Jump Takeoff w. Ground Roll

[ltcolroger]

Hi all,

I'm a first time poster but have been browsing this site for a little bit. Love it. Anyways, to the meat of it:

I'm expanding the equations John Wheatley developed for an autogyro jump takeoff in NACA TN-582 to include the case where the vehicle does a ground roll and builds up forward speed before it jumps.

Before I continue any further with my code, I'd like to do a sanity check. I'd like to know how a ground roll and then jump takeoff compares in performance to purely a jump takeoff. Would we expect it to attain a greater maximum height off the ground by the time the rotor blades slow down to flight RPM? Or what would be different?

I'll be the first to say I'm not well versed in autogyro or rotorcraft theory, but I am studying aerospace engineering, so I do understand the fundamental language.

Thanks!

[WaspAir]

In practice in the A&S 18A jump gyroplane, a ground roll will cost you some rotor rpm because you can't roll with the rotor drive engaged. As a result, the rotor rpm decays while you're accelerating down the runway with the blades in flat pitch. That means that the jump itself will be less energetic, because you have less energy to extract from the blades when you put in collective pitch at the time of the jump. However, the extra airspeed you get from the roll will make for a better initial climb.

When all is said and done, you get a different climb profile in the two cases.
No roll - jump high from full rpm, but need to accelerate thereafter before establishing a good climb rate.
Roll first - lower jump from lesser rpm, but you have some climb speed already established when you lift off.

P.S. The reason you can't roll with the rotor drive engaged is that there is no torque compensation. Sitting still, the tires will resist any rotation of the airframe. But if you're rolling forward, and spinning a 35' diameter rotor with 150 pounds of blades at 370 rpm, the torque of the spin-up system will pull you into a quick turn right off the runway (not a good thing!). You must de-clutch the rotor drive before releasing the brakes, roll to your desired forward airspeed, and then pop in the collective pitch to hop off the ground. You can easily lose 10% or more of your rpm while rolling, and that's a big step in stored energy.

[groundhog]

Soooo ... dick degraw"s partially powered or the lightly powered (benson ?) that stay engaged need to be declutched for take off roll ?I had not realized this.

[WaspAir]

I don't know the details of his systems. A pre-rotator strong enough to get, say, 150% of flight rpm with blades weighted to store jump energy (as on the 18A) is unmanageable if you try to roll with it engaged because the torque is just too much. On the 18A, you hit the clutch release before releasing the brakes, or you'll find yourself headed for a ditch. Besides, if you want rapid airframe acceleration to get translational airspeed, you want the engine power going into the prop, not into the rotor. With the clutch fully engaged on the 18A, there's a direct relationship between engine rpm and rotor rpm, and there shouldn't be any slippage.

If you had light blades and a much weaker pre-rotator the torque might be low enough that you can run with it and still steer effectively, but that's not a description of a jump system. A jump system has to drive way past normal flight rpm with substantial mass in the blades to store the energy that you extract when you jump.

[groundhog]

I should have caught on at the word "Torque". Dick Degraws system is set up like the rear end of a vehicle where any resistance dumps the power (Torque)to the other disk (prop).There is also a preferance ratio to the prop. Way more to it but I don't have chuck's or your ability in explaining concepts.
the lightly powered unit was only a few hp so yes not enough to overpower the rudder.

[ltcolroger]

Thank you both for your input, I appreciate it! Seems there's a reason designers like fairey, cartercopter, and groen mostly do jump takeoffs from a standstill.

[Doug Riley]

It's perfectly possible to steer a gyro having the usual fractional-RPM prerotator (hydraulic, flex shaft, and so on) thorugh takeoff with the prespin engaged. It requires some nosewheel and rudder technique, but the reward is a much shorter takeoff. The prop unloads as you pick up airspeed in your roll, leaving more power for the prespinner to build RRPM.

[kolibri282]

I take it you already have come across this one:
http://www.rotaryforum.com/forum/sho...1&postcount=21
Hope it helps!