Dive Bombing: USN vs USAAF/USAF

[wuzak]

Tomo Pauk

Both F7F an F8F used single-stage supercharged engines, without intercoolers.

I beg to differ,
http://www.wwiiaircraftperformance.org/f7f/F7F-1_80262.pdf
Performance charts clearly list low and high blower

Means it is two speed, not two stage.

All of those four USN fighters mentioned used water-alcohol injection, the Hellcat and Corsair starting from early 1944 on.

That's correct, and that allowed more horsepower to be produced

It allowed more boost to be used and thus more power. But also that would be at a lower altitude.

[KJ_Lesnick]

Wuzak & Tomo Pauk

I'm confused about a couple of things

Regarding the F4F

• I've received all sorts of conflicting information on the F4F Wildcat
  
• One website says the F4F was thought to be the first aircraft to use a twin-speed, twin-stage supercharger
  
• "Wings of the Navy" by Captain Eric M. Brown lists the F4F to have a twin-speed, twin-stage supercharger with inter-cooling
  
• Other sources seem to state a twin-stage, single-speed supercharger
  
• The http://wwiiaircraftperformance.org page seems to show performance curves for the F4F that indicate a single-speed supercharger
  

Regarding the F7F

• I'm curious why the F7F would use a single-stage supercharger when the F4F, F6F, and F4U used twin-staged
  
• I'm also curious why they'd do away with inter-cooling
  
• I figure either would jack up the critical altitude well above the amount specified
  

[tomo pauk]

The engines, whether Cyclones or Twin wasps on the F4F, were always 2-speed supercharged. Most of the people don't separately state that any 2-stage engine have had an intercooler (reasoning it is understood as such), and the 2-stage, 2-speed version of the Twin wasp featured a pair of intercoolers indeed. The F4Fs with single stage Twin wasps were with suffix 'a' - ie. F4F-3a. Sometimes, the gearing on the 2-stage P&W engines is described as 3-speed, since the 1st stage could be de-clutched - only in low altitudes, where it is not needed, hence leaving more engine power to drive the prop.
No ww2 aircraft engine that matter used only 1-speed gearing when was outfitted with a gear-driven 2-stage supercharger; the US engines did use 1-speed for engine-stage impellers, though (and multiple gears for 1st stage) - they did not featured both impellers on same drive shaft as it was common by RR 2-stage engines.

The sigle stage radial engine is very difficult to intercool/aftercool, there is no single intake manifold that leads from compressor to the cylinders. The US 2-stage radials used intercooler to cool down the compressed air that went from 1-st stage to 2-nd stage of the compressor system. RR 2-stage engines cooled down the air/fuel mixture that went through both stages, one after another.
Why no 2-stage engines on the F7F (and F8F)? Probably a matter of 'it's good enough between SL ad 20000 ft'? The 2-stage engine will weight more, will have more drag, and will be bulkier. The USN already have had the F4U to cover high altitudes anyway. The F8F featured, vs. the Hellcat & Corsair, a smaller fuel tankage, smaller weight of armament, smaller fuselage and wings, so the single stage R-2800 was seen as a good fit.

[KJ_Lesnick]

tomo pauk

The engines, whether Cyclones or Twin wasps on the F4F, were always 2-speed supercharged. Most of the people don't separately state that any 2-stage engine have had an intercooler (reasoning it is understood as such)

So, it should be implied...

Sometimes, the gearing on the 2-stage P&W engines is described as 3-speed, since the 1st stage could be de-clutched - only in low altitudes, where it is not needed, hence leaving more engine power to drive the prop.

Strange though the power-curves: Usually when there is a two-speed set-up, you usually see the brake horsepower staying about the same until you reach the critical altitude (true airspeed of course keeps going up), then you get a drop off in horse-power (and some speed falls off a little) until the next speed clutches on and then it stays there for awhile (true airspeed keeps on going up) and then above the critical it falls off and so on.

