# P-51C at ~70% scale as ultralight?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by J.L. Frusha, Apr 30, 2019.

1. May 6, 2019

### Victor Bravo

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Making a 70% scale P-51 and bringing it in under 254 pounds is technically possible, but only if you are a university or a large aerospace company. The financial and technical resources that are required to succeed at this will be huge.

Paul MacCready could have done it. Lockheed and Boeing can do it. One of the Technical Universities in Europe, or a German Akaflieg could do it. Our own BobK could do it if Bill Gates and Jeff Bezos both bring their checkbooks.

The vast majoity of private homebuilders are not in the position to do it successfully.

Then, once it is done, and built, and weighs 254 pounds, it will fly like crap. Not like a Mustang, not like an ultralight. If you drop a pencil accidentally, the pencil will poke through the wing skins or fuselage.The engine that you developed (from a clean sheet) will produce about 40 horsepower for 20 minutes, until is siezes or breaks, because the engine had to weigh 12 pounds. The tires will be about 8 inches in diameter and look ridiculous, the same way as most ultralights look like they are on toy wheels.

I'm sorry to be a naysayer and a party-pooper. My agenda here is not to spoil your dream, but to prevent you from spending many years and huge financial expenses pursuing something that is unrealistic.

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2. May 6, 2019

### Jerry Lytle

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Heavier alunimum, larger bulkheads, added structural members, wheel pants, it appears that you are leaving the ultralight arena before you have even started.

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3. May 7, 2019

### pictsidhe

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The windwagon is 273lbs, it stalls at 40mph, which seems about right for a plane with 54sqft of wing area. A 103 is going to need double that with largeish flaps. The 103 limit is 103lbs and 28mph stall. You want to take the windwagon design, make it significantly bigger while using thicker aluminum and think you can lose 20lbs at the same time?

I have done a fair bit of calculating on my Hurricane Mk103 Despite the naysayers, I'm pretty sure that I can make it work, but I am having to pare absolutely everything to the bone to do it. There is no room for TLAR engineering or making it stronger (heavier) than needed in lieu of that. I have nearly scale wheels that weigh about the same as BMX wheels, I have found paint that covers well at 1/4 oz/sqft. But I have a test panel in the yard to see if it will keep the UV nasties out of my material...

Yes, I think a 70% p-51 is quite possible. But it's a lot of work. The easy way is to take a proven 103 that just needs a few cosmetic tweaks and the right paint (or tinfoil) to look about right. A tapered wing and the right tips would really add to the warbird look of any ultralight design, but will take some engineering, especially if you want to lose the struts. My project won't be ready to adapt for a few years I don't even know if anyone is actually selling the much hyped linear generators you want to use yet.

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4. May 7, 2019

### J.L. Frusha

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I did NOT say build the entire thing of heavier Aluminum. I SPECIFICALLY stated the Bulkheads and additional stringers would be and why.

The Wright Brothers were told THEY couldn't and look where we are.

WHY MUST I increase ALL parts to use the heavier guage aluminum I only specified for bulkheads and certain stringers?

Skinning and aircraft with tissue paper HAS been done, though there are sturdier materials and the process is called "Dope and Fabric"... Presuming I plan to use something that thin is utterly, stupendously ridiculous.

How many times have I expressed the intent to use other materials?

MacCready used aluminum, wrapped it in fiberglass at 2 different angles, (45° and 60°, with 50% overlap and in opposite directions, for torsional strength, if I remember correctly), dissolved the aluminum with acid and filled the fiberglass tubing with expanding foam to reinforce it..., but, what would I know of composite construction specifics for the Gossamer Condor, right?

Why MUST I create a new engine at your ridiculous @$$umed 12 pounds? Your @$$umptions are incorrect.

______________________________________________________

NEVER @ume that someone asking for help and advice has no knowledge of that which they are asking. Even NASA Engineers ask each other for suggestions and advice. No, I am NOT one of them, but I'm not some starry-eyed backwoods kid that has no knowledge whatsoever.

