190V-18

[Mace2004]

Would this concept be similar, but improvement to what the SR-71 engine answer?

The principle is similar but the results are wildly different.  In each case, air enters the inlet, is decelerated (in the Mustang by expansion and in the SR-71 by inlet shock waves), energy is added (in the Mustang by the radiator, in the SR-71 by fuel burned in the afterburner) and the resulting heated gas is then accelerated out the back producing thrust.  The SR-71 produced tremendous amounts of trust due to the extreme heat involved but the Mustang produced only a tiny amount of thrust and only at high altitude and high speed.  In fact, the "thrust" produced by the Mustang's cooling system only offsets some of the drag inherent in any radiator based cooling system, the thrust never completely offsets the cooling system drag so the net result is still increased drag.  In other words, the Mustang would be faster if it had no cooling system at all but this is obviously a difficult proposition.  The P51 is unique in that the cooling system design is very visible (the large, underbelly scoop) as well as effective but as Krusty mentions many other WWII aircraft used the same principle for cooling.  

The main part that's relevant to the discussion of cooling effectiveness at high altitude is the issue of heat transfer from the radiator to the airstream.  A radiator is affected by aerodynamics as much as any other part of the plane.  All parts exposed to airflow develop a boundary layer of stagnant air near the surface.  This boundary layer increases in thickness at low pressure.  The same happens in a radiator and this boundary layer acts as an insulator which inhibits the transfer of heat and actually becomes a barrier inhibiting airflow.  By slowing the air and increasing its pressure the boundary layers within the radiator are reduced resulting in more effective heat transfer.  

[dirtdart]

Mace thanks for explaining what I am incapable of explaining. krusty the contrast of the two p40s is great but that is also 20k which way different than 35k.

Another interesting example of trading heat for thrust is an old racing plane called the beguine (named after a song). It had wingtip radiator arranged like "ramjets".

Mace where do you reckon the cooling for the mustang was optimized.  There was a YouTube link I saw recently about two mustangs flyin cross country to set the record back in the 40s. I wonder what their flight profile was would probably answer the optimum speed altitude for that airplane.

[Debrody]

What, radiator creating thrust?
Thats how?
At the alt of 10km, the air temperature is around -50/-70 Celsius, whats around 210 Kelvin.
Even a pressurized water cooling system cant be warmer than 130-150 celsius (no more than 450 kelvin anyway). There is no way the radiator could warmen up the 210 Kelvin air to like 300-350 (what would mean a temperature-propotional expansion, effectively the air would exhaust 350/210 times faster than it enters the radiator). Since the air spends only a moment in the radiator, its physically next to impossible to build an effective radiator like this. Also, the more effective the radiator is, the more drag its structure indicates (inside that box under the belly).
So, it will have some effect, especially at high altitudes where the difference in the temperature is larger, but i dont think it can generate "thrust" compared to a clean airframe (true, theres no place for the cooling then...)

[oakranger]

The principle is similar but the results are wildly different.  In each case, air enters the inlet, is decelerated (in the Mustang by expansion and in the SR-71 by inlet shock waves), energy is added (in the Mustang by the radiator, in the SR-71 by fuel burned in the afterburner) and the resulting heated gas is then accelerated out the back producing thrust.  The SR-71 produced tremendous amounts of trust due to the extreme heat involved but the Mustang produced only a tiny amount of thrust and only at high altitude and high speed.  In fact, the "thrust" produced by the Mustang's cooling system only offsets some of the drag inherent in any radiator based cooling system, the thrust never completely offsets the cooling system drag so the net result is still increased drag.  In other words, the Mustang would be faster if it had no cooling system at all but this is obviously a difficult proposition.  The P51 is unique in that the cooling system design is very visible (the large, underbelly scoop) as well as effective but as Krusty mentions many other WWII aircraft used the same principle for cooling.  

The main part that's relevant to the discussion of cooling effectiveness at high altitude is the issue of heat transfer from the radiator to the airstream.  A radiator is affected by aerodynamics as much as any other part of the plane.  All parts exposed to airflow develop a boundary layer of stagnant air near the surface.  This boundary layer increases in thickness at low pressure.  The same happens in a radiator and this boundary layer acts as an insulator which inhibits the transfer of heat and actually becomes a barrier inhibiting airflow.  By slowing the air and increasing its pressure the boundary layers within the radiator are reduced resulting in more effective heat transfer.  

OK, i knew thatthe SR-71 compacts the air giving it the ability.  Thanks   

[beau32]

What, radiator creating thrust?

Known as the Meredith Effect....

Here is a forum post on it. Quite intresting to read.

http://www.ww2aircraft.net/forum/flight-test-data/meredith-effect-p-51-a-16845.html

[Krusty]

Mace, you're right about boundary layers, but not about the end conclussion. The reason the P-51 scoop is offset from the fuselage with a fillet is to set it out from the boundary layer to prevent any problems. The layers is small, even at lower altitudes. The P-51 scoop has no problems at any alt unless it recieves a bullet. Then it's going down one way or another, and fast.

