I did it to my ElectroHub because there was too much effect on pitch/roll when yawing heavily. I have 10" props, which I suspect is part of the problem. I think smaller props would spin up/down faster and work better, but I like the efficiency of the larger props.
It worked great. Yaw performance was better. But if you use the FT tilt mounts, the motors will angle both inwards and forwards, because the booms extend diagonally from the center-line. The effect is that the copter hovers slightly nose-up. A more subtle effect is that yaw inputs induce forward motion. So if you are hovering and you spin in a circle, the copter will actually want to move in a spiral. This actually can be nice in FPV flight. I found it to require less control input to maintain a coordinated turn with the angled X config, compared to a configuration with a T-boom for the rear motors, which causes them to be angled only inwards, not forwards. If you think about how you do a turn, as you yaw, you also have to roll and tilt the copter into the turn. With the angled-forward motors, you basically only had to roll, unless the turn was very sharp. Ultimately, though, I switched to a T-style boom, because I wanted to isolate the inputs from each other. I didn't like that when I spun the quad around during hover, it would start to drift.
I prefer an A-tail configuration because it keeps the motor logic on yaw the same. With the default MultiWii motor directions, if you want to yaw to the right, the back left and front right motors spin up, and the other two spin down. If you think about the alignment of the motors in an A-tail quad, spinning up the back left motor yaws the quad to the right. If you think about a V-tail quad, the opposite is true. So with an A-tail quad, all you have to do is adjust the motor gains, but you don't have to reverse any motor wires or motor/prop directions or anything like that.
Some resetting of your motor mixes is appropriate. First, calculate the decrease in downward thrust do to the angle, and add that back in to the ptich gains on your rear motors. So you might have 100% pitch gain on the front motors, and 120% pitch gain on the rear motors.
Second, set the yaw gain on the rear motors to 100%, and decrease the yaw gain on the front motors. This will let the tilted thrust do most of the yaw work, and rely on the motor torque less. This is good because using motor torque to yaw can introduce unwanted pitch/roll effects, especially on slower motors with larger props. I found that I could mix about 25% to 35% yaw in to the front motors without producing unwanted effects.
Third, decide how you want to handle roll. With tilted motors, yaw inputs will also induce roll, and roll inputs will induce yaw. What can happen is that when you yaw, the copter starts to roll, so the FC spins up the opposing motors to counteract the roll. When the opposing rear motor spins up (to counteract the roll) it also fights the yaw that you were trying to accomplish. You either get the motors fighting each other, which reduces efficiency, or you get slower yaw performance, which is the whole reason you went to a V-tail in the first place. So what I suggest is putting 100% roll gains on the front motors and 0% roll gains on the rear motors. That way when the FC "fixes" the unwanted roll, it's only using the front motors, which have less effect on torque.
so the question is, why do you want angled arms on a spider ?
My ElectroHub is around 450 size, but it's no "camera ship". I fly it fast proximity FPV. I put angled arms on it because yaw performance was poor in the flat configuration. If I had it all to do over again, I would build a 250 mini quad, but I didn't, so I fly what I've got. If it was a camera ship, then I would agree with you. A camera ship should not be doing fast yaw at all. It should be flown slow and steady and smooth. That being said, I was having some problems with stability even under relatively moderate yaw, and going to an A-tail setup fixed it.
