Meredith Effect and the P-51

Very cool!

A David Lednicer, who is or was an aerodynamicist at Analytical Methods, Inc, Redmond, WA. has written some interesting papers on how the Mustang performed aerodynamically. One of these, comparing the aerodynamics of the P-51, Spitfire and Fw 190 was available as a pdf document on the net until recently. I have a copy which I will post - when I have some spare time. He had some interesting things to say about the Spitfire v Mustang here back in 1996.

NZTyphoon said:

A David Lednicer, who is or was an aerodynamicist at Analytical Methods, Inc, Redmond, WA. has written some interesting papers on how the Mustang performed aerodynamically. One of these, comparing the aerodynamics of the P-51, Spitfire and Fw 190 was available as a pdf document on the net until recently. I have a copy which I will post - when I have some spare time. He had some interesting things to say about the Spitfire v Mustang here back in 1996.

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cool,i am waiting for your post ~

Some interesting points; it would seem that all three fighters had 2° of washout on the wings, although the distribution of the twist varied. Although the Spitfire radiators were supposed to have been influenced by Meredith's research there were problems with the boundry-layer seperating shortly after entering the duct, creating extra drag. The windscreen was sloped back too steeply creating another area of airflow seperation which took at least 12 mph off the top speed.

Anyway, this should be of interest.

NZTyphoon said:

A David Lednicer, who is or was an aerodynamicist at Analytical Methods, Inc, Redmond, WA. has written some interesting papers on how the Mustang performed aerodynamically. One of these, comparing the aerodynamics of the P-51, Spitfire and Fw 190 was available as a pdf document on the net until recently. I have a copy which I will post - when I have some spare time. He had some interesting things to say about the Spitfire v Mustang here back in 1996.

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I have posted it many times - notably in a Fw 190D vs P-51D thread. David is still a practicing aero and one of the top consultants for the Big Bore racers.

drgondog said:

I have posted it many times - notably in a Fw 190D vs P-51D thread. David is still a practicing aero and one of the top consultants for the Big Bore racers.

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I haven't had much time to look through all of the threads, but I'm not surprised that this article has been posted before; still, it does make it more available to those who haven't been here long, like KimJong. It would be interesting to know if this type of computer analysis has been done on other WW II fighters, such as the 109, P-38, P-47 etc...

Thanks for the report. Ray Wagner wrote on his "Mustang Designer" that in the initial quotation presented to the BPC in 1940 included calcurated performance figures based on a wing having naca 230 series sections. I am sure somebody had tried this assumption already somewhere. Could some of you guys locate that?

http://www.ww2aircraft.net/forum/flight-test-data/p-51-performance-thread-12670.html

This thread has the Lednicer report in Pdf.

NZTyphoon said:

A David Lednicer, who is or was an aerodynamicist at Analytical Methods, Inc, Redmond, WA. has written some interesting papers on how the Mustang performed aerodynamically. One of these, comparing the aerodynamics of the P-51, Spitfire and Fw 190 was available as a pdf document on the net until recently. I have a copy which I will post - when I have some spare time. He had some interesting things to say about the Spitfire v Mustang here back in 1996.

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There is an excellent reference to the Spit windscreen and the pressure stagnation build up on it in contrast to very little stagnation of the 51B and Fw 190 - and even less on the 51D

drgondog said:

There is an excellent reference to the Spit windscreen and the pressure stagnation build up on it in contrast to very little stagnation of the 51B and Fw 190 - and even less on the 51D

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Bill
wrt to the earlier statement (can't remember who made it), I thought the Spitfire's windscreen wasn't swept back enough, rather than being 'too swept back'

Edit: post #125

Colin1 said:

Bill
wrt to the earlier statement (can't remember who made it), I thought the Spitfire's windscreen wasn't swept back enough, rather than being 'too swept back'

Edit: post #125

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According to Lednicer the Spitfire's windscreen was angled at 35°, compared with 22° for the 190 and 31° for the P-51;

Evidently the Spitfire's windscreen is too steep. An experimental windscreen, rounded and of shallower slope was fitted to a Spitfire IX in 1943 produced [sic] a speed increase of 12 mph at a mach number of .79.

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NZTyphoon said:

According to Lednicer the Spitfire's windscreen was angled at 35°, compared with 22° for the 190 and 31° for the P-51;

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Umm, yeah
that's pretty much how I saw it, maybe I read you wrong

Thanks for the article! I've been looking for that information for close to five decades.

What an interesting discussion! Has all the human element and emotion of a bad soap opera! It's length is staggering and quite a read. But definitely worth the wade.

I would like to point out that any discussion about" the Merredith effect" in connection with Mustang developement cannot discount the personalties of the participants. IMHO, quite a bit like the afformentioned soap opera.

It is no secrest that Atwood and Schmued did not get along. Former finally forcing latter to resign and made a few acidic quotes about Scmued's lack of designing ablilites. Atwood has apparently tried to distance Edgar Schmued's participation in the Mustang story and seldem, if ever, mentions him in any historical effort. Atwood has also often taken personal credit for introducing "the Merredith Effect" into the mix, tout it above all else for the design's ultimate scuccess . On the otherhand, it should be noted that neither Schmued nor Horkey gave it that much credit. Both indicated the infamous Curtiss info was NEVER consulted by them. Horkey maintaining, IIRC, "No mathematical formula by a British proffessor was ever used by us".

