Dive Bombing: USN vs USAAF/USAF

[wuzak]

Boost is the pressure above standard atmospheric pressure at sea level.

You could go without intercooling, using just ADI as most Allison 2 stage engines did, but that would probably not work so well for an air cooled engine. You cannot use an aftercooler Merlin style as you would need 7 (for the R-4360) or 9 (for the R-3350). Also, the US preferred method was air to air intercoolers, especially for air cooled engines.

Given that you have to add intercoolers you will end up with more drag. And the extra weight will also cause more drag. So you will penalise the aircraft in the region where it operates teh majority of the time - low level.

You could probably calculate how much drag falls off with altitude. Look through http://www.wwiiaircraftperformance.org/ at some different aircraft and compare engine power at altitude with speed at altitude.

The formula you need is P = k * V^3. P = power, V = speed and k is the constant which encompasses drag and propeller efficiency.

See if you can find some aircraft operation manuals on the web. They will have fuel consumption at different weights, speed and altitudes.

To learn of mission profiles you will have to find aircraft specific books, or operations record books or similar.

While I'm curious: How far could the P-47D-25 and P-47N fly with 1,000 and 2,000 pounds of bombs?

Not far.

[KJ_Lesnick]

Boost is the pressure above standard atmospheric pressure at sea level.

And this was measured in inches mercury right?

You could go without intercooling, using just ADI as most Allison 2 stage engines did, but that would probably not work so well for an air cooled engine.

Why?

You cannot use an aftercooler Merlin style as you would need 7 (for the R-4360) or 9 (for the R-3350).

This has to do with the number of cylinders correct?

Also, the US preferred method was air to air intercoolers, especially for air cooled engines.

What level of preference for the engine manufacturers and the USAAF and USN?

Given that you have to add intercoolers you will end up with more drag.

The radiator -- still couldn't that be covered by the oil-cooler radiator?

And the extra weight will also cause more drag.

How much extra weight would be added by an extra-stage of supercharging and an intercooler?

So you will penalise the aircraft in the region where it operates teh majority of the time - low level.

True enough, however the P-51B/C/D all seemed to perform better at low altitude as well as high.  They seemed to get progressively faster and farther flying all around -- I figure whatever would work for the fighters would work for this.

You could probably calculate how much drag falls off with altitude. Look through http://www.wwiiaircraftperformance.org/ at some different aircraft and compare engine power at altitude with speed at altitude.

The formula you need is P = k * V^3. P = power, V = speed and k is the constant which encompasses drag and propeller efficiency.

I need to know what units to put in: This sounds stupid but there are many ways to measure the following

See if you can find some aircraft operation manuals on the web. They will have fuel consumption at different weights, speed and altitudes.

I'll look

To learn of mission profiles you will have to find aircraft specific books, or operations record books or similar.

How would I find operational record books provided they're not secret.

Not far.

Would you say either could fly greater than or less than 800 miles?

[wuzak]

Boost is the pressure above standard atmospheric pressure at sea level.

And this was measured in inches mercury right?

It could be measured with any pressure units.

The British used pounds per square inch (lb/in² or psi) to measure boost.

The Americans used Manifold Absolute Pressure (MAP) and measured them in inches of mercury (inHg).

MAP = standard sea level pressure + boost.

So for a Mustang running 18psi boost the MAP is 14.7psi + 18psi = 32.7psi ~ 67inHg MAP.

You could go without intercooling, using just ADI as most Allison 2 stage engines did, but that would probably not work so well for an air cooled engine.

Why?

It's kinda obvious if you think for a minute....

Air cooled engines were often limited by cylinder head temperature. Running air straight from the supercharger without intercooling or aftercooling will mean the air is hotter going into the cylinders and will make controlling temperature harder.

ADI helps a little, but not by nearly as much as intercooling.

Liquid cooled engines are more tolerant of increased intake temperatures.

You cannot use an aftercooler Merlin style as you would need 7 (for the R-4360) or 9 (for the R-3350).

This has to do with the number of cylinders correct?

