Why was the BF109 so slow compared with the P51?

[DarrenW]

Aerodynamic refinements do count. The K-4 introduced the more stremlined HMG installation, wheel well covers, and reintroduced retractable tailwheel - all of that accounted for 20+ km/h more than 109Gs with DB 605ASM and 605D engines. The K-4 was upposed to do 710 km/h at 7.5 km with 1565 PS (441 mph at 24600 ft with 1543 HP), that is a ballpark with P-51D that used ~1500 HP to do the same.

Hmm, I don't know if 12 mph is such a monumental increase, given the extent of the refinements you mentioned. I'm also wondering if suspected poor quality control negated a lot of the possible performance improvement that these changes could bring. And wasn't that speed only an estimate, as no real-world test data exists for the K series? I'm fully prepared to be wrong about this.

 

[tomo pauk]

True. The fact that the three guns alone still enabled it to be a potent fighter is not in dispute, though. The canoe guns did affect handling and manoeuvrability, however; virtues that the F maintained, but were lost on subsequent variants.

What enabled the 109F4 (and less applicable to the F1 and F2) was that big & powerful engine was installed on smalll, light and reasonably well streamlined fighter.

Does the zero lift drag coefficient take into consideration the size of the object (like the wingspan) or is it just like a per meter kind of thing?

Is there a list of the drag coefficients for different aircraft by type?

The 109 had a lot of protrusions, tail wheel and open wheel wells and everything but it was such a small aircraft. That is why it had relatively low drag.

Wasn't until the ultra streamlined P-51 showed up that they arguably needed to focus more on reducing drag further.

The low drag wing of the P51 seems to have lost a little bit and lift because it doesn't turn all that well despite being pretty big, or maybe that's just a factor of weight

Cd0 does not take into consideration size of object.
List of Cd0 for different aircraft can be made from different sources, eg. the Cd0 for the 109F4 is/was 0.0232 per manufacturer data. 109E were 'dirtier', 109G5s were also 'dirtier'. British have an useful list, posted here, for many A/C. NACA reports have a lot of data, too. Germans were probably aware that Bf 109 will be over-shadowed by new Allied fighters, hence Bf 209 and 309 fighters, plus jets.
Lift-to-drag (L/D) ratio of the wing profile choosen for the P-51 wasn't greatest indeed, that combined with usually high weight didn't made it into a turning champion.

 

[DarrenW]

Does the zero lift drag coefficient take into consideration the size of the object (like the wingspan) or is it just like a per meter kind of thing?

Total drag (CD) - induced drag (CDi) = zero lift drag (CD0)

Both total drag and induced drag take wing area into account.

Equivalent flat plate area (or "drag area") is zero lift drag x wing area. Some say that this is a true representation of an aircraft's "foot print" and how much thrust is needed to overcome total drag.

 

[tomo pauk]

Hmm, I don't know if 12 mph is such a monumental increase, given the extent of the refinements you mentioned. I'm also wondering if suspected poor quality control negated a lot of the possible performance improvement that these changes could bring. And wasn't that speed only an estimate, as no real-world test data exists for the K series? I'm fully prepared to be wrong about this.

12 mph was certainly not a monumental increase, but it shows that improvement in streamlinign counts. The Bf 109K4 was still saddled with 3 radiators, the ram air intake is still 'outside' of airframe, the 2R1 wing profile was no longer competitive even if it was thin on the 109s. The 441 mph probably represented the best case scenario indeed.
There is a number of charts and tables for the K4 on the 'net, how good/bad (= based on the tests or manufacturers estimate) they are really is anyone's guess I suppose. 441 mph figure is from LW data sheet.

 

[swampyankee]

Does the zero lift drag coefficient take into consideration the size of the object (like the wingspan) or is it just like a per meter kind of thing?

Is there a list of the drag coefficients for different aircraft by type?

The 109 had a lot of protrusions, tail wheel and open wheel wells and everything but it was such a small aircraft. That is why it had relatively low drag.

Wasn't until the ultra streamlined P-51 showed up that they arguably needed to focus more on reducing drag further.

The low drag wing of the P51 seems to have lost a little bit and lift because it doesn't turn all that well despite being pretty big, or maybe that's just a factor of weight

Yes. It's a non-dimensional coefficient, so it provides a comparison that removes size differences.

If I remember, the P-51 had a higher wing loading.

 

[DarrenW]

but it shows that improvement in streamlinign counts.

Yes, off course streamlining counts but in my view the negative aerodynamic characteristics you mentioned above had a far greater impact on it's speed than the lack of wheel-well doors and the like. It just seems like a band aid attempt to overcome some truly poor aerodynamics by 1944 standards.

