# A-36 Apache



## NightHawk (Jan 31, 2005)

Did anybody hear about this plane ?
A-36 Apache, from what i have heard its the ground attack version of the P-51, all info will be welcomed,


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## Lightning Guy (Jan 31, 2005)

The A-36 was the initial american development of what would be the P-51 Mustang. It was intended to be a USAAF dive-bomber. These sites have some useful info in the type.

http://www.wpafb.af.mil/museum/air_power/ap21.htm

http://home.att.net/~jbaugher1/p51_6.html


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## Anonymous (Feb 1, 2005)

It's a P-51A variant. The USAAF had no more budget for fighters, but they did have budget for dive bombers. But the Generals wanted more fighters (and fighter-bombers) and didn't care what Congress had allotted the money for, so they ordered the A-36 which was just a P-51A with dive brakes.

=S=

Lunatic


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## KraziKanuK (Feb 1, 2005)

Lightning Guy said:


> The A-36 was the initial american development of what would be the P-51 Mustang. It was intended to be a USAAF dive-bomber.



The A-36 came 9 months after the P-51(no designation letter)/Mustang I. The first order for P-51s was in July 1941 when the USAAF ordered 150. The A-36 was not ordered until April 16, 1942.


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## plan_D (Feb 1, 2005)

Used in Burma and Italy. I heard they had to wire the dive brakes shut because they caused stability problems.


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## Lightning Guy (Feb 1, 2005)

I've read that wiring the dive brakes was a rumor but that their use was restricted because of staibility issues.


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## the lancaster kicks ass (Feb 1, 2005)

nice pic.........


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## DaveB.inVa (Feb 1, 2005)

I've got two books about the P-51 and both pretty much agree that the whole dive brake thing is a myth. Both books and the Baugher site mention that the dive brakes were beneficial and very effective resulting in better stability and accuracy. The dive brakes were used frequently in all theatres of operation. From what I have read the wired shut story comes from some officers in training believing the dive brakes would be useless and that they should be wired shut.. then the story ran rampant from there!


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## Anonymous (Feb 1, 2005)

plan_D said:


> Used in Burma and Italy. I heard they had to wire the dive brakes shut because they caused stability problems.



That is incorrect. Sometimes later in their lives the dive brakes were wired shut because they didn't want to maintain them and just closed off that part of the hydrolic system. If they were working properly, and were in the closed position, they had no effect on stability. They also worked pretty well as dive brakes.

=S=

Lunatic


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## HealzDevo (Feb 3, 2005)

The whole story was that the A-36 Apache was originally developed as an escort fighter by the USAF. However, when the plane proved to be unworkable in its intended role it was turned into a dive-bomber, until the lend-lease agreement saw some A-36 Apaches go overseas to Britain where they were modified by the addition of a better engine to become the P-51 Mustang. It wasn't so much the airframe that was wrong but just that the plane was underpowered. Once the British put in a more powerful engine it worked and the plans for that engine were bought by the US to enable production of one of the most potent escort fighters the world has ever seen.


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## Anonymous (Feb 3, 2005)

HealzDevo said:


> The whole story was that the A-36 Apache was originally developed as an escort fighter by the USAF. However, when the plane proved to be unworkable in its intended role it was turned into a dive-bomber, until the lend-lease agreement saw some A-36 Apaches go overseas to Britain where they were modified by the addition of a better engine to become the P-51 Mustang. It wasn't so much the airframe that was wrong but just that the plane was underpowered. Once the British put in a more powerful engine it worked and the plans for that engine were bought by the US to enable production of one of the most potent escort fighters the world has ever seen.



That is incorrect. The P-51A (and A-36) were fine at low altitudes. The P-51A was not specifically developed as an "escort fighter", the British wanted to buy P-40's but Curtis's production capacity was fully untilized. Therefore they went to North American and wanted them to manufacture P-40's for them under license to Curtis. NA wanted to build its own fighter design, and got the go-ahead assuming they could demonstrate a prototype in 120 days, which lead to the Mustang I which filled the same operational requirements as the early P-40, but had much better range and speed.

As I said before, the USAAF budget did not include any more allocations for fighter purchases. The USAAF got around this by ordering the A-36 as a dive-bomber, which they did have funding for, rather than as a fighter. But it was just a P-51A (Mustang II) with dive brakes.

The P-51A was a screemer below 13,000 feet, one of the fastest low-altitude planes of the war. It was faster than the P-51B or D, and it was faster than the Spitfire, at such altitudes.

See http://www.vectorsite.net/avp511.html#m5 for details.

=S=

Lunatic


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## cheddar cheese (Feb 4, 2005)

Does anyone have some info on combat records of the A-36? How successful it was etc?