No ww2 aircraft engine that matter used only 1-speed gearing when was outfitted with a gear-driven 2-stage supercharger

Was this because of the fact that you'd over-boost too easily?

the US engines did use 1-speed for engine-stage impellers, though (and multiple gears for 1st stage)

I'm not sure I understand you, I will try and make sure we're on the same page

• The US did use 1 speed for 1-stage superchargers in some cases
  
• The US did use 1 speed superchargers with 2 or more speeds in some cases
  

Am I right or wrong?

they did not featured both impellers on same drive shaft as it was common by RR 2-stage engines.

I was under the impression that the F4U was laid out like this, while I'm not sure here, my impression was as follows

• First stage supercharger is integral to the engine and single-speed and this is what they called "neutral blower".
  
• Second stage supercharger seems to be de-clutched at this point, and then is clutched into low and then high speed
  

I was under the impression that other than that (and possibly the P-63) they were tacked onto the same shaft.

The sigle stage radial engine is very difficult to intercool/aftercool

Well technically the word intercooler/aftercooler are all based on their position: To have an intercooler you have to have at least two stages of compression as I understand it, it would be an after-cooler or pre-cooler otherwise.

there is no single intake manifold that leads from compressor to the cylinders.

You mean several airflow paths which go into each cylinder?

The US 2-stage radials used intercooler to cool down the compressed air that went from 1-st stage to 2-nd stage of the compressor system.

Which makes sense: The key word being "inter" cooler, which means in between.

RR 2-stage engines cooled down the air/fuel mixture that went through both stages, one after another.

As I understand it the engine had an after-cooler that included cooling of the air-passageways between the first and second stage as well: An after-cooler that also doubled as an intercooler too...

Why no 2-stage engines on the F7F (and F8F)? Probably a matter of 'it's good enough between SL ad 20000 ft'?

True, but for a WHIF design the F7F might very well have had the potential for a good escort: Consider the following

• The original intention was for Grumman to develop a twin-engined fighter that would be built for the Navy and the Army
  
• The USN version would use traditional supercharging, and an unpressurized cockpit; the USAAF version would be called the XP-65 and utilize a turbocharger and a pressurized cockpit (possibly differences in armament)
  
• The two designs grew so different that eventually the XP-65 was cancelled
  
• It should be noted that the USN's then design and the USAF's XP-65 were substantially different from the later F7F-1
  
• The USN continued to develop the design into the refined F7F-1
  
• While the P-38 was turbocharger-equipped, it was not often operated at the high-altitudes used for bomber escorts: The intercoolers and/or oil-coolers provided reliability issues and were damage-prone; though the P-38J did away with that, new problems popped up that took some time to be fixed
  
• An F7F-1 had a critical altitude of around 23,400 to 24,000 feet would be a little low for bomber escort (which would typically be around 25,500 to 31,500 feet, with a median of 28,500, being that 3,000 to 5,000 feet was favored over the bomber)
  
• The F7F-1 was more maneuverable than the P-38 (sans maneuvering flaps at least, and prior to the -J in terms of roll-rate at speed) and both had excellent climb-performance
  
• The baseline speed, and climb-rate performance of the F7F could be boosted by removing the tail-hook (which isn't needed for land-based operations), the wing-fold (un-needed for land-based operations)
  
• The baseline range performance could be improved without engine modification by doing the following: Removing the 4 x 20mm cannon and laying tankage in that area, and/or laying additional fuel-tankage outboard of the normal fold-line
  
• Engine improvements would drive up critical altitude however: Twin-stage, twin-speed with intercooler would do the job, alternately using a single-stage hydraulically clutched system (similar to the P-63) behind a single-speed integral supercharger with some form of cooling.
  

[tomo pauk]

The reason people were using multi-speed supercharger drives is that this is a far more flexible thing then to have just one speed drive - it will suck less power down low (hence more power goes to the prop), while at higher altitudes the supercharger system will receive more power in order to improve the engine's power at the desired altitude range.
Overboosting was imited by throttling, in real world.

I'm not sure I understand you, I will try and make sure we're on the same page

    1# The US did use 1 speed for 1-stage superchargers in some cases
    2# The US did use 1 speed superchargers with 2 or more speeds in some cases

Am I right or wrong?

