Overboosting the P-47's engines: need some clarifications

[tomo pauk]

While trying to read more about the P-47, this information popped up at another forum. An interview with Robert Johnson, US ww2 pilot commander:

R.Johnson--Sure. My second Jug, a D-5 was the best P-47 that I ever flew, and I flew them all, including the P-47M which the 56th got near the end of the war.

CCJ: What made this one Thunderbolt so fast?

RSJ: Several things. My crew sanded every joint smooth, and waxed it to a high gloss. Factory technical reps showed my crew chief, Pappy Gould, how to adjust the wastegates to keep the boost pressure higher than normal. My D-5, which I named Lucky, had water injection. I never used the water injection in combat. I didn't need it. From time to time I'd switch it on, push the throttle up to 72" of manifold pressure and the head rest would smack me from behind. I would let her run for a few minutes just for the fun of it.

He further goes to say this Jug was making above 300 mph indicated at 32000 ft, or almost 470 mph TAS, with B series R-2800. Thus beating the official value by some 60 mph*, or, as same as the P-47M with C series R-2800.
During the Spring of 1944, all Jugs were operating with overboosted engines, per interview?

D9 in 1942 | Forums - Page 2

So I'd like to know more about this increase of engine power plane's performance, was it ever officially tested and approved, what were the combat experiences etc. Informations appreciated in advance.

* the flight test data from Williams' site gives 'only' this for the regular P-47D (graph here):

At 70.0" Hg., water injection, a maximum speed of 444 MPH was obtained at 23,200 feet.

... at 32000 ft making 400-410 mph.

 

[davparlr]

Shortround6 would probably be the expert here. I feel the R2800 was quite a robust engine, but it might vary with model, A, B, or C.

 

[Jabberwocky]

Fit and finish could have a major impact on level speed. I've seen Spitfire Mk V tests where the aircraft made almost 15 mph better than official figures thanks to a full sand down and strip, repainting of the aircraft, filling all the joints and cutting back external protuberances upsetting the airflow.

From what I've read, P-47's were regularly overboosted to various levels. Modifications to the turbocharger and wastegate were done in the UK with the help of local Pratt Whitney representatives. I've seen claims ranging from 70" of manifold pressure all the way up to 100" of manifold pressure - all with the caveats that this was a strictly "30 seconds and it blows up" sort of a deal.

That said, Robert Johnson seems more than a little prone to exaggeration, so I'd take the "300 mph at 32,000 ft" with a couple of very large grains of salt. Remember, first hand accounts are the least reliable of all forms of evidence. Doubly so if is a combat situation.

Also, a indicated airspeed figure from a WW2 fighter isn't going to take into account calibration and compressibility, which could have easily thrown the airspeed readings out.

EDIT: Found the original interview:


CCJ: I remember reading where you thought that your P-47 was the fastest fighter in the ETO.

RSJ: I still believe that it was.

CCJ: Really?

RSJ: Sure. My second Jug, a D-5 was the best P-47 that I ever flew, and I flew them all, including the P-47M which the 56th got near the end of the war.

CCJ: What made this one Thunderbolt so fast?

RSJ: Several things. My crew sanded every joint smooth, and waxed it to a high gloss. Factory technical reps showed my crew chief, Pappy Gould, how to adjust the wastegates to keep the boost pressure higher than normal. My D-5, which I named Lucky, had water injection. I never used the water injection in combat. I didn't need it. From time to time I'd switch it on, push the throttle up to 72" of manifold pressure and the head rest would smack me from behind. I would let her run for a few minutes just for the fun of it.

CCJ: 72 inches!? Did you ever take note of your airspeed during one of those runs?

RSJ: Of course.

CCJ: And....... how fast did it go?

RSJ: I've seen just over 300 at altitude.

CCJ: 300 indicated?

RSJ: Yes.

CCJ: What was your altitude?

RSJ: I guess it was right around 32,000 feet.

CCJ: Geez, thats well over 450 mph!

RSJ: Oh, I figure closer to 470.

CCJ: Maybe you did have the fastest fighter in the ETO after all.

RSJ: Like I said, Lucky was the fastest.

CCJ: What ever happened to Lucky?

