USAAF Attack & Light Bombers: Needs & Desires

[Shortround6]

It used a variable speed drive. The "high" altitude gearing for single stage supercharger was often around 15,000ft, give or take. Gear ratio was 7.52 max.

Please note that the boeing XF8B-1 used a two stage supercharger but the engine weighed about 350lbs more, this includes the contra rotating prop gear box. You pick your intended altitude band and get an engine with supercharger to suit.

Big intercoolers need lots of air.

 

[wuzak]

Apparently the XA-41 had a three-speed single-stage supercharger, I'm not sure why you'd gear the high-speed setting for around 15,000 feet.

I don't know about the XA-41's engine specifically, but Pratt and Whitney superchargers often had high and low gears, plus neutral, for the auxiliary compressor.

 

[Zipper730]

It used a variable speed drive. The "high" altitude gearing for single stage supercharger was often around 15,000ft, give or take.

Understood, though in my personal opinion, it would have probably been best to have pushed it up to at least 24,000-25,000 feet based on the need to deal with improvements in enemy fighter aircraft.

Gear ratio was 7.52 max

What was the minimum for the design?

You pick your intended altitude band and get an engine with supercharger to suit.

True, but I figure the fighters that were doing this mission were largely able to fly faster because of their higher altitude capability (with jets this would become more extreme), so designing this plane to go up a bit higher would compensate for the nature of changing air-defenses would probably be fairly smart.

Please note that the boeing XF8B-1 used a two stage supercharger but the engine weighed about 350lbs more, this includes the contra rotating prop gear box.

All that only added 350 pounds?

 

[wuzak]

Understood, though in my personal opinion, it would have probably been best to have pushed it up to at least 24,000-25,000 feet based on the need to deal with improvements in enemy fighter aircraft.

I think it was more of a limitation of the supercharger design than the desired altitude.

 

[Zipper730]

I think it was more of a limitation of the supercharger design than the desired altitude.

If I recall right, the F7F Tigercat could do around 24000 feet with a single-stage supercharger

 

[Shortround6]

If I recall right, the F7F Tigercat could do around 24000 feet with a single-stage supercharger

Please be more specific.
F7Fs did not all use the same engine.

However the F7F-1 & -2 used R-2800-22W engines , these were single stage "C" series engines and were good for 1600hp at 16,000ft in high gear, no ram.
With RAM the critical altitude goes a somewhat higher and the Navy arranged for water injection to be used at any altitude. This helped at the higher altitudes but nowhere near what it did at heights below the critical altitude.

The F7F-3 used the R-2800-34W engine and these were capable of 1700hp at 16,000ft dry without RAM. The -34 engine used a new diffuser and had better high altitude performance than the -22 engine, but that still meant critical altitude with no RAM was below 20,000ft.

These are pretty much post war engines. Please note both engine were good for 2100hp at low altitude in low gear.

 

[Zipper730]

Please be more specific.

What I was trying to point out was that with ram, the critical altitude of the F7F was around 24,000 feet. The point is that even with a properly designed single-stage, twin-speed supercharger, and a good intake, one should be theoretically able to achieve 24,000 feet.

As for the XA-41, with a variable speed set-up it seems easier to do...

 

[Shortround6]

The critical altitude, even with ram, was under 24,000ft. Nobody picked up 8,000ft of altitude with RAM.
P & W says the -34W engine had a critical altitude of 16,000ft in high gear, US Navy says 16,600ft, performance charts show max speed of a F7F-3 was at 23,000ft using military power.

The -22W engine gave the F7F-2 max speed at about 22,500ft in Military power and using WEP it was a bit over 20,000ft. At 20,000ft the F7F-2 was about 20mph faster than using military power but at about 22,500 and up the difference was about 10mph. The WEP required water injection.

 

[Zipper730]

The critical altitude, even with ram, was under 24,000ft. Nobody picked up 8,000ft of altitude with RAM.
P & W says the -34W engine had a critical altitude of 16,000ft in high gear, US Navy says 16,600ft, performance charts show max speed of a F7F-3 was at 23,000ft using military power.

The -22W engine gave the F7F-2 max speed at about 22,500ft in Military power and using WEP it was a bit over 20,000ft. At 20,000ft the F7F-2 was about 20mph faster than using military power but at about 22,500 and up the difference was about 10mph. The WEP required water injection.

Thanks for the correction in figures

I'm curious if you know how the performance would compare with a variable-speed supercharger with good ram?

 

[Shortround6]

Thanks for the correction in figures

I'm curious if you know how the performance would compare with a variable-speed supercharger with good ram?

You have two components to the supercharger, the supercharger itself which is just an air compressor. and the drive system, which can be simple, a single fixed gear ratio. or complicated as in multiple gear ratios (although more than 2 almost never saw service) or hydraulic/variable drive.

P & W didn't seem to use the variable speed drives with the same compressors (or indeed on the same engines ) as they used with the single speed, two speed and even the two stage engines so it gets very difficult to say.
The R-2800-22/W engine used a two speed drive and the gears were 7.29 and 9.45. The -30W used a variable speed drive with 7.29 being the lowest speed and 10.55 being the high limit. However the -30 wasn't a "C" series engine, it was an "E" series and further complicating things it used a new supercharger and a new carburetor. It also weighed 200lbs more than the -22W.
There were only two "E" series engines built. The -30W used in the F8F-2 and the -32W used in the F4U-5 which used a variable speed drive to the TWO impellers running in parallel in the first stage of the two stage system.
The New Supercharger gave the -30W a considerable increase in altitude performance, much more than can be attributed to just the change in impeller speed due to the different top limits. The supercharger on the -32W engine is quite possibly the most sophisticated mechanical drive supercharger used on a piston engine fighter in squadron service.