Last edited: May 7, 2019
5. May 7, 2019

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I don't think anyone considers you an idiot, but your proposed project is so far out of the "norm" that it's hard not to have doubts about its feasibility. To say that what you want is "hard" is a huge understatement. However, the good news is that no one really gives a hoot about whether an ultralight is strictly legal or not. As long as it flys like an ultralight and looks like an ultralight it is an ultralight....but of course what you want won't look anything at all like an ultralight.

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6. May 7, 2019

### J.L. Frusha

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There is no part of FAR 103 describing your presupposed 'looks-like' or 'flies-like' an ultralight, only a set of specifications. Either an aircraft IS an ultralight, or it is not.

The Airdrome Aeroplanes Fokker E-III 3/4 scale WW1 replica along with the Earlybird 2/3 scale Curtiss JN-4 Jenny are BOTH recognizable fighter aircraft designs made as ultralights. I have plans for a couple of others. Hell, I have a briefcase FULL of stuff directly pertaining to ultralight aircraft, few of which are easily recognized as ultralight aircraft, and I have a locked/licensed pdf set of plans for the Raceair Zipster that can be built as either an ultralight, or an aerobatic light sport aircraft (but it looks the same, either way)...

Being stuck in some complacency zone isn't my problem. Being a 'conformist' simply because someone says they THINK it SHOULD LOOK LIKE "X" isn't my problem.

I can see why so many people simply give up before they get started... If I ever get around to building a Hovey Whing Ding II style ultralight, it will be the South American Pamperito, with ailerons, which I have plans for.

Be that as it may, THIS topic is about an approximately 70% scale P-51 MustangB/C, possibly even as a carricature-style, but still recognizable P-51.

7. May 7, 2019

### BBerson

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I have Hovey's 1976 Ultralight Design book. Not much about FAR103 types since 103 was created in 1982.
I thought the Wing Ding was extremely light and unreliable and only used on dry lake beds.
A full body "ultralight" with "bulkheads" and skin seems heavy and nothing like a Wing Ding.
Would need fabric for sure, I think.

The 12hp Wing Ding didn't need to climb much. Nor did McCready's designs.
A short strip capable modern ultralight needs quite a bit of power for climb over the trees.
What sort of hp is planned?

Last edited: May 7, 2019
8. May 7, 2019

### 12notes

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A few items I want to express, but I want you to know upfront that I want you to succeed at this. It's a cool venture, and I'm on your side. However, there's significant danger in the approach you're taking. There are people with significant experience (not me, but I've read and learned a lot in my time here), and, while you might not like their advice, that doesn't mean you shouldn't listen and learn.

The Wright brothers built a wind tunnel, corrected the lift equation and did extensive testing before they attempted to build their airplane. If you're planning on doing that, go ahead and compare your plan to theirs, otherwise, it's comparing an apple to an orchard.

I'm fairly sure the nosegear Windwagon and the tailwheel Hummelbird have the same thickness in their bulkheads and fuselage skins. I'll check this when I get home, I have both plans. The fuselage of both are oval at the front (like the Mustang), which tapers to a circle (not like a Mustang) past between bulkhead C & D, bulkheads D & E are circles. Changing bulkheads D&E's shape is not something anyone would notice, especially with the turtledeck extending the oval shape from bulkhead C to the start of the vertical stabilizer. Changing the bulkhead shapes and using thicker metal is a risk to safety at the expense of weight and effort, with very little to gain.

You'll be hard pressed to modify the Windwagon plans to an ultralight safely without design and engineering work. There's a small chance you can make it fly safely at that weight without calculations, but a far greater chance that you'll end up with something that has a critical vulnerability. Morry Hummel spent years changing the design to go from the Hummelbird to the ultralight Ultracruiser, and it barely makes weight for part 103. It would save you a lot of effort to just buy the Ultracruiser plans rather than working off the Windwagon ones and trying to lose the weight. They are complete plans, not a list of modifications to the Hummelbird or Windwagon plans.

