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superstring
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Steel As An Aircraft Material

Mon Nov 19, 2007 4:48 pm

Well this question has already been discussed in a thread before however I want to ask it again to cover several points in one question:

Let's start with the main question:

---Why is no airframe manufacturer using steel to build the fuselage?---
(for examle to make the shells of the fuselage/hull out of steel...and why is Aluminum used so often)

So let's now cover several answerers:

->Weight: ALuminum for example has only a density of 2.7 km/dm^3 (compared to 7.8 kg/dm^3 for steel) HOWEVER if one compares the Young modulus of AL to steel (70 GPa for AL vs. 210 GPa for steel) or the Tensile strength (up to 1500 N/mm^2 for steel vs. about 400 N/mm^2 for AL) then the density factor is not an advantage any more: Steel structures could be thinner because of its higher strength of the material (compared to AL))

->Corrosion: Well, stainless steels offer supreme qualties, (AL for example also has to deal with corrosion)
->Price: Well can anybody explain why steel should be so much more expensive than AL, or carbonfiber structures (consider the development costs for such CFP structures)

So what is the answer to my question?
 
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Scooter01
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RE: Steel As An Aircraft Material

Mon Nov 19, 2007 6:26 pm

Maybe this will give an ansver to your question: http://flyingtigerline.org/history/1940s.htm

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2H4
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RE: Steel As An Aircraft Material

Mon Nov 19, 2007 6:30 pm

The Budd RB-1 (C-93) Conestoga was stainless steel:


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Filton
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RE: Steel As An Aircraft Material

Mon Nov 19, 2007 6:35 pm

Stiffness would be one reason.

In theory, you could make a Steel structure lighter than an Aluminium one that carries the same load. However, in practice it would be so thin it would flex so much that it would be useless. Imagine a fuselage made of foil.


Ductility is another.

High strength steels are quite brittle and tend to fracture rapidly after reaching the yield point. Aluminium deforms more plastically after yield but remains intact. Essentially it bends before it breaks, which is preferable for impact situations.
 
scrubbsywg
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RE: Steel As An Aircraft Material

Mon Nov 19, 2007 7:22 pm

well, you have to remember youngs modulus also shows how stiff a material is. E, young modulus, is the slope of the stress strain curve. Steel is much stiffer, and since airplanes do flex, its probably not the best. Since E is the slope, for the same incremental strain(elongation) the stress of steel rises more than that of aluminum.


Another factor is probably workability. Have you ever tried drilling steel vs. aluminum? Often times you will wear out a drill used on steel faster than on aluminum. I would imagine the benefit of the smaller size can be detrimental in some cases as well. If you need a quater inch thick piece of aluminum, sure you may be able to replace it with a steel piece 1/16" thick, but now you have a much smaller piece. Less room needed for a crack to propagate, tolerences may have to be tightened increasing cost, machining smaller peices can take more time than larger ones. Sometimes bigger is better.

Yeet another is ductile to brittle transition. While steel is a ductile material at room temperature, it transitions to brittle when it gets cold. While the temperature that aluminum and steel cross is quite low, it can also be another factor.

Stainless i can imagine is not really used due to the cost. Stainless must cost more than the aluminum alloys, i must assume.


These are just some points that i remember from materials classes. They may or may not be contributing factors in airplane design. I would still imagine that the weight is the major thing.
 
KELPkid
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RE: Steel As An Aircraft Material

Mon Nov 19, 2007 10:50 pm



Quoting 2H4 (Reply 2):
The Budd RB-1 (C-93) Conestoga was stainless steel:

The Budd Manufacturing Co. also specialized in the manufacture of stainless steel passenger railroad cars and busses. They used full-monocoque construction, where the skin was the structure, except on the ends  Smile This made their passenger cars significantly lighter than their competitors...
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MrFord
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 1:09 am



Quoting KELPkid (Reply 5):
The Budd Manufacturing Co. also specialized in the manufacture of stainless steel passenger railroad cars and busses.

I didn't know they tried their way into airplanes... But then, it does look kinda like an R-32 fomr the New York subway system  Wink
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hangarrat
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 2:25 am



Quoting KELPkid (Reply 5):
The Budd Manufacturing Co. also specialized in the manufacture of stainless steel passenger railroad cars

I ride to work on derivatives of those every day. And, man, are some of them ancient.

http://en.wikipedia.org/wiki/Image:SEPTA_Silverliner_II.jpg
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tdscanuck
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 3:09 am



Quoting Superstring (Thread starter):

---Why is no airframe manufacturer using steel to build the fuselage?---
...
->Weight:
...
So what is the answer to my question?

