Turbo-supercharging and aircraft performance

[HoHun]

Hi everyone,

Trying to analyze aircraft performance, I found that turbo-supercharged aircraft are both poorly documented and enigmatic in their documentation.

The problems are simple:

- (US) power graphs for turbo-supercharged engines give power as constant from sea level up to critical altitude.

(This is unrealistic, considering that most turbo-supercharged engines had a two-speed, mechanically-driven supercharger stage, too. This stage would subtract a certain amount of power in low gear, and a greater amount of power in high gear, thus creating a drop in power at altitude.)

- Speed graphs/data points (which are few) for the P-47 show that the plane is not using the constant shaft power engine curve. The data does indeed suggest that the power is increasing with altitude.

(This is the opposite of what one should expect since a) the engine doesn't provide exhaust thrust, b) the useful power drops due to decreasing propeller efficiency, c) the mechanically driven stage subtracts more power).

- Performance graphs/data points usually show a hard break in the speed curve at critical altitude, but no such hard break in the climb graph at critical altitude. Climb also seems to drop more quickly than the assumption of constant or even increasing power with altitude suggest.

In addition to the effects described above, I also believe that even without them, the turbo-supercharger wouldn't yield the constant power-over-altitude curve we're used to. The overspeed tendency of the WW2 US turbochargers at high altitude is one indication (reduced ambient air pressure makes them more effective), some German power graphs for turbo-supercharged BMW801 versions reinforce it:

http://hometown.aol.de/HoHunKhan/BMW801Turbo.jpg

I'd say that the R-2800 power curves probably should have a very similar look to the BMW801J power curve (disregarding the exact figures).

However, that would result in considerably different performance graphs for the P-47 than those I have seen.

Does anyone have an idea how to solve this puzzle? In my attempt to achieve a break-through, I seem to have hit an impenetrable wall of contradictions :-)

Regards,

Henning (HoHun)

[SunTracker]

Couldnt you just extrapolate a P-47s speed at sea level to its speed at 30,000 feet, and then check that against its real life speed?

[pasoleati]

Henning, while the aircraft had a mechanical stage as well, they were single speed blowers, not two speed ones! This is at least so for all US installations. Honestly speaking, having two speed mech stage would be very silly.

Another point you must remember is that is some installations it might have been necessary to throttle the engine at very low level (therefore slightly less hp at SL) while at higher up the throttle would be allways wide open and the boost would be determined solely by waste gate control. In can be safely assumed that any high power operation above 10,000 ft of the P-47  would be with the throttle fully open with the MAP being controlled via waste gate.

[Bodhi]

Originally posted by pasoleati
Henning, while the aircraft had a mechanical stage as well, they were single speed blowers, not two speed ones! This is at least so for all US installations. Honestly speaking, having two speed mech stage would be very silly.

Another point you must remember is that is some installations it might have been necessary to throttle the engine at very low level (therefore slightly less hp at SL) while at higher up the throttle would be allways wide open and the boost would be determined solely by waste gate control. In can be safely assumed that any high power operation above 10,000 ft of the P-47  would be with the throttle fully open with the MAP being controlled via waste gate.

well said

[HoHun]

Hi Suntracker,

>Couldnt you just extrapolate a P-47s speed at sea level to its speed at 30,000 feet, and then check that against its real life speed?

Have a look at this estimate, showing that constant power would yield too high speeds near sea level:

http://hometown.aol.de/HoHunKhan/P-47Dcalculation.gif

(It's not as bad as I thought it would be :-)

However, if you look at the climb rate, I calculate a much higher climb rate than it is reported for the P-47D in the F4U-4 comparison report which I used as a data source. What's more, I calculate a definite knee at critical altitude while other sources, including Aces High (http://www.hitechcreations.com/ahhelp/models/p47d25.html) and America's Hundred Thousand, show a rounded climb graph without a knee.

Regards,

Henning (HoHun)

[HoHun]

Hi Pasoleati,

>Henning, while the aircraft had a mechanical stage as well, they were single speed blowers, not two speed ones!

Hm, I checked an overview of the R-2800 types, and you seem to be right. Strange, I had read anecdotes from P-47 pilots and they described automatic gear changes - must have been when they flew P-51s later, or I misunderstood them entirely :-)

>Honestly speaking, having two speed mech stage would be very silly.