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## evangilder (Feb 4, 2005)

Well, I could write something up, or lift it from Joe Baugher, so here goes:

_Only after Pearl Harbor did the US Army finally agree to order the Mustang for its own use. General H. H. "Hap" Arnold, Chief of Staff of the USAAF, was instrumental in breaking up the bureaucratic log-jam and getting the Army to relent and order the Mustang for its own use. On April 16, 1942, the Army finally ordered 500 NA-97s. The NA-97 was a ground attack version and was designated A-36A (in the attack series rather than the fighter series). Serial numbers were 42-83663/84162. 

The A-36 seems to have been known by several different names--it was initially called *Apache*, which was the name that the Army initially assigned to the P-51, but there was an effort to change the name to Invader following the invasion of Sicily. However, the name Mustang was generally applied by most people to the A-36. 

The A-36A differed from previous Mustang versions in having a set of hydraulically-operated perforated door-type dive brakes mounted at approximately mid-chord on both the upper and lower wing surfaces outboard of the wing guns. The brakes were normally recessed into the wings, but were opened to 90 degrees by a hydraulic jack to hold diving speeds down to 250 mph. A rack was fitted under each wing for a 500-pound bombs, a 75 US gallon drop tank, or smoke-curtain equipment. A built-in armament of six 0.50-inch machine guns (two in lower fuselage nose, four in the wings) was fitted, however the two nose guns were often omitted in service. The wing guns were moved closer to the main landing gear strut in order to minimize stress under taxi and takeoff conditions. The engine was the Allison V-1710-87 (F21R), rated at 1325 hp at 3000 feet. Normal and maximum loaded weights rose to 8370 pounds and 10,700 pounds, and the maximum speed in clean condition fell to 356 mph at 5000 feet and 310 mph with the two 500-lb bombs fitted. With the bombs, range and service ceiling were 550 miles and 25,100 feet respectively. 

The first A-36A flew on September 21, 1942. Deliveries of the A-36A were completed by the following March. The A-36A equipped the 27th and 86th Fighter Bomber Groups based in Sicily and in Italy. They initially were painted in olive-drab and light-gray finish and were painted with yellow wing bands and yellow circles around the national insignia. Both of these Groups arrived in North Africa in April of 1943 just after the end of the Tunisian campaign. They saw their first action during aerial attacks on the island of Pantelleria, with the first sortie being flown on June 6, 1943. The A-36A was involved in the taking of Monte Cassino, and participated in the sinking of the Italian liner Conte di Savoia. 

The only other A-36 user was the 311th Fighter Bomber Group, based in India. It saw extensive use in the China-Burma-India theatre. 

Several sources list the Invader as not being particularly effective during combat. It seems that this is not strictly correct. Although losses during low-level attacks were rather high, the A-36 was actually a good dive bomber and it was a stable and effective ground strafer. The engine was very quiet, and it was often possible for an A-36 to get nearly on top of an enemy before he realized that an attack was imminent. Dive bombing was usually initiated from an altitude of 10,000 feet to 12,000 feet, with bombing speed held to around 300 mph by the dive brakes. The bombs were dropped at an altitude of 3000 feet, and pullout was at approximately 1500 feet. The Invader was fairly rugged and easy to maintain in the field. The A-36 could consistently stay within 20 feet of the deck and could easily maneuver around trees, buildings, and other obstacles while strafing. The A-36A was able to take a considerable amount of battle damage and still return to base. Nevertheless, a total of 177 A-36As were lost in action. 

The A-36s did not see very much air-to-air combat, since it was optimized for low-altitude operations and lost its effectiveness above 10,000 feet altitude. It was generally believed that the A-36 Invader was no match for the Messerschmitt Bf 109 at high altitudes, and that it was therefore best for A-36 pilots to avoid such encounters if at all possible. If air-to-air combat was unavoidable, it was thought best to force the battle down to altitudes below 8000 feet, where maximum advantage could be taken of the A-36A's excellent low-altitude performance. Although it was not a fighter, the Invader claimed 101 enemy aircraft destroyed in air-to-air combat. One of the pilots of the 27th Fighter Bomber Group, Lt Michael T. Russo, became the only ace in the Allison-engined Mustang, although several other of his colleagues did score victories as well. 

A sort of urban legend has sprung up about the A-36A's dive brakes. According to some stories, the dive brakes of the A-36A were next to useless and were deliberately wired shut at the manufacturers so that they could not be used. This story is totally incorrect. On the contrary, the dive brakes proved to be quite effective in combat, and the aircraft was so stable with the dive brakes extended that bombing while in a dive was particularly accurate. The origin of this legend seems to have been in the United States, at a time before the Invaders first went overseas. It seems that A-36A pilots were told by their officers in the USA that their dive brakes would be all but useless in combat and it would be best if they simply wired them shut. This turned out to be incorrect, and the dive brakes were used to great effect throughout the Sicilian campaign and the Italian invasion. 

