P-47M/N?

[Widewing]

Originally posted by gripen:
Widewing,
The R-2800 C-series was rated at 2800rpm, I don't know how it was rated at the XP-47J. Also the propeller of the XP-47J seems to be (according to the pictures) similar large one as in the later D models, 13ft or more (spinner is also huge). Then it also should be noted that compressebility starts to affect to the propeller around tip speed mach O.8 see  this NACA report.

gripen

This report must be evaluated within the context of its scope. The purpose was to determine the effect of compressibility on props during takeoff and climb. It establishes that prop pitch angles are in the 20°-25° range, not nearly as flat as would be found for maximum speed. This increase in the angle of attack will induce compressibility effects at a considerably lower speed. Moreover, the blade designs tested represent those available long before the Curtiss 834 blade design (used on the XP-47J) existed. This prop was slightly smaller in diameter than the standard unit used on production Jugs. Many historians report that the prop measured 13 ft in diameter, but Higginbotham, who headed the Powerplant Engineering Department insisted that it measured 152 inches in diameter. It had rounded tips, but a chord to thickness ratio of just 4.5% at the outboard stations.

Therefore, NACA report 639 is not directly applicable, and will be misleading when applied to paddle blade props in flat pitch.

One item overlooked when considering the performance of the XP-47J was exhaust thrust. Unlike the P-38 installation, the Jug's turbo layout was not sensitive to back-pressure. Other considerations include the fact that Test Pilot Richie's notes include reference to pulling 72 in/hg MAP on his speed run. Engineers extrapolating the horsepower curve believed that Richie had managed approximately 3,200 hp during the run. We know that the R-2800-57 generated between 400 and 650 lbs of exhaust thrust, which certainly augmented speed capability. In the case of the XP-47J, this was extrapolated up to nearly 800 lbs of thrust at 76 in/hg (72 in/hg is normal WEP rating).

A call to Warren Bodie resulted in his telling of Republic Test Report No. 51 dated January 1945. This test report states that 502 mph was attained after correction. This report was signed off by General L.C. Cragie, Chief of the USAAF Engineering Division. The "Offical Performance Summary of the XP-47J aircraft" should be available via the Air Force Historical Research Agency at Maxwell AFB.

I flew as a Flight Engineer on Convair 240 (C-131F) type aircraft in the mid 1970s. This aircraft was powered by a pair of R-2800 engines and incorporated thrust augmentation utilizing exhaust gases routed rearward through a venturi and diffuser system. A genuine increase of 22 knots airspeed was gained by this harnessing of exhaust gas velocity. You can expect at least that much gain in the XP-47J, perhaps considerably more.

My regards,

Widewing

[gripen]

Widewing,
If you want to argue that the propeller of the XP-47J acted significantly different at high speeds (something like 80% efficiency around tip speed mach 1) than propellers in that NACA report, why don't you show some verifyable evidence? (please, not your own articles)

Your exhaust thrust argument is also interesting. AFAIK most of the turbocharged engine's exhaust thrust is used for the turbo and wasted in the long piping. Again, evidence please.

But now you claimed at least one reference which at least might be available, thank you.

gripen

[Widewing]

Originally posted by gripen:
Widewing,
If you want to argue that the propeller of the XP-47J acted significantly different at high speeds (something like 80% efficiency around tip speed mach 1) than propellers in that NACA report, why don't you show some verifyable evidence? (please, not your own articles)

Go to: http://naca.larc.nasa.gov/reports/1952/naca-tn-2859/naca-tn-2859.pdf  

 

Your exhaust thrust argument is also interesting. AFAIK most of the turbocharged engine's exhaust thrust is used for the turbo and wasted in the long piping. Again, evidence please.

This data can be found in the document listed earlier. Get a copy. Oops, I forgot, you want everyone else to to do your leg work, right?

