Solved Tri motor VTOL w/out yaw

Ryan O.

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


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.
Yes, now that I think about it for both options it may be better to yaw it into whatever orientation it can maneuver in without a weird yaw effect (roll or pitch) and then do a maneuver.
 

JasonK

Participation Award Recipient
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?

As described, you would not have yaw control in hover mode (other then wind wind vaning you into the wind). you need airflow over a rudder for it do do anything... and unless your moving forward with some speed, you wouldn't get any airflow over the rudder for it to give you stability.
 

Scotto

Elite member
This sounds like a job for darpa and about $5 million, but you can probly do it for $500 or less. Im not sure why but I like the idea of the front ones pointing out a little. So the thrust line is like a 3 legged stool in hover. How big are you thinking this would be? Nice drawings btw
 

Hondo76251

Legendary member
This sounds like a job for darpa and about $5 million, but you can probly do it for $500 or less. Im not sure why but I like the idea of the front ones pointing out a little. So the thrust line is like a 3 legged stool in hover. How big are you thinking this would be? Nice drawings btw
Haha, yeah, hope i can do it for somewhat less than 5 mil 🤣

Haha, well, doodling always was what i did best in school! 🤪
 
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Hondo76251

Legendary member
So, understanding that controlling yaw via rudder isnt ideal, but working within that constraint, the question becomes can one reach a point where the torque from the small motors is counteracted evenly by the single rear motor in static hover.
20220429_215744.jpg

If the CG is moved aft, motor B works harder and it would yaw clockwise while remaining level, conversely if moved fwd, the A motors would work harder and it would yaw counterclockwise.

I know theres some engineering wizzards that can do the maths and probably get some equations but I'm not that smart so I think building a frame of roughly the proportions and weight i want and testing to see what cg gets it to hover with the least yaw will be my first step. From there I think i can design an airframe around that, matching the weight and cg to what i know has the best chance of working.
 

CampRobber

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

Have you looked at the "rudders" on coaxial helicopters? The rudder looks normal at its neutral position, but instead of yawing to give it AoA against the forward-flight airflow, it rolls to give it AoA against the rotor downwash.

I'm not sure how you'd add that to your design. Either the rudder hinge is angled, or the rudder is actuated by a longitudinal shaft and bevel gears, and the shaft is a foil-shape that passes through the downwash? It'd be tricky with the aft prop vectoring though.
 

Hondo76251

Legendary member
Have you looked at the "rudders" on coaxial helicopters? The rudder looks normal at its neutral position, but instead of yawing to give it AoA against the forward-flight airflow, it rolls to give it AoA against the rotor downwash.

I'm not sure how you'd add that to your design. Either the rudder hinge is angled, or the rudder is actuated by a longitudinal shaft and bevel gears, and the shaft is a foil-shape that passes through the downwash? It'd be tricky with the aft prop vectoring though.
Yeah, you could go the vectoring route i suppose. A titling set of rudders wouldn't be impossible. Could have some on the front too.
20220430_075123.jpg

Of course, then you're right back into typical tricopter operation at that point and a second tilt axis on the rear motor would be about the same and probably lighter.

And some form of vectoring might actually be the only way to get it stable, but I'm gonna give it a shot without it anyway and see what happens.
 

LitterBug

Techno Nut
Moderator
You could use something like the 3D printable impossible tilts originally designed for David Windestål's tricopters.
https://www.thingiverse.com/thing:1035497

I have used them on a variety of projects.
RCExplorer Tri-copters. (baby, Mini)
Snow skimmer
RCExplorer BiCopter
Misc self designed bi/tricopters.

I've always limited the servos to +-40 degrees travel (total 80 degrees) for "XXXcopter reasons", but you could probably adapt them to work for a single or dual tilt arrangement on your design.

Video of one in tailtune/calibrate mode

Example on snow skimmer


Think.... Mount horizontal instead of vertical. Then have the motion from striaght ahead to rear facing. Two on the front would give you a bunch of forward thrust, differential thrust, plus be able to hover, do yaw control, fly backwards, etc....
 
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L Edge

Master member
Looking at landing in a confine area as your result, what will kill you is look at the transition from fwd flight to hover. With the prop going vert to horizontal(at an angle) you have force components going fwd and up. the fwd lessens but the vert is increased which deals with pitch change. How do you handle that?
As the rear prop becomes horizontal, how do you handle headwinds/sidewinds as it pushes you around?

