Making Bad Designs Good

[tomo pauk]

I'm afraid that Airacuda is as bad as it gets - P-38 makes a far better use of turbo V-1710s, and as a bomber the A-20 will make much more sense.

The P-39 was always outfitted with supercharged engine. The collapsing nose gear is probably a myth, though a tail dragger will save not just save weight but also the volume, many times a necessity on a small fighter - P-39 was the smallest of both US fighters, plus smaller than Spit or Hurri. The performance was better then of P-40 of same era (P-39 was lighter than P-40), let alone of what Soviets or Japanese were producing in respective years. It will outpace any Hurricane or Wildcat, though these will climb better above 15000 ft.
The range was barely better thank what European SE fighters were doing, but less than P-40 or Japanese machines.
Much of the P-39s problem was the weight of the armament - similar of what P-38 carried. So the 1st step might be what Soviets did - get rid of the wing .30s and the ridicoluos 4000 ( fout thousand) rounds for those, saves 350 lbs and gains little bit of speed. Move the ram air intake into 10 or 2 o'clock position, for better use of ram effect that should, hopefully, improve a bit altitude performance. Replacing the 37mm with a belt-fed Hispano (another 100 lbs saved) will improve the duration of the cannon fire, it will also match the ballistics of the BMGs. Need be, add one or two BMG in the nose, where the P-39C have had synchronised .30s.

[KJ_Lesnick]

tomo pauk

I'm afraid that Airacuda is as bad as it gets

Yup, but with the modifications I specified it was essentially a different design...

P-38 makes a far better use of turbo V-1710s, and as a bomber the A-20 will make much more sense.

I personally prefer the P-38 over this, but this design was available two years earlier; the A-20 seemed to be only limited to around 3g maximum (ultimate might have been 4.5g) which is inadequate for a fighter.

The P-39 was always outfitted with supercharged engine.

The XP-39 was fitted with the integral supercharger, and a supercharger second-stage with intercooler; the turbo was removed in an effort to reduce weight and drag which was mentioned earlier on this thread.

The collapsing nose gear is probably a myth

Dunno, Weaver seems to say otherwise

a tail dragger will save not just save weight but also the volume, many times a necessity on a small fighter

Agreed

The range was barely better thank what European SE fighters were doing, but less than P-40 or Japanese machines.

Yeah, that was a major short-coming.  Don't know how the turbo would have worked out but it could carry drop-tanks.

[Gondor]

Surely we can boil this down into a few simple statements.

Designers do make a mess of things at times, either because they were given an impossible set of criteria to design to or they simply f#€k£d things up

Making the designs better is the job of the designers and engineers

We are model makers, we can with our imagination take anything regardless of how well it worked to begin with, or not, and make something wonderful from it.

That what we do and that's what this site is for.

Simple!

Gondor

[Weaver]

The P-39 was always outfitted with supercharged engine. The collapsing nose gear is probably a myth,

As you might have guessed, I've got a book that does a good job of separating myth from reality for the P-39. The nose gear problem was real. It's not a generic problem of all nose gears, just a fault in this particular design.

Much of the P-39s problem was the weight of the armament - similar of what P-38 carried. So the 1st step might be what Soviets did - get rid of the wing .30s and the ridicoluos 4000 ( fout thousand) rounds for those, saves 350 lbs and gains little bit of speed. Move the ram air intake into 10 or 2 o'clock position, for better use of ram effect that should, hopefully, improve a bit altitude performance. Replacing the 37mm with a belt-fed Hispano (another 100 lbs saved) will improve the duration of the cannon fire, it will also match the ballistics of the BMGs. Need be, add one or two BMG in the nose, where the P-39C have had synchronised .30s.

Agreed. Couple of related points:

1. If the 350lbs from the wing guns is worth saving then surely the 300+lbs from the nosewheel is worth saving too.

2. Most P-39s has two 0.50cal BMGs in the upper nose. I think that by losing the nose gear, you can get a third one on the lower centreline. I'm not sure a fourth one is doable: the prop gearbox still needs some hefty support however you arrange the fuselage structure.

[Weaver]

Designers do make a mess of things at times, either because they were given an impossible set of criteria to design to or they simply f#€k£d things up

While designers do indeed make straight up mistakes, my impression from all the accounts of successful and unsuccessful aircraft that I've read is that the following account for the majority of bad aircraft:

1. The aircraft's performance pushes the limits of knowledge and it hits problems that nobody could reasonably have expected before somebody went there.

