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Old Sep 20, 2012, 05:20 PM
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Idea
Tilt Rotor Project

Hello all,

I thought I'd share my VTOL project with this forum and gather some constructive criticism and help from you guys

So let's look at where I am now...

RC Tilt Rotor VTOL: Part 2 (2 min 44 sec)


This is the first untethered hovering test where I just did a few hops to prove everything is working as it should. You can see at 2:10 I managed a pretty stable forward-moving hover.

The goal of this project is to make the transition from hover to forward flight and back as simple as possible. Not only to keep the complexity of the build down, but also to let me use my dx7 without having to do many extreme mixes that it just can't do.

I guess the simplest way to explain how everything works together is to break it down for you guys...

-The two main motors on the wingtips are tp bell outrunners spinning 8x4s- Each producing about 20oz of thrust. You can see that they extend past the wing itself and none of the wing rotates with the motors for the transition (:50 in the video)They rotate 90 degrees from hover to forward flight and stop at about 55 degrees for mid-transition by using the 3 position flap switch.

-Roll control in a hover is by differential thrust of the wingtip mounted motors. I used a detrum gy48v gyro in rate mode for stability.
You can see the dihedral in the wings and motors. This gives even more stability in a hover. The motors blowing air outward gives the aircraft a larger area to sit on when hovering so the platform is self-leveling. I almost don't need the gyro with the dihedral (gain is set quite low)
The dihedral is also crucial in forward flight. It will be RET, so it will need to be self correcting with only a rudder for steering. This also means that I can utilize the differential throttle from hovering in forward flight for control - no mixing required. The "rudder" will still be on the aileron stick because it was being used for roll in the hover, so it will be just like flying a RET plane.

-Pitch control reminds me of the F-35. The elevator control surface has the rear motor (emaxx spinning 6x4 on 2s) directly attached to it. I used the elevator-flap mix in the dx7 to get the control surface to vector down to about 50 degrees when the wingtip mounted motors are at 90. It vectors back to level when the wingtip mounted motors are level as well for forward flight (best seen at :50). I especially like this system because all 3 motors will be used all the time. I won't have to toggle the rear motor for transition, etc.

-As of now, actual control of pitch is achieved by simply using the movement of the control surface to let the tail either slide out (pull up) or be pushed up (forward). The exact same elevator movement is used for hover and forward flight- No mixing again!
So when I pull up (to pitch backwards), the elevator control surface moves upward like a normal plane. In hover, this causes the tail to lose lift since the motor is no longer supporting the back. Pushing forward to go forward causes the control surface to move further down, giving the tail even more lift and thus pitching forward.
Stability as of right now is achieved by using a cheap esky gyro hooked up to the elevator servo. The movement it gives is not sufficient enough to keep the pitch perfectly stable, I'm doing most of the work in the video. The plan is to hook the gyro directly up to the motor to help stabilize pitch, but I'll still be using the mechanical control method. Worst case scenario in forward flight, the gyro will try to correct pitch by speeding up/slowing the rear motor but nothing will happen

-Yaw in a hover is very simple. Just a thrust vectoring motor on the the elevator surface. No gyro because it doesn't need it.
The yaw servo will always be active, so nothing will happen in forward flight because I simply won't use the rudder stick since it's an RET plane. No mixing again.


Soooo I think that's it... Any questions? lol
I'd love to hear any constructive criticism from you guys. I'll be slowly working on this over the next few months.
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Old Sep 27, 2012, 06:29 AM
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I think it looks great. I'd recommend a servo slow device on the tilt servos. Even with the motors moving very slow while transitioning your aircraft will want to pitch up immediately and very quickly to the point of trying to loop. Have you considered using a MultiWii for doing mixing that the DX7 can't?
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Old Sep 27, 2012, 06:17 PM
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That is definitely on the list. Thanks for the heads up. I hope that the thrust vectoring control I have for pitch will allow me to effectively counteract any pitching while transitioning, but I'm positive that without any servo slowing, it will be too sudden to handle.
I've looked into stabilization boards and the multiwii controller. I think they're just too complicated for my young mind to grasp... For now I'm just trying to simplify everything and use methods that are already known to me.
Thanks for the input
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Old Nov 04, 2012, 04:39 PM
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Thrust vectoring motor mount(suprisingly simple stuff) (1 min 15 sec)
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Old Nov 04, 2012, 04:47 PM
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Thanks for the video. The rear vectoring motor on this is directly attached to the elevator control surface with a servo for yaw mounted on the control surface as well. Keeping it simple like this actually works
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Old Nov 22, 2012, 06:51 PM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
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tilting mechanism

may i know what mechanism u used to tilt the rotor horizontal to vertical.

and what is the weight of ur plane averagely...
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Old Nov 24, 2012, 06:01 PM
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Quote:
Originally Posted by jyothibasu1k View Post
may i know what mechanism u used to tilt the rotor horizontal to vertical.

and what is the weight of ur plane averagely...
Attached is a rough sketch of the tilt assembly at the tips of the wings. The 1cm square wing spar is cut at the point of tilt and joined by a 3mm metal rod glued into a channel on the side of the motor. The rod is secured in the groove on the other side by a thin plywood stripped glued on top, allowing it to rotate freely. The rod is bent at 90 degrees where a ball joint connected to it allows the servo to rotate it. I've found this to be the strongest way of doing it, but certainly not the lightest. If there's any confusion with this, let me know and I can try to explain further.

