Hi Bill and Shortround,
I think that somethings may be being confused here.
OVERALL drag between a Mustang and a 109 may very well be similar.
We would have to compare specific models at the same altitude and know the power output for both engines at that specific altitude.
However, even if the 109 is equal or even slightly faster than the Mustang using the same power it is still a "dragger" airframe because it is a smaller airframe. The Mustang with it's bigger wing and larger fuselage is more streamlined for its size. THe larger size allows it to carry the extra fuel and heavier weight of armament. I don't want this to veer off into tangent but I think we can all agree that that the Mustangs .50 cal MGs and ammo weighed more than the 109 normal internal armament.
So you have a small but higher drag airframe (the 109) vs a larger but lower drag airframe (Mustang) giving the same TOTAL drag.
I understand the difference between absolute drag values (ie. equivalent flat plate area) and relative drag (usually expressed as a coefficient related to some other area value, ie. total surface area or more often, related to wing area only). My problems with the statement are really the following:
Generally I don't believe expressing aerodynamic 'cleaness' with a wing area related coefficient gives you realistic comparison values, since in this case we are comparing the total drag of the aircraft (fuselage, radiators, antenna etc) to a single element of the drag, that is responsible only for the fraction of the total drag, ie. Hoerner assumes that the 109 wing assembly was responsible for 37.5% of the total drag. Such comparison, by its nature always gives better coefficients for aircraft with larger wing areas.
For a simplified example, lets assume that the total drag of the 109 is 100, and its wing has an area of 16. The wing thus is responsible for 37 units of drag (see above); the coefficient would be 100 drag / 13 wing area, ie. a drag coefficient of 6.25.
Now, double the wing area of the 109, to 32 units, and leave everything else alone. The wing's drag component is now 74 (ie. 2x37), the remainder is still the same as with the original wing, 63 (ie. 100-37), a total of 137 units of total drag,
but the coefficient now is 4.28, almost 50% 'better' than the original 109... even though there were absolutely no improvements to skin friction, the fuselage or anything else... its exactly the same aircraft, just with oversized wings. I understand that it may be somewhat more complicated in practice, but you get the point I guess: It would be nonsense to state that the same 109 with bigger wings is cleaner than a 109 with smaller wings, and its only seems so because our basis of relation (wing area) takes account only 1/3 of the total picture.
Wing area related coefficients of drag are NOT a measure of quality of aerodynamic cleaness (I believe stated many times by NACA papers as well), but just that, a mere coefficient for engineers. I believe from factor is probably better expressed in front area related form factor coefficients, not to mention, given an aircrafts multitude of flying conditions (near stall, fast flight, climb, landing, flaps/slats deployed etc.) there's a myriad of drag coefficients for each condition..
drgondog said:
From Hoerner: At 380mph, 22,000 feet at 6700 pounds gross weight, 1200 Hp:
Building up all the drag factors, applying Thrust calculated with 140 pounds of exhaust thrust at 380mph and propeller efficiency of .85 and getting total drag yielded:
The Drag Area of the 109G is approximately 6.2 sq ft with Cd of .036 at that speed and altitude.
The total wetted area of the 109G, according to Hoerner, is 590 square feet. Also according to the example calculations using the 109G Hoerner's calculated CDwet = .0105. (Divide Drag Area by Wetted Area). Compare against the following values derived from Flight and Wind Tunnel tests:
The tables in Lednicer's WWII Fighter Aerodynamics present the following at ~360kts, 15,000 feet.
