Hurricane IIc vs. B-17s B-24s

[renrich]

SR, I had forgotten that the P47 had an engine driven supercharger in tandem with the turbo supercharger. That must have been what Linnekin was talking about " the combination of drag and mechanical power bleed-off" at low altiudes.

 

[renrich]

SR, I had forgotten that the P47 had an engine driven supercharger in tandem with the turbo supercharger. That must have been what Linnekin was talking about " the combination of drag and mechanical power bleed-off" at low altiudes.

 

[Shortround6]

SR, the F4U4 had a service ceiling in excess of 41000 feet. The F4U5 the same except it developed more HP. Please enlighten me as to what a "sidewinder supercharger" is.

The F4U-5 used the R-2800-32W engine.

See patent drawing:
http://www.ww2aircraft.net/forum/engines/radial-engine-2-stage-superchargers-r-2800-etc-17593.html

The two sideways (to american thinking) superchargers were run in parallel as the first stage and after the air went through the intercooler it was feed into the second stage impeller mounted in the usual place at the back of the engine. You can see the induction pipes leading from the supercharger casing to the cylinders in the drawing.

This set up was rated at 1800hp at 30,000ft Military power compared to the 1800hp at 23,000ft military rating of the R-2800-18W used in the F4U-4. Or the 1500hp at 30,000 normal compared to the 1550hp at 26,000 normal rating of the R-2800-18W.

 

[Shortround6]

SR, I had forgotten that the P47 had an engine driven supercharger in tandem with the turbo supercharger. That must have been what Linnekin was talking about " the combination of drag and mechanical power bleed-off" at low altiudes.

Except that the R-2800 in the F4U had an almost identical supercharger as it's second stage to supply boost at low altitude. The Navy two stage superchargers disconnected the 1st stage at low level to save power.

Without some sort of supercharger there is no way on god's green earth (or in his blue sky ) that you are going to get over 30in of manifold pressure in an engine.

 

[drgondog]

For those who are interested there are several charts over on the Spitfire performance website.

One for the P-47 that is rather interesting: scroll down to the Oct 11 1943 test for the P-47D.

P-47 Performance Tests

This test lists altitude, manifold pressure, horsepower, turbo RPM, exhaust back pressure, and carb temperature in addition to speed.

This gives a pretty good idea of the operation of the turbo charger. It was basically idling at sea level with just about all pressure coming from the engine driven supercharger. The work a supercharger does goes up with the square of it's speed so at sea level the turbo was doing about 1/9 the work it would be doing at 31,000ft when it hit 22,000rpm.
.

SR - that test was performed with a D-10 w/WI but it was unclear whether the paddle prop was installed

 

[drgondog]

While not a rule of thumb, it was not rare at all to have enemy aircraft approaching at 28,000ft and higher. I pulled these in about 15 minutes.


.

It was common to see the 'escort' 109s come from 30-32000 feet and for 51s (or 47s) to spot them and climb to that altitude to engage. My father met one at 30K and chased him to the deck before he got it..

 

[DAVIDICUS]

Then it was more common than I thought.

I also noticed that ME-109's would fly top cover for FW-190's.

 

[Shortround6]

SR - that test was performed with a D-10 w/WI but it was unclear whether the paddle prop was installed

I was more interested in the relationship between altitude and the manifold pressure/power of the engine and how it related to the exhaust turbine speed and exhaust back pressure than in the actual performance. For instance you may have noted that at a steady manifold pressure of 56 in the power output varies in relation to the exhaust back pressure.

It illustrates how the turbo allowed a steady hp output instead of the saw tooth profile of a multi speed geared supercharger. It also illustrates the the higher altitudes the turbo allowed compared to even very good mechanical 2 stage engines.

Later aircraft with improved propellers and and later model engines and with improved turbines/compressors using higher manifold pressures can, of course do much better

 

[Vincenzo]

SR - that test was performed with a D-10 w/WI but it was unclear whether the paddle prop was installed

there is the name of propelelr in the test need only know what are i think they are the not paddle (i'm talking of 11th ocotber '43 test of 43-75035 if you talking an other test sorry)

 

[Nikademus]

dup post. server issues.

 

[Nikademus]

...is pretty much the perfect answer
The lack of combat at 30,000ft, for me, can be attributed to a lack of opposition rather than any lack of plausibility.

I disagree. It appears to me that the prime determinant of chosen altitude was driven primary by the German ground defenses, (aka "flak") balanced against the need for accuracy, particularily in the case of the Eighth AF with it's stated goal of "precision" bombing. The average altitude settled upon for this was around 21,000 feet. (RAF had started bombing in 39 at 9-11,000 but quickly got schooled)

Despite this lofty height, German Flak defenses took a terrible toll on the Eighth over the course of the campaign in terms of damage, casualties and outright losses. Additionally the defenses had a measurable impact on accuracy. In spite of this, the USAAF continued to attack around this altitude band. If accuracy could have been mainained at much higher altitudes while reducing losses and damage vs. said defenses, I think Eaker and company would have done so.

 

[drgondog]

Then it was more common than I thought.

I also noticed that ME-109's would fly top cover for FW-190's.

I haven't found an 'exact date' for the LW changes to tactics but the 8th (particularly the Target Escorts) began seeing the 'top cover' Me 109s in the April 1944 timeframe when it became clear the pre-Sturm Fw 190s were struggling against the Mustangs at bomber altitudes - and nearly mandatory when the Sturm Fw 190A8's reached combat levels of deployment. It was quite common in June/July and then the rest of the war.