1# - Yes, vast majority of V-1710s, and all turbocharged and 2-stage engines were employing 1-speed drive for the intergral supercharger/impeller
2# - It is either one speed or two speed (or 3 speed like in Jumo 213E/F, or infinite number of speeds like in DB engines) drive, can't be both. Sometimes the drive for auxiliary supercrager of the 2-stage P&W engine was described as 3-speed: neutral (de-clutched), low gear, high gear.

I was under the impression that the F4U was laid out like this, while I'm not sure here, my impression was as follows

   1# First stage supercharger is integral to the engine and single-speed and this is what they called "neutral blower".
    2# Second stage supercharger seems to be de-clutched at this point, and then is clutched into low and then high speed

I was under the impression that other than that (and possibly the P-63) they were tacked onto the same shaft.

1# - As above, the term 'neutral' applies here on the auxiliary supercharger (1st stage). The integral supercharger (2nd stage) have had 1-speed drive and was always turning when engine was running.
2# - The auxiliary supercharger is the 1st stage here, and indeed was not clutched in in low altitude.

The auxiliary supercharger on the 2-stage V-1710s didn't share the drive, or the shaft with integral supercharger, but was provided with hydraulic coupling.

Well technically the word intercooler/aftercooler are all based on their position: To have an intercooler you have to have at least two stages of compression as I understand it, it would be an after-cooler or pre-cooler otherwise.

The term is based on the position of the observer - in the UK, people called the device 'inter-cooler' (as between supercharger and engine), the term 'after-cooler' (as after the supercharger) was mostly used in the USA.

You mean several airflow paths which go into each cylinder?

Each cylinder has it's own intake manifold, going directly from supercharger.

True, but for a WHIF design the F7F might very well have had the potential for a good escort:

Agreed

[KJ_Lesnick]

Tomo Pauk

The reason people were using multi-speed supercharger drives is that this is a far more flexible thing then to have just one speed drive - it will suck less power down low (hence more power goes to the prop), while at higher altitudes the supercharger system will receive more power in order to improve the engine's power at the desired altitude range.

Correct, plus it's performance more closely mirrored a turbocharger.

1# - Yes, vast majority of V-1710s, and all turbocharged and 2-stage engines were employing 1-speed drive for the intergral supercharger/impeller

Okay

2# - It is either one speed or two speed (or 3 speed like in Jumo 213E/F, or infinite number of speeds like in DB engines) drive, can't be both.

Well, what I was trying to get at was all 2-speed superchargers were two-speed, but one stage superchargers could be one speed, two speed, etc.

Sometimes the drive for auxiliary supercrager of the 2-stage P&W engine was described as 3-speed: neutral (de-clutched), low gear, high gear.

Okay, that I understand.

2# - The auxiliary supercharger is the 1st stage here, and indeed was not clutched in in low altitude.

Now that is unexpected and I'd probably have to see it to visually grasp the layout though I grasp the basic idea.

The auxiliary supercharger on the 2-stage V-1710s didn't share the drive

Why?

The term is based on the position of the observer - in the UK, people called the device 'inter-cooler' (as between supercharger and engine), the term 'after-cooler' (as after the supercharger) was mostly used in the USA.

I thought it was relative to the impellers...

Each cylinder has it's own intake manifold, going directly from supercharger.

Got it

Agreed

I think the USAAF had too much of a predilection for turbochargers: I could understand with the B-17, B-24, and P-38, and P-47's: However it wasn't largely necessary for everything.

[tomo pauk]

Correct, plus it's performance more closely mirrored a turbocharger.

Generaly it did, if the supercharger was of 2-stage variety.

Well, what I was trying to get at was all 2-speed superchargers were two-speed, but one stage superchargers could be one speed, two speed, etc.

Yes, all 2-speed superchargers were two-speed   The 2-stage superchargers have had multi-speed drives.

Now that is unexpected and I'd probably have to see it to visually grasp the layout though I grasp the basic idea.

