Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: MiloMorai on August 12, 2004, 09:47:26 AM
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Commentary by Lee Atwood.
What made the P-51 Mustang the fastest fighter of World War II (before the German jet came along)? The answer depends on whom you ask. The editors at Air & Space/Smithsonian asked the late Lee Atwood, vice president at North American Aviation when the fighter was born, and the question started an interesting correspondence. Atwood's explanation of what gave the Mustang the edge over the Spitfire and Curtiss P-40 was the design of its cooling system, especially the radiator duct's variable exit.
In an address to the Yorkshire Air Museum in June 1998, Atwood described the effect of the design on the Mustang's performance. An excerpt from his speech follows, along with drawings he made to illustrate his point to the editors.
In 1940 we had a young, energetic, and first-class engineering department with competence in aerodynamics, structures, materials, and thermal technologies as developed up to that time, and the factory had a nucleus of expert machine shop, tooling, and production personnel. We gave the Mustang design credit to Edgar Schmued who led the design room effort and brought the components together under the direction of Raymond Rice, who succeeded me as a chief engineer, and the technical specialists.
All these and many others contributed significantly to the project, including Colonel, now General (Retired), Mark E. Bradley who directed the installation of the 85-gallon fuselage tank. He then demonstrated that the longitudinal instability created by this weight behind the pilot could be managed by the combat pilots, and the effective endurance could be increased by some two hours.
In considering the speed performance of the Mustang, which is really its primary advantage and distinction, it is necessary to adjust one's thinking and point of reference to a rather early period in the science of aerodynamics. In the 1930s, there was no jet propulsion, and by any measure of comparison, the technical resources, personnel employed, test equipment and financial expenditures were really insignificant when compared to the aerospace establishment of today. Of course, the basics were there--which involved derivations of Newton's laws and Bernoulli's hydraulic principles--and aero sciences had been basically defined by Prandl, von Karman, and many other mathematical and scientific authorities, but applications to actual aircraft were relatively crude and empirical. Wind tunnel models were the primary proving element in design, and there were still many elements of such testing that had to be estimated or extrapolated with opinion and hope.
In these circumstances it is not very surprising that, among these early practitioners of aeronautical engineering, there were discontinuities of information and differences of opinion on various fine points in the application of general aerodynamic science, as then known, to actual airplane design. This was most apparent in one of the critical aspects of airplane design during the period of reciprocating engines and propeller-driven airplanes. The liquid-cooled designs favored in England and Germany--and also used in the United States and other countries--were generally considered of lower drag because of their in-line cylinder configuration. Air-cooled engines were generally of radial design, with all cylinders facing the cooling air stream, and the diameter was considerably larger.
The well-known radiator became the automobile standard early on, and everyone in the pre-war era had various experiences with these installations and their belt-driven fans. The common experience usually involved adequate cooling at cruising speeds, with frequent over-heating on mountain grades or slow traffic, and the fans were not always adequate to control the temperature. Generally, most people had an occasional bad experience with an overheated engine.
Airplane radiators had a lot of the same troubles, and while separate cooling fans were not seriously considered, ground cooling from propeller circulation alone was frequently inadequate. Basically, the radiators were designed to cool the engines at full power in a climb--which was usually something like half the maximum possible level flight speed with the same power--so at high speed, the cooling capacity was much more than needed.
Now it is clear that we were then quite sure that, as in an automobile, there was no reasonable dynamic use for the warm air discharged from a radiator, and a low and medium speeds, up to say 200 miles per hour, that was quite true. The temperature rise was small, and the expansion was correspondingly modest, and heat energy recovery was insignificant.
However, as engine power increased and better aerodynamic shapes were developed in monoplane designs, we were all slow to realize that, with a normally ducted radiator at high speed, we had at our disposal a really remarkable air pump.
This air pump, like all pumps, had three elements--a compressor stage, a metering or valving stage (radiator core), and a discharge function through an air outlet. This began to be a considerable pumping action as speeds approached 300 miles per hour--and at 400 miles per hour, it had a large potential and could be a considerable fraction of the airplane's total power equation, since the pumping pressure increases as the square of the speed. To make this automatic pump effective, only one thing was required, and that was to choke the outlet enough to keep the pressurized airflow through the radiator just adequate for cooling and to discharge this compressed air at the highest speed possible.
