P-38 vs P-51: Full internal fuel dogfighting

[drgondog]

I did the math and got a different delta and numbers than you did

P-51B-1NA
Empty: 6988 pounds
Guns & Ammo: 7686 pounds (delta: 698 lbs) 270 (guns) + 422 (ammo) at 1 pound per 3 rds plus links
Fuel in Wings: 8790 pounds (delta: 1104 lbs) Correct for 184 gallons wing only

P-51D
Empty: 7205 pounds Includes 55 pounds for the actual fuselage tank
Guns & Ammo: 8239 pounds (delta: 1034 lbs) Correct 414 (guns) + 620 (ammo) " " " for 1880rnds
Fuel in Wings: 9343 pounds (delta: 1104 lbs) Correct
Center Tank: 9853 pounds (delta: 510 lbs) Correct

P-51D had 1043 pounds vs 900 pounds on the P-51B (I've found a multitude of different weight figures for the P-51B also, I'm not sure what to make of them), though in truth it would be more like 533-713 because in combat you would have already either drained the center tank or had 30 gallons at max present.

Combat came at unexpected times. Center tank usage SOP was to burn 20-25 gallons in climb up to cruise and then switch to externals. On Very Long missions center tank was conserved until after drop tanks released.

As for turning performance, the P-51D had a slightly different wing and less drag. With the turning circle can I just vary the weight difference, or is this going to require more complicated calculations due to the wing and drag?
I'm not sure if I missed something, but why would you want to reduce FTH on the P-51D's over the P-51B/C if they're being used as bomber escorts?
.

The P-51 wing from XP51 through P-51B/D was the same wing/airfoil NAA/NACA 45-100. Only the root chord was changed on the P-51D to accommodate the new Gear Door and wheel uplock design. You may be thinking about the XP-51F/G/J and P-51H which had straight leading edge and different (but same T/C) NACA 66-(1.8) 15.5 airfoil.

If you are playing with 'textbook' turning radius calculations, Drag and Engine/Prop Efficiencies and HP as f(altitude) are not factors, and not realistic nor consistent with real world. A factor missed by the gamers, and very elusive, is the Form Drag increase as a function of Angle of Attack which is derived from wind tunnel and for axis symmetric flight only. Asymmetric flight conditions with side loads, rudder and elevator trim increments also introduce Trim Drag.

Basically, if you are serious, you need to also add cooling drag for the P-51 which is significant as high speed exhaust thrust neutralized the Cooling Drag but in Climb was a Major factor, The latter factors contribute to increments to Power Required in addition to incremental form drag and trim drag discussed above.

To the Power Available from the manufacturer charts (which sometimes include ram effect), you also need to calculate Exhaust gas Thrust. To take into consideration the Engine/Prop efficiency you have to carefully look at the airspeed and altitude because the Prop efficiency falls off in high AoA/Low speed envelope (as in high G turn).

 

[bobbysocks]

If you are playing with 'textbook' turning radius calculations, Drag and Engine/Prop Efficiencies and HP as f(altitude) are not factors, and not realistic nor consistent with real world. A factor missed by the gamers, and very elusive, is the Form Drag increase as a function of Angle of Attack which is derived from wind tunnel and for axis symmetric flight only. Asymmetric flight conditions with side loads, rudder and elevator trim increments also introduce Trim Drag.

Basically, if you are serious, you need to also add cooling drag for the P-51 which is significant as high speed exhaust thrust neutralized the Cooling Drag but in Climb was a Major factor, The latter factors contribute to increments to Power Required in addition to incremental form drag and trim drag discussed above.

To the Power Available from the manufacturer charts (which sometimes include ram effect), you also need to calculate Exhaust gas Thrust. To take into consideration the Engine/Prop efficiency you have to carefully look at the airspeed and altitude because the Prop efficiency falls off in high AoA/Low speed envelope (as in high G turn).

Bill, you need to write a synopsis of the calculations gaming developers need to use....in all you free time of course..haha.

I always get into discussions with these players because they are under the impression that the flight models in the games are accurate when in fact they are woefully wrong. I don't think the developers intentionally do this but in reality have no clue as to what metrics to use for the formulation of those flight models. they basically build off of someone else's work....which is wrong to begin with.

 

[GregP]

What?!

You mean real airplanes DON'T have unlimited fuel? And you CAN'T fly them at full throttle for the entire flight? And they DON'T have unlimited ammunition, either?

Next you'll be telling me tailgraggers don't take off straight, either!

 

[bobbysocks]

and they are wheels up with full load out after only rolling out 100 feet....

 

[GregP]

And you can start doing rolls right after liftoff with EASE ... and gain altitude at the same time.