Schematics of the supercharger system on the Hellcat: http://www.mediafire.com/view/wpz44qeyz01kdbq/stager.jpg#
The stages are numbered as the air (that they are supposed to compress) encounters them. Hence the auxiliary supercharger is the 1st stage, the integral (or main, or engine-stage) is the 2nd stage. Please note that, with auxiliary stage de-clutched (inoperative, 'neutral'), the air flow goes directly to the carburetor and then into the main stage.

(The auxiliary supercharger on the 2-stage V-1710s didn't share the drive)Why?

No need, and it does not offer anything.  

I think the USAAF had too much of a predilection for turbochargers: I could understand with the B-17, B-24, and P-38, and P-47's: However it wasn't largely necessary for everything.

It was not used on everything - many important aircraft flew well without it, and so did the ones that used turbo

[KJ_Lesnick]

Tomo Pauk

Generaly it did, if the supercharger was of 2-stage variety.

Wait, if it was a supercharger (single speed) with a variable-speed tacked-on: Wouldn't it behave like a turbocharger with a supercharger too?

Schematics of the supercharger system on the Hellcat: http://www.mediafire.com/view/wpz44qeyz01kdbq/stager.jpg#
The stages are numbered as the air (that they are supposed to compress) encounters them. Hence the auxiliary supercharger is the 1st stage, the integral (or main, or engine-stage) is the 2nd stage.

Okay, I got it

Please note that, with auxiliary stage de-clutched (inoperative, 'neutral'), the air flow goes directly to the carburetor and then into the main stage.

Okay

No need, and it does not offer anything.

Why did the British use them?

It was not used on everything - many important aircraft flew well without it, and so did the ones that used turbo

Well... the following fighter-planes didn't

• P-36: They did toy with the idea of fitting a turbo in it, but it was unreliable
  
• P-40: It was mostly intended to operate around 15,000 feet and didn't require it
  
• P-39: It was intended to use a turbocharger, but it was removed for a number of reasons to the dismay of many who flew it
  
• P-51: It was developed by NAA internally, but was launched by the British Purchasing Commission and built around RAF specifications; it originally used a V-1710, and later a V-1650-3 or -7
  
• P-61: Though a night-fighter was seen as useful in the US, the aircraft was largely developed to RAF specifications for a turret night-fighter with an endurance of 8-hours; turbochargers were omitted to save weight and volume (it was realized it would add 50 mph and 10,000 feet altitude))
  
• P-63: It was basically built to correct the deficiencies of the P-39, and for obvious reasons they weren't going to use a turbocharger...
  
• P-75: It was basically a prototype and while the V-3420 was designed to carry a turbocharger, it didn't for some reason even when escort requirements were added: No idea of what kind of superchargers were used.
  

Regardless, the P-37 was fitted with one (even though it looked absurd and part of me speculates they built it largely to get a sufficient demand of V-1710's); the P-38's and P-47's had it, and the intention was for the P-39 to use it.

Pretty much all the attack planes didn't use it because they weren't designed for high altitude operation.

The bombers that didn't used them included

• The B-18
  
• The B-25
  
• The B-26
  

The B-18 was built in relatively small numbers and often used for missions at low altitudes such as MPA duty; the B-25 and B-26 were medium altitude planes; regardless, the B-17, B-24, B-29, the proposed XB-28, and B-36 all did; the XB-42 however didn't appear to

[wuzak]

Generaly it did, if the supercharger was of 2-stage variety.

Wait, if it was a supercharger (single speed) with a variable-speed tacked-on: Wouldn't it behave like a turbocharger with a supercharger too?

Similar, but not the same.

The variable speed drive had less of a range of rpm than the turbo, and the drive losses increased with altitude (as the compressor was spun faster to give a higher pressure ratio).

The graph would show a more gradual drop off in power than an engine with distinct supercharger gear ratios.

The way the V-1710 supercharger was set up was such that it was a bolt on. Except for a few models that mounted the carburettor on the auxiliary supercharger, the core engine for a two stage V-1710 was the same as that for a turbocharged model.

(The auxiliary supercharger on the 2-stage V-1710s didn't share the drive)Why?

No need, and it does not offer anything.

Why did the British use them?

