P-47N/M vs P-51H

[Howard Gibson]

I am working on a project here at home, and things are thoroughly out of hand. I have a graph that is appropriate for this discussion. These all are aircraft that were fast, late in WW2.

All of these curves are from WWII Aircraft Performance, except for that of the Yak-3. I got those numbers from the Soviet Air Force Fighters Part 2 WW2 Aircraft Fact Files, by Green and Swanborough. A P-51H at 487mph is running on 150 octane fuel and water injection. On 100/130 octane, it should be somewhat faster than the P-51D shown. The P-47M had performance similar to that of the P-47N. Note the War Emergency Power. I assume that is methanol water injection.

The Mustang, Spitfire and the Thunderbolt all have two stages of supercharging, as you can tell at 30,000ft.

 

[Acheron]

I don't see how weight can "have nothing with dive speed and dive acceleration"
Weight is literally the reason why you can dive at higher speed and higher acceleration than you can fly horizontally. When diving, weight does have a force vector in the direction of travel, the steeper the dive angle, the bigger this force component will be, basically increasing your "thrust"

I think GregP might be referring to this: Galileo's Leaning Tower of Pisa experiment - Wikipedia

 

[drgondog]

I don't see how weight can "have nothing with dive speed and dive acceleration"
Weight is literally the reason why you can dive at higher speed and higher acceleration than you can fly horizontally. When diving, weight does have a force vector in the direction of travel, the steeper the dive angle, the bigger this force component will be, basically increasing your "thrust"

Perhaps you wish to explain why the limit dive speeds of the Mustang and the P-47 are the same. And why the F6F and F4U are 'slower' in dive than the Mustang? Greg's point was that 'weight' per se is not the primary factor for achieving high speed in a dive.

And explain why the P-47 is faster in dive than say a P-38 (which weighs more).

Yes the force vector due to the attraction by gravity on a mass is a factor for acceleration to point where Drag=Thrust+Weight.

That said, Drag is the key... up to the point where the aircraft pushes the limit load - and enters the 'no can hold it together zone'. Note that the combined HP of a B-17 far exceeds that of any WWII fighter and the GW far exceeds any fighter. It will transform into a shower of alminum confetti and spare parts long before reaching dive speeds suggested by its very large gross weight.

 

[Shortround6]

The engine has to pump air out of the cylinders through the turbine. I am not totally into turbochargers. They may be more efficient than engine driven superchargers. They still use significant power

ROFLMAO

"May be more efficient than engine driven superchargers?"

May be more efficient?

P-47 makes about 2000hp at 25-27,000ft using it's single engine stage supercharger and it's turbo supercharger and using 52in of manifold pressure.
An F4U makes about 1650hp at 23,000ft (using RAM) using it's two stage engine driven supercharger and using 53in of manifold pressure.

Granted the F4U has a bit more exhaust thrust from the exhaust pipes (but that can be changed by the engine installations)

Yes the P-47 paid a penalty for the "higher efficiency " due to the higher installed weight and higher installed volume of the air ducts (But the F4U did have intercoolers and some duct work).

P-47 was making over 350hp to the prop (17.5%) over and above the cylinders pumping the air out of the cylinders to the turbine.

 

[mig-31bm]

Perhaps you wish to explain why the limit dive speeds of the Mustang and the P-47 are the same.

Ok, I don't have exact number so i will use this ">" symbol to explain "greater" , more ">" symbol illustrate more magnitude of greater
P-47 vs P-51
Because top speed is reached when thrust = drag
P-47's drag >>> P-51's drag
P-47's engine power at sea level >> P-51's engine power at sea level
=> P-47's horizontal speed at sea level < P-51's horizontal speed at sea level
P-47 engine power at 30k ft >>>>P-51 engine power at 30k ft
=> P-47's horizontal speed at 30 kft > P-51's horizontal speed at 30 kft

When diving , the mass of the aircraft add into the "thrust", because gravity has a component in direction of travel when aircraft is diving
and since P-47 weight >> P-51 weight
it is not a big surprise that it would be enough to close the gap in drag

And why the F6F and F4U are 'slower' in dive than the Mustang?

same as P-47, they are draggier than P-51, but they don't have enough additional mass to compensate to that in a dive
and their engine output at altitude is less than P-47 as well

Greg's point was that 'weight' per se is not the primary factor for achieving high speed in a dive.

