This has always been of great interest to me so recently I built a series of mid-motor deltas to settle the issue. One design has a slot which is barely big enough to clear the prop. It produces a loud (but cool) noise and requires 8 of 16 throttle clicks to stay aloft. Another has nothing behind the prop and bigger space in front of and around the slot (like Jacko). It is almost dead silent and runs on 6 clicks. A couple in-between configs perform as expected.
Noise requires energy which is robbed from the prop. I also found this to be true for a full size pusher ultralight aircraft I fly which makes a lot of noise when the prop is nearer the TE and requires noticeably more throttle.
Any vibration or noise from things like unbalanced engine/redrive or badly tracked props take away from thrust. More than you might think.
Originally Posted by leadfeather
Has any one done or know of any research on the slot for a "prop in slot" configuration.
What I'm interested in is minimizing the thrust reduction due to the slot, and also minimizing the noise caused by the slot. My guess is that a wider slot with more clearance behind the prop might be better. I also would guess that shaping the aft edge of the slot might help too.
Any data, research or theories out there?