Supercharger Development & Aircraft Design Policy (USAAC)

[tomo pauk]

The rather thick fuselage of the F6F can be attributed to Grumman's design philosophy and comments made by BuAer's Lt Cdr A. M. Jackson, who directed the manufacturer to raise the height of the cockpit even further than what was seen on the F4F. His goal (along with Grumman's) was to increase pilot visibility, where he's famously quoted as saying "you can't hit 'em if you can't see 'em". There was never a need to use the spare fuselage space for additional fuel because the airplane more than met the design criteria for range set forth by the US Navy.

Thickness of fuselage spanned in two directions - upwards and downwards. Requirement for visibility had nothing to do with additional thickness of fuselage under the engine and aft - something that was not present on F4U, F4F or on F8F.

 

[P-39 Expert]

Thickness of fuselage spanned in two directions - upwards and downwards. Requirement for visibility had nothing to do with additional thickness of fuselage under the engine and aft - something that was not present on F4U, F4F or on F8F.

Agree with Darren somewhat on this subject. The F6F fuselage was made thicker upwards by raising the cockpit for visibility over the nose. Compare it with a P-47 cockpit which is not raised at all .

 

[Zipper730]

Nobody has mentioned that the advantage of the Merlin 60 series for a fighter was compactness. Aux superchargers take more space. OK with a radial but problematic with a V12. Turbos are even harder to fit in. That's why there were only three operational turbo fighters in WW2. They were designed around the turbos. The turbo defined the shape. The turbo made the P-38 twin-boom, at the expense of versatility and made the P-47 so big.

That is actually a good argument for twin-stage superchargers.

I'm not sure if the USAAC had enough money to fund twin-superchargers and turbochargers. I think the turbochargers proved invaluable on certain aircraft such as...

1. The B-17
   • While the Avro Lancaster was trundling along at 16000-19000 feet, the B-17's were able to reach altitudes often in excess of 25000 feet.
   • When the Avro Lancaster was given high-altitude supercharging (B.VI), it's maximum speed increased from the high 280's to around 317-318 mph (similar to the B-17E)
2. The P-47
   • Every variant I know of could exceed 400 mph in level flight
   • While they weren't always so good at low altitude their performance was respectable once they were up high
3. The B-29
   • The B-29's were capable of routine operation at 31500'

The F4U Corsair was nicknamed hose-nose because the cockpit was far back on the fuselage and the visibility wasn't very good over the nose.

From what I recall, the length had to do with the addition of fuel capacity. The original design didn't have spectacular visibility, but it's nose was quite a bit shorter.

When they went from 4 x 0.50 to 6 x 0.50, they ended up removing some fuel-capacity in the wings, and that meant that the plane couldn't meet range targets. So they figured they could increase fuel capacity by moving the engine forward and the cockpit aft.

 

[Shortround6]

From what I recall, the length had to do with the addition of fuel capacity. The original design didn't have spectacular visibility, but it's nose was quite a bit shorter.

When they went from 4 x 0.50 to 6 x 0.50, they ended up removing some fuel-capacity in the wings, and that meant that the plane couldn't meet range targets. So they figured they could increase fuel capacity by moving the engine forward and the cockpit aft.

On the prototype there were two guns in the cowl, and one in each wing.

The wing fuel tanks were not self sealing.
There were bomb cells inside the wing for dropping small bombs on enemy bomber formations.

 

[DarrenW]

Thickness of fuselage spanned in two directions - upwards and downwards. Requirement for visibility had nothing to do with additional thickness of fuselage under the engine and aft - something that was not present on F4U, F4F or on F8F.

Sorry Tomo, I may have been talking a little out of my butt here and assumed way too much. I'm only going by various second hand sources so it's not something that I would defend to my last dying breathe....