One A-36A was supplied to the RAF in March of 1943 for experimental purposes. Its RAF serial number was EW998. 

There are very few A-36As still surviving today. A-36A Ser No 42-83665 is on display at the WPAFB Museum in Dayton, Ohio. 42-83731 is with the Lone Star Flight Museum in Texas. 43-83738 is currently undergoing restoration as a P-51B at the Warhawk Air Museum in Boise, ID. Another A-36A is with the Collings Foundation, where it is undergoing restoration. 

Serial numbers of the A-36 were 42-83663/84162.
_

Source: http://home.att.net/~jbaugher1/p51_6.html

It is always a good idea to get second sources on all facts, however, Joe is usually pretty accurate and does have soem well-researched articles.


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## cheddar cheese (Feb 4, 2005)

Ah nice, thanks 8) Been wondering about this pln for a while now...


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## evangilder (Feb 4, 2005)

Me too, after seeing another pic of it, I figured I would look and see what Joe B. had written up. Now we both know! 8)


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## KraziKanuK (Feb 4, 2005)

RG_Lunatic said:


> The USAAF got around this by ordering the A-36 as a dive-bomber, which they did have funding for, rather than as a fighter. But it was just a P-51A (Mustang II) with dive brakes.


 

The P-51A/Mustang II was an A-36 with the dive brakes removed and a different engine(Allison V1710-81 instead of a -87). The P-51A/Mustang II did not have the lower cowl mgs which the P-51/Mustang I and A-36 all had.


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## wmaxt (Feb 4, 2005)

Lightning Guy said:


> The A-36 was the initial american development of what would be the P-51 Mustang. It was intended to be a USAAF dive-bomber. These sites have some useful info in the type.
> 
> http://www.wpafb.af.mil/museum/air_power/ap21.htm
> 
> http://home.att.net/~jbaugher1/p51_6.html



The original P-51/A-36 was initiated by the British as a P-40 replacement/helper because North American was unwilling to build P-40s. That's also why the A-36 had a low blown Allison engine in the begining it was designed for low altitude attack missions.


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## Anonymous (Feb 4, 2005)

Evanglider,

I agree multiple sources are best. I often don't post them all for obvious reasons. Joe Baugher's info does not conflict with my statement about why the A-36 was ordered, it simply ignores that info.

Here are some P-51 sources (in no particular order):

http://home.att.net/~jbaugher1/p51.html
http://www.chuckhawks.com/mustang_P-51.htm
http://www.vectorsite.net/avp511.html
http://www.vectorsite.net/avp512.html
http://nasaui.ited.uidaho.edu/nasaspark/safety/cover.htm
http://www.airspacemag.com/asm/mag/supp/jj99/Mustang.html
http://www.airspacemag.com/ASM/Mag/Index/1996/AS/wmtm.html
http://www.flightjournal.com/p-51.asp
http://www.flightjournal.com/plane_profiles/p-51_mustang/p-51_mustang_history.asp#p-51d and k
http://www.ctie.monash.edu.au/hargrave/cache_002.html
http://www.wpafb.af.mil/museum/research/p51.htm
http://www.aviation-history.com/north-american/p51.html
http://freespace.virgin.net/shermanic.firefly/must1.html
http://www-rcf.usc.edu/~nagle/development.html
http://www.parkerinfo.com/ap30.htm
http://members.tripod.com/Air_Museum_ww2/id23.htm
http://www.nasm.si.edu/research/aero/aircraft/NAP-51D.htm
http://www.acepilots.com/planes/p51_mustang.html
http://www.icon.co.za/~pauljnr/history.htm
http://www.cebudanderson.com/viewfromtheline.htm (worth the read!)
http://www.zenoswarbirdvideos.com/P-51.html
http://www.ipmsstockholm.org/magazine/2000/01/stuff_eng_p51late.htm
http://www.ipmsstockholm.org/magazine/1999/11/stuff_eng_p51early.htm

=S=

Lunatic


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## evangilder (Feb 4, 2005)

I wouldn't say he ignored the info, he just might not have had a source for the info. But I get the feeling that you are a P-51 fan.


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## Anonymous (Feb 5, 2005)

evangilder said:


> I wouldn't say he ignored the info, he just might not have had a source for the info. But I get the feeling that you are a P-51 fan.



Well, it depends on who's info you listen too. The USAAF does not present things from a point of view that would make higher up's look bad. You have to look at info from the engineers and exec's of the manufacturing company to see the how's and whys of many of the decisions that were made to satisfy often stupid rules and requirements. A good example is the P-39 - some high mucky muck in the USAAF or Congressional purchasing dept. wanted a highly streamlined design - thus the turbo supercharge (or 2nd stage supercharger) was removed to remove the required scoop.