In a vacuum then sure , being heavier doesn't matter. But in atmosphere, being heavier basically give you more "thrust" to overcome the drag when diving.
Let me put it this way. If we have a piece of paper 4*4 cm, and a piece of metal 4*4 meter, then obviously the piece of metal will have more drag due to much higher surface area.
However, when we drop them, the piece of metal will still fall to ground much faster than the piece of paper

And explain why the P-47 is faster in dive than say a P-38 (which weighs more).

P-38 control surface at lock up at high speed as far as i know, and it probably much draggier than P-47 due to the twin boom as well

 

[pbehn]

The Spitfire had the highest dive speed, it could even dive faster than its own propeller, you dont get faster than that.

 

[Howard Gibson]

P-38 control surface at lock up at high speed as far as i know, and it probably much draggier than P-47 due to the twin boom as well

Read your Eric Brown. A P-47 had a "tactical Mach number" of 0.71. A P-38's is 0.68. The Bf109 and Fw190 were 0.75, and the P-51 0.78. The P-47 may have better acceleration in a dive, but the Mustang can achieve a higher speed, safely anyway. Brown described Spitfires as "easily controlled" up to Mach 0.84. The Spitfire would have less acceleration in the dive, and the highest ultimate speed.

 

[pbehn]

If all of the above aircraft were to fly at their maximum speed at 30,000ft then dive vertically down, which would break up first which would reach the highest speed and how long would it take?

 

[Howard Gibson]

pbehn,

Obviously, not the Yak-3.

 

[SaparotRob]

Turning out to be quite an educational thread for me.

 

[pbehn]

pbehn,

Obviously, not the Yak-3.

I think the Yak can sit this one out.

 

[Howard Gibson]

If all of the above aircraft were to fly at their maximum speed at 30,000ft then dive vertically down, which would break up first which would reach the highest speed and how long would it take?

You have forced me to check my sources.

In early 1944, the Americans were losing escort fighters when they dove down on Germans at high speed. I presume their controls locked up. I assume bomber interceptor interception took place at a bit over 30,000ft. Brown's terminology is interesting. He talks about tactical (manoeuvring) and critical (loss of control) Mach numbers. The tactical Mach numbers of the P-38H and the P-47C were 0.68 and 0.71, respectively.

I am a bit puzzled by all this. Roland Beamont tested Hawker Typhoons by diving from 30,000ft, I think at around 45°. Their controls locked up. It appears to me that a Typhoon would have difficulties doing much more than 300mph at 30,000ft. At around 15,000ft, the speed of sound would increase sufficiently to drop the Mach number to the point where the aircraft could be controlled. Beamont died of old age in 2001.

I don't understand why the Thunderbolts and Lightnings were unable to pull out. If they pull out at 15,000ft, they would be vulnerable to Fw190s. Otherwise, they slowly or quickly (respectively) climb back up the bomber altitude, or they fly home. It would be wise for the Thunderbolt to get his ass up above 25,000ft.

Diving from top speed at 40,000ft, the stripped and instrumented SpitfireXIs started to gently shake at Mach 0.83. Does this translate to tactical Mach number?

