Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by J.L. Frusha, Apr 30, 2019.
Using what I can find doesn't go far enough, but right at 100 sq ft for 24 ft span.
When you scale anything, the area changes by the square of the scale and the volume by the cube of the scale. So a simple 2 x 2 x 2 unit cube has a surface area on one face of 4 square units and a volume of 8 cubic units. When I scale that cube down 1/2 or 50% (1 x 1 x 1 unit) the area of one face will decrease to just 1/4 or 25% (1 square unit) and the volume to 1/8 or 12.5% (1 cubic unit).
When I suggested earlier that you look at 500 lb gross weight and 160 sq ft of wing area as typical of normal Part 103 ultralight design (extrapolated from the Sky Pup, one of the few legal Part 103 ultralights with cantilever wings), I got to the suggested scale by taking 160 sq ft/235 sq ft = .681 or 68.1%. If you take the square root of that area scale you get .825 or 82.5% for linear scale, so your stock 37' P-51 span would go down to 30' 6".
Interestingly, the Sky Pup span is...31'. Whether you want to adopt the Sky Pup construction method or not, it seems like a pretty good place to start conceptually. Really what you're trying to do is more akin to building a giant scale foam model of a P-51 and you should keep that in mind--just the look, not the form.
If I were to choose a WWII fighter for a Part 103 ultralight replica, then it would be the Yak-3, arguably the best Soviet fighter of the war and mount of the Free French Normandie-Niemen Fighter regiment. It was also one of the smallest and lightest fighters of the war, with a span just over 30' and a wing area of...160 sq ft. ;-)
Cool...fulll size ultralight replica!
If you're willing to pay the price all this sort of fantastic stuff is probably doable. The guy who won the Alaska STOL contest for a couple of years running built a super lightweight Cub replica to do it. He said that the first pound of weight saved cost a dollar or so, but the last pound saved cost several thousand dollars.
If I were Russian, I'd build a 7/8 Yak-3
Japanese, a 2/3 Zero,
American, a 2/3 P-40.
But I'm British, so it has to be a 2/3 Hurricane.
The above choices include a dose of reality. The pilot needs to be close to the cg without butchering the profile too much. Something like a spitfire with the pilot way back is going to be very sensitive to pilot weight changes.
Not to mention the classic lines and look of the P-40.
The great thing about the P-40 (in addition to the iconic lines) is that served with so many different forces you have lots of colors to choose from. This are the profiles available from Wings Palette http://wp.scn.ru/en/ww2/f/375. The P-51 list is even longer but I suspect that it includes a lot more post-WWII examples where the P-40s were pretty much scrapped after the war. The problem with the P-40 in this case is the drag from that iconic chin scoop.
Curtiss P-40 Warhawk/Kittyhawk/Tomahawk
Great Britain 
China (Nationalists) 
New Zealand 
France (Free) 
South Africa 
Funny thing about the Mustang. It was built for England, by contract, first, then became a staple of the USAAC/USAAF
What i didn't include in my 'quick and dirty' calculations, was the HT down load. As your design is 'short coupled' - distance between wing and tail, the short Lever Arm will necessitate a larger HT and VT. The larger HT will impart a higher load on the wing, which will increase the weight the wings need to carry. This does increase your flying weight. For a aircraft with the suggested 3 x Wing Chord (25% to 25%) by about 5%, yours might be 10%. That will push the wing area up from 119 sq. ft to 131 sq.ft.
Needless to say this is CL dependent as there is differing opinions on CL landing without flaps. However I like to be conservative.
This could be further refined with all the necessary data by an Engineer.
North American was asked to build P40s and NA said we can do better. Something like 109 days from contract to flying prototype.
It took a bit longer to get the engine that it needed to be great, though.
Numbers game, using online calculators for what I've found so far...
65% scale P-51B/C Mustang Mark III (Going back to stock wing and Fowler Flaps, to begin with...)
Slightly high on stall at @ 27 knots...?
24 ft wing span - 101 sq ft
Vert Stab - (calc as wing divide area by 2)
Horiz Stab -
FAR 103 Calc -
No, I have not recalculated for the increased tail areas, as suggested. Establishing the baseline for a 65% scale first.
These are two different designs. One is the German carbon fiber UL and the other is an all metal Czech (I think) design. The metal airplane has a Rotax 912 and weighs nearly double the German design.
Slight stretch to the tailplane, (~+10% area) still within scale appearance...