Be that as it may, my personal beliefs concerning the positioning of the supercharger/oil cooler intakes on the chin of the engine cowling were due to at least these two reasons:

A) by placing the oil cooler and it's associated intake/plumbing below the engine it could more easily be protected by heavy armor plate, making it less far vulnerable than the wing mounted variety (example - Corsair and early Wildcats).

B) if these particular intakes were placed anywhere else on the cowling there probably would have been less forward and/or downward vision from the cockpit.

Unfortunately I wasn't there during the design phase so I can't defend my assumptions with any level of concrete certainty. But as you stated earlier, all of this certainly added to the overall thickness of the fuselage.

 

[P-39 Expert]

Sorry Tomo, I may have been talking a little out of my butt here and assumed way too much. I'm only going by various second hand sources so it's not something that I would defend to my last dying breathe....

Be that as it may, my personal beliefs concerning the positioning of the supercharger/oil cooler intakes on the chin of the engine cowling were due to at least these two reasons:

A) by placing the oil cooler and it's associated intake/plumbing below the engine it could more easily be protected by heavy armor plate, making it less far vulnerable than the wing mounted variety (example - Corsair and early Wildcats).

B) if these particular intakes were placed anywhere else on the cowling there probably would have been less forward and/or downward vision from the cockpit.

Unfortunately I wasn't there during the design phase so I can't defend my assumptions with any level of concrete certainty. But as you stated earlier, all of this certainly added to the overall thickness of the fuselage.

I always wondered why Grumman didn't put the supercharger/oil cooler intakes in the space above the engine since the space was there after the cockpit was raised to help visibility over the nose.

 

[Zipper730]

On the prototype there were two guns in the cowl, and one in each wing.

I didn't know that, but I can see it clearly in the image.

The wing fuel tanks were not self sealing. . . . There were bomb cells inside the wing for dropping small bombs on enemy bomber formations.

So, I guess they removed the cowl guns, removed the non self-sealing guns in the wings, the fuel capacity for them and possibly ate into some of the space where the bomb-cells were located?

 

[P-39 Expert]

As I understand it, the production Corsair deleted the cowl guns and wing fuel tanks, moved the cockpit back and put the main fuel tank where the cockpit was. Some of the wing tanks may have been retained.

 

[DarrenW]

I always wondered why Grumman didn't put the supercharger/oil cooler intakes in the space above the engine since the space was there after the cockpit was raised to help visibility over the nose.

I believe that placing the intakes above the engine would have widened the top and sides of the engine cowling and forward fuselage, providing the pilot less overall vision. Besides the integration of a downward sloping nose, the designers wanted to keep the cowling as trim as possible to allow for improved aerial gunnery.

 

[P-39 Expert]

Intake would have been between the engine and canopy in that sloped area. Would have eliminated the intakes below the engine and reduced the frontal area while still allowing the raised vision line.

 

[DarrenW]

Intake would have been between the engine and canopy in that sloped area. Would have eliminated the intakes below the engine and reduced the frontal area while still allowing the raised vision line.

Interesting "re-engineering" concept, not sure if Grumman looked at that as a possibility or not. They would have to re-position the oil tank down at the bottom of the engine and rearrange some things in the accessories bay as well. The supercharger duct work was enormous when compared to the Wildcat but it might have been feasible.

 

[wuzak]

Intake would have been between the engine and canopy in that sloped area. Would have eliminated the intakes below the engine and reduced the frontal area while still allowing the raised vision line.

The lower duct also fed the oil coolers and the intercooler.

0-Line-Drawing-F6F-5-Hellcat-01

 

[Shortround6]

I always wondered why Grumman didn't put the supercharger/oil cooler intakes in the space above the engine since the space was there after the cockpit was raised to help visibility over the nose.

Well, if take those ducts/inlets off the bottom and put them on the top your visibility over the nose goes to pot. granted the outer sections are the intercooler inlets so may be both bottom and top intakes?

 

[DarrenW]

Those lower intakes are what gave the Hellcat it's distinctive grin so how about we just learn to live with some possible extra drag, ok?