 

[pbehn]

When 10% of people using sims die in training and deaths using sims are at a similar level to those in combat then sims can be accepted as realistic. I have taken on many a Bf 109 in a head on attack and not once has a bullet come through my screen (But then I am a sim ace).

 

[KiwiBiggles]

When 10% of people using sims die in training and deaths using sims are at a similar level to those in combat then sims can be accepted as realistic. I have taken on many a Bf 109 in a head on attack and not once has a bullet come through my screen (But then I am a sim ace).

A bit unfair. I don't think air forces require that their simulators kill 10% of their trainees, but they still seem to find them useful. Simulators don't have to be realistic in every detail to be useful and interesting in many ways.

 

[Zipper730]

Somewhere around that.

The most efficient cruise is typically how much altitude or percentage of altitude above the critical altitude?

But you are overlooking the fact that when the bombers were attacked the escorts chased after the fighters which, in most cases, saw the lose altitude.

Of course, and then afterwards they'd probably have to climb back into the bomber formation right?

The P-51 wing from XP51 through P-51B/D was the same wing/airfoil NAA/NACA 45-100. Only the root chord was changed on the P-51D to accommodate the new Gear Door and wheel uplock design.

Still doesn't that have an effect on the performance of the wing? Did the wing-area or aspect-ratio change at all (Most sources say no, but the shape changed so it seemed a good question to start with).

If you are playing with 'textbook' turning radius calculations, Drag and Engine/Prop Efficiencies and HP as f(altitude) are not factors, and not realistic nor consistent with real world.

So just multiplying the difference in weight (85% so multiplying turn-radius by 0.85) is not realistic?

A factor missed by the gamers, and very elusive, is the Form Drag increase as a function of Angle of Attack which is derived from wind tunnel and for axis symmetric flight only.

Okay, I can find the form drag formula on wikipedia; I'm not sure how to determine this data based on the presence of AoA (unless the formula already includes it).

Asymmetric flight conditions with side loads, rudder and elevator trim increments also introduce Trim Drag.

I'm not sure where I'd be able to even find all that data.

Basically, if you are serious, you need to also add cooling drag for the P-51 which is significant as high speed exhaust thrust neutralized the Cooling Drag but in Climb was a Major factor, The latter factors contribute to increments to Power Required in addition to incremental form drag and trim drag discussed above.

No idea where I would find that?

To the Power Available from the manufacturer charts (which sometimes include ram effect), you also need to calculate Exhaust gas Thrust.

That varies with speed and altitude right?

To take into consideration the Engine/Prop efficiency you have to carefully look at the airspeed and altitude because the Prop efficiency falls off in high AoA/Low speed envelope (as in high G turn).

I don't actually know how to determine propeller efficiency....

Bill, you need to write a synopsis of the calculations gaming developers need to use....in all you free time of course..haha.

Agreed!

 

[drgondog]

Form Drag as a function of AoA was not suited to an analytical approach in WWII - all wind tunnel. Wikipedia is worthless for useful Physics of flight.

Power Available as a function of altitude can be found on many sites. Spitfireperformance.com is a good site for the Power curves - some have adjustments for ram effect.

Cooling Drag is wind tunnel focused.

There isn't enough time in the day to explain how to derive Propeller Efficiency. The definition is Eta= THP/SHP where THP is Useful Work Done on Airframe and SHP is Work Output of the Power Plan. There are many texts that will lead you down the path - but you have to have the Activity Factor - a non-dimensional value which is the capacity of the prop to absorb power. It can be derived via numerical integration. It is done empirically and also adjusted for profile drag of the prop and compressibility effect. One text for you to look at is Gerald Corning's Supersonic and Subsonic Airplane Design.

Also look to NACA Restricted Report, dated September, 1943, "Representative Operating Charts for Propellers Tested in NACA 20 ft. Propeller Research Research Tunnel".

Calculating Exhaust Gas Thrust (and drag) of exhaust stacks requires Po Outside static airpressure, A = Total stack opening area, Me+ Engine Charge Consumption in slugs/sec, Po/Pm = ratio of static to manifold pressure. A couple of charts are necessary. Charge Consumption for both High and Low Blower are required as a function of Brake HP.

Engine air momentum loss must be calculated to account for corrections to Power Available which require Engine Air Consumption, Engine Charge Consumption and the Fuel to Air ratio.

I have spent way too much time on this. If I am silent regarding future questions on the 'unknowable by simple calculations', please give me some slack.

Also NACA Restricted Report, dated November, 1942 "Effect of Exhaust Stack Shape on Design and Performanceof the Individual Cylinder Exhaust Gas Propulsion System"

 

[pbehn]

I have spent way too much time on this."

I for one appreciate the time you have taken, I know I know next to nothing about aerodynamics, what I didnt know is how many fields of knowledge there are which I didn't even know existed.

