Prop diameter to fuse diameter relationship?

[Pierre Audette]

Is there a rule of thumb as to the relationship, ratio or overhang a prop needs to be when the nose of the plane is wide and blunt? I'm always amazed on how can a plane like the GeeBee actually gets airborne.

[Quacker]

>1:1 ?

[steve lewin]

I'm sure I remember reading somewhere that a prop makes the vast majority of its thrust from the outer 25% of the blade. That suggests that you don't need much more than that overhanging e.g. 6" dia fuse, 8" prop. I could have the percentage wrong but it was around there and I'm sure someone will have the right numbers.

Steve

[Haldor]

Most of the pull from a prop comes from the low pressure area infront of the blade. The area behind the prop only need to be enough to get the air away.

Thrust comes from lift generated by the propblades and the acceleration of air molecules, all these events happen in front of the prop and at the propdisk itself. No work is generated behind the prop disk.

[Pierre Audette]

I can understand that much of the work happens up front, but there's a lot of drag behind that works against it. I guess a flat nose should have rounded edges to minimize that drag. I would also think that blade design at the tip could be a factor.

[Haldor]

Everything play together, as long as the fuselage shape do not restrict "exhaust" airflow too much it will work.

[Tim Green]

If you want to find out, do an experiment with one of your electric motors, a prop, and some type of stiff, thin material - perhaps a sheet of really stiff cardboard.

Make a disc from the sheet of stiff material, which has a larger diameter than the prop. Mount it between the prop and the gearbox or motor. It would probably be best if it didn't spin.

Hang on to the motor, turn it on, and see what kind of pull you get, compared to when there's no disc right behind the prop.

Experiment more, by extending the space between the prop blades and the material - and note the change in the pull as the distance changes.