The weight ready to hover is around 35-36oz. Each wingtip motor puts out 19-20oz of thrust and the rear motor puts out about 12oz. It will hover at a little less than 3/4 throttle, maybe less (haven't flown it in quite some time )
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Old Nov 25, 2012, 07:13 PM
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you lost the 1/2 thrust because the wings
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Old Nov 25, 2012, 08:03 PM
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Quote:
Originally Posted by van_kq View Post
you lost the 1/2 thrust because the wings
How so? I'm using 8x4 props and the center of the motors are mounted 5 inches from the tips of the actual wings.

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Old Nov 26, 2012, 12:58 PM
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You are not losing any significant thrust on the main rotors, and a very small amount of thrust on the tail rotor, actually
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Old Nov 26, 2012, 04:51 PM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
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u have used a servo motor instead of stepper motor ?


can i use the same mechanism for this type of rc plane, the wing is made of thermocol sheet.(weight of the plane in picture is 650grams )


my imagination plane weights about 1.5kg

i just want to modify the wing, remaining body is same. over the wing i want to keep two fans on either side of the wing as u kept in the 1st video. i don't want to keep a third motor at the rudder/elevator. i want to move it with 2 motors only. (i will also remove the motor that is at the front in the picture)

hope u under stand me
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Old Nov 26, 2012, 05:27 PM
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Yes I'm using these servos to tilt the wingtip mounted motors http://www.horizonhobby.com/products...-servo-JRPS821

You could try adapting that plane to tilt the motors similar to mine. The (aluminum?) wing spar could be replaced with something longer that runs the entire length of the wing. Then that could be adapted to a similar setup as mine.

Pitch stability would be eliminated in hover with only 2 wingtip mounted motors. You'd need some serious gyro stabilization without the rear motor. Roll stability would also be worse than mine because I can see your plane has no dihedral.
I'm not saying it's impossible, but it just might be a challenge for you to get the same mechanical stability as I'm getting with your completely adapted project. This VTOL was completely designed around many mechanical stability/control concepts I've had in mind for a while. Everything you see on my project is there for a very specific reason.
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Old Nov 27, 2012, 09:00 AM
NoT iNtElLiGeNt
India, Andhra Pradesh, Guntur
Joined Jan 2011
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so can't we attain stability using rudder's,ailerons and elevators without using 3rd motor.

can u tell me in detail about dihedral.
tell me in detail about various mechanical stability/control concepts in detail.

u said i can use the similar mechanism as u used to tilt can u explain it in brief how can i how use it over thermocol sheet or any other metal
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Old Nov 27, 2012, 05:32 PM
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Canting the two rotors inward should have a negligible effect on stability. The dihedral effect works due to sideslip which causes one wing to have a higher AOA than the other. But it takes a pretty large translational speed for a rotor to start developing translational lift and be affected by rotor "dihedral".
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Old Nov 27, 2012, 06:03 PM
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Quote:
Originally Posted by jyothibasu1k View Post
so can't we attain stability using rudder's,ailerons and elevators without using 3rd motor.

can u tell me in detail about dihedral.
tell me in detail about various mechanical stability/control concepts in detail.

u said i can use the similar mechanism as u used to tilt can u explain it in brief how can i how use it over thermocol sheet or any other metal
You could, by tilting the motors slightly to deflect the thrust for yaw and pitch control in hover, but pitch would be very unstable. This is because the entire plane is balancing on the 2 motors, leaving the airframe to tilt forward/back freely on the pitch axis since there's very little holding it in place. The introduction of a third motor, making the whole rig essentially a tricopter, will hold up the tail and prevent it from getting into an uncontrollable swaying motion.
This is why tricopters are very stable while bicopters require far more stabilization.

Dihedral helps level a plane in forward flight because the plane is basically hanging on the wing. It creates a self-leveling effect. My theory is that introducing dihedral to the motors of a hovering platform will let the plane rest (in a hover) on more air, or more space for its size. Think of it this way:

Straight mounted motors, no dihedral: l_l________________l_l air blowing straight down from motors

Motors with dihedral: /_/_________________\_\ Air blowing outward. Gives platform more area to "sit" on in a hover

Some people say it is counter-productive and will actually make it LESS stable, but I've found that I need a lot less gyro stabilization with the dihedral implemented to sustain stable roll control. Leadfeather proved this
Self Leveling Tricopter (2 min 29 sec)


Other methods I used in conjunction to create a practical VTOL are mentioned in the original post. I will list the most important here again though:

-Dihedral in wings and motors for roll stability

-Weak rear motor to keep the tail from getting into uncontrollable swaying
***Pitch control achieved by mechanically moving motor to allow it to "push up" tail for forward movement; backwards pitch achieved by allowing the tail to slide out on itself when the rear motor stops giving lift...... Full forward pitch=vertical motor... Full rearward pitch=horizontal motor.
***Motor in normal hover is at 45 degrees pointed down. Allows for above control method ^^^ as well as helps with stability by creating a larger area for the plane to "sit on" (like the dihedral!)
***Pitch stability still achieved by gyro stabilization of rear motor speed

The main thing you have to realize is that transitioning from hover to forward flight is the HARDEST part. This project was completely designed around these principles to allow for the easiest mechanical transition. My stability techniques may not be the most straight-forward approach to hovering, but they allow for an easy transition from hover to FF. For example the rear motor. The reason it is not fixed mounted like a real tricopter is to allow its use in forward flight as well. It is directly mounted on the elevator control surface so I also have thrust vectoring control in forward flight. It also gives me full control of pitch during a transition and no periods of no control.

I think that you could implement these design features into your platform, but it would certainly not be ideal. I think you should start from scratch and build the plane around some of these features. You can see that my plane isn't pretty, but it does what it's supposed to do. That's because I disregarded all aesthetic features and completely designed it around my ideas of easy hovering, transitioning, and forward flight.
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