This intuitively easy to follow and was also logical from a general streamlined design point of view--which all designers tried to follow as a matter of course. The potential magnitude of this effect was more difficult to appreciate, however, and since little or no data were available, these possibilities were overlooked in most cases.
In the case of the Mustang, the air duct pumping system at full speed at 25,000 feet was processing some 500 cubic feet of air per second, and discharge speed of the outlet was between 500 and 600 feet per second relative to the airplane. This air jet counteracted much of the radiator drag and had the effect of offsetting most of the total cooling drag. To offer some approximate numbers, the full power propeller thrust was about 1,000 pounds and the radiator drag (gross) was about 400 pounds, but the momentum recovery was some 350 pounds of compensating thrust--for a net cooling drag of only some 3% of the thrust of the propeller.
This air discharge had what can actually be called a regenerative effect. Maximum aircraft speed is the point where the line of power available, created in the engine and delivered by the propeller, crosses the line of power required to propel the plane through the air. Since the propelling force of the pressurized air from the radiator discharge increases as the square of the speed, we have the favorable situation where the faster you fly the more help you are getting from this regenerative air pumping system.
Since this high speed phenomenon could not be effectively measured by regular wind tunnel model test, it was viewed as ephemeral or even imaginary by many in the engineering practice. Actually, it is quite real and has a close relationship with jet propulsion.
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Lee Atwood on the P-51, pt 2
Regarding the Mustang, I have always referred to the work of F. W. Meredith of the RAE, whose report (RAE No. 1683) of August 1935, greatly influenced me as chief engineer for North American Aviation to offer the British Purchasing Commission the ducted radiator design configuration in 1940. That report showed how the momentum loss in the cooling radiator could be largely restored when excess cooling air was being forced through the radiator at high speed. As noted before, this involved closing the air exit enough to get a substantial back pressure behind the radiator which largely restored the momentum loss--which was quite large as described above. This was possible, in Meredith's words, because the outlet was "adjusted to suit the speed,o and back pressure was available accordingly.
Here again, while Meredith's analysis was coherent and mathematically instructive, he failed to convey the practical aspects through an example or two, although he did offer a chart showing drag reduction for various discharge area ratios and conditions. The point I am making was that his work was generally in unfamiliar mathematical terms and was poorly understood. In fact, in two cases I know about, it was described in terms of mild ridicule. In any case, some if not most of the designs of wartime aircraft, including the Spitfire, failed to get the full advantage of this available air pump.
It should be pointed out here that the controversy and misunderstanding of the Meredith Effect on the performance of the Mustang developed largely because it was essentially impossible to get a reasonable measure of the effect from wind tunnel models at the time. The mass flow and momentum could not be accurately measured on a scale model, and no large tunnels were fast enough--200 to 400 miles per hour--to get meaningful results.
It has been reported that Messerschmitt made extensive efforts to determine the reason for the low drag of the Mustang, but his wind tunnel measurements did not disclose the restoration of momentum to the radiator cooling air, and most probably could not have done so with the wind tunnel equipment available at the time.
At this point I would like to interpolate what is , to me, a most fascinating element in Meredith's 1935 report. As you may have noted, I have made no reference to the thermal element in the momentum recovery of the radiator cooling air and at the temperatures involved, the air expansion was relatively small and could be neglected. Real jet propulsion, however, involves fuel burning, and the velocity of the gases and heated air is greatly augmented by this high temperature.
In his report, undoubtedly independent of Whittle's jet engine work, Meredith suggests piping the engine exhaust heat and gases to discharge behind the radiator to heat the discharged air just as burning fuel would do. This would have increased the volume and velocity of the discharged air at the same back pressure and increased the favorable thrust force.
Of course, the thrust of the short stack exhausts had been recognized by Sir Stanley Hooker of Rolls-Royce in his book, NOT MUCH OF AN ENGINEER, and others, but Meredith's suggestion might have produced a much more powerful effect, but it involved complications and practical difficulties. As far as I can determine, it was never tried on any airplane.
This brings me to the Spitfire comparison, although that is probably a poor choice of words. That airplane was in a class by itself and at the top level of defense against the Luftwaffe in 1940, and was undoubtedly the most important defensive weapon in history. It was some 1,000 pounds lighter than the Mustang and was at the peak of interceptor efficiency and was essentially in classic conformity with the objectives of the RAF fighter command. It overmatched its opposition and was there when most needed.