I'm a fan of a lot of WWII planes. The P-38, Corsair, P-51, FW190A6, Bf109F, Spitfire IX, La7, Yak3, Ki-84-Ib, are all "favorites" 8) 

I've just studied the P-51 a bit more than the others, I used to have a web-page about it. When I was in college, I was at Cal-Tech doing some research (on an unrelated topic) and there happened to be a section of the library doing a presentation on the P-51 and other NA accomplishments achieved with Cal-Tech support. I was able to read all kinds of interesting info about the plane and its development in documents laid out under glass display cases, and spent about a third of my 2 day research visit "wasting time" in that section of the library.

But to say I'm a fan of it.. no not really. I like a lot of planes, I just think the P-51 tends not to get a fair shake because of how its numbers match up in the traditional comparisons such as wing-loading and climb rate. Judged by those figures it was rather weak, but that does not do justice to the P-51 at all.

=S=

Lunatic


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## cheddar cheese (Feb 5, 2005)

190A-6? How comes? I can understand the other but preferring the A-6 to an A-8 seems a little strange to me.


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## Anonymous (Feb 5, 2005)

cheddar cheese said:


> 190A-6? How comes? I can understand the other but preferring the A-6 to an A-8 seems a little strange to me.



The A-6 was the best fighter variant of the A series. It was lighter and more manuverable than the A-7 and A-8 which had much more armor and tended to be setup with 4 wing cannon rather than 2. As a dogfighter, the A-6 was the better plane.

=S=

Lunatic


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## cheddar cheese (Feb 6, 2005)

Ah right. Always a fan of the A-8 myself 8)


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## Anonymous (Feb 6, 2005)

cheddar cheese said:


> Ah right. Always a fan of the A-8 myself 8)



The A-8 was more of a bomber killer. Matter of personal preference.

However, I think it's pretty clear that an A6 would beat an A8 in a dogfight far more often than not.


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## cheddar cheese (Feb 6, 2005)

Yes it probably would. Am I correct in saying the A-8 had more guns and armour, causing the lack of manoeverability? If so, thats probably why I like it.


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## Anonymous (Feb 6, 2005)

The A-8 had a slightly more powerful engine and quite a bit more armor than the A-6. The only difference in the gun package was the A-6 had 2 x 7.9 mm machine guns in the cowl where on the A-8 these were replaced by two 13 mm guns (which were very weak for their caliber) - not much of an improvement.

Some later models of the A-8 had the two outer 20 mm guns replaced with 30 mm MK108's, but these were even heavier than the 20 mm guns and nearly useless for dogfighting.

On the dogfighter configuration the outer pair of guns were usually not fitted.

=S=

Lunatic


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## DerAdlerIstGelandet (Feb 16, 2005)

As was said before the A-8 was more of a bomber killer. The 30mm MK108s were used for bomber killing not dog fighting so yes they are useless.


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## KraziKanuK (Feb 16, 2005)

DerAdlerIstGelandet said:


> As was said before the A-8 was more of a bomber killer. The 30mm MK108s were used for bomber killing not dog fighting so yes they are useless.



Don't tell the Kurfurst(109) pilots that. 

The A-8 could have a much more powerful engine than the A-6, if the engine used in the A-9 was fitted, and it was fitted. Many A-8s had the aux 115 l tank removed.

Depends on which /R kit is fitted but the standard A-8 had the same armour fitted as the A-6.


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## Anonymous (Feb 16, 2005)

KraziKanuK said:


> DerAdlerIstGelandet said:
> 
> 
> > As was said before the A-8 was more of a bomber killer. The 30mm MK108s were used for bomber killing not dog fighting so yes they are useless.
> ...



The base weight of the A-8 is several hundred pounds heavier than that of the A-6, the only armament difference being the MG131's vs. the MG17's. Most of the difference is armor builit into the cowl, in front of the oil cooler ring, and around the cockpit, and the thicker armor-glass windscreen. The engine was a little more powerful in practical usage, not a lot - it was after all just an twin-wasp derivative that had been pushed about as far as it could be pushed already.

=S=

Lunatic


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## KraziKanuK (Feb 17, 2005)

All 190A a/c had 50mm thick windscreen glass. The standard A-8 did not have any armour around the cockpit. It was the /R8 that had it added.

The standard A-8 with the D2 engine had 6.5mm oil cooler armour. On the A-8/R8 and the A-8 with the TU engine, this was increased to 10mm.

The standard A-8 with the D2 engine had 5.5mm oil tank armour. On the A-8/R8 and the A-8 with the TU engine, this was increased to 6.0mm.