The added parasitic drag of the leading edge slats must have not helped its case either.

 

[Schweik]

Yes. It's a non-dimensional coefficient, so it provides a comparison that removes size differences.

If I remember, the P-51 had a higher wing loading.

It would at takeoff, but after climbing to altitude, forming up, and flying an hour or three to the target area maybe not.

 

[drgondog]

Does the zero lift drag coefficient take into consideration the size of the object (like the wingspan) or is it just like a per meter kind of thing?

The drag coefficients for a specific RN (see below) are individual component drag ("force) values observed in the wind tunnel as each individual component is isolated as a change, then divided by wing area. Only Wetted Area calculations are derived as Total Drag/Total surface area.

Is there a list of the drag coefficients for different aircraft by type?

There are for US aircraft in NACA reports for 1940 and 1945. The Brits complied one list that I know of about 1942-1943. I don't have similar documents for USSR, Germany or Japan

The 109 had a lot of protrusions, tail wheel and open wheel wells and everything but it was such a small aircraft. That is why it had relatively low drag.

The wing and radiator/cooling drag components were by far the most significant, but all the other factors combined were important.

Wasn't until the ultra streamlined P-51 showed up that they arguably needed to focus more on reducing drag further.

For a long time, the result of the low drag for the Mustang remained 'unexplained' - but it wasn't laminar flow. There Was some positive value of the shape of the airfoil after wake flow max T/C to trailing edge was scrutinized with trailing edge drag rakes, but the separated flow aft max CP was determined to be a factor of the relatively lower velocity gradient from LE to max T/C relative to say, the NACA 23016. The adverse pressure gradient was less 'malignant' until the flow was past max T/C ---------------> resulted in lower wake area in front of max t/c. This is why the Pressure Drag of the Mustang wing was lower. It is also why it performed so well in high speed dives.

The low drag wing of the P51 seems to have lost a little bit and lift because it doesn't turn all that well despite being pretty big, or maybe that's just a factor of weight

The CL for the Mustang wing and 109 and Fw 190 wing in level flight were all pretty close, but the LE slats for the 109 as stall approached were a definite benefit in medium speed/high G turns. Climb is strictly a function of Power Available - Power Required and Wing Loading. Obviously Drag and Weight are important factors.

First - the Comparison of minimum parasite/profile drag requires that you research the RN for which the drag values were extracted in the Wind tunnel.
Second - the RN is a function of Velocity, Kinematic Viscosity and Mean Aerodynamic Chord(based on the wing). E.G. the RN for a Bf 109G-6 at 300 mph at 25000 feet is totally different (lower because the m.a.c. of 109 wing is much less) from P-51 at same speed and altitude in a parallel run. RN=V(m.a.c.)/v where v = kinematic viscosity.

Third, Parasite/profile drag components have certain values that correlate with RN, (wing, fuselage, empennage, cockpit, carb duct, exhaust stacks, external radiator duct) and some that are constant with RN as they relate to pressure drag (gun ports, antenna, bomb rack, leaks/gaps, surface roughness). Also included in the Drag build up are the values of total drag introduced by changes of CL (as one must when altitude increases in order to maintain constant altitude in cruise).

Cooling Drag (internal losses in Radiator/Intercooling system), and prop vortex pressure drag are also independent of RN).

For any given flight profile all of the above factors are added together - then multiplied by the Compressibility factor which changes with increasing Mach No. (beginning somewhere above incompressible fluid region of approx. .3 Mach. THIS FACTOR is critical to comparisons between aircraft at high speeds. Bumps, slopes of windscreens, etc are important - but the WING is to core of this factor.... both airfoil shape and thickness play a role. E.G. The Spit wing was considerably 'thinner t/c ratio' than the Mustang and dived slightly faster without structural damage, but the Mustang wing with max t/c and quasi-wedge shaped airfoil reaching maximum value at ~37.2 % (vs 25-26% for most other WWII fighter) had both lower profile drag as well as delayed drag rise.

Last value added is Induced Drag which is added to all the Parasite/pressure drag elements corrected by compressibility factor.

 

[Peter Gunn]

I will devote a lot of time in the Apendices of my new book on this very subject:

Start with Power Available vs Power Required as the fundamental set of calculations once the drag parameters are derived (wind tunnel testing);

'The Wing is The Thing' to begin the discussion.
The achievement of (Close to) net zero drag for the internal cooling system is an assumption based on the composers (Horkey and Ashkenas) of the Performance Calculations contained in NA-5534 on the P-51B-1-NA airplane - for high speed Cooling Drag. In this assumption the Pressure and internal friction Drag compared to Thrust generated by the design often referred to as the Meredith Effect is specifically stated as "0 for high speed cruising at Critical Altitude" BUT the Delta CDp for 35,000 feet = .0004 and at 40,000 feet = .0010. In other words Net Zero Internal CDp for only a region at high speed in a narrow altitude envelope.

