Why did Mooney use steel framing for the cabin?

[scottfromiowa]

To get strong inside?...or was that to get to the other side?....

[Shadrach]

Quote: RobertE

Someone asked if I thought Mooneys suffered more from corrosion than other aircraft.  I believe the answer is yes because a) two dissimilar metals in electrical contact give the opportunity for galvanic corrosion and aluminum oxide is dense enough to tend to stop much corrosion once it begins but steel isn't and, so, you gotta paint those tubes to provide a physical barrier, early on with the zinc chromate and, lately, with tougher epoxy paint.  These aren't unknown or trivial problems so there must have been a reason and my guess is cost.  Anyone else got an opinion other than cost given that aluminum structures can be made just as strong?

[RobertE]

Hmm.  I guess this is a tough group.  I plead guilty that I don't know of a single instance of galvanic corrosion.  I seem to have implied that brother Al would be so base as to have allowed matters of commerce ever to have entered into his thinking.

I also asked the group if, other than cost, anyone had an idea why steel was used, mindful that within a number of years the factory changed to using epoxy paint to cover the tubes.  My guess is that perhaps corrosion was the reason for the change?  Had an aluminum structure been used instead of steel I doubt that a typical pre-purchase inspection would be so focused on determining the type of paint applied to the tubes and the type of insulating material (the early type that retained water or the later type that retained less).  So, again, anyone got a good theory of why, other than cost, steel is preferable to another material?

[KSMooniac]

Quote: aviatoreb

Materials wise - these days Ti is not nearly as expensive as you think.  One of my bikes I had built in China directly from the OEM.  I was going to buy from a US company that makes a big deal about made in America, which I really believe in, but I got annoyed when it turns out that they outsource many of their jobs.  I believe in the American worker but I do not believe in the American middle man - meaning I was happy to be my own middle man on this one.  So I found that this company makes parts for dozens and dozens of bicycle manufacturers in the US and Europe,

http://www.xacd.com.cn/product.asp?rootcl=1

A domestic built custom Ti frame cost $5-$6k when I purchased about 5 years ago.  These guys built me my frame for $450.  Fully custom, and I worked with the engineer and the CAD drawing to make it exactly as I wished.  It was so inexpensive I was worried that the quality of the built and also the materials would be low.  No - it is PERFECT.  The welds are gorgeous, the angles are PERFECT, the machining, everything is gorgeous.  $450 for a Ti frame is crazy inexpensive and that is what our middle men get (even better since they buy in bulk) and then charge $5k.  Can't blame them....  But can't blame me for being my own middle man.  It was a learning experience about the international trade economy and I had them make me a "29'er" mtn bike frame.

They also make parts for the international aerospace industry and other industries too.  They also sell raw tubes to your spec (thickness, which Ti alloy, diameter, etc).

I believe that the materials cost would be small/lost in the noise relative to the STC cost.

Welding Ti is a special artisan skill separate from welding steel. But there are plenty of folks who are very skilled in it, including in the bicycle industry that is one industry that I think the skill would transfer well to building Mooneys.  I am not sure if a replacement of steel tubes for Ti would even be necessary since Ti must be welded in an O2 free environment, and I do not know if it can be welded to other parts that are not Ti - like steel.  Can one replace every single tube in a Mooney?!  I will guess no.  ANd even if yes, the materials cost would be miniscule compared to the STC cost.  And labor would be crazy high too.

Now building a mooney from the ground up in Ti tubes would be a different matter.   That would be technically quite doable.  I presume that would require some kind of major change to the type certificate.  Wouldn't that be COOOL!  And Ti skins too....  You wouldn't even need to paint the airplane, just like a bicycle - a beautiful finish for a Ti bike is polished - brushed is also nice.

[KSMooniac]

Quote: RobertE

I also asked the group if, other than cost, anyone had an idea why steel was used, mindful that within a number of years the factory changed to using epoxy paint to cover the tubes.  My guess is that perhaps corrosion was the reason for the change?  Had an aluminum structure been used instead of steel I doubt that a typical pre-purchase inspection would be so focused on determining the type of paint applied to the tubes and the type of insulating material (the early type that retained water or the later type that retained less).  So, again, anyone got a good theory of why, other than cost, steel is preferable to another material?

[Shadrach]

Quote: RobertE

Hmm.  I guess this is a tough group.  I plead guilty that I don't know of a single instance of galvanic corrosion.  I seem to have implied that brother Al would be so base as to have allowed matters of commerce ever to have entered into his thinking.

I also asked the group if, other than cost, anyone had an idea why steel was used, mindful that within a number of years the factory changed to using epoxy paint to cover the tubes.  My guess is that perhaps corrosion was the reason for the change?  Had an aluminum structure been used instead of steel I doubt that a typical pre-purchase inspection would be so focused on determining the type of paint applied to the tubes and the type of insulating material (the early type that retained water or the later type that retained less).  So, again, anyone got a good theory of why, other than cost, steel is preferable to another material?

[jwilkins]

I'm not sure where the idea that steel was choosen for 'cost reasons' came from.

The early Mooneys were engineered for strength and safety within the constraints of being able to manufacturer them. The very early tube fuselages had the tubes filled with oil to prevent rusting. 

