Author Topic: Oh my god the pony is awful! (Read 9306 times)

Widewing · « **Reply #105 on:** February 25, 2008, 12:21:02 AM »

Quote

Originally posted by Widewing
When I have a little more time, I'll calculate acceleration for these two, and I'll give the Tempest an extra 5% more prop efficiency when calculating thrust. The lower the speed, the greater the available thrust.

Acceleration in feet per second, per second = Thrust - Drag/Weight/32.2

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

My regards,

Widewing

A quick calculation based upon 80% prop efficiency shows me that acceleration is as follows.

Beginning at 150 mph

Tempest: 8.44 ft/sec/sec
P-51D: 8.69 ft/sec/sec

Tempest at 85% prop efficiency: 9.21 ft/sec/sec

At 300 mph, the rate of acceleration slows to:

Tempest: 2.32 ft/sec/sec
P-51D: 2.93 ft/sec/sec

This makes good sense as the greater profile drag of the Tempest begins to tell more at higher speeds.

This is based upon weights of 11,480 lb for Tempest and 9,611 lb for the P-51D.

Total drag used for the Tempest is 1,350 lb, 845 lb for the P-51D (based upon profile drag coefficients, wing area and dynamic pressure at sea level, meaning that increases in altitude will result in decreased drag).

Horsepower depends on boost.

I used 1,720 hp for the V-1650-7 and 2,180 for the Sabre IIA at 9.0 lb boost. Greater boost means greater power. However, I'm going by standard boost of 67" MAP for the Packard Merlin and 9.0 lb for the Sabre.

By the way, if I add the 2,800 hp P-47M to this, you would see that it runs away from both of the others.

One other factor that bugs me is the P-38J/L. Using the same calculation, it should accelerate faster than the P-51, Tempest or P-47M. Currently in the game, it's about even with the P-51D, which means it accelerates rather slowly despite 1/3 more hp than the Tempest and a total drag only slightly greater than the P-47M. It strikes me as very odd. I have no idea how profile drag is coded into the game, so I can't even begin to guess why this exists. It is what it is and we have adapted to it.

My regards,

Widewing

Lumpy · « **Reply #106 on:** February 25, 2008, 12:29:46 AM »

Then perhaps your calculations are missing something vital. Distilling something so complex as the physics of powered flight down to math cannot be simple or easy.

Widewing · « **Reply #107 on:** February 25, 2008, 12:37:47 AM »

Quote

Originally posted by Lumpy
"Even though the USAAF had cleared the P-51 for 75" Hg., the Eighth Air Force chose 72" Hg as the P-51's War Emergency Rating."

Let's finish the paragraph...

"Apparently there is more to the story, however, as Encounter Reports demonstrate that 75" Hg was used operationally."

This shows that the modifications resulted in 75" regardless of preferred rating.

Nonetheless, you are right in that these boost pressures are not available in game. However, the AH2 Tempest seems to run at considerably greater boost than the standard 9.0 lb. It looks like 10.5 lb to me.

This chart shows performance with 150 octane fuel.

My regards,

Widewing

Widewing · « **Reply #108 on:** February 25, 2008, 12:47:31 AM »

Quote

Originally posted by Lumpy
Then perhaps your calculations are missing something vital. Distilling something so complex as the physics of powered flight down to math cannot be simple or easy.

This calculation is middle-school simple if you have the required drag, weight and power figures. These I have as they are published. The only estimate is propeller efficiency. Usually, this is plugged in as 80%. See Dean's explanation for non-engineers in America's Hundred Thousand.

My results agree with the AFDU chart, btw.

My regards,

Widewing

Lumpy · « **Reply #109 on:** February 25, 2008, 01:16:27 AM »

Yes I'm sure there were many pilots who flew with higher than officially allowed boosts. I know a few P-47 pilots did that. Still, the 8th only allowed 72" for the Pony, and even 75" isn't +25 lbs, so the RAF documents are still useless without knowing what boost they represent.

If your calculations are middle-school simple and rely on only three factors then I can safely say that your results won't be accurate within any reasonable margin. And I can also say with some conviction that the calculations of Hitech Creations are likely much more complex and accurate.

Lumpy · « **Reply #110 on:** February 25, 2008, 01:18:44 AM »

"However, the AH2 Tempest seems to run at considerably greater boost than the standard 9.0 lb. It looks like 10.5 lb to me."

If that is the case than surely the Tempest should out perform the Pony?

Stoney · « **Reply #111 on:** February 25, 2008, 02:18:06 AM »

Quote

Originally posted by Lumpy
Then perhaps your calculations are missing something vital. Distilling something so complex as the physics of powered flight down to math cannot be simple or easy.