My regards,

Widewing

[ 08-18-2001: Message edited by: Widewing ]

[gripen]

Widewing,
That NACA document really contains data about a propeller which could do very well at high tip speeds and the propeller of the Thunderscreech or Tu-95 might do even better. But actually I was interested about the proppeller of the XP-47J which resembles those propellers in the NACA 639 (blade shape, thick profile at the root). Also your own statement that the proppeller of the P-47 (similar late Curtiss design?) worked like a air brake at mach 0.83 speaks against your own arguments.

Which mentioned source supports your claim that exhaust thrust gained at least 22knots for the XP-47J after losses in the turbo and piping?

BTW a book called the Secret Years by Tim Mason claims that the Mustang III did it's top speed (450mph@28k) at reduced rpm (2800) due compressibility.

gripen

[ 08-19-2001: Message edited by: gripen ]

[Widewing]

Originally posted by gripen:
Widewing,
That NACA document really contains data about a propeller which could do very well at high tip speeds and the propeller of the Thunderscreech or Tu-95 might do even better. But actually I was interested about the proppeller of the XP-47J which resembles those propellers in the NACA 639 (blade shape, thick profile at the root). Also your own statement that the proppeller of the P-47 (similar late Curtiss design?) worked like a air brake at mach 0.83 speaks against your own arguments.

This is exactly what I expected from you. Let's be honest, if an American stated that the sun rises in the east, you would require documentation and argue "how can you you prove this if it's overcast?" Just another Luftwaffe Mafia troll.

I present you with verifiable documentation, you pooh-pooh it stating that doesn't represent the Curtiss design (as if you actually had a clue what your were talking about, but we know better). You refer back to a report of tests with obsolete, mid- 1930s propeller designs. The report I cite was declassified and released in 1952, but written in 1947, is a rewrite of an earlier report declassified and released in 1948 but written in 1943. There is usually a 3 to 5 year lag between the internal report being written, and it's being released to the public. Indeed, some reports released in 1947 discribe tests conducted in 1941.

For the balance of your uninformed comments, I suggest you review the drag rise curve for various propellers, and you will see that virtually every design suffers at speeds above Mach .80, and this always becomes the greatest factor in limiting speed.

 

Which mentioned source supports your claim that exhaust thrust gained at least 22knots for the XP-47J after losses in the turbo and piping?

Your reading comprehension skills require some polishing. I never made any such claim. I reported the speed gain via exhaust thrust augmentation for the Convair 240.

For reference, see Bodie's "Republic's P-47 Thunderbolt", page 232. Also see Republic Test Report No.51.

 

BTW a book called the Secret Years by Tim Mason claims that the Mustang III did it's top speed (450mph@28k) at reduced rpm (2800) due compressibility.

I do not know where Mason gets his data from, but the USAAF defined the Mustang's (P-51B) maximum speed at 430 - 450 mph depending which test report you read. However, each test reports the use 3,000 rpm. Furthermore, the fact that the Mustang's propeller measures just 11'2" or 11'1" in the case of the Aero Products prop (on the Dallas built P-51D, designated P-51K), it seems highly unlikely that compressibility was a noteworthy factor.  

My regards,

Widewing

[niklas]

Widwing, i think you know very well that your 2859 report is no help in a discussion about a high speed flight in 35k.

They tested the propeller at forward mach numbers of ~0,6, while the XP-47J claims a mach number of 0,76.
They even mention in the report:
"The highest tip speeds ... were obtained at comparatively low forward speed, and it should be pointed out that such data may not be adequate when used in estimation efficiencies for higher forward speeds because the radial variation of Mach number will not be correct"
If you look at real flight tests with higher forward mach numbers (usually 0,7) and high tip mach numbers, you can see a significantly higher loss of propeller efficiency (1784 report, or L4L07)
Furthermore they tested in a wind tunnel near ground under ideal conditions. No thin air, no icing problems, no yaw movements etc..

400lb exhaust thrust for a usual supercharged engine with 2000hp in 30k is realistic. 800lb with 3000hp for a tubocharged is imo a bit too much, the energy for the turbo is not for free.

