P-39 Airacobra as a dive bomber?

jahenders

Banned
The 37mm gun was not a good tank killer (one of the great myths about the P-39 is that the Soviets loved it in the ground attack role because of the 37mm, not true, the Soviets used them as a fighter). The M4 cannon had a muzzle velocity of 2,000 FPS, about 1/3 lower than the M3 anti-tank gun. The cannon was also both hampered by a small magazine capacity (30 rounds) and a slow rate of fire, neither of which is useful in a ground attack aircraft.

What the P-39 was very, very good at was medium level dogfighting (under 15K) and in temperate climates. At that altitude it was one of the best handling aircraft of the entire war.

Good point. I had thought that myth true for years. However, you're right that the Russians loved it for air-to-air use and had many aces using it, including Pokryshkin with 47 kills in a P-39.

I've seen several P-39 pilots say that the P-39 was a good air-air plane as long as you knew it's strengths and weaknesses and played to its strengths.
 
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The A-36 Apache was ordered not because a dive bomber was needed but because the fighter budget for the fiscal year couldn't justify another fighter and there were a few bucks available in the attack budget. Weird but true. When the A-36 dive brakes were found faulty, they were wired shut, rather than repaired or modified. That's how much they needed or wanted a dive bomber. An early version of the Mustang was equipped with 4 20mm cannons, probably providing a superior ground attack armament to an M4 and 2 .50s. I don't know why they didn't follow that up.

All good points, but the bolded part - really a myth?
 
Good point. I had thought that myth true for years. However, you're right that the Russians loved it for air-to-air use and had many aces using it, including Pokryshkin with 47 kills in a P-39.

I've seen several P-39 pilots say that the P-39 was a good air-air plane as long as you knew it's strengths and weaknesses and played to its strengths.

You can say that about most fighter planes.
 
All good points, but the bolded part - really a myth?
They weren't always left wired up, but they performed poorly and were occasionally faulty, and were never fixed or modified. Had the fighter/dive bomber concept shown merit, they would have modified the dive brakes for subsequent models.
 
I don't believe it would be possible to install a bomb trapeze to the centerline bomb shackle due to space restriction, unless some clever engineer invents a totally new device. The King Cobra did have the wing stores stations installed, and plumbed. Installing a full-span fuel tank could be interesting, but would be subject to some possible bad effects, to handling, servicing and repair, and vulnerability.

The A-36 Apache was ordered not because a dive bomber was needed but because the fighter budget for the fiscal year couldn't justify another fighter and there were a few bucks available in the attack budget. Weird but true. When the A-36 dive brakes were found faulty, they were wired shut, rather than repaired or modified. That's how much they needed or wanted a dive bomber. An early version of the Mustang was equipped with 4 20mm cannons, probably providing a superior ground attack armament to an M4 and 2 .50s. I don't know why they didn't follow that up.


The new tanks in the outer wing would be separate from the inner tanks to avoid slosh and total fuel loss if a wing tank is hit. I should have clarified that. So now we have 4 internal fuel tanks and the provision for 3 drop tanks. Just imagine what the fuel tank selector knob must look like. : )

I must admit that my P-39 fighter/dive bomber would have been a niche predator but surely a more effective plane or at least a smaller mistake then some of the dogs that were produced. Faint praise indeed. Besides, carry some Holy Moses on the outer bomb shackles and no need to dive bomb at all.
 
They weren't always left wired up, but they performed poorly and were occasionally faulty, and were never fixed or modified. Had the fighter/dive bomber concept shown merit, they would have modified the dive brakes for subsequent models.
My understanding is that it's completely untrue - there were issues with the dive brake actuators, and actual diving was discouraged until they were resolved - and that the A-36 was a good dive bomber. It seems to have been well-regarded in action in Italy, but the airframe was more valuable as a fighter than as a dive bomber.
 
It's getting harder to promulgate a good myth these days, with everyone being so sharp, and google and all.
 