The Delta CDp for CLIMB condition = .0064 at ALL altitudes independent of Reynold Number.

The INITIAL base CDp for RN =1.8x10^^ for the Wing = .0074 (note: the same initial value for same wing at 2.0x10^^6 for the P-51D wing =.0070). This is a value that considers the profile drag of the wing immersed in a flow - INDEPENDENT of the friction drag.

This value is far below that of the same relative thickness wings of the Fw 190/F6F/F4U/P-47/Bf 109 using NACA 230xx airfoils at same RN.

The second significant base Delta CDp values for comparison, but less than the wing in importance, are the form drag totals for Fuselage, empennage, carb duct, cockpit enclosure build ups. If you wish to ask why, consider that the entire airframe Lines were developed using Descriptive geometry for which the contour from nose through cockpit enclosure was essentially an ascending shape area of low gradient change aft of the card intake duct (including Allison).

(Note that ALL Mustang variants placed the 'disruptor' (scoop, ducting for radiator/oil cooler, intercooler, and exhaust exit) aft of the CP of the wing and aft of the boundary layer separation from fuselage and wing.)



The next significant CDp factor was the Friction Drag Delta CD = .0008 + 'Gap' Drag Delta CD = .0004 = .0012 which is Constant for All RN - a Very low value speaking to the following: 1.) Extremely good production design co-ordinated with factory processes to achieve extremely good sheet butting, flush riveting, excellent jigs and fixture design, priming/sanding/painting of upper and lower wing surfaces through ~ 40% wing chord, and placement of important panels at or aft of that region.

These values for CDp were the very best in comparison with all the values I have seen presented for other aircraft.

Looking to Hoerner 14-4 (b.) Drag of the Wing ------------> the comparable Friction drag for paint irregularities, gaps, bumps, sheet edge and rivet losses @ RN=1.1X10^^7 (For ONLY the wing) ----------> .0035 (skin friction) plus .010 (gap/surface imperfections). Compare to .0008 plus .0004 for the P-51 values above. More than 10x Bf 109G total skin friction and surface imperfection components for the wing drag over P-51B drag.

Hoerner does not present the individual components of parasite drag as a function of RN like NAA. That said, the clues for his Parasite Drag as function of RN @ 4x10^^6 (Wind tunnel at Chalais-Meudon for either Bf 109E or F in 1941) = CDp .030 Without momentum drag of engine air intake of that of the tail wheel.

The comparable CDp for RN=4x10^^6 for the P-51B = .0190 -----------> ~ 63% of the Bf 109E

Comparing CDp for his calculation of .028 @RN=2^^7, the comparable P-51B CDp = .0155 ----------> 55% of the Bf 109E

In Chapter 14 he 'reverse calculated total drag based on T=D;
For his Calculated Total Drag (CDt) at Vmax (= 380mph at 22,000 feet) = 0.036 [includes base parasite/form drag, vortex drag + high speed cooling drag) multiplied by Mach No. correction +Induced drag. For the P-51B, the same CDt = 0.24 ------------> 66% of the Bf 109G

Last note - the High Speed/Low Drag 45-100 wing while loosely described as Laminar (not so much), was extremely low drag in two areas of discussion. First, the low velocity gradient from LE to max T/c at 37.2 % was much lower than equivalent thickness 23016 airfoil which delayed Mach drag rise and major CP shift, and second - the form drag of the wing was simply lower.

I HAVE not spent a lot of time looking at the Hoerner discussion, but the P-51B drag values for CDp and CDp1(combined gun ports, radio mast, gaps/leaks, friction, etc) are extracted from the P-51B Report, compared to P-51D and P-51H to check for unexplainable variances (none other than P-51H had lower wing parasite drag)

Postscript. The Cooling drag build up for climb (worst case) was .0064 for a combination of internal pressure drag/flow losses of the scoop/plenum/oil cooler/radiator/exit plenum and open exit gate. So, if we take Horkey and Ashkenas at their technical competency, then for Net Zero high speed Cooling drag, the CD to be recovered by incremental Meredith Effect jet thrust = .0064. Thus, if Jet THRUST = 0 and all internal drag components are not neutralized, the the total drag build up for RN=2x10^^7 = 0.31 --------> 86% of the Bf 109G-6 calculated total by Hoerner.

Take what you want, leave the rest.

Man, I really wish you'd actually explain this stuff with fact, formula and sources and not just these apocryphal stories, superficial speculation and SWAG's...