~32 HP 54 mph Max air speed
@Lendo I would appreciate you running this through your spreadsheet.
RJW.....I dont doubt it a bit.....the pics on my phone are very small when selecting what file to post and I was in a rush so I didnt double check my post....my apologies.
JJ, Sorry should be JL.
I can't read the that Data your put up, the print is too small for my declining eyesight.
Just to clarify some things 3 times the MAC between the wing at 25% chord and the HT MAC at 25% chord, is considered (on average) to provide the optimum sized Tail feathers (HT and VT). The opinion being that the fuselage length is smaller in weight and Parasite drag, than the extra HT and VT areas, not to mention the extra load on the wing with the extra HT area for short-coupled aircraft.
There is a formula for that and it's MTOW in Lbs.=[1+0.67(St/Sw)]i.e. St is Area HT and Sw is Area wing. (answer in Lbs.)
Now Mr Roncz's area formula wing area (S) is Total weight including HT download; S (wing area)/ (CL x 0.5 x P (density at sea level 0.002378) x Velocity ^2.)
To get the wing area at stall, which is what we are aiming for the Velocity is the Target stall speed.
Now CL is the most difficult, as it depends upon Airfoil data, however Roncz suggests you should get 0.1096623 Cl (2D) for each degree angle up to stall which provides total Cl (which is 2 D) lift. to convert from 2D to 3D x .73. so in your case I don't know the Airfoil, so I used stall at 14 degrees approx 1.5 Cl x .73 = 1.095 CL (3D). BTW the 0.73 is provided by Mr. Roncz.
Now if you run up a Spread sheet in Excel, you run run the numbers to your hearts content, varying your Lever Arm (25% to 25%), your HT area to get your Wing Area.
This is so easy to do, but more difficult for me to interpret what your intending.
BTW there are different Cl for different Flap designs for stall with flaps (1 -1.3 Raymer). I would add 1.5 to 1.2 (average) = 2.7 x 0.73 = 1.971. Stall speed is considered to be stall in Landing configuration, being practical it's best done at some altitude (over 3,000 ft for safety) and adjust for air density, when being demonstrated/ tested.
Thank you. I don't understand it, but I'll try. As for running a spreadsheet... Uh... I haven't touched them since I locked up the system on Lotus 123 MANY many years ago... I passed the final by default... That "Print Screen" box printed over my work... That was the end of that.
George, is that full span flaps? Normally full span flaps (flaperons) can only go down perhaps 20°, or less.
The problem of flaps on ultralights is that flaps are only useful for landing (and AC103-7 calcs).
The real world operations problem is takeoff. And the limited power possible under the 254 pounds mandate.
That's why the majority forgo flaps and use the larger span and area, for less induced drag for takeoff. Makes for safer failed engine sink rate also.
The old underpowered T-craft, Aeronca, etc. we're without flaps. The high powered Citabria, Super Cub etc. can use some flap for takeoff.
Medical requirements, additional options:
A person may pilot a glider or motorglider without a medical or drivers license. 61.23(c) does not apply.
An experimental P-51 with a large span could be registered as a motorglider and flown without a medical or drivers license.
B.B. haven't researched Flaperons, as I have never considered needing to use them. However as you mentioned it, I will put it on my list to find out.
J.l. something you probably need to know is the Volume (the term used) of your Horizontal Tail and your Vertical tail, once you have settled on your wing Area. Naturally HT area will affect the down load calculations.
Volume Co-efficient (VC) is a term used by all texts books.
HT -VC is between (0.45 to 0.5 - considered for safe control) take your pick, I would suggest look at both.
HT Area (sq./ ft.)= HT-VC * Wing Area (sq. ft.) * MAC (ft)/ Lever Arm (Ft.)
VT Area (Sq./ ft.) = VT-VC * 0.045 * Wing Area (s) (sq./ ft)/ Lever Arm ( ft).
Also a good Guide by Roncz is to target your HT AR to be approx 60% (min) of your wing AR.
Excel is very easy once you understand the process, get someone who knows to show you or look it up on the Net. It just makes your life so much easier!
There is nothing hard in anything I've explained, just different than what you have been used to, I would expect.
I offer these formula and approach to save you going around in circles.
J.L. Did I offer an explanation of the Lever Arm measurement, it's commonly the 25% wing Chord to 25% HT chord. Use ft. for everything.
Once you have these calculations in Excel you can vary inputs and they will automatically vary outcomes and saves a lot of time on calculations, we all have to start somewhere.
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