The D-2 and TU a/c had 12mm head armour which was increased to 20mm on the /R8.

The 801TU put out over 2000hp. Some say 2300hp.

The 115 l aux tank, which was often removed, added 200lb to the A-8.


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## DerAdlerIstGelandet (Feb 17, 2005)

KraziKanuK said:


> DerAdlerIstGelandet said:
> 
> 
> > As was said before the A-8 was more of a bomber killer. The 30mm MK108s were used for bomber killing not dog fighting so yes they are useless.
> ...



Oh dont take me wrong. Any Fw-190 was a very formidible fighter aircraft and even the allied pilots would agree that seeing the butcher bird in the skies was not something to shrug off. I just think the A-8 was better suited for knocking out bombers then dog fighting.


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## Iskandar Taib (Mar 24, 2005)

The "Mustang" name actually came from the British, I believe. They wanted the Allison Mustangs for "rhubarbs". Fitting a Merlin into it wasn't so easy. It didn't fall in - there's a book available from the RR Heritage Trust about the subject. They had to enlarge the air scoop and radiator duct, and there was all sorts of plumbing that needed to be routed. The smaller air scoop was most likely the reason why the Allison Mustang was faster on the deck.

(There's something weird going on.. why are posts from the FW-190 topic in the A-36 topic??)


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## the lancaster kicks ass (Mar 24, 2005)

don't worry this always happens...........


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## Anonymous (Mar 24, 2005)

Iskandar Taib said:


> The "Mustang" name actually came from the British, I believe. They wanted the Allison Mustangs for "rhubarbs". Fitting a Merlin into it wasn't so easy. It didn't fall in - there's a book available from the RR Heritage Trust about the subject. They had to enlarge the air scoop and radiator duct, and there was all sorts of plumbing that needed to be routed. The smaller air scoop was most likely the reason why the Allison Mustang was faster on the deck.



No, the Allison's single stage supercharger and its prop were better suited to low altitudes. The scoop size didn't matter, as its drag was canceled out by the radiator thust.

=S=

Lunatic


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## KraziKanuK (Mar 24, 2005)

The early P-51/A-36 radiators were not as efficient as the the later Merlin engined P-51s were.There was a complete re-design of the intake.


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## DerAdlerIstGelandet (Mar 24, 2005)

Interesting that I did not know.


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## Anonymous (Mar 24, 2005)

KraziKanuK said:


> The early P-51/A-36 radiators were not as efficient as the the later Merlin engined P-51s were.There was a complete re-design of the intake.



The gutter gap was found to be insufficient at high speeds, espeically at higher altitudes where boundary layer seperation occures more easily.

I was not saying the P-51A radiator was as efficient as the Merlin powered models, it didn't have to be. But the point is it was not faster because of the smaller scoop, since in both cases the drag from the scoop is canceled out by the thrust from the radiator.

=S=

Lunatic


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## DerAdlerIstGelandet (Mar 24, 2005)

That is pretty much what I have read.



> The radiator scoop for the later model P-51’s was well designed. While the scoop caused some profile drag, the heated air exiting the exhaust expanded and created enough thrust to compensate for the drag anytime the air was heated above 170° F. The stock P-51 radiator is large, being 14’ deep and 21" wide by 28" high, and it was sized to cool the engine at 3800 RPM and 55 lbs. of manifold air pressure (MAP). Racers go to 4,000 RPM and 120+lbs. of MAP, so additional cooling is required. Rather than add an even larger radiator, racers added cooling water spray bars in front of the radiator to accomplish the same thing. Spraying cooling water onto the P-51 radiator is necessary but reduces the thrust created in the exhaust because it also cools the air, decreasing hot air expansion.
> http://www.aafo.com/racing/news/98/intrepid.htm


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## Iskandar Taib (Mar 24, 2005)

From what I understand, you never get as much thrust out of the Meredith Effect as it takes to overcome the drag of the radiator inlet. You can recover quite a bit, but it is never zero sum or better. In any case, there is only so much thrust that can be generated out of the heat produced by the engine. Make the inlet larger than is optimal, and you get more drag. The inlet size really does matter - the bigger it is, the bigger the drag. You make it just as big as it needs to be to cool the engine, no bigger. If the size didn't matter, then you could make the inlet as big as you wanted. This also applies to radials, by the way.