In the cold illumination of hindsight, however, and probably for reasons I have outlined above, it missed the opportunity to restore much of the air flow momentum to the radiator cooling air and, with it, a possible speed increment of more than 20 miles per hour. The late Jeffrey Quill, Supermarine test pilot, describes the incorporation of the Meredith Effect in the Spitfire in his book, SPITFIRE, A TEST PILOT'S STORY, and that the radiators were enclosed in ducts under the wings. Here I would like to quote from an article "The Mustang Margin" I wrote for the AIR POWER HISTORY JOURNAL which involves some background and detail on the subject. It will, of course, be glad to try to answer any questions you may have at the end of my presentation.
"The most notable and probably the first application of the Meredith Effect was incorporated in the Supermarine Spitfire, one of the world's most successful airplanes. Over 20,000 were built in various models, but the Mark IX, with the Merlin -61 engine, was typical of the later wartime production, and a sketch of this model with detail of the radiator installation is shown. Two aspects of this design are significant. First, the radiator outlet has two positions--that is, fully open and partly closed--and cannot be progressively 'adjusted to suit the speed.' Second the inlet upper wall is a continuation of the lower surface of the wing and expands the duct cross section by rapidly curving upward.
"The first, the non-adjustable exit, of course, is a deviation from Meredith's dictum and precludes the progressive build-up of pressure behind the radiator with increasing speed. However, the second can only be judged in hindsight, from an airplane design point of view. The inlet seemed to be configured properly to recover the ram air pressure, and the first Mustang design had a similar entry opening. It was later apparent that the thin boundary layer of air flowing along the lower surface of the wing was progressively thickening ahead of the duct opening, and that the flow would break away at a point on the upward curve of the duct wall. While the resulting turbulent unsteady flow apparently did not create a serious vibration, it certainly reduced the efficiency of the radiator and prevented a more complete closure of the exit opening, which is necessary to develop the jet thrust. Very interestingly, the R.A.E. Subcommittee on Aerodynamics in 1936--in commenting on the Meredith and Capon reports--rather accurately predicted this problem: 'Experiments upon air-cooled engines in the 24-foot tunnel have shown that it is necessary to pay particular attention to the design of the entrance to cowlings and the cooling ducts in order to avoid loss of energy by the formation of eddies.' (Somewhat easier said than done at that time.)
"In the case of the Mustang, the duct volume was larger and flow instability more violent, creating an unacceptable vibration and rumble. Resourceful engineers at North American, working with wind tunnel models, overcame the problem by lowering the intake upper lip below the wing surface boundary layer, thus beginning a new upper duct surface. In this design, the flow expanded gradually as the duct velocity decreased, and the pressure at the radiator face was reasonably uniform. This permitted the appropriate closure of the exit with a temperature-controlled power actuator, and a minimum pressure drop across the radiator consistent with efficient radiator function and cooling demand.
"As a result, the cooling drag was estimated at only 3 percent of the total and used only something like 40 horsepower for cooling purposes. While the comparable power used for cooling by the Spitfire is not available to me, the measurements made by Rolls-Royce show a total power required for the same speed (400 mph) as 200 horsepower more for the Spitfire than for the Mustang.
"Records show the P-51D's speed was 437 mph and the Spitfire Mk IX speed was 405 mph. While the Spitfire had exposed tail wheel and other small differences from the Mustang, most of the speed difference was in the cooling drag. The Mark VIII with retracted tail wheel is rated at 414 mph at a somewhat higher altitude. Advanced models of both airplanes with higher performance were produced late in the war, but were not available in significant numbers before V-E Day, May 8, 1945.
"It seems that most other contemporary airplanes attempting to take advantage of the Meredith Effect failed for one reason or another to combine an efficient duct system with a properly designed and regulated exit-closing mechanism and did not develop the energy recovery inherent in the Meredith method. They generally used 10 percent or more of their power available at high speed to overcome cooling drag. A notable exception was the DeHavilland Mosquito multi-purpose plane with the same Rolls-Royce engines and which used a wing leading edge radiator mounting with a short and direct inlet duct. The controllable exit opening had a minimum area little more than half that of the Spitfire, and while it was a larger two-engine airplane, it had a speed of 425 mph.
"Since jet engines do not require cooling systems of the type described here, the subject has become moot and of little current importance. There was a time, however, when this rather insignificant subject made a critical difference."