[Debrody]

Known as the Meredith Effect....

Thanks!
Trying to find a mathematical formula to calculate the effectiveness, but still there are too much unknowns...
In ideal case: (mass-flow_{cooling water})*(specific heat_water)*(deltaT_water)=output thrust.   right?    
There are middle phases as the expansion of the airflow, the losses cant be as high there...
The overall effective thrust may be a decent number... but the radiator itself causes some drag too, no im not sure.

[dirtdart]

The layers is small, even at lower altitudes. The P-51 scoop has no problems at any alt unless it recieves a bullet. Then it's going down one way or another, and fast.

The scoop is optimized to provide the required amount of air to cool the radiator, the flow of air is regulated by the exhaust vent behind the radiator.  If the radiator is hit, then OK plane is hurting, the scoop...eh who cares.....

http://www.air-racing-history.com/aircraft/Beguine.htm

As mentioned by mace, the word thurst is used very loosely.  It means it overcame drag, but bu no means a "engine". 

Great discussions gents.

[Krusty]

Read the article posted in the linke beau32 provided, dirt.  (if you haven't already)

It's basically 1 step shy of a ramjet. It's a very measurable thrust. Very interesting stuff.

[dirtdart]

Dammit...all the security stuff on our computers is not letting me see the videos.

http://history.nasa.gov/SP-445/ch5-5.htm

http://contrails.free.fr/refroid_meredith_en.php (Authors original work)

"Mustang, this jet of heated cooling air reduced cooling drag to almost
nothing. It did not eliminate it entirely, but it reduced it to the
point where cooling drag was merely "3% of the thrust of the
propeller." Corky Scott

From what I can read on the net, at any rate, leads me to believe the best a radiator based system could do was overcome drag induced by the scoop/inlet/exhaust.  Merediths study did give rise to the exploration of the ramjet.  This is very interesting and I think I may head down to the library (I am at the US Army Engineer School) and see what I can dig up on this.  We actually have a decent library. 

[Mace2004]

Mace, you're right about boundary layers, but not about the end conclussion. The reason the P-51 scoop is offset from the fuselage with a fillet is to set it out from the boundary layer to prevent any problems. The layers is small, even at lower altitudes. The P-51 scoop has no problems at any alt unless it recieves a bullet. Then it's going down one way or another, and fast.

You misunderstand.  I was not talking about the fuselage boundary layer, I'm talking about the boundary layer created IN the radiator core itself.  Basically, a radiator core is a series of flat tubes and the air flows through the slots between.  A boundary layer forms on these tubes as well insulating them from heat transfer (a thermal boundary layer) and, when it gets bad (i.e., lower pressures), it effectively reduces the amount of space between them to the point that the airflow is restricted.  Also consider the increasing parasitic drag which rising exponentially relative to speed.  In effect, the radiator essentially becomes just a big speed brake with very little cooling capability. 

One option is to move the tubes further apart but then you reduce the cooling density (amount of cooling fluid that can flow through a specific size radiator) and you still have the issue of increased form drag relative to speed.  The radiator can be increased in size (or multiple radiators used) but then you have more cooling drag to deal with and more need for greater horsepower and then more need for cooling, etc., etc.  It's a typical issue with aircraft design, tradeoffs must be made.  In this case, the additional complexity (and weight) of a cooling "system" of scoops, plenums, and exhausts can be employed to increase the efficiency of a smaller radiator by employing Bernouli's law to reduce the speed and increase the density of the air at the radiator face to allow it to operate efficiently while also reducing cooling drag.

[PR3D4TOR]

As HoHun points out in that thread It would in fact more properly be called the "Junkers effect" as Junkers patented the diffusor-radiator-jet combation as "Düsenkühler" ('jet cooler') in DRP 299799 on 17 January, 1915. (Von Gersdorff et al., "Deutsche Flugmotoren und Strahltriebwerke, p. 196.)

The principle obviously was well-known in the English-speaking part of the aviation industry as well. "Fundamentals of Fighter Design" by Ray Whitford notes (p. 61): "In 1926 it was realized that airflow through the radiators on liquid-cooled engines could, if properly ducted, eliminate the cooling drag and even produce a little thrust at speeds above 260 kts (483 km/h)."

The P-51's radiator design might have been more effective than most, but all WWII radiator designs were "jet-coolers".

[icepac]

The scoop provides measurable thrust but it only partially offsets the drag created by it.

This is why the reno planes are starting to show up without scoops at all.

[nrshida]

Anybody got any detailed pictures of the P-51D scoop, especially the internal structure and the radiator itself? Pages from a specific book illustrating the key features would be most helpful.

[Charge]

Google is your friend: http://www.pprune.org/aviation-history-nostalgia/387708-napier-heston-belly-scoop-first-one.html

-C+