I'm in the middle of building a Hummelbird. I've made a few minor changes that have been done before. I have read everything I can by the people who have done those changes. I would estimate that each change has still added at least a month of build time to the project, and nothing I have done is on the scale of what you're wanting to do. I'm recommending that you modify an Ultracruiser not because I'm brand loyal or have anything to gain, but because it will get you flying much (as in years) sooner and will result in a much more reliable, safer plane when done without sacrificing much of the look you're going for.

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9. May 7, 2019

### BBerson

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It does look like the Ultracruiser is a good start. To get the same wing area with the tapered wing needs a bit more span.
So 26 feet or something. But I think the skin is .016" so your proposed plywood/foam would need to be less weight than .016" (.23lb/sq.ft.).
Or consider a D-cell with fabric like the \$88k ultralight Corsair from Germany.

10. May 7, 2019

### BJC

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Take a look at radfordc’s avatar. You may have missed the message from him.

It seems that you came here looking for an argument. Perhaps it is time to stop being so belligerent and start building.

BJC

11. May 7, 2019

### BBerson

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It looks to me the Ultracruiser would exceed the FAA103 top speed rule, unless it has some throttle limiter as described in AC103.7. WW1 scale have a bunch more drag and well suited to ultralight scale.

Ignore posts that you disagree with. (I need this advice)

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12. May 7, 2019

### pictsidhe

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I'm with 12notes. I can't see it being at all easy to adapt the windwagon plans. They are not a good starting point. There is absolutely no way you can safely adapt them without doing a lot of calculations, by which time you are completely redesigning it, anyway. Thinking that it will be easy is why people are skeptical of your chances of success.
My project is very similar in outline to yours. It would be cosmetically modifiable to a P-51. So, I'm going to throw you some bones.
I've been down a few lot of dead ends with my project. A hard skin is very tricky on a 103. By the time it is light enough to make weight, it is pretty thin and fragile. Composites would need very light cloth, which is very pricey. Thin foam cores also get very pricey. I gave up on composites after pricing them and realising that by the time they are light enough, they are going to be very prone to hangar rash and other accidents... Aluminium is doable at much lower cost, but it's going to be very thin and dentable, too. It would need to be semi-monocoque. Which gets increasingly tricky to design as the skin gets less stiff. I eventually hit upon Coroplast. Don't worry, just about everyone here thinks I'm completely cuckoo for trying to use that stuff. But, my numbers are still looking promising so I am ignoring their pleas for sanity advice. I am starting for scratch with a novel and new aircraft material, so I have even more work to do than designing, say, an Ultracruiser. You may find my basic structural design useful: It most resembles the Hawker Typhoon. Monococque behind the cockpit, an aluminium tube centre section frame much like the steel Hurricane one and stub spars for part of the outer wing panels before transitioning to monocoque tips. I want detachable or Fairey Firefly type folding wings, so I ruled out carbon spars as a hardpoint nightmare. If I was going to make a single piece wing, I'd probably use graphlite spars. Mine will be 7075 caps with a truss shear web. 7075 caps don't work out too much more expensive than 6061 and save several lb. A sheet shear web won't work due to shear buckling. I am hoping to be able to use 6061 for the bulk of the tubing as it is way cheaper than 2024. I am also seriously considering making a tube end squisher to square up the tubes, then rivet them. Those bolts add up! I would like to kit this eventually, and riveting a ready made set of tubes with flat fishplates will make the structure very quick to build.

Attached are my 21% root and 13% tip airfoils. Change the .txt to .dat and they work in xflr5. I spent a while creating a pair that should avoid the nasty stall that tapered wings are so good at creating. The p-51 has the same taper ratio of 0.5. I may yet change mine to 0.55 to make it that little bit tamer.

If you still want to go monocoque and don't already have it, I'd suggest getting a copy of

Stresses in aircraft and shell structures
Kuhn, Paul

There is an online pdf floating around. I bought an Indian reprint, which is usable, if not great. I wish that I had found it before I started calculating, it has some lovely shortcuts for some very tedious calculations.