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Very interesting.
Still, the Spitfire Mk. XIV had a top speed in the 440's, and the Ta-152 had a top speed in the 470's.
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To give an idea of how much the radiator affected performance, if you flew with the automatic system OFF and the outlet door closed, the P-51D's top speed dropped to like 410 MPH (and suffered horrible overheating problems as well).
While the late-model Spitfire and 152 had evolved from earlier designs, don't forget that the Mustang evolved too. Later production Mustangs had top speeds approaching 490 MPH.
J_A_B
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Originally posted by Fruda
Very interesting.
Still, the Spitfire Mk. XIV had a top speed in the 440's, and the Ta-152 had a top speed in the 470's.
Keep in mind though, the P-51D's merlin engine was essentially the same as the Spitfire Mk. IX's merlin. They ran similar boost levels, and had similar power outputs. The Spit XIV, 190D-9, and 109G-10 all use substantially more horsepower to achieve similar airspeeds.
The postwar P-51H had a more powerful 2200hp water injected Merlin, and was capable of 444 mph at 5000 feet, and 487 mph at 25,000 feet
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>However, as engine power increased and better aerodynamic shapes were developed in monoplane designs, we were all slow to realize that, with a normally ducted radiator at high speed, we had at our disposal a really remarkable air pump.
Well, I'd say that shows lack a regrettable of appreciation of the research done elsewhere. Hugo Junkers' "jet radiator" patent showing how to exploit that "really remarkable air pump" dates from 1915, and the jet radiator was already implemented in his WW1 monoplanes.
>Since this high speed phenomenon could not be effectively measured by regular wind tunnel model test, it was viewed as ephemeral or even imaginary by many in the engineering practice.
In 1937, Willy Messerschmitt made a speech at a German aviation conference, addressing "The Problems of High-Speed Flight":
"The introduction of the jet radiator by Junkers made it possible to drastically reduce the drag of the radiation system. The adjustable jet radiator has the advantage of providing constant cooling capability independend of the speed, thus rendering the power required for radiation purposes constant. The most beautiful implementation of the jet radiator appears to be the installation in the aircraft wing."
(Quoted from "Flugzeug Classic" 6/2004.)
>It has been reported that Messerschmitt made extensive efforts to determine the reason for the low drag of the Mustang, but his wind tunnel measurements did not disclose the restoration of momentum to the radiator cooling air, and most probably could not have done so with the wind tunnel equipment available at the time.
I'm afraid that in the light of the 1937 Messerschmitt speech, this report has to be considered as "slightly" inaccurate :-)
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A prototype Tempest first flew on 24 February, 1943, and by September it had been pushed to 472 mph in level flight,
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While not as fast as the P-51, the Mosquito also had a net thrust gain from it's radiators. That feature was not unique to the Mustang.
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Originally posted by MiloMorai
Lee Atwood on the P-51, pt 2
"A notable exception was the DeHavilland Mosquito multi-purpose plane with the same Rolls-Royce engines and which used a wing leading edge radiator mounting with a short and direct inlet duct. The controllable exit opening had a minimum area little more than half that of the Spitfire, and while it was a larger two-engine airplane, it had a speed of 425 mph.
I wish we had a 425 MPH Mosquito.......
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The most beautiful implementation of the jet radiator appears to be the installation in the aircraft wing
Yep, I was just going to post the same thing as apparently messerscmit said.
IMO the most ideal radiator would be a wing leading edge mounted design, like mossie or maybe even an in wing like in the P39 that utilized the pump effect. I'd have two of them, one in each wing and also place the oil coolers in there too. The two cooling sytems would be independant and the pilot would be able to isolate either one from the cockpit in the cae of battle damage so he could have a limp home safety feature.
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Heyas:
HoHun and I posted a very little bit about this some months ago. I said at the time I'd never seen any reference to the Mossie rads adding speed, though I'd seen positive comments re: effect on lift. Of course, about 6 weeks later that, I read that in 1938, when the Mossie was just a gleam in de Havilland's eye, the radiator setup was described to the Air Ministry as adding more to speed than to drag.
So there.
Cheers,
Scherf
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To give an idea of how much the radiator affected performance, if you flew with the automatic system OFF and the outlet door closed, the P-51D's top speed dropped to like 410 MPH (and suffered horrible overheating problems as well).