Weight.

On an aircraft, the loads are fixed. So the key is how much weight do you need to carry a particular load. Young's modulus (E) doesn't have anything to do with how much load you can carry, so it's not the major factor. Yield strength is. The ratio of yield strength to density is much much better for aluminum than for most any other widely available metal. Also, for a lot of aerostuctures 2nd moment of inertia is very important, which is primarily a function of density, and low density gives you better 2nd moment of inertia.

Tom.
 
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jetmech
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 3:43 am

Quoting Tdscanuck (Reply 8):
Also, for a lot of aerostuctures 2nd moment of inertia is very important, which is primarily a function of density, and low density gives you better 2nd moment of inertia.

Are you talking about mass moment of inertia (i.e. resistance to angular acceleration), or the area moment of inertia (i.e. a beam cross section's distribution of material about neutral axis)? I presume you are talking about the former.

Quoting Tdscanuck (Reply 8):
Young's modulus (E) doesn't have anything to do with how much load you can carry, so it's not the major factor.

True, but Young's modulus does become important when designing for compressive loads and buckling of structures subjected to compression. If everything else is kept the same, doubling (E) allows you to carry twice the compressive load before buckling occurs.

Nonetheless, that ratio of material properties to density is important as you have mentioned. Although steel has a Young's modulus three times higher than aluminium, it is also 2.88 times more dense. Given that the area moment of inertia is calculated with fourth powers, an aluminium column supporting a given compressive load may still be somewhat lighter than an equivalent steel column, although it's outer dimensions would be larger.

Quoting Superstring (Thread starter):
Weight: ALuminum for example has only a density of 2.7 km/dm^3 (compared to 7.8 kg/dm^3 for steel) HOWEVER if one compares the Young modulus of AL to steel (70 GPa for AL vs. 210 GPa for steel) or the Tensile strength (up to 1500 N/mm^2 for steel vs. about 400 N/mm^2 for AL) then the density factor is not an advantage any more: Steel structures could be thinner because of its higher strength of the material (compared to AL))

As an example, a square section column made of aluminium would need to have external dimensions that are the fourth root of three times larger (1.326) to take the same compressive load without buckling compared to a steel column. This would make the aluminium column have a greater volume to the tune of the square root of three (column height does not scale). Thus, the aluminium column would have 1.732 times the volume of an equivalent steel column. Given the density ratios of the two materials, the steel column would still be 1.66 times heavier.

Regards, JetMech

[Edited 2007-11-19 19:45:30]
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mark5388916
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 4:46 am

IIRC the MiG-23 Flogger had much of its structure made of steel due to complexity of titanium at the time. Please correct me if I'm wrong.

Mark
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redflyer
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 5:02 am



Quoting Mark5388916 (Reply 10):
IIRC the MiG-23 Flogger had much of its structure made of steel due to complexity of titanium at the time. Please correct me if I'm wrong.

I think it was the MiG-25 Foxbat that had a steel structure (but I could be wrong). Also, I believe much of the XB-70 was made of steel as well.
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57AZ
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 5:25 am



Quoting HangarRat (Reply 7):
I ride to work on derivatives of those every day. And, man, are some of them ancient.

Try working on one of them-they're a real pain. The Rail Diesel Car was a good idea financially, but was only really suited to longer runs and not commuter service. Their twin 8 cylinder Detroits were a real weak spot-couldn't generate enough power to get the car up to speed in a short distance.

I was aware of the attempt by Budd to use stainless steel in the Conestoga. I should also point out that stainless steel (when not used in full monocoque construction) can create hidden internal problems. The railroads that bought Pullman Standard's first stainless steel cars found that out when they realized that the fluted stainless steel sides were poorly sealed and the substructures of the cars were suffering severe rusting due to moisture getting trapped.

However, technology has advanced with carbon fiber materials that essentially allow the aviation industry to obtain a building material that has the strength of steel, but the weight and flexibility of aluminum. That said, I would suggest that with the exception of a few applications, the discussion of steel in aircraft manufacture is mostly academic.
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tdscanuck
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 5:26 am



Quoting JetMech (Reply 9):
Quoting Tdscanuck (Reply 8):
Also, for a lot of aerostuctures 2nd moment of inertia is very important, which is primarily a function of density, and low density gives you better 2nd moment of inertia.

