Tight Slot

Inq

Elite member
I wrecked my Mini F-22 and want to make another. I'll look into doing the larger version. Anyway, the wing slot for the propeller is pretty large - ~6.75" x ~1.5". I was wondering if anyone had tried trimming the slot to just slightly larger than the propeller??? Say 1/10" around it. Something like this image shows... instead of cutting the full rectangle, only cut around the blade... keeping the yellow portion in foamboard also. Would it reduce the thrust or increase the thrust? My logic doesn't help me - (1) being close interferes and hurts thrust OR (2) being close straitens out the flow sooner and increases thrust. Any thoughts?


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luvmy40

Elite member
I could be wrong, but the slot size has more to do with noise than efficiency. I think there is what's called, or referred to as a bow tie slot to make things even quieter.
 

Inq

Elite member
I could be wrong, but the slot size has more to do with noise than efficiency. I think there is what's called, or referred to as a bow tie slot to make things even quieter.

I was just looking at the larger F-22 FT version. Is this what you're talking about? I was wondering. So... it makes it quieter?

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luvmy40

Elite member
Evidently. I've read this. No first hand experience. My single FT F22 has never left the Bat Cave to date.
 
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Tench745

Master member
The bow-tie slot like on the F-22 helps reduce noise. The area of the prop from about mid-span of the blade out towards the tips is what moves the most air and has the most pressure differential between the front and rear faces of the prop. If the slot is too close to the blade the pressure zones around the prop will collide with the structure causing a lot of noise. And we all know noise is a form of energy; which has to come from somewhere. It eats at efficiency heavily.
Another point to make, the underside of the plane has a high pressure zone and the top is low-pressure. These two pressures will try to equalize through the prop slot, causing turbulence, which hurts efficiency. But, if the slot isn't big enough to allow room for this pressure differential to equalize a little, one of the propeller blades will be smashing into the high pressure while another swings through the low, but when both blades are in line with the wing, they're seeing roughly equal load for a split second. This means there is a cyclic loading on the motor and prop which will cause more noise and vibrations. Having the propeller spinning in the cleanest air you can will be the most efficient.
 

Merv

Site Moderator
Staff member
...Would it reduce the thrust or increase the thrust?...Any thoughts...
I agree with luvmy, a narrow slot will likely be more noise.
But more noise equals less power going to thrust. You may notice the noise, I doubt you will notice any change in thrust. It may lead to slightly shorter flight time.
 
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Inq

Elite member
I agree with luvmy, a narrow slot will likely be more noise.
But more noise equals less power going to thrust. You may notice the noise, I doubt you will notice any change in thrust. It may lead to slightly shorter flight time.

I did note that the F-22 was far noisier than the Storch. And that is a patently perfect observation... Conservation of energy. If energy is going into making noise, it certainly isn't going into making thrust. - Brilliant!

Bowtie it is - I guess we go formal.
 

Inq

Elite member
The bow-tie slot like on the F-22 helps reduce noise. The area of the prop from about mid-span of the blade out towards the tips is what moves the most air and has the most pressure differential between the front and rear faces of the prop. If the slot is too close to the blade the pressure zones around the prop will collide with the structure causing a lot of noise. And we all know noise is a form of energy; which has to come from somewhere. It eats at efficiency heavily.
Another point to make, the underside of the plane has a high pressure zone and the top is low-pressure. These two pressures will try to equalize through the prop slot, causing turbulence, which hurts efficiency. But, if the slot isn't big enough to allow room for this pressure differential to equalize a little, one of the propeller blades will be smashing into the high pressure while another swings through the low, but when both blades are in line with the wing, they're seeing roughly equal load for a split second. This means there is a cyclic loading on the motor and prop which will cause more noise and vibrations. Having the propeller spinning in the cleanest air you can will be the most efficient.

WOW! This is all intuitively obvious AFTER someone explains it. I am certainly losing brain cells in old age.