Well, not really as you get better high-altitude performance while avoiding low-altitude throttling losses.

But I'm glad you pointed out I was in error there as it reduces the magnitude of the problem I'm facing consiberably :-)

>Another point you must remember is that is some installations it might have been necessary to throttle the engine at very low level (therefore slightly less hp at SL) while at higher up the throttle would be allways wide open and the boost would be determined solely by waste gate control. In can be safely assumed that any high power operation above 10,000 ft of the P-47  would be with the throttle fully open with the MAP being controlled via waste gate.

Very good point! I had thought about this, too, but couldn't find any data on it. It would certainly help to explain the 10 km/h gap at sea level which I get from the constant power assumption.

(Von Gersdorff also mentions boost air bleeding as secondary throttling technology, used in addition to waste-gate controlled exhaust gas bleeding - boost air was bled at lower power settings. I'd imagine it was more responsive and perhaps more effective, while waste gate control was more efficient at high power.)

Thanks for your help! :-)

Regards,

Henning (HoHun)

[pasoleati]

Henning, where is the P-47 vs. F4U-4 comparison report to be found?

E.g. the Jumo 211F/J has a system where in conditions of high rpm, low boost, excess boost air is bled out before the throttle valve. The operatorīs manual says it is to prevent induction air overheating.

[HoHun]

Hi Pasoleati,

>I had thought about this, too, but couldn't find any data on it. It would certainly help to explain the 10 km/h gap at sea level which I get from the constant power assumption.

To get my calculations match the P-47D from the F4U-4 comparison report, I've to reduce power at sea level from the rated 2600 HP to 2520 HP.

Of course, this has an impact on the climb rate as well so that I drop below the quoted initial climb rate. Since my calculation shows a much better climb than the F4U-4 report, the rest of the climb rate graph is safely above the reported data of course.

Do you have any data on the throttling losses (or a hint how to get such data)? There are so many variables in turbocharger performance that I'm hesitant to guess anything :-)

The next thing to compare it to is Baugher's data for the P-47D-25RE. The high critical altitude indicates that his data is actually for 2300 HP, while I have a power plant chart for the type indicating a WEP rating of 64" Hg, which should yield the oft-quoted 2535 HP. On the other hand, this power plant chart is dated 11-1-44 (which I take to be November 1944), so it might be that when the D-25RE came out, it was only cleared for 2300 HP. On the other hand, Baugher explicitely quotes 2535 HP. No way his speed curve matches this! Unfortunately, the climb rate he quotes  is in the middle between those I'd predict for 2300 HP and 2535 HP, so that's no help either.

The Aces High P-47D-25 seems to have 2535 HP, too, judging from the climb rate. It's considerably slower than one might expect from both Baugher's data and the F4U-4 report, though. Surprisingly, the Aces High D-30 climb better at the same power, weight and only very slightly improved speed. I wonder if this could have something to do with the paddle blade propeller?

The Aces High D-11 seems to be the fastest of the Aces High Thunderbolts, but as it also has the worst climb rate despite being 1000 lbs lighter, it looks like it has only 2300 HP and the speed gain is due to the better aerodynamics of the razorback fuselage. Or does it have the toothpick propeller that harms climb rate?

Unfortunately, I don't have any real-life data on the razorback to compare this to. Which Thunderbolts had which propeller type, anyway?

Regards,

Henning (HoHun)

[MiloMorai]

Originally posted by pasoleati
Henning, where is the P-47 vs. F4U-4 comparison report to be found?

E.g. the Jumo 211F/J has a system where in conditions of high rpm, low boost, excess boost air is bled out before the throttle valve. The operatorīs manual says it is to prevent induction air overheating.

found it here,
http://mywebpages.comcast.net/markw4/index1.html

[HoHun]

Hi Pasoleati,

>Henning, where is the P-47 vs. F4U-4 comparison report to be found?

I believe F4UDOA posted it once, but I'm afraid I don't have the link. As it has more than 5 MB, I'd offer to send it per email if your inbox can take it :-)

>E.g. the Jumo 211F/J has a system where in conditions of high rpm, low boost, excess boost air is bled out before the throttle valve. The operatorīs manual says it is to prevent induction air overheating.