How do you think HTC does it then? Its a vector-based flight model, so it has to use math. That equation that WW used is the basis of just about every thrust/drag calculation presented in current aerodynamics textbooks.

Stoney · « **Reply #112 on:** February 25, 2008, 02:31:17 AM »

Quote

Originally posted by Lumpy
I don't think the Tempest had a laminar flow wing. Not sure though.

Looking at the profile on the UIUC database, it doesn't appear to be in the laminar family, certainly not the NACA 6 digit series. It almost appears to be a modified NACA 14XX series with the position of maximum thickness pushed back (they list it as 37% which would put it between the NACA 63XXX and NACA 64XXX series). Perhaps a NACA 1414/1410 with the maximum thickness adjusted. Dave Lednicer lists it as a H-1414 root/H-1410 tip. Some Hawker airfoil I suppose. I could run it on X-Foil and compare it to the P-51 under the same conditions as a comparison. Its the same as the airfoil used on the Sea Fury.

Neil Stirling · « **Reply #113 on:** February 25, 2008, 04:10:45 AM »

War time Tempest V's were fitted with two different marks of Napier Sabre.
The Sabre IIa +9lbs boost 3,700rpm, upped to +11lbs for V1 interception and the Sabre IIb +11lbs boost 3,850 rpm.

Those Sabre IIA's having Mod. No. Sabre/158 or 297 ("strengthened propeller reduction gear assembly") were converted to IIB's by the incorporation of a new boost control cam (Mod. No. Sabre/433) and a new boost control capsule (Mod. No. Sabre/435).

More in detail here:-

http://www.wwiiaircraftperformance.org/tempest/tempest-V.html

Neil

Neil Stirling · « **Reply #114 on:** February 25, 2008, 04:29:24 AM »

Lumpy

RAF documents are still useless without knowing what boost they represent.

Do you mean these?

http://www.wwiiaircraftperformance.org/sl-wade.html

If so http://www.wwiiaircraftperformance.org/wade-data.jpg

Neil.

Lumpy · « **Reply #115 on:** February 25, 2008, 09:55:36 AM »

Quote

Originally posted by Stoney
How do you think HTC does it then? Its a vector-based flight model, so it has to use math. That equation that WW used is the basis of just about every thrust/drag calculation presented in current aerodynamics textbooks.

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

Lumpy · « **Reply #116 on:** February 25, 2008, 09:57:43 AM »

Neil Stirling: Do you mean these?

No. I meant the documents Widewing posted, unless they are the same.

Neil Stirling · « **Reply #117 on:** February 25, 2008, 10:48:29 AM »

They are.

Neil.

SgtPappy · « **Reply #118 on:** February 25, 2008, 04:39:47 PM »

Quote

Originally posted by Neil Stirling
War time Tempest V's were fitted with two different marks of Napier Sabre.
The Sabre IIa +9lbs boost 3,700rpm, upped to +11lbs for V1 interception and the Sabre IIb +11lbs boost 3,850 rpm.

Those Sabre IIA's having Mod. No. Sabre/158 or 297 ("strengthened propeller reduction gear assembly") were converted to IIB's by the incorporation of a new boost control cam (Mod. No. Sabre/433) and a new boost control capsule (Mod. No. Sabre/435).

More in detail here:-

http://www.wwiiaircraftperformance.org/tempest/tempest-V.html

Neil

Lol that's what I put in the Tempest Engine thread and no one listened to me

The Tempest is using 150 grade as seen from the document posted. Thus +10.5 lb. boost is generated from our in-game Tempest's Sabre IIA, not +11 lbs. You can check the maximum boost from the E6B. I think most Tempests used the Sabre IIB (at WEP: +11 lbs.) which actually had better power... 2,420 hp by the documented standards compared to the Sabre IIA's lower 2,180 hp I believe.

Widewing · « **Reply #119 on:** February 25, 2008, 05:42:41 PM »

Quote

Originally posted by Lumpy
If your calculations are middle-school simple and rely on only three factors then I can safely say that your results won't be accurate within any reasonable margin. And I can also say with some conviction that the calculations of Hitech Creations are likely much more complex and accurate.

Well, you're not safe and your conviction is misplaced. These calculations represent an excellent relationship between the two aircraft. The AFDU shows the P-51 having better initial acceleration, which means that the acceleration at higher speed ranges should be even more in favor of the P-51 until a maximum is approached... The effect of drag is not linear.

So, the P-51 should walk away, with the Tempest eventually catching up after a long run.

One could argue that the AH2 Tempest is boosted above standard. However, this opens a Pandora's box or sorts; because everyone could begin arguing that their favorite aircraft should be boosted to something beyond standard that was actually used. I wouldn't bother to argue that, but would rather see acceleration more closely match the theoretical numbers without over-boosting or higher octane fuel.

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

My regards,

Widewing