# Thread: Steel Tube Fuselage Design

1. ## Steel Tube Fuselage Design

I have seen a lot of posts about aircraft design and different things about steel tube fuselages, but I am looking for specific information on designing a fuselage for a particular airplane.
I have decided to change my Corsair design to be built as a steel tube fuselage with composite skins. Maybe a steel main spar as well.
I am a composite guy, and I don't know anything about steel tube design. Is there a book or program that will spoon feed a math idiot through the design process?
I know where I need to attach the spar, engine mount, tail structures, and tail wheel. I just don't know where to go from there.

2. ## Re: Steel Tube Fuselage Design

I dont have an answer to the original question, but I was thinking that a Corsair would be pretty good as a mix of construction. Behind the cockpit alu monocoque, engine through cockpit, tube. Aztecs and Mooneys are made that way; might copy that.

3. ## Re: Steel Tube Fuselage Design

You've got to be proficient in basic trigonometry and the buckling loads formula (but there are nice spread sheets from the EAA that make the buckling loads part easy). Once the trig is understood (if not already) then it is a relatively simple-but-tedious process of diagramming each joint and resolving the horizontal and vertical components of the loads by various techniques. A simple explanation is about all that can be done on this forum; Peery's book is worth the money just for the truss analysis section and Schaum's Statics an Dynamics is a great intro to truss analysis also.

This thread contains a very simple truss joint problem, specifically post #11, the material and fastening method won't matter because the basic analysis is the same and the tubes are considered pin-jointed:

Aluminum Tube Sizing?

4. ## Re: Steel Tube Fuselage Design

Can't help you out with your original question, but if I remember right and you are looking at selling kits it might be profitable for you to design the truss structure so you could modify the skins to make it into a Super Corsair for those who were interested. That way you could add a second seat and take a friend along. The full scale Super Corsair was only about 2 inches longer than the stock F4U and with the exception of the wingroot intakes used the same wings and horizontal tail as the stock F4U. Even the vertical tail was a stock unit set on top of the extension. Just something to consider. If you would like reference on the F2G Super Corsair I have quite a bit that I would be happy to send you.

5. ## Re: Steel Tube Fuselage Design

I am first going to worry about getting one in the air. If it doesn't kill me, I will think about kits.
You make a great point, since there were quite a few variations of the Corsair. If I kit it, I will try to make it easy to modify to be the model the builder wants to own.

6. ## Re: Steel Tube Fuselage Design

Steel tube works well for that kinky Corsair spar also. Just weld it together, shape hardly matters with steel tube.

7. ## Re: Steel Tube Fuselage Design

I found the easiest way to analyze and size tube fuselages was using a demo version of NASTRAN or a similar program. (LISA for example is free to download). You simply model the frame using coordinates from you plans, assign nodes to each junction, apply constraints so your model doesn't go spinning off into space and apply all the loads you you calculated. "Simply", yeah right! But it is a hell of a lot easier than doing it by hand and way more accurate. I do not mean a full finite element analysis that does all the buckling calcs etc for you - I just use it to find the loads in the individual members and plug those into the standard buckling formula (spreadsheet makes this easy). You can consider everything pinjointed, but Bruhn and others allow a fixity factor for welded joints.

You need to do a lot of learning to get the software to work, but if you have a basic understanding of engineering, it isn't that hard, just time consuming. My NASTRAN demo would handle 300 nodes and since you only need one node per joint, that is plenty for a fuselage or spar. My version ran on Win98 and was very user friendly, later version were far more tedious to use (splitting up the modelling and analysis into two programs etc).

8. ## Re: Steel Tube Fuselage Design

I was going through your site today and saw your wing spar that you built to part strength to test and see if your design had any merit. The design I was thinking of for my spar is very similar and I was wondering how that worked out and if you had done any static tests on it yet? I'm also curious what motivated you to abandon composites and start looking at steel truss structures?

9. ## Re: Steel Tube Fuselage Design

Oh boy. I could write a book to answer those questions.
Just got home and it's late. Will answer ASAP.

10. ## Re: Steel Tube Fuselage Design

Just to be a downer, a steel tube truss is easy to build (just weld it up, he said),
but it's a VERY difficult thing to build to any exact dimension.

When heated, steel expands.
The hot part does anyway.
The cool parts not so much.
That makes the truss warp something awful.
Yes, there are techniques to minimize that,
but no sure way to completely eliminate it.

11. ## Re: Steel Tube Fuselage Design

The older EAA design books and those by Tony Bingelis, William Fike and Raoul Hoffman all have lots of interesting tips and tricks for tubular structure designs--how to route loads around holes for doors or cockpit access, etc.