Any other details ? What outlet door position was optimal for high speed level flight ?
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Not the radiator thrust topic!
Wishful thinking on the part of both Spitfire and 109 fans. Come on, the physical size of the "thrust" producing chamber on both a/c was so small that the radiators on those planes produced way more drag than any "thrust".
Only the P51 has a hope of making the "thrust" from the radiator argument. Nice article in Jan. '99 "Fighters of WWII Special Edition" in Aviation History Magazine on this topic.
It's an extremely contriversial topic among aeronautical engineers with supporters on both sides of the fence. It is niether proven nor disproven for the P51.
The P51 did have a LARGE radiator chamber fed by a small opening and ejected out a smaller opening in the rear. As I understand it, pressure differences created "thrust".
I agree with the middle of the road argument that it's "thrust" was neglible but it greatly reduced overall drag of the radiator by removing it from hanging out in the slipstream.
Either way the radiator is the key to the P51's great aerodynamics.
The Luftwaffe experimented with the same "thrust" producing design on the FW-190C in the "Hirth turbo blower to advantage". Notice the exhaust extensions and outlet to contribute to the "thrust".
http://www.asd05.com/default_zone/fr/html/page-1285.html
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Good luck Crumpp. Here's another thread with the same discussion in much greater detail (pg. 8 onwards): http://www.hitechcreations.com/forums/showthread.php?threadid=100221&referrerid=5405
BTW, when this topic reappeared I sent in an E-mail to Aerosud in S. Africa that did some radiator redesign work for a P-51 project there that suggested some modern analysis of the P-51 cooling system. I don’t know how much they got into the duct geometry vs. the specific radiator itself, but if they bother to respond I'll post it. These are actual aerospace engineers who do a lot of inlet geometry type work.
Also, I came across a reference for a new P-51 book months ago that suggests an accidental application of “area rule” during the design was the primary reason for the p-51’s atypical performance. A quick search failed to relocate the blurb, but maybe somebody else has happened across it.
Charon
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Good luck Crumpp.
:cool:
Not even gonna touch this one. All anybody can offer at this point is an opinion.
I would love to see the reply Charon!
Thanks
Crumpp
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Hi Charon,
>I don’t know how much they got into the duct geometry vs. the specific radiator itself, but if they bother to respond I'll post it.
Thanks, that would certainly be interesting!
>Also, I came across a reference for a new P-51 book months ago that suggests an accidental application of “area rule” during the design was the primary reason for the p-51’s atypical performance.
Hm, I guess something was lost in translation there. The area rule actually deals with supersonic airflow:
http://en.wikipedia.org/wiki/Area_rule
Regards,
Henning (HoHun)
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Crump,
Are there actually Spitfire and 109 fans that claim their pet fighters got a net thrust gain from their radiators?
I thought it was common knowledge in this hoppy, especially in the Spitfire's case, that their radiators were horribly draggy and one of their worst features.
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"The most notable and probably the first application of the Meredith Effect was incorporated in the Supermarine Spitfire, one of the world's most successful airplanes. Over 20,000 were built in various models, but the Mark IX, with the Merlin -61 engine, was typical of the later wartime production, and a sketch of this model with detail of the radiator installation is shown. Two aspects of this design are significant. First, the radiator outlet has two positions--that is, fully open and partly closed--and cannot be progressively 'adjusted to suit the speed.' Second the inlet upper wall is a continuation of the lower surface of the wing and expands the duct cross section by rapidly curving upward.
From above. Looks like someone laying the foundation for a jet pack asisted Spit.
And you know that 109's will follow because it's radiator was adjustable with speed.
Then I will have to pipe up with something on the 190...
:eek:
Crumpp
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>Also, I came across a reference for a new P-51 book months ago that suggests an accidental application of “area rule” during the design was the primary reason for the p-51’s atypical performance.
Hm, I guess something was lost in translation there. The area rule actually deals with supersonic airflow:
[edit: Your area rule link does note: Even at high subsonic speeds, local supersonic flow can develop in areas where the flow accelerates around the aircraft body and wings due to the Bernoulli effect, FWIW]
After much searching (and frankly I don't know how I came across this particular review in the firstplace), I found the reference. I don't know about the accuracy of the reviewer, but the book seems to get good reviews for accuracy (but dry reading) in some of the many other reviews I came across in the search.
