v tail quad

[luv]

guys.....i'm trying to make the v tail quad. i saw the video also but i'm confused that the speed control circuit can be replaced by some other circuit? and one more thing that the rpm of the back rotors should be more than the front rotors or not?
please clarify......
thank you

 

[Balu]

Hi luv,

welcome to the forum. I'm going to move this to the multirotor newbie section where probably more people will find it


Yes, you can use different ESCs for a V-tail quad as long as it has enough power for your motor.

You can also use the same motors from front and back. You will just loose a little lifting power. The flight controller will compensate for the less effective back props by slowing down the front motors too.

 

[luv]

okay, i got it. but as the video shows .....the back rotors need more thrust or something like that ( i got little bit confused in the settings of the ese) and if i use different propellers in front and back, will it help?

 

[Craftydan]

Hy Luv,

Welcome to the forum!

V-tails are a bit of a complex beast when you start playing with them. Short answer is I wouldn't worry too much about it -- you can make up for some of the losses by moving your CG a little more forward to give your rear motors a bit more of a lever arm.

Somewhat longer answer is:

Look at the goal of a V-tail as a configuration. No, not "make something wicked cool looking" . . . although it is . . . It's to make a Y/T-copter configuration that uses side-thrust differential from two rear motors to generate yaw, instead of a tilt (traditional T/Y copter) or motor torque differential (the coaxial-tail'ed Y-4 copter). Like any Y/Tcopter, all four motors contribute to pitch, but only the front motors contribute to roll. Like the tilt version, the yaw is handled exclusively in the tail.

The big headache with the V-tail is a nasty roll/yaw coupling that's caused by the tail rotors. If you mix them into roll, as you roll left the left rear slows and right rear speeds up and you end up with a strong yaw (direction will depend if it's a V or A tail -- really annoying when it leans out of a turn).

Fine. So we don't mix them into roll . . . problem solved? not quite -- we've only covered half of the coupling. The same thing will happen when we Yaw . . . one motor slows, another speeds up and boom! Coupled roll. So what do we do about it? Keep it to a minimum, meaning keep rear motors as close together as we can afford . . . but the props get in our way of this.

The boom angle and tilt angle on the V-tail batbone is pretty well fixed by the batbone geometry, so not much you can do to move the motors closer to each other, except moving the motor on the boom . . . farther forward makes them closer together which is better for reducing roll/yaw coupling, but farther back is better for load balancing . . .

If you haven't bought the motors yet, having a higher KV motor on the rear will help you with this balancing act (they can throw a shorter prop with maybe a little more thrust), but if they're all the same, moving the CG forward with a camera or battery will have much of the same effect.

 

[joshuabardwell]

The first thing I would say is don't over-think it. I built the ElectroHub spider quad and flew it for a long time with a standard X-quad configuration in my flight controller, no problem. Then I converted it to V-tail, and again, no problem. The Flight Controller will go a long way towards automatically figuring out what the motors need to do to keep the quad flying properly. You can tweak things to get more performance, but no need to stress at first.

Once you start tweaking, you will be working with the motor mixes or gains, depending on what your flight controller calls them. Assuming you are using the same motors and props on all four arms, you will start with equal gains on all motors, then adjust the gains based on the geometric position of the motors relative to the CG of the craft. This will result in "correct" mixing if the copter was a flat quad instead of a V-tail.

From there, make the following adjustments:

• Add pitch gain to the rear two motors proportional to the degrees of tilt. This can be calculated by taking the SIN of the angle. So for example, a motor with 20 degrees tilt would have an additional SIN(20)=0.34=34% of pitch gain in the rear motors. This will prevent the copter from pitching when you throttle up rapidly. EDIT: See below. I stink at trig.
• Set roll gain in the rear two motors to zero to avoid yaw coupling on roll input. Only the front motors will control roll. Since the rear motors should be as close together as possible, they shouldn't have much roll aut
• Reduce yaw gain in the front two motors to reduce pitch/roll coupling on yaw input. Setting the front motors' yaw gain to zero will minimize coupling, but will result in slower yaw performance. Adding some yaw gain back in to the front motors improves yaw performance. To test this, set your front motors' yaw gain to something like 50% and then hover, and yaw aggressively back and forth. If the copter becomes destabilized in the pitch/roll axis, reduce yaw gain in the front motors. If you have small props and good ESC's (like KISS ESC with regenerative braking) you may be able to run 100% yaw gain in the front motors without a problem.

If you are using different motors/props on the front and rear, then you need to take that into account in the first step, when you get the "correct" mixing for a flat quad, but the subsequent steps are the same.

You can get some of the calculations done for you using this spreadsheet: https://dl.dropboxusercontent.com/u/23394347/quad mix.xlsx

 

[Ace2317]

Like everyone else has said, for now don't overthink it. Just let your control board compensate for you. What everyone means by saying that is that your control board at level is going to feed in a little more throttle to the back two motors than it will the front. Ergo, your top end power is going to be a little lower because it has to limit the front motors a little to keep it level.

Bottom line, don't worry about it for now. You'll eventually tear it up and you can change it then.

 

[joshuabardwell]

Sorry for my confusion above on the trig. I have crunched some examples and I think I have it figured out. The COS gives the percent of thrust going vertically. In that case, you would need to add in additional gain in the amount of 1 - COS(angle), I believe. So for a 20 degree angle, COS = 0.94, so you would add in 6% pitch gain to the rear motors.

Hopefully another smart person on the forum will point out if I have made another mistake.

 

[luv]

thank you everyone for your support and suggestions. I'll try to work it out and tell the outcome.