Ethan - question of Drag comparisons between 109K and P-51D

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Perhaps it might've been a good idea if the different models of the Bf 109 are not lumped together. for example, it looks like the Bf 109F-4 have had the 'high speed Cd' (= Cd0?) of 0.023, as listed at the lower left corners of the tables from here: link
 
I posted that before Is
I posted that before I saw this: Kurfürst - Kurz-Betriebsanleitung für Flugzeugführer und Bodenpersonal für GM 1-Anlagen in Bf 109 G.
A friend who studying German told me it has difficulties with installation,must be activated above 8000m which is much higher than DB605's critical altitude,also has limited time of use (especially in summer).I don't know if that is very successful as high altitude interceptor.
 
Hi Ethan,
The GM-1 is used above the critical altitude exactly in purpose. Its job there is to raise the power back up to near its rating at max height - when it is actually above that level. Exactly the same technique was used in the BMW801 engine fighters, only used above rated altitude. For various reasons which I wont go into here, the German engines were thermally loaded near their maximums - so usually the officially used boosting systems such as GM-1 and MW-50 were used only well above, or well below rated height.

So in other words if you ask if GM-1 was useful for interception, no it was not in the climb useful, MW-50 was used for climbing, and if GM-1 was used depends entirely on at which altitude the engagement occurred at.

Each use was supposed to be recorded by the pilot, and unfortunately the Luftwaffe squadron records are not yet in my possession, so I cannot comment if the official "rules" were actually adhered to or not by the pilot.

Unfortunately Kurfurst have not dated their document. Which makes it a lot less useful- maybe they will add that sometime.
 
Tomo - I can't read German and can not comment on the value presented. For example, is it the value of the wing contribution? is it a wind tunnel model that is 'clean', as in a model with no gaps, antenna, etc? or is it a full airframe with gun ports and control surfaces taped? or a full airframe with no treatment including standard camo?

As an Illustration, the combination of standard surface finish, including filling/priming and painting the first 50% of the Mustang wing, NMF fuselage with flush rivets, very aerodynamically smooth canopy, sealed ailerons, etc contributed a value of .0019 CD which was constant throughout the speed range independent of RN.

What RN? or what airspeed and density altitude?
 
The data sheet is about the 109F-4, so that would've been the Cd0 of the whole, service-worthy aircraft.
 
tomo pauk said:
The data sheet is about the 109F-4, so that would've been the Cd0 of the whole, service-worthy aircraft.
Click to expand...
Not unless it so states... very few aircraft were tested that way unless trying to make comparisons with wind tunnel models. NACA did so for the Mustang, the F4U, F4F and P-63. I think also the P-40.

NAA went a step farther, in conjunction with NACA, to test a P-51B without prop to perform dive tests to compare with wind tunnel data up to .75 M
 
I assume a propellerless dive test was done using an unmanned model? Because I know I'd be hard pressed to volunteer for that flight.

Seriously, to satisfy the curiosity of the layman (me) how did they actually do a test like that?
 
Swampyankee has it correctly. The P-51B-1 without prop was equipped with an array of flight instruments to record velocity, inertial accelerations, pressure distributions. It was towed to altitude and tow rope disconnected. The Test pilot maintained a 1 G dive while the instruments were recording. To account for 'real world' effects of non perfect manufacturing processes leading to more friction on airframe surface, the surface of the wind tunnel model was altered with fine dust and very fine sandpaper.

The agreement between the test dives (to M=0.75) CD and the wind tunnel CD was very close.

The Mustang was towed to altitude by a P-61, flown straight and level while still under tow to gather airspeed vs altitude at a fixed known velocity to set the baseline for RN - at different altitudes as RN varies with density, then tow cable disconnected and entered the dive.
 
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Thanks both of you, I'm going through the pdf, good stuff, much of the discussion admittedly is over my head but still fascinating. I haven't gotten too far yet but will be interested to see any mention of the glide characteristics of the Mustang. I've read tons on the F-86 and its ability to glide, curious as to how its ancestor did in the same area.
 
Wings/Airpower did an article speculating on how to make the 109 a 400mph class fighter with disrupting the production line. The listed straight forward items like eliminate the mass balance horns, retract the tail wheel, fully enclose the undercarriage, bare metal, basically an aerodynamic clean up. If i can find the article, i'll post.
 

Granted, some of the marks of the Bf 109 were not as streamilned as another, but then there was plenty of 109s that easily topped 400 mph out from the box. Retractable tail wheel was present in many of 109s produced.
 
tomo pauk said:
Granted, some of the marks of the Bf 109 were not as streamilned as another, but then there was plenty of 109s that easily topped 400 mph out from the box. Retractable tail wheel was present in many of 109s produced.
Click to expand...

I would expect that power and altitude would overcome some of the drag handicap. Differing conditions can lead to comparing apples and picked cherries.
 
I know there are a lot of people who think that nobody could do synching anything better than the Germans. I think a bit better examination of the data show that this was not the case. Certainly, there were some very good German aerodynamicists, but I think there is more than enough evidence to show they were not, on average, significantly better than those from any other country with a major aero industry.