P-51 Mustang: Development of the Long-Range Escort Fighter
Paul Ludwig
$56 US
Here's the part about area rule:
An interesting technical revelation is in Paul’s telling of the reasons for the re-engined Mustang being faster in level flight than the Spitfire Mk.V equipped with the same Merlin 28 engine. We’ve heard a lot over the years about the Laminar Flow Airfoil. More recently we’ve been hearing quite a bit about the Meredith Effect producing “jet-thrust” from the radiator exit. Now Paul shows us that the major reason for the Mustang’s lower drag lies in the fact that from their placement of the wing, cockpit canopy and radiator housing relative to one another, North American unwittingly applied the Whitcomb Area Rule to their airplane years before Richard Whitcomb of the NACA defined the effect. They struck it lucky!
http://www.ipms-seattle.org/newsletters/2003August.pdf
The review is on page 11.
Charon
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"Any other details ? What outlet door position was optimal for high speed level flight ?"
Well, the optimal setting is "automatic" :)
J_A_B
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"Automatic" means that door will be automatically set to the best position. But what position is best ?
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Hi Charon,
[edit: Your area rule link does note: Even at high subsonic speeds, local supersonic flow can develop in areas where the flow accelerates around the aircraft body and wings due to the Bernoulli effect, FWIW]
Well, we're talking about compressiblity territory here which the Mustang won't reach in level flight. (Compressiblity makes itself felt at about Mach 0.75 for the Mustang, while it tops out at around Mach 0.60.)
>"North American unwittingly applied the Whitcomb Area Rule to their airplane years before Richard Whitcomb of the NACA defined the effect."
No way this would have made the Mustang faster. The first generation jet fighters were designed in ignorance of the area rule, too, and it didn't make the slightest difference. The Me 262, capable of reaching Mach 0.73 in level flight, actually out-performed the calculations (while the F-102 fell far below the predictions until fixed by application of Whitcomb's discovery).
Regards,
Henning (HoHun)
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Whitecomb seems to use mainly german research results for his own reputation.
From the book "Die Deutsche Luftfahrtforschung", P. 172.
"K.A. Kawalki, (DVL Berlin Adlershof), proposed 1940 fast-flying airfoils, which had as characteristics elliptic nose, far back positioned thickness maximum (up to 50%) and "flat" top and bottom (symmetrical airfoils). With this design the flow speeds over the airfoil were significantly reduced. And while Kawalki just did research with subsonic speeds, the airfoils he found were IDENTICAL WITH THE PROPOSED OVERCRITCAL AIRFOILS FROM T. WHITECOMB in the USA at the Beginning of the 50ies. [...]
"B. Göthert and K.A. Kawalki report 1944 from measurments in transall. the report of allied combined intelligence objectives sub-comittees (CIOS) over Focke-Wulf let know, that 1944/45 all aspects of overcritical flight machnumbers, including the problems of wind tunnel simulation, were known to all industrial design teams. [...] . Klaus Oswatitsch citates in this "Gasdynamik" numerous theoretical works to the overall problematics, and the search for an overcritical airfoil with isentropic recompression, that means a fallback to subsonic speeds without shock was continued until the 70ies. Today, for practical reasons, they were left out. The first aircraft flying with an overcritical airfoil was thea Airbus 310
The arearule was vom decovered 1943 from Otto Frenzl in an experiment at Junkers in Dessau, using a selfbuilded Transsonic canal, and was patented together with Heinrich Hertel and Werner Hempel.
[...]
Friedrich Keune and Klaus Oswatitsch succeeded at the beginning of the 50ies in proofing the area rule theroetically with the "Äquivalenzsatz", [...] while R.T. Whitecomb confirmed at this time the area rule experimentally.
niklas
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Originally posted by Karnak
Crump,
Are there actually Spitfire and 109 fans that claim their pet fighters got a net thrust gain from their radiators?
I don`t know if there were net thrust gain in the 109 (doubt there were any serious in the Mustang either), only actual testing can give definiteive answer to that.
Both the Spit`s and the 109E`s radiator is quite bad to get a net thrust gain. No variable inlet, large baggy radiators, and could not be even adjusted automatically to small outlet, and I have seen aerodynamic CAD-testing on the Spit`s radiator revealing boundary layer seperation at the inlet.
