How about a UL Buhl Pup replica?

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Tiger Tim

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I’m not sure anything will come of it, but I enjoyed a previous UL design thread I started for my own education and I want to do it again. At this point I’m just looking to learn things and spend some time without spending money on layovers at work. I figure in this thread all my books will once again be open to all in case anyone wants to double check my figures or take the ball and run in their own direction with it.

So why a Buhl Pup? Well, I like them and I think one would be neat. There are projects out there that could probably be made available but I want to design something so why not? It’s a classic design, looks simple enough, and as a bonus it’s one of the only mid or low wing designs from the era that has anything resembling a rollover structure.

1652729663881.jpeg

I want to start by studying the similarities and differences between other existing airplanes with that rough configuration and performance to see if the thing even looks feasible. I don’t have Raymer’s book in my hands right this instant but IIRC that’s where he starts. Call it benchmarking, a feasibility study, or simply a reality check. I’ve started a table in my notebook covering various parameters of the original Buhl Pup, Minimax UL (successful braced shoulder wing), Hummel Ultra Cruiser (has that comparable aluminum can fuselage), output of the FAA 103 Appendices (to check the honesty of other ULs studied), and my own goals.

Are there any other comparable airplanes that should be added to that list? I seem to remember Matt posting a light, reasonably modern French(?) plane a little while back that was a wire braced midwing but can’t find it again. Anyways don’t be afraid to post anything in this thread, it’s all for learning after all.
 

TFF

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Similar ideas are the Church mid wing and Heath mid wing. It’s not middle but a Flybaby is the same idea. Move wing and wires up and you get the same thing.
 

Victor Bravo

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HTF do you climb into that thing? Is there a trap door on the bottom? Do you have to crawl up the side of that smooth plywood skinned fuselage behind the wing? Do you step up on the tire and then strain yourself through the landing wires like a 'cheese slicer'???
 

BJC

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I love the Buhl Pup. Start looking for a Verner that someone has now, because you know that you will want to build it.

A configuration copy could be lighter if it uses stringers and thinner sheet metal for the fuselage.

The V-Witt has a wire braced wing, but the upper attach point is enclosed in the fuselage. Check out Witt’s spar-to-fuselage connection.


BJC
 

Wanttaja

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It’s not middle but a Flybaby is the same idea. Move wing and wires up and you get the same thing.
A bit of a sketching exercise from long ago....
mid-wing Fly Baby.JPG
The bulkheads that the Fly Baby wing spars attach to are parallel, so, structurally, one could mount the wing at any distance from the bottom. Structurally, this is pretty good...the flying wires under the wings attach at a lot steeper angle, and thus support better.

Three problems exist with this concept. First, like the drawing shows, one would need a kingpost above the fuselage to attach the landing wires. Not a major issue; The fittings to hold the cabane struts for the Fly Baby biplane would work nicely.

Second, as has been mentioned, access to the cockpit will be difficult. Steps would have to be added to the fuselage, as well as hand-holds. Possibly, the steps could be put in FRONT of the wing, and crossbars added to the kingpost as handholds.

Third, the Fly Baby has pushrod-driven ailerons. This setup has the aileron pushrods in front of the pilot's chest; much more difficult to interface with the control stick. One could modify the wing to have the aileron control come from below (even as cables) but that loses the attraction of using nearly-stock wings.

Ron Wanttaja
 

Bill-Higdon

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HTF do you climb into that thing? Is there a trap door on the bottom? Do you have to crawl up the side of that smooth plywood skinned fuselage behind the wing? Do you step up on the tire and then strain yourself through the landing wires like a 'cheese slicer'???
Plywood, Plywood, Plywood, We don't need no stinking Plywood. The Buhl Pup was a monocoupe aluminum fuselage
 

Tiger Tim

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Can we get one on floats?
You can get one on whatever gear you want as long as you build it yourself.

HTF do you climb into that thing?
I have no idea. Must be a step or something on the fuselage behind the wing. How do you get into a Minimax?

Do you have to crawl up the side of that smooth plywood skinned fuselage behind the wing?
Plywood? Believe it or not that fuselage is metal, as is the vertical fin. Kind of an elegant way to build a light airplane, or so it seems. That’s why I’m kind of keeping one eye pointed to however Hummel builds a fuselage.

Would be sweet with a Verner three-banger.
That was the hope when this thing first started flying around in my head a few years ago. Nowadays maybe a half-VW for power? I’m kind of hoping whatever the engine of choice is can just bolt up to a nose bulkhead but the exercise of even choosing a power plant is a few steps down the road.

A configuration copy could be lighter if it uses stringers and thinner sheet metal for the fuselage.
That’s kind of where my head is at. Surely we’ve learned things about aluminum construction since the original Pup was designed. Heck, we probably learned orders of magnitude more about aluminum in the first decade after it was designed.

A bit of a sketching exercise from long ago....
View attachment 125571
That’s very cool but a Fly Baby is a lot bigger and boxier than I’m shooting for this time around.
 

TFF

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It could be put at the panel pretty easy. Then again you could put it anywhere and let the pilot contort.
 

Wanttaja

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Except for when the spar feed-thru ends up.
No separate spar feed-thru on a Fly Baby. There's a bulkhead in the front of the cockpit (where the panel is) and the pilot sits with his back against another. The wings pin to these two bulkheads. As the bulkheads are parallel, it's possible to attach the wing at any height. The bulkheads would need to widen the side portions, that's all.

COCKPIT.jpg
Ron Wanttaja
 

Tiger Tim

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Here’s a quick comparison of the real deal against some examples of popular monoplanes in the UL world. I tried crunching some numbers on UL limits so I have a far end to stay within but it must be getting late and my math seemed way off. I’ll get back on it in a day or two. I have to admit I’m blown away by the published useful load of the Ultracruiser.

Buhl PupMinimax
Max-103
Hummel
Ultracruiser
AC103
limits
Mine
BEW (lbs)550250252254254
Load (lbs)300250323251
MTOW (lbs)850500575500
Fuel (gal)105555
Stall (mph)26282727
Cruise (mph)785563
Max (mph)98636363
Vne (mph)98?9095
Span (ft)302522.5
Area (ft^2)109.4112.5101
Length (ft)191616
HP45283742
EngineSzekely SR-3Hirth F-33Half VWVernier 3V
Engine
Weight (lbs)
147458581

At this very early stage I’m picturing a UL Buhl Pup as basically a Minimax with an aluminum fuselage. Wing and tail construction is still very much up in the air but I suspect it would be whatever material is lightest within reason. First thing is to come up with the exterior shape and dimensions before filling in the structure… I think.
 

Tiger Tim

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After some thought, I’m going to go with one of Casler’s more base-model half VWs for calculation’s sake while keeping one eye on the three cylinder Verner as a possibility. The half VW comes out at four pounds heavier (who’d have guessed?) and can be spec’d to the same power so call it a wash I guess. Many real Buhl Pups have been re-engined with horizontally opposed four cylinders so a horizontal twin won’t be much of a crime.
1653186756406.jpeg
Close enough.

Next up I guess is to really start thinking about the size of the thing. In my mind the laziest starting place is to lift the areas and moments directly off the Minimax and start working from there to validate it all unless somebody has a better plan.
 
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