P-51 Replica. It's a Slow Day, so Let's Design One.

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Tom Kay

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Hi Replica Lovers;

I'm interested in going through the exercise of designing a Mustang replica, despite its having been done before. It would be the D variant. So far, I like the look of the Stewart and the Thunder Mustang. Both of those are off the market, and massively expensive if they were on the market. I'd prefer the Stewart, because I would like to build in metal, not carbon fiber, if I were a rich man.

Then there is the Titan-51 Mustang. There's a lot of good things about this aircraft, too numerous to mention, but a few items I'd prefer to slightly improve on, if this project ever happens. Most of the items are cosmetic, so with a little work, should be do-able. Such as the shape of the chin, the canopy and the tail.

I would like to approach the design like this:

1. Steel tube fuselage with added bulkhead formers for contour shaping.
2. Metal skin all around, like the Titan. No major carbon fiber work, except possibly wingtips and tail tips.
3. As scale as possible to the P-51, probably around 75% size. Most of the visible seams in the sheet metal would be present to give the illusion of it being a real P-51.
4. Blind pop rivets for most of the skin work, probably Avex rivets, either countersunk or dimpled like the Zenith aircraft method. This is to make it possible for one man to build the airframe, not two guys always needed to bang rivets. In some cases, solid rivets may be needed, especially on the wings if it's a stressed skin design.
5. Titan-like performance, gross weights and load limits. I'm no ace pilot, so let's say similar flight numbers as the titan, which is mid forties stall (mph) and 150+ cruise. No hot rod like the Stewart or Thunder Mustang. Decent cruise, gentle touch downs.
6. Two-seater is an absolute must.
7. Built from plans (as they emerge), not a kit. The Titan kit is $55,000. Ouch (Yes, I know this is cheap for some people).
8. Built from simple, readily available materials. The landing gear will be a challenge!

There would be more to it than this, obviously, and I invite assistance from all of you. Please respect the general concept of not using composites or wood for the major components. I'm sure these are fine materials, and have tons of fans, but metal is my preferred way to go.

So why not just buy a Titan kit? First, big outlay in initial cost, and I DO enjoy this type of developmental work, even if I am green at it. And as you'll see, I AM green at it.

Thanks, and I'll follow with some early design concepts and pics. I hope more people enjoy this than just myself. Remember, there are no stupid questions, except the ones I might ask.

Have fun. Tom Kay.
 

Tom Kay

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So let's start with the basic shape. I'll upload two pics to start, and this is how I've obtained the basic side shape, from readily available images on the web.

As you can see, I've started to add the "bones" or steel tubes. These can be modified to be any shape, moved around, etc. For example, the front vertical tubes, near the firewall, might be inclined back so that I can add an inclined bulkhead there and make an inclined metal skin seam, just like the original P-51. So, none of this is cast in aluminum yet.

The second picture is the same, but I moved the Titan-51 down a bit to more clearly show the image behind it. If you bounce your eyes up and down from the Titan to the drawing above it, I think you'll see a few areas that could be cleaned up and made more to scale. For me, the purpose of building a replica, would be to make it as close as possible, cosmetically, to the original.

I'd have to accept some concessions with the propeller, as it will be smaller than scale, given a smallish engine. Also, notice that I've slid the canopy forward (drawn in blue lines), as this should help with balance. The Titan also has been altered like this.

Tom.
 

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Tom Kay

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Throughout this process, I'll ask a few zillion questions, presumably. To start, how do people add an outside shape or contour to a typically boxy space frame, made from 4130 chrome-moly tubing?

I know I've seen a few examples, but would this method work?

The first shot shows a basic tube frame. I haven't added every member, and all of the diagonals are missing, but you get the idea. Again, as preveiously mentioned, even the tubes might be shaped or attached differently than pictured, as in the case of the vertical tubes near the firewall.

Notice the addition of little steel tabs, welded right onto the frame cross-members (and any other areas they may be needed).

The second image shows the bulkhead formers. These would be aluminum, 6061-t6 or 2024-t3. Please assume that there would be NO galvanic action between the steel and aluminum, as I'd isolate them, with coatings, plastic washers, whatever. It's hard to see, but there are little tabs on the edge of the front bulkhead. These would exist all around the outer edges of all bulkheads, and be the rivet-attach points for the skins.

Is this an acceptable way of adding outer contour to a tube frame? Any concerns about the tabs not being strong enough?

