If anyone's paying attention, I'm in my second epoch as a prospective aircraft buyer. It feels like an 80's movie where a 50-something adult goes back to college thinking they know everything, but everything is actually different, and once they learn to open their mind, their preconceived notions of life and reality begin to melt away, revealing more confusion, but with an optimistic embrace of the crazy.

But which is a better way to build an airplane? All-aluminum? Or steel? What about both?

Now, I've spent my fair share of hours commanding aircraft from both of the predominant aircraft structural methods of the WW2 era:

1) Semi-monocoque aluminum structures comprised of die-formed bulkheads, stringers, and skins/panels fastened with rivets—the Cessna.

2) Welded steel tubing fuselage covered in fabric, with a lightweight wing structure covered in fabric—the Piper/Aeronca/Taylorcraft.

3) Hybrids—steel fuselage with fabric and an all-aluminum wing—the Maule/Bearhawk/Sedan

For the purposes of this conversation, I'm choosing to ignore the wood construction preceding the WW2 era, save for maybe wing spars.

The other day I was discussing old airplanes with a pilot friend, who felt there was simply no way to accept the age of steel fuselages built in the 1940s. The uncertainty of what lurks inside those tubes is pure Schrödinger's Cat—it can't actually be evaluated non-destructively so it assumes a condition of both vulnerable and structurally sound simultaneously. Most damage from corrosion is not discovered until recover time, so how long do you just fly it with possible structural compromise? And what's that mean for your investment?

Accordingly, his feeling is that structural aluminum is forthcoming in indicating its condition. Corrosion occurs on the surface—intragranular type not withstanding but that is such a fluke anyway. For this reason he is much more comfortable with the advanced age of a 1950s Cessna. I was on the other side of the argument: once the aluminum structure of a Cessna starts to corrode, it's either bandage it by fogging with protectant products, or dive into resecting the affected areas, which can be a major project. I find it extremely intimidating.

As for strength. having run both over rough ground with not-big-enough tires, I can say I like the way a steel tube fuselage handles it. My old 170B would kind of twist and groan, especially without the seaplane V-brace.I always imagined rivets smoking and popping during those moments.

But as history has shown, these old airplanes do stand the test of time, each style. And they do it in some pretty rough environments—moist and rainy Alaska/Canada, near the coast, extreme temperature swings, rough ground, turbulence, on floats—and a good portion spared some other calamity are still flying. Save for that one 1997 American Champion Scout I saw for auction in 2004 that had total lower longeron rot—it had been operated by some government entity on floats in the saltwater of SE AK.

A similar argument began in the late 90's in the motocross community as Honda debuted its new CR model with an aluminum perimeter chassis—anybody remember that? The first year design was stiff as hell, causing champion riders to jump ship to other brands still using the more flexible steel backbone chassis. I wasn't fast enough to complain, I thought it ripped. But 2 decades later, the rift persists—Japanese brands all went to aluminum perimeter, and the big Austrian brand stayed with steel. Wins have been mixed so there's not much consensus.

But as a guy who's been around airplanes for a while, I have become more comfortable in just liking what I like and being okay with that. I'm drawn to a welded steel tubing fuselage. I like tubes I can grab onto and clamp things to. I like the idea of refreshing an airplane by stripping its disposable skin, blasting and re-coating its tubing, and starting anew. For a while, a more ignorant me thought a fabric wing would just bounce big haul off like a trampoline, but I was quickly corrected. A Cessna benefits from that golf ball dimpling in cruise speed but pays for it later in resale value.

That Mr. Maule decided an all-aluminum wing and fabric fuselage was the best idea for his models, I find interesting. Was it inspired by the Aeronca Sedan? What pushed those engineers in 1947 to design a wing skinned in aluminum?

So what's your opinion on the pros and cons of construction types? And what has been your experience and observation for longevity operating in the field over rough ground and parking outside?

Bearhawk, Tube and Rag fuselage with a metal wing for the win

Zzz wrote:

The other day I was discussing old airplanes with a pilot friend, who felt there was simply no way to accept the age of steel fuselages built in the 1940s. The uncertainty of what lurks inside those tubes is pure Schrödinger's Cat—it can't actually be evaluated non-destructively so it assumes a condition of both vulnerable and structurally sound simultaneously. Most damage from corrosion is not discovered until recover time, so how long do you just fly it with possible structural compromise? And what's that mean for your investment?

I have to disagree with your "pilot friend". Non-Destructive Inspection (NDI) is employed regularly across industries and specifically called out to ensure structural integrity of tubular structures in aircraft by manufacturers and is mandated in type specific Service Bulletins and AD's. The Beaver is one such example where we use an Ultrasonic Thickness Tester to confirm wall thickness of tubular structures such as the forward tubular frame (birdcage), rudder pedal torque tubes, and forward fuselage (doorpost) struts.

Although not mandated, I just completed mapping the entire tubular frame wall thicknesses on our '54 PA-18A as I have it stripped for re-cover. I desired to know the wall thickness of my entire frame as now is the time to remove and replace tube if required. UT is an invaluable tool to identify wall thickness allowing one to determine if the tube continues to meet design strength criterion. I would certainly employ UT on a tubular frame during pre-purchase inspection. Yes, there are areas of "cover" that are not accessible, however, there are enough accessible locations (longerons/rudder post for example) providing enough data to make a walk away or proceed assessment. Do not fear the tubular structure, there is a reason it is an enduring engineering practice.

TR

Owning and flying both right now

If they are good examples, no corrosion and good shape fabric, I’d say it’s a toss up, dealers choice

If I was going to get a project, I’d rather rag & tube.

Since you live around here and the vast majority of backcountry flying opportunities are on floats, one thing to consider is the tail.

You will often be parking on a beach, and the water level varies with rain.

