20/20 Hindsight - different armament?

[Dac]

There's some interesting information on this subject at this site:

http://www.geocities.com/CapeCanaver...n/fgun-in.html

[Jank]

The XP-72 was ordered by the USAAF in two armament variants. One was to have six .50's. The other was to have four 37mm cannons.

The project was dropped because the writing was on the wall with respect to what the USAAF wanted and that was extreme range. The P-47N fit the bill nicely with it's 2,300 mile maximum range and souped up performance.

[Grampa]

Originally Posted by syscom3

Not Good. That would take away some of the fuel cells and decrease range considerably.

Ah well It looks like you have a point here when I see on a X-raydrawing on the P-38. the ammo is no problem if you place them behind the pilot but for the gun It looks like It have to make bout the main thank and the reserve port thank 5-10 % smaller wide. that mean losing 12,5-25 Imp gal (56,8-113,6 litres) fuel. I wonder if there was possible to place some or even more of the lost fuel betwen the supercharger and the engine coolant. like make some modifications in a way when they did on the P-51 B Mustang. Putting a Extra fuelthank in the rear. would that solve the problem whit 5 cannons on the P-38?

[delcyros]

If You ask me, the XP-72 armed with four 37 mm would be good for some bad surprises.

We may discuss this more in detail, if You desire.
-impact of altitude-
low latitude=high airdensity=higher drag
-stabilising the airframe due to more airflow, the plane gets "stiffer".
One reason why the Ju-87 could use two 3,7cm BK, however, as such, the plane wouldn´t really execute prolonged sustained firing.
high altitude=low airdensity= low drag, rapidly increasing due to earlier Mach effects
-from hi alt dogfights -or interception- reports it is obvious that the recoil played a huge and often unnoticed role. There are several reports of hi alt engagements with Spits armed with two 20 mm guns. The most often recorded failure was loss of controll due to excessive recoilforces.

A six 20 mm upgrade for the P-47 would therefore have more worse tradeoffs than benefitials, except for the use as specialized ground attack plane. The Hi alt performance would be terrible when shooting. The recoil force is also much more pointed: originally: 350.7 Kp distributed to eight structural fixpoints in the wing (each 43.84 Kp), post refit: 823.68 Kp distributed to only six structural fixpoints (each 137.28 Kp). This causes a TRIPPLING of powerimpact for structural issues.

I do not question that it might be possible to fit 6 20mm guns into the wing of a Thud but I am confident that this could only be achieved with structural reinforcements of the midwing section, thus further increasing the weight and lowering the performance.

[R988]

Wouldn't a mix of armament like a Spitfire provide a reasonable gun load, four 20mm is a lot of firepower against a fighter, though I guess you want a lot of quick killing power in a boom and zoom aircraft. 2x 20mm and 2x or 4x .50cal with extra ammo would be a good mix of armament for many situations. Many other fighters got on fine with 2 cannon and 2 mgs, the 109E and Zero for example, they also had tons of mg ammo.

[Jank]

"A six 20 mm upgrade for the P-47 would therefore have more worse tradeoffs than benefitials, except for the use as specialized ground attack plane."

I thought it was clear that that was what we were talking about.

Since the P-47 did just fine in the air to air role with eight .50's, (Almost all of the other fighters in the U.S. arsenal did fine with six .50's.) an outfit of six 20's would have been an improvement for the air to ground role which the P-47 assumed heavy responsibility for later in the war.

Anyway, in light of the little Hurricane's ability to handle the recoil of four 20mm's, it wouldn't have been difficult for the P-47 to have handled six. The trade off in increased weight on performance would have been a benefit in the air to ground role.

R988 has an interesting take as well. Perhaps some mix of cannon and machine gun.

It was no engineering feat to make the Hurricane (originally designed to be outfitted with .303 guns) work with four 20mm's. Any re-engineering, if required per your opinion, would have been easily effected. There aren't any fuel tanks in the wings of the P-47D.

[Squeelig]

delcyros -

It's easy to get caught up in equations and mathematical formulas. Sal Monela asked earlier how the Hurricane fared under your analysis with 4 x 20mm guns. I'm curious myself but am gonna go out on a limb here and say I bet it would have failed far worse than the Thunderbolt with 6 x 20mm guns and yet, it was a success as far as I know.

The Hurricane had less weight for each of its four 20mm guns than the Thunderbolt had for each of six and the Thunderbolt wing was already designed to handle far, far greater stress. It often carried 1,000 pound bombs under each wing. I don't think the the Hurricane could even handle 500 pounders.

Anyway, maybe an armament of 2 x 20mm's and 4 x .50's would have been a better trade off along the lines of what R988 said.

[Glider]

Re the discussion of the P47 with 6 x 20 I suspect your missing one vital factor. WHICH 20mm.
If you choose the Russian B20 that weighs less than an .50 M2, you can probably have 6 with ease and maybe 8 at a push

[delcyros]

Since it is matter of interest, lets have a look to the Hawker Hurricane MK IIc:

exteriors:
flight envelope: low-med altitudes, GROUND ATTACK PLANE
airfoil: 19% at wingroots, 12% at wingtips (actually thicker than a Pe-2 wing)
ergo:
unlike the P47 wing, the Hurricane has a huge cross sectional wing airflow area = stiff wing stability

interiors:
steel pipes as structural elements, MKIIc wings were REWORKED METALWINGS
guns in mid wing position
loaden weight: 8250 lbs (3746 Kg)
recoil of 4 HS MK II 20mm: 457.6 Kg
gas effects: 91,52 Kp
total recoil energy: 549 Kp
recoil/weight ratio: 0.146 for a full loaden and 0.18 for a lightly loaden plane

Conclusion: This specialized ground attack plane exceeds the 0.1 margin in recoil/weight ratio, which would effect the fighter role of this plane severely.
For a GROUND ATTACK plane it has a mediocre recoil/weight ratio.
Do not mix up fighter with ground attack issues, the statements that low alt strafing was no problem doesn´t mean that precise prolonged firing in dogfights, esspecially at altitude wasn´t a problem either.
Attention, the original Hurricane wings had to be modified AND STRUCTURALLY REINFORCED IN ORDER TO DEAL WITH THE RECOILFORCES. This added 129 lbs weight to the wings (not counting the weight for guns and ammo)! The results of these reinforcements were suboptimal, so further structural wing modifications of the MK III and subsequent MK IV and MK V had to be executed.