Later, when there was enough warning to assemble and form up, the Fw 190s would try to stage at 28-27,000 feet and go slightly 'downhill' (still with top cover w/109G A/S) to try to make one company front attack and then head for the deck.

 

[drgondog]

I disagree. It appears to me that the prime determinant of chosen altitude was driven primary by the German ground defenses, (aka "flak") balanced against the need for accuracy, particularily in the case of the Eighth AF with it's stated goal of "precision" bombing. The average altitude settled upon for this was around 21,000 feet. (RAF had started bombing in 39 at 9-11,000 but quickly got schooled)

Despite this lofty height, German Flak defenses took a terrible toll on the Eighth over the course of the campaign in terms of damage, casualties and outright losses. Additionally the defenses had a measurable impact on accuracy. In spite of this, the USAAF continued to attack around this altitude band. If accuracy could have been mainained at much higher altitudes while reducing losses and damage vs. said defenses, I think Eaker and company would have done so.

In 1945 there were far more missions flown at 18-20K, particularly when cloud cover was 10/10 over the continent, than 1944. Radar directed blind bombing was more effective and the optical tracking AA was ineffective.

However the prime altitude for the lead groups in the middle of the stack was ~ 24-25,000 feet for B-17s and was SOP for clear days even in 1945... lower for the B-24s.

 

[Colin1]

...the average altitude settled upon for this was around 21,000 feet

Despite this lofty height, German Flak defenses took a terrible toll on the Eighth... ...If accuracy could have been mainained at much higher altitudes while reducing losses and damage vs. said defenses, I think Eaker and company would have done so.

Someone got their sums wrong, the 88mm could range between 32 and 35,000ft.

Maybe Arnold thought so too, he relieved Eaker for Doolittle in 1943.

The next round of aircraft design was being built, among other things, to fly higher. I'm not even sure if the B-29 was next round or next generation. The air war was being pushed upwards as a natural reflex in avoiding the defences of the enemy. I don't really understand why you can't envisage combat at 30,000ft+ just because the war didn't last long enough for that to happen.

Lauding the P-47N for its high altitude performance whilst ignoring its shortfalls at altitudes lower, no more gives a 'skewed picture' than would pushing an F8F up to 30,000ft to expose its shortfalls; horses for courses.

 

[DAVIDICUS]

In the Korean war, our pilots would enter Mig Alley in successive four aircraft flights spaced five minutes apart. These flights would enter the airspace staggered between 30,000 and 35,000 ft. Migs, with their higher operational ceiling, would usually bounce from high above, often approaching at altitudes over 45,000ft. Once the bounce was seen, the rest of the four ship groups would converge in a counterattack.

 

[davparlr]

Someone got their sums wrong, the 88mm could range between 32 and 35,000ft.

Some interesting information on gun coverage and altitude. A 35k ft range antiaircraft gun could cover a radius of 3.3 miles (34 sq. miles) at 30k ft, 4.6 mile radius (66 sq miles) at 25k ft., or 5.4 miles (92 sq miles) at 20k ft. An aircraft, flying directly overhead at 30k ft. and a ground speed of 250 mph, will be exposed to 22 rounds of Flak at 15 rds/min, at 25k, it will be exposed to 35 rds, and at 20k, it will be exposed to 38 rounds.

A bomber flying at 20k ft, say a B-24, will have an 8% higher risk than a B-17 at 25k, and 72 % higher risk than a B-29 flying at 30k. The B-17 will have a 60% higher risk than the B-29. A plane flying above 30k would, for all practical purposes, have to be directly overhead to be hit by the gun.

Altitude also provides benefit against fighters. While bombers typically do not maneuver, fighters do, and altitude affects maneuverability similar to adding gross weight. A nimble fighter at 25k could be quite sluggish at 30k.

It also should be noted that the B-29 is much faster than the B-17/24, also significantly reducing the exposure time.

For the bomber, even more so than a fighter, airspeed and altitude is life. It is easy to see the lure of high altitude, high speed bombing.

Countering all this is bombing accuracy.

These are all rough estimates.

 

[Colin1]

It is easy to see the lure of high altitude, high speed bombing.

Countering all this is bombing accuracy.

These are all rough estimates.

Good info Dave
agree with your conclusions

 

[Milosh]

Someone got their sums wrong, the 88mm could range between 32 and 35,000ft.

88mm Flak 18, 36 and 37

Maximum range :
Horizontal 16,200 yd
Vertical 39,000 ft
Maximum effective ceiling 25,000 ft (at 70-deg elevation )

from http://www.lexpev.nl/downloads/tme9369agerman88mmgun.pdf

 

[Shortround6]

Maximum effective ceiling

Can be the altitude at which a gun get get off a reasonable number of shots at a target flying straight and level.

It is often governed by the shells fuse. Most if not all AA shells having a self destruct that functions at the end of a certain time period. Since all time fuses have a + and - tolerance and since the shells have slowed considerably by the time they get to 25,000ft or better getting the shell to burst at the right height (even if the line of flight intersects the bombers path) gets harder and and harder as the height goes up.

 

[Milosh]

It is my understanding is that is the distance/height at which there was a reasonable chance of making a hit.

Other effective ranges:

20mm > ~1100m
37mm > ~1600m