P-47 acceleration

[Jenisch]

Hello,

Given the heavy weight of the plane, I was wondering how the P-47 accelerated in level flight as compared to other fighters of the era (both Allied and Axis). Any ideas?

 

[OldSkeptic]

Hello,

Given the heavy weight of the plane, I was wondering how the P-47 accelerated in level flight as compared to other fighters of the era (both Allied and Axis). Any ideas?

Depended on the flight regime. It was heavy and climbed poorly (though that got better with later more powerful engines and different props, but still still not great).

The design was optimised for high altitude missions. At 30.000+ it was a hot ship and surprisingly, in fact very manoeuvrable despite it's size and weight. At that height it's acceleration was acceptable (still plenty of engine power and drag had gone down with altitude).

The lower you went the worse it got. Not only heavy but pretty draggy (it's design really predated the 190 like tight, aerodynamic cowling designs of later air cooled designs). Again the change to the paddle props (which really traded speed for more 'bite' for better climb ...basically) and more engine power helped, but at 15,000ft or less it was not the fastest thing around in any way (climb, speed, acceleration).

P-47 escort fighter pilots learned very quickly not to get too low in any fights with 109s or 190s.

So the answer to your questions is, like all the fighter aircraft of the time, depends on the flight regime. For example in the late '43 versions, at 30,000ft it would run rings around a 190A. But at 20.000ft (let alone 10.000ft) the balance swings the other way.

 

[stona]

P-47 escort fighter pilots learned very quickly not to get too low in any fights with 109s or 190s.

I don't disagree with your points about the P-47 but this statement is simply not true. P-47 pilots confidently took on their Luftwaffe adversaries even on the deck. I have many combat reports which support this contention. Of the first five I looked at (at random out of a total of about 300) three describe low level combats.

First a report describing several low level combats undertaken with no hesitation and a Luftwaffe pilot abandoning his aircraft BEFORE he was actually engaged, something that started to happen more often towards the end.

And a couple of others.

3 out of 5 out of 300 at random! The USAAF was operating at low level because it was engaged in attacking targets on the ground at this time. I don't see any evidence in these, or any of the other many reports that I have read, that this caused the pilots undue concern in terms of engaging the Luftwaffe at low level. They were far more concerned about flak when attacking defended targets.

Cheers

Steve

 

[tomo pauk]

Depended on the flight regime. It was heavy and climbed poorly (though that got better with later more powerful engines and different props, but still still not great).

The design was optimised for high altitude missions. At 30.000+ it was a hot ship and surprisingly, in fact very manoeuvrable despite it's size and weight. At that height it's acceleration was acceptable (still plenty of engine power and drag had gone down with altitude).

The lower you went the worse it got. Not only heavy but pretty draggy (it's design really predated the 190 like tight, aerodynamic cowling designs of later air cooled designs). Again the change to the paddle props (which really traded speed for more 'bite' for better climb ...basically) and more engine power helped, but at 15,000ft or less it was not the fastest thing around in any way (climb, speed, acceleration).

P-47 escort fighter pilots learned very quickly not to get too low in any fights with 109s or 190s.

So the answer to your questions is, like all the fighter aircraft of the time, depends on the flight regime. For example in the late '43 versions, at 30,000ft it would run rings around a 190A. But at 20.000ft (let alone 10.000ft) the balance swings the other way.

What would be the Cd numbers for the P-47 and Fw-190A? How big (if any?) was the speed penalty once the paddle blade props were installed on the P-47?

 

[Shortround6]

There are a couple of charts at the back of "America's Hundred Thousand" that are calculated at 250mph and sea level and in both early and late lists the P-47 is second to the P-38 in acceleration. While it was heavy it also had a lot of thrust. Acceleration changes with speed, altitude and exact model.

 

[tomo pauk]

The tables at AHT assume that all props have the efficiency of 80% - that would tend to skew results? Also, there is no exhaust thrust taken into account.

 

[tyrodtom]

I know there's a lot more to acceleration in aircraft than just power to weight, but if you look at just that, the P-47D and Fw190A8 had the approximately the same power to weight ratio, about .20 hp per lb.

 

[tomo pauk]

The exhaust thrust of the non-turbo engines,with well executed exhaust stacks, was able to add ~10% more thrust to the prop at sea level. At ~25000 ft, it was circa 15%, but the BMW-801D was loosing much of the power above 20000 ft, while the turbo R-2800 in P-47D did not until above 25000.
Probably more important was that drag (as force, not the coefficient) was bigger for the P-47, due to it's bigger size.

 

[Shortround6]

The tables at AHT assume that all props have the efficiency of 80% - that would tend to skew results? Also, there is no exhaust thrust taken into account.

Most props are going to be within a few percent of 80% at sea level. At 20,000ft or so were the air is about 1/2 as dense prop size and design becomes more important.

Exhaust thrust doesn't quite equal power, it varies in efficiency with the speed (and altitude) of the aircraft. At 250mph and at sea level it contributes quite a bit less than it does at 20,000ft and 325-375mph.