Highly interesting, that seems to be the exact equivalent of what von Gersdorff mentioned with regard to turbo-superchargers! :-)

Regards,

Henning (HoHun)

[pasoleati]

Milo, wrong report! I am looking for the P-47 vs F4U-4 report.

Henning, you may send the report to paso.leati@@reppu.net (remove extra @).

[Grendel]

Turbocharger:

[joeblogs]

Did a bunch of this a while back. See if these help.

http://mysite.verizon.net/vze479py/index.html

Originally posted by HoHun
Hi Pasoleati,

>I had thought about this, too, but couldn't find any data on it. It would certainly help to explain the 10 km/h gap at sea level which I get from the constant power assumption.

To get my calculations match the P-47D from the F4U-4 comparison report, I've to reduce power at sea level from the rated 2600 HP to 2520 HP.

Of course, this has an impact on the climb rate as well so that I drop below the quoted initial climb rate. Since my calculation shows a much better climb than the F4U-4 report, the rest of the climb rate graph is safely above the reported data of course.

Do you have any data on the throttling losses (or a hint how to get such data)? There are so many variables in turbocharger performance that I'm hesitant to guess anything :-)

The next thing to compare it to is Baugher's data for the P-47D-25RE. The high critical altitude indicates that his data is actually for 2300 HP, while I have a power plant chart for the type indicating a WEP rating of 64" Hg, which should yield the oft-quoted 2535 HP. On the other hand, this power plant chart is dated 11-1-44 (which I take to be November 1944), so it might be that when the D-25RE came out, it was only cleared for 2300 HP. On the other hand, Baugher explicitely quotes 2535 HP. No way his speed curve matches this! Unfortunately, the climb rate he quotes  is in the middle between those I'd predict for 2300 HP and 2535 HP, so that's no help either.

The Aces High P-47D-25 seems to have 2535 HP, too, judging from the climb rate. It's considerably slower than one might expect from both Baugher's data and the F4U-4 report, though. Surprisingly, the Aces High D-30 climb better at the same power, weight and only very slightly improved speed. I wonder if this could have something to do with the paddle blade propeller?

The Aces High D-11 seems to be the fastest of the Aces High Thunderbolts, but as it also has the worst climb rate despite being 1000 lbs lighter, it looks like it has only 2300 HP and the speed gain is due to the better aerodynamics of the razorback fuselage. Or does it have the toothpick propeller that harms climb rate?

Unfortunately, I don't have any real-life data on the razorback to compare this to. Which Thunderbolts had which propeller type, anyway?

Regards,

Henning (HoHun)

[HoHun]

Hi Joe,

>See if these help.

Hm, not immediately. I had thought I might be able to see from the specific consumption whether 2300 HP required water injection (which I'm not certain of), but the turbo-supercharged engine are only graphed up to 2000 HP. Do you happen to have R-2800 power plant charts similar to this one?

http://hometown.aol.de/HoHunKhan/p47-d-30-power_small.jpg

The key is that we have boosts and rpms given along with a date - but unfortunately, it's November 1944 so it doesn't tell us much about the timeline of P-47 performance.

If we had more of these, with earlier dates, we could probably figure out the exact history of the P-47 pretty well :-)

Regards,

Henning (HoHun)

[joeblogs]

I've got some engine calibration curves for P&W A&B series engines. Let me see what these say. The dates on the curves are earlier (1942 if memory serves me). But those are not for the turbocharged engine.

-Blogs

Originally posted by HoHun
Hi Joe,

>See if these help.

Hm, not immediately. I had thought I might be able to see from the specific consumption whether 2300 HP required water injection (which I'm not certain of), but the turbo-supercharged engine are only graphed up to 2000 HP. Do you happen to have R-2800 power plant charts similar to this one?

http://hometown.aol.de/HoHunKhan/p47-d-30-power_small.jpg

The key is that we have boosts and rpms given along with a date - but unfortunately, it's November 1944 so it doesn't tell us much about the timeline of P-47 performance.

If we had more of these, with earlier dates, we could probably figure out the exact history of the P-47 pretty well :-)

Regards,

Henning (HoHun)