1941: the best airframe for a single engined fighter

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I'm not that fluent with this, so bear with me
A table from Lednicer's analysis shows Cdswet of the P-51s as being in league of it's own, while the Spit IX and Fw-190D-9 are comparable - meaning D-9 is not that close second?
 

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the 51 out perform in max speed but not in all others performances

taking for good your power
power/weight 51 1150/8000= 0.14 HP for lbs
power/weight 1099F 1165/6400= 0.18 HP for lbs

over 25% advantage for 109F so what planes has best acceleration or best climb?
 
tomo pauk said:
I'm not that fluent with this, so bear with me
A table from Lednicer's analysis shows Cdswet of the P-51s as being in league of it's own, while the Spit IX and Fw-190D-9 are comparable - meaning D-9 is not that close second?
Click to expand...

This is tricky in one sense.

Flat Plate Drag is all about dynamic pressure 'q' and S=wing area as they relate to Total Drag - and used to provide a reasonable comparison between two aircraft. This is all about resolving Total Drag (incl Zero Lift Drag of the wing, parasite drag, friction drag, profile drag and even compressibility drag) against the usual major component - the wing). Hence the wing area reference.

CDwet is about referencing Total Drag as a function of the entire surface of the aircraft, more of a funtion of the comparison of all the forces 'pulling' on the aircraft.

Remember both the Mustang and Spit had wing areas of 16 and 20% greater than the 190A and D models so if all of these birds were compared at the same speed and altitude (q1=q2=q3=q4) then it (Flat Plate Drag) says something about the relative amount of power (Thrust=f(hp/V)) required to maintain that speed - all other things being equal (which they are not).

In the above analysis the FW 190-D at 4.77 Sq ft is only 2% above the P-51D while the Spit at 5.4 sq ft is 16% more flat plate drag, so the FW190D-9 is indeed a close second relative to total drag expressed as a function of wing area and dynamic pressure.

While the Flate Plate drag is referenced to dynamic Pressure it gives you a nice sense of relative 'delta' Hp requirements to increase speed, accelerate, versus another airframe and if the Flat Plate Drag is calculated at top speed the contrast between that value and another airframe gives you a nice relative airframe aerodynamic efficiency.
 
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Bill, for the sake of discussion, let's 'increase' the Fw-190D's wing to the size of Spit or P-51. Beside the wetted area that would increase, what happens to the 'f' and Cdwet?
 

The 109 will have an advantage in climb, it will have an advantage in acceleration from low speed.

The advantage in climb can be large but perhaps not as much as you think. What matters is the surplus power available after you take out the power needed to maintain a given airspeed. Best climb speeds are usually down under 200mph if not closer to 150mph. It may only need 250-350hp to fly the plane at those speeds with the rest of the power being available for climb. the lower drag plane needs a bit less power for the same speed and so has a a slightly higher percentage of power avaialbe for climb. nowhere enough to out climb the 109 but as speeds go up things even out.

If you are at an altitude where the 109 can do 325mph and the P-51 can do 340mph, the 109 cannot climb at all without loosing speed. The P-51 if it is doing 325mph has the power difference between 325mph and 340mph available for climb. If both planes are doing 300mph the 109 has the power difference between 300 and 325 mph and the P-51 has the power difference between 300 and 340mph available.
The slower the aircraft start out, the bigger advantage the 109 has with it's better power to weight ratio. The faster the aircraft are going the closer the "surplus" or available power for maneuver or climb becomes.
 
time to 20k for 51 8.82 minutes (that with 1150 hp)
time to 6km (19,685) for F-4 6.1 minutes
time to 5k for 51 1.87 minutes
time to 2km (6,562) for F-4 1.8 minutes
time to 25k for 51 13.77 minutes
time to 8 km (26,247) for 109F-4 9.3 minutes

oh yes the 109F-4 can not climb at S.L. flying to 325 mph, and why should do? and yes the 51 can climb very slowly at S.L. flying to 325 mph
 
Tomo - if you maintain the same drag on the larger wing (VERY doubtful unless you change from conventional to laminar), both values will go down.