1. Spit IX Drag Area = 5.4 sq Ft, Wetted Area = 831.2 sq ft, CDwet = .0065
2. P-51B Drag Area = 4.61 sq ft Wetted Area = 874..0 sq ft, CDwet = .0053
3. P-51D Drag Area = 4.65 sq ft Wetted Area = 882.2 sq ft, CDwet = .0053
4. Fw 190A-8 Drag Area=5.22 sq ft, Wetted Area = 735.0 sq ft, CDwet = .0071
4. Fw 190D-9 Drag Area=4.71 sq ft, Wetted Area = 761.6 sq ft, CDwet = .0063
I believe Hoerner's study was discussed a number of times. I don't believe for a minute that Hoerner's method would be unprofessional in many details, in fact I am pretty sure his study is as perfect as it can be. However I believe its purpose is misunderstood - its certainly not a serious study specifically on the 109s aerodynamics, rather than a generic example provided how to calculate such things, and what factors are present in drag, and how it can be improved. Also, the following has to be taken into account:
Any calculation is just as good as the basic numbers it uses. Hoerner's study however uses ad hoc number s for its calculation, the - rather optimisitic imho - 85% propeller efficency is an assumption, with no actual data on the actual VDM propellers effiency curves - and we know too well that some of these were optimised for climb speeds, some for high speed, high altitude flight (this is obvious looking at climb curves for normal altitude G-14 and high altitude G-14/AS with equivalent power levels, but different props).
Exhaust thrust is yet another assumption with a some 20% margin of error in what actual thrust is available for the aircraft, and this foundamentally effects the accuracy of the calculations. In both propeller and exhaust thrust, I believe Hoerner's guess are a bit on the optimistic side with regards of the actual thrust available.
Secondly, he assumes a top speed of 380 mph (612 km/h) for his '109G', which is waaay lower than what was actually measured on the 109G at these power levels (1200 HP at altitude relates to 1.3ata Kampfleistung, ie. 30 min ratings). The actual measured/nominal speeds were, at this rating, atltidue:
G-1 through G-4: 660 km/h / 410 mph
G-5/G-6 : 630 km/h / 391 mph
Needless to say, underestimating top speed by as much 50 km/h or 30 mph (!!!), or 10% will yield drastically worser drag values, since power requirements increase on the cube for higher top speed. Ie. Hoerner's fantasy like 109G does 380 mph at 1200 HP; the real one did 410 mph on the same 1200 HP; the get Hoerner's 109G from 380 mph to 410 mph, ie. the speed actually achieved by the real one, it would take an 1500 HP output engine at that altitude, ie. in practice, the most powerful engine in the 109K.
So I believe Hoerner's study, while theoretically perfect, in practice are flawed by his wrong base data (the weight figures BTW are also curious, 6700 lbs would be a correct weight for an early 109G-2, but the G-6 was some 200 lbs heavier, yet still faster than Hoerner's example, unless it had gunpods... which would add another 500 lbs.... so, what is this Franken109 of Hoerner's really is?)
They are also at odds with the drag coefficients and polars stated by the wartime Messerschmitt AG papers, which all state a drag coefficient of 0.023 for the Bf 109F/early G, which I posted on this board quite a few times btw..
drgondog said:
The Drag Area of the 109G is approximately 6.2 sq ft with Cd of .036 at that speed and altitude.
Moreover if we take Hoerner's calculated drag area for the '109' seriously, its also at odds with common sense and reasoning: simply an aircraft with higher drag and lower power output can't be as fast or faster as another, ie.
drgondog said:
Compare against the following values derived from Flight and Wind Tunnel tests:
The tables in Lednicer's WWII Fighter Aerodynamics present the following at ~360kts, 15,000 feet.
1. Spit IX Drag Area = 5.4 sq Ft, Wetted Area = 831.2 sq ft, CDwet = .0065
OK, so we have Spit IX with drag Area = 5.4 sq Ft, 1595 HP at critical altitude, doing 404 mph at critical altitude (21 k feet).
Please explain, how its possible, that a 109G, which
according to Hoerner, has a drag area 6.2 sq ft (but 4 sq. ft according to Messerschmitt AG...), manage to do about 405 mph at 21 feet, with an 1200 HP powerplant... while being draggier, and having about 2-300 HP less. I would say its either a miracle, or Hoerners drag coeff is flat out wrong, as is the base date he used.
Same comparison BTW at Sea levels, ie. compare Spit IX F/LF speeds to 109G-2/G-6 speeds and the corresponding powers.. the 109G was considerably faster (by ca 30 km/h if we look at nominal specs, ie. ) near the ground than the Merlin 61 powered Marks, despit having slightly less power available. And it was about as fast as the Mark IX LF, which had considerably more (by some 300 HP!) available...