For now I just ordered a set of the larger engine mount plates from Rusty (along with another mini quad frame
). That will get the motors mounted and allow me to use a brace between the top and bottom without having to jury rig anything to do it. I don't mind using the X mount, that's how I have the current motors mounted. The advantage is you can install the motor mount plates and put the motors on afterward whereas it would be pretty much impossible to do that when using just the tapped holes on the bottom of the motor.I've seen the discussion on RCG about the various shapes of the arms, I look at it this way, on a Y6 with big motors and props the airflow over 3 1/2 inch square arms isn't going to amount to a huge issue. If it was a flat hex or octo, then yeah, maybe I'd think more about using round rather than square but probably still use square. The decision to use the 1/2 inch square aluminum was one of practicality, first I already have a bunch I bought a while back, and second, since this frame is to be used for APV I want to be assured the motors are going to stay as straight and square to the frame and each other as possible to make it fly as smooth as possible. I've found with round arms, no matter what you do to locate and secure them, there's always potential and a high probablilty that they're going to be off square by some amount. I think that is going to be more of an issue than the shape of the tubing for the intended use, if a motor isn't perfectly straight its going to be thrusting at an angle which ultimately will have to be compensated for by the flight controller. Have several of them not straight and the problem is magnified, and that's super easy to do with round tubing. I see it even on my Droidworx frame, the motor mount assembly is not pinned perfectly in place and can move to either side a little bit. Bang the motor or mount loading it onto the car and that motor is likely to be off square on the next flight. On Rustys frame compound the problem by using an arm material like carbon fiber where you really can't crank down on the bolts to try and keep the mount from moving and not having any kind of a through bolt to keep things in line and now you need to be continually checking the motors to make sure they're lined up properly, I see that continually on my mini quad with the round C/F arms. In the case of the mini its not that big of a problem as its better for the motor to twist when it hits the dirt and absorb some energy that otherwise might bend or break an arm, not a feature I need on a larger APV craft.
Is all that worth worrying about the aerodynamic difference between a 1/2 inch square tube vs. a 12 MM round? Not to me, I'll accept a little extra turbulence and probable loss of a tiny bit of motor efficiency to not have to be constantly checking motor alignment. Should it turn out to be a major issue, I'll get some really thin aluminum sheet and make some airfoil shaped sleeves to fit over the square arms, though I doubt the difference it would make in the recorded video would make it worth the effort, a bit of post process stabilization if required is a lot less work IMO. For larger sizes of tubing or square arms that might be a different matter, but the 1/2 inch isn't that much different than the 10mm square arms on my standard MK Hexa and it doesn't seem to affect the MK much if at all.
Ken