 

[Schweik]

Is there such a thing as a list of parasitic / profile drag for a variety of major fighter types, or is that going to be in the book? When is the book available I'm sold!

 

[DarrenW]

Is there such a thing as a list of parasitic / profile drag for a variety of major fighter types, or is that going to be in the book? When is the book available I'm sold!

Having comparable trustworthy drag figures for the majority of WWII fighters is a moving target at best. Even for one aircraft type you'll find variations. Problem is to have comparable numbers the same wind tunnel and test conditions must best used, which is obviously not possible in every case.

But I guess one can still get a basic idea of how "clean" or "dirty" an aircraft is by using the data that's currently out there, so there is some value in that I suppose. You first just have to make sure that you are comparing apples to apples. A value for total drag (CD) of one aircraft should not be compared with another's zero-lift drag coefficient (CD0), as total drag is always greater than zero-lift drag.

By the way, early P-40s had a fairly low total drag number, as noted in NACA wind tunnel test results from the period:

NASA Technical Reports Server (NTRS) - Review of drag cleanup tests in Langley full-scale tunnel (from 1935 to 1945) applicable to current general aviation airplanes

But please note that even NACA insisted that it would be unwise to compare data between the various aircraft tested, and only the drag reduction results per type are useful to the engineer. Also note that many of these airplanes were early examples of production aircraft so I am sure there were some incremental improvements made to aerodynamic efficiency along the way.

 

[nuuumannn]

What enabled the 109F4 (and less applicable to the F1 and F2) was that big & powerful engine was installed on smalll, light and reasonably well streamlined fighter.

Not saying it wasn't, my point is that despite its three guns it still was a formidable fighter.

 

[Dan Fahey]

The ME 109 K could get up to the 440 mph speed.
Read where the speeds got past 400mph the ME 109 was a good bit harder to maneuver.

 

[spicmart]

I had read a max speed figure of 727 kmh/452 mph. Anyone knows about this?

 

[spicmart]

Messerschmitt always had a clear focus on performance, speed and light build for his designs. Given this premise one might say that he was not better than other designers and his creation, with the exception of the Me 262, were not faster than other planes and often inferior in their characteristics. So his planes were not on the forefront of aerodynamic progress.

Someone said that laminar flow profile was not feasible because of the surface structure of the planes deteriorating thus negating the laminar flow. But I read that the Me 309 featured a laminar flow wing and the later Ta 152 variants as well.

 

[Shortround6]

I may have to tread lightly here, there seem to to be a multitude of definitions of "laminar flow" at the time (and even now) in that what can be achieved in the laboratory/wind tunnel is one thing. What can be achieved in the real world, even with a carefully crafted and tended high performance glider with a fiberglass/composite wing, what could be achieved with a metal wing (even with 20 coats of paint sanded between each coat and so on and so on................
There seems to have been something of a misunderstanding in regards to what was needed in surface finish instead of just changing the air foil in order to even get "laminar Flow" to occur over 30-40% of the wings surface. Some wings were referred to as "laminar flow" when they changed the maximum thickness of of the wing from about 30% of cord back to 40-50% of cord. It was a step and some of the best surface finish in the world wasn't going to give much of an improvement in laminar flow if the airfoil wasn't correct but the best airfoil in the world (in the lab/wind tunnel) wasn't going to work in the real world without surface finish standards that were pretty much unobtainable on a service aircraft.

The P-51 did about the best and a 'good' P-51 maintained laminar flow to almost the 40% of cord mark, which is not laminar flow at all according to some people because it didn't maintain it all the way (or even 90%).

The Me 309 or late Ta 152 might have been designed with the intention of getting laminar flow, or at least getting more laminar flow (keeping it non turbulent) over higher percentage of the wing than previous wings but achieving that intention/goal was pretty much beyond the capabilities of the German aircraft industry at the time.

So the question maybe were they designed with the intention/hope of getting laminar flow (or at least a significant reduction in drag) rather than if they achieved it.

 

[tomo pauk]

I had read a max speed figure of 727 kmh/452 mph. Anyone knows about this?

That would be a speed value for the K-14, the version powered by DB 605L engine that was outfitted with a 2-stage supercharger.

 

[spicmart]

That would be a speed value for the K-14, the version powered by DB 605L engine that was outfitted with a 2-stage supercharger.

The source gave it for the K-4 actually.

 

[spicmart]

So the airfoils of a lot of WW2 fighers were obsolete/not competitive at the end? Afaik as the Me 109, Fw 190, F4U Corsair, F6F Hellcat, F7F Tigercat, F8F Bearcat et al are concerned.

 

[tomo pauk]

The source gave it for the K-4 actually.

What source?