Besides, the concept the P-39 Expert is suggesting is somewhat revolutionary for an aircraft that's was far less on innovative concepts and more about using existing proven technology. That's why the gestation period for the fighter was extremely short and there were far less modifications required during it's existence than with any other American fighter of it's time.

 

[P-39 Expert]

View attachment 567583
Well, if take those ducts/inlets off the bottom and put them on the top your visibility over the nose goes to pot. granted the outer sections are the intercooler inlets so may be both bottom and top intakes?

The Hellcat cockpit was raised substantially to provide the best possible view over the nose for carrier landings. Remove the ducts below the engine for a smooth line from engine to wings to tail and reduce frontal area. Put the new intake scoop on top between the extended cooling louvers just aft of the engine. Change the upper engine cowl to come straight back from the engine instead of being part of the angled raised forward fuselage creating an opening for the scoop. Move the oil radiator from below to behind the engine ahead of the oil tank. Rest of the scoop goes down each side to the engine air intakes. Or put oil coolers in the wing leading edges inboard of the main landing gear.

 

[Zipper730]

I would note that the Navy and P & W were developing a single speed engine supercharger with a two speed plus neutral (unused at low altitude) auxiliary supercharger.

I should have posted this earlier, but I thought the supercharging gearing for Pratt & Whitney was...

• Neutral Blower: Aux Blower offline, main-stage blower in low-gear
• Low Blower: Aux and Main-Stage blower in low-gear
• High Blower: Aux and Main-Stage blower in high-gear

Contrast this to the Merlin two stage in which both impellers were turning at all times and the first and 2nd stage turned at the same speed, the two speed drive shifted both impellers at the same time.

P&W's idea seemed to be that you'd have less power taken off the shaft in neutral-blower, so you'd do better at low altitude; at low-blower you'd do worse at lower altitudes, but better at intermediate altitudes, and high-blower at high altitudes.

Also note the P & W two stage engine that flew in th eUS 1939 fighter trials was good for 1050hp at 17,500ft in high gear

So they gained 1500' feet by the time the F4F-3 came online?

The US had a dearth of supercharger designers at this time

While this might sound silly... do you have any idea why?

Simply stated - Material Division under leadership of Oliver Echols, did not believe that a two speed-two stage supercharged engine was feasible.

From what I've found, Oliver Echols didn't become the leader of the Material Division until 1940, he was the Chief Engineer in 1934, and (either) Deputy or Vice-Chief in 1939. I'm guessing this decision was made when he was the Chief Engineer based on the chronology of the P-38 and P-39.

If I had to guess, from an engineering standpoint: The reason would have likely been

1. A twin-stage supercharger would remove too much shaft power for the amount of critical altitude added to the overall engine: That said, the exhaust thrust is higher in a supercharger, and I'm not sure when people in the USAAC began to seriously look into that. That said, I'm pretty sure that the fact that pistons generated thrust out their exhaust shortly after piston engines were first used. It's just that...
   • Thrust levels were often low because it was tied to horsepower at a given altitude
   • Critical altitudes were low: If critical altitude can be kept nice and high, one would be able to generate thrust levels that were significant.
   • Thrust levels are more significant when one is moving at higher speed: From what I recall, at around 374-375 mph, 1 lbf = 1 hp.
2. A turbocharger theoretically removes no horsepower from the engine (there is, apparently a small loss to backpressure), and has the ability to vary the RPM in flight (reduces throttling loss): In practice, there are very few aircraft that used single-stage turbochargers, and I don't know of any operational American design that used a twin-stage turbocharger. Generally speaking, you'd see an engine with a turbocharger with a main-stage engine-driven blower, and a turbo-driven auxiliary stage blower.

The Ge Turbo was required at that time to push the P-38 performance envelope above 15,000 feet enough to qualify as a high altitude 'Interceptor'. The P-39 was described by Kelsey as 'the small solution' and never achieved promised capability until the P-63 limped out with the Allison auxiliary second stage - requiring a major redesign to accommodate the engine.