The problem with the P-47 is that the plane had a comparably light airframe weight and much weight for fuel, ammo, engine and loadings. However, only the airframe deals with the recoils (except for engine mounted guns, which do share a good percentage with the engine as well) at all. A fully loaden P-47(20mm mod) will have a recoil/weight ratio of 0.097, which is ok. So from this point of view, You are right. However, for a lightly loaden plane this ratio quickly rises to 0.18.
Next problem is P-47 design. The ship is litterally designed around a huge and powerful turbosupercharger for hi alts. This isn´t necessary for a low alt ground attack plane and adds only dead weight. An optimized P-47 redesign therefore would be a major issue and could include the removal of the supercharger units, making the plane lighter. It is possible.

I stay to my statement, the wing would need a structural reinforcement to deal with the recoilforces as did the Hurricanes wing. The plane as a whole may sustain the recoils, I admit.

[Glider]

Re the statement
Conclusion: This specialized ground attack plane exceeds the 0.1 margin in recoil/weight ratio, which would effect the fighter role of this plane severely

The records that I have seen comparing the IIB and the IIC showed that the extra firepower made precious little if any difference to the planes performance as a fighter. Can you amplify that asspect

Some strengthening is of course to be expected, you cannot expect to go from one of the most lightly armed fighters around to the most heavily armed fighter without some support but the figures were very similar.

Small point, the first IIC wings were reworked metal wings, but the vast majority were of course built that way. The switch to metal wings I always believed to be in hand for some time and you are not going to try to fit 4 x 20mm in a canvas wing, its asking for trouble.

[Sal Monella]

Glider, it was my understanding as well that the Mk IIC's performance was not hampered (at least not enough to warrant any comments) in the air to air realm by the realtively massive upgrade from .303's to 20mm's.

Delcyros, do you have any information on any purported loss of performance attributed to the 20mm's?

[delcyros]

My mistake with the wings, I should go sleeping (tomorrow have to see the World Championship opening in Berlin, yeah!).
My database contains 89 types and subtypes of ww2 fighter and fighterbomber. I recalculated the recoil issues for each of them and noticed that very few FIGHTER PLANES exceeds a recoil / weight ratio of 0.1. (those to exceed have fuselage mounted or inner wing mounted guns)
Several planes had to be redesigned because of unproper recoil estimations (He-162 is a good example, the two fuselage mounted MK108 exceed the critical margin heavily to 0.143, hence the plane got a pair of 20 mm MG 151, which fit well (0.06). The later A-2 subtype was heavier and structurally enforced to carry the MK 108 again (0.11). Nethertheless this subtype had a lower performance, of course)
However, to be more correct I would need to recalculate against the airframe weight (structural parts). This task is ongoing.
The wing / fuselage positions also play a role, the more offcenter a recoil, the more worrisome it will be (ergo, the Yak9T´s and P-39´s 37 mm centerline mounted guns produce comparably low recoil issues) generally spoken.
The VVS analyzed MK IIc and found the plane to be well suited in the ground attack and very worse for dogfights (the report quotes that prolonged deflective shooting is very difficult with the heavy recoilforces). Eventually the VVS DOWNGRADED their few MK IIc Hurricane to 0.50 cal guns which again fitted the critical recoil/weight margin of 0.1!

[Sal Monella]

"The problem with the P-47 is that the plane had a comparably light airframe weight ..."

Huh? Look at the unloaded weight of the F4U Corsair, F6F Hellcat and the P-47 Thunderbolt. (The Corsair and Thunderbolt are very close in size)

No, the P-47 did not have a comparably light airframe weight. In fact, The Thunderbolt weighed about 1,800lbs more than the Corsair in empty trim.

Both planes sported R-2800's and the airframe of the Corsair was very robust as it was designed to crash land onto carrier decks.

[pbfoot]

an interesting sidenote this little snippet is from Jim Coyne Dfc 263 Squadron who flew Whirlwinds " The crop of new pilots was posted in and the Sqn moved to Zeals to work up . This airfield was as a training ground used by the "airfield commandos" ground crews who would accompany the aircraft to europe after D Day they would be trained to defend the airfield from attack as well a refueling an servicing. The commando unit CO happened to mention that his people needed real practice at refueling and arming . Since they had no aircrew on strength he suggested that I might be interested in flying some of his a/c .Always game I did a few flights in his
Hurricane MkIV then had a go with his Spit IX both without benefit of handbook or even good advice . I took the Spit down to the gunnery range in the channel and fired away and was startled when the Spit went into a great slide I immediatly ceased firing and pulled out of the dive realizing the slide had been caused by the a cannon jamming "

[Sal Monella]

That's a really interesting account. The Hurricane Mk. IV, by the way, sported two 40mm guns!

I wonder if anyone else knows of accounts where small fighters with wing moiunted cannons experiences such problems. Based on that account, one would expect the Hurricane Mk. IIC to have had serious problems.