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Old Jan 20, 2013, 05:16 PM
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Thoughts about transitioning a tilt-rotor

I've got the parts for my next tilt-rotor which will be able to transition to forward flight. I've been thinking about the autopilot for transitioning from hover to forward flight.

Often a hurdle is that at some point between hover and forward flight, the autopilot must adjust the controls, because roll transitions to yaw and yaw transitions to roll. The solution is often that at some arbitrary point in the transition, the controls swap.

I propose that you could have the autopilot mix the channels, using a little trig, and combine roll and yaw proportionally to the angle of the rotors. At any angle, you would could combine moments on the airframe into the single moment that is being commanded.

Any thoughts?
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Old Jan 21, 2013, 07:10 AM
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Check out the mixing cams in the Canadair CL84 patents. It is an ingenious system that adjusts the mix of the control inputs based on the wing tilt angle so that yaw, roll and pitch inputs produce the correct deflection of control surface and rotor pitch to "map" to yaw, roll and pitch responses. All done mechanically, with no computer help, though the inputs were mixed with mechanical dampening gyros before going through the mixer unit.
Here's a link to the drawings section: http://www.google.com/patents?id=bxV...20vtol&f=false
I think the same could be done in a model, perhaps using something other than an ider+cam setup
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Old Jan 21, 2013, 09:54 AM
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Thanks for that post. I'm amazed at what you can find on the internet. I think that proves that it can be done. If it can be done mechanically, then it problem can be done electronically. I have my goal.
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Old Jan 21, 2013, 10:08 AM
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The one problem you might find with an electronic mixer setup, as opposed to a mechanical mixer, is the input delay caused by the mixing. This is what leads to oscillations when using gyroscopes, for example.
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Old Jan 21, 2013, 10:27 AM
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I may need to brush the dust off of my control theory book and come up with something to simulate a bicopter in all three axis.

My gut tells me that if the craft has sufficient phase/gain margins and is stable in both hover and forward flight, and is stable through the transition, that adding mixes won't necessarily affect the stability. Worth further investigation.
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Old Jan 21, 2013, 11:28 AM
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I think you will find that rather than doing realtime trigonometrical operations it will be more advantagious to store pre-calculated curves and interpolate as needed. If you have enough resolution then you can just interpolate linearly between the closest stored curve coordinates. It makes it also easier to adjust the curves on the ground, and allows for more flexibility than a matematical model.
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Old Apr 10, 2013, 02:15 PM
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Seems to me the best new way to transition is to not have to transition at all. If you can go from hover to forward flight without having to change anything about the control system that would be the safest way to do it. By that I mean a flat transition into forward flight. Maintain your lift in hover as you apply forward thrust then turn off hover motors.... Ive just about got it.....
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Old Apr 10, 2013, 02:18 PM
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Seems to me the best new way to transition is to not have to transition at all.
Where is the fun in that?
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Old Apr 10, 2013, 03:09 PM
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I think it can absolutely be done, and while using sines and cosines would be mathematically correct a simple linear mix would probably suffice. For example, lets say that your tilt servo was controlled by a value from 0 to 1 with 1 being hover mode or straight up, 0.5 being 45 degrees, and 0 being forward flight mode, or tilted 90 degrees from vertical. Then you simply multiply your roll input (as well as any stability feedback input) by that value. if you wanted, you could multiply the yaw input by 1 minus that value, though in forward flight you probably just want pure throttle so it's not necessary.
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Old Apr 10, 2013, 05:00 PM
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What I meant in my post was essentially to approximate the job done mechanically in the Canadair CL 84 Dynavert. Take for example the horizontal stab: in a hover it is kept horizontal. Trough the conversion regime it's tilted up to 45 degrees, so that it will be mostly parallel to the airflow and contribute to pitch control, and when the wing is entirely tilted it goes back to horizontal. You could work out a formula to do this, but if you have to adjust it so that it ramps up faster in the first half of the transition and goes down slower in the second half, you have again to find another formula that fits the expected movement. you could instead just store a list of positions at specific steps, matching specific wing angles. If the wing angle is between these two steps, you can just interpolate linearly. linear interpolation is easy to calculate, and does not take many cycles.
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Old May 12, 2013, 05:45 PM
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Quote:
Originally Posted by cwleveck View Post
Seems to me the best new way to transition is to not have to transition at all. If you can go from hover to forward flight without having to change anything about the control system that would be the safest way to do it. By that I mean a flat transition into forward flight. Maintain your lift in hover as you apply forward thrust then turn off hover motors.... Ive just about got it.....
Yes... but.

Now you have to have motors for lift, and motors for thrust. Most of the time only one set will be working, and the other set will be extra weight that's just going along for the ride.

You also have the drag of the hover propellers when you are going forward. You might be able to use them as a gyrocopter and gain some lift, but now my brain starts to hurt.
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Old May 14, 2013, 01:40 PM
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Originally Posted by Ad_Astra View Post
Yes... but.

Now you have to have motors for lift, and motors for thrust. Most of the time only one set will be working, and the other set will be extra weight that's just going along for the ride.

You also have the drag of the hover propellers when you are going forward. You might be able to use them as a gyrocopter and gain some lift, but now my brain starts to hurt.
Agreed. And worst of all, it costs more!
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Old May 25, 2013, 01:04 AM
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Where is the fun in that?
Actually, now that Ive had a chance to fly it a little, it turns out its a lot of fun. Its the same thing as flying a really fast quad copter. Only forward flight is the same as flying a conventional aircraft. I have a couple more bugs to work out on my esc arrangement. Im not an electrical engineer and so I'm not sure I have things hooked up in the most efficient way. I need to cut ALL power to the hover motor after transitioning to full forward flight so that Im not losing any juice after shutting it down. But if I cut off all power using a relay type device then I have to re arm the esc. I think Ive got it worked out on paper though. Some guys at dimension helped me out.
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Old May 25, 2013, 01:11 AM
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Originally Posted by Ad_Astra View Post
Yes... but.

Now you have to have motors for lift, and motors for thrust. Most of the time only one set will be working, and the other set will be extra weight that's just going along for the ride.

You also have the drag of the hover propellers when you are going forward. You might be able to use them as a gyrocopter and gain some lift, but now my brain starts to hurt.
One small motor for forward and one larger motor for hover. Don't use the larger for very long, then the smaller is ultra efficient. No additional drag from hover props because they are buried in the wing. They don't create drag in forward flight. Only time both motors are used simultaneously is from hover to forward flight transition and that's only for a moment. On "landing" or returning to hover, the forward flight motor is shut off and the hover motor isn't turned on until closer to the ground. It actually glides quite well.
The battery is a little small for hovering but a little large for forward flight. The airframe is VERY light. I think the final version will have a thrust vectoring fan that can be reversed for braking or backwards flight.......
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Old May 25, 2013, 01:18 AM
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Agreed. And worst of all, it costs more!
Not true, one battery, two esc and two motors. That's less cost for primary flight controls than most vtols. plus its cheaper to buy one larger and one smaller motor than two larger.
Im using an arduino mini, wmp and nunchuck, magnetometer, and barometer for a total fc cost of 48 bucks.

Not sure what everyone else is paying for airframes and such but I figure im in mine for less than 200.00. That's not too bad is it?
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