Rotor acceloration

[WHY]

Since I don't have a lot of hours with the gyro, I would like to ask a question to see if my observation is correct.

It seems to me like rotor rpm on takeoff is a lot like "getting up on the step" in hydroplane boating, that is to say it take quite a bit of power to get up to the "step" on the boat hull but once "on the step" speed comes on QUICK !

About what rotor speed would you say is equal to the "step" mentioned above, 200 rpm ?????

Finally, what would you say a 50 rpm increase in "below the step" speed would do to shorten the takeoff run, anything worth while ???

Tony

[Penguin]

Tony:
You make an interesting analogy to a planing hull.
For me, I prerotate to about 200rrpm which falls to about 160rrpm as I release the prerotator and rollout. (I avoid stressing the prerotator needlessly)
At about 225rrpm, the nose gear comes up. Sort of like a planing hull starts pointing to the sky.
Stick forward brings the nose down just like trimming the outboard brings the bow back down. You don't really want to be pointing to the sky when you're trying to get up on a plane.
Around 275rrpm, Punkin flies. Just like in the boat, even though you are planing, you trim down a little lower to gain speed.
Once up to flying rrpm - about 360 for me - then you trim for best cruise just like in a boat.
Odd I've not thought of this analogy before. It's a lot like planing a boat.

An additional 50rrpm would be a big boost, of course. But at what cost? Lamely reverting to the boat analogy, I can prop a planing hull to be on a plane in little more than its length but the top end will suck as will fuel economy On a gyro, additional equipment or maintenance are big factors.

[WHY]

Larry

Thanks for your reply, your comparison and experience with the hydroplane was what I was looking for. SOoooo--- for your 23 ft Dragon wings "the step" occurs at about 225, by evidence of the nose wheel coming off the ground. I think this will be a excellent refference point for "the step RPM" on any set or rotorblades regardless of brand or length or airfoil, when the nose comes up you are at "STEP RPM" . I now think that once a set of blades reaches "STEP RPM" the distance from "STEP RPM" to lift off will remain constant under zero wind conditions. Therefore the place that needs to be improved is in the area before the "STEP RPM" is reached.

I can remember when the first pre-rotor goal was to get a pre-rotor that would get "above flap RPM" which apparently is around 100 to 125 (prefer 125). Since I am looking at the "electric" end of the pre-rotor, there should be no problem with the engine or prop combo. Thanks again for the conformation on the "step" analogy.

Tony

[Alan_Cheatham]

Tony,

Have a read of this thread, especially the last couple of pages:

http://www.rotaryforum.com/forum/showthread.php?t=30495


Also, our experience on 23ft Dragonwings is rotor rpm over 200 allows full power from the main engine without much risk of flapping the blades, so 200 plus is going to give you the shortest take off distance.

On a tricycle landing gear, as most gyros are, the "STEP RPM" as you call it is also determined by landing gear geometry, the more the main gear is behind the center of gravity the greater will be the amount of rotor lift needed to raise the nose wheel off the ground.
.

[WHY]

Hi Alan

Thanks for your reply, and yes that thread link sure does relate to my question. It does look like the "200 RPM" mark is a good minimum rpm to shoot for on pre-rotate to get the most efficient short takeoff. If one can get to this rpm on pre-rotation (or close) then full throttle can be easily applied to get to flight speed, which will probably vary with different brands of blades. Of course us poor tractor guys can't use the "nose pop up" as a indicator


Tony

[Alan_Cheatham]

Originally Posted by WHY

Of course us poor tractor guys can't use the "nose pop up" as a indicator

Not a nose pop up but Ron Herron says he eases the control stick slight forward when the main gear gets light and extends against the shock absorber spring.

.

[WHY]

I think if there ever gets to be enough tractors to have a good representation at the flyins, it will be an interesting topic of discussion on the difference in ground handling techniques and takeoff techniques. It will probably take some time before there are any instructors on the tractor as well.

Tony

[WHY]

I have a special "personal" interest in the electric pre-rotor since I am a tractor nut, just about any "mechanical" engagement mechanisim for a pre-rotor is going to require some kind of shaft going right up in front of you, which I would like to avoid. There is of course the hydrualic system as well.

Tony

[WHY]

8-05-12 -----chat room till 8:45 ct

Tony