 

[BiffF15]

Ok, I don't have exact number so i will use this ">" symbol to explain "greater" , more ">" symbol illustrate more magnitude of greater
P-47 vs P-51
Because top speed is reached when thrust = drag
P-47's drag >>> P-51's drag
P-47's engine power at sea level >> P-51's engine power at sea level
=> P-47's horizontal speed at sea level < P-51's horizontal speed at sea level
P-47 engine power at 30k ft >>>>P-51 engine power at 30k ft
=> P-47's horizontal speed at 30 kft > P-51's horizontal speed at 30 kft

When diving , the mass of the aircraft add into the "thrust", because gravity has a component in direction of travel when aircraft is diving
and since P-47 weight >> P-51 weight
it is not a big surprise that it would be enough to close the gap in drag

same as P-47, they are draggier than P-51, but they don't have enough additional mass to compensate to that in a dive
and their engine output at altitude is less than P-47 as well


In a vacuum then sure , being heavier doesn't matter. But in atmosphere, being heavier basically give you more "thrust" to overcome the drag when diving.
Let me put it this way. If we have a piece of paper 4*4 cm, and a piece of metal 4*4 meter, then obviously the piece of metal will have more drag due to much higher surface area.
However, when we drop them, the piece of metal will still fall to ground much faster than the piece of paper

P-38 control surface at lock up at high speed as far as i know, and it probably much draggier than P-47 due to the twin boom as well

MiG-31BM,

FYI you are arguing aerodynamics with a guy who has both a BS and a Masters in Aerodynamics…

Cheers,
Biff

 

[mig-31bm]

MiG-31BM,

FYI you are arguing aerodynamics with a guy who has both a BS and a Masters in Aerodynamics…

Cheers,
Biff

So?. Even if iam wrong, it still better to say what i think, so that people can correct my mistake. I would rather be a fool once than remain silent and be a fool forever

 

[mig-31bm]

. A P-51H at 487mph is running on 150 octane fuel and water injection. On 100/130 octane, it should be somewhat faster than the P-51D shown.

This is what still kinda confuse me till now.
In this chart, is the F-51H running with 130 octane or 150 octane fuel?

The army P-51H manual seem to show 80 Hg MP and 130 octane fuel only, but that manual doesn't show the top speed

 

[Metrallaroja]

In this chart, is the F-51H running with 130 octane or 150 octane fuel?

115/145 and 90''

http://www.alternatewars.com/SAC/F-51H_Mustang_SAC_-_3_July_1950.pdf

 

[drgondog]

Ok, I don't have exact number so i will use this ">" symbol to explain "greater" , more ">" symbol illustrate more magnitude of greater
P-47 vs P-51
Because top speed is reached when thrust = drag
P-47's drag >>> P-51's drag
P-47's engine power at sea level >> P-51's engine power at sea level
=> P-47's horizontal speed at sea level < P-51's horizontal speed at sea level
P-47 engine power at 30k ft >>>>P-51 engine power at 30k ft
=> P-47's horizontal speed at 30 kft > P-51's horizontal speed at 30 kft

When diving , the mass of the aircraft add into the "thrust", because gravity has a component in direction of travel when aircraft is diving
and since P-47 weight >> P-51 weight
it is not a big surprise that it would be enough to close the gap in drag

Thank you for helping me forget all that I learned while studying for my MS in Aero. While I desparately clear the cobwebs trying to re-learn the CDparasite of paper and cannonballs I usually go look at the Wing first for 'hidden clues'. Maybe you should also. While strolling through those references, seek insight on Drag Rise as CDp varies with Mach No. As you peruse through CDp vs Mcr (and beyond) seek clues regarding thickness to Chord ratio and location of Max T/C for each of the wings. Seek further elucidation on the % of Drag contribution of the Wing v Fuselage v Empennage.

