Internal weight and max speed

[dtango]

Stoney:

Compressibility Drag:
I haven't verified it but I'd be surprised if they didn't model compressibility drag.

4400 lbs of difference in weight in the P-47N's you tested is a considerable amount.  The tests were done at 34k as well.  Lastly there are 2 more gun barrels sticking out in the wind at high speed (8 guns vs 6 guns).  Total drag increases with the square of velocity which means changes in overall drag coefficient get magnified.  The combination of all these could translate to the differences in max level speed you're seeing for the P-47N's you tested.

EDIT: Last but not least, the AH flight model is more sophisticated than my calcs.  My calcs were done to illustrate the principles.  Lift-dependent profile drag is not estimated which I know is in the AH flight model.  Also my compressibility drag polar I'm estimating may be different than how HTC is implementing their's.

Minimum Drag:
The point of lowest drag is where induced drag = parasite drag.  What use is this?

(1) For a jet, because the thrust doesn't vary with airspeed the maximum excess thrust is at the airspeed for lowest drag.  This is the speed for the best-rate-of-climb for a jet.  This is not true for a piston-prop because thrust varies with velocity.

(2) Knowing the velocity for lowest drag for a prop or jet plane tells you the velocity for the best glide ratio.

(3) Min drag (which occurs at max L/D ratio) also tells us the max endurance for a jet (how long it can remain airborne).  Note that endurance and range are not the same thing. [Tanget: Of course the tricky thing here is that weight changes with fuel burn therefore you have to either reduce your airspeed with reducing weight or keep changing AoA to maintain L/Dmax!].  This is not the case for a prop-plane.

(4) Velocity for Min drag (L/Dmax) will give us the maximum range for a prop aircraft.

Tango, XO
412th FS Braunco Mustangs

[Knegel]

Hi,

the different "speeds of total drag" of different planes and at different loadout are one of the most important factors, if it comes to determining plane performence relations.

Best would be to have the curves for two extremes to compare them, unfortunately i dont know this curves for the A6M2 and FW190A8(probably upper and lower end of the scale).

But its safe to assume, the A6M2 had a very low "speed of smalest drag", while the FW190A8 had a rather high "speed of smalest drag".

It must be something like this(only a assumption of course):
 


This explain the good climb and acceleration of the Zero at slow speed and the good acceleration of the FW190A in medium speed. And this explain why the FW190 didnt climb that good and dont had a good Vmax, while it had a very high cruize speed and why it could outclimb the F4U-1 and the P47 at medium speed, while it couldnt do this at slow speed.  This also explain why the inlunce of weight to the Vmax is rather big.

Would be nice to have real datas regarding this, at least we have a new FW190A8 to test it in a wind tunnel.


Greetings,

Knegel

[Shiva]

Originally posted by Stoney74
CG can have an affect on Vmax as well.  Planes loaded near to the aft limit fly faster than the same plane loaded near the forward limit, assuming gross weight is equal.  Some of you guys may be able to correct me or expound on this, but it might be interesting to determine the CG of a B-17 with bombs versus right after they get dropped.

If I remember the standard design parameters for aircraft correctly, the center of lift of the main wing is behind the center of gravity of the aircraft, resulting in a pitch-down moment; the horizontal stabilizer provides a downforce to balance the pitch-down moment. Because the amount of drag from the horizontal stabilizer goes up with the lift produced by the stab, as an aircraft is loaded to move its CG aft, reducing the pitch-down moment from the main wing and the compensatory lift required from the stab, decreasing its drag, resulting in a higher speed, whereas moving the CG forward increases the pitch-down moment, requiring more compensation from the stab, increasing the drag, and reducing the aircraft's speed.

[dtango]

Here are the estimates for the B-17G:

B17G Sea Level: assuming 1200hp ea. eng.
Alt %F Weight rho Di CD0 Dp Dtotal Thrust V mph Mach
0 100 65500 0.0023 1333 0.032 5305 6639 6642 216.0 0.275
0 75 61330 0.0023 1143 0.032 5423 6567 6569 219.0 0.278
0 50 57160 0.0023 974 0.032 5528 6503 6507 221.0 0.281
0 25 52990 0.0023 823 0.032 5623 6447 6451 223.0 0.283

B17G 25k: assuming 1200hp ea. eng.
Alt %F Weight rho Di CD0 Dp Dtotal T V mph Mach
25000 100 65500 0.0010 2084 0.032 3394 5479 5475 263.0 0.33
25000 75 61330 0.0010 1725 0.032 3596 5321 5319 270.0 0.339
25000 50 57160 0.0010 1430 0.032 3768 5198 5196 277.0 0.347
25000 25 52990 0.0010 1182 0.032 3916 5099 5097 282.0 0.353

Impact of increased drag due to weight, the effects are definitely seen more at altitude than at sea level.