See:

http://www.supercoolprops.com/ARTICLES/gwhite.htm
http://www.supercoolprops.com/ARTICLES/ducted_cooling.htm

I don't think the Allison was making more power than the two stage Merlin, even on the deck. If they did, people would be dumping the two stage supercharger for racing! Instead, they finesse the inlet, and spray liquid on the radiator core. Incidentally, Supercool is wrong about the Spit IX. It did use the Meredith effect (see Quill's book), though the radiator design wasn't as efficient as on the Mustang, and there were other sources of drag. The Mustang manages to hide most of the bulk of the radiator core inside the fuselage - it takes up a large amount of space below and behind the pilot, while the scoop and inlet represent a comparatively small "bump" in the fuselage profile.


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## DaveB.inVa (Mar 24, 2005)

Iskandar Taib said:


> I don't think the Allison was making more power than the two stage Merlin, even on the deck. If they did, people would be dumping the two stage supercharger for racing!



True, but the warplane version wasnt running in high blower on the deck either! Youve gotta compare apples to apples here. The configuration that a P-51D would run on the deck in 1945 is way way different than a racing Mustang at Reno today.

In 1945 configured the way they came from the factory an Allison engined P-51 compared VERY favorably with a Merlin engined P-51 up to about 10000'. Somewhere above that the Merlin shifted into high blower. Ive said it before and Ill say it again.. it wasn't the Merlin in its self that was so great it WAS the fact that the Merlin could get enough air at higher altitudes that made it a performer. Try to run it on the deck in 1945 in high blower and youd surely get a lot more power... but not for long!!


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## Iskandar Taib (Mar 24, 2005)

What I'm getting at is that you can't turn off the second blower stage. What you CAN do is change the gears so that the two stages run faster (at high altitude) or slower (at low altitude). The blower absorbs a lot of power that can otherwise go to the prop. At low altitudes, you use the low gear because, if you don't, 1) you get more boost than you need, and 2) the amount of added power you generate doesn't make up for the amount you lose turning the blower. The reason you have two stages is that at high altitudes you need them to get the needed boost. But you have to gear them lower at lower altitudes. 

So it would make sense, if the second blower stage didn't really give you an advantage close to the ground, to delete it if you're not flying at high altitudes. But in fact, it does. Note that the two stage Merlins gave more output than the single stage ones even at low altitudes. Part of this had to do with the intercooler, part of it that the two stage Merlins were built stronger to take the added boost. So the Merlin should have had higher output than the Allison did regardless of the altitude - just the fact that it needed a bigger radiator testifies to that. But even with less power the Allison Mustang was as fast or faster up to 10,000 feet. Why? I suspect the inlet scoop area had something to do with it. The Meredith effect helps, but it can't cancel out all of the radiator intake drag, and intake drag would be severest at low altitude.


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## Anonymous (Mar 24, 2005)

Iskandar Taib said:


> From what I understand, you never get as much thrust out of the Meredith Effect as it takes to overcome the drag of the radiator inlet. You can recover quite a bit, but it is never zero sum or better. In any case, there is only so much thrust that can be generated out of the heat produced by the engine. Make the inlet larger than is optimal, and you get more drag. The inlet size really does matter - the bigger it is, the bigger the drag. You make it just as big as it needs to be to cool the engine, no bigger. If the size didn't matter, then you could make the inlet as big as you wanted. This also applies to radials, by the way.
> 
> See:
> 
> ...



I have to say I'm dubious of the articles above (especially the first one). It claims the meredith effect was tweaked in the Cal Tech wind tunnels. The effect cannot really be evaluated in a static test of that nature. The engine must be running and the cooling system operational for the effect to occur. Certainly the shapes were studied in the wind tunnel, but if you research the P-51 development you will see these guys did most of their work utilizing mathematics rather than physical tests. Physical tests were conducted to confirm or refute the mathematical conclusions, but the process was not one of trial and error as implied.

Also, he seems to take Lee Attwoods story about the P-51 at face value. If you study the P-51, you will see that there was a scism of sorts between Atwood, who was an excutive on the P-51 team, and Edgar Schmued and Ed Horkey, who were the actual designers. Attwood tried every trick in the book to claim the P-51 design features were his ideas, but the truth seems to indicate otherwise. For instance, he did not make claim of having brought the info about the "Meredith Effect" to the team from the British researcher F.W. Merdith done in 1935 until after Edgar Schmueds death in (I think) 1986. But this is refuted by the fact that the heat pump design of the P-51 cooling system had been on the table at NAA for years within Schumed's design team, long before Atwood returned from his sales trip to England. 

It is kind of telling that Atwood waited until after Schmued's death to start making his claims as "father of the P-51". Ed Horkey has disputed Atwoods claims from the moment he made them. I tend to believe Schmued was the force behind the P-51's design, not Atwood. Atwood's baby was the B-25 - a plane sorely lacking in any kind of innovation. Schmued's babies, the P-51 and the F-86 were loaded with innovations, and Schmued was the one whose team was designing hypothetical fighters back when they were tasked to design the AT-6 trainer when Atwood got the B-25 project. Atwood had no such reputation of zeal to design a fighter - he was more of... a businessman/salesman. 