Here is the circular arguement. At max speed, parasite drag is at maximum and induced drag is minimum. When you increase wing area, holding weight/airfoil constant, the you will have a very slight decrease in AoA (a good thing) to maintain the Lift to offset the Weight in equilibrium. On the other hand increasing the wing area, for same airfoil, will increase wetted area - and hence increase parasite drag.

If you increase the wing area by increase the chord and holding the span, you will both reduce AR and possibly change Oswald efficiency for the worse and necessarily increase Induced Drag, along with CDo - a very bad thing at max speed or range. On the other hand, lengthening the chord, even with shorter span, may decrease the parasite wing drag at Mach numbers from say .55 to .65M for net positive benefit. Too much incremental span increase is also a major issue structurally resulting weight increases, starting the circular calcs once again.

You have to play with the variables.

When you increase wing area by holding the span constant and increase the chord, your manuever performance will suffer because the reduced AR and e will necessarily lower the CLmax and increase CDo - which will also affect low to medium speed performance, as an example, as well as maximum operational ceiling
 

Ok but then, how do you explain, Mustang Mk.I being 30mph slower with similar power rating like P51A on the deck?
While at that, can you explain what does "the most aerodynamically efficient airframe of any major propeller fighter of WW2", exactly mean?
 
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Which engines EXACTLY were used in each aircraft and what boost LIMITS were being used.

Allisons had a nominal power rating of 1150hp but varied, depending on supercharger gear, on the full throttle height. the -39 engine used 8.80 gears and was good for 11,500-12,000ft (no ram), later engines used 7.48 gears (9,600ft ?) and 9.60 gears (15,000-15,500ft).
The -39 engine offered 1150hp for take-off and could be over boosted. the later engines had structural improvements and could take more over boost. Depending on the date of the test, WEP settings were not initially approved, an Allison could give 1150 to 1540 (?) hp at sea level (or 1000ft. ) but his varied with the supercharger gear, the low altitude 7.48 gear offering the most usable boost at sea level.

"aerodynamically efficient airframe" may mean the lowest total drag for it's size.

A P-26 was more "aerodynamically efficient" than a P-12 but the P-12 paled in comparison to a Lockheed Vega and the P-26 was way behind the Lockheed Orion in efficiency. They all used the same basic engine although the newer planes did get a bit more power.
Getting low drag by using a small airframe is one thing, getting the same or lower drag while using a bigger airframe is efficiency.
 
Cola - what do have in mind relative to such low Drag (outside Rutan's wonders) in a reciprocating engine (or turboprop) aircraft that has been designed and flown in production since the P-51B/C/D/K/H?
 
Shortround6 said:
Which engines EXACTLY were used in each aircraft and what boost LIMITS were being used.
Click to expand...
Well, we do know that V-1710-39 (Mustang Mk.I) developed 1150 hp at SL (installed) with boost (Military setting) and we also know V-1710-81 (P51A) developed similar power without boost, but was apparently 30mph faster.
So, you still didn't answer my question, because obviously power was equal and you insisted, that was enough to make comparison.

"aerodynamically efficient airframe" may mean the lowest total drag for it's size.
Click to expand...
It may, but usually doesn't, since "aerodynamic efficiency" mainly refers to L/D ratio.
P51A had a 5% smaller AR and 10% higher wing loading then 109F4, so the actual amount of drag reduction the Mustang's laminar wing produced in relation to higher alphas flown over almost all speeds, makes your claim very dubious, at best.
It's probably the other way around, but I wouldn't speculate at this point.

drgondog said:
Cola - what do have in mind relative to such low Drag (outside Rutan's wonders) in a reciprocating engine (or turboprop) aircraft that has been designed and flown in production since the P-51B/C/D/K/H?
Click to expand...
Mustang was pretty sleek at high speeds.
But as I said, no such thing as "best" airframe in such broad terms.
Airframes that fly good straight, usually suck in turn and the other way around.
All planes are results of a KPP instilled compromises.
 
Mustang I (1 pair of 20mm cannons) was using 1150 HP to make 363 mph at 5100 ft, P-51A (2 pairs of HMGs) was using 1125 HP to make 357 mph at 5000 ft. All data from Mike Williams' site.
The P-51A's engine is using more power to drive it's supercharger, having FTH 2600 ft higher - that explains the 25 (B)HP difference. Giving the P-51A another 100 ft, thus making both planes flying at the same altitude - the speed achieved is within measurement tolerances.

added: The addition of wing racks skews the comparison - P-51/Mustang I was (usualy) without them, wile the P-51A/Mustang II was carrying them on regular basis. The wing racks cost 12 mph, according to 'US 100 hundred' book.
 