I take it what Tomo meant was that the two impellers were not mounted on the same shaft Rolls-Royce style. The reason being that you would end up with both impellers spinning at the same speed, and the variable speed function of the auxiliary supercharger would be lost.

The reason why Rolls-Royce did it that way was because it was compact, and limited the extra length added to the engine.

• P-36: They did toy with the idea of fitting a turbo in it, but it was unreliable
  

Really? Or was this the XP-37, which was adapted from the P-36 to experiment with a V-1710/turbo combination?

• P-40: It was mostly intended to operate around 15,000 feet and didn't require it

[/list]

Not actually the case.

The turbo in the X/YP-37 was woefully unreliable, and created compromises that limited the practicality of the aircraft. Don Berlin, therefore, requested that a non-turbo V-1710 be provided, teh P-40 becoming a more practical and usable aircraft.

• P-51: It was developed by NAA internally, but was launched by the British Purchasing Commission and built around RAF specifications; it originally used a V-1710, and later a V-1650-3 or -7
  

Really, source for that?

• P-61: Though a night-fighter was seen as useful in the US, the aircraft was largely developed to RAF specifications for a turret night-fighter with an endurance of 8-hours; turbochargers were omitted to save weight and volume (it was realized it would add 50 mph and 10,000 feet altitude))
  

It may have also been thought that the extra speed was not required.

• P-63: It was basically built to correct the deficiencies of the P-39, and for obvious reasons they weren't going to use a turbocharger...
  

The P-63 was built around the two stage V-1710.

• P-75: It was basically a prototype and while the V-3420 was designed to carry a turbocharger, it didn't for some reason even when escort requirements were added: No idea of what kind of superchargers were used.
  

The V-3420 was designed to have 1 stage supercharge, 1 stage supercharger with turbo or two stage supercharger.

The V-3420 would require two B-series turbochargers, the C-series not being large enough.

I believe the P-75 had two stage V-3420s.

Regardless, the P-37 was fitted with one (even though it looked absurd and part of me speculates they built it largely to get a sufficient demand of V-1710's); the P-38's and P-47's had it, and the intention was for the P-39 to use it.

As I said above, the USAAC wanted to have a liquid cooled fighter (after seeing the Spitfire and Bf 109 performance), and their preferred system was to use a turbocharger.

The V-1710 was supposedly designed to be turbocharged, but the theory behind the turbocharger in pre and WW2 aviation was that the engine would essentially not know it was turbocharged. The carburettor intake would see sea level pressure and the exhaust back pressure would be the same as one with a standard exhaust at sea level.[/list][/list]

[tomo pauk]

I can agree with Wuzak's comments, here are mine:

Wait, if it was a supercharger (single speed) with a variable-speed tacked-on: Wouldn't it behave like a turbocharger with a supercharger too?

The shape of the power graph can be somewhat similar as with a turbocharged engine power graph, but the power itself will be far smaller. Eg. the turboed V-1710 was good for some 1425-1600 HP at 25000 ft without any tricks in 1943-44, the single stage V-1710 with the best single stage compressor conceivable in ww2 will give maybe 1000 HP at 25000 ft. The ~50% more HP the turbo has is a major difference, even if we account for the drawbacks of the turbo (like the greater volume and weight of installation, no useful exhaust trust).

Well... the following fighter-planes didn't [used turbo]

<snip>

See what I've told you

Pretty much all the attack planes didn't use it because they weren't designed for high altitude operation.

We're in agreement here.

The B-18 was built in relatively small numbers and often used for missions at low altitudes such as MPA duty; the B-25 and B-26 were medium altitude planes; regardless, the B-17, B-24, B-29, the proposed XB-28, and B-36 all did; the XB-42 however didn't appear to

B-18 was produced before the turbo matured, and indeed as an MPA didn't needed a turbo. The XB-42 used the two 2-stage V-1710s.

[KJ_Lesnick]

Was the V-1710 even capable of accommodating a liquid-air intercooler?

[tomo pauk]

It was. The XP-51J was outfitted with water-to-air intercooler on it's V-1710.

[KJ_Lesnick]

Thanks