More P-51 Design Information

[MIflyer]

A while back I think someone asked if any other aircraft used the positive thrust exhaust approach employed by the Mustang. I recalled an article that addressed that and today finally found it, along with a related article. Both are by J. Leland Atwood, former Chairman of North American Aviation. These appeared in the Friends Journal, a very nice "newsletter" that has grown into an interesting magazine sent out to supporters of the USAF Museum, almost 25 years ago and probably have not been seen before by very many people.

 

[Peter Gunn]

Awesome, thanks.

 

[nuuumannn]

Good to see, thanks for posting, although Atwood had essentially the same article published in Aeroplane Monthly back in the mid nineties. I have a copy of it somewhere. A guy I once knew restoring a CAC Mustang pointed it out to me, having met Atwood in the past.

 

[GregP]

We all thank you!

 

[MIflyer]

I recalled seeing what looked like much the same article in a magazine back then. I thought it was Wings/Airpower or Air Classics but I guess it might have been Aeroplane Monthly.

Considering the vast amount of information that has been published on the Mustang it is surprising to me that new material still comes to light, even 25 years ago.

 

[chuter]

Note no mention of the laminar flow wing. In a speech on the Mustang at an SAE convention in the '50s (or '60s ... idr) Atwood said believed there was little realistic payoff from the wing due to dirt, bugs and unavoidable field caused surface irregularities/dents, not to mention the gun and ammo bay doors, that largely defeated the wing design. Alternatively, Supermarine (and Messerschmitt) who couldn't seem to get the Meredith Effect to work figured the Mustang's secret must have been the wing. Supermarine's frustrating experience with the Spiteful's laminar wing would eventually give them reason to rethink things. Atwood also said, as he alludes here, that he believed the only WW2 combat planes to take full advantage of Meredith were the Mustang and Mosquito.

 

[tomo pauk]

Germans measured the drag of P-51's wing, it came out as 30% (give or take) less draggy, as coeeficient, than of Bf 109 and Fw 190. Despite being thicker.
With Spitfire, a move towards the laminar flow wing didn't yielded that much of advantage due to the wing already being the thinnest in military service. A dealof CL was sacrified, and wing was of smaller area than on the Spitfire, with weight going up - not a recepy for happy stall & turn characteristics.

P-51 was either better or equal in each item, drag wise, than it's competitors, thus high speed even on low power and it's not small stature and weight.

 

[Timppa]

In the article above Atwood took the credit of applying Meredith effect to the P-51.
Schmued said the "negative drag" was accidental as the P-51 was found faster than the wind tunnel tests predicted. Explained by the chief thermodynamicist Joe Beerer.
Horkey (aerodynamicist) said the real achievement was the design of the radiator air inlet (proper location of the air intake below the fuselage and the boundary-layer gutter)
Schmued and Horkey did not mention Atwood at all.

 

[MIflyer]

Note that the later P-38's added a lip to the radiator air intakes to separate the boundary layer. And the later higher powered engines with the better intercooler still used the same size engine coolant radiators as did the earlier models - they just got more efficient use of the radiators.

Omission of the boundary layer splitters from the first P-80's is hard to understand, when you think about it.

By the way, it is true that laminar flow wings are adversely affected by surface imperfections. But even then they still have lower drag than other wing sections.

 

[gingerbob]

Edit: nothing to see here, sorry!

 

[Ivan1GFP]

Note that the later P-38's added a lip to the radiator air intakes to separate the boundary layer. And the later higher powered engines with the better intercooler still used the same size engine coolant radiators as did the earlier models - they just got more efficient use of the radiators.

This is not completely true. The Radiators on the late model P-38 were a different shape from the early radiators. They were a bit wider at the core from what I remember seeing in photographs. The early radiators had a higher opening top to bottom.
The other consideration is that the Allison engines in the P-38 used oil coolers to reject heat more so than some other engines and I believe the size of the oil coolers was also increased up front where the intercoolers were.

Yes, this is a two year old thread, but the discussion is still interesting.

 

[MIflyer]

The shape of the P-38 radiator "cowlings" was larger, in order to move them further out into the airstream, which is my point. But actual radiators under the cowlings were the same. A friend of mine, Lt Col Ward Duncan, rebuilt an F-4 using P-38J parts. See:
9th PHOTO RECON SQUADRON

 

[Ivan1GFP]

Hello MIFlyer,

That is a really cool story. Thanks for posting the link.
The story doesn't actually emphasize that the radiators are the same size. There is a bit of a difference in the design of the two in that there is a boundary layer splitter in the later model, but if you look at photographs, you can see that it is physically a much larger unit.
The inlet was not very big in order that the velocity of the air through the radiator core (the widest part) remain lower for better heat transfer.
Here are a couple photographs for a comparison. Unfortunately the best photograph of an early model is of Glacier Girl which is a rebuilt aircraft.

- Ivan.

 

[MIflyer]

I wrote the Duncan's Hot Rod story and Ward told me that they were the same size radiators, but moved further out to get them away from the boundary layer. I believe him.

 

[Ivan1GFP]

Hello MIFlyer,

Very good story. Pity there are no pictures of the Hot Rod.

Regarding Radiators on the Lightning..... Here are some frontal shots of an early P-38 as compared to the late P-38.
There is obviously no question that the Radiators are much wider on the later aircraft and the shape is more rounded on the outside.

As you can see from the following photographs of what the core looks like inside the housing, it is pretty obvious that the core is not a smaller unit moved outboard with a gap but is quite rounded and fills the entire space inside the housing. It simply would not have fit inside the housings of the early Lightning.

 

[Ivan1GFP]

Hello MIFlyer,

Here is another view of the bare core of a late model Lightning as compared to the housing of a P-38F (Glaclier Girl).
The outboard contours of the core do not match the housing.

- Ivan.

 

[Jugman]

I wrote the Duncan's Hot Rod story and Ward told me that they were the same size radiators, but moved further out to get them away from the boundary layer. I believe him.

Not according to the parts catalog or the E&M manual. Early radiators are P/N# 224165 and J/Ls are 197325. In both cases the radiators are mounted flush against the boom sides.

 

[Jugman]

That tube and fin radiator is a post war replacement. All P-38s had bundled tube radiators.

 

[Ivan1GFP]

That tube and fin radiator is a post war replacement. All P-38s had bundled tube radiators.

Hello Jugman,

Thanks for pointing that out. I had not caught that even though I have seen photographs of the radiators being assembled.

- Ivan.