Improve That Design: How Aircraft Could Have Been Made Better

[drgondog]

Hello Tomo Pauk,
This seems like a pretty minor difference for some potentially great reductions in drag: The Mustang and other aircraft with radiators and oil coolers located very far aft on the fuselage all have longer tubes for oil or coolant but the aft radiator location seems to be the optimal setup. Note that this was done even for the oil cooler on the La-7 which had its oil cooler moved quite a bit further aft than on La-5FN.
The P-38 Lightning also had rather long coolant lines back to its radiators.



Hello drgondog,
I can't recall the source, but I have seen a mention that at the same cruise power settings, the earlier aircraft were noticeably faster so there was a significant difference in drag in the new intercooler setup. The maximum speed of the later aircraft was much increased because they were able to run higher sustained maximum power settings because of the increased efficiency of these same improved intercoolers.

- Ivan.

Define "noticeably faster" and separate Induced Drag increases due to more GW? Stick with P-38J-5/-10 comparison because earlier versions couldn't exploit the extra HP yet.

 

[Zipper730]

Major re-design and re-tool - perhaps a year's delay in production that War Production Board would never approve.

That bad?

 

[pbehn]

That bad?

There were a huge number of plane types that weren't improved when they could have been because it would have resulted in less planes in service. Changes to design especially when un planned and not researched can cause huge losses in production.

 

[Zipper730]

drgondog

Could the tailcone extension have been fitted? That seems fairly simple, and would have increased performance quite a bit.

As I understand it, the problems with the P-38 involved compressibility, which was effectively caused by...

• Airflow reaching and exceeding the speed of sound on the wings and tails, which eventually serves to produce turbulent flow that reduces stabilizer/elevator effectiveness: This was exacerbated by the fact that, predominantly, the gondola caused the airflow to accelerate over it and the wings to a combined sonic/supersonic velocity earlier than each would alone, and; the stabilizer was fairly sensitive to turbulent airflow, as evidenced by the fact that opening the canopy windows could cause buffeting
• The filleting to the inboard wings basically smoothed out the junction between gondola and stabilizer and reduced the interference effects between the two surfaces (the description seems almost to sound like area-ruling, whether that was the intention or not): I could also imagine that the blending would reduce turbulent flow not just through the increase of critical mach-number, but also by preventing the two airflows from interacting badly with one another (i.e. less turbulence, particularly over the stabilizer).
• The gondola tailcone-extesion as done on the P-38 Swordfish and earlier wind-tunnel proposals in 1942 (far as I remember) seemed to reduce interference effects at the trailing edge of the wing, and would reduce turbulence to structures located behind them.

I remember being told the tail-cone extension improved the diving ability of the P-38 Swordfish (not sure how much also had to do with the outboard wings fitted with laminar flow foils).

 

[Zipper730]

I was also thinking of something else: The XP-49 was an all new design, and originally built to use the X-1800 engine (H-2240/2600), and ended up using the XI-1430. I'm curious why they'd put the XI-1430 in there? It was less powerful than the X-1800 (XI-1430 = 1600 HP), which produced around 2000 horsepower.

Why not put the H-2470 in there? It was actually a little bit shorter than the X-1800 (91.13 vs 107) and produced more power (2300).

 

[wuzak]

I was also thinking of something else: The XP-49 was an all new design, and originally built to use the X-1800 engine (H-2240/2600), and ended up using the XI-1430. I'm curious why they'd put the XI-1430 in there? It was less powerful than the X-1800 (XI-1430 = 1600 HP), which produced around 2000 horsepower.

Why not put the H-2470 in there? It was actually a little bit shorter than the X-1800 (91.13 vs 107) and produced more power (2300).

The H-2470 may not have existed at that time.

The XP-49 was not an all-new design since it used the P-38's wing.

 

[wuzak]

XP-49 prototype was to have XH-2600s installed, but production aircraft were to have the R-2160 Tornado.

"Both the USAAC and Lockheed soon came to realize that with either the Pratt & Whitney XH-2600 or the Wright R-2160 engines, the XP-49 would be seriously overpowered."

"Two-thirds of the XP-49 airframe components were common with the P-38."

Lockheed XP-49

 

[Zipper730]

The H-2470 may not have existed at that time.

I think you might be right, my source (admittedly, Wikipedia, but I think it's still right) seems to say that the H-2470 was conceived when the O-1230 proved uncompetitive in power-output.