RSJ: She was lost in a mid-air collision over the North Sea. I don't recall the pilot's name who was flying her on that ramrod. I was very upset. Lucky got at least 24 enemy aircraft and was the best Jug I ever flew. She was trouble free and I never had a single abort while flying her.

 

[tomo pauk]

About that 300 mph indicated at 32 kft: how likely is that compressibility at such speed altitude skews the on-board speed indicator's measurements?

 

[GregP]

The P-47 was limited to 52 inches of MAP in WWII except for WEP, and 2800 rpm max. There was no 70" of MAP back then. Whoever claims that is just plain old wrong.

That is directly from John Maloney, who flies the R-2800 on a VERY regular basis, and also races at Reno, flies Bearcats frequently, and all R-2800-powered aircraft we fly.

The extra inches of MAP they run today for racing are NOT what was run in WWII.

Yes, they were tested at higher MAP, but not released for it in operation except for WER, which was an "Oh my God, save my life!" type of sitution for WER use, particularly if your aircraft was some distance away from a friendly base.

 

[MikeGazdik]

CCJ: What ever happened to Lucky?

RSJ: She was lost in a mid-air collision over the North Sea. I don't recall the pilot's name who was flying her on that ramrod. I was very upset. Lucky got at least 24 enemy aircraft and was the best Jug I ever flew. She was trouble free and I never had a single abort while flying her.

Have to figure out how to find the thread. The one that was about "which aircraft had the most kills" I believe the thread was about any individual aircraft. 24 kills from one plane. Interesting.

 

[davparlr]

The P-47 was limited to 52 inches of MAP in WWII except for WEP, and 2800 rpm max. There was no 70" of MAP back then. Whoever claims that is just plain old wrong.

That is directly from John Maloney, who flies the R-2800 on a VERY regular basis, and also races at Reno. Bearcats frequently, and all R-2800-powered aircraft we fly.

The extra inches of MAP they run today for racing are NOT what was run in WWII.

Hi, Greg, how's that P-59 coming along. Got a schedule of when it will fly yet?

According to "America's Hundred Thousand", the P-47M/N was rated at 2800 hp at 32.6k ft using 72" Hg. Also, the 100/150 Grade Fuel test indicate that the R-2800-63 could be operated, using 44-1 fuel and water injection, at 70" Hg. for short periods of time.

150 Grade Fuel

This report from Johnson and a comment by an ace on "Dogfights" indicate that there were some tinkering with operational directions and engines to "soup-up" aircraft performance. Since American gear heads are noted for tinkering with engine specs to step up hp, and there were probably quite a few in flight line maintenance, I wonder how prevalent was "hopping up" aircraft on the flight line?

 

[Shortround6]

Something seems off. From this report on Mike Williams site

http://www.wwiiaircraftperformance.org/p-47/P-47D_43-75035_Eng-47-1652-A.pdf


it looks like turbo was maxed out at 56in at 31,000ft. Now pilots and mechanics can play fast and loose with pressure regulators and over boosting at altitudes where the superchargers have extra capacity but once the superchargers have hit their limits there isn't much that fiddlingwith the controls is going to do. Radial engines don't take to being over revved much and turbos tend to come apart with a bang if over revved by much so there are some definite limits as to what can be accomplished. P-47D was certainly capable of pulling 70" MAP, just not at 32,000ft or close to it.

The American standard for WEP was that the trail test engine had to run for 7 1/2 hours at the WEP power level, in 5 min increments. 90 five minute periods separated by 5 min "cool down" periods at idle or a cruise setting. There was obviously some room to "play" with without immediate disaster but again, subject to some limits. Over revving the engine to get more boost was really going to screw things up.

Another report on the site has a P-47D pulling 65in at 25,400 without water, see this chart:

http://www.wwiiaircraftperformance.org/p-47/p47d-44-1-level.jpg

This is with 150PN fuel. The P-47D supercharger system simply runs out of breath. Later P-47s had a different turbo that allowed higher rpm and boost. ANd they had a "C" series engine that allowed an extra 100rpm without over revving beyond factory recommendations.

 

[tomo pauk]

Thanks for the feedback, people.

Now if we return to the original interview, maybe some claims can be taken separately, in order to solve this puzzle (sorry if some things in the post look like repeating):

RSJ: Sure. My second Jug, a D-5 was the best P-47 that I ever flew, and I flew them all, including the P-47M which the 56th got near the end of the war.

