Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)

[Ivan1GFP]

Hello Gentlemen,
It seems to me that it is again a debate on how much of a difference there needs to be before an aeroplane is considered a new design. There are certainly quite a few differences between the P-51D/K and P-51H, but if one goes back a little bit, there are just about as many differences between the P-51/P-51A and the P-51D. The Wing was a different section to allow the guns to be upright, the Armament was increased, the Fuselage changed for a Bubble Canopy, the Radiator is much larger and shaped differently. The entire Fuselage is deeper by about 3-4 inches. Design principles are the same but nothing beyond a few shapes and construction techniques is really quite the same.

Regarding the Whirlwind to Welkin: Eric Brown once made a few unfavourable comments about the Whirlwind and how the design had no ability for improvement because it could not mount any larger engines. How much had to change to make room for larger engines? Is the result just another clean slate design for a twin engine fighter?

Regarding the weights of the P-39C as versus the P-400, obviously some of it was equipment but some of it was also what each service considered "Normal Loaded" weight for the aircraft. The US Army had a habit of specifying only partial loads of fuel, ammunition and even engine oil as "Normal". The British practice may not have been quite the same.
I remember going through these differences early last year and finding the difference to be several hundred pounds.
Just Fuel alone accounts for 120 pounds.
Another amusing thing about the Airacobra is that although the armour tended to be (mostly) in the same places, it often was not the same thickness or weight depending on the exact version of the aircraft.

- Ivan.

 

[drgondog]

Hello Gentlemen,
It seems to me that it is again a debate on how much of a difference there needs to be before an aeroplane is considered a new design. There are certainly quite a few differences between the P-51D/K and P-51H, but if one goes back a little bit, there are just about as many differences between the P-51/P-51A and the P-51D. The Wing was a different section to allow the guns to be upright, the Armament was increased, the Fuselage changed for a Bubble Canopy, the Radiator is much larger and shaped differently. The entire Fuselage is deeper by about 3-4 inches. Design principles are the same but nothing beyond a few shapes and construction techniques is really quite the same.


- Ivan.

Ivan the airfoil for the wing for X73 through P-51K was NAA/NACA 45-100. Only the root chord and inboard leading edge of the wing changed for the D/K.The 50 caliber guns could have been mounted vertically in A-36, P-51A/B/C but probably were angled to preserve option to install 20mm (angled) from P-51-NA to A-36. Discussions between Material Command and NAA regarding adding 20mm to P-51B and D were finally killed in November 1943 and the gun mount to 50 caliber was changed to vertical for the P-51D and Subsequent. At that time there were still on-going discussions about P-51F (interceptor- production) with Oldsmobile 20mm guns that would have imposed significant change to LE spar and aft spar for the evolving NACA 66 series wing on the future P-51F, and cancelled.

 

[drgondog]

The bottom line is the British specified a plane too heavy to meet spec and then tried to cancel the order. The main reason they didn't want the plane was they won the Battle of Britain in late 1940 and there would be no German invasion. Now their internal fighter production would be sufficient and they for sure didn't want to pay for them.

Not true, They bought the Mustang I, IA, II, III and IV, the P-47D, the F4U, F4F, F6F, and the P-40. They didn't buy the P-39 because it didn't serve a niche that a.) the Spitfire served, b.) it wasn't superior enough to the Hurricane II in CAS role, and c.) it wasn't deemed superior to any of the others named above.

A second point is that the British were quite well aware in 1941 that the P-39 was a dead end evolution and the P-63 was two+ years away with an unproven engine that had no other airframe to match to simplify logistics. The P-40 (and Mustang I with essentially the same engine until the 1650-1 was installed in the F) was a much better all around choice for the Commonwealth.

 

[Milosh]

The Wing was a different section to allow the guns to be upright,

Can't believe this myth still gets stated. I guess it was forgotten that 4 20mm cannons were fitted in Allison powered P-51s without the wing being changed.

 

[Kevin J]

Why would the British, who were in desperate need for front line fighters, reject an aircraft which out-performed the Spitfire?

The Spitfire V had a higher critical altitude, so it is not surprising that the P-39 lightweight special out-climbed it at low altitude.

Because the threat of invasion had gone when they transferred them to the USSR. The British Cobra would have been a useful asset in repelling a German invasion, it had a cannon and much more armour than the P-39D.

 

[Peter Gunn]

Going back to the P-51H, I was under the impression that it was a whole different design than the D and shared very few, if any, components. Wing, tail and if I'm not mistaken, even the thrust line was altered to give better over the nose visibility for deflection shooting.

If I'm wrong I will stand (well, sit really) corrected.

 

[wuzak]

Because the threat of invasion had gone when they transferred them to the USSR. The British Cobra would have been a useful asset in repelling a German invasion, it had a cannon and much more armour than the P-39D.

The P-39D had a canon too.