...The P-39s effectiveness as a fighter-bomber could have been increased with relatively minor modifications mainly to the wing. Removing the 4 .30 machine guns from the wings and using the space for fuel tankage would have increased the range maybe 30 to 40%. In the dive bomber role the .30 guns are not useful.
Check! However I have an Evil Plan for the wings that would take up some of this freed up volume. Maybe not too much of it though.
The wing structure would need to be beefed up to take the loading the A-36 style air brakes would impose during the vertical dive and to withstand the high-g pull outs.
Evil plan might possibly eliminate the need for these air brakes, or anyway lighten them.
Bomb shackles could have been installed just outside the propellor arc for bombs and plumbed for drop tanks as well.
EP totally eliminates the shackles! MWA-HA-HA-HA! Seriously those shackles, no doubt found on the A-36 as well as on planes designed to be dive-bombers must mess up the aim of the dive bombs a bit, which is the whole point of having dive-bomb capability, is it not? Precision targeting? What if we can snug the bomb up closer to centerline (or if we like, replace it with a heavy rocket for punch against appropriate targets) and the pilot's intuitive pointing of the airplane is much closer to the physics of a released bomb's trajectory, or near straight line of a powerful rocket's accelerated path.
...What the P-39 dive bomber would have had that the A-36 didn't was a 37 mm cannon in the nose as well a 2 .50 cal Brownings. The 37 mm cannon would have been a powerful defensive fire suppressant particularly during the vertical dive attack.
I like how clear you were about the value you think the cannon would have, and then how everyone ignored it and circled around vulture-like saying "nuh-uh, the cannon is no good against tanks! Stop listening to myths about tank-killing...." You didn't mention tanks, you mentioned giving AA wielding defenders of a point under attack a hard time and at least startling them into spoiling their aim.

I'd think any sort of cannon fire would rip right through the protective armor plating an AA gunner is lucky to get. Misses are more likely than hits--but misses from a heavy cannon shell, especially one getting gravity assistance on a near-vertical downward trajectory, make for shrapnel. AA gunners certainly don't get all-round protection; a near miss may hurt the gunner almost as much as a direct hit. A direct hit might bounce off a well-armored tank (its glacis anyway, riggerrob does well to point out armor against plunging fire can't be as good or the damn turret and body would be too heavy) but it surely would demolish a machine gun.

Anyway, not to worry. Evil Plan offers options of replacing the guns pretty easily!
Other advantages the P-39 would have is the engine better protected in its' mid plane location and the coolant radiator and oil coolers buried with in the fuselage under the engine. This is especially so if the extra armour is installed around the rads and coolers as Bell did with some Photo-recon versions of OTL P-39s. And perhaps a little more armour in the nose and lower nose area to cover some gaps though ofcourse weight will be a main consideration. The engine has to be protected as much as practicable as liquid cooled engines and the coolant rad and lines are more vulnerable then radial air cooled engines.
Excellent point! Evil Plan does not address front armor directly but loading up lotsa guns up front would tend in this direction.
...So the P-39 based dive bomber would have been a little more robust then the OTL A-36 with a more powerful punch with the 37mm cannon. Much cheaper then the 2 man Vultee dive bomber and much more capable of protecting itself I think the Australians in New Guinea would have greatly appreciated such a plane. Of course it would not need to have been used only in the dive bomb role but all fighter bomber type operations would have been possible as well.
Again I think you made a clear reasonable point people missed. The idea is not to make the Perfect Ultimate WWII Dive Bomber, the idea is to repurpose a more appropriate airplane than the P-51 to be an auxiliary light dive bomber that can also serve pretty well in its modified form as a near-standard performance fighter of its type.
...In other theatres like the CBI and the SWPA the dive bomber still had a place. The flak was lighter and need to precisely bomb targets hidden in jungle or camouflaged existed. The RAAF used the Vultee Vengeance in New Guinea and RAF also used the Vengeance in Burma to good effect.

Planes like the A-36 were not Stukas. They were well capable of protecting themselves. The only proviso being that high cover might have been needed to avoid getting bounced on the way in to the target while burdened with the bomb load. This could have been provided by other flights from the same squadron.

