How about a UL Buhl Pup replica?

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Bill-Higdon

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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.
View attachment 125728
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.
a"Tin Can"Minimax
 

Tiger Tim

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A question on the AC 103 Appendices: is that all that must be demonstrated for a UL to be legal? Just a set of calculations, some very arbitrary? I’m asking not with the intent to cheat but I do recognize that an awful lot of modern pilots weigh more than 170 pounds and stall speeds go up accordingly, for example.
 

Vigilant1

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From what I've read, the stall speed doesn't >have< to be demonstrated at all, as long as the plane meets the formulaic requirements of Appendix 2.
You can >choose< to demonstrate the stall speed in flight, if you'd like.
 

TFF

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It doesn’t have to be demonstrated unless you are cornered by the FAA. They have nothing to care about, unless you push yourself into that corner. If you flew it into the side of the FSDO, they might care. In the middle of a corn field within no one around, never.
 

challenger_II

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Most Fed's do not want to be bothered by Ultralights. Incur their rath, and they will search until they find an infraction...

What I have observed is they look at fuel capacity, engine size, and wing size. If a plane looks heavy, they will, grudgingly, drag out the scales.
 

Tiger Tim

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It doesn’t have to be demonstrated unless you are cornered by the FAA.
I understand that applying to ground measurable things like weight and fuel capacity but I’m less convinced that can be applied to performance figures. Will they come meet me on an ICAO standard day and do I get to pick the place? Who is responsible for ASI system calibration to ensure position error is eliminated or at least taken into account?
 

TFF

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Nope nope nope. They will not certify that your thing is legal. Your problem. There is a set of rules; you are expected to follow them. End. If you crash one and don’t maim or kill, they will ignore it. They only deal with one if they suspect there is gross oversight. I know if at least three local crashes. No one hurt. When the FAA arrived and they found out they were ultralights, the got back in their cars and left. Not their problem.
 

Tiger Tim

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Your problem. There is a set of rules; you are expected to follow them. End.
I was mostly being facetious, I’m a big letter-of-the-law guy so if it says it needs to be calculated to need X wing loading at only a single given pilot weight to satisfy the stall requirements that’s fine by me. Likewise the other calculation-based stuff in the appendices.
 

TFF

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It’s hard to comprehend that the government ignores something they clearly could put their finger in, but they do. All you need to fly a 103 UL is an awe shucks Andy Griffin smile and something that looks like it will fall apart and you’re in. The hard part is keeping it that way. Stir the pot is how everything gets worse. There is an honor system. It should be followed. There is always the debate of letter versus spirit. Past spirit is the real problem.
 

Tiger Tim

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Added another one to the comparison chart based on similar configuration and overall objective.

Buhl PupMinimax
Max-103
Hummel
Ultracruiser
Airdrome
E-III
AC103
limits
Mine
BEW (lbs)550250252238254254
Load (lbs)300250323242251
MTOW (lbs)850500575480500
Fuel (gal)10555?55
Stall (mph)2628322727
Cruise (mph)78556363
Max (mph)9863826363
Vne (mph)98?909582?
Span (ft)302522.523.2
Area (ft^2)109.4112.510192
Length (ft)19161618
HP4528375242
EngineSzekely SR-3Hirth F-33Half VWRotax 503Verner 3V
Engine
Weight (lbs)
147458581

The Airdrome Fokker E-III seems to be in the same ballpark, though their admitted stall speed is probably more realistic if incriminatingly honest.

Am I remembering right that someone has said, maybe even here, that the UL Fokker Eindecker really benefits from longer wings?
 

RJW

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I drew a fairly detaled sketch of a metalized Minimax. I even called it a Metalmax. As usual each iteration diverged more and more from the original idea ending in what is attached. I even put some numbers to this "design". It wouldn't make UL weight but would be close. Also it would be quite a bit harder to build than a Minimax or even an Ultracruiser I think. Change the tail and it would look somewhat like the Bhul Pup.

Rob

MM1.jpg
 

Tiger Tim

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Oh yeah, that’s most of the way there. Did you run any numbers on it? Specifically I’m curious if it could get away with the shallow strut angle as the Pup will have similar issues but with cables.
 

RJW

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The driving reason for struts on a small airplane like this is to make room for the pilot. The wing could be cantilever but the carry through for a thick and light wing doesn't leave much room for a pilot. (See the lengths that Hummel went to to build a sufficient (heavy) carry through on the Ultracruiser and still leave room for the pilot.)

The angle that is chosen for the strut or wires will determine how much structure is needed to react the forces on them for a given load. Quick example for the strut. The strut must react, say, 1800lbs lift. If the angle the strut meets the spar is 45 degrees, then the force on the strut will be 1800lbs/sin 45 degrees or about 2546lbs. A shallower angle of say 30 degrees will result in a force of 1800lbs/sin 30 degrees or 3600lbs. So the shallower strut and associated structure will end up heavier to react the same load. (Hopefully I did this right. And yes, this is a simplified method with lots of assumptions.)

