P-47N/M vs P-51H

drgondog · Dec 6, 2021

Howard Gibson said:
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.

Without diving deep into the weeds, on or about Mcr a lot of bad things begin to alter the mechanics of flight associated with the wing as a result of the chosen airfoil. Most conventional airfoil/wings had a max thickness to chord ratio (fat part of wing) at 25-30%. The velcity gradient from LE to the place on the wing (usually Max T/C) reaches Mcr at the thickest part and creates a shock wave as the airflow transitions from subsonic to supersonic. When that occurs the Center of Pressure of the wing is moved aft, creating a Mach Tuck or Nose Tuck in which the pitching moment overwhelms the ability of the elevator to restore pith control... further complicated by wake turbulence immersing the elevator.

The Mustang with unique high speed airfoil/wing design had the max T/C further aft, the velocity gradient was less (delaying formation of shock wave) and movement of Center of Pressure.

Howard Gibson said:
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.

Same phenomena as described above.

Howard Gibson said:
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.

They were usually chasing the Fws so the Fw 190 was delighted to be set free.

Howard Gibson said:
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?

As described, yes.

mig-31bm · Dec 6, 2021

drgondog said:
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

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.

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

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.

That a lot of information about airfoil. But iam not entirely sure what you are trying to convey to me though? That weight is literally have nothing to do with dive speed like greg said?. How come though?. The same airfoil, the same engine are still on these respective fighter when they fly horizontally. The only different between a dive and horizontal fly is just the additional gravity that add force to direction of travel (and might be less lift induced drag) so how come weight play no role?.

tommayer · Dec 6, 2021

mig-31bm said:
How does the last massed production P-51 version compared to the last mass production P-47?
In term of acceleration?
Top speed?
Turn rate?
Climb rate?
Overall which is the better fighter? and more suited to deal with FW-190D-9 and TA-152C/H?
P/s: how come the p-47 so insanely fast? it look so draggy. And what is the benefit of curvy elliptical wing and tail on P-47, Splitfire compared to square/trapezoid wind like on P-51, FW-190?
View attachment 650409
View attachment 650410

Suggest you go to Greg's Airplanes and Automobiles. He has done very good videos that address everything you are interested in. Quick and dirty guesses, made w/o reference to the manuals: The 47N would have been able to deal with the TA-152 up high, cause it had the highest critical altitude. I should think the Grummans would have slaughtered the German under 20K, and 51 Hotel would have had little problem with it from 20-30 K.

Howard Gibson · Dec 6, 2021

tomo pauk said:
How many were they losing due to, I presume, insurmountable problems in dive?

Eric Brown does not say. It was bad enough that General Jimmy Dolittle asked the Royal Aircraft Establishment and Eric Brown for help.

pbehn · Dec 6, 2021

mig-31bm said:
That a lot of information about airfoil. But iam not entirely sure what you are trying to convey to me though? That weight is literally have nothing to do with dive speed like greg said?. How come though?. The same airfoil, the same engine are still on these respective fighter when they fly horizontally. The only different between a dive and horizontal fly is just the additional gravity that add force to direction of travel (and might be less lift induced drag) so how come weight play no role?.

You are discussing the maximum speed in a dive. In horizontal flight at maximum sped a plane is in equilibrium between thrust and drag. In a dive it goes to a new equilibrium but the increase in drag at high speed caused by compressibility and shock waves etc is huge and the weight of the plane makes no difference. As previously stated the heaviest of the planes discussed have the lowest mach numbers, while the fastest in a dive was the Spitfire which is among the lightest.

tomo pauk · Dec 6, 2021

Howard Gibson said:
Eric Brown does not say. It was bad enough that General Jimmy Dolittle asked the Royal Aircraft Establishment and Eric Brown for help.

Thank you. When was he asking them, like in what month? Their reply and/or help?

Howard Gibson · Dec 6, 2021

tomo pauk said:
Thank you. When was he asking them, like in what month? Their reply and/or help?

Read Wings on my Sleeve, by Eric Brown. Doolittle visited the RAE in early 1944. The decision was made to switch escort fighters over to Mustangs.

tomo pauk · Dec 6, 2021

Howard Gibson said:
Read Wings on my Sleeve, by Eric Brown. Doolittle visited the RAE in early 1944.

Roger that.

Howard Gibson said:
The decision was made to switch escort fighters over to Mustangs.

Because the USAF was loosing fighters that entered a dive in early 1944? For those that believe that, I have Tower Bridge on sale, real cheap.

drgondog · Dec 6, 2021

mig-31bm said:
That a lot of information about airfoil. But iam not entirely sure what you are trying to convey to me though? That weight is literally have nothing to do with dive speed like greg said?. How come though?. The same airfoil, the same engine are still on these respective fighter when they fly horizontally. The only different between a dive and horizontal fly is just the additional gravity that add force to direction of travel (and might be less lift induced drag) so how come weight play no role?.

Gross Weight plays an initial role in the introduction of the a/c in a dive, in the initial acceleration - until the Drag Force far exceeds the ability of Gross Weight and propeller Thrusr HP to accelerate the moving body. Weight Does play a role in adding the acceleration vector to THP to achieve higher velocities in a dive than in level flight (or Climb), but THP DECREASES with V. Gross Weight is factor in Wing Loading and Induced Drag - all important in different performance calculations - but not so much for Maximum Dive speed.

That isn't the answer to the question "Why does a Mustang dive so much faster than an aircraft with more HP"?

The Drag Rise CDm as f(M) is Non-linear.
The Propeller/engine system Thrust HP Decreases with Velocity.

