Author Topic: P-51: Maximum Speed / High Speed (Read 786 times)

niklas · « **on:** November 10, 2003, 02:48:50 PM »

Hi

I just ran across a document that was sent to me by an american institue some time ago.

It´s about the XP-51, the prototype of the P-51. It´s in bad shape, so i didn´t have a close look into it so far. The Allision powered prototype isn´t of interest anyway.

BUT NOW is spotted an really interesting note: They distinguish between Maximum Speed and High Speed. And do you know what the difference is?

High Speed : Wing gun holes open for firing

Soooooo does this mean that american maximum speed numbers are for sealed wing gun openings, and probably other sealings too.... very interesting....

The exact table looks like this:
***Level Fligh Speed at Design Altitude of 130000 ft with Design Gross Weight at 7934

Maximum Speed: 382 mph at 3000rpm with 1110 bhp (MAX rated)
*High Speed: 370 mph at 3000rpm with 1110 bhp (MAX rated)
.....
* Win gun holes open for firing
*** Data obtained from M.R. PHQ-M-19-1330-A Dec. 27. 1941

a nice difference of 12 mph due to the open gun ports with same power....VERY interesting...

niklas

Wilbus · « **Reply #1 on:** November 10, 2003, 03:04:12 PM »

Seems to be alot of difference for something that small, must have distured the laminar flow ALOT if it slows it down by 12mph.

mos · « **Reply #2 on:** November 10, 2003, 03:46:20 PM »

I can believe it. Supposedly, the P51 gained most of it's speed from its superb aerodynamic shape.

F4UDOA · « **Reply #3 on:** November 10, 2003, 11:51:29 PM »

Niklas,

Remeber the Xp-51 had gun ports in the nose as well as the wings. Like the P-40.

Very draggy.

niklas · « **Reply #4 on:** November 11, 2003, 11:34:03 AM »

The gun installation itself didn´t changed. They just had some coverings over the muzzles imo. So the installation itself has no influence, the general installation drag is also included in the Max speed variant. Btw, the XP-51 had 2 in the nose, 2 in the wings, the D 6 in the wings...

It´s noteworthy that according to american official claims the P-51B did "only" 425mph , and in british tests the P-51 performed also some 15mph slower than in some american max speed charts.
Maybe closed gun ports was the reason for the difference between max speed claim and service "high" speed claim.

niklas

dtango · « **Reply #5 on:** November 11, 2003, 02:44:52 PM »

Howdy niklas:

That's a good question. I personally don't have P-51 flight test reports so it's hard to say. However I'll make a guess based on some other data.

Peter Stickney drafted a reply regarding the topic of max level speeds for the P-51. Here's the link to the discussion thread:

P-51 Max Level Speeds

To summarize a couple of points out of there. We have 4 reports:

Brit CFE (1946): Mustang III with V1650-7 engine, Military Power (61"/3000R)
Test Weight: 9200#
Vmax: 438 mph @ 27500'
412 moh @ 14000'
These numbers give a CdF of 0.0179, and a Ram Recovery of 95%

USAAF 1943 Eglin: P-51B with V1650-3 engine, Emergency Power (67"/3000R)
Test Weight: 9690# (Pylons attached)
Vmax: 435 mph @ 27000'
420 mph @ 13100'
This gives us a CdF of 0.0185, and a Ram Recovery of 85%

USAAF Late 1944 EE 393: P-51B with V1650-3 engine, Emergency Power
Test Weight 9200#
Vmax: 450 mph @ 28200'
430 mph @ 15300'
This gives a CdF of 0.0175, and a Ram Recovery of 95%

Pax River: P-51C with V1650-3 engine, Emergency Power
Test Weight 9423#
Vmax: 450 mph @ 29200'
426 moh @ 15600'
CdF 0.017945 Ram Recovery 97%

(1) All the CD0's (listed as CdF's) are approximately the same with the biggest difference with the pylon attached P-51.

(2) The Pax River report we do have a copy of that online. Here it is:
Pax River F4U-1 vs P-51B Comparison

This report mentions that drag modifications for the P-51B in question were minor and consisted of some antenna change and sanding of the wing.

(3) Because the CD0's are all similar either all the various flight tests had their gun ports taped or they didn't. My guess is that they didn't because of the drag condition stated for the Pax River report.

Peter Stickney in his write up does state that the CD0's are consistent with drag clean up including waxing and taping gun ports. I'm guessing however that if the gun ports were taped then mention of them would be given in the reports.

My $.02. Actually Pyro may know the answer.

Tango, XO
412th FS Braunco Mustangs

davidpt40 · « **Reply #6 on:** November 11, 2003, 06:21:11 PM »

EE 393: P-51B with V1650-3 engine, Emergency Power
Test Weight 9200#
Vmax: 450 mph @ 28200'

WHOA!

niklas · « **Reply #7 on:** November 12, 2003, 12:58:04 PM »

What Stickney writes has nothing to say and is also in a special case incorrect.

The Langley report he speaks about is available, for example at least partly here:
http://mitglied.lycos.de/luftwaffe2/NACA/NacaDragResearch.pdf

So it´s not just (Beat-up condition) and (Cleaned up, with cowling gaps sealed.) but WHOLE aircraft faired and sealed and aircraft in service condition.
A CD of 0.017x for an aircraft in service condition is absolutly inconsitent, actually the CD of slightly over 0.2 fits much better to the Dive tests. It was, however, the minimum CD reached, the mean drag coefficient was more likely around 0.022

About the other tests: They´re pretty useless, because they don´t contain temperature, static pressure and mach correction.
AND they´re tests of single machines. The quote about the multi-plane test is very interesting, it fits with a more realistic CD like 0.020 - 0.23

niklas

dtango · « **Reply #8 on:** November 12, 2003, 11:40:01 PM »

niklas:

Quote

What Stickney writes has nothing to say and is also in a special case incorrect.

Not sure I agree with you there but I don't think it really matters anyway.

As for posting the NACA L-108 report, I've been looking for that report! Do you have the full version of it? If so could I get a copy of it? I only have part of the summarized version done in 1976 by NASA - report TN D-8206.

I never understood the CD values in the report. I thought they were listing CD0's and not total CD values but it's not apparent what they are talking about.

I do have the other report regarding the P-51B dive tests. The CD values given there are total CD, not the CD0 values. The CD values include the lift-dependent portion of the drag. Using CD0 values of .0176 for the P-51 and using a CL of .1, AR=5.815, and e=.87 I get a total calculated CD of about .0182. There is a difference there as you have pointed out. The P-51B dive test report was available. I'd like to know why the .018-.017 values are still used. In my opinion I don't think there is an agenda to mislead anyone with this data.

Tango, XO
412th FS Braunco Mustangs

niklas · « **Reply #9 on:** November 15, 2003, 04:20:54 AM »

I think i could organize the full version, but afaik it´s not very informative. Many pics, few text. The most important is given in the table.

From 0.0182 to 0.020-0.023 is still a large margin. Only the model reached less than 0.02.

0.017 sounds very nice, but it´s completly unrealistic for real flights. Except highly polished prepared machines flying in america´s desert-good-weather-atmospheric-condition.

niklas