However the 109F/G/Ks radiator scheme has all the properties the author of the above article believes to be neccesary for the gain effect, including automatically adjustable radiator inlet and outlet flaps according to cooling needs (temp/speed dependant). It has a 'combustion chamber' after the small inlet takes the air in and allows it to expand while it warms up during passing the radiator. At high speeds the radiator outlet closes to about 40mm width, so it`s an effective discharger of the hot air, creating some thrust. And it was already out in 1940, so as others pointed out, such radiator design is by no means a Mustang speciality, altough it`s a very good radiator design, no doubt.
(http://109lair.hobbyvista.com/techref/systems/cooling/f_airflow.jpg)
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I knew about the 109's radiator set up. Nice diagram, though!.
I have to point out though that the chamber's physical size is rather small. Any "thrust" would be negliable and IMO wouldn't come anywhere near equalizing the drag of the radiator.
It just doesn't pass the "common sense" test.
I don't think the "Meredith Effect" was a major factor in any aircraft's performance. Radiators were draggy, bottom line. The P51's was just less draggy than other inline engined fighters.
IMO of course.
Crumpp
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I was just putting out something I came across in passing. I've not read the book, and likely won't be buying it, so I don't know how this is stated in the book and what it was in relation to as far as performance is concerned.
It would seem that the amount of time the P-51 spent at mach .7 or higher would be far short of that required to have any significant impact on operational range.
Charon
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To me it makes perfect sense.
A certain amount of COLD air with certain amount of momentum is let inside the radiator where it warms EXPANDING in volume and is let out from the rear part of radiator the exit being LARGER than the inlet negating much of the drag caused by the radiator. With a radiator of equal sized entry and exit the expansion of air is not very effective resulting in lots of drag. I'm not sure if it really can produce thrust enough to overcome the drag of the inlet but coming close to negate the drag of the inlet is good enough in my opinion.
SO the most important thing is to have a radiator, not big with plate area, but of deep design giving enough effective travel for air to heat as much as possible.
So the optimal relation of inlet and outlet area is probably achieved by measuring the best cooling in different altitudes and from that data make the automatic regulator which function suits the most temperatures where the a/c is flown.
To me it seems that Messerchmitt had the same idea, probably just failing to understand that in his design the boundary layer separation was not done properly as it was done in P51 by moving the intake more away from the fuselage.
-C+
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Hi Charge,
>To me it makes perfect sense.
It does :-) It's just that the mis-application of the technical term "area rule" caused some confusion.
>To me it seems that Messerchmitt had the same idea, probably just failing to understand that in his design the boundary layer separation was not done properly as it was done in P51 by moving the intake more away from the fuselage.
Actually, Messerschmitt designed a boundary layer bypass duct for his radiator, so he certainly understood the problem. The Me 109K-4 and the P-51B (faster than the P-51D) get virtually the same speed at that altitude where their engines yield identical power. As the Mustang is the larger and more capable aircraft, this suggest a slight aerodynamical advantage on its part, but it could be literally anything - no reason to suspect it was the radiator.
(And even if you do, it's a complex problem. For example, I could claim that the Me 109's wing radiator actually was superior to the P-51's belly radiator, but the Messerschmitt's less sophisticated oil cooler blew the advantage. No way we will ever be able to tell for certain, but a lot of fun to speculate about all the possibilities :-)
Regards,
Henning (HoHun)
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I always wondered how big a drag penalty that huge DB600 series oil cooler was on 109... But it does make the 109 look cool. :)
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Hi Grünherz,
>I always wondered how big a drag penalty that huge DB600 series oil cooler was on 109... But it does make the 109 look cool. :)
The Jumo-engined Bf 109C was cool, too - one photograph that's puzzling me is a "Bf 109C-2 of I/JG71 (later to become II/JG51) with Tigershark markings".
I knew the Tigershark design was pioneered by the Germans, but it already has the perfect modern Tigershark look and there is nothing crude about it like I would expect from first attempts at a new design.
In fact, that makes me suspect they design originated elsewhere, evolved from crude beginnings to the form that survived the decades, and only then was borrowed for the Me 109.
Maybe one day curiosity will make me go on a quest for the oldest Tigershark Design photo ever. No idea what that might bring up :-)
Regards,
Henning (HoHun)
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The early Jumo 109s are a pretty clean design except for the horizontal tail supports..
The 109E IMO was a design disaster...