Nevertheless, if you compare the thrust "produced" by the weight during dive tests (14000lb) to 800lb, it still canīt explain imo the high mach number of 0,76. I know that the gradient of drag/mach is steep after the critical mach number, but the difference between 0,76 and 0,833 is imo too small to be realistic.

niklas

[wells]

A normal P-47 that produced 2800 hp @ 34000' would be expected to reach 530 mph.  The fact that it *only* does 504 or whatever, means that compressibility or loss of propeller efficiency *is* in effect.

The 530 mph figure was extrapolated from a P-47 being capable of 420 mph @ 25000' with 2000 hp in *standard* atmosphere.

[gripen]

Widewing,
The propeller described in the NACA 2859 is very much different if compared to one in the XP-47J and this can be verified simply from the pictures.

Your statement about exhaust thrust can be verified above ("You can expect at least that much gain in the XP-47J, perhaps considerably more.")

The sources used by Mason are well known and publically available; A&AEE reports which are available from the Public Records Office or the library of the Boscombe Down.

Mason writes:
"The effect of propeller tip speed was measured and a reduction of 200rpm (reducing tip Mach No from 1.07 at 3000rpm to 1.03 at 2800) gave an increase of 10mph (true)."

There is also similar claim about the Corsair Mark II (F4U-1D) but no exact values for the speed increase or engine ratings:
"It was found that reducing to climb rpm increased high level speeds and climb, caused, it was thought, by high propeller tip speed of mach 1.165 at 33000ft and low intake efficiency, particularly in the climb."

gripen

[niklas]

yes wells, and if we try to explain everything with extrapolations and simple formulas, weīll come f.e to the result that with 800lb thrust from the exhaust gases, the P47 wouldnīt have needed a propeller at all in 35k to fly  

btw, thatīs the reason why i donīt understand that some people donīt want jets. Every prop driven plane, especially with turbocharger, is so close to a jet, if you compare the mechanical design. And they used the backpressure of the exhaust gases, which made them all "little" jets. But this is OT for this thread  

Itīs obvious that the mach effect was the limiting factor. I just donīt want to believe 0,76. And i want to see the original report  

niklas

[Widewing]

Originally posted by niklas:
Widwing, i think you know very well that your 2859 report is no help in a discussion about a high speed flight in 35k.

They tested the propeller at forward mach numbers of ~0,6, while the XP-47J claims a mach number of 0,76.
They even mention in the report:
"The highest tip speeds ... were obtained at comparatively low forward speed, and it should be pointed out that such data may not be adequate when used in estimation efficiencies for higher forward speeds because the radial variation of Mach number will not be correct"
If you look at real flight tests with higher forward mach numbers (usually 0,7) and high tip mach numbers, you can see a significantly higher loss of propeller efficiency (1784 report, or L4L07)
Furthermore they tested in a wind tunnel near ground under ideal conditions. No thin air, no icing problems, no yaw movements etc..

400lb exhaust thrust for a usual supercharged engine with 2000hp in 30k is realistic. 800lb with 3000hp for a tubocharged is imo a bit too much, the energy for the turbo is not for free.

Nevertheless, if you compare the thrust "produced" by the weight during dive tests (14000lb) to 800lb, it still canīt explain imo the high mach number of 0,76. I know that the gradient of drag/mach is steep after the critical mach number, but the difference between 0,76 and 0,833 is imo too small to be realistic.

niklas

Niklas, I doubt that the forward velocity is significant when the sum of rotational and forward velocity are considered.

You would be surprised at the relatively low loss of velocity in a turbo system. A properly designed turbo system, that is. According to one source, the turbo hood was instrumental in increasing exhaust velocity, while the CH-5 turbo, being designed for high back-pressure systems, had only a minor effect on said velocity. Our own work with centrifical type systems clearly demonstrated a significant pressure rise in front of the unit, but little change in velocity down stream. Granted, these were not turbo systems, but the comparison seems to survive reasonably well. Again, the system vent is critical to function. Generally, there is little available to aid in the research of the design of the P-47's turbo hood.