You can get a glimps into the whole debate by reading the following articles:

http://www.airspacemag.com/asm/mag/supp/jj99/Mustang.html

http://www.airspacemag.com/ASM/Mag/Index/1996/AS/wmtm.html

-------------------------------

I certainly agree there is an optimal size for the scoop. My understanding is that the radiator thrust system overcame 90-100% or more of the cooling system drag, depending on altitude (pressure), ambient temperature, and how hot the particular plane was setup to run. Sometimes they used a Prestone formulation that allowed temps up to 350 degrees F, but these were highly caustic and hard to handle for ground crews, and usually lower temp solutions were utilized.

From the Spitfire thread:

-----------------------------------

First off lets look at the Bf109 scoop/cooling design:







As you can see the radiators are indeed quite small. The "boundary layer diverter" mechanism was a fix for a problem discovered on the E models. The radiator is mounted to the bottom of the scoops and the boundary layer is allowed to flow through a space between the top side of the radiator and the wing. This helps to avoid injestion of the turbulent boundary layer which makes the radiators more efficient than they would be if there were larger but no space was provided for the boundary layer. Because the boundary layer has to make a significant turn upward to follow the diverter, this is only partially effective and at high speeds as the boundary layer gets thicker and has more mass and as pressure builds in the scoop, the boundary layer still lifts off the bottom of the wing and around the scoop entirely resulting in the "gulping" effect. The boundary layer diverter design helps to get a little more efficiency out of the small scoops of the 109 but it hardly "solves" the issue. There is no significant thrust generated for a number of reasons which I'll cover further down.

Now lets look at the P-51 radiator-thrust design:






First off, as is quite apparent, the P-51 radiator is HUGE compared to the two scoop radiators of the Bf109. Furthmore, the radiator has three to four times frontal area of both Bf109 radiators combine, which makes it inherantly more efficient for transfering heat.

Next, lets consider the boundary layer diversion method. On the P-51 the scoop is spaced more than an inch and a half away from the bottom surface of the wing (this varied a bit through different models). This means the boundary layer misses the scoop inlet entirely and encounters no obstruction that could rip it away from the scoop inlet until it is well past the inlet. The problem is completely solved.

Now lets look at how the thrust system works. First high speed cold air enters the scoop and proceeds down a widening passage which acts as an expansion chamber. The expansion chamber futher cools the air, slows its velocity, and increases the pressure (I know this is counter-intuative but its true). Then the (relatively) slow moving air passes through the radiator grilling, which is designed in the form of little ">" shapped fins stretched over the tubeing to form a sort of one-way valve. The heated air then passes into a narrowing passage which acts as a compression chamber.

When air passes through the radiator it is heated unevenly. Air molecules which make contact with the radiator fin elements are super-heated. Those that pass mid-way between the elements are much less heated. These molecules exchange heat in the compression chamber. One hot molecule and one cold molecule take up less volume than two warm molecules (assuming the total heat energy level is the same). So the air in the compression chamber is being compressed by its momentum into the narrowing passage and at the same time it is expanding as the heat in the molecules is transfered from the hottest molecules to the cooler molecules.

Finally, the hot air is vented through the thrust nozzel, which is designed and regulated for pressure. This provides thrust. At medium-high to high speeds, the jet of air comming out the thrust nozzel is supersonic, which provides usable thrust beyond speeds where a prop is no longer able to provide much thrust. Not only that, but the stream of hot expanding air is directed right into the wake of the fuselage. This wake is where parasitic drag normally "sucks" the plane back, and is the biggest part of an airplane's drag. Just like a tracer bullet, the P-51's exhaust fills the vacuum wake and reduces drag.

The Bf109 cooling system lacks both an expansion chamber and a compression chamber. The cold boundary layer air is re-introduced into the radiator exhaust in the space behind the radiator, virtually eliminating the chances of producing much thrust from expanding air. The cooling flaps at the back of the scoops are not designed to sustain high pressures behind the radiator, nor to control the outflow to generate a supersonic thrust stream, they are there simply to regulate the radiators to prevent excess cooling, primarily in dives. And finally, the radiator exhaust does not flow into the fusealge wake to help cancel out the parasitic drag.

The 109 has no meaningful "Meredith effect" thrust!

Note: The "Meredith effect" explanation is incomplete, trying to attribute all of the advantage of the radiator design to the thrust generated. A good portion of the advantage was the projection of super-heated air into the vacuum wake of the fuselage, nullifying parasitic drag.