So pick your angle and design the structure to support the loads they will be subjected to.

Rob
 
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Tiger Tim

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I had a few minutes so I worked my way through AC103-7 Appendix 2 to determine stall speed based on wing loading and airfoil, or rather working it backwards to determine wing area required to make Part 103 max stall speed. Based on the heaviest allowable BEW of 254lbs, the example pilot they give at 170lbs and 30lbs for a full 5 gallon fuel tank I get 116ft^2 with large flaps and 146ft^2(!) without flaps. Those answers are startlingly higher than the examples I used for comparison earlier in this thread so what gives?

Is the AC103 Appendix just being very liberal with their figures or do the Max-103, Ultra Cruiser, and Airdrome Eindecker use some sort of magic airfoil? Maybe they have advantageous pitot error? I’m not above that last one.

I’ll see what I have in my library to try and come up with a more technical and mathematical stall speed solution than the Appendix gives to compare but in the meantime what do you folks think?
 

BJC

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Have you looked / calculated the Cl for the wing as a whole to see how believable the reference stall speeds are There is as much BS in stall speeds - from ultralights to fast cruisers - as there is in stated top speeds.

The only numbers that I have confidence in are the C.A.F.E. APRs, and they never tested an ultralight.


BJC
 

TFF

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Back to how you want to claim compliance. Paper or demonstration. A Legal Eagle has 107 sqft. A Legal Eagle XL has 120 sqft. They don’t have crazy thick airfoils, but probably a couple percent more than normal. The Hummels are very thick. Back to the letter of the law or spirit of the law. Back to being into the minimalist engineering exercise or fitting something as close as willing to stick the neck out. I just watched a video of a plane being called a UL. Fit and finish seemed a little too nice, definitely not worried about paint weight. Fancy details were not weight conscious. There was UL intent, but I bet it was no 254.
 
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I get 116ft^2 with large flaps and 146ft^2(!) without flaps.
That is what I remember getting too. The numbers in the AC are quite accommodating.
Solving for Cl:
@146 ft^2 = 1.54
@116 ft^2 = 1.94 (average for the flapped and un-flapped portions)

Both kind of reasonable numbers..............for a 2D wing of infinite span.
 

challenger_II

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The TEAM Mini Max has a wing area of 112sqft. With a Rotax 277 and gear drive, or a half VW, she will make 254#, and stall below the 103-posted stall speed, without flaps. Wayne Ison added flapperons once people started using larger, heavier engines, to help maintain a low stall speed.
The Ultracruiser uses a Ribblet airfoil and does well at 254#.

Both aircraft use a fairly thick-section airfoil.

One thing that many people overlook, as regards stall speed, is how clean the airframe is. As an example, take the Mooney M20: the stall speed gear-up/flaps-up is higher than gear-down/flaps-up.
Gear-down/flaps-down is slower still. When it comes to slow stall speeds, drag is your friend.
Ison used a blunter radius airfoil for the express purpose of making more drag, to help with the stall, and to help keep the flying speed under the required 63mph. A simple change to a less blunt leading edge will allow the max speed to jump to 75mph, with the same engine/propeller combination.

I had a few minutes so I worked my way through AC103-7 Appendix 2 to determine stall speed based on wing loading and airfoil, or rather working it backwards to determine wing area required to make Part 103 max stall speed. Based on the heaviest allowable BEW of 254lbs, the example pilot they give at 170lbs and 30lbs for a full 5 gallon fuel tank I get 116ft^2 with large flaps and 146ft^2(!) without flaps. Those answers are startlingly higher than the examples I used for comparison earlier in this thread so what gives?

Is the AC103 Appendix just being very liberal with their figures or do the Max-103, Ultra Cruiser, and Airdrome Eindecker use some sort of magic airfoil? Maybe they have advantageous pitot error? I’m not above that last one.

I’ll see what I have in my library to try and come up with a more technical and mathematical stall speed solution than the Appendix gives to compare but in the meantime what do you folks think?
 

Vigilant1

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One thing that many people overlook, as regards stall speed, is how clean the airframe is. As an example, take the Mooney M20: the stall speed gear-up/flaps-up is higher than gear-down/flaps-up.
Gear-down/flaps-down is slower still. When it comes to slow stall speeds, drag is your friend.
Why would that be? As I understand things, a wing stalls when it exceeds its critical angle of attack. That AoA of that wing will provide lift which varies according to airspeed (and density, which we normally set as standard day and disregard). If we know the lift required (equals weight of acft, assuming wings level, unaccelerated level flight) and we know the critical AoA, then stall speed is the airspeed needed to produce the lift required (acft weight) at the wing's critical AoA. By this line of reasoning, airframe drag is immaterial to stall speed.

Note: I'm just trying to understand the principles, I'm not saying that there aren't POHs that say landing gear position changes stall speed.
 
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