As to the difference in drag between the two numbers under discussion, examine a drag rise curve. You will see that drag rise between .76 and .83 is huge.

My regards,

Widewing

[Widewing]

Originally posted by gripen:
Widewing,
The propeller described in the NACA 2859 is very much different if compared to one in the XP-47J and this can be verified simply from the pictures.

Yada, yada, blah, blah, blah.....

The Curtiss 834 was designed specifically to minimize drag rise at high sub-sonic speeds. Fisher wrote on the extensively in the late 1940s. Moreover, the 836 paddle blade used on production aircraft also followed this approach.

 

Your statement about exhaust thrust can be verified above ("You can expect at least that much gain in the XP-47J, perhaps considerably more.")

You're right. However, this is not rocket science. An aircraft with more than twice the the flat plate area, and three times the weight, producing about 700 lbs of exhaust thrust gained 22 knots in airspeed. Is it not reasonable to expect the XP-47J to gain at least this much? Even if you limit the total thrust to 400 lbs, it should still gain more than 22 knots. Whether or not the calculated value of 765 lbs was attained, is irrelevant to the fact that 400 lbs should produce a substantial speed increase, that is not effected by prop tip speed, but by power output alone.

 

The sources used by Mason are well known and publically available; A&AEE reports which are available from the Public Records Office or the library of the Boscombe Down.

Mason writes:
"The effect of propeller tip speed was measured and a reduction of 200rpm (reducing tip Mach No from 1.07 at 3000rpm to 1.03 at 2800) gave an increase of 10mph (true)."

Once I was able to find some time, I ran Mason's numbers and produced the following
theoretical tip speeds at 440 and 450 mph at 28,000 ft.

440 mph, 3,000 rpm (1,500 rpm for the prop) rings out as Mach 1.079. 450 mph at 2,800 rpm (1,400 for the prop) shows Mach 1.041.

This tells me two things.
1) Mason's numbers are spot on.
2) The theoretical formula is very accurate.

So, this tends to indicate that the XP-47J might have gone even faster had Richie reduced his rpm..... Not exactly the point you were hoping to make, is it?  

My regards,

Widewing

[gripen]

Widewing,
You should show evidence that the propeller of the XP-47J did well at Mach 1.18, so far you have not. But the A&AEE tests show that the problem existed at much lower tip speeds.

Your exhaust thrust statement was very much of from reality.

Your tip speed calculations are again wrong, because you are using wrong reduction gear ratio (the Mustang III had 0.479 reduction gear).

And the tests of the A&AEE clearly prove that propellers (1943 era) loss large amount of their efficiency a lot before mach 0.8, therefore there can't be huge difference in propeller efficiency (from positive to negative) between Mach 0.76 and 0.83.

You are wasting bandwidth.

gripen

[Toad]

Originally posted by gripen:
You are wasting bandwidth.
gripen

 


Well, somebody sure is. I don't think it's WW, though.

[Widewing]

Originally posted by gripen:
Widewing,
You should show evidence that the propeller of the XP-47J did well at Mach 1.18, so far you have not. But the A&AEE tests show that the problem existed at much lower tip speeds.

Why is it that you cannot consider the possibility that the speed recorded reflected prop thrust loss? After all, the speed has been accepted by virtually every aviation authority extent. Obviously you have an agenda that precludes rational thinking if it undermines your "a superior race produces superior aircraft" mentality.

 

Your exhaust thrust statement was very much of from reality.

Nonsense. Kindly prove otherwise.

 

Your tip speed calculations are again wrong, because you are using wrong reduction gear ratio (the Mustang III had 0.479 reduction gear).

Then Mason's numbers are, in fact too high.