Are you talking about mass moment of inertia (i.e. resistance to angular acceleration), or the area moment of inertia (i.e. a beam cross section's distribution of material about neutral axis)? I presume you are talking about the former.

Actually, I was talking about the latter (area moment of inertia). In several applications, primarily fuselage and wing stringers, that is the major factor in determining the load on the stringer.

Quoting JetMech (Reply 9):
Quoting Tdscanuck (Reply 8):
Young's modulus (E) doesn't have anything to do with how much load you can carry, so it's not the major factor.

True, but Young's modulus does become important when designing for compressive loads and buckling of structures subjected to compression. If everything else is kept the same, doubling (E) allows you to carry twice the compressive load before buckling occurs.

Good point, although I think that would be for columns of equal I (equal cross section), which would be considerably heavier for a steel column than an aluminum one.

Tom.
 
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jetmech
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 5:59 am



Quoting Tdscanuck (Reply 13):
Good point, although I think that would be for columns of equal I (equal cross section),

Yep, that statement was referring to columns of equal I value. The example I gave with numbers was for columns of equal capacity to carry a certain compressive load and not buckle.

Quoting Tdscanuck (Reply 8):
Also, for a lot of aerostuctures 2nd moment of inertia is very important, which is primarily a function of density, and low density gives you better 2nd moment of inertia.

Do you mean that low density indirectly gives you better second moment of area (i.e. lower density would indirectly require more cross sectional area, which if placed correctly would increase the I value)?

Regards, JetMech
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airfoilsguy
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 12:49 pm



Quoting ScrubbsYWG (Reply 4):
Stainless i can imagine is not really used due to the cost. Stainless must cost more than the aluminum alloys, i must assume

This is what I usually pay when I buy steel for my company. I don't buy aluminum so I don't know what the cost is.

Carbon steel -----------------.23 to .32 dollars a pound
4340 300 M steel .-------- .56 to 2.13 dollars a pound
8822 .---------------------------.39 dollars a pound
410 stainless .---------------.61 dollars a pound
9310 ----------------------------5.45 dollars a pound
Waspaloy --------------------20.8 dollars a pound
Titanuum ---------------------24.77 dollars a pound
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superstring
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RE: Steel As An Aircraft Material

Tue Nov 20, 2007 10:34 pm



Quoting JetMech (Reply 9):
As an example, a square section column made of aluminium would need to have external dimensions that are the fourth root of three times larger (1.326) to take the same compressive load without buckling compared to a steel column. This would make the aluminium column have a greater volume to the tune of the square root of three (column height does not scale). Thus, the aluminium column would have 1.732 times the volume of an equivalent steel column. Given the density ratios of the two materials, the steel column would still be 1.66 times heavier.

Indeed this is a very good example, however there is one point which should be mentioned here as these calculations using the Second moment of area have a flaw: Consider the fuselage of an aircraft: Indeed everyone who has ever witnessed how such a shell is made (of course I speak of "traditional" Aluminium) will agree that the hull itself is just AL without any stiffness (I mean it would flex enormously). It are the stingers and frames which make the structure stable.

Quoting Filton (Reply 3):
Stiffness would be one reason.

In theory, you could make a Steel structure lighter than an Aluminium one that carries the same load. However, in practice it would be so thin it would flex so much that it would be useless. Imagine a fuselage made of foil.

So the point is that the 2nd moment of area of the fuselage hull is mainly the result of the stingers and frames and not of the sheet itself. So let's again compare the density quotients of steel vs AL (7.8 kg/dm^3 vs 2.7), the quotient is ~2.9. However comparing the tensile strengths of steel vs. AL (lets assume 1600 vs. 400 N/mm^2) the factor is ~4. For the thickness of the hull's sheet the shear stress is the important factor --> So the thickness could be reduced by the factor of 4, density disadvantage is 2.9 --> So in this case the steel structure would win as its mass would only be 0.73 x that of the equivalent AL structure....
 
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jetmech
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RE: Steel As An Aircraft Material

Wed Nov 21, 2007 12:40 am



Quoting Superstring (Reply 16):
Indeed everyone who has ever witnessed how such a shell is made (of course I speak of "traditional" Aluminium) will agree that the hull itself is just AL without any stiffness (I mean it would flex enormously). It are the stingers and frames which make the structure stable.

True, but you must also remember that it is the material that is furthest away from the neutral axis that makes the greatest contribution to the second moment of area. Indeed, the general rule when designing a beam is the get as much of the material as far away from the neutral axis as practical. I do agree however that the skin being so thin may not contribute that much, especially on the bottom of the fuselage which is under compression.