The XP-49 was not an all-new design since it used the P-38's wing.

Proposed in 1939

XP-49 prototype was to have XH-2600s installed, but production aircraft were to have the R-2160 Tornado.

What problems affected the Tornado?

 

[wuzak]

Proposed in 1939

That is correct. It is also before the first YP-38s had been built.

 

[pbehn]

Could the tailcone extension have been fitted?

Not may post.

 

[Zipper730]

Okay, here's something a bit older: The Y1P-37.
The concept was actually not all that out there: A P-36 with a V-1710 and a turbocharger. The method of implementation, however left a lot to be desired.

Looking at the basics of the design...

1. The wings and landing-gear seem basically the same as the P-36: That's good. They were aerodynamically and structurally sound.
2. The turbocharger didn't appear to be placed in a bad location: This is good, it takes up little space
3. Main fuel tanks appear to be in the inboard-wings: This is good. It seems to provide adequate capacity, while simultaneously not taking up excessive space in the plane.
4. The auxiliary fuel tank is positioned behind the intercooler, and in front of the cockpit: This is bad. While it was based on the P-36, the P-36 had a compact radial engine up front, not a long inline engine; it also did not have any intercoolers (or radiators) to deal with.
   • Possible remedy: Reposition the auxiliary tank behind the pilot, as was done on the later P-40. While I'm not sure what the USAAC regs were over the years, there is physical room for it.
5. The intercooler/radiators are positioned behind the engine: This is bad as this adds length behind the engine, and ahead of the nose

I'm not sure if the intercooler is liquid/air or air/air because, while air/air is normal; the two are so close to each other, I can't really tell the exact details.

 

[gumbyk]

Okay, here's something a bit older: The Y1P-37.
Looking at the basics of the design...

1. The wings and landing-gear seem basically the same as the P-36: That's good. They were aerodynamically and structurally sound.
2. The turbocharger didn't appear to be placed in a bad location: This is good, it takes up little space
3. Main fuel tanks appear to be in the inboard-wings: This is good. It seems to provide adequate capacity, while simultaneously not taking up excessive space in the plane.
4. The auxiliary fuel tank is positioned behind the intercooler, and in front of the cockpit: This is bad. While it was based on the P-36, the P-36 had a compact radial engine up front, not a long inline engine; it also did not have any intercoolers (or radiators) to deal with.
   • Possible remedy: Reposition the auxiliary tank behind the pilot, as was done on the later P-40. While I'm not sure what the USAAC regs were over the years, there is physical room for it.
5. The intercooler/radiators are positioned behind the engine: This is bad as this adds length behind the engine, and ahead of the nose

I'm not sure if the intercooler is liquid/air or air/air because, while air/air is normal; the two are so close to each other, I can't really tell the exact details.

Putting the aux fuel tank so far aft will result in a large shift in C of G as fuel is burnt off. It is best if all consumables (fuel, ammunition) are located as close to the centre of gravity as possible.
Increasing intercooler pipe length adds inefficiencies to the system, shortening it means that you have the issues you noted - its a trade-off in design that I'm sure they would have taken into account.

 

[Zipper730]

Putting the aux fuel tank so far aft will result in a large shift in C of G as fuel is burnt off. It is best if all consumables (fuel, ammunition) are located as close to the centre of gravity as possible.

The P-40 had the auxiliary tank behind the pilot. I'm not sure if it was designed to use this tank in combat or for ferrying. Regardless, it worked okay, from what it appears.

Increasing intercooler pipe length adds inefficiencies to the system

Well, yeah, but you have to see over the nose too...

S Shortround6 W wuzak
Is that intercooler liquid/air or air/air? Looking at the diagram, I can't really tell as it's so close to the main radiator.

 

[wuzak]

The P-40 had the auxiliary tank behind the pilot. I'm not sure if it was designed to use this tank in combat or for ferrying. Regardless, it worked okay, from what it appears.

The pilot is so far back that moving a fuel tank behind the pilot would be bad.

Is that intercooler liquid/air or air/air? Looking at the diagram, I can't really tell as it's so close to the main radiator.

It is an air-to-air intercooler.


The big improvement to the YP-37 would be changing the position of the coolers. You could put the intercooler ahead of the turbo, under the chin. The radiator would then have to go under the wings or on the leading edge of the wings.

