Oh my god the pony is awful!

[SgtPappy]

Well did the Sabre IIB-equipped Tempest V's have a higher boost level anyway? If anything, I think our Tempest is underboosted. Just pulling some info off of spitfireperfromance.com

[Stoney]

Originally posted by Lumpy
Somehow I think Hitech Creation's flight model is a bit more complicated than that.

Sure, its crunching a lot of numbers simultaneously, but the equations used to compute the multiple vectors used to simulate the 3D aerodynamic effects on the models, are all computed with either the standard aerodynamic equations for each vector, or variations of the same.

For example, the top number of all the vectors displayed along the wing looks to be pounds of lift at each station.  Those numbers would be substituted in for a number of equations running simultaneously.  They could be used to determine the rolling moment created by the propwash on the left wing, whether or not the spar reaches its ultimate load as a result of lift, etc.  All of these factors that act upon the plane must be a result of some equation.  Otherwise, HTC would be relying on an FM that used Computational Fluid Dynamics (CFD) to determine the forces that acted upon each aircraft.  Most of us have computers that would crash shortly after engine start if that was the case.

My point being, that WW's use of a common, simple thrust/drag formula to represent the relative difference between two aircraft is a proper and rational argument.  After all, its what HTC does in order to make the FM work.

[Stoney]

Originally posted by Widewing
The P-38J/L should be one of the premier drag racers of the plane set, but isn't.

It had a flat plate area over twice as large as the Pony (8.78 to 3.80), with a Cd over 60% greater than the Pony (.0268 to .0163) with not quite twice the power.  I appologize for fanning the flames without doing the math first.  (Numbers pulled from this)

Its possible that the 85% eta-P for the Tempest is a bit generous.

Anyone have the Root/Tip Chord or MAC of the P-51D and Tempest?  Having a hard time finding them for airfoil comparisons.  (Need the lengths in order to generate the proper Reynolds number).

[Widewing]

Originally posted by Stoney
It had a flat plate area over twice as large as the Pony (8.78 to 3.80), with a Cd over 60% greater than the Pony (.0268 to .0163) with not quite twice the power.  I appologize for fanning the flames without doing the math first.  (Numbers pulled from this)

Its possible that the 85% eta-P for the Tempest is a bit generous.

Anyone have the Root/Tip Chord or MAC of the P-51D and Tempest?  Having a hard time finding them for airfoil comparisons.  (Need the lengths in order to generate the proper Reynolds number).

I'll answer the last question first as I have this handy.

Root Chord @ C/L is 103.99"
Wing Tip Chord at Station 215 (where the wing tip cap is screwed on) is 50.0".

As to the the P-38/P-51 comparison, there's a few things your source omits, if you will..

They have the CDo at .0163. This is for a P-51B/C and it reflects no stores pylons and taped over gun ports. For the P-51D it should be .0176 with pylons and untaped gun ports.

Also to be considered is that the hp rating for the P-38 reflects MIL power, not WEP. It gets tricky because the AAF rated the V-1710-89/91 engines at 1,600 hp in WEP, and applied the same rating to the V-1710-111/113 of the L model. However, the L model was delivered with a factory rating of 1,725 hp. Tony LeVier and Kelly Johnson have both stated that the L models were delivered and accepted by the AAF with the full 1,725 hp available in WEP. However, it was merely an adjustment to the prop governors to limit RPM to 3,000 and 1,600 hp. The full 1,725 hp occurred at 3,200 RPM (according to LeVier). So, the AAF could have readjusted the RPM during assembly and rigging at the Burtonwood or Langford Lodge depots. Of course, the Lockheed and Allison reps immediately reset the RPM when the aircraft were ferried to the combat units (according to several P-38 crew chiefs).

As to the P-51, we have the same issue in that the MIL rating appears in your source rather than the 1,720 hp WEP rating.

If we do the math, we find the following using full internal fuel weights:

P-38J: 375 x .8 x 3,200 / 150 mph = 6400 lb thrust
So, 6,400 lb thrust - 1676 lb drag / (16,480/32.2) = 9.23 ft/sec/sec

P-38L: 375 x .8 x 3,450 / 150 mph = 6,900 lb thrust
So, 6900 lb thrust - 1676 lb drag / (16880/32.2) = 9.97 ft/sec/sec

P-51D: 375 x .8 x 1,720 / 150 mph = 3,440 lb thrust
so, 3440 lb thrust - 845 lb drag / (10,208/32.2) = 8.18 ft/sec/sec

At 250 mph:

P-38J: 4.22 ft/sec/sec
P-38L: 4.70 ft/sec/sec (4.13 ft/sec/sec for 1600 hp rating)
P-51D: 3.85 ft/sec/sec

Much depends upon weight. Early P-38Js didn't have leading edge tanks, so you could subtract the weight of 110 gallons, or 660 lb.

My regards,

Widewing

[Stoney]

Originally posted by Widewing
As to the the P-38/P-51 comparison, there's a few things your source omits, if you will..

Obviously, those differences would be significant.  I assume all of those are Sea Level comparisons.  Given that the P-38 is turbo-supercharged, I'm assuming the Allisons made 1725 up to altitude (a la P-47).  I'm curious as to standard day altitude comparisons that could create an advantage for the sleeker Pony.

I'm having some trouble getting XFoil to accept the coordinates for both the Tempest and P-51 airfoils as listed on the UIUC website.  I'll play around with it to see if I can get some results.

[SD67]

Stoney, if you could send me the coordinates for the airfoils I can try my software to see if I can generate a set of polars for you to compare.

[Stoney]

Originally posted by SD67
Stoney, if you could send me the coordinates for the airfoils I can try my software to see if I can generate a set of polars for you to compare.

PM sent...

[TimRas]

Originally posted by Widewing
IIRC, the Tempest has a profile drag coefficient of about .0245, with the P-51D coming in at .0176 (with pylons, .0168 without)

0.0245 doesn't really match with known power and speed.
As the Tempest has both better power loading and better top speed (at low altitude), it seems obvious even without calculating that i has better acceleration at all speeds.

[Tilt]

Originally posted by Widewing
This calculation is middle-school simple if you have the required drag, weight and power figures.
Widewing

So if I have a Cx of 0.0228 (trimmed) and 0.0273 (untrimmed)

an all up weight of 7115 lbs
and a max take off power of 1850 hp

(36.6lbs/ft^2 wing loading)

How would you compare them to the P51 and the Tempest under study above.

[SgtPappy]

Not to shoot down your statements, Widewing, but if they purposely limited the RPM which lowered the maximum power output, would that not mean there was a reason for the lowered RPM; i.e. they lowered the RPM (and consequently, power) for a reason.

Now did the pilots flying these P-38L's commonly manage 1,700 + hp using the Allison F-30's? If not, that would be the reason as to why we do not have the max. power output. Whether the F-30's had the ability and potential to produce that power is irrelevant... it's whether it was used commonly or not... I think. I could be wrong. I'm not sure how HTC controls how they add features to planes.

[Lumpy]

Same reason we don't have a 2200 hp C3 fueled uber 109K SgtPappy.

[Lumpy]

However if it can be proved that these boost levels were used at least by one pilot perhaps they can be added as perk "ace planes". Would be fun I think.

[moot]

Those would be fitting rewards for a successful pilot career in CT.

[MiloMorai]

Originally posted by Lumpy
Same reason we don't have a 2200 hp C3 fueled uber 109K SgtPappy.

1 PS does not = 1 HP

[Lumpy]

I know. PS ratings are approx. 98% of a similar HP rating. Also the 109K doesn't develop exactly 2200 hp or PS. I was generalizing.