Any other simpler ideas to adding a shaped skin to a boxy frame?

Tom.
 

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Mac790

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Tom Kay said:
Any other simpler ideas to adding a shaped skin to a boxy frame?
Tom,
I don't have other simpler ideas, besides these few pictures, unfortunately it's hard to see details. Maybe Avi was right, maybe it's a good idea to get Jurca plans, I saw Jurca Mustang plans on the ebay about 2 months ago for 400-500$.

Seb

pic 1,2,3 Jurca Spitfire, pic 4 Mustang (probably Jurca).
 

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Tom Kay

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Hi Mac790;

Thanks. It looks a fair bit like what I was planning. Of course I'll give Jurca and Titan the credit, as they worked on this eons before I did.

It's hard to get good detail from small pics, but it's definitely a tube fuselage with bulkheads added. How they're attached is a little uncertain, based on the pics. I have noticed that Jurca did not use verticals in his Mustang fuselage frame, just mostly diagonals. I guess a truss is a truss, huh? Just as long as it's mathed out to be strong enough for the intended loads.

Thanks for the pictures. Tom.
 

PTAirco

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Hi Mac790;

I have noticed that Jurca did not use verticals in his Mustang fuselage frame, just mostly diagonals. I guess a truss is a truss, huh? Just as long as it's mathed out to be strong enough for the intended loads.

Thanks for the pictures. Tom.
The reason for using only diagonals (a Warren Truss) as opposed to an N-truss is to keep the individual tubes as short as possible, thus increasing their buckling load. Buckling goes up with the square of the length, so it's worth doing.
 

AVI

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Tom,
I don't have other simpler ideas, besides these few pictures, unfortunately it's hard to see details. Maybe Avi was right, maybe it's a good idea to get Jurca plans, I saw Jurca Mustang plans on the ebay about 2 months ago for 400-500$.

Seb
Mac790: As Bartyl & James would say, "Thanks for your support, Seb."
I'm staying out of this one. :)-
 

Mac790

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Mac790: As Bartyl & James would say, "Thanks for your support, Seb."
I'm staying out of this one. :)-
AVI, You welcome:),

A few years ago I was thinking about similar warbird for myself, that's why I have all these pictures etc, but I grew up and changed my mind, now I rather think about smooth composite plane. But before I make a final decision, I'll probably build 2-3 cars, maybe one copy (I prefer reverse engineering) of existing car and one or two my own ideas/designs will see.
I put a simple rendering in "What software do you use" thread" but it's still long way to go.
http://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/5025-what-software-do-you-use-4.html#post41428

Seb
 
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pepsi71ocean

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Tom,
I don't have other simpler ideas, besides these few pictures, unfortunately it's hard to see details. Maybe Avi was right, maybe it's a good idea to get Jurca plans, I saw Jurca Mustang plans on the ebay about 2 months ago for 400-500$.

Seb

pic 1,2,3 Jurca Spitfire, pic 4 Mustang (probably Jurca).
Is that a full sized P-51B replica with a Maxim bubble canopy?

If so, Tom that is the way to build a sturdy airplane. The way they rib those is to prevent torquing the frame. I think you have an interesting idea with your drawings, but i think thicker sheets cut out on a CNC or plasma cutter machine would be able to make the cuts easily. and the hollowness would hep with weight reduction, which from what i see your plane's design could have issues with using tubes and bolting arches on the square frame, seems to complicated to me. i would be afraid that one of those bolts would come loose or break free if somethign else were to happen.


Any ideas on a Powerplant yet?
 

Mac790

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pepsi71ocean said:
Is that a full sized P-51B replica with a Maxim bubble canopy?
Pepsi

Pic 1,2,3 it'a a Spitfire replica Mk.9, 1:1 scale with the Allison engine, wooden wings, tubular fuselage covered with aluminium skin (attachments for more details) check out this page for two movies in flight
Avions - Marcel Jurca - Aircraft - MJ-100 Spitfire #DeFord- N1940K
Pic 4 it's a P-51D probaly 75% scale I don't remember exactly.

btw Are you familiar with Stewart P-51 manuals
S51 Construction Page

Seb
 

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pepsi71ocean

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Pepsi

Pic 1,2,3 it'a a Spitfire replica Mk.9, 1:1 scale with the Allison engine, wooden wings, tubular fuselage covered with aluminium skin (attachments for more details) check out this page for two movies in flight
Avions - Marcel Jurca - Aircraft - MJ-100 Spitfire #DeFord- N1940K
Pic 4 it's a P-51D probaly 75% scale I don't remember exactly.

btw Are you familiar with Stewart P-51 manuals
S51 Construction Page

Seb
On second thought looking at some of the photos you can see the spitfire look in the tail.