Of course, the EAA welding book and others have lots of advice on how to reduce stress and deformation and it can be managed. I would think that the method you are describing of composite over a steel truss would work well if you make allowances for some variation in the way the composites are actually attaced to the underlying structure.

I would think that a T-shaped tubular structure from firewall to the back of the cockpit and serving as wing spars from the roots to the landing gear attach points (a droopy T to the wing kink in the case of a Corsair) would make a lot of sense. All the heavy loads would pass through the tubular structure, reducing point loads on the composite parts and incidentally providing a rugged crash cage around the pilot. Some 1920s, 1930s and early 1940s aircraft used similar construction with steel tube forward fuselage and sheet alloy or wooden construction rear fuselage.

Another option to consider is to use bolted and riveted construction with gusset plates--whether in steel or aluminum. It is less common now as it is both heavier and more time consuming, but it was quite common up to WWII--Westland, Hawker, etc. It avoids any of the distortion issues with welding and the metal retains its full strength and it is easier to repair if you are expecting any battle damage. ;-) If you kept your truss design as simple and symmetrical as possible, you would be able to get away with a few standard gusset designs and just vary the tubing wall thickness and gusset thickness as needed.

PS--Here are a couple of relevant examples from the FLIGHT magazine archives showing the General Aircraft Cygnet. I am quite sure that some of inter-war and minor WWII fighters used similar construction, but I can't find any images right now.

1936 | 3380 | Flight Archive
1939 | 1- - 0145 | Flight Archive

12. ## Re: Steel Tube Fuselage Design

For sure, We wanted to build 4 Bearhawks in my shop/hanger, so we built a large heavy fuselage jig ( 19' long and weighed about 1500 lbs) and hard tooling for almost all of the airplane, (engine mount, LG, rudder pedals, stick assembly, etc. So in the future if one of the aircraft need a part, we could make an identical part from the jigs. Even with all of the fuselage tubes clamped down, and 95% of the welding finished in the jig, there is still a little warp that is present after the tubes were unclamped. Dan R.

Originally Posted by cavelamb
Just to be a downer, a steel tube truss is easy to build (just weld it up, he said),
but it's a VERY difficult thing to build to any exact dimension.

When heated, steel expands.
The hot part does anyway.
The cool parts not so much.
That makes the truss warp something awful.
Yes, there are techniques to minimize that,
but no sure way to completely eliminate it.

13. ## Re: Steel Tube Fuselage Design

Good info, guys.
The reason for my change to a steel frame is just to help me get the project going.
I designed the landing gear and built the prototype about six years ago. I had one machine shop keep it for three years, before he sent it back (with my money and some parts he made).
The next machine shop, well, I can't say anything yet, the law suit is not completely done yet, but a judge ordered him to return my prototype (again, after three years).
My thinking is, if I build a tube frame and center section main spar, I can get the systems and controls done, and even hang the engine. I can use a fixed main gear during this process and later install the retractable gear.
I have the dimensions for the gear, so I can plan on the re-fit.
After the first airplane, if I kit it, I can stay with the frame, or go all composite.

I bought a Stearman frame. It is in sad shape, but almost the right size for my fuselage. I am building a frame inside my composite fuselage, using cheap conduit. I will extract that frame, then build one out of 4130, using the conduit frame as a template for size and shape and using the Stearman frame as a template for tube sizing and bracing.

How's that as a plan?
Any other ideas?
Thanks!

14. ## Re: Steel Tube Fuselage Design

Originally Posted by Will Aldridge
I was going through your site today and saw your wing spar that you built to part strength to test and see if your design had any merit. The design I was thinking of for my spar is very similar and I was wondering how that worked out and if you had done any static tests on it yet? I'm also curious what motivated you to abandon composites and start looking at steel truss structures?
I never got to test the spar, but on paper it works.
I went to a spar design class. It was for gliders, but a spar is a spar... not really.
The spar had to be designed taking flex into consideration. Gliders have very flexible wings (spar design is lower G and optimizes for weight). An airplane can't have that much flex (depending on the designed aircraft type).
I had to design the wing spar so it wouldn't flex so much as to lock up the ailerons in a high G turn. I ended up with a 19 G spar. It only weighed about a pound more than a glider type spar would have.
I later found out that a Money spar is also a 19 G spar. Maybe for the same reason?

15. ## Re: Steel Tube Fuselage Design

Originally Posted by Corsair82pilot
I never got to test the spar, but on paper it works.
I went to a spar design class. It was for gliders, but a spar is a spar... not really.
The spar had to be designed taking flex into consideration.
Hi Corsair,

What class is this that you went to? Is this still offered? EAA? Or University? I am very interested in the later.

Regards from Spain and good luck on your project,

JM