OTOH a jet maintains thrust. If a Spitfire or Mustang somehow crammed a J47 into either, they could probably achieve a dive speed , M<1, at which they would disintegrate - but NOT attain or exceed M=1.

Drag is the thing. The Wing is the 'first suspect' when one airframe with similar HP exceeds performance of another -

drgondog · Dec 6, 2021

Howard Gibson said:
Read Wings on my Sleeve, by Eric Brown. Doolittle visited the RAE in early 1944. The decision was made to switch escort fighters over to Mustangs.

Howard - the P-47B and P-38E (or F) experienced catastrophic structural failures of empennage due to dive pull outs in 1942. The knowledge of compressibility as a guide to airframe dsign was in its infancy. I explained the cause somewhere today, and ultimately the addition of dive flap on lower surface of the wing to disrupt airflow in the transonic region enabled better control in the dive.

The P-38J was first production P-38 with dive flaps, introduced in mid 1944. Ditto for P-47D-30 (?) in summer 1944.

The problem was known and worked on a year (in ETO via Cass Hough) )before Jimmy Doolitle took control of 8th AF.

The decision to switch from P-47D and P-38J to P-51B for primary LR escort was both for range and performance considerations.

tomo pauk · Dec 6, 2021

drgondog said:
The P-38J was first production P-38 with dive flaps, introduced in mid 1944. Ditto for P-47D-30 (?) in summer 1944.
The problem was known and worked on a year (in ETO via Cass Hough) )before Jimmy Doolitle took control of 8th AF.
The decision to switch from P-47D and P-38J to P-51B for primary LR escort was both for range and performance considerations.

Now, wait a second. Are you implying that it was not RAE & Eric Brown that were instrumental for the decision that Mustang is to be the escort fighter, after the USAF was loosing fighters from dives in early 1944?

I'm shocked.

wuzak · Dec 6, 2021

drgondog said:
Gross Weight plays an initial role in the introduction of the a/c in a dive, in the initial acceleration - until the Drag Force far exceeds the ability of Gross Weight and propeller Thrusr HP to accelerate the moving body.

When pilots in WW2 described their aircraft as a "good diver" were they describing the acceleration in a dive, or the speed that could be attained in the dive? Or, perhaps, the stability in the dive and the ability to pull out?

Howard Gibson · Dec 6, 2021

wuzak said:
When pilots in WW2 described their aircraft as a "good diver" were they describing the acceleration in a dive, or the speed that could be attained in the dive? Or, perhaps, the stability in the dive and the ability to pull out?

That is three very different performances.

Macandy · Dec 7, 2021

The H model Mustang was not a 'good' warplane.
It was a refined lightweight thoroughbred, it was fast, very fast, but it traded the Mustangs robustness for the Spitfires fragility.

tomo pauk · Dec 7, 2021

Macandy said:
The H model Mustang was not a 'good' warplane.
It was a refined lightweight thoroughbred, it was fast, very fast, but it traded the Mustangs robustness for the Spitfires fragility.

There is no greater curse than not to be a good warplane - just like the fragile Spitfire.

pbehn · Dec 7, 2021

tomo pauk said:
There is no greater curse than not to be a good warplane - just like the fragile Spitfire.

Especially a fast one.

tomo pauk · Dec 7, 2021

pbehn said:
Especially a fast one.

Yep. Horrors await for a pilot that flew that version into a war zone.
*shudders*

pbehn · Dec 7, 2021

tomo pauk said:
Yep. Horrors await for a pilot that flew that version into a war zone.
*shudders*

They no longer do squadron photos with half the air base staff on a bombers wings, because todays planes are flimsy thoroughbred rubbish.

drgondog · Dec 7, 2021

wuzak said:
When pilots in WW2 described their aircraft as a "good diver" were they describing the acceleration in a dive, or the speed that could be attained in the dive? Or, perhaps, the stability in the dive and the ability to pull out?

I think all three were important. It was so rated in the Fighter Conference questionairre - I know the last part was most critical to the participants.

drgondog · Dec 7, 2021

Macandy said:
The H model Mustang was not a 'good' warplane.
It was a refined lightweight thoroughbred, it was fast, very fast, but it traded the Mustangs robustness for the Spitfires fragility.

Ah No. It would have been a superb warplane - better than predecessors in all respects save rough field ops. Its landing gear was designed for 4G compared to 7G for all the other operational variants.

The P-51H in fact was more robust at design Combat Weight of 9600 pounds. Its design Limit load was 7.3G (same as Spitfire) at 9600 pounds. These changes to design philosophy were made after comparing P-51B to Spit IX in October/November 1942 - leading to Schmued's visit to Britain to look and learn. The XP-51F was the first derivative of his activities. The 7.3/11G Limit/Ultimate Load design for Stress margin allowables as the XP-51F grew to XP-51G (heavier by 800 pounds) to P-51H heavier by 2000 pounds over XP-51F at max internal Combat weight.

The P-51D at the same Weight of full P-51H Combat weight (but 600 pounds less than full P-51D Combat Weight of 10,200 pounds) had a limit load of 6.7 G.

The Design Limit was not altered from the NA-73 through the P-51D. 8G at 8,000 pounds Gross Weight. That said, there were several mods made to beef up empennage and wing (i.e Doublers at WS 75) that improved the actual limit load for the B/D above "6.3G" at 10200 pounds - but no part by part, assembly by assembly component re-design was made to re-stress the total airframe to maintain "8G Limit Load" as it grew in weight over four years.

There were individual component re-designs for rib/spar in form of doublers to account for load increases via bomb racks as the rack limit increased to 1000 pounds and accounting had to be made for sway brace and rack loads imposed by scooting a 165 gallon combat tank along at 400mph with 1G lateral combined with fore/aft Q loads.

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