Ive found that to be the best website for the S-51, and yes i am familiar with the manual, ive read the engine part several times over. Some are in MS word and other in PDF. but its well worth reading.

Tom read the whole S-51 manual, its several sections but i would recommend it.
 

Tom Kay

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Hi Pepsi;

I think I should have added more to my drawing, because I had intended to use a steel tube frame, then alum. bulkheads with bent edge flanges, then a few lengthwise stringers, then finally the skin. So really I was just looking at a way to transition from a boxy tube fuselage to a nice rounded exterior of alum. skin. So my bad for not explaining that part. In effect, the Jurca Spitfire is what I was thinking about, assuming that it has little 4130 tabs welded to the fuselage frame tubes. And, funnily enough, so does the Titan, and I wasn't aware of that at first.

You're probably right to be concerned about some of the bolts that attach 4130 tab to alum. bulkhead vibrating loose. So locking or plastic nuts would be used. They could be quite small, as they carry minimal load (aerodynamic forces, perhaps).

Mac and Pepsi, yes I've certainly had a good look at the Stewart 51 info. The PDF's are very informative. One thing that becomes immediately clear, is that the S-51 is a beefier, more "serious" Mustang replica than the Titan. Thicker skins, heavier build, bigger engine, etc. I'd like to stay in the realm of the Titan, lighter, simpler, lower performance, you get the concept. Although, who wouldn't want to roll up to the parking area in his S-51? Beautiful machine, and since I'm into all metal, I prefer it to the Thunder.

For now though, low and slow. One truth I am continually running into, is that the Titan seems to be a fairly well thought out, reasonably elegant design. It's just the up front cost that is the show-stopper (translated as "killer"). Any cosmetic mods, such as re-shaping the chin cowling a bit, could be done to the existing kit, I would assume.

Ideally, I'd like to change a few concepts to be simpler than the Titan, but not just for the sake of being different. So I'll keep plugging away. For the moment, it's the fuse I'm working on. Shortly, it'll involve how the H-stab and V-stab are attached, then the wings. Again, Titan seems to be the logical way to go, the way they've arranged their parts final assembly.

Cheers boys, and those Jurca views are appreciated.

Tom.
 

Tom Kay

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Here's a more detailed look at the concept. I've added stringers and small bulkheads to the side of the fuse.

I haven't drawn it perfectly, so to clear up something, the fuse tubes would be about 1/8" to 1/4" inch away from touching the inside of the skin. The skin would be carried fully by the bulkheads and stringers, and would never be "draped" around the steel tubes to help create "corners. "

The bulkheads and stringers would be shaped (inside the skin) so that rivets and skin joints would occur where the real P-51 D would have its joints. This would help it look more accurate.

So you can see that we're probably all talking about a slight variation of the same thing.

Based on this, any obvious weaknesses?

Thanks again, Tom.
 

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bmcj

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Based on this, any obvious weaknesses?

Hi Tom,

What you're asking here is really too broad a question to answer. You may have to provide some more specific design information in order for someone to give you a definitive answer.
 

Tom Kay

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BMCJ;

Yes, fair enough. I guess what I'm asking at this point, is about any weakness in the concept that may stick out in someone's mind. Certainly no number crunching at this point. And I'd be hard-pressed to ask anyone to do that for fun.

Conversely, if anyone has alternative methods for making a boxy tube fuselage, into a nicely shaped fuselage with a metal skin, I'd like to see them. I'd like to stay all metal, not foam, composite, wood, etc.

So I'll try to get to specifics when I can, but for now I acknowledge your point.

Cheers, Tom.
 

wsimpso1

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Someone has to explain this to me. Why would an airplane designer, when designing and building a new airplane, build a steel tube fuselage and then add all of the work to make a monocoque fuselage? They have FS ribs and longerons and skin stiffeners...

It seems to this engineer that if you are going to have all of the moncoque stuff, by adding a little more weight, you could make all of that the structural and skip the steel tube fuselage, saving a bunch of weight.