CCJ: What made this one Thunderbolt so fast?

Here several things come to play: sanded waxed planes are faster than 'plain' ones. Jaberwocky has noted that Spit Vs were faster 15mph after the similar treatment, so an already fast P-47 can easily turn into a world beater. Another thing is overboosting of the R-2800 into what could be called as 'WER dry', ie. no water injection. As noted in interview, P&W technicians teched the crew chief(s) how to do that. It goes without saying that such a topic deserves it's own thread (hopefully it's covered in the book about R-2800 (Dependable masterpiece)). It's also not known how much of the boost was achievable with 100/130 grade fuel, ie. before 150 grade fuel became available. Use of such an overboost that's mentioned in encounter reports should help?

Further:

RSJ: Several things. My crew sanded every joint smooth, and waxed it to a high gloss. Factory technical reps showed my crew chief, Pappy Gould, how to adjust the wastegates to keep the boost pressure higher than normal. My D-5, which I named Lucky, had water injection. I never used the water injection in combat. I didn't need it. From time to time I'd switch it on, push the throttle up to 72" of manifold pressure and the head rest would smack me from behind. I would let her run for a few minutes just for the fun of it.

Mentioning of the ADI in the same answer with the 'WER dry', almost as a side note, does not do the justice to the important addition for the R-2800s. It was with ADI that enabled 72" of the manifold pressure (covered above by Greg and SR6).

Then:

CCJ: 72 inches!? Did you ever take note of your airspeed during one of those runs?

RSJ: Of course.

CCJ: And....... how fast did it go?

RSJ: I've seen just over 300 at altitude.

CCJ: 300 indicated?

RSJ: Yes.

CCJ: What was your altitude?

RSJ: I guess it was right around 32,000 feet.

CCJ: Geez, thats well over 450 mph!

RSJ: Oh, I figure closer to 470.

CCJ: Maybe you did have the fastest fighter in the ETO after all.

RSJ: Like I said, Lucky was the fastest.

The critical altitude for 70" MAP, using water injection and 44.1 (=150 grade) fuel was at mere 21500 ft, and FTH for 64" map was 23700 ft (pdf here). So if it was possible to attain 72", it was certainly way under 32000 ft (again covered by SR6). We still lack the information about the fuel used.
The last thing here is the compressibility messing the speed sensor (noted above by Jaberwocky). Eg. German tests of the Bf-109F-4 claim that plane was going circa 410 mph when using Steig Kampfleistung, only to find out those speed numbers are most likely at 390 mph, once the compressibility effects are accounted for.

 

[GregP]

I spoke with John Maloney about this question yesterday. According to John, short of WER and water injection, WWII R-2800's were limited to 52" of MAP and 2,800 rpm. They run more at Reno, of course, but Reno isn't WWII and they didn't exceed these ratings often (except for the odd individual pilot who might or might not have made it home) except for WER operation. They were TESTED at higher ratings, but not cleared or operated there.

John should know. He flies everything at the Museum, races at Reno, and is generally one ot the best pilots around, especially when it come to operating a big radial.

According to John, 70" of MAP in WWII is remembering with "rose colored glasses."

 

[GregP]

Hi Dave,

The P-59 is coming along well and all we need is money to fly it! I'm taking a temporary YP-59A break and am working on a sliding canopy for the F4U-1A (it has rails across the top instead of being a completely blown sliding canopy). Our Corsair is the oldest one flying, started life as a birdcage Corsair, and this is the correct canopy for it since it was upgraded to F4U-1A configuration, so we're going to put it on the aircraft.

Our guys fly our P-47G, Tigercats, Bearcats, Hellcats, and Corsairs, and our pilots say none of these were cleared for 70" MAP ... and they all have the dash one manuals for the birds, including the P-47M manual.

Could it make 70"? Probably, and it was probably tested at 70", but it was NOT operational policy and I doubt if any were flown at that power rating in WWII. I'll ask some WWII pilots, too. We get a lot of them through the museum. We have a former WWII P-47 pilot (Enrico Botteri) at Chino who has a hanger just a few door down. He'd certainly know ... and also has the dash ones for several versions of it.