Pre-Rotator RPM Questions

[cburg]

Originally Posted by cburg

Here's an Atlas 240 sq/ft clip. Note the 320 sq/ft performs even better.

Oops:

STOL Trike 240 sq/ft wing - YouTube

[birdy]

I was eluding to the fact that you don't need a tail rotor nor zero collective.
If you want to achive flight rrpm [ 1Gs wprth of lift] then yes, you do need a TR or collective, or youll just rotate the machine wen traction is lost.

We studied this a bit when we were testing the machine with floats. When you prerotate the frame would spin a bit so we added a large enough water rudder to counter the effect....and it works
But you couldnt TO virticaly.

The ferels prespinner can get to bout 280rrpm [ no rotor tac].
Any more n that and it starts to dance.
But, this setup means i can leave the spinner engauged for as long as i need to clear wotever im flyn over.

[Master Roda]

Originally Posted by birdy

If you want to achive flight rrpm [ 1Gs wprth of lift] then yes, you do need a TR or collective, or youll just rotate the machine wen traction is lost.

Nope. Keep trying. Jim has lifted off with the prerotator engaged. With enough tail you dont need TR or collective. As long as no more than a certain amount of power is transfered.

Originally Posted by birdy

But you couldnt TO virticaly.

Nope...no gyro can without jump capability...even then it isn't really Vertical is it?

Originally Posted by birdy

The ferels prespinner can get to bout 280rrpm [ no rotor tac].
Any more n that and it starts to dance.

I'm sorry it dances??

[birdy]

Nope. Keep trying.
Keep tryn wut??

Jim has lifted off with the prerotator engaged.
But not virticaly, which is wot would happen if you were at flight rrpm [ your initial claim].



As long as no more than a certain amount of power is transfered.
Duh.
And that 'certain amount of power' isnt enuff to lift virticaly is it.

IOW, he's only do'n wot the ferels been do'n for years.

Nope...no gyro can without jump capability...even then it isn't really Vertical is it?
I never said JUMP.
Im questioning your claim that he can get FLIGHT RRPM with the spinner, while not moveing.
Soons you add horisontal airflow, all bets are off.
BTW, wot do you call 'virtical TO' if wot Degraw dose isnt virtical take off??

If Jim is breaking ground, with no roll, then youv just created the first tailrotorless single rotor heli.
Congradulations.

Simple fact is, if you have flight rrpm, your flyn,and can TO virticaly.
But you dont, so you cant.

[Master Roda]

Originally Posted by birdy

Im questioning your claim that he can get FLIGHT RRPM with the spinner, while not moveing.


Simple fact is, if you have flight rrpm, your flyn,and can TO virticaly.
But you dont, so you cant.

Originally Posted by Master Roda

That still torques the airframe does it not?

Our M912 and the SCII both get close to flight RRPM (280-300max), but still need a short run to lift off.

This is my initial claim Birdy.
I can't translate Australian, but I think we're saying the same thing.

ALL rotorcraft need forward movement, heli's included. Hovering is the exception.

Watch the first 20 seconds of this clip Birdy.....this is what I meant. I didn't claim we could take off vertically, just very short and leaving the rotor engaged without spinning in a circle There's a good headwind of about 8MPH and he prerotates over 280. The rotors are 8x27'

Sport Copter M912 STOL - YouTube

[ckurz7000]

NOTE: Although I just realized that my post doesn't exactly fit with the current topic I'll leave it here unless the moderator wants to move it...


I just wanted to present two traces of the rotor rpm during prerotation and take off roll. The discussion applies to a prerotator which is disengaged at commencement of the take-off roll.



The two take-offs are quite different in that the blue line prerotates to 255 rrpm whereas the red line goes only to 232 rrpm. After disengaging the prerotator I applied full throttle with the stick fully back in both cases. I did examine other 4 other cases which pretty much fall inbetween those two. Unfortunately, conditions were probably quite different among the set so that it becomes difficult to draw any grand conclusions.

The drop in rrrpm during the take-off roll between the two presented cases is quite different. The blue case shows a drop of 30 rrpm while the red case with lower prespin drops only 8 rrpm. Also interesting to note is that they both drop to the same rrpm of 232 rrpm (but that turned out to be very likely accidental as suggested by other traces I looked at).

What you can see is that there are two time scales involved here: (1) is the rate at which the rotor loses energy after disengaging the prerotator, and (2) is the rate at which the gyro gains speed and thereby transfers energy to the rotor. My data suggest that it takes 4-8 seconds, after commencing the take-off roll, until the rotor has come back up to the initial prerotation speed, and the average drop in rotor rpm is 15 rrpm.

There seems to be a point of diminishing return beyond which any increase in prerotation speed will not shorten the take-off run appreciably anymore. The reasons are these two:

(1) The rate of energy loss from the rotor is higher the faster it rotates.

(2) The acceleration of the gyro is slower with a faster spinning rotor.

So the best possible strategy for a short take-off run seems to be to prerotate above a certain threshold (about 230-240 rrpm in the ArrowCopter), then disengage the prerotator and flatten the rotor disk (i.e., push the stick forward). This makes for a much higher acceleration of the gyro, since rotor drag is greatly reduced. When the gyro reaches flying speed (about 80 km/h in the ArrowCopter) you gently pull back on the stick and lift-off.

This procedure is actually an effective short field take off technique but comes with some risk attached to it because of the danger of blade flapping if not performed correctly.

-- Chris.

[gyronutjoe]

I have a question. Can the prerotator be kept engaged untill the main wheels leave the ground as to me this means the rotor is at flying speed? Wouldn't that create a short almost vertical takeoff?

[Master Roda]

Thats what I was saying. The rotors will usually spit the bendix out once they exceed the prerotator max speed.....whatever it may be geared for. It doesnt care if it's on the ground or airborne.
If your prerotator is only geared for 250, it doesnt make sense to leave it engaged if your flight speed is 325.

[MichaelBurton]

Originally Posted by CLS447

I was looking for some videos of Trikes doing short takeoffs, I didn't find many good ones. The guy I used to hangar with, had a Toucan with a 503...he could just about do a jump takeoff. With 2 onboard it was almost as impressive !

Gyos just don't takeoff as fast. But look at this plane ....

Short field Take Off, Turbo Zenith 801 - YouTube

Hey that's my buddy next door. He just sold that aircraft. He could get off in a shorted distance than I could but the gyro landing distance was shorter. We had a contest and I touched down and rolled back, he conceded.

[birdy]

Please accept my appoligy M R , i have confused your post with Doug Barkers.
Yes, WE are sayn the same thing.
I thought it was you who said it could prespin to flight rrpm.

Can the prerotator be kept engaged untill the main wheels leave the ground as to me this means the rotor is at flying speed?
Yes, if your prespinner is setup for it.
Electric ones are usualy out run by the rotor by the time they reach lift off speed, and alot of machanical typs aint designed to be engauged after the roll starts.
But if you have a moderatly powerfull mechanical prespinner that is still driveing after lift off rrpms are reached [ correct gearing] then this setup will get you the shortest TO roll.
Iv held mine on till iv cleared 60' trees more than once, and its almost like taken off ina heli, streight up.

[Master Roda]

Originally Posted by birdy

Please accept my appoligy M R , i have confused your post with Doug Barkers.
Yes, WE are sayn the same thing.
I thought it was you who said it could prespin to flight rrpm.

No worries mate. had me for a minute though