Billski
 

pepsi71ocean

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Someone has to explain this to me. Why would an airplane designer, when designing and building a new airplane, build a steel tube fuselage and then add all of the work to make a monocoque fuselage? They have FS ribs and longerons and skin stiffeners...

It seems to this engineer that if you are going to have all of the moncoque stuff, by adding a little more weight, you could make all of that the structural and skip the steel tube fuselage, saving a bunch of weight.

Billski

I agree, i was thinking the same,
 

bmcj

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I'm not sure if Tom's intention is to have a load bearing skin, but with all of the structure, it would be an easy jump to make and he could then emiminate the welded truss substructure. Unfortunately, most part time designers and a lot of builders feel more comfortable with the ol' tried and true metal tube framework. Designing a monocoque structure and building one from scratch takes a little more experience, or at least different experience, than most people have.

Besides, mix a welded tube structure with a monocoque structure... talk about statically indeterminate!!!

Bruce :)
 

Tom Kay

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Hi Billski and Pepsi;

That question has certainly occurred to me too. It seems wasteful, but, it is also very common in homebuilts, especially replicas. I've seen oodles of examples.

I can't answer for everyone, but I'll try to answer for the Mustang replica that I am re-working.

First, I'm no engineer. I have an Aerospace Technology diploma, which qualifies me to work somewhat on aerospace stuff, but certainly doesn't make me capable of calculating anything related to airframe loads. I might have been qualified to do this, had I been a MUCH better student, 25 years ago at Ryerson (Toronto) but stress analysis was my worst subject, and the day after I graduated, I wouldn't have been qualified to design a true moncoque structure. For that matter, I'm not qualified to design a truss tube fuselage either, but my intentions are to use the work of others and "design" an aircraft with a weight and structure similar to others. However, I acknowledge your good point. And with changes I want, I'll still have to illustrate the strength of any structure on paper before building. God knows how I'm going to do that, because I sure don't.

Second, sheet metal forming can be truly difficult. I haven't used an English Wheel, have you? If you have, how'd it go? Making compound curves in aluminum, and ensuring that they are now capable of carrying loads would be WAAAY beyond my abilities.

Third, the truss tube just acts like a good place to hang things (other hardware) like bulkheads, control sticks, engine, tail, etc. It's like the skeleton. Just add meat and skin. I make it sound so easy, don't I?

Forth, like the Hawker Hurricane, a truss tube construction should be simpler for a one-off build. The Spitfire, a better aircraft in many respects (but not all) was much harder to build, and took more man-hours (and woman hours, sorry Rosie) than the Hurricane, even as production became "routine." So I think a space frame of tubes is more beginner-friendly for the homebuilder.

Fifth, riveting. I had hoped to stay away from solid rivets, for the most part, since this now takes additional equipment, at least some level of additional practice in riveting, two people ( a real disadvantage if you can't find a buck-bar volunteer), and makes real noise (neighbours would love me). I'd like to use Avex rivets, as these are one man items. The skin wouldn't really be load-carrying, so it just has to withstand the aerodynamic forces in flight. The wings are a different matter, I realize, and they might indeed be stressed skin. So I might have to go through these motions. I haven't dived into the whole wing-thing yet.

Lastly, weight. A distinct disadvantage to use a truss frame under the skin. You're right that a not-so-carefully designed structure would add weight and be inefficient, but, I will be without guns, armour plating, oxygen system, and the fuel to make it all the way to Berlin. I might want to take off and make it all the way to the breakfast meet at the nearest airport, or some sort of modest cross-country, but that's about it.

I can't speak for Titan, since I am not involved with the company, but I think I can see why they've gone this route and used small pieces of fuselage skin with simple 2-D curves, as opposed to fully wheel-formed 3-D curves. The average home builder has no skills for this work, or the equipment. So they approximate a curve by using a few pieces to make a chin cowl shape, a top nose shape, or a wing root fillet, rather than rely on real skill to form each piece. And, the more I see of the kit, the more respect I have for some of their design decisions, even though I'd like to see an aircraft that looks more like a D-model Mustang. Especially when they've gone to all that trouble to make an airplane kit for the masses. A few simple tweaks would even improve the replica look, especially in the V-stab area.

Thanks gents, for the points you've made. As always, feel free to point out flaws in my way of thinking. It all gets sifted and computed.

Cheers, Tom.
 

Tom Kay

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Bruce has the gift of minimal gab. He stated it well, based on my (assumed) comfort level of aircraft building. Assumed, because I've never built one yet.

T.K.
 
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