Solved Tri motor VTOL w/out yaw

Hondo76251

Legendary member
Ive been kicking around the idea of a VTOL build for some time now and I have an idea to run by the quad builder crowd.

Can i build a fixed wing, tri copter platform with a vertical tail as the only yaw control?

My thinking is the front two motors on the wings would be hard mounted in the hover position and would not be used in FWD flight. A larger rear motor would tilt to transition between fwd and hover. Rather than have it on a gimbal to provide yaw i would like to keep the rudder as the yaw control for both flight modes. It seems to me that most fixed wing vtol craft want to wind vane pretty badly anyway, sometimes to the point of instability if the FC is fighting to correct for it. Since i wouldn't be looking for a lot of maneuverability in hover mode anyway i think it could potentially simplify the build.

Anyone know if something like this has been tried?
 

L Edge

Master member
Ive been kicking around the idea of a VTOL build for some time now and I have an idea to run by the quad builder crowd.

Can i build a fixed wing, tri copter platform with a vertical tail as the only yaw control?

My thinking is the front two motors on the wings would be hard mounted in the hover position and would not be used in FWD flight. A larger rear motor would tilt to transition between fwd and hover. Rather than have it on a gimbal to provide yaw i would like to keep the rudder as the yaw control for both flight modes. It seems to me that most fixed wing vtol craft want to wind vane pretty badly anyway, sometimes to the point of instability if the FC is fighting to correct for it. Since i wouldn't be looking for a lot of maneuverability in hover mode anyway i think it could potentially simplify the build.

Anyone know if something like this has been tried?
 

L Edge

Master member
Could you sketch your setup. You must include having control in three axis(roll,pitch,yaw) in hover, transition and forward flight.
Hardest is with all the interference with ground turbulence(taking off and landing).

Only thing harder is a bi-copter where the 2 motors/props has to be handling the roll,pitch, yaw in all three phases.
 

Hondo76251

Legendary member
20220429_141701.jpg
 

Hondo76251

Legendary member
I understand that without the single rear motor being able to tilt left or right it will not have yaw authority in hover. It will naturally wind vane itself into the direction of motion or of existing wind if in a static hover.

For what i want to build this would be fine because all im trying to do is make a very slow controlled landing in a confined area
 

Hondo76251

Legendary member
The biggest problem I see is that you will have two props spinning the same direction. This with mean that the craft will tend to want to rotate in a static hover. I thought this might be mitigated somewhat with some tuning of the thrust angles.
 
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FlyerInStyle

Elite member
I don’t think that the rudder would control it during vertical flight, because the rudder only works if there is airflow.
you could have the motors slightly tilted, so that the front motors contradict the tilt of hte back motor, but the back motor still pushes some air over the rudders. it would take a lot of complex math to calculate all of the needed angles though
 

Ryan O.

Out of Foam Board!
In theory, if you wanted to keep the three motor with only a single pivot idea you could have all the motors in a line down the center of the aircraft. The center motor, which would be right on the cg, would spin one way and the motors in front and behind it would spin the other way. The center motor would have to be larger, and you would have no roll control so it would have to be inherently stable. I couldn’t think of a way to make it work in forward flight without control surfaces though. The other big problem, it would be a huge pain to program. Perhaps using the same motors for all three with throttle limits on the front and back motor could work.
What I would do is kind of cheating, but put a contra rotating motor on the back, like the one below and turn it into a y4.
https://usa-m.banggood.com/DZP30-Co...9SHxGZZS0kigMMncyJ2mKMMdEfRL3gEMaAoYNEALw_wcB
 

Hondo76251

Legendary member
I don’t think that the rudder would control it during vertical flight, because the rudder only works if there is airflow.
Agreed, this wouldn't be a true, "fly around like a drone when in hover mode" kind of craft but, if i can work it out, it would be the "line up for a normal landing, nose into the wind, transition to hover mode, land like a STOL plane" kind of craft
 