 

And the tests of the A&AEE clearly prove that propellers (1943 era) loss large amount of their efficiency a lot before mach 0.8, therefore there can't be huge difference in propeller efficiency (from positive to negative) between Mach 0.76 and 0.83.

There is a huge difference... Here's a novel idea, try researching it beyond superficial reading.  

 

You are wasting bandwidth.

There's bandwidth a'plenty. However, it does appear that I am wasting my time discussing anything with you. Again, you have an agenda, and will ignore anything that conflicts with it. I suppose the next thing I can expect from you is a denial that the holocaust happened, right?

Indeed, you are the typical weasel who behaves rudely in the safety of cyberspace. So, do us all a favor and sod off.....

Widewing

[gripen]

Widewing,
Well, if you read carefully what I have written in this discussion, you will notice that I have not questioned 500mph speed claim by the Republic. Infact I'm mostly having fun and also collecting nice written material for future discussions about the supposed 3200rpm rating of the P-38L and propeller tip speeds in that case. That exhaust thrust discussion just came as a nice little bonus. And actually some respected aviation writers seem to have some doubts about that mach 0.76 claim like F.H. Dean.

And then to the exhaust thrust. First it should be noted that you made a statement so it's generally your problem to prove it. But anyway, here you are. The Mustang could gain in the best case 30-40mph for it's top speed by the exhaust thrust which in that case was somewhere around 300-400lbs. It should be noted that the in-line engines can utilize exhaust thrust better than the radial engines because the exhaust stacks can be made very short, which means that the thrust pulse from the cylinder can be used more efficient without hammering this pulse to the walls of the long piping typical for the radial engines. This is probably one of the main reasons why mechanically supercharged in-line engines did better than similarly supercharged radial engines at high altitudes despite both utilized exhaust thrust. So in the case of the XP-47J we can assume that in the best case there is something like 600-800lbs thrust available before the piping and turbo. Normally the turbo charged engines can't utilize exhaust thrust very well but in the case of the XP-47J the exhaust of the turbocharger was constricted to create backpressure to utilize exhaust thrust at least in some degree (as you correctly noted). So to this point everyone agrees, but now to the losses. First this backpressure itself causes a little loss in the cylinder exhaust, but not much, maybe couple %. More important loss is caused by the nature of the closed system itself when varying pressure of the exhaust pulses is transformed to the steady backpressure, this means that a large part of the peak pressure of the exhaust pulse is hammered to the walls of the pipes. Again it's difficult to estimate amount of the loss but 10-30% might be close. Then comes that long piping itself which cause some loss, not very much, couple % again. Then comes the turbo which needs something like 700-800hp at 34k, and that might eat 40-50% of the remaining exhaust thrust. So good estimate for the available exhaust thrust should be somewhere around 200-400lbs, probably closer smaller value.

So in the best case there was about same exhaust thrust available as for the Mustang but much higher drag coefficient (XP-47J about 0.036 at mach 0.75 vs P-51D about 0.022 at mach 0.70) and about 50% more drag area. The net gain might be around 10-20mph in the very best case. But why to speculate when there is data available, the USAAF published at 1946 a report which discussed these issues (Final Report of Developement, Testing and Acceptance of the Republic XP-47J airplane). Actually one of the main issues discused in the report was back pressure and temperature of the exhaust system which caused a lot problems and also might caused performance differences between the tests of USAAF and manufacturer. Anyway, the difference between these test was about 12mph which fits to the estimates very well.

You still need some exercise in the tip speed calculations. You must know conditions to calculate exact mach values.

Well, actually I was hoping to see at least 50 lines about how great knowledge you have about these things and how minimal my knowledge is. But I was not disapointed when I saw that nazi and weasel stuff, thanks.


Toad,
I admit that I have been wasting bandwidth here and I also admit that I have been kidding someone. But as you probably know, discussing about the XP-47J is overall waste of bandwidth because it probably never flys in the AH. BTW did you know that I teached tip speed and roll rate theories to CC/Widewing? Want a prove? Try Google...

gripen