here, having maintained a silence that befits my ignorance, I would politely disagree.

Air coming from above the disc that proceeds through the disc will reduce RRPM acting as a brake.

The rotor will slow, losing blade rigidity, and, compromises the ability of a gyro to fly.

When on the ground, without a rotor brake, and, having allowed a minus 1 degree forward movement of the rotor on our Hornet, I could by putting the stick full forward and tilting the rotor 1 degree down, while facing the wind, then decelerate the blades very rapidly.

Air comes through the rotor from below on take off with the stick back, at no time should any air be coming through the disc from above during any take off!!!

Any views to the contrary will leave me as confused as many of have been during this discussion.

 

[All_In]

Yes, air sucked from above the disk... All that air pushed down by the rotor has to come from somewhere, and it obviously comes from above the disk... That flow of air from above will, btw, slightly reduces the AoA of the blades...

Anyway, it's clear that there's a downdraft, and a strong one. It has even been proved by experiment, as mentioned above in several posts of this thread...

Xxaiver. It's not a helicopter. It's being deflected like all wings deflect the high pressure from the bottom of any wing. Just because it is spinning does not change the deflection of the relative wind any more than a sycamore seed does. Take a hose and squirt a high-pressure stream of water on the bottom of the blades and you will see exactly how it is deflected and at what angle to the ground.

 

[XXavier]

During the autotation, the air commes from front and then deflected down by the rotor. But not enough to cross the disk from above to bottom.
Crossing from bottom to above is the prerequisite for autorotation. it increases the A.o.A of the blade.
See my sketch

I was mentioning the conditions during the much-discussed pre-rotación, with no wind anda the gyro stopped on the runway...

 

[XXavier]

Pls s

Xxaiver. It's not a helicopter. It's being deflected like all wings deflect the high pressure from the bottom of any wing. Just because it is spinning does not change the deflection of the relative wind any more than a sycamore seed does. Take a hose and squirt a high-pressure stream of water on the bottom of the blades and you will see exactly how it is deflected and at what angle to the ground.

Pls see muy post to JC...

 

[XXavier]

Here, having maintained a silence that befits my ignorance, I would politely disagree.

Air coming from above the disc that proceeds through the disc will reduce RRPM acting as a brake.

The rotor will slow, losing blade rigidity, and, compromises the ability of a gyro to fly.

When on the ground, without a rotor brake, and, having allowed a minus 1 degree forward movement of the rotor on our Hornet, I could by putting the stick full forward and tilting the rotor 1 degree down, while facing the wind, then decelerate the blades very rapidly.

Air comes through the rotor from below on take off with the stick back, at no time should any air be coming through the disc from above during any take off!!!

Any views to the contrary will leave me as confused as many of have been during this discussion.

During the pre-rotation, with no wind, and gyroplane stopped, the rotor blows downwards, and the air comes to the rotor dial from above...

 

[XXavier]

Ian now sitting no mu ELA warming up, soI cannot reply no due form, bit I think its now colear that i was writing about the rotaron pre-rotation...

 

[Resasi]

During the pre-rotation, with no wind, and gyroplane stopped, the rotor blows downwards, and the air comes to the rotor dial from above...

Xavier I apologise,, with the expanded explanation above, I agree that in this case, air can conceivably be being drawn in from above the rotor disc.

The minute he pulls the stick back, releases brakes, and begins forward movement then that is no longer the case for the remainder of the flight.

 

[XXavier]

Xavier I apologise,, with the expanded explanation above, I agree that in this case, air can conceivably be being drawn in from above the rotor disc.

The minute he pulls the stick back, releases brakes, and begins forward movement then that is no longer the case for the remainder of the flight.

No need for apologies... It was a misunderstanding due to my very brief, unqualified statement...

Anyway, in this interesting thread, there is an important point that concerns the moment (or the point of the takeoff run) when the rotor switches from the 'antirotative mode' acquired in the pre-rotation to the 'autorotative mode' necessary for lift. It's probably a very complex thing, with many variables...

 

[DavePA11]

in short this all my instructors tought me not to do ...

Unless you are flying a Sportcopter M912 with the large suspension and 100HP engine. I wouldn’t recommend trying that with any two seater.

 

[All_In]

PRA next year will sponsor the senior EE students to make a mini wind tunnel and a gyro rotor system. It's too late to submit it this year.
Then we will all see the high-pressure deflection from the bottom of the wing and it's not a downdraft like a helicopter. The use of the term downdraft makes all laymen think of power rotors with a collective. Why not call it what it is?
Going to use the largest fan we can get for the size, a smoke system, and cut boxes of soda straws in the middle and then stack them in front of the fan to create streams of laminar airflow filled with smoke.