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13. May 7, 2019

### BBerson

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I think 7075 caps save just a couple pounds. Because the caps need a certain thickness anyway, at least in the outer spar. I find little buckling difference with 7075 in the thin caps. Some savings possible in the thicker inner caps. The taper needed is a challenge. 7075 caps is also hard to find.

14. May 7, 2019

### Victor Bravo

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I wasn't assuming you are an idiot, or uneducated, or incapable of figuring things out. Perhaps you made a few assumptions about my intentions and level of comprehension as well.

I made a valid argument that was intended to save you a lot of wasted time and money, and I stand by every point I made. Be my guest, prove me wrong, best of luck to you.

A 12 pound engine was an intentional reference to the fact that you would have to push every part of the aircraft to its limit, including reducing the weight of the engine down to the "grenade" level. We do have 12 pound racing engines in the model airplane world that can produce a huge amount of power for a very brief time, by the way.

I'm always game for a good mud-flying debate, even one where there is fertilizer mixed in with the mud, but I'm going to let discretion be the better part of valor in this case. I look forward to seeig your Mustang replica fly safely.

15. May 7, 2019

### J.L. Frusha

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I did miss the avatar.

No, I did not come looking for an argument, but having people spout ridiculous things like needing 12 pound 40hp custom-built engines when a 1/2 VW was the originally specified powerplant for the aircraft in question (Watson Windwagon), or assuming a person cannot increase a bulkhead thickness and find a way to reduce that weight (without even LOOKING at the bulkhead design), then presuming an increased thickness of ALL the metal, simply BECAUSE of that suggested increased bulkhead thickness would tend to START arguments.

I will not apologize for defending myself against ridiculous, baseless assumptions used to make me appear too stupid to know a thing about the subject.

I may not BE a sheetmetal worker, but I have worked with it in Union and non-union jobs and I DO know things that nobody has bothered to even ask about, before making baseless assumptions after one simple statement.

My father WAS a career sheetmetal worker and I surprised HIM by devising a simple sheetmetal brake out of 2x4s for clean, straight 90° bends. It worked and got the job done, where we did not have access to (nor funds for) a sheetmetal brake (longest angle was ~4ft long).

I prefer stainless steel pop-rivets, over blind rivets and know how to seal them, as well. I freely admit that I despise crimping sheetmetal and will find ways to avoid it, if at all possible.

16. May 7, 2019

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Perhaps you are right about the EIII being capable of being built as a legal ultralight....but I tried and mine weighs 350 lbs. But I've only been building and flying ULs for 25 years so my skills may not be as developed as others. Good luck on you're P-51, for sure.

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17. May 7, 2019

### cluttonfred

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I would think a Sky Pup could form the basis for an ultralight, cantilever low-wing design that could look cosmetically like a WWII fighter if you stand far enough back.

18. May 7, 2019

### J.L. Frusha

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Thank you and yes, I am familiar with RC engines and heavily doped fuels sometimes used for racing them. Adapting one for this would be a bad idea, anyway. The fuel alone is so dangerously flamable and I find the fumes to be nauseating. At least nobody is using Castor Oil anymore.

Until I see a set of the Hummel Untracruiser plans, I cannot judge them against the Windwagon plans, which I do have, as well as plans for several other sheetmetal aircraft, including the aforementioned Zippster. I won't commit to a set sight-unseen.

As stated earlier, a sudden change of priorities in just the past few days has made delaying this a neccesity, however, if the change works out, it will allow MORE funds for building, so, it can be a real win-win for me.

I AM the R&D guy for a Biogas startup and will be cutting and welding plastic for the time being. fwiw, I use Nitrogen to prevent oxidation from causing brittle welds. If anyone here understands plastic welding, most find Nitrogen to be a tad expensive and tend to use dried air.

I DO want to continue this discussion. At this point in time, it has suddenly become my escape valve.

19. May 7, 2019

### J.L. Frusha

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Perhaps it would make for a Junkers W33 or F13...

20. May 7, 2019

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