Yes. And the way that the induction is fed from the supercharger to the cylinders.

A V-12 only needed one outlet. An H-24 (Sabre) or X-24 (Vulture) required 2.

Also, the US preferred method was air to air intercoolers, especially for air cooled engines.

What level of preference for the engine manufacturers and the USAAF and USN?

The only US engines to use liquid cooled intercoolers as far as I am aware are the Packard Merlins and one model of V-1710 (the -119). Oh, and the XR-7755 may have used them too, but that didn't even get into an airplane.

Given that you have to add intercoolers you will end up with more drag.

The radiator -- still couldn't that be covered by the oil-cooler radiator?

Um, no.

You need additional cooling for the induction air. That means extra radiator area.

Also, for an air-cooled engine it is likely that a larger oil cooler is necessary, as air cooled engines did reject a significant amount of heat through the oil.

So you will penalise the aircraft in the region where it operates teh majority of the time - low level.

True enough, however the P-51B/C/D all seemed to perform better at low altitude as well as high.  They seemed to get progressively faster and farther flying all around -- I figure whatever would work for the fighters would work for this.

They kept adding fuel (drop tanks, rear tanks). Improved engines (-7 replaces the -3 in the B/C/D/K for better low down performance, particularly climb) and improved aero (in the H).

The range performance was with little or no low altitude component.

You could probably calculate how much drag falls off with altitude. Look through http://www.wwiiaircraftperformance.org/ at some different aircraft and compare engine power at altitude with speed at altitude.

The formula you need is P = k * V^3. P = power, V = speed and k is the constant which encompasses drag and propeller efficiency.

I need to know what units to put in: This sounds stupid but there are many ways to measure the following

It doesn't matter what units you use, so long as they are consistent (always use the same unit for each input). Calculate the % change in the k value. That's really all you need to know.

To learn of mission profiles you will have to find aircraft specific books, or operations record books or similar.

How would I find operational record books provided they're not secret.

USAF archives. US national archives. Squadron histories.

For British information it is the National Archives. Some items are free, some can be purchased directly, but others you will need to copy them yourself or have them do it for you (for a fee).

Not far.

Would you say either could fly greater than or less than 800 miles?

I would say less than 800 miles for a D, since that is approximately the range for a clean D without drop tanks. The N would go further than the D on account of extra fuel, but it would still be less than 800 miles.

[KJ_Lesnick]

It could be measured with any pressure units.

What would you use?

The British used pounds per square inch (lb/in² or psi) to measure boost.

The Americans used Manifold Absolute Pressure (MAP) and measured them in inches of mercury (inHg).

MAP = standard sea level pressure + boost.

So for a Mustang running 18psi boost the MAP is 14.7psi + 18psi = 32.7psi ~ 67inHg MAP.

How do you convert MAP to PSI?

It's kinda obvious if you think for a minute....

Air cooled engines were often limited by cylinder head temperature. Running air straight from the supercharger without intercooling or aftercooling will mean the air is hotter going into the cylinders and will make controlling temperature harder.

ADI helps a little, but not by nearly as much as intercooling.

Okay

Yes. And the way that the induction is fed from the supercharger to the cylinders.

Okay.

The only US engines to use liquid cooled intercoolers as far as I am aware are the Packard Merlins and one model of V-1710 (the -119).

Why did the V1710-119 use liquid cooling?

Was the USAAF or P&W averse to such a design if it offered an advantage?  Did an air-to-air cooled cooler provide more ruggedness?

You need additional cooling for the induction air. That means extra radiator area.

Well any intercooler requires a radiator for itself to dispel heat (I'm not sure how much bigger it'd have to be), a liquid cooled intercooler wouldn't require additional induction air though (the coolant is liquid) right?

They kept adding fuel (drop tanks, rear tanks).

Would you speculate the 445 gallon capacity listed was due to a 95 gallon tank in the bomb-bay, or do you think it was more like the P-51/P-47?

Improved engines (-7 replaces the -3 in the B/C/D/K for better low down performance, particularly climb)

Still what was the critical altitude of the -7 engine versus the -3?