Spitfires were held back from overseas deployment well into 1941, even 1942, to keep a strong defence.

Instead, Hurricanes and P-40s were shipped off to other theatres and Commonwealth nations.

If the P-39 was the equal of, or superior to, the Spitfire V the British would have laid their hand on as many as they could get.

It may have been used in ground attack roles, but that was not the primary reason the British were planning to acquire the P-39. It possibly wasn't even a consideration.

 

[Shortround6]

Because the threat of invasion had gone when they transferred them to the USSR. The British Cobra would have been a useful asset in repelling a German invasion, it had a cannon and much more armour than the P-39D.

By the fall of 1941 the British were fitting two 20mm cannon in the Spitfire and the Hurricane IIc was starting to show up with four 20mm cannon, and AIracobra I with a single 20mm (and drum fed) was hardly a wonder weapon for anti-invasion duties. The British never ordered or got an AIracobra with a 37mm.

I am really wondering about the much more armor bit too. The P-39D, when ordered, had about 250lbs of armor. Since you have P-39Ds, D-1s and D-2s not to mention the reposesed P-400s (which the British had paid for cash) it is quite possible the armor fit changed a bit. AHT (Dean) says that a P-39 D had 262.2lbs of armor and BP glass, a P-39D-2 had 246.4lbs and a P-400 had 265.6lbs while a Q-1 had 193lbs.

Now these are the weights form a specific aircraft and are going to fluctuate a bit from aircraft to aircraft on the procution line so I wouldn't pay attention to the tenths of pound or even a difference of 2-5 lbs out of 260lbs. 1-2% variation being common. Some sources say the British planes had a bit over 290lb which even if true is only 30lbs more than a P-39D. It alos makes you wonder what the US took out when they got the British Airacobras and just called them P-400s?

 

[tomo pauk]

The P-51H used pretty much the same wing, at least aerodynamicly, did it not?
Yes they got rid of the little wing root extensions (which helped house the landing gear) but wasn't the airfoil the same? or slightly stretched to straighten out the leading edge?
Wing span the same, (or within a 1/4 in) wing area is within 1-2 sq ft. fuselage is a 2 feet longer and the tail is taller (but a lot of planes got bigger/longer tails as they got more powerfuel engines.
Yes you can't make a P-51H out of a P-51D airframe (or at least not without a lot of work) but they are pretty close to each other.

Lightweigth Mustangs used different airfoil series, that was also thinner (% - wise) with 15.5% TtC at root; flaps and airlerons were of different size, different sweepback, washout and MaC. New wing also had bigger fuel tanks.

 

[MIflyer]

The XP-37's critical altitude was 20,000 ft and the YP-37's critical altitude was 25,000 ft. That is where their highest performance was, not at 10,000 ft, but up where the B-17's were flying. Highest speed was 331 MPH, so they missed the promised 340 mph, and the USAAC lost interest, choosing instead to buy the P-40 with the V-1710 and no turbo. But while the basic bare bones prototype P-40 could hit about 365 MPH, that was at a much lower altitude. The YP-37 was much faster above 20,000 ft, probably about 50 mph faster and had a higher maximum ceiling as well.

The Vulture was 2592 cubic inches so putting that supercharger as the first stage of the two stage Merlin gave that "little" engine (smaller displacement than a V-1710 or even the engine used in the A6M3) world class performance..

 

[MIflyer]

The RAF tested the 37MM of the P-39 and found that it would not penetrate the armor of the German tanks, while the Vickers 40MM gun on the Hurricane IID did. The USAAF said that the P-39's gun could crack tanks but the actual tests proved that it could not.

The "tank-busting" P-39 is a myth.

 

[Shortround6]

The Vulture was 2592 cubic inches so putting that supercharger as the first stage of the two stage Merlin gave that "little" engine (smaller displacement than a V-1710 or even the engine used in the A6M3) world class performance..

The Vulture supercharger impeller happened to be handy and about the right size for a proof of concept test rig. After that it's usefulness rather diminishes. It gave "close" to the desired airflow at the altitudes they were interested in.

I would note that the RR had a pretty good idea of what the Merlin could or could not do as a basic engine back in 1938/39 when they worked on the "Speed Spitfire"

 

[Shortround6]

The RAF tested the 37MM of the P-39 and found that it would not penetrate the armor of the German tanks, while the Vickers 40MM gun on the Hurricane IID did. The USAAF said that the P-39's gun could crack tanks but the actual tests proved that it could not.

The "tank-busting" P-39 is a myth.

Well, judging the ability of your anti-tank guns against your own tanks might lead to some faulty conclusions

More seriously. armor penetration is strongly related the amount of energy that can be delivered per sq in (sq cm ) of target area (area of the projectile).

The American AP projectile for the 37mm aircraft gun weighed 753 grams and had 116,000 Joules of energy at the muzzle.