The T/C comparisons IIRC
Spit NACA 2412 12% at approx 29% C - I Can't remamber exactly because I am feeble minded but seem to recall airfoil and T/C - but Spit wing bay far the 'thinnest'. This is important
P-51 NAA/NACA 43-100 (T/C 16% at 37.5% C AKA High Speed and Laminar airfoil in which the velocity gradient signifcantly less than convention for same V
P-38 NACA 23016 (same T/C as P-51)
P-47 Republic S-3 (T/C 15% at approx 29% C)
F4U NACA 23018 (T/C 18% at approx 25% C
F6F NACA 23015.6Mod (T/C 1.6% at approx 25%)

So Fattest Wings F4U then P-51
Thinnest wings Spit, then P-47
Fastest divers Spit, P-51/P-47 then F4U Spit = lightest<P-51<F4U,P-47 ---------> Lightest airfames, least HP
Slowest diver P-38 ------> Greatest GW, Greatest HP

Conclusions: Airfoil selection critical - fat wing/low drag = P-51. Thin wing/conventional drag = Spitfire, P-47

Look to drag characteristics of the airfoil/wing - Best L/D = P-51

same as P-47, they are draggier than P-51, but they don't have enough additional mass to compensate to that in a dive
and their engine output at altitude is less than P-47 as well

Draggier is 'imprecise' hand waving. How about comparing dive speed well under FTH for turbo'd R-2800 where the dive HP of P-47 is same as F6F and F4U and far less than P-38.

In a vacuum then sure , being heavier doesn't matter. But in atmosphere, being heavier basically give you more "thrust" to overcome the drag when diving.
Let me put it this way. If we have a piece of paper 4*4 cm, and a piece of metal 4*4 meter, then obviously the piece of metal will have more drag due to much higher surface area.
However, when we drop them, the piece of metal will still fall to ground much faster than the piece of paper

Oh, how could I lose sight of that comparison while discussing Drag of WWII fighters?

P-38 control surface at lock up at high speed as far as i know, and it probably much draggier than P-47 due to the twin boom as well

Both the P-47 AND P-38 experienced 'nose tuck' at or near MCr in which dive recovery was delayed until reaching higher density altitude. And for the record, the CDp of the P-38> P-47 but reached Mcr at or near 0.68M. P-38, F4U and F6F all had CDp nearly the same.

 

[pbehn]

You have forced me to check my sources.

In early 1944, the Americans were losing escort fighters when they dove down on Germans at high speed. I presume their controls locked up. I assume bomber interceptor interception took place at a bit over 30,000ft. Brown's terminology is interesting. He talks about tactical (manoeuvring) and critical (loss of control) Mach numbers. The tactical Mach numbers of the P-38H and the P-47C were 0.68 and 0.71, respectively.

I am a bit puzzled by all this. Roland Beamont tested Hawker Typhoons by diving from 30,000ft, I think at around 45°. Their controls locked up. It appears to me that a Typhoon would have difficulties doing much more than 300mph at 30,000ft. At around 15,000ft, the speed of sound would increase sufficiently to drop the Mach number to the point where the aircraft could be controlled. Beamont died of old age in 2001.

I don't understand why the Thunderbolts and Lightnings were unable to pull out. If they pull out at 15,000ft, they would be vulnerable to Fw190s. Otherwise, they slowly or quickly (respectively) climb back up the bomber altitude, or they fly home. It would be wise for the Thunderbolt to get his ass up above 25,000ft.

Diving from top speed at 40,000ft, the stripped and instrumented SpitfireXIs started to gently shake at Mach 0.83. Does this translate to tactical Mach number?

Maybe in a combat situation pilots got into a situation that they couldnt get out of. 25,000ft is 4.7 miles and at 480MPH you are doing 1 mile every 7.5 seconds, if controls lock up at high speed when do you have to start pulling out of a dive steeper than 45 deg.

I think my question is easy to ask and difficult to answer.

 

[Metrallaroja]

Btw SAC performances of P51H are also calculations...
Does actually someone have a flight test source at 90'' and clean?
Best flight tests I could find are this ones but they are with bomb and rocket racks:

P-51H Performance Test

 

[tomo pauk]

In early 1944, the Americans were losing escort fighters when they dove down on Germans at high speed. I presume their controls locked up. I assume bomber interceptor interception took place at a bit over 30,000ft.

How many were they losing due to, I presume, insurmountable problems in dive?