The P-51 Meredith Effect

[MIflyer]

A little article from the Winter 1995 issue of the USAF Museum Friends Newsletter by Leland Atwater on the Meredith effect as used on the P-51.

 

[tomo pauk]

Thank you very much

 

[swampyankee]

Stan Miley is probably the leading expert (at least leading living expert) on cooling aircraft piston engines. In one of his articles (I believe it was Miley, S.. (1988). Review of liquid-cooled aircraft engine installation aerodynamics. Journal of Aircraft - J AIRCRAFT. 25. 222-228. 10.2514/3.45581. The aerodynamic behavior of liquid-cooled aircraft engine cooling installations is reviewed. Design considerations for inlets, diffusers, and exits are discussed. It is shown that the design of an efficient liquid-cooled installation is a technically sophisticated problem. This problem should not be underestimated in the development program of liquid-cooled aircraft engines. Questions are raised concerning the availability of suitable radiators for aircraft installations.) made the statement that there was no evidence that the Mustang's cooling installation provided significant thrust.

Just to stick a number to that: using optimistic assumptions, the P-51's Meredith effect would be about 0.2 lbf per square foot of radiator exhaust nozzle exit area. This assumes no pressure loss across the radiator and 100% radiator effectiveness as a heat exchanger.

 

[gingerbob]

Do you have the spring '93 article referred to at the very start of this article?

 

[MIflyer]

Yes, and I have already posted a scan of the Spring 1993 article by Atwood some time back. Do not recall the title but if you do a search for posts by me you might find it. I could repost it but I am away from home right now and do not have access to it.

I wonder about the true nature of the Meredith Effect. Anyone who has driven a pickup truck with a rear window you could open knows that while driving the air comes in through the rear window and the goes out the side windows, rather than the opposite. There is a powerful suction that develops when you have an abrupt decrease in vehicle shape. In a recent AOPA magazine they describe how they took a 300 mph Lancair and turned it into a 400 mph airplane. They found that the rather abrupt decrease in diameter aft of the wing was causing more drag and faired the area in.

Injecting air into such an area, such as behind the cooling airscoop could have reduced the suction that tends to develop there and reduced drag considerably.

 

[wuzak]

The Meredith Effect refers to the use of the engine's wast heat to add energy to the air in the cooling duct and provide thrust.

Don't think they were injecting this air into areas of low pressure.

 

[MIflyer]

Yes, the claim is that the hot air expanding out behind the intake created thrust. Problem is, you would need a converging diverging nozzle - like a rocket engine has - in order to do that.

And if you have something sticking out into the airstream, you have a low pressure area behind it.

 

[wuzak]

Diverging - converging nozzle.

The exit and intake are smaller in cross-section than the radiator.

The duct doesn't exit into an area of such a sudden cross-section change as a pick-up has, nor does it have a further blockage downstream like the tailgate.

 

[swampyankee]

Yes, the claim is that the hot air expanding out behind the intake created thrust. Problem is, you would need a converging diverging nozzle - like a rocket engine has - in order to do that.

And if you have something sticking out into the airstream, you have a low pressure area behind it.

A convergent-divergent nozzle isn't necessary unless exit velocity is supersonic. Given the low pressure ratio and low temperature in the radiator duct, there wouldn't be a chance of supersonic exhaust

I'm going to stand by my estimate that exhaust thrust is less than 0. 2 lbf per square foot of exhaust nozzle area.

 

[gingerbob]

I think that this is the previous article. (and the same one posted here)
Thanks, MIFlyer!

bob

p.s. As some of you will know, there was a bit of a battle between Atwood and Schmued/Horkey (?) in later years about who gets credit for which "most significant" element. Rather a pity.

 

[pbehn]

I am lost with the Meredith effect I must admit, most things I can see as logical but I have read a lot of the things posted and still don't get "it". If it actually produces thrust why do Reno racers dump it? If the radiator exhaust on a P-51 produces thrust then surely that slows the flow of air over the radiator? To me the best system for the Meredith effect restricted the flow at the inlet to the minimum allowed the best flow over the radiator and then let the heated exhaust out to compensate for any turbulence caused by the inlet.

 

[wuzak]

Do Reno racers dump it?

The Meredith effect doesn't produce more thrust than the drag of the system, but it does offset some of the drag.

Some Reno racers delete the radiator for a total loss system, and remove the cooling drag completely.

 

[pbehn]

Do Reno racers dump it?

The Meredith effect doesn't produce more thrust than the drag of the system, but it does offset some of the drag.

Some Reno racers delete the radiator for a total loss system, and remove the cooling drag completely.

That's what I meant.

 

[swampyankee]

The Meredith Effect is a low-pressure ratio, very low temperature ratio ramjet: adiabatic compression, in the inlet diffuser, constant pressure heat addition in the radiator, and adiabatic expansion back to the free stream. In reality, of course, there will be losses in the inlet (probably about a 2% loss in total pressure), radiator (a significant pressure loss, probably around 10%), and in the nozzle (probably about 1% in total pressure).

 

[wuzak]

Except the outlet is not expanding, but converging.

The air velocity is slowed in the inlet, and increased at the outlet.

The duct is a divergent-convergent type, as the flow is subsonic.

Ramjets are convergent-divergent type ducts for supersonic flow.

 

[swampyankee]

Except the outlet is not expanding, but converging.

"Expansion" refers to a reduction in static pressure and "compression" to an increase

The air velocity is slowed in the inlet, and increased at the outlet.

The duct is a divergent-convergent type, as the flow is subsonic.

Ramjets are convergent-divergent type ducts for supersonic flow.

All ramjets -- and the Meredith effect is essentially a very low-temperature subsonic ramjet -- are Brayton cycle engines: compression (provided by slowing the incoming air through the inlet, which is a diffuser), heat addition (with burning fuel in a ramjet, but by radiator* for the Meredith effect), and expansion to ambient pressure (more or less) in a nozzle. If the pressure ratio is high enough (and it won't be for a ramjet at subsonic speed), the expansion would be to supersonic speed, which would require a convergent-divergent nozzle.

----

* Pedantically, radiation is a trivial part of the heat transfer from a "radiator" to the air flowing past; it's convection. Why it's called a "radiator" is a mystery.

 

[ThomasP]

Technically, since there is no exchange of material between the fluid in the interior of the radiator and the air bouncing off the radiator, the majority of heat transfer is conductive, not convective. Once the air molecules have bounced off the radiator surfaces and absorbed some of the heat from the fluid inside the radiator (conducted through the metal surfaces to the air molecules) the heat is then dispersed by convective effect from the "bounced" air molecules to the surrounding air molecules (by bouncing off of other air molecules). (edit: The air being forced through the radiator by the aircraft's forward movement can be considered the equivalent of convective heat transfer also.) Note that there is also some radiative transfer via infrared photons from the metal to the air.