But yes why bother with another type of aircraft? North American built 500 A-36s. Those planes might have been better utilized it they had been built as P-51s. The P-39 was built in large numbers and not all were given to the Soviet Union. The ones that remained in U.S. service were under utilized. Adding these design improvements would have opened new operational uses for this airplane in Western Allied service.

More good points and a clarification. Perhaps some of your critics did not realize that aside from Soviet theaters, one place the P-39 was deployed to and used a lot was in the Pacific theater, in New Guinea and Melanesia generally. Uncle Sugar had paid good tax/war bonds for these Bell planes, and then dithered around not knowing what to do with them except maybe foist them off onto low priority theatres--or give them to Ivan, what the hell.

In the context of your point that this is not a big expensive project but an economical expedient to wring extra value cost-effectively out of underutilized resources, hence accomplishing thrifty value recovery, my Evil Plan will be, well, Evil. I'm talking about a fundamental redesign of an already weird plane design into something even weirder, so weird I don't think anyone has ever done it. And yet it seems like a good idea to me.

It digresses somewhat from your own ATL proposal but it does feed back into the possibility of this variant you have in mind.

The P-39 I believe we all know was odd in that the engine was placed in the center of the airplane, behind the cockpit. This puts a heavy structural element, I guess the densest thing there is on an airplane but the fuel tanks and possibly ammo hopper, right onto the center of lift! Nifty! It also complicates things in that now the propeller is separated from the engine, a drive shaft being required to bring power to it. (Actually I've read the shaft was not a simple shaft but an array of six small ones, in case one or two got shot up or otherwise snapped, the others could still deliver at least some torque to the prop; I think this was also meant to give some assurance to the pilot a warped drive shaft would not thrash through the cockpit, treating his lower body much the way a Braun whipping blender might if it had 1300 HP driving it). An engine in dead center also has issues with cooling, though for a liquid cooled engine they are not so very hard to deal with.

Now then--here's the damn EP at last.

What if, instead of running a shaft forward to a single prop on the nose, one were to run two shafts (or one shaft sticking out two ways) transversely, conformal with the wing leading edge let us say, and at a suitable distance from the fuselage we have on each side a gearbox that turns the torque 90 degrees and drives two wing-mounted propellers on the sides? The thing would look like a mini-DC-3, or really more like a Lockheed plane except with a single tail. (Lockheed's niche in the airliner market was to make smaller capacity planes that went faster, hence their more streamlined and futuristic look versus the more pedestrian Douglas form. To be sure the Gooney Bird was actually very well formed indeed, perfect for speeds well below compressibility--but Lockheed was trying to push the envelope on performance. Their idea was that very rich passengers would pay a big premium to get where they were going as soon as possible, so they did not have to provide a lot of capacity; after all their target customer would prefer not to be stuffed like a sardine in with a crowd. First class all the way).

Now we've taken the crucial drive shaft and moved it so it too has all its mass on the center of lift, pretty much. There is no danger whatsoever of the shaft wreaking mayhem on the cockpit. The pilot has an absolutely clear, unobstructed view (well, 5 and 7 o'clock are less clear than on the OTL version, but 12 o'clock is wide open blue skies).

As with the P-38, you put all the guns dead center in front of the pilot; aim is simplified and weight permitting, you can pack an arsenal there!

The critics say, the 37 mm cannon was not up to snuff (for some tasks anyway; I gather it worked well in its intended role as a plane-killer)? Well, then, unmount it, modify the ammo hopper and feeder, and swap in another cannon that can get the job done! Don't like cannons at all? Want lots of smaller machine guns for rate of fire and faster trajectory little bullets instead? Ditch the cannons and put in as many machine guns as you can cram in there! Properly aligned they are all pumping lead in a tight pattern right on the bore sight, none of this fussy triangulation of wing guns!

OTL P-38s, at least those deployed in the Pacific theatre (despite the generally excellent qualities of the P-38 it too was treated like a red-headed stepchild by the AAF and tended to get shunted into the lower priority Pacific theatre, where its long legs and the comfort of having two engines in