My tail often ends up in the sticks, and you can trim them back ect but the stuff grows fast and the water comes up so inevitably the tail makes contact with some sticks.

I would be pretty nervous with a fabric tail. You would probably have to nose in and tie down, then cut like a maniac well beyond any where the wind might blow the branches into your tail. Then spin the plane and tie down again.

That government Scout was a Fish & Wildlife Scout that was sent to Kodiak, not Southeast. Kodiak is tough country. The tail on that brand new airplane was junk after a year.....AC hadn't corrosion proofed the tail at all..... The rest of the lower fuselage wasn't well corrosion proofed. I'm hoping that ACA has learned to do a much better job of preserving their fuselages.

For perspective, I flew a 1969 Piper Super Cub, that was based in Kodiak since new, from 1978 till 1986. This airplane was regularly in salt water, always parked outside, including all winter.....salt water, wind, etc. To put it politely, the plane was not pampered. The plane was, however, covered in cotton, which apparently was standard in 69. So, in the winter of 1985, they told me to bring the plane up to Center's outfit in the Valley for recover. I really suspected I'd never see the airplane again.

A month or so later, I was in Anchorage for business, so I drove up to Center's place to check in.....fingers crossed. First thing they said was: "I thought they said this thing had been in salt water it's whole life." I confirmed that, in Kodiak......but why the question? They took me back in the shop to look at the (now naked) airframe. It was squeaky clean, uniformly coated in zinc chromate, and straight as a string. I complimented them on their refurbishment skills..... Their response: "We haven't done ANYthing except stripped the fabric off.....that is precisely the way we found it.....virtually perfect."

I'm not sure they ever believed me that the airplane was in salt water a LOT, and the only way to wash it after salt was to land in fresh water and splash around a bit, which I did routinely.

But, the point: Piper had PROPERLY prepared that airplane for whatever it might encounter. Center drilled a couple of fuselage tubes to get a sense of the interior of the tubing.....linseed came out.

That's a long way of saying that, properly prepared, a rag and tube airplane will be fine.

I've worked both types plus a few hundred hours in several Maules, plus a lot of time in various all metal types.

I'd let your MISSION dictate the airplane, not worries about materials.

MTV

Z, what kind of project do you have now and what keeps you from making progress on it?

(last I was aware you had a Tripe)

I think I have responded to this or similar question before but that's OK.

I was flying into Northway Alaska during a sheep hunt, the wind was really crappy so I was pretty busy with the landing.
As I was rolling out after touchdown I noticed a wrecked plane just off the side of the runway, it was really balled up badly!

When I was at the gas pump the man there commented to me that I had made a much better landing than the last guy, I didn't think I had done all that great but whatever.

He said the wreck was a Cessna, I think a 180 or 185 I think, it had got caught in the turbulence and had crashed just a very short time before I got there.

I looked at the remains of the Aluminum tube structure and reached up and patted the steel tubing that surrounded me in my old Stinson.

I might have got in trouble landing just like the other guy had but I think I would have been sitting in a steel cage when it was all over, I don't know if the Cessna pilot was hurt but it sure didn't look like anyone would have come out un damaged.

I can see benefits and issues with either style of construction. What I'm wonder about is repairing damage and the required skills. Both will likely require a gig for major repairs, straitening bends etc. Which style of airframe is better suit to the smaller, field repairs, the art of welding vs rivet toppling might be the decider in the equation. We have all likely seen the pictures (could have been photoshop I guess) of a Cub frame all wrapped up in duct tape but that said I've seen a 4x8 sheet of aluminum pop riveted on stress skin in a pinch.

The other question is cost to manufacture; my sense is that aluminum is the winner on this front. I suspect it lends itself to mass production better than welding however that was before robotics. If robotics could be used to cut the tubing, tack weld and final weld a steel tube airframe maybe I could finally afford a Top Cub, Husky, Maule, Scout since taildragger Cessna's are no longer in production.

TR wrote:I just completed mapping the entire tubular frame wall thicknesses on our '54 PA-18A

TR

What is this device called?

Edit: GE Inspection Technologies CL5 Ultrasonic Thickness Gauge. I see a number of similar devices out there that are described as "coating thickness measuring devices" but unsure if that will penetrate .035" steel.

Geeze, thanks. Another gizmo I would sure like to have. how muchy monies ?????

shorton wrote:Geeze, thanks. Another gizmo I would sure like to have. how muchy monies ?????

The unit pictured is no longer in production, however, it is available and as long as one carries out the calibration procedures prior to use, it does exactly what it was designed to do. This unit was recently available for $1000 as I recall from a company called Instrumart which sells and rents test equipment. This unit was in its rental arsenal and being phased out. There is a lot of science behind NDI in general and Ultrasonic Test (UT) specifically. Generally, if NDI is mandated it must be accomplished by a level II technician or above which is beyond the skillset of the typical A/P and IA. Having said that, if not mandated, have at it. I recommend some training on the particular unit and press on.

TR

For normal and successful ops, I'd agree it is a toss-up. When things go sideways, I like the idea of a steel roll cage surrounding the cockpit. I've seen the aftermath of two Maules that flew into trees (failed go-arounds) and while things were twisted badly, the cockpit area was intact. In both cases, the pilot escaped unscathed. I've only see one Cessna that hit trees and there were a lot of sharp edges from torn metal in the cockpit area. I don't know the fate of the pilot in that instance. Still, I find comfort in a stout steel tube structure around my seat.

I have always thought the chrome moly fuselage with aluminium wings is the way to go. If I was to build anything now it would be a Badlands traveller, or a Bearhawk. I could even go aluminium tail cone, but added complexity.

I saw a bare Stinson the other day and the fuselage is very solid. Definitely a safer structure in an impact.