Hondo76251

Legendary member
In theory, if you wanted to keep the three motor with only a single pivot idea you could have all the motors in a line down the center of the aircraft. The center motor, which would be right on the cg, would spin one way and the motors in front and behind it would spin the other way. The center motor would have to be larger, and you would have no roll control so it would have to be inherently stable. I couldn’t think of a way to make it work in forward flight without control surfaces though. The other big problem, it would be a huge pain to program. Perhaps using the same motors for all three with throttle limits on the front and back motor could work.
What I would do is kind of cheating, but put a contra rotating motor on the back, like the one below and turn it into a y4.
https://usa-m.banggood.com/DZP30-Co...9SHxGZZS0kigMMncyJ2mKMMdEfRL3gEMaAoYNEALw_wcB
Ive seen the contra motors and that would certainly work but im still trying to keep kind of simple. (Well, relatively speaking🤣)

I think you're on the right track trying to figure a way to balance the torque induced yaw from the motors. Brings me to my other theory:

If both the front motors actually rotated the same direction, and the large one in the back the other, if all were equidistant from the CG fore and aft, i think you could reach a relative equilibrium by carefully adjusting that distance from the CG.
 

Ryan O.

Out of Foam Board!
Ive seen the contra motors and that would certainly work but im still trying to keep kind of simple. (Well, relatively speaking🤣)

I think you're on the right track trying to figure a way to balance the torque induced yaw from the motors. Brings me to my other theory:

If both the front motors actually rotated the same direction, and the large one in the back the other, if all were equidistant from the CG fore and aft, i think you could reach a relative equilibrium by carefully adjusting that distance from the CG.
I think that would work too, but you would lose pitch control which may or may not end up being harder to balance.
 

FlyerInStyle

Elite member
In theory, if you wanted to keep the three motor with only a single pivot idea you could have all the motors in a line down the center of the aircraft. The center motor, which would be right on the cg, would spin one way and the motors in front and behind it would spin the other way. The center motor would have to be larger, and you would have no roll control so it would have to be inherently stable. I couldn’t think of a way to make it work in forward flight without control surfaces though. The other big problem, it would be a huge pain to program. Perhaps using the same motors for all three with throttle limits on the front and back motor could work.
What I would do is kind of cheating, but put a contra rotating motor on the back, like the one below and turn it into a y4.
https://usa-m.banggood.com/DZP30-Co...9SHxGZZS0kigMMncyJ2mKMMdEfRL3gEMaAoYNEALw_wcB
It would probably be cheaper to get 2 smaller power motors and mount them one right side up and the other upside down, and fc's that support betaflight would take it. You could alos then use the fc to switch into airplane mode
 

Hondo76251

Legendary member
Ive done a lot with my pixhawks, but never drone/vtol stuff. Seen a lot of different ways work, these FCs are pretty incredible.

I think that would work too, but you would lose pitch control which may or may not end up being harder to balance.
You'd still have pitch, two clockwise motors fwd of CG and one counterclockwise one aft, you'd just have some variance in the yaw as, say, the front motors speed up to pitch up they would produce more yaw relative to the aft motor, but again, I'm not looking to get a super stable hover, just a slow landing.
 

Hondo76251

Legendary member
It would probably be cheaper to get 2 smaller power motors and mount them one right side up and the other upside down, and fc's that support betaflight would take it. You could alos then use the fc to switch into airplane mode
I guess I should mention that I want this as an FPV platform. I need a lot of gear on this plane and a fair amount of battery. Im shooting the gap between the longer flight time of fixed wing and the deployability of a large drone.

More motors would absolutely make it work, that's been done, ive watched a ton of builds like that. The idea is VTOL with the minimum of added weight.
 

Hondo76251

Legendary member
So, to continue the rest of the story, I have been working on ironing out the "auto land " sequence with the pixhawk runing ardupilot. Hard not to damage the airframe because, even when it works, i don't exactly have runways here, let alone if i have to initiate a failsafe and autoland off runway. Hence the desire to try VTOL.
Mind you, the best plan may just end up being a simple parachute and a good hand lauch sequence, but this would be WAY cooler! 🤣
 

Ryan O.

Out of Foam Board!
It would probably be cheaper to get 2 smaller power motors and mount them one right side up and the other upside down, and fc's that support betaflight would take it. You could alos then use the fc to switch into airplane mode
That’s what I was thinking but then pivoting to forward flight would be difficult.