2. The armed services asked for the wrong thing and the designers dutifully gave them what they asked for.

3. The armed forces asked for the right thing, the designers wanted to give it to them, but the politicians/beaurocrats wouldn't pay for it and insisted on cutting the budget until what was left was fatally compromised.

You can, of course, mix and match any or all of the above.....

[sandiego89]

Designers do make a mess of things at times, either because they were given an impossible set of criteria to design to or they simply f#€k£d things up

While designers do indeed make straight up mistakes, my impression from all the accounts of successful and unsuccessful aircraft that I've read is that the following account for the majority of bad aircraft:

1. The aircraft's performance pushes the limits of knowledge and it hits problems that nobody could reasonably have expected before somebody went there.

2. The armed services asked for the wrong thing and the designers dutifully gave them what they asked for.

3. The armed forces asked for the right thing, the designers wanted to give it to them, but the politicians/beaurocrats wouldn't pay for it and insisted on cutting the budget until what was left was fatally compromised.

You can, of course, mix and match any or all of the above.....

Good list Weaver, I would add 4. Aicraft that were let down down by unreliable engines, and/or enigines that did not produce specified/called for power/thrust. Especially true of some early jets. Many listed power ratings were more hoped for than actually met.

[scooter]

Good list Weaver, I would add 4. Aicraft that were let down down by unreliable engines, and/or enigines that did not produce specified/called for power/thrust. Especially true of some early jets. Many listed power ratings were more hoped for than actually met.

Like the Cutlass.  Interesting design, had the potential for being a great design, but the tiny Westinghouses just made it gutless.

[Weaver]

Designers do make a mess of things at times, either because they were given an impossible set of criteria to design to or they simply f#€k£d things up

While designers do indeed make straight up mistakes, my impression from all the accounts of successful and unsuccessful aircraft that I've read is that the following account for the majority of bad aircraft:

1. The aircraft's performance pushes the limits of knowledge and it hits problems that nobody could reasonably have expected before somebody went there.

2. The armed services asked for the wrong thing and the designers dutifully gave them what they asked for.

3. The armed forces asked for the right thing, the designers wanted to give it to them, but the politicians/beaurocrats wouldn't pay for it and insisted on cutting the budget until what was left was fatally compromised.

You can, of course, mix and match any or all of the above.....

Good list Weaver, I would add 4. Aicraft that were let down down by unreliable engines, and/or enigines that did not produce specified/called for power/thrust. Especially true of some early jets. Many listed power ratings were more hoped for than actually met.

Yes, I agree. It happened plenty of times in the piston era too. Look at the Manchester/Vulture, Whirlwind/Peregrine, He 177/Db 606, Beaufort/Taurus etc...

[Mossie]

I think that applies to the F11F Tiger.  Good airframe, let down by it's J65 engine.  When they put a J79 (F11F-1F) in it, they got something world class, then on your list of 1-4, I think number 17 happened!

[Weaver]

I think that applies to the F11F Tiger.  Good airframe, let down by it's J65 engine.  When they put a J79 (F11F-1F) in it, they got something world class, then on your list of 1-4, I think number 17 happened!

According to Corky Meyer, there was a proposal to put an Avon in it for Germany in competition with the Starfighter too. Probably not quite a fast (the Avon was in between the J-65/Sapphire and the J-79) but it's a lovely thought, nonetheless. I have an Avon Tiger with 2 x ADEN cannon and AI.23 on my to-do list of alternative FAA types.

[tomo pauk]

As you might have guessed, I've got a book that does a good job of separating myth from reality for the P-39. The nose gear problem was real. It's not a generic problem of all nose gears, just a fault in this particular design.

Interesting - what is the book's name?

Agreed. Couple of related points:

1. If the 350lbs from the wing guns is worth saving then surely the 300+lbs from the nosewheel is worth saving too.

Per 'America's hundred thousand', whole undercarriage of a P-39 weights 515-517 lbs ( vs. the U/C of the P-40 being at 630-650 lbs, for comparison sake). Nose gear representing maybe 1/3rd of that - 170-180 lbs? But, granted, every little bit of shaved weight will help with climb rate.

