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Feb 26, 2013, 05:00 PM
Registered User
Thanks for the input and interest,

I'm researching the many multiwii boards right now before buying one. Until then, I've purchased a simple orangerx flight stabilization board to mess around with and see if it can get me anywhere near a hover. From what I understand, the board uses 3 simple rate gyros for full 3-axis stabilization, which is essentially the setup I had in the video (only with individual heli gyros in rate mode). I've looked into the hk kk2 board but don't think it's ideal compared to other options though.

That is a very good idea for testing... The final version I'm sure will have landing gear which prop the nose up, but I didn't think of doing that! I'm hoping that in a hover, the nose will be slightly pitched up, cancelling out the angle of the rear motor which is pushing it forward.

I've changed many aspects of the control system from the original video. All shared channels (between hover and FF) have a y-harness going out from the rx, allowing one channel for say, an unstablized rudder control surface, and the other for stabilized roll control on the wingtip mounted motors.
It seems you fully understand the simplicity that I'm trying to incorporate into this project. The lack of mixing between hover and forward flight, I believe, is amazing. The dihedral in the wings allows roll input from a hover to be turned into a simple RET plane in FF- no swapping of channels and their functions and no ailerons. The elevator surface with the motor mounted to it acts as a control surface in forward flight, and vectors down to nearly 90 degrees via simple elevator-flap mixing for hover. From there, the movement of the surface is togglable (off for hover so it doesn't move in response to elevator input) while the rear motor controls pitch by altering its speed. The effects or the rear motor being used for pitch would be minimal or not even noticeable in FF. In transition mode, the aircraft will begin gaining forward momentum, which is when the elevator surface will be re-enabled to react to stick input. This allows for simple thrust vectoring under my control, while the pitch is still being stabilized by the motor mounted on that surface. Like you said, effects of the yaw control of the rear motor will again be minimal if not noticeable in FF- Simple!
All of this leaves me with one extra channel on my dx7.. So I will use that for gyro gain-either off or on. This is just precautionary in case the gyro sensitivity in hover is too much in forward flight and wobbles occur.

The airframe you see is just the first stage of this project. I mainly built it just to test the mechanics and principles. I don't think it will ever be capable of forward flight. However, once I can achieve a successful hover with it, I will take careful notes on dimensions, etc. and build a second airframe that is more suitable. The cg on this model is about an inch and a half from the trailing edge of the wing on the wing itself. No doubt tail heavy, but still could fly nevertheless. I will soon begin researching and designing airframes that have a cg located in a more appropriate location and decide on one to rig up for VTOL

I'm really trying to put together a video on the flight controls so it's easier to understand what is going through my head. All those words can get confusing ^^^ lol.

Once again, thanks for the input!
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Feb 26, 2013, 06:35 PM
Registered User
Ran D. St. Clair's Avatar
The multiwii boards will give you more flexibility long term if you want to learn to play with code. The KK2 is less flexible, but would probably get you going a bit quicker.

If you want to keep it really simple, you can get rid of the rear motor and you would have a nice little bicopter. Just move the CG directly under the wing spar, which would also solve your tail heavy problem in forward flight.

You already have separate servo's for the front rotor tilt, and the pivot point is already nice and high above the CG, so you have everything you need.

The only other control surface you need is the elevator. That and you would need to add a vertical stabalizer. A rudder would be nice but not absolutely necessary.

If the Right prop turns CW and the left prop CCW, then the application of left roll will automatically give a little left yaw in hover. That, and as you begin to translate to the left the vertical stabalizer will drag the nose to the left, so you don't absolutely need differential tilt on the motors for yaw control.

Tilting both motors fore and aft will give fore and aft translation control in a hover, similar to pitch, but not really pitch.

As the motors move forward, and you pick up a little speed, it becomes a normal airplane, with just elevator and differential thrust for yaw, which the dihedral turns into roll.

It would be about the simplest fully capable VTOL I can imagine, just 2 motors and one servo. I haven't built one that way myself, but I can't see any reason it would not work.

Good luck and have fun.
Feb 26, 2013, 07:26 PM
Registered User
I'm not sure bicopters are the way to go.. Sorta like trying to balance a table with only 2 legs. Tricopters are much more dependable especially for something as complex as VTOL.
I've heard and seen that bicopters tend to get into a pendulum effect that magnifies itself every oscillation. It requires a large amount of precise stabilization to keep this under control from my understanding. My goal with this project is to utilize as much mechanical stabilization over gyro stabilization as possible to decrease the amount of frustration going into electronic setup. Sort of a lazy approach on my part
We'll see what works out in the end....
Feb 26, 2013, 08:55 PM
Will fly for food
ilektron's Avatar
For me, being a software guy, doing everything in software is the lazy way out

Who wants to spend the time building a nice airframe when you can slap some gyros and a microcontroller on there and call it a day?
Feb 27, 2013, 01:19 PM
Registered User
Ran D. St. Clair's Avatar
Yes, I agree, a tricopter is a more solid approach. It's just that you expressed a desire for maximum simplicity, and also your current airframe has some limitations as a tri-copter, but as a bi-copter it is pretty much ready to fly. (just remove the tail motor and shift the CG forward). I also hear you about the pitch oscillations. You have a reasonably long fuselage with a horizontal stabalizer to help damp those out, something a lot of bicopters lack. Anyway, you should have fun however you like. I am just making conversation.
Feb 27, 2013, 02:12 PM
Registered User
If I had more experience with programming and code, I'd be building a bi in a second Unfortunately my best area right now is mechanical engineering so I'm working on taking advantage of that stored knowledge. I wish I had the time to sit down and learn the basics of arduino and multiwii, but school keeps getting in the way. The capabilities of the multiwii boards to simplify things is reeling me in. One of these day I'm gonna cave in

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