Right or wrong, Atwood comes off as vindictive, sour grapes, if you will, and claiming import at meetings with British, as if he alone were there. Yet Scmued says that he prepared the original three-view after the initial meeting and presented it with others to the British delegation, a day or so later. Atwoods accounts often are at odds with the recollections of others.

In my opinion, the techno-historical presentation of facts regarding "Merredith Effect" within the context of its use in Mustang development is observably split along the personal lines and predudices of the participants.

The " the Merredith effect" was first put out in a paper read in 1935. It was the same paper that advanced the idea of jet thrust from the engine exhaust. The theory was hardly a secret.

The problem was in turning the theory into reality.
You needed a radiator core that was of the right size (cross section) and thickness to have the low pressure drop through the radiator and not so large or thin to limit the amount of heat transfered to any given unit of air.
You also needed air ducts leading to and from the radiator of the right size and the right change of cross section to keep the pressure changes and air flows with in the limits they needed to be order to get any usable thrust from the whole system. Or even a significant reduction in drag.

too short a duct or too abrupt a change in cross section of the duct could ruin the anticipated results.

With a real shortage of wind tunnels and such to experiment with before the war and little experience to go on it was going to take a while (or inspired guesses) to get it right.

If someone has already made this point, I apologize.

Five hundred cubic feet of air raised 200 degrees F per second corresponds to 47 horsepower.

Forty-seven horsepower into the system and 47 horsepower out.

If the so-called Meredith effect could make 100% use of the temperature rise of the air going through the scoop and cooler, it would at best reduce the drag of the scoop by 47 horsepower.

In the real world, the Meredith effect, if it exists at all, probably does not work at 100% efficiency; and so we should expect a less than 47 horsepower benefit from the Meredith effect.

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The designers of the scoop meant for the scoop to capture a large volume of high velocity air with a minimal cost in frontal area.

Because the scoop originally functioned in the turbulent boundary area, they did not get the flow and volume they expected.

The designers lowered the scoop in order to get it into clear air, but with the knowledge that they paid a price in increased frontal area in order to do so.

A radiator that exchanges heat with the air works best with air travelling at a certain velocity, meaning, with air that spends enough time in contact with the radiator.

The designers directed the high velocity air from the scoop into a larger volume area where the air expanded to fill the space, slowed down and decreased in pressure.

The original volume of air that came into the system through a small scoop then passed through a radiator with a much large cross-section area, at a slower, more optimal velocity, so that the air spent more time in the radiator and thus absorbed more heat (in fact, 200 degrees F per second).

As this hot air passed through the radiator, it expanded while still in the radiator; and, since the dimensions of the radiator constrained the air's expansion, the air could only expand in the direction of travel, which results in an increase in velocity.

The increased velocity air then enters the second or exit chamber, where it has room to expand.

Even after expanding, the air has more energy than it had when it entered the scoop; and, in fact, a potential of 47 horsepower of extra energy.

Here I run into trouble with some of Atwood's math.

In his article, he states that at 1000lbs of propeller thrust, the radiator drag equates to 400lbs of thrust lost to drag, or 40%.

He further states that the Meredith effect recovers 350lbs of that thrust, or 35%, for a 5% difference.

However, he then describes this difference not as 5% but as 3%.

Let's work with the 5% figure.

The P-51D has a max continuous horsepower rating of 1380 horsepower, and a War Emergency Power rating of 1720 horsepower.

One of those two horsepower ratings corresponds to Atwoods' use of the the phrase "full power."

Five percent of 1380 horsepower equals 69 horsepower, or 22 horsepower more than the 47 horsepower potentially available to a perfect system under optimal conditions.

Five percent of 1720 horsepower equals 86 horsepower, or 39 horsepower more than the 47 horsepower potentially available to a perfect system under optimal conditions.

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To me, the brilliance of the P-51's radiator system involves the small cross-section scoop pulling in high velocity air and then slowing it down by directing it into a large-volume, high cross-sectional area, low-velocity radiator system.

Maximum cooling for minimum frontal area.

This, and not exit thrust, explains the effectiveness of the P-51's radiator system.

Perhaps the system produced some small amount of exit thrust that compensated in a small way for frontal area, but the real benefit happened before the hot gas exited the system.

The placement and dimensions of the scoop, the dimensions of the pre-radiator chamber, and the dimensions of the radiator itself made the biggest difference, and not so much what happened after the radiator other than basic streamlining.

A huge low-pressure, low-velocity radiator fed by a small high-velocity scoop:brilliant designing.

I would agree all your points. IMO Atwood was blowing smoke and Schmeud categorically denied the math Atwood presented.

The key elements of the design - going from P-51/A36 to the P-51B was a.) improved flow into the radiator, and b.) reduced drag. The original design, founded on the new radiatior cowling design obtained from Curtis, was fine for the P-51 to match the cowl design before the P-51B caused the wing to drop some 3-4".

The P-51H design was closer in appearance to the earlier Mustangs.

Placing the radiator inside the fuselage instead of tacking it on to a wing or embedding it in a wing allows a smaller inlet to feed a much larger radiator, provides the enclosed volume necessary for the high velocity inlet air to expand and slow down, allows the air to spend more time in contact with the ultra-large radiator, and thus transfers more heat to the air that came into the system through the comparatively small (compared to the size of the radiator) scoop.

understand the distinction - merely commenting about changes within airframe evolution - not between different aircraft altogether.

drgondog said:

understand the distinction - merely commenting about changes within airframe evolution - not between different aircraft altogether.

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I didn't mean my post as a rebuttal or a correction.

I have struggled to understand and write what I wrote for some time, and now that it has started coming out of me I can't stop.