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Best fuselage shape

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Eugene

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Now Eugene, on to your problem. You have a strong taper going into the prop inherent to the layout. This tends to cause separation that is not only draggy by lowers prop efficiency a bunch too. Ouch, Ouch!

Billski
Thank you. I need some time to process this information and I will come up with many questions. But all my questions I need to back up with pictures, so there's no confusion in my head. I am sure this is like a walk in the park for most of you guys, but not for this Russian. I need my pictures.

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The way I understood, that part of fuselage under the wing in front of Propellers is under constant vacuum during cruise and with a few vortex generators = problem is fixed.

Picture below is the shape, cross section right under the wing. I was told that this is almost acceptable.

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Picture below, is cross-section of fuselage at propeller tip. I was told that it doesn't really look that bad after VGs installed for 80-100 MPH cruise. Wrong?

IMG_2628.jpeg

The bottom part of fuselage really need help and there's no questions about it. One time I was thinking about different shapes. This is one of my pictures below

IMG_3591.jpeg
 

wsimpso1

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I still do not understand why you think fattening the shape aft of the prop more than is needed for structure has any use. Please tell us what this is supposed to do.

Billski
 

Eugene

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I still do not understand why you think fattening the shape aft of the prop more than is needed for structure has any use. Please tell us what this is supposed to do.

Billski
It was explained to me that I should keep fuselage as wide as possible all the way to the prop line to prevent airflow from the bottom to be pooled up by propeller. Peter Garrison agreed with that.

How to finish this tail boom is big question. How to go from 48 inches to 6 with no separation? As soon as I start flying again will do more tuft testing. Picture below is attempt to finish it as "flying wing". Peter was telling me that every cross section should look like symmetrical airfoil.

IMG_3579.jpeg
 

Eugene

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In your case it is the shape from the bulkhead at the seat back to the upright just in front of the prop that is critical to drag and airflow into the prop and over the tail surfaces. A side by side pusher is the worst possible arrangement and I don’t think that there is any magic shape that will produce a smooth airflow around that steep taper.
So, tuft testing picture below after VGs installation doesn't really telling us that problem is fixed?

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Eugene

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Drag is more than just skin friction or planes would be perfect Spheres
The faster you go the more elongated the body should be in the axis of flight.


It seems like there should be a simple graph describing the shape based on the maximum width and speed requirements.
Alternatively the minimum enclosed volume and the maximum speed.

Not being an expert on these things I can’t say for sure but I really suspect there will be no concave curves in the optimum shape.
Yes, thank you! Before we start talking about this I was for sure thinking that fuselage shape directly related to aircraft design speed, but I didn't find support for this in my books. Something like 3:1 for 80 MPH, 4:1 for 160 MPH, 5:1 for 300 MPH
 

Eugene

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I still do not understand why you think fattening the shape aft of the prop more than is needed for structure has any use. Please tell us what this is supposed to do.

Billski
In my mind supposed to provide gradual transition from very wide fuselage to the tail pipe small diameter

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Pilot-34

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Another thing to keep in mind is how long and arm you need for the tale to be effective?
I suspect a nice smooth Cone shape all the way back is probably superior to a convex shape and tiny tail boom even though it may offer slightly more wetted surface.
I’m sure someone will be a long to explain the details of the trade-offs soon
 

Pilot-34

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My intuitive thought is a nice ice cream cone shape is the most aerodynamically efficient.
The question for me is how pointed should the ice cream be?

People are saying that the body should be vertical from the widest point to the bottom of the wing ?
 

TFF

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The width should be no wider than the cabin. Your problem is engine behind. That limits much of what you can do and make sense. Your direction with the covered fairing is probably the best reasonable way to go. Have you tuffed the nose? The nose may be improved.
 

Riggerrob

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Definitely read through Strojnik's books.
I have one that I will cheerfully send you for the cost of mailing.

Also take a look at Janowski's pod and boom pushers. Only his first J-1 had high wing. All Janoski's later airplanes had mid or low wings.
A frequent problem - with high wing pushers - is balance. In order to balance, the propeller often needs to be forward of the trailing edge. This forces designers to cut out a chunk of trailing edge or sweep the wings aft (later Spencer Air Car) or install a drive shaft extension (Republic Seabee). Another solutioin to balance is installing the front pilot's seat a long way forward: Curtiss Pusher, Woody's Pusher and Air Cam.
One advantage of a low wing and bottom boom is the ability to overlap the prop with the wings' trailing edge. An extreme example is the 193 vintage Luton Buzzard low-wing, low boom ultralight airplane.

The other problem is aerodynamic. How do you taper from a two-person wide bulkhead (48") to a slim trailing edge in a mere 96")? That is always a challenge on small pushers. Glass Goose had problems with random yaw until they installed vortex generators to equalize airflow on both sides of the aft cabin. Vortex generators always a poor "fix" to a problem that could not be solved by more conventional aerodynamics.
 
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crusty old aviator

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1:3 is the most efficient ratio in fluid dynamics (unless you throw in the psi principle, fibonacci, etc., but that’s for Jay at Pax Scientific to explain). That’s why the Questair and the Strojnik and Kolb designs are so efficient.

Watch Mike Arnold’s video, Why it Goes so Fast, on youtube, and you will gain a good understanding of nozzle effect, etc.
 

Eugene

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After reading to all replies I start thinking little different. Thank you. Sorry for being so slow. English is my third language and you’re talking altogether different English here. So it is number four for me.

I am looking at the picture below and thinking, that maybe this was a mistake to make afterbody as wide as possible all the way to propeller line. Maybe I should look at fuselage and landing gear separately. After all fuselage itself is only 48 inches wide, but landing gear outside dimension is 64 inches. I believe this is what Peter Garrison was trying to tell me, but I did listen. Like I always do. He was trying to tell me that landing gear part that in red, should be finished as symmetrical Airfoils. Maybe if I try to go this road would be easier to create something much more acceptable.

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Eugene

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Have you tuffed the nose? The nose may be improved.
I asked this question already and I was told that it would be a waste of time. I was told that nose look fine. And who I am to argue? Every time when I am talking to someone who actually designed and build at least one airplane, I feel very small and very stupid in comparison to him.

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henryk

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cross-section of fuselage at propeller tip
-do you know Goldsmied fuselage drag reduction ?


 
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Eugene

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Stoped by airport today and did some measuring just for fun. With aircraft in level flight position (bottom of the wing level) at prop tip line fuselage is 40 inches. From the door (widest point) to propeller = 57 inches and 50 inches to the back side of fuselage. So, pretty abrupt change.

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