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What about the flying conditions and the individual conditions at the time of the tests. The fuels used, the atmospheric conditions, engine wear, the paint job. whether in military condition or clean. There are many variables. Drag can be affected by many factors not seen in the basic drawings. Same for engine power. Engine power outputs can be quite massively affected by seemingly minor, sometimes undetectable variations to its construction. Everything built by human hands works to tolerances, and sometimes those tolerances work to lower or heighten performance by huge margins
 
The 'measurement tolerance' should take care for that.

Claiming this:
Well, we do know that V-1710-39 (Mustang Mk.I) developed 1150 hp at SL (installed) with boost (Military setting) and we also know V-1710-81 (P51A) developed similar power without boost, but was apparently 30mph faster.
Click to expand...
is misleading.
The -81 was equipped with supercharger tailored for greater altitudes, in order to exploit the advantage (= less drag) of the thin air at higher altitudes. So, above ~16000 ft, the P-51A was really faster than the P-51. Under 15000 ft, the P-51 should be faster - it has 25 HP more (because it's supercharger sucks less power, supercharger drive having 8,80 ratio vs. 9,60 for P-51A). All of this for military power.

All ww2 fighters were using the boost, so the claim that P-51A was developing similar (or any) power without the boost is simply not correct - supercharger was in function all the time.
 


Agreed on the compromises required by the designer - trying to figure out what the Contractor REALLY wants to pay money for.
 
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Cola said:
It may, but usually doesn't, since "aerodynamic efficiency" mainly refers to L/D ratio.
Click to expand...

The L/D ratio relates to the aerodynamic efficiency of the wing, not the airframe as whole.

Which is the more aerodynamicly efficient airplane, a single seat fighter (with one .50 and one .30 cal gun) that can do 234mph at 8000ft or a single engine airliner with a pilot and 6 passengers that can do 220mph at sea level using the same engine?
 
@drgondog,
At the end of the day, while HP is the rule of thumb...
Click to expand...

Exactly and this is why assessments done on the hp basis alone is misleading.
Now, the P51A's engine characteristic, from Shortround6's chart, looks more like Merlin 45 than V-1710-81 a standard engine for P51A and this combination certainly doesn't belong to '41 and comparison against 109F4.
Then, there's the matter of supercharger tuning and propeller, which differs for those two engines and particularly for 109, so again claiming superior drag performance based on different engine/propeller setup is also misleading.
This is why I said 109G-1 would be better comparison to P51A (with Merlin 45) from the chart.

As said, I won't speculate, since assumption is mother of all f*ck-ups.
If you get your hands on measured or manufacturer's data (not computer simulations) and time permitting, we can make a thorough analysis and comparison.
I agree that friction drag was major player in drag at top speeds, but there is also a matter of wetted area, which is higher for Mustang at all speeds (higher alpha and larger size), so for the rest of envelope Mustang wasn't as good lifter, at least theoretically, as 109.


@Shortround6,
look, you said "aerodynamic efficiency" and I told you what does it refer to and it's not Cd/aircraft size...and L/D doesn't refer to wing only, but the whole aircraft.
What you said, Cd/aircraft size is more in line with fineness ratio.
 
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All in - the 51 was faster on the deck at a higher gross weight, had a lower oefficients of total drag, induced drag and lower parasite drag - i did the above calcs in a hurry but you are free to challenge and present your own from the two reports cited.
 
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fineness ratio is the comparison of the length of an object (boat, airplane, car) and it's width. While a useful number it does not really describe the coefficient of drag. Two fuselages, each with a length of 48 ft and a diameter (or width/height) of 8 ft will both have a fineness ratio of 6 to 1 even if one is a box with square corners and flat front and flat rear and the other is a gradually tapering ellipse in side view and of circular cross section.

You can stick a highly efficient wing on a box of a fuselage:




and still wind up with a high drag airplane.
 
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