The range performance was with little or no low altitude component.

It made difference up high mostly, in other words.

It doesn't matter what units you use, so long as they are consistent (always use the same unit for each input).

As a jump off point

• What units would you use
  
• If you used MAP: What would go with that
  

Calculate the % change in the k value. That's really all you need to know.

Not to sound stupid but what's the k-value as a basic constant (pi for example is around 3.1416); as for percentage -- is that propulsive efficiency?

The N would go further than the D on account of extra fuel, but it would still be less than 800 miles.

Okay, so the predominant reason for the cancellation of the XA-41 was that they simply didn't want a dedicated dive bomber that wasn't two engined?

[wuzak]

It could be measured with any pressure units.

What would you use?

I would tend to use Pa, kPa, MPa or bar if I were calculating something. Which is irrelevant to this discussion.

Otherwise I would tend to use the units used back in teh day - psi boost for British a/c, inHg MAP for american and ATA for German a/c.

How do you convert MAP to PSI?

Seriously, you can't figure that out for yourself?

btw MAP is not a unit. MAP is an absolute pressure, boost is a gauge pressure, except it is compared to standard atmospheric pressure at sea level.

The only US engines to use liquid cooled intercoolers as far as I am aware are the Packard Merlins and one model of V-1710 (the -119).

Why did the V1710-119 use liquid cooling?

I believe it was the only one that had an intercooler of any description.

Was the USAAF or P&W averse to such a design if it offered an advantage?  Did an air-to-air cooled cooler provide more ruggedness?

Probably because they didn't want to carry another fluid.

I don't think that either had more ruggedness. If a liquid intercooler radiator was hit the engine power would have to be reduced.

If an air to air intercooler was hit it would lose cooling efficiency but would also leak the compressed air, so the engine would become somewhat starved.

You need additional cooling for the induction air. That means extra radiator area.

Well any intercooler requires a radiator for itself to dispel heat (I'm not sure how much bigger it'd have to be), a liquid cooled intercooler wouldn't require additional induction air though (the coolant is liquid) right?

An air to air intercooler has to be stuck into the air stream somehow to get the cooling effect.

A liquid to air intercooler needs a radiator to cool the working fluid (usually glycol/water in WW2). That radiator needs to be exposed to the air flow too. In theory it can be smaller than an air to air intercooler.

They kept adding fuel (drop tanks, rear tanks).

Would you speculate the 445 gallon capacity listed was due to a 95 gallon tank in the bomb-bay, or do you think it was more like the P-51/P-47?

Wouldn't have a clue.

Improved engines (-7 replaces the -3 in the B/C/D/K for better low down performance, particularly climb)

Still what was the critical altitude of the -7 engine versus the -3?

http://www.wwiiaircraftperformance.org/mustang/mustang-III-ads-3.jpg
http://www.wwiiaircraftperformance.org/mustang/mustang-III-ads-7.jpg

Near the bottom.

S= fully supercharged (Hi in USAAF terms)
M = medium supercharged (Lo in USAAF terms)

It doesn't matter what units you use, so long as they are consistent (always use the same unit for each input).

As a jump off point

• What units would you use
  
• If you used MAP: What would go with that
  

I would use the units from the source data.

MAP has nothing to do with it. You need Power (hp, kW) and speed (mph, kph, m/s, leagues per year). That's it. They have to be from the same altitude to have any meaning, though.

Calculate the % change in the k value. That's really all you need to know.

Not to sound stupid but what's the k-value as a basic constant (pi for example is around 3.1416); as for percentage -- is that propulsive efficiency?

No.

k covers a number of factors such as drag coefficient, cross sectional area and propulsive efficiency.

It is largely meaningless, and really only an approximation. It will, however, give you a feel for the effect of altitude on performance - if you use it as a comparison between the same aircraft at two different altitudes.

The N would go further than the D on account of extra fuel, but it would still be less than 800 miles.