Witness, for example, how many German aircraft used NACA airfoils.
 
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I think they were pretty good, but not necessarily any better than anyone else.

Their planes were excellent and had many very good characteristics for their class. So did most military planes of the day. If a plane did NOT have overall good characteristics, it usually didn't make production.

If I am not mistaken, the Bf 109 and Spitfire swapped the title of best ETO fighter back and forth throughout the war, until very late in the war. In the end, the Bf 109K might or might not have been as good as a late-model Spitfire, but it certainly wasn't an opponent to be taken lightly if the 109 pilot(s) was(were) any good at all. I'm somewhat partial to the Spitfire, but the late BF 109 was not a second-tier fighter in any meaningful way.

Had the Allies and Axis swapped major warplane designs, I bet neither side would have been overly disappointed except for the lack of long-range German heavy bomber types produced. Sure, they BUILT a few, but never put them into production. And I'm not so sure any of their heavy prototypes were as good as the B-29 / B-24 / B-17 / Lancaster / Halifax group were.

But with fighters, both sides had some good ones. One area the Bf 109 might have been a major disappointment would be range. But the British got along with the Spitfire that didn't have all that much more range, so perhaps it would have been OK, at least for the British. The U.S.A. could NOT have escorted heavies to Berlin and back with Bf 109s, that's for sure.

Maybe the Bf 109Z? I've never seen a range projection for it, and the prototype was damaged before it flew. Ya' never know, might have been a good one. Maybe not.

 
In going over the NACA report posted by swampyankee (Thanks BTW - Good stuff) I noticed a lot of attention at the end paid to "dust" on the windshield, nose and leading edges, I assume this was done to simulate an aircraft that had some use on it and was no longer "factory fresh"?
 
Active aircraft don't stay "fresh" for long in the field, especially when filters are slow coming from the supply chain. When they sit outside, planes get a bit tacky pretty quickly, and would likely be "war weary" within maybe 3 - 6 months, depending on use.

Hangared, and with perfect maintenance, they last a LOT longer. But they didn't GET that. Sitting behind another plane taking off from dirt would wear the windscreen pretty quickly. Also flying low over a battlefield where artillery was impacting. And definitely a volcano, which DID happen in the war.

The performance numbers we see in the "performance section" are for factory-fresh aircraft flown by experienced test pilots. Nobody else got that performance for all that long ... they got a bit slower and less responsive as time went on. Never got to be slouches, but DID degrade, absolutely. Why do you think a B-17 formation cruised at 170 mph? They cruised with the slowest planes, at the least fuel consumption! A formation is no faster than it's slowest member who is stretching range to make the mission.
 

The objective of the Tests were to determine whether Wind Tunnel testing correlated to real world measurements on a production Mustang, and by inference other production aircraft. The wind tunnel model was made as smooth as possible to obtain minimum CDo for each of the components save friction and surface imperfections.

In an attempt to correlate with real airframe fine dust and sandpaper were applied to the leading edges of the wing of the wind tunnel model to approximate (by RN scale) the real world surface conditions of the dive test P-51B.

The Wind tunnel testing also derived RN vs CL, an important Delta to level flight CL as it increased with altitude. The dive tests at .755 M also produced validation for the Compressibility Correction factors which must be applied to the Parasite Drag build up from low speed/low RN wind tunnel values.

The two major factors in Mustang performance were a.) Lowest CDo of all the major fighters, unmatched until the YP-80 and Me 262, due to the strict adherence of second degree curve/conical sections for the fuselage and the low drag wing, and b.) the happy circumstance of high M divergence of the NACA/NAA 45-100 Low Drag airfoil with laminar flow cross section pushing the max T/C to 45% in contrast with all conventional airfoils at ~25% T/C for max thickness.

The result was the lowest CDm/CDo factor due to Drag Rise as function of Mach no. This factor was the dominant reason why P-38s basically hit a brick wall at .65 M.
 
Thanks both of you. I see where I need to polish up on my reading comprehension skills though. I was looking at the photos of the dust on the leading edges of the P-51 and confused it with the wind tunnel model. I totally missed in the narrative earlier where they talked about the P-51B getting dusty on takeoff after the paired P-61/51 got past the oiled part of the dry lake they were using as a runway. Then I see where they lacquered the model to simulate that, my mistake and my apologies.

No to mention the tow rope broke on the fourth flight and the Mustang was a washout. Man, Mr. Nissen must have been one hell of a pilot.

greg - Yup, good stuff, not to belittle your post but yes, I agree, kinda' knew that already but appreciate your answer. I did grow up on an airfield after all.

drgondog - Your explanation is much appreciated, now I must admit, I "get" it as much as a guy who does not have an aeronautical degree can. Which means I was with you pretty much all the way, things got a little hazy at times but with the help of google I understand the gist of the whole issue. And make no mistake, I am very grateful you took the time to explain it, I find it fascinating and informative, thank you.

Did this for a living didn't you?
 
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