Solved Tri motor VTOL w/out yaw

JasonK

Participation Award Recipient
Like, a normal plane? I have lots of those lol
no, that can 'nose up' and hover and do vertical take of and landing (that is what you were trying to to get yes? your going to need a FC for any hovering mode anyways. and if the primary goal of the hover is just to make take off and landing take less space, that should get you what you need).

Example of what I was talking about that FT made
 

Hondo76251

Legendary member
Yeah, I've seen that one. Its pretty cool!
That one uses the tilt of the front motors for yaw control though 😉

no, that can 'nose up' and hover and do vertical take of and landing

your twins don't do that already? 🤔 jk. 🤪

FC for any hovering mode anyways. and if the primary goal of the hover is just to make take off and landing take less space, that should get you what you need

It would certainly have an FC, most likely one of my pixhawks. There are several neat planes (like the one @skymaster just showed) that i could buy if VTOL was all i was after. There are easier ways to do scratch built VTOL that have a higher probability of success but this one is just fun from the, "can it be done" side of engineering.
 

L Edge

Master member
Actually, this is what you are looking for. The 2 mounted servos in front of the leading edge allow you to handle on sitting vertical all three axis as you take off. Has long flight times as video shows.

You have differential thrust for yaw, both going up/down for pitch, and one up/other down for roll. Notice the rudders are behind the prop flow and act as a vertical stand for launch. No FC is needed. Flies in a tight area and can land regular. Large wing area(semetrical airfoil) that allows you to fly slow. As you flare, hit the switch that not only shuts the motor off, but flips the servo up so props don't hit. Made this in 2012.


Reused it for a VTOL in 2017. Landings were tough with wind. It is like X-Vert that was on market.
 

Hondo76251

Legendary member
While I love seeing cool designs and talking about them it seems we're kind of straying from the original topic.

The question is, "Can an FC be programmed to control a trimotor aircraft if forced to use only the rudder for yaw." And i think the answer is, above 5kts fwd airspeed, absolutely. Now, would it be able to perform a static hover for more than a few seconds? I have my doubts. Would it be stable long enough to gently land? I believe so.

Rather than building an entirely different airframe i can simply put a L/R tilt servo on the rear motor and it would be just like several other proven tricopter designs ive seen. If it proves to be too unstable that is the route i would probably go.
 
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Hondo76251

Legendary member
I've been using the pixhawk cube for a while, in part because it seems to be favored by professionals. This was a good explanation video i found when researching.

 

Hondo76251

Legendary member
I found the idea of using two different battery systems pretty clever. High C batteries for hovering and LiIon for flight.

Also, the peter and the FT guys did a similar build a long time ago.

 

JasonK

Participation Award Recipient
yup, but that is the normal tri-copter-plane with tilt front rotors and possibly tilt on the tail rotor
 

NickRehm

Member
The question is, "Can an FC be programmed to control a trimotor aircraft if forced to use only the rudder for yaw." And i think the answer is, above 5kts fwd airspeed, absolutely. Now, would it be able to perform a static hover for more than a few seconds? I have my doubts. Would it be stable long enough to gently land? I believe so.

An out-of-yaw-control vtol that's yawing in circles will still be stabilized in the roll and pitch axis in stabilized/angle controller modes. So yes you will be able to take off, uncontrollably spin in circles, and hope that when you go to transition, the airplane is pointing in the right direction as yaw control authority is restored with airflow over the rudder. The concept of commanding roll and pitch in hover to translate around will be almost completely removed, unless you're really good and fast at changing from flying nose in / nose out at 2-3 times per second.

You can design for this anticipated yawing and try to reduce it if you match the hovering thrust of the same direction spinning motors. This will very closely match the yawing torque--no need for calculations on motor/prop size...just need to make sure your front 2 same-spinning motors are producing the same amount of thrust as your rear motor in a trimmed hover. You will need to account for cg location to figure out where the motor locations to achieve this should be. It would be easier to determine by trial and error with a variable length boom you mount the forward or rear motor on.

But, for roll and pitch stabilization, the rpm of all three motors will still be changing, inducing seemingly random yawing moments on the vehicle. So for example commanding a roll in one direction will cause a yaw in some direction, and vice versa. You might notice this effect on some tricopters that do not have feed-forward terms in the controller... a rapid rolling command will cause a bit of a yaw wiggle before the IMU can measure the wiggle from the delta in torque from the roll maneuver, command the servo, and correct the unwanted yaw coupling from the roll input.

Will it fly? Yes absolutely. Will it be controllable in hover and and fun to fly? You be the judge and report back

To offer a solution, you could put a yawing control vane under one of the forward motors...doesn't matter which one. This eliminates the mechanical problem of the rear motor needing to pitch and yaw. Yaw authority won't be very good, but at least it will be controlled
 
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Hondo76251

Legendary member
To offer a solution, you could...
Thats an idea, and thanks for the input, but I was more wanting to just focus on the physics at play rather than to build a craft with propper yaw control.

More like this:
An out-of-yaw-control vtol that's yawing in circles will still be stabilized in the roll and pitch axis in stabilized/angle controller modes..........................2-3 times per second.

2-3 TIMES A SECOND?
20220501_174538.jpg
Mind you, i was a helicopter mechanic and I've actually seen what a loss of tail rotor authority looks like, i still cant imagine it ever getting that wildly out of control and still being in the air long enough to get to that kind of rotational velocity. Helicopters do that because they only have a single blade and the tail rotor is the only resistance to the torque it generates. Without it things go very bad very quick.