Okay, so the predominant reason for the cancellation of the XA-41 was that they simply didn't want a dedicated dive bomber that wasn't two engined?

Probably it was cancelled because they didn't want a dive bomber full stop, and eh AD-1 did everything else better.

[KJ_Lesnick]

I would tend to use Pa, kPa, MPa or bar if I were calculating something.

Makes enough sense

Otherwise I would tend to use the units used back in teh day - psi boost for British a/c, inHg MAP for american and ATA for German a/c.

Okay

Seriously, you can't figure that out for yourself?

Instinctively I'd just divide the two: I just wanted to make sure so all my answers weren't fecked up

MAP is an absolute pressure, boost is a gauge pressure, except it is compared to standard atmospheric pressure at sea level.

That's actually quite interesting and does make MAP seem better...

I believe it was the only one that had an intercooler of any description.

What aircraft was it used on?

Probably because they didn't want to carry another fluid.

That makes sense, and reduces weight: However, one has to consider that liquid cooled intercoolers are smaller

I don't think that either had more ruggedness. If a liquid intercooler radiator was hit the engine power would have to be reduced.

If an air to air intercooler was hit it would lose cooling efficiency but would also leak the compressed air, so the engine would become somewhat starved.

Starved sounds worse...

An air to air intercooler has to be stuck into the air stream somehow to get the cooling effect.

A liquid to air intercooler needs a radiator to cool the working fluid (usually glycol/water in WW2). That radiator needs to be exposed to the air flow too. In theory it can be smaller than an air to air intercooler.

Smaller is a good idea for compactness, the question is the weight.

Wouldn't have a clue.

Okay -- I wonder where I'd go to sort this out?

http://www.wwiiaircraftperformance.org/mustang/mustang-III-ads-3.jpg
http://www.wwiiaircraftperformance.org/mustang/mustang-III-ads-7.jpg

Near the bottom.

S= fully supercharged (Hi in USAAF terms)
M = medium supercharged (Lo in USAAF terms)

Okay, so

• Merlin 3: 25,000 Low; 28,000 High
  
• Merlin 7: 11,000 Low; 24,500 High
  

.
The F4U-4's critical altitude was around 26,000 in High-gear, and 20,000 in low-gear correct?

I would use the units from the source data.

MAP has nothing to do with it. You need Power (hp, kW) and speed (mph, kph, m/s, leagues per year). That's it. They have to be from the same altitude to have any meaning, though.

Okay

No.

k covers a number of factors such as drag coefficient, cross sectional area and propulsive efficiency.

It is largely meaningless, and really only an approximation. It will, however, give you a feel for the effect of altitude on performance - if you use it as a comparison between the same aircraft at two different altitudes.

Should I just treat it as solving for x?

Probably it was cancelled because they didn't want a dive bomber full stop

LOL, true enough

[wuzak]

• Merlin 3: 25,000 Low; 28,000 High
  
• Merlin 7: 11,000 Low; 24,500 High
  

No.

Merlin 3:
M: 1,710hp @ 11,000ft
S: 1,470hp @ 23,000ft

Merlin 7:
M: 1,680ho @ 5,500ft
S: 1,470hp @ 18,750ft

[KJ_Lesnick]

Wuzak

No.

Merlin 3:
M: 1,710hp @ 11,000ft
S: 1,470hp @ 23,000ft

Merlin 7:
M: 1,680ho @ 5,500ft
S: 1,470hp @ 18,750ft

Okay, I read the wrong item on the list.

Just to be curious is that with or without ram?

[wuzak]

Wuzak

No.

Merlin 3:
M: 1,710hp @ 11,000ft
S: 1,470hp @ 23,000ft

Merlin 7:
M: 1,680ho @ 5,500ft
S: 1,470hp @ 18,750ft

Okay, I read the wrong item on the list.

Just to be curious is that with or without ram?

It is not clear.

[KJ_Lesnick]

Wuzak

It is not clear.

Understood

While on a tangentially connected subject: The F4U-4's critical altitude was around 26,000 in High-gear, and 20,000 in low-gear correct?