2. Most P-39s has two 0.50cal BMGs in the upper nose. I think that by losing the nose gear, you can get a third one on the lower centreline. I'm not sure a fourth one is doable: the prop gearbox still needs some hefty support however you arrange the fuselage structure.

Fuselage and engine section of the P-39 weighted less than fuselage and engine setcion of the P-40 - 620-630 lbs vs. 690-720 lbs, depending on the model.

The P-39C was the only version that featured a pair of synchronised MGs of .30 calibre, the receivers of those MGs being located, roughly, under the barrels of the BMGs. There should be a place for the 3rd synchronised BMG instead of the two .30s even without change of U/C layout.
Historically, the P-39C lost the synchronised .30s when P-39D emerged, that one received the four .30s in the wings, as well as protection for the tanks, now having 120 gals instead of 170 gals in unprotected tanks of the P-39C.

[sandiego89]

Designers do make a mess of things at times, either because they were given an impossible set of criteria to design to or they simply f#€k£d things up

While designers do indeed make straight up mistakes, my impression from all the accounts of successful and unsuccessful aircraft that I've read is that the following account for the majority of bad aircraft:

1. The aircraft's performance pushes the limits of knowledge and it hits problems that nobody could reasonably have expected before somebody went there.

2. The armed services asked for the wrong thing and the designers dutifully gave them what they asked for.

3. The armed forces asked for the right thing, the designers wanted to give it to them, but the politicians/beaurocrats wouldn't pay for it and insisted on cutting the budget until what was left was fatally compromised.

You can, of course, mix and match any or all of the above.....

Good list Weaver, I would add 4. Aicraft that were let down down by unreliable engines, and/or enigines that did not produce specified/called for power/thrust. Especially true of some early jets. Many listed power ratings were more hoped for than actually met.

Yes, I agree. It happened plenty of times in the piston era too. Look at the Manchester/Vulture, Whirlwind/Peregrine, He 177/Db 606, Beaufort/Taurus etc...

For the jet era, I nominate the Allison T-40, the coupled turboprop as the most flawed "production" engine. Used in 8 aicraft designs, none really successful.  J-40 runner up perhaps.  Granted this was the dawn of big turboprops and big tubojets, but they let down a few promissing aircraft. 

[KJ_Lesnick]

tomo pauk

Per 'America's hundred thousand', whole undercarriage of a P-39 weights 515-517 lbs ( vs. the U/C of the P-40 being at 630-650 lbs, for comparison sake).

I don't think it's a big deal

Fuselage and engine section of the P-39 weighted less than fuselage and engine setcion of the P-40 - 620-630 lbs vs. 690-720 lbs, depending on the model.

Still, having a different canopy that slid back or even one more like the P-38 would be preferable.

The issue then comes down to the turbo-layout: It seems better than nothing, however was the poor airflow based on the length of the plane?

[tomo pauk]

The 'total engine accesories' drag (including the unfaired(!) turbo and four waste gates, along with coling drag) was found to be excessive on the XP-39 - Cd0 of that was 0,0152, vs 0,0063 for the same thing on the XP-38, vs. Cd0 of 0.0176 for the whole P-51D in combar trim!
The capacity of intercooler was found to be sorely lacking, no doubt due to the lack of any control of boundary layer, along with absence of any flap that will control the air flow. The cooling was of too low a capacity for climb, too high above the simulated 300 mph in the wind tunel.

[KJ_Lesnick]

tomo pauk

The 'total engine accesories' drag (including the unfaired(!) turbo and four waste gates, along with coling drag) was found to be excessive on the XP-39 - Cd0 of that was 0,0152, vs 0,0063 for the same thing on the XP-38, vs. Cd0 of 0.0176 for the whole P-51D in combar trim!

Firstly

• I know Cd is coefficient of drag: What's Cd0? 
  
• As for the numbers are they linear or exponential?
  

Secondly

• How many waste gates are normally used in an aircraft turbocharger of that era?
  
• Was the turbos normally faired?
  
• Coling drag = Cooling drag or Cowling Drag?
  

The capacity of intercooler was found to be sorely lacking, no doubt due to the lack of any control of boundary layer, along with absence of any flap that will control the air flow.

Okay

• What methods were normally used to control the boundary layer?
  
• When you say "any flap that will control the air flow" do you mean a diverter?
  

Was the intercooler an air-to-air cooler or a liquid-to-air cooler?