Proper Motor Angle for V tail Quad?

[mlutze]

I am in the process of building a V-tail. I am inspired by the Mini V tail from Simplecopter. But figure it would just be too easy to buy one, so will be DIYing it. My first try was sized for 5X3 props and had a motor angle of 20 degrees. It flies, but not particularly well, especially at the altitude where I live (about 6000 ft). It hovers at about 70% throttle, and does not have enough vertical acceleration to make me even think of trying flips, and has about the same yaw rate as a waterbed. So I have some 6X4.5 props on the way, but will need to scale up the frame to fit the new props, so figure I should also put in some more motor angle to get it to yaw.

So to get to my question, what have people been using for motor angles? I started with 20 degrees for no particular reason. The flitetest angle motor mounts are set at 40 degrees, so is that the correct angle? Is there such a thing as a correct angle?

Any words of wisdom would be very appreciated!

Thanks

 

[xuzme720]

You want a nice balance of lift and yaw. More angle will give you more yaw. Less angle will create more lift. You need to tune to your liking and given your altitude, I'd tend to go a bit flatter to start than 40. Maybe try 30-35 and see how that does. But truly there is no "correct" angle...

 

[joshuabardwell]

I liked 20 degrees myself.

 

[Balu]

Most Vtails quads I have seen have something around 30ish degrees.

 

[Craftydan]

I'm with Josh.

I run 20 on my big V-tail (technically A-tail). I loose 6% upward thrust and gain GOBS of yaw. At around 500mm, she's a nimble monster . . . haven't taken her out flying recently . . . I really should.

 

[joshuabardwell]

I run 20 on my big V-tail (technically A-tail). I loose 6% upward thrust and gain GOBS of yaw. At around 500mm, she's a nimble monster . . . haven't taken her out flying recently . . . I really should.

Yeah. That's the thing--at 20%, I gained more than enough yaw, so why go further? It's not like we're trying to spin in circles here. Just want "enough". I also think A-tail is the best choice if you're going with standard (at least for MultiWii) motor directions, because it means that yaw torque is working with thrust instead of against it.

 

[Tritium]

25% is what my Armattan V-Tail is set to.

Thurmond

 

[joshuabardwell]

BTW, if you are going to do a V-tail or A-tail, I recommend customizing motor mixes (if your FC's V-tail or A-tail mix doesn't already do this automatically). Here's what I suggest:

Yaw - 100% in rear motors. For A-tail, 25% in front motors, since torque is working with vectored thrust. For V-tail, 0% in front motors since torque is working against vectored thrust.

Pitch - 100% in all motors. If you want to split hairs, 106% in rear motors (assuming 20% angle), to make up for the loss of thrust to the angle mount, but the difference is small enough that the FC can likely handle it, and the ultimate effect will be that the quad will pivot around a point slightly off of its CG, which hardly matters for this small of a difference.

Roll - 100% in front motors, 0% in rear motors. The issue here is that when you yaw, you will also slightly induce roll. If roll is mixed into the rear motors, then when you yaw, one rear motor spins up to push you around in yaw, which induces roll, which causes the other rear motor to spin up to counteract the roll, but this also counteracts the yaw. So the rear motors end up "fighting" a little bit, which compromises efficiency and yaw performance. By putting roll only on the front motors, there is greatly reduced coupling between yaw and roll.

 

[mlutze]

Very interesting discussion. Thank You very much.

I am not quite sure I fully understand the advantage of A vs V tail. I had thought that with a V tail the prop wash would be out away from the craft and give cleaner air. While with an A tail the motors would be 'blowing' into each other and cause more turbulence. Am I off completely off base with this?

Also I am using a Flip-32 with cleanflight. Do the A/V tail setups work well? Or will I need to customize the mix?

 

[joshuabardwell]

I am not quite sure I fully understand the advantage of A vs V tail. I had thought that with a V tail the prop wash would be out away from the craft and give cleaner air. While with an A tail the motors would be 'blowing' into each other and cause more turbulence. Am I off completely off base with this?

Imagine that you have a regular flat quad. It produces yaw due to the torque created by the motors. Every action has an equal and opposite reaction, so when a prop spins CW, it produces CCW torque. By spinning the two CW motors faster and the two CCW motors slower, the FC can yaw the quad CCW, and vice versa.

With the standard MultiWiii/Naze motor directions on a flat quad, in order to yaw to the right, the front-right and back-left motors spin faster, and vice versa. Think about an A-tail now. In order to yaw right, which of the rear motors spins faster? The back-left. Now think about a V-tail--it's the opposite. So one advantage of an A-tail is that the thrust vector is working with the motor torque instead of against it. With a V-tail, the thrust vector can and will overcome the motor torque, but it creates some inefficiency. The motor has to spin faster and use more energy to create a desired yaw rate.

Of course, an industrious person could switch the motor directions on their V-tail to fix this, but most people won't. So one nice thing about an A-tail is that you can run it using a standard Quad-X motor mix and you will get better performance than a V-tail under the same circumstances. Bear in mind also that if the FC has a proper A-tail or V-tail mix, it may take all of this into account. I don't have personal experience with the Naze32 mixes, so I can't speak to that.

If we assume that the FC is mixing the motors optimally, then the question becomes whether it's better to "cross the streams" on the intake side of the motors (V-tail) or on the output side of the motors (A-tail). I don't have any data to this effect, but my gut instinct is that it doesn't matter. It makes sense that an A-tail will have more turbulent air on the under-side of the props, where the streams are colliding with each other, but it's hard to see how that would affect flight--especially when the quad is moving forward, away from the turbulent air. The other thing to keep in mind is that the air stream exists on both sides of the prop. It's easy to imagine the interaction between the outgoing air in the thrust column, but the exact same interaction is occurring on the intake side. For every unit of air that comes out of the prop on the bottom, a unit of air has to come into the prop on the top. So an argument could be made that it's better to have turbulent air on the output side of the prop than the input side, since at least on the output side, the air is not interacting with the lifting surface. But I'm no aerospace engineer... I'm just a dog on the Internet.

 

[mlutze]

Very good explanation, makes sense now. You might be a dog on the internet, but you are a clever one certainly.

 

[joshuabardwell]

Very good explanation, makes sense now. You might be a dog on the internet, but you are a clever one certainly.

Well, that's the danger, and why I warn you. It's easy to ignore the blatherings of an idiot. But a clever dog might fool you into thinking he's smart.

 

[mlutze]

I guess that I will need to test then. So I will make some sawdust, and perhaps a frame as well. A tail and I think 25 degree motor pitch.

 

[mlutze]

So without a doubt I am a convert to the A-tail! The maiden today was much more responsive then the V-tail configuration. Need to get the 6inch props on, and spend some time tuning PIDs. But I am very happy with the feel.

Just a note to anyone using cleanflight with an A-tail. The standard V tail and A tail mixes are actually the same. But the motor ordering is swapped for the tail motors. So be careful when you setup the A-tail since it does not use the standard as the rest of the models.

 

[joshuabardwell]

For an A-tail mix, one approach is to simply use a Quad-X mix, and then adjust the motor gains manually. That's the approach I used in MultiWii.