The German 37mm AP projectile as used by the JU 87 (and others) weighed 680 grams but it's higher velocity gave 218,000 Joules.

The Russian NS-37 gun used a 760 gram AP projectile and had 294,000 Joules at the Muzzle.

The British Vickers S gun used a bit bigger diameter shot but but the MK I AP weighed 1,130 grams and had 214,000 Joules while the MK VI shot weighed 1,360 grams and had 221,ooo Joules.

A 37mm needs to make a hole about 3.4 times bigger than a 20mm, so the American 37mm was actually a worse hole puncher than some 20mm cannon.

 

[drgondog]

Going back to the P-51H, I was under the impression that it was a whole different design than the D and shared very few, if any, components. Wing, tail and if I'm not mistaken, even the thrust line was altered to give better over the nose visibility for deflection shooting.

If I'm wrong I will stand (well, sit really) corrected.

IIRC, the P-51H shared zero assemblies with the P-51D/K, was longer, thrust line different, wing different, empennage different, lower cooling system cowl different , scoop design different, ~13 common parts (brackets, switches, plates), tires smaller, tall tail, wing area greater, airfoil different, plan form of wing different -easier to maintain, faster, climbed better, higher ceiling - but the name remained Mustang.

 

[pbehn]

IIRC, the P-51H shared zero assemblies with the P-51D/K, was longer, thrust line different, wing different, empennage different, lower cooling system cowl different , scoop design different, ~13 common parts (brackets, switches, plates), tires smaller, tall tail, wing area greater, airfoil different, plan form of wing different -easier to maintain, faster, climbed better, higher ceiling - but the name remained Mustang.

Well it looked like a Mustang so it was called a Mustang, what else would you call it? If you look at the variations that constitute a "Hurricane Mk I" or a Spitfire Mk I and then the whole Spitfire/Seafire series, what constitutes a new "marque" is just a matter of looks/fashion and bureaucracy

 

[FLYBOYJ]

Well it looked like a Mustang so it was called a Mustang, what else would you call it? If you look at the variations that constitute a "Hurricane Mk I" or a Spitfire Mk I and then the whole Spitfire/Seafire series, what constitutes a new "marque" is just a matter of looks/fashion and bureaucracy

In the case of the P-51H - "A letter"

 

[pbehn]

In the case of the P-51H - "A letter"

In the case of a Hurricane Mk ! you can change the engine, the prop, the wings from dope to metal, armour as long as it still looked like a Hurricane that's what it was a Hurricane MkI. Even more so for a Spitfire, they even changed the prop rotation, engine, wings and pretty much the whole thing, but somehow they all look like Spitfires.

 

[Shortround6]

The thing is the Hurricane, Spitfire and Mustangs (Allison's to Merlins and the D/K to the H) and a few others the changes were somewhat incremental, the planes had the same general purpose and the wings, when changed didn't "fly" much different than the old wings. See complaints about the Supermarine Spiteful.

When you go from the Whirlwind to the Welkin you not only nearly double the weight, you go from a 45 ft wing span to 70ft (one foot less than a big wing B-26 Marauder ) the wing area goes up 72%, we are not talking about clipping wing tips or fitting extensions here. You also nearly double the power in one jump.

A lot of people claim the grumman F6F was developed from the F4F but a quick look at the companies unbuilt paper studies shows that they had given up any hope of developing the f4F airframe/wing started over with a clean sheet of paper. This does not mean that some details or general shapes were not the same.

 

[GrauGeist]

The RAF tested the 37MM of the P-39 and found that it would not penetrate the armor of the German tanks, while the Vickers 40MM gun on the Hurricane IID did. The USAAF said that the P-39's gun could crack tanks but the actual tests proved that it could not.

The "tank-busting" P-39 is a myth.

Where were they aiming?
Mantle, Glacis, Skirt, engine compartment (rear) or top plating?

 

[Shortround6]

Most logical targets would be the sides and rear of both the hull and the turret. Aiming at the top is near useless, if you are in a 30 degree dive then your impact angle on the top deck or turret top is 60 degrees, The physical path through the top armor is doubled and when you add in the tendency of projectiles to skid/ricochet then the armour will act 2 1/2 to 3 times is nominal thickness, if you are less than 30 degrees it just gets worse fast. If you are diving at more than 30 degrees how close can you get before you have to pull out of the dive?

The low velocity 37mm just doesn't have the punch. Please note that the American 37mm anti-tank and tank guns (and the 37mm AA gun) used heavier projectiles at higher velocities.

they had 317,000 to 326,000 Joules of ME depending on which load and exactly which gun.

If you think you can aim at and hit an engine grate your ego is probably writing checks your actual marksmanship can't cash. The M4 37mm gun only fired about 2.5 rounds per second so you aren't going to get many chances and at 300mph the plane is moving 440 feet per second. Plane is moving 176 ft between each round fired?