I'm curious if the provision had existed for a twin-turbo set-up that would involve two small turbochargers instead of one big one mounted so that the exhaust goes out the side rather than out the bottom.

General Malin Craig was Army Chief of Staff and retired days before the Germans attacked Poland, not a staunch supporter of AAF vision but also not regarded as a meticulous bean counter looming over Arnold's shoulder regarding approved budgets for R&D.

That said, I'm surprised there wasn't some money available for twin-stage supercharging, since the USN was working on it too.

The USN & USAAC often were at each other's throats for obvious reasons, but they did sometimes agree to work together. A great example was the V-1710: Both agreed to sponsor the engine development.

They had both developed quite a number of inline engines... just off the top of my head, I can think of the following

1. Army
   1. Continental Hyper Engine
   2. Lycoming O-1230
2. Navy
   1. Lycoming H-2470
   2. Pratt & Whitney H-2240/2600
   3. Pratt & Whitney H-3130/3730

And probably quite a number more. I can't say they were all good designs, but I figure if there were less engines being developed and more depth of effort allocated to each specific engine design, things would have worked better.

As for the Continental Hyper Engine: I'm not sure what latitude at the time existed for the USAAC to change the design specs as they proceeded from, say individual cylinder to monoblock construction. I could easily envision that would have made for more compact engines and lighter ones too.

 

[Sid327]

Didn't the F6F have the engine mounted so that it canted nose-down a few degrees (?)

This would have made it fly more nose-up in S&L flight compared to just about all other aircraft where the engine was mounted at 90 degrees(?) to the vertical (bulkhead).

 

[wuzak]

I should have posted this earlier, but I thought the supercharging gearing for Pratt & Whitney was...

• Neutral Blower: Aux Blower offline, main-stage blower in low-gear
• Low Blower: Aux and Main-Stage blower in low-gear
• High Blower: Aux and Main-Stage blower in high-gear

Actually, the main supercharger had a fixed gear ratio to the engine, while the auxiliary supercharger had high, low and neutral.

 

[wuzak]

The USN & USAAC often were at each other's throats for obvious reasons, but they did sometimes agree to work together. A great example was the V-1710: Both agreed to sponsor the engine development.

They had both developed quite a number of inline engines... just off the top of my head, I can think of the following

1. Army
   1. Continental Hyper Engine
   2. Lycoming O-1230
2. Navy
   1. Lycoming H-2470
   2. Pratt & Whitney H-2240/2600
   3. Pratt & Whitney H-3130/3730

And probably quite a number more. I can't say they were all good designs, but I figure if there were less engines being developed and more depth of effort allocated to each specific engine design, things would have worked better.

As for the Continental Hyper Engine: I'm not sure what latitude at the time existed for the USAAC to change the design specs as they proceeded from, say individual cylinder to monoblock construction. I could easily envision that would have made for more compact engines and lighter ones too.

The Lycoming H-2470 was two O-1230s joined together.

The H-2600 or X-1800 was intended for Army types. I'm not sure if an H-2240 existed, though that's what the Sabre would have been.

The H-3120 and H-3730 were based on the X-1800 design.

When you say that the Army and Navy developed these engines you really mean, mostly, they sponsored the engines.

Of those listed, only the Continental I-1430 had direct Army involvement in the design and development of the engine. The I-1430 was never a monoblock design.

The Army had complete control of the specs of that engine, so if they wanted a monoblock they could have made it a monoblock.

 

[wuzak]

From what I've found, Oliver Echols didn't become the leader of the Material Division until 1940, he was the Chief Engineer in 1934, and (either) Deputy or Vice-Chief in 1939. I'm guessing this decision was made when he was the Chief Engineer based on the chronology of the P-38 and P-39.

It was the Materiel Division.