I used to assist at a shop and got to see a lot of aircraft in for annulas and repairs. I have seen in person and photographs of rust on tubes and none of these few examples were obviously galvanic. If you had an aluminum and steel assembly submerged in an electrolytic solution (like salt water) the galvanic action would appear at the mating surfaces. 

The rust I've seen was in areas that got wet from leaking windows and stayed wet or moist. I saw a spar that was corroded from passengers spilling coke, but the corrosion was not near the fuselage tubes.  Some aircraft that spent time near the coast showed corrosion but not necessarily between steel and aluminum; just where the spar was exposed to moist salty air. Mice peeeing in tail cones leaves very obvious surface corrosion.

As was pointed out, when these planes were new, there was no '50 year' corrosion or rust data. Steel tubes were choosen because they were strong, resilient, and the welds a can be as strong as the base structure. Aluminum tubes fracture easier thna steel and wodul not absorb as much impact as the steel.

Bonanzas have had Magnesium skins fall off because they deteriorated where the factory layout guys used a pencil to layout the skin. The graphite reacted with the magnesium and, in a couple cases. ate through the skin to the paint. There was absolutely no data available to predict this, and teh fact that a corrosion issue later developed certinaly does no tmean the design decision touse Mg skins was made for 'cost reasons'.

I think in 50 years we will very possibly see major issues with the composite aircraft being manufacturered now. 

Engineers do the best job they can with the materials and information available at the time. Designs also must be manufacturable and meet cost production targets. 

If you read more of the history about Mooney, Piper, Beech, and Cessnas early days you'll have a better feeling for how the engineering decisions were made. Most of the time it was NOT 'cost reasons'. I believe the contemporary development of composite aircraft may have been driven by cost and produciton constraints more than the original M/P/B/C aircraft but that is just an impression based on articles about how efficient these aircraft are to make compared to our conventional ones.

As a manufacturing person and an engineer I take exception to the statement that steel fuselage tubes were a 'cost' decision.

I think it was an engineering decison which,like the Bonanza magnesium skins, had some long term effects whcih were not predicted at the time. Even if the rusting over 50 years had been predicted I'm not sure what would have been done differently.  Has everyone here had their plane corrosion treated? Do all the TKS planes have the tanks removed every year to check for corrosion?  As owners we don't always do everything we coudl tomitigate oem of these issues, either.

Steel fuselage tubes are a good choice. I'd much rather fly inside a steel fuselage structure than a plastic composite. Exotic material such as Ti and Al alloys may or may not have other unintended consequences. I'll leave that up to the engineers.

Jim

 

 

 

 

[RobertE]

I could certainly be wrong in my guess that cost was the driver.  I do, of course, know that steel is used in every aircraft (my first job out of college was as a salesman for a major, integrated steel mill so I got familiar enough with its properties to be dangerous!) but I think it's routinely used in those situations in which toughness is needed (engine web, portions of the landing gear) that are unlikely to be continuously exposed to moisture.  I also admit to being only knowledgeable enough about galvanic corrosion to be dangerous there too, but wouldn't any corrosion suffered by the tubing due to contact with damp insulation be accelerated by virtue of the numerous contact points the steel frame has elsewhere with the aluminum skin?  That is, wouldn't galvanic corrosion exist not just at the junction between, say, the steel and aluminum, but anywhere in which an electrolite comes in contact with the steel tubing?

 

[Shadrach]

Quote: RobertE

I could certainly be wrong in my guess that cost was the driver.  I do, of course, know that steel is used in every aircraft (my first job out of college was as a salesman for a major, integrated steel mill so I got familiar enough with its properties to be dangerous!) but I think it's routinely used in those situations in which toughness is needed (engine web, portions of the landing gear) that are unlikely to be continuously exposed to moisture.  I also admit to being only knowledgeable enough about galvanic corrosion to be dangerous there too, but wouldn't any corrosion suffered by the tubing due to contact with damp insulation be accelerated by virtue of the numerous contact points the steel frame has elsewhere with the aluminum skin?  That is, wouldn't galvanic corrosion exist not just at the junction between, say, the steel and aluminum, but anywhere in which an electrolite comes in contact with the steel tubing?

[jwilkins]

Let's shift this discussion just a little:

The steel tubing can rust (which is  a form of corrosion).

The aluminum can corrode.

Both are accelerated by introducing moisture and electrolytes (salt or pollutants)

In theory Galvanic Corrosion could occur where steel and aluminum are in direct contact with an electrolyte.

Electrolysis effects (long distance voltage potential from displaced areas of contact) are probably not a significant risk.

Steel was most likely choosen as the best material available that met the engineering and product requirements for tensile strength, weight, cost, and ability to manufacture and repair. 

All of this dscussion is really important if it gets owners to think about corrosion inspections and periodic treatment.

Bruce Jaeger (former owner of Wilmar) has photos of Mooneys that are scrap due to unchecked corrosion.

It's REALLY important that we pay attention to this potential for corrosion; SB208 and periodic corrosion treatments.

Jim

[MooneyMitch]

I recently asked Mooney legend Bill Wheat if Mooney ever filled the steel tubing framework with any type of oil for corrosion prevention.  Below is his reply:

The Mites and M20 serial # 1001 (The prototype M20 series) were.  After they were treated with Linseed oil they were drained and the holes had Drive Pins driven in the drain holes to block them.

 

Could have saved a lot of problems if that had been continued.

 

Bill Wheat