 

[Zipper730]

You have two components to the supercharger, the supercharger itself which is just an air compressor and the drive system, which can be simple, a single fixed gear ratio. or complicated as in multiple gear ratios (although more than 2 almost never saw service) or hydraulic/variable drive.

What gear-ratios do variable-speeds usually work along?

P & W didn't seem to use the variable speed drives with the same compressors (or indeed on the same engines ) as they used with the single speed, two speed and even the two stage engines

Why?

The R-2800-22/W engine used a two speed drive and the gears were 7.29 and 9.45.

Ratio difference is 2.16

The -30W used a variable speed drive with 7.29 being the lowest speed and 10.55 being the high limit.

Ratio difference is 3.26

the -30 wasn't a "C" series engine, it was an "E" series

Which seemed to be available in 1945 at best?

The supercharger on the -32W engine is quite possibly the most sophisticated mechanical drive supercharger used on a piston engine fighter in squadron service.

Is there any advantage in having two impellers working in parallel than in seres?

 

[swampyankee]

What gear-ratios do variable-speeds usually work along?
Why?
Ratio difference is 2.16
Ratio difference is 3.26
Which seemed to be available in 1945 at best?
Is there any advantage in having two impellers working in parallel than in seres?

The advantages may be related to packaging and gearing; there are disadvantages, in greater parts count and possibly lower achievable efficiency; dynamic compressors tend to be less efficient as size decreases.

 

[Shortround6]

I would note that the centrifugal compressors tend to both flow air and require power to drive in relation to the square of the tip speed.
So the Change from 7.29 to to 9.45 is more like the change from 53 to 89 or about 68% more airflow and 68% more power required to drive the supercharger.
Please note that most of these compressors operated in the 70% range for efficiency with the extra 25-30% of the power driving going directly into heating the air over and above the heat generated by the simple compression of the air.
This may be one reason they didn't shift to the high gear ratio of the variable speed drive until they redesigned the supercharger itself. This is supposition on my part. The variable speed drive was used an a different base model of engine and a different compressor design. I think they changed the diffuser but I am not sure and have no other details on what else may or may not have been changed. This makes it very hard to draw comparisons or to say "they should have done......." sooner.
Using an intake charge (air and fuel) 100-200 degrees hotter in high gear than in low gear can really push you closer to detonation limits and put an extra load on the engine cooling requirements.

 

[Zipper730]

The advantages may be related to packaging and gearing

When you say packaging, do you mean volumetrics? As for the gearing, it would be simpler in layout?

there are disadvantages, in greater parts count and possibly lower achievable efficiency; dynamic compressors tend to be less efficient as size decreases.

The greater parts count, I get.

I would note that the centrifugal compressors tend to both flow air and require power to drive in relation to the square of the tip speed.
So the Change from 7.29 to to 9.45 is more like the change from 53 to 89 or about 68% more airflow and 68% more power required to drive the supercharger.

I follow...

Please note that most of these compressors operated in the 70% range for efficiency with the extra 25-30% of the power driving going directly into heating the air over and above the heat generated by the simple compression of the air

. . .

This may be one reason they didn't shift to the high gear ratio of the variable speed drive until they redesigned the supercharger itself.

Which had to do with the shape of the compressor and the diffuser? What's the diffuser do exactly?

 

[fubar57]

Internet...."A diffuser -- a set of stationary vanes that surround the impeller -- converts the high-speed, low-pressure air to low-speed, high-pressure air. Air molecules slow down when they hit the vanes, which reduces the velocity of the airflow and increases pressure."

 

[Zipper730]

"A diffuser -- a set of stationary vanes that surround the impeller -- converts the high-speed, low-pressure air to low-speed, high-pressure air. Air molecules slow down when they hit the vanes, which reduces the velocity of the airflow and increases pressure."

So basically, the redesigned diffuser was able to better increase the pressure without increasing temperature?

 

[swampyankee]

So basically, the redesigned diffuser was able to better increase the pressure without increasing temperature?

Kind of.

The impeller increases the velocity of the air; the diffuser slows it down and increases pressure. See this site for some more detail.

 

[Zipper730]

Done, though I personally prefer the image on page 1

I know exactly what the function is... I'm kind of a person who has to see things

 

[wuzak]

Ratio difference is 2.16
Ratio difference is 3.26

What would we do without you Zipper?

 

[wuzak]

Internet...."A diffuser -- a set of stationary vanes that surround the impeller -- converts the high-speed, low-pressure air to low-speed, high-pressure air. Air molecules slow down when they hit the vanes, which reduces the velocity of the airflow and increases pressure."

The vanes are there for guiding the air.

The diffuser is the expansion of volume.

A diffuser is "a device for reducing the velocity and increasing the static pressure of a fluid passing through a system". Diffusers are used to slow the fluid's velocity while increasing its static pressure. The fluid's static pressure rise as it passes though a duct is commonly referred to as pressure recovery. In contrast, a nozzle is often intended to increase the discharge velocity and lower pressure while directing the flow in one particular direction.

A typical, subsonic diffuser is a duct that increases in size in the direction of flow. As the duct increases in size, fluid velocity decreases, and static pressure rises. Both mass flow rate and Bernoulli's principle are responsible for these changes in pressure, and velocity.

Diffuser (thermodynamics) - Wikipedia