Someone in the Power Systems group inquired about how to make a nitro driven quad. Some members said that the response time of nitro engines would be too sluggish to maintain stability.
I don't know if servos would be too sluggish as well compared to ESC's, but I came up with an idea of how to use servos to stabilize a nitro quad: By moving baffles in and out of the prop wash of the various props. Referring to the following video, ailerons, for example, would be controlled by a baffle connected to a servo that would decrease the efficiency of one prop while increasing the efficiency of the other. Pitch would be controlled in a similar manner. Yaw would be treated the same as with electric quads by rotating the engine around the flight axis.
I wonder what you folks think about this arrangement.

http://www.youtube.com/watch?v=JY5JgMlWr8U

Here's the link to the Power Systems thread
http://www.rcgroups.com/forums/showthread.php?t=1840171

Or you could have four belt-driven variable pitch rotors, and gain inverted flight capability in the bargain.

Originally Posted by Jkflorida (in the Power Systems forum):
"I wanted to know this too. I wanna build a nitro powered quad. "
My comment (same as Brandano):
"One way to make that work would be servo-controlled variable pitch props. Throttle control would have much too slow response. "

You could also have the gas motor drive one single large lift rotor and a few additional helicopter tail rotors for control. that way you would have immediate response from the blade pitch adjustment, while running a more efficient main lift source, and reducing the stresses on the more complex rotor heads.

Who needs gyros or complicated heli parts?

http://svsm.org/gallery/FlyingPlatform

Hiller Flying Platform (3 min 17 sec)

Kevin

Brandano, you're right, but what do you think of the baffle idea?

kcaldwell, I think that the hover craft levelled itself by ground effect; the side that got closer to the ground generated more lift. That's why it could only fly a few feet from the ground.
Also, the fan itself acted like a gyro. That would be tricky because when the pilot leaned forward trying to bring the front down, the gyro effect would tilt the vehicle sideways as well.

In the following video at 2:01 you can see this vehicle being controlled by controlling the airflow from the fan (or fans).

Nope, it easily went higher than ground effect. They just tended to keep it close to the ground since the engine failure procedure was pretty terminal from altitude. There are two control vanes above the rotors. The counter-rotating props cancelled any gyro effect.

The later one I posted a photo of had full controls, with the pilot strapped in a chair. You can see the four control vanes underneath. It also had collective with constant speed rotors, more like a helicopter.

There were a whole series of similar things commissioned by the US Army and Navy at that time. Some were more successful than others.

Kevin

I think that partializing the output from the rotor is a good way to waste energy, actually. Also, just propping a baffle in the prop wake will create all kinds of hard to predict turbolence. I'd use something like a drag rudder in the prop wake instead, or a variable geometry duct. Still, apart from the all important "to see if it can work", I can't see a reason to go this route. If I ever were to scale something like this to full size, I'd go with variable pitch rotors, to keep at least a chance of autorotating to a landing.

The virtue of the baffle idea is that it's very simple and inexpensive.

This maybe offers a design that simplifies the co-axial propeller drive - one engine above the propellers, and one below?

It looks like they are using vanes in the airflow for control, and I guess varying RPM for height.

http://www.gizmag.com/airburr-crashi...481/pictures#3

Kevin

Very interesting. They don't need an antirotation prop like helis do.

That is the configuration that Heinkel meant to use in their Lerche VTOL fighter (http://en.wikipedia.org/wiki/Heinkel_Lerche). The earlier turboprop version (the Wespe) was meant to have a single rotor, though, I imagine with stator vanes to counteract the torque.