If, all things being equal (theoretical realm here) it was in a static hover, it could be stable if the cg were exacly correct. The instability in the real world will come from the cg being less than perfect and the FC adjusting the motors to keep the craft level, changing the torque forces generated, but even then they will still be close to balancing each other most of the time. Obviously it will eventually begin to drift clockwise or counterclockwise and that is where the debate lies. How long will that be and will it be sufficient time to get fwd airspeed?
And keep in mind, this craft is not meant to achieve stable static hover... I'm betting 2 to 3 seconds before its uncontrollably yawing but im betting it can be moving fwd with sufficient speed in as little as a second and a half.

It would be easier to determine by trial and error with a variable length boom you mount the forward or rear motor on.

Agreed. Pretty much the approach I plan to do, trial and error.

Will it fly? Yes absolutely. Will it be controllable in hover and and fun to fly? You be the judge and report back

Thats the plan but obviously im still in the theoretical physics and napkin drawing stage! 🤣
 

Hondo76251

Legendary member
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.
View attachment 225057 View attachment 225060

Current version is a boat. Im working up the courage to attempt 20 landings with it for the bush challenge.
View attachment 225058 View attachment 225059
I loved your do29 btw, can't wait to see the water version in action!
 

Hondo76251

Legendary member
Found one!

Tilt rotor Bicoper with both motors mounted to the same shaft and operated by one servo.

Unless you want to see the code he used in the arduino, skip to 9:30
 

Baron VonHelton

Elite member

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Hondo76251

Legendary member
Seeing that one gives me some confidence that this might actually work. He did a very good job considering how touchy a tilt rotor like that is about CG and the fact that he was able to get to transition with just a few lines of code, and on the maiden no less. 👏 hats off to him. Im cheating by comparison as i plan to use a good FC and run adrupilot so all the code has been done by people far better at that than myself.

Ive been using mission planner for a while and the painless360 channels videos were a great help. I'm happy to see that Hes doing a VTOL series now.

Obviously when i get to the point I'm putting expensive equipment in it I will have proper yaw control. I plan to apply this thought experiment to the initial test build as I'm going to try to keep it in the sub 250 range, so fewer moving parts would be a great help.

I'll do a build thread on this project when i get to it later this summer.
 

L Edge

Master member
While I love seeing cool designs and talking about them it seems we're kind of straying from the original topic.

The question is, "Can an FC be programmed to control a trimotor aircraft if forced to use only the rudder for yaw." And i think the answer is, above 5kts fwd airspeed, absolutely. Now, would it be able to perform a static hover for mpre than a few seconds? I have my doubts. Would it be stable long enough to gently land? I believe so.

Rather than building an entirely different airframe i can simply put a L/R tilt servo on the rear motor and it would like just like several other proven tricopter designs ive seen. If it proves to be too unstable that is likely the route i would go.
Seeing that one gives me some confidence that this might actually work. He did a very good job considering how touchy a tilt rotor like that is about CG and the fact that he was able to get to transition with just a few lines of code, and on the maiden no less. 👏 hats off to him. Im cheating by comparison as i plan to use a good FC and run adrupilot so all the code has been done by people far better at that than myself.

Ive been using mission planner for a while and the painless360 channels videos were a great help. I'm happy to see that Hes doing a VTOL series now.

Obviously when i get to the point I'm putting expensive equipment in it I will have proper yaw control. I plan to apply this thought experiment to the initial test build as I'm going to try to keep it in the sub 250 range, so fewer moving parts would be a great help.

I'll do a build thread on this project when i get to it later this summer.

He has a better and later version of a plane where VTOL that can even handle the (cross)winds. But if your goal is to keep it below 250, good luck.
You will handle problems like vibration, stress on servos and structural(bouncing when landing) problems plus tuning(crashes).

I would suggest before you try, practice the FC tuning so you know what how and what to do before, otherwise you will learn by crashes. If you do the printing approach, get your tolerances close for the gearing arrangements. Weight is going to be your problem.
 

Hondo76251

Legendary member
He has a better and later version of a plane where VTOL that can even handle the (cross)winds.

Yeah, id seen his videos before but hadn't noticed that this version was run by a single servo running the tilt on both motors. It obviously has limitations but it answers the question of hovering with just a rudder.

But if your goal is to keep it below 250, good luck.

Ever the optimistic cheerleader you are @L Edge ! 😂

Like i said, still in the napkin doodling phase, but i believe the sub 250 attempt will be done with parts from my Lidl glider scrap pile. Ive been crashing the snot out of them for years doing sub 250 FPV stuff. Seems like a decent place to start.

I would suggest before you try, practice the FC tuning so you know what how and what to do before, otherwise you will learn by crashes. If you do the printing approach, get your tolerances close for the gearing arrangements. Weight is going to be your problem.

I'm pretty familiar with using FCs at this point. I use my stuff on the ranch to check pastures, water, fences, bogs, look for strays etc...
I got into long range stuff some time ago (back when that was still kosher) and, though i typlically build fixed wing stuff, i have quads that i use as well. Like i said, I'm a big fan of using ardupilot. It is such an impressive program. You can do anything with it it seems.

I've been researching VTOL for a while and ive been tempted to buy one now that there are several airframes available at a reasonable price, and i still might, but I really want to build my own.

The advantages for this type of work with UAVs that are VTOL capable are too many to ignore. Rather than compromising the wings profile and building more like a STOL craft you can build for low drag speed and endurance and use VTOL rather than having to land at mach 3 with expensive equipment on board.
 
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