 

[GregP]

Being as rusty as I am on it, I appreciate the time spent as well. I find I can spend several hours looking up something, reviewing it, and formulating an opinion ... somewhat begrudge a lot of additional time to document it for a one-sentence question like, "how did you arrive at that conclusion?" especially considering how lousy a typist I am, not to mention how slow.

Some of you guys are very thorough at it though. We all know who they are, and I appreciate the effort expended, whether or not I see it the same way. A concise explanation goes a long way to help understanding. Well, maybe except for noconshooter's "thrust column" theory ...

So, thanks for the time investment, guys.

 

[Shortround6]

I have spent way too much time on this. If I am silent regarding future questions on the 'unknowable by simple calculations', please give me some slack.

A very well said post.

If simple calculations would actually give even close to real answers then people wouldn't have to go to school for years to get a handle on even one or two aspects of flight performance.

There is no "Fight Performance Calculations for Dummies" book available anymore than there is a "Career in Wall Street Investing for Dummies" book or "Brain surgery in 4 and 1/2 weeks".

Aeronautical companies didn't employ engineers by the dozens (and by the time of jets by the hundreds) if simple calculations would do the job.

 

[Zipper730]

Power Available as a function of altitude can be found on many sites. Spitfireperformance.com is a good site for the Power curves - some have adjustments for ram effect.

Sounds good

Cooling Drag is wind tunnel focused.

Ok

There isn't enough time in the day to explain how to derive Propeller Efficiency.

It's in the John D. Anderson book though?

One text for you to look at is Gerald Corning's Supersonic and Subsonic Airplane Design.

When some more money flows in and my courses are finished, I might get it...

Also look to NACA Restricted Report, dated September, 1943, "Representative Operating Charts for Propellers Tested in NACA 20 ft. Propeller Research Research Tunnel".

That sounds good

Calculating Exhaust Gas Thrust (and drag) of exhaust stacks requires Po Outside static airpressure, A = Total stack opening area, Me+ Engine Charge Consumption in slugs/sec, Po/Pm = ratio of static to manifold pressure. A couple of charts are necessary. Charge Consumption for both High and Low Blower are required as a function of Brake HP.

Engine air momentum loss must be calculated to account for corrections to Power Available which require Engine Air Consumption, Engine Charge Consumption and the Fuel to Air ratio.

Beyond my current pay-grade, I'll have to read up on this

Also NACA Restricted Report, dated November, 1942 "Effect of Exhaust Stack Shape on Design and Performanceof the Individual Cylinder Exhaust Gas Propulsion System"

Fascinating

A very well said post.

If simple calculations would actually give even close to real answers then people wouldn't have to go to school for years to get a handle on even one or two aspects of flight performance.

Actually I've had cases where a person gave me one piece of information and it made it possible to make sense of the rest...

There is no "Fight Performance Calculations for Dummies" book available anymore than there is a "Career in Wall Street Investing for Dummies" book or "Brain surgery in 4 and 1/2 weeks".

I see your point...

 

[swampyankee]

When 10% of people using sims die in training and deaths using sims are at a similar level to those in combat then sims can be accepted as realistic. I have taken on many a Bf 109 in a head on attack and not once has a bullet come through my screen (But then I am a sim ace).

Certainly, the death rate in service wasn't that high. After all, aircrew had almost a 50% chance of surviving 50 missions. (8th Air Force bomber crews had casualty rates greater than infantry units).

 

[pbehn]

Certainly, the death rate in service wasn't that high. After all, aircrew had almost a 50% chance of surviving 50 missions. (8th Air Force bomber crews had casualty rates greater than infantry units).

These are not the same thing at all. Fifty missions is fifty flights. From starting training to becoming a qualified front line pilot with two hundred hours on P51s takes a lot more than 50 flights and the trainee is permanently learning, as he becomes proficient on one type he then gets to fly something more difficult. Loss rates are an average, while it may be true over the whole war that a crew had a 50% chance of surviving 50 missions, it is also true that they had less than 50% chance of surviving one mission like Fairey Battles in France or some unescorted RAF daylight raids. US losses on some raids were around 30% while on other raids late in the war the loss rate purely reflected the reliability of the aircraft itself.

Bomber Command losses September 1939 - May 1945: 47,268 men killed on operations. Another 8,303 killed in flying or training accidents, total 55,571. Another 1,570 groundcrew and WAAFs lost their lives from other causes.

From the table here
United States World War II Aircraft Loss Statistics during Flight Training
13,621 people died in 5533 fatal accidents while training in the US air force.

Flying at the time was dangerous in itself especially from airfields in the UK when fog is common and low cloud perfectly normal.