prop distance from fuse

[dalbert02]

I am scratch building a twin boom pusher style airplane. I was curious what the minimum distance from the prop to the fire wall should be. My fuse is a 4" square. I would like to use an 11 or 12" by 6 or 7" prop. I would assume the closer the prop is to the fuse, the less effecient it would be, but are there any hard and fast rules to design by? Thanks,
-dave

[MarkusN]

You can pretty much forget the area shaded by the firewall for efficiency, regardless of distance.

a thing to consider is the over-swing of the prop blades, if you are using a folding prop. Maybe think about hard stops for the blades.

And noise may be a concern; pusher props behind obstacles are loud; distance can affect this. If you are concerned about this a few test may be in order.

[eflightray]

As MarkusN says. I believe the center area of a prop doesn't do a lot of work anyway, some people have used very big spinners on scale models with little loss of power.

Certainly try to keep the prop out away from the fuz, and if possible do something like the picture below on the right. It wont particularly improve things much other than looks.

Don't forget to include adequate cooling for the motor.

How about more details of the model?


Edit - forgot the picture, do a page refresh if it isn't showing.

[Brandano]

If you want o reduce the noise from the prop make sure that the wake of the wing won't strike both blades at the same time. If this isn't possible try using a 3 bladed prop instead. The tail rotor of the Apache is an interesting adaptation to reduce noise, as is the odd blade spacing on the Eurocopter EC135 fenestron. I have seen one of these take off, and it's amazing how quiet it is if you compare it to older designs.

[dalbert02]

Thanks. I will try to incorporate the tips! :)
-dave