----------------------------

The Spitfire lacked the boundary layer diverter portion of the Bf109, so it may have generated more thrust than the Bf109, at the cost of injesting the boundary layer. To overcome the cooling inefficiency caused by boundary layer injestion, the British simply made the scoops and radiators larger and larger. But without the expansion chamber, compression chamber, and pressure nozzel, not much thrust would be developed. And because the radiators don't feed into the vacuum wake behind the fuselage, the reduction in parasitic drag is minimal.

Finally, I'd also point out that the Mossie ustilized the Meredith effect, and it did so rather effeiciently - but only at two speeds (one being maximum)where the cooling flow was just right.

Also the Japanese Zero utilized the effect, and the this was stolen and transfered to the later model Corsairs. But the effect on these planes was much less (1/3rd?) than on the P-51. I'm not really sure how it was done.

=S=

Lunatic


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## DaveB.inVa (Mar 24, 2005)

The Baugher site has the Allison in the A-36 rated at 1325hp at 3000' and the P-51B rated at either 1400 or 1450 hp depending on the engine. Not that great of a difference.

Im not so sure about them being built stronger either. Read a little and youll find that the Allison was regarded as being a bit stronger than the Merlin. In fact Diego Red's connecting rods are from an Allison.

There are plenty of other variables in there as well besides the scoop, how about weight and for instance as RG mentioned the prop.


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## Anonymous (Mar 25, 2005)

I don't know if the Allisons were stronger than the Merlins or not. However, I do know the Packard Merlins were stronger than their Rolls Royce cousins. They were made to stricter tolerances using better materials and the cores were cleaned up more. RR was under tremendous production pressure and lacked the materials and time that Packard enjoyed.

=S=

Lunatic


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## Iskandar Taib (Mar 25, 2005)

DaveB.inVa said:


> The Baugher site has the Allison in the A-36 rated at 1325hp at 3000' and the P-51B rated at either 1400 or 1450 hp depending on the engine. Not that great of a difference.



750 to 1025 HP is fairly significant. 



> Im not so sure about them being built stronger either. Read a little and youll find that the Allison was regarded as being a bit stronger than the Merlin. In fact Diego Red's connecting rods are from an Allison.



Yup! But those were just the rods, and the rods were from a late model Allison. And Allisons aren't used in racing - if the basic engine was good enough, you could hop it up like they hop up the Merlins. 

This makes for VERY interesting reading:

http://www.supercoolprops.com/ARTICLES/gwhitegearheads.htm
http://www.supercoolprops.com/ARTICLES/gwhite_reno.htm



> There are plenty of other variables in there as well besides the scoop, how about weight and for instance as RG mentioned the prop.



Weight, yes. I don't think there was much of a difference in the rest of the fuselage between the A and B (the A had its air intake on the top, like a P-40 did, because that's how the Allison was built), don't think there was much of a difference, if any, in the wings. Not sure how the prop would have mattered - they used a four blader on the B because the Merlin's power needed to be dissipated, so using the A's prop wouldn't have made it any faster, I don't think.


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## KraziKanuK (Mar 25, 2005)

Iskandar Taib,

the Allison is used for in modern racing. There is an Unlimited hydro (U-3) that uses a dual turbo charged Allison. It has even won some races against the T-55 engined boats.


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## DaveB.inVa (Mar 26, 2005)

Iskandar Taib said:


> DaveB.inVa said:
> 
> 
> > The Baugher site has the Allison in the A-36 rated at 1325hp at 3000' and the P-51B rated at either 1400 or 1450 hp depending on the engine. Not that great of a difference.
> ...




Yeah 750 to 1025hp is significant... but 75 to 125 isnt. I think you've injected a couple zeros into your calculation.


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## the lancaster kicks ass (Mar 27, 2005)

or just miss-calculated............


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## DerAdlerIstGelandet (Mar 28, 2005)

seriously miss-calculated


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## the lancaster kicks ass (Mar 28, 2005)

dude i have to do 6+5 on the calculator, to me what he did was complicated maths, and i'm in top set maths at GCSE level!!


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## DerAdlerIstGelandet (Mar 28, 2005)

Im just keeping the convo going wiht what I said. I dont have anything to add to what they are saying.


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## CharlesBronson (Mar 29, 2005)

North American A-36A Invader
514th FS, 27FBG, 12th AF USAAF
Tunisia, 1943


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## the lancaster kicks ass (Mar 30, 2005)

very nice..........


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## DerAdlerIstGelandet (Mar 30, 2005)

Yeah good drawing.


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## stug3 (Jun 8, 2013)

Bump for a very informative thread from the past about my favorite plane.


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## GregP (Jun 9, 2013)

We built one for Tom Friedken at Chino (Steve Hinton's Fighter Rebuilders). It's at his ranch in Texas. Flies great and the dive brakes work just fine.


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