I would use the units from the source data.

okay

MAP has nothing to do with it. You need Power (hp, kW) and speed (mph, kph, m/s, leagues per year). That's it. They have to be from the same altitude to have any meaning, though.

Okay

No.

k covers a number of factors such as drag coefficient, cross sectional area and propulsive efficiency.

It is largely meaningless, and really only an approximation. It will, however, give you a feel for the effect of altitude on performance - if you use it as a comparison between the same aircraft at two different altitudes.

Should I just treat it as solving for x?

Probably it was cancelled because they didn't want a dive bomber full stop

LOL, true enough

[jcf]

The XA-41 order of November 10, 1942 was amended April 30, 1943, changing the specification from dive-bomber to ground-attack.
So long before it's first flight on February 11, 1944 it was already not a dive-bomber, and it was not a dive-bomber when the contract was
cancelled.

[KJ_Lesnick]

The XA-41 order of November 10, 1942 was amended April 30, 1943, changing the specification from dive-bomber to ground-attack.

Actually it was changed from dive to level-bomber

So long before it's first flight on February 11, 1944 it was already not a dive-bomber, and it was not a dive-bomber when the contract was cancelled.

Correct, but I'm not sure if any changes were made to it that prevented it from executing the mission

[wuzak]

While on a tangentially connected subject: The F4U-4's critical altitude was around 26,000 in High-gear, and 20,000 in low-gear correct?

Something like that.

http://www.wwiiaircraftperformance.org/f4u/f4u.html

You will find some information on "aircraft critical altitude" in there. Ram is included in their numbers, as the critical altitude varies with loading.

It also varies with engine setting - normal,military and combat. As is the case with all supercharged piston aero engines of WW2, the critical altitude is reduced the higher the boost used.

It is largely meaningless, and really only an approximation. It will, however, give you a feel for the effect of altitude on performance - if you use it as a comparison between the same aircraft at two different altitudes.

Should I just treat it as solving for x?
[/quote]

k = P/V^3

Simple as that.

[KJ_Lesnick]

Okay I did some figures for the F4U-1 @ 11142 lbs: I got the following

• 311 mph @ 0 feet -w-  takeoff-power (2000 hp) main-blower: 0.000664889 for k-constant
  
• 336 mph @ 7400 feet -w- mil-power (1800 hp), low-blower: 0.00004745 for k-constant
  
• 333 mph @ 8250 feet -w- mil-power (1800 hp), low-blower:  0.00004875 for k-constant
  
• 378 mph @ 20000 feet -w- mil-power (1650 hp), high-blower: 0.00003055 for k-constant
  
• 395 mph @ 23800 feet -w- normal power (1550 hp), high-blower: 0.00002515 for k-constant
  

I'm not sure if I'm doing everything right here, I assume I'm supposed to compare these figures against other designs to determine efficiency at speeds and altitudes. 

BTW: How do you compute ram, and is it even important?

[wuzak]

Okay I did some figures for the F4U-1 @ 11142 lbs: I got the following

• 311 mph @ 0 feet -w-  takeoff-power (2000 hp) main-blower: 0.000664889 for k-constant
  
• 336 mph @ 7400 feet -w- mil-power (1800 hp), low-blower: 0.00004745 for k-constant
  
• 333 mph @ 8250 feet -w- mil-power (1800 hp), low-blower:  0.00004875 for k-constant
  
• 378 mph @ 20000 feet -w- mil-power (1650 hp), high-blower: 0.00003055 for k-constant
  
• 395 mph @ 23800 feet -w- normal power (1550 hp), high-blower: 0.00002515 for k-constant
  

I'm not sure if I'm doing everything right here, I assume I'm supposed to compare these figures against other designs to determine efficiency at speeds and altitudes. 

BTW: How do you compute ram, and is it even important?

I suppose you could compare with other designs, but I originally said to do this to see how drag varied with altitude.

Interesting that the drag is higher at 8,250ft than 7,400ft. I'd suggest, though, that the power isn't the same at both altitudes.