The scariest thing is to look at the spars of wings. These often have very beefy, extruded caps, yet the web between the spar caps is often no more that sheet aluminium!

Regards, JetMech
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tdscanuck
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RE: Steel As An Aircraft Material

Wed Nov 21, 2007 1:37 am



Quoting JetMech (Reply 14):
Quoting Tdscanuck (Reply 8):
Also, for a lot of aerostuctures 2nd moment of inertia is very important, which is primarily a function of density, and low density gives you better 2nd moment of inertia.

Do you mean that low density indirectly gives you better second moment of area (i.e. lower density would indirectly require
more cross sectional area, which if placed correctly would increase the I value)?

Yes, that's what I meant. For a given load, you need more area of aluminum to handle it (due to lower yield strength). That inherently gives you a higher I (assuming you shape the beam intelligently).

Quoting Superstring (Reply 16):
So the point is that the 2nd moment of area of the fuselage hull is mainly the result of the stingers and frames and not of the sheet itself. So let's again compare the density quotients of steel vs AL (7.8 kg/dm^3 vs 2.7), the quotient is ~2.9. However comparing the tensile strengths of steel vs. AL (lets assume 1600 vs. 400 N/mm^2) the factor is ~4. For the thickness of the hull's sheet the shear stress is the important factor --> So the thickness could be reduced by the factor of 4, density disadvantage is 2.9 --> So in this case the steel structure would win as its mass would only be 0.73 x that of the equivalent AL structure....

Except you always assume that the skin doesn't carry bending loads, only the stringers. Likewise, you assume the stringers carry no shear, only the skin. Somewhere I have the notes that go with this, but it basically comes down to the fact that you end up with a density squared term in the strength of a shear panel, which hugely disadvantages steel.

I'll see if I can find this or rederive it tonight.

Tom.
 
rwessel
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RE: Steel As An Aircraft Material

Wed Nov 21, 2007 11:45 pm



Quoting Superstring (Reply 16):
Consider the fuselage of an aircraft: Indeed everyone who has ever witnessed how such a shell is made (of course I speak of "traditional" Aluminium) will agree that the hull itself is just AL without any stiffness (I mean it would flex enormously). It are the stingers and frames which make the structure stable.

That's a bit oversimplified. The skin in semi-monocoque construction very definitely contributes to the stiffness of the structure. Without the skin, the frame would be quite floppy too, although in different directions. In a very real sense, the frame prevents the skin from bending, and the skin prevents the frame from bending.

Of course purely framed aircraft are possible too, although they tend to be covered in fabric.
 
KELPkid
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RE: Steel As An Aircraft Material

Thu Nov 22, 2007 2:14 am



Quoting 57AZ (Reply 12):
I was aware of the attempt by Budd to use stainless steel in the Conestoga. I should also point out that stainless steel (when not used in full monocoque construction) can create hidden internal problems. The railroads that bought Pullman Standard's first stainless steel cars found that out when they realized that the fluted stainless steel sides were poorly sealed and the substructures of the cars were suffering severe rusting due to moisture getting trapped.

Corrosion induced by dissimilar metals (i.e. a Galvanic reaction)?  Wink
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tdscanuck
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RE: Steel As An Aircraft Material

Thu Nov 22, 2007 5:57 am



Quoting Tdscanuck (Reply 18):
Somewhere I have the notes that go with this, but it basically comes down to the fact that you end up with a density squared term in the strength of a shear panel, which hugely disadvantages steel.

I'll see if I can find this or rederive it tonight.

OK, I ran myself through the derivation and now I know where it comes from.

Most aircraft structures are beams and shear panels. For a given load (ignoring buckling for the moment), the mass of a beam or shear panel is proportional to the density divided by the square root of the yield strength. If you want the derivation, let me know. I remembered there was a power involved, I just couldn't remember what it was before.

What this means, in practical terms, is that the density dominates the overall mass much more than the yield strength.
If you look at the ratio of density to square root of yield strength for typical aluminum and steel alloys, you find that for aluminum it's about half of what steel is.

If you bring buckling back into the mix, buckling depends on E and I. E for steel is about 3 times that of aluminum, but I goes as the reciprocal of the yield strength to the 3/2 power, which has about a 2:1 advantage for aluminum. Net result is that steel is better in buckling, but not so much better as to override the basic advantage aluminum has from the basic loading.

Tom.

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