There are 3 other approaches to solve short takeoffs and short landings.
 

Hondo76251

Legendary member
You could use something like the 3D printable impossible tilts originally designed for David Windestål's tricopters

I really liked his tricopters, watched all that. Obviously they are not capable of the same high levels of speed and precise control that a quad is but I'm interested in them from the design and engineering side.

Think.... Mount horizontal instead of vertical. Then have the motion from striaght ahead to rear facing. Two on the front would give you a bunch of forward thrust, differential thrust, plus be able to hover, do yaw control, fly backwards, etc....

It's not that I cant think of a way to control yaw, its that im trying to do it from only a conventional rudder. I don't need to skew, yaw, or fly backwards 😉

With the prop going vert to horizontal(at an angle) you have force components going fwd and up. the fwd lessens but the vert is increased which deals with pitch change. How do you handle that?
I'm not trying to do this manually. Im hoping i can get the FC to manage that. If the FC is still trying to keep the airframe level i can have the rear pusher motor (that faces down in hover) tilt slightly aft. This will begin to push the aircraft forward and the FC, still trying to keep level, will increase power. When sufficient airspeed is reached that the wings have lift then the transition to full forward flight mode will occur.



how do you handle headwinds/sidewinds as it pushes you around?
In intial take off/hover you dont.
Nose into the wind for liftoff then, perhaps only a few feet from the ground, the pusher motor tilts slightly to begin building FWD airspeed. Only then do you begin to have rudder control.

There are 3 other approaches to solve short takeoffs and short landings.
True, lots of ways to do STOL and every design is a compromise to suit its intended purpose. I'm just pondering an idea for another...

From a bicopter perspective....
Yeah, that's awesome! I'd love to try a bicopter design some time. I got out of the Marines before I got to fly in an Osprey, though there was quite a few years you wouldn't have wanted to fly in one! 🤣🤪
 
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Hondo76251

Legendary member
So just to visualize what i think the physics look like, and correct me if im missing something, but start with a for and aft "bicopter" (like a CH 46) and then remove the front rotor and replace it with two smaller ones. Now, instead of pitching nose down and increasing power to go forward, imagine pitching only the rear single blade and increasing power to go forward (but the airframe remaining level) then imagine this chimeric 46 had wings so once it reached sufficient speed with this method it could achieve lift and fly without using the front motors.
20220430_120149.jpg
 

Baron VonHelton

Elite member
So just to visualize what i think the physics look like, and correct me if im missing something, but start with a for and aft "bicopter" (like a CH 46) and then remove the front rotor and replace it with two smaller ones. Now, instead of pitching nose down and increasing power to go forward, imagine pitching only the rear single blade and increasing power to go forward (but the airframe remaining level) then imagine this chimeric 46 had wings so once it reached sufficient speed with this method it could achieve lift and fly without using the front motors.
View attachment 225033

Ok, that's just evil.........
:ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO::ROFLMAO:
 

L Edge

Master member
I really liked his tricopters, watched all that. Obviously they are not capable of the same high levels of speed and precise control that a quad is but I'm interested in them from the design and engineering side.



It's not that I cant think of a way to control yaw, its that im trying to do it from only a conventional rudder. I don't need to skew, yaw, or fly backwards 😉

I'm not trying to do this manually. Im hoping i can get the FC to manage that. If the FC is still trying to keep the airframe level i can have the rear pusher motor (that faces down in hover) tilt slightly aft. This will begin to push the aircraft forward and the FC, still trying to keep level, will increase power. When sufficient airspeed is reached that the wings have lift then the transition to full forward flight mode will occur.



In intial take off/hover you dont.
Nose into the wind for liftoff then, perhaps only a few feet from the ground, the pusher motor tilts slightly to begin building FWD airspeed. Only then do you begin to have rudder control.

True, lots of ways to do STOL and every design is a compromise to suit its intended purpose. I'm just pondering an idea for another...


Yeah, that's awesome! I'd love to try a bicopter design some time. I got out of the Marines before I got to fly in an Osprey, though there was quite a few years you wouldn't have wanted to fly in one! 🤣🤪

You may be lucky to reef the plane into the air to beat the ground effects for takeoff, but landing it without 3 axis control(unless your a jedi) will trash the device.

