I have a scratch build rc ufo. weighs about 44 oz and is 38" diameter. the cg is well below the propeller line (by about 4-6") but when i try flying it, it will rock bakd and forth to such an extreme that it is uncontrollable. any ideas what may be causing this?

Inadequate lateral stability... what is the shape?

circular. how do i fix it? i thought by cg was low enough the center of the props are about 24" apart and the tips about 14" apart.

Clam:
bigger fin, rudder might do it.

there is no fin. it is a vtol aircraft, with the propellers up top and the cg down below.

Are they indivual electric motors for each impeller.?

yes. BTW, after some thought, i dont think fins would work anyway because my airspeed is just about zilch. i have mixed them so i control them in pairs for aileron and elevator, all together for throttle, and linked in pairs so i can speed one pair and slow down the other causing the torque imbalance to turn the model. they are counter rotating props. i do have some control but it does not wayt to stay vertical. it just rocks back and forth until it falls out of the sky (about 1-2 cycles)

You need stability augmentation, try gyro's
Bob

already got them, i forgot i had turned them all the way down, but they are mounted. (it is fun to be me) but i am still curious as to why it does that. it should act like a pendulum and senf center i should think... i would like to try and solve the problem without the aid of gyros as much as possible, but when it starts to require extreme measures i will just let the gyros pick it up.

Limeybob beat me to it.

i know, but i would like to know what is causing it.

This machine appears to have its CG close to its geometric center. To be stable in normal, translational flight, like a regular airplane, it needs to have its CG ahead of about 25% of the mean aerodynamic chord, unless you have active stabilization by gyros and very fast servos, and it also needs a vertical tail, unless it has active yaw stabilization spoilers as on the Stealth Bomber. Otherwise, just stick to hovering and very slow flight, and have fun.

I would suspect 4 unmatched motors for your source of the wobble.
A speed governor that keeps all of them at the -same- rpm is probably needed.

they are bushless. will they still be unmatched? how would i create a governer?

.
the second part is harder... first, start a career in sports....
For a governor..
The rpm of each motor would need to be sensed and compared to a desired value. The motor(s) not at that value would need a controller to get them to that value.
It's probably doable with the electronics we have available now.

that sounds like it would be a complicated circuit. i obviously have a speed controller for each motor, but i would have to put it in front of every speed control, and it would not be something as simple as a comparitor circuit. it has to pe perportional, and not compare 2 items, but compare 4 items to each other item (the motors) but what i was asking when i said how may i create a governer, i meant "how do i build it" or "do you have a schematic" or that kind of thing. i understand that each motor would have to be sensed, most likely using a tachometer type light sensor.

How High does it fly?

How fast does it go?

couple mph at most, range is about 1000 feet. that doesnt help though...

Heres a ship like yours. It dosnt really wobble. It gets difficult to fly now and again, but its still fun to fly. Its fun to try to spot land it. The engines are geared.

Another option you might try is bleeding air through tubes, out to the sides and front and back, with gates to control the airflow. The Harrier uses this system for zero airspeed stability. Yours looks somewhat like the fullsize Avro Saucer from the 60s. If you look that up on the net, you may find more ideas.

Here's the real answer: ADD DIHEDRAL

No wait, I know you are not flying an airplane.

However, it sounds like you need to add lateral stability - like someone earlier said. If you look at the fan forces, if they are all thrusting straight up, there is nothing to restore the UFO to a level hover when disturbed. WHat you need to do, is tip each fan unit so the exhaust is pointed slightly outward from center. Either that or modify the exhaust slightly outward using a vane of some sort. In this case lateral and longituinal stability are the same mechanism. THIS (angled jets) is what the Harrier uses for stability - the puffer jets are for control.

It works on the same principle as dihedral, to a certain extent. The "down" side automatically experiences more lift and the "up" side gets less -> return to level flight.

that is a good idea. i will try putting a small angle that vectors a small portion of the thrust away from each motor. do you think that will make a difference? i cant really tilt the whole fan unit becuase it is alreayd epoxied in place and it would be huge major surgery to break them off and i would most likely have to replace the entire CF structure to do that...

SpaceClam

Is this UFO your own creation? If not could you please tell us about the device. Where is it made, if you have a URL for the manufacture, the cost, or any other details. This looks like a very interesting device.

You may also like to look at the TurboPlane, a VTOL aircraft from Austria. It was made in the 1980. You can check out the original craft and photos of my copy of the device at:

http://community.webshots.com/user/poppyparrot

You can see movies of the craft in flight at the English/German site as well as more photos of my craft at:

http://www.turboplan.de/

Poppy Parrot
>[email protected]

what would you expect from a UFO?!

Hey Clam, how's it working for ya these days?