 

[Shortround6]

The P-40 had the auxiliary tank behind the pilot. I'm not sure if it was designed to use this tank in combat or for ferrying. Regardless, it worked okay, from what it appears.

The P-36/Hawk 75 had the the auxiliary tank behind the pilot. The radial engine plane was shorter and lighter and the tank was a true ferry tank, combat and acrobatic maneuvers were prohibited with fuel in the tank. The P-40s had no such prohibition but then the fuel tank was being used as a counter balance to the longer heavier engine (this varied somewhat depending on version).
The regular fuel tanks were were in the wing center section (under the fuselage/cockpit).
Putting fuel tanks some distance from the CG can cause a lot of trouble.

 

[Zipper730]

The pilot is so far back that moving a fuel tank behind the pilot would be bad.

Yes, but if the radiator and intercooler layout were altered, it would allow the cockpit to be more conventionally forward. This would produce a shape similar to the P-40.

It is an air-to-air intercooler.

Thanks

The big improvement to the YP-37 would be changing the position of the coolers. You could put the intercooler ahead of the turbo, under the chin.

I assume you mean the intercooler radiator? Sounds silly, but air-to-air intercoolers do have an intake.

The radiator would then have to go under the wings or on the leading edge of the wings.

That looks like it would be workable.

 

[wuzak]

I assume you mean the intercooler radiator? Sounds silly, but air-to-air intercoolers do have an intake.

I mean the intercooler. With an air-to-air there is no need for a secondary radiator.

I was thinking along the lines of the intercooler placement on the P-38J/L.

 

[Zipper730]

I mean the intercooler. With an air-to-air there is no need for a secondary radiator.

As I understand it, an intercooler includes air drawn in from the outside, and cycled through tubes/grills/passageways (a radiator) which absorb the heat from the air passing through the carburetor intake on the way to the engine. The heat from the carburetor is transferred to the airflow in the intercooler, which then is routed overboard.

So the intercooler is a radiator, it gets it's coolant from the outside if it's an air/air intercooler.

I was thinking along the lines of the intercooler placement on the P-38J/L.

Makes sense, but I'm figuring the turbo has hot exhaust gas flowing out of it. If the duct was like the P-38J, wouldn't the air get cooled off, then heated up by a whole bunch of hot exhaust gas blowing in (this might sound stupid, but it's 2 AM so my thinking might not be all that perfect)?

Would it be possible to bifurcate the duct around it without too much increase in frontal area, or mount the turbo on its side?

 

[wuzak]

As I understand it, an intercooler includes air drawn in from the outside, and cycled through tubes/grills/passageways (a radiator) which absorb the heat from the air passing through the carburetor intake on the way to the engine. The heat from the carburetor is transferred to the airflow in the intercooler, which then is routed overboard.

So the intercooler is a radiator, it gets it's coolant from the outside if it's an air/air intercooler.

I meant to distinguish between the air-to-air intercooler, which is a single heat exhchanger, with the liquid-to-air intercooler which has two heat exchangers - the intercooler and the radiator.

Makes sense, but I'm figuring the turbo has hot exhaust gas flowing out of it. If the duct was like the P-38J, wouldn't the air get cooled off, then heated up by a whole bunch of hot exhaust gas blowing in (this might sound stupid, but it's 2 AM so my thinking might not be all that perfect)?

Would it be possible to bifurcate the duct around it without too much increase in frontal area, or mount the turbo on its side?

You may note that I said the intercooler would be mounted ahead of the turbo.

On the YP-37 the turbo was mounted below the engine, with the turbine section at the bottom.

The compressor sat above that, and would feed forward to the intercooler. It would, most likely, not cross the path of the exhaust system. The outlet would feed back to the carburettor, and with careful design it should be reasonably clear of the exhaust system.

The exhaust from the turbo would dump below the aircraft on the centreline. This would be heading away from the intercooler, but precludes the use of a belly scoop radiatior like that in the P-51 (or XP-40). Thus the radiator would have to be positioned elsewhere - the wings, perhaps.

Having the radiator and intercooler and oil coolers under the nose would be impractical. It could be made to work with a liquid-to-air system, with the intercooler mounted at the back of the engine and the intercooler radiator mounted with the engine coolant radiator.

 

[Zipper730]

Deleted: I hate it when I think of something, then forgot it...