Looking at your sketch, how high do you think the ground effects will be that plane?
Here is a pic with my VTOL design(rudder, aileron and elevator) are all controlled with the exhaust by heli parts so I had the experience of facing hovering and landing.
I even approached it with another total new design not having rudder control(had rudder) and it was a failure.

Did you know that in most FC's, you have gyro drift as it heats up and doesn't hold very well. Ask me how I know?
P1010013.JPG


Same type of airplane as you.
 

Hondo76251

Legendary member
You may be lucky to reef the plane into the air to beat the ground effects for takeoff, but landing it without 3 axis control(unless your a jedi) will trash the device.

Thats entirely possible, maybe even likely! 🤣

Looking at your sketch, how high do you think the ground effects will be that plane

Entirely depends on the scale and power required, certainly several feet.

Here is a pic with my VTOL design(rudder, aileron and elevator)... Same type of airplane as you.
That's quite the project! Way cool!

To me, thats a thrust vectoring jet. An aicraft balancing on a directed column of air. I believe mine would behave much more like a tricopter, a vehicle that balances on 3 colums of air.

It's not that this design isnt 3 axis, its just limited yaw control at complete hover (which i really dont intended for it to do)

Perhaps I'm oversimplifying my idea but i think your project looks like much more of a challenge, at least to my mind.

Perhaps its a poor example as the original didnt work well (and i believe someone here tried to build one also) but i want this to work a lot more like the DO-29 STOL prototype.

20220430_141528.jpg


Now that plane, i think could be made to work if given a bicopter brain... but that looks even more unstable than my idea, at least to me...
 

Baron VonHelton

Elite member
Thats entirely possible, maybe even likely! 🤣



Entirely depends on the scale and power required, certainly several feet.

That's quite the project! Way cool!

To me, thats a thrust vectoring jet. An aicraft balancing on a directed column of air. I believe mine would behave much more like a tricopter, a vehicle that balances on 3 colums of air.

It's not that this design isnt 3 axis, its just limited yaw control at complete hover (which i really dont intended for it to do)

Perhaps I'm oversimplifying my idea but i think your project looks like much more of a challenge, at least to my mind.

Perhaps its a poor example as the original didnt work well (and i believe someone here tried to build one also) but i want this to work a lot more like the DO-29 STOL prototype.

View attachment 225053

Now that plane, i think could be made to work if given a bicopter brain... but that looks even more unstable than my idea, at least to me...
Someone here on the forum already did that plane, or something similar.

:coffee::coffee::coffee::coffee::coffee:
 

Scotto

Elite member
i believe someone here tried to build one also) but i want this to work a lot more like the DO-29 STOL prototype
I tried that. It worked really good Im just a bad pilot. I was about to say the motors on flaps would make transitions smooth. I think youd need a canard prop out front, and if it was on that swivel that Litterbug linked youd have it made. Or blowing the wrong way and on the tail.
20210820_215821.jpg 20210820_014105.jpg

Current version is a boat. Im working up the courage to attempt 20 landings with it for the bush challenge.
20220211_014329.jpg 20220211_014301.jpg
 

JasonK

Participation Award Recipient
Any reason you specifically want a tri copter?

- you could do a flying wing with 2 motors and tail land (even less moving parts).
- you could have the front 2 motors tilt instead of the tail, and you could then use those same tilt mechanics to get some yaw control
- you could do a bicopter transition thing (ok... this one is harder, really picky about CG in hover mode)
- you could do 4 motors and have all 4 tilt for the transition - then you could have normal quad logic for hovering (you could do this with equal size front and rear lift surfaces - where the whole surface rotates).
 

Hondo76251

Legendary member
- you could do a flying wing with 2 motors and tail land (even less moving parts).

Like, a normal plane? I have lots of those lol

- you could have the front 2 motors tilt instead of the tail, and you could then use those same tilt mechanics to get some yaw control

That is also an option that ive considered. The main reason i was thinking tricopter with one large aft motor was for swinging a large efficient prop for longer flight times. The front motors would just big enough to take off. And 1 tilt in a big motor is lighter and less complex than gimbals on two.

you could do a bicopter transition thing (ok... this one is harder, really picky about CG in hover mode)

Yeah, maybe someday... not up for that one yet.

you could do 4 motors and have all 4 tilt for the transition - then you could have normal quad logic for hovering (you could do this with equal size front and rear lift surfaces - where the whole surface rotates).
Also have considered this, actually a flying wing with a big pusher motor and 4 lift motors would be by far the easiest, but again, im going for minimum weight...