I’ve found rules of thumb that suggest this distance should be as much as 1/3 the prop diameter - which much further away - about 80mm. That seems way too large.
I’m curious what sort of thoughts anyone might have about this topic. Of course I’ll give it a try and see.
In particular, I’m curious to know how greater distance from the prop to the body might effect tuning.
BTW - these motors and props are from a Hexsoon EDU-450 - where the distance from the prop to the body is 15mm.
Here is my first Arducopter based multirotor vintage 2014. Mostly collects dust but it still flies. Note the distance to the arm which is fairly wide being a mono-shell.
Decades ago, when I took classes in aerodynamics, I learned that the “work” of a propeller happens when the blade strikes the air. The airflow after that is far less consequential.
In the case of a quad copter there are other factors - such as the effects of the prop wash acting both on the body and from the different propellers. Intuitively, I think greater prop spacing which diminishes prop wash effects is likely a good thing. The question is how much is enough - and what are other possible effects.
My guess is that quadcopters have a short of a “wheelbase” to make it easier to handle the thing on the ground. And longer arms require greater strength and rigidity. But if you didn’t care about ground handling and the arms were sufficient - how much efficiency is gained by having longer arms and where is the point of diminishing return.
I think it’s not so much about the prop itself, but
clearance between the prop tip and base plate or fuselage
clearance between the prop and the arm (and shape of the arm)
Both of these relate more to vibration than anything else, in my opinion.
I would estimate that clearance between the prop tip and fuselage would be nice to be about 10% of the prop size.
So a 10 inch prop should have 1 inch clearance to the fuselage.
I agree. Now, a longer arm would completely change characteristics of the copter, but not due to the clearance of the prop.
A slight overlap of the prop tip over the base is pretty meaningless in terms of power losses, but it could produce vibrations, especially if the vertical clearance between prop tip and the base is reduced. I would expect possible pulsating vibration there, as the tip passes over the base, the tip would compress air between the blade and the base and give an upward pulse on the prop (Z vibe).
So my position is that there could be almost zero horizontal clearance or even a slight overlap if there is some vertical clearance. That is especially true for VTOLs, where the arm length is often complicated.