I've updated the P-51D estimates as well.  I had thrust set constant but modified it to vary with airspeed as it does in reality.  Here they are:

P-51D SL: 1650hp + 200lbs exh. Thrust
Alt %F Weight rho Di CD0 Dp Dtotal Thrust V mph Mach
0 100 12100 0.0023 122 0.019 1441 1564 1564 362 0.476
0 75 10606 0.0023 93 0.019 1462 1555 1554 365 0.479
0 50 9486 0.0023 73 0.019 1475 1548 1548 367 0.481
0 25 8739 0.0023 62 0.019 1483 1545 1545 368 0.483

P-51D 24K: 1250hp + 200lbs exh. Thrust
compressibility drag factored
Alt %F Weight rho Di CD0 Dp Dtotal T V mph Mach
24500 100 12100 0.0010 220 0.021 891 1111 1112 411 0.616
24500 75 10606 0.0010 163 0.0213 933 1097 1097 418 0.627
24500 50 9486 0.0010 129 0.0216 959 1088 1090 421 0.631
24500 25 8739 0.0010 108 0.0218 981 1089 1086 423 0.634

P-51D 24K: 1250hp + 200lbs exh. Thrust
no compressibility drag
Alt %F Weight rho Di CD0 Dp Dtotal T V mph Mach
24500 100 12100 0.0010 203 0.019 872 1075 1076 428 0.642
24500 75 10606 0.0010 149 0.019 909 1059 1058 437 0.655
24500 50 9486 0.0010 117 0.019 930 1047 1048 442 0.663
24500 25 8739 0.0010 98 0.019 942 1040 1042 445 0.667

On the issue of compressibility drag, I used the following curves to estimate the amount to add based the NACA XP-51 compressibility drag curves.



There are other sources that have more generic compressibility drag polar estimations that have drag rise starting at a higher mach number than the polar I was using as seen here:



My gaming PC is dead at the moment (ugh!!!) so I haven't been able to flight test anything.  For the P-47N tests that Stoney performed, if AH has compressibility drag modelled they may be using a model like the generic one above which means drag rise doesn't factor in until beyond Mach .7.

Tango, XO
412th FS Braunco Mustangs

[Stoney74]

Very nice information...

[gripen]

Originally posted by dtango

On the issue of compressibility drag, I used the following curves to estimate the amount to add based the NACA XP-51 compressibility drag curves.

The effects of the compressibility tend to vary really a lot depending on plane. As an example in the RAE wind tunnel tests on the Spitfire and Typhoon (about 1:6 scale models), the Cd0 of the Spitfire raised from about 0,02 to about 0,045 when the mach number raised form about 0,5 to 0,78 while in the case of the Typhoon the Cd0 raised from about 0,02 to about 0,15 at same mach number increase. The AoA for the Cd0 stayed nearly constant -0,5deg for the Spitfire while in the case of the Typhoon the AoA for the Cd0 raised from about -2deg to +4deg. Most of this difference is caused by much thicker wing profile of the Typhoon.

[dtango]

Good info as always gripen.

Tango, XO
412th FS Braunco Mustangs

[Stoney74]

What characteristics tend to affect compressibility effects?  I saw the mention of wing thickness.  Are their others?

[gripen]

There is probably an endless list of things affecting compressibility; Reynolds number, temperatute, profile characters,fuselage shape, air intakes, tail surfaces etc. In the case of the P-38, the wing alone did fine up to above mach 0,7, but when connected with the fuselage and the engine nacelles, the airstream accelerated between the fuselage and nacelles (venturi effect) causing loss of lift in that section and so called "tuck under" starting around mach 0,68.

[Charge]

I think the wing profile may be a rather significant factor of drag behaviour in high speeds and high loads.

If we look at P51 and 190 the the difference is that the P51 uses a symmetrical profile which does not "force" the change in AoA in speed change the same way as 190 as the 190 has an asymmetrical profile which forces the a/c to change its attitude more in flight when speed changes.

I think that especially when flown laden and high the P51 is in its own element, where as the 190 starts to suffer from its weight/wing-area ratio so that the wing is forced into AoA where the profile starts to create excess drag to keep up the lift (when compared to symmetrical profile). Maybe this explains the drop in top speeds between A5 and A8? Down at the deck there is almost no difference but at higher altitude the difference in weight starts to kick in.

BTW is the test flight data always corrected to standard atmosphere and temperature etc? I'd think that it has a significant effect on results.

-C+