Lee Atwood on the P-51, pt 1

[HoHun]

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)

[Karnak]

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.

[Crumpp]

"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...



Crumpp

[Charon]

>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

[J_A_B]

"Any other details ? What outlet door position was optimal for high speed level flight ?"

Well, the optimal setting is "automatic"  


J_A_B

[VooDoo]

"Automatic" means that door will be automatically set to the best position. But what position is best ?

[HoHun]

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)

[niklas]

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

[VO101_Isegrim]

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.

[Crumpp]

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

[Charon]

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

[Charge]

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+

[HoHun]

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)

[GRUNHERZ]

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.

[HoHun]

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)