The only figures I have are for a Hurricane II with a Merlin XX engine. Charge airflow though the engine at 15,00ft is 140.5lb/min and at 20,000ft it is 144lb/min which is fairly close (at 25,000ft it is 129lb/min) but due to lower atmospheric pressure at the exhaust tips the exhaust gas velocity changes from 1395fps to 1695fps with the change in altitude ( 25,000ft brings 1840fps) and there is only a 10mph difference in speed with 25,000ft splitting the difference. Exhaust horsepower is 86.5 at 15,000ft, 113 at 20,000ft and 107 at 25,000ft. IF the accompanying chart is correct and if you continue the lines down to below 5,000ft it looks like the Merlin XX is making under 50hp in exhaust "power" below 5,000ft. Or a change from providing 10.5% of the prop power to 4% or under of the prop power at sea level.

Very few radial engines had exhaust stacks as well executed as a a V-12. Please also remember that exhaust thrust is dependent on exhaust mass ( fuel, air and water/alcohol) early aircraft without water/alcohol/over rich mixtures and the like have less mass than later engines (or even two stage engines) , exhaust speed and to turn into useful power , the aircraft speed. the closer the match of the exhaust speed to the aircraft speed the more useful the exhaust thrust becomes. One of the great advantages of the variable pitch prop. At low pitch (take-off) the prop may be trying to accelerate the air to around 1/2 the speed it would be if it was stuck in high pitch. (Spitfire I Rotol) prop had a bit over 23 degrees pitch at low pitch and about 58 degrees at high pitch.

There may be a some minor effect or change due to exhaust thrust or a change in ranking if the positions were close but it is not a big game changer at low altitudes and speeds.

 

[tomo pauk]

Most props are going to be within a few percent of 80% at sea level. At 20,000ft or so were the air is about 1/2 as dense prop size and design becomes more important.

Guess you're right.
Though, we can observe that a 14500 lbs heavy P-47D with paddle blade props will climb just a tad better than a 13500 lbs 'light' P-47C/D with toothpick prop at 1000 ft.

Exhaust thrust doesn't quite equal power, it varies in efficiency with the speed (and altitude) of the aircraft. At 250mph and at sea level it contributes quite a bit less than it does at 20,000ft and 325-375mph.

Agreed, I've mentioned the altitude as an important factor. The BMW-801D gives ~270 lbs of exhaust at 5.7 km, and ~180 lbs at 1 km, despite 300 HP less power at higher altitude; all for 'Notleistung'. Speed is not specified, though.

The only figures I have are for a Hurricane II with a Merlin XX engine. <snip>

Thanks. Here is the chart for the DB-601A, where the speed is specified, 600 km/h, for the horsepower equivalent of the exhaust thrust. Gives max of ~120 HP at 4.5 km. Chart kindly provided by krieghund:

Very few radial engines had exhaust stacks as well executed as a a V-12. Please also remember that exhaust thrust is dependent on exhaust mass ( fuel, air and water/alcohol) early aircraft without water/alcohol/over rich mixtures and the like have less mass than later engines (or even two stage engines) , exhaust speed and to turn into useful power , the aircraft speed. the closer the match of the exhaust speed to the aircraft speed the more useful the exhaust thrust becomes. One of the great advantages of the variable pitch prop. At low pitch (take-off) the prop may be trying to accelerate the air to around 1/2 the speed it would be if it was stuck in high pitch. (Spitfire I Rotol) prop had a bit over 23 degrees pitch at low pitch and about 58 degrees at high pitch.

The engine installation of the Fw-190A was one of that few, employing rear-facing, individual exhaust stacks. It would be interesting to know two things about the P-47: was there any exhaust thrust via partially-opened waste gates, and, was there any exhaust thrust emanating from rear-facing turbo exhaust.

There may be a some minor effect or change due to exhaust thrust or a change in ranking if the positions were close but it is not a big game changer at low altitudes and speeds.

Probably. Small details can add up, though.
For the Fw-190A vs. P-47 low-alt match-up in acceleration, the size and weight were probably the main determinants.

added: a quote from a design analysis of the F4U-1D (here):
Six separate exhaust manifolds, each serving three cylinders, discharge the exhaust backwards under the fuselage and create at full throttle a reactive force of 190 lb. At high speed, this amounts to approximately 210 hp, and the arrangement adds about 20 mph to the airplane's top speed. As compared with the utilization of the exhaust energy in a turbosupercharger, this arrangement has the advantage that the power thus made available need not be absorbed by the propeller.
Unfortunately, the altitude is not specified - seems like those extra 210 HP need to be added to the ~1970 HP available through WER at ~17000 ft?

 

[mhuxt]

I don't disagree with your points about the P-47 but this statement is simply not true. P-47 pilots confidently took on their Luftwaffe adversaries even on the deck. I have many combat reports which support this contention. Of the first five I looked at (at random out of a total of about 300) three describe low level combats.

(snippage)

Cheers

Steve

Steve,

Do you have any reports for the 359th FG on 29 January 1944? If no, can you tell me how I'd go about getting them?

Thanks in advance...