Prop in fuselage slot?

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BBerson

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Anyone have any documents or info on effects of a prop operating in a pusher fuselage slot?
I see this thread about a slot in wing Slot in wing for pusher prop??? but a wing has a lift factor, not the case for a narrow fuselage. I imagine a slot in a narrow fuselage would be less problematic. I found some slot threads on RC forums. But wondering about NACA type data.

I am familiar with the fuselage slot work of Fritz Raab and HB in Austria: https://en.m.wikipedia.org/wiki/Brditschka_HB-3
 
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bmcj

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I suspect that it will be no different than the slot in wing... it has lift-induced vertical flow (even more than slot in the wing because the air is finally able to spill off the wing into the down wash), and it has structure behind the disc to block (or possibly straighten) the flow. It would inherit the negatives from both pusher and tractor configurations.
 

BBerson

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I suspect that it will be no different than the slot in wing... it has lift-induced vertical flow (even more than slot in the wing because the air is finally able to spill off the wing into the down wash), and it has structure behind the disc to block (or possibly straighten) the flow. It would inherit the negatives from both pusher and tractor configurations.
I think placing the wing below the prop would fix that. Sort of like the Custer channel.
Wondering about the drag penalty of the slot.
 
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Dan Thomas

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Vibration from the prop blast would be severe and the structure would need to be designed for it. The extra weight might offset any gains.
 

cluttonfred

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My key conclusion from the "pseudo-jet" thread was that I wanted to avoid long prop shafts subject to torsional vibration issues or cutting slots in wings or tail surfaces. I still think it would be fun to have a jet look and Cub performance but I I'd go with the "profile scale" solution with essentially a single boom fuselage.
 

BBerson

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Vibration from the prop blast would be severe and the structure would need to be designed for it. The extra weight might offset any gains.
Right. But fabric is flexible, AC 43.13 only requires extra stitching in the prop blast zone.
 

BBerson

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My key conclusion from the "pseudo-jet" thread was that I wanted to avoid long prop shafts subject to torsional vibration issues or cutting slots in wings or tail surfaces. I still think it would be fun to have a jet look and Cub performance but I I'd go with the "profile scale" solution with essentially a single boom fuselage.
Right. The prop needs to be direct drive and forward of the tail.
 

cluttonfred

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I didn't want to go with an actual structural connection like the Raab types, you could definitely play with a hollow cylindrical "female" spinner and a spinner-like "male" fairing on the fin (or vice-versa) to give the illusion of a connection. That and narrowing the slot would help, but I don't know at what point the slot starts to significantly degrade prop effectiveness.
 

BBerson

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but I don't know at what point the slot starts to significantly degrade prop effectiveness.
Right. That's why I wanted more technical details. (but may have to experiment)
It's really only "half" a slot, since the prop spins freely in clean air above and most of the circumference.
Also, an after body would certainly increase wetted area drag. But it also increases directional stability.
An after body behind the prop might actually reduce separation drag.
 

cluttonfred

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Something like my sketch above is always going to be a less-than-optimal configuration, but I think it could still be popular if affordable and easy to build and fly. The core of the plane would be a fuselage cage surrounding the occupants and providing a roll bar, engine sub-mount (to which various engines and mounts can bolt), and attachment points for the seats, harnesses, wings, landing gear and boom. The whole thing could be less than 8' long. Make it simple: straight parallel sides, vertical bulkheads, prewelded steel tube sections bolted together or maybe bolted/riveted aluminum tube or sheet aluminum with pulled rivets. Add a simple truss or sheet aluminum boom (too many designs have been rendered obsolete because a particular boom or spar extrusion was no longer available). For the wings and tail surfaces, I'd go with a simple aluminum structure covered in fabric. The wingtips, nose cone, engine cowling, etc. would dress it up. I would aim for a "generic advanced jet trainer" look rather than a replica of any specific plane, only the color scheme need change to go from Blue Angels to Frecce Tricolori to Vietnam-era F-5 to pseudo-MiG.
 

BBerson

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I am not looking for the jet look. Rather I want the typical fishlike sailplane look but with an engine placed somewhere other than the front, and without the typical pusher pod and stick boom look.
 

BBerson

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Have you considered an asymmetric
Not really. I like symmetry. And I think the engine is best set inside the fuselage for low drag.
I suppose I could put a single tailboom offset to one side? But that would look very odd....

I guess your suggestion is to put the engine in a pod like a Lake Amphib, but on the side.
The problem is the pod is extra drag.
A typical tractor like a C-150 has a teardrop body fairing around and behind the engine.
A Vari-Eze has the fairing in front of the engine but nothing behind.
A Molt Taylor Imp or BD-5 has both ends faired in nicely. That's ideal but requires a shaft.
The idea is to get a fairing in front and back of the prop as big as the engine without the long shaft.
 
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rotax618

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A Prop in Slot shouldn’t be too radical, the tall tail Gyros suggest that control surfaces work fine behind the prop, without excessive vibration or noise. Mind you, pushers are always noisier than tractors.
My experience with the Boorabee suggests that 2 and 4 bladed props should be avoided where the prop is near to the trailing edge of the wing, there are unusual harmonics and increased vibration as 2 blades cut through the wake of the wing at the same time.
This aircraft didn’t suffer from excessive noise or vibration in spite of having only 2 bladed prop.E306F35B-0E91-4921-8EB4-9B7F0374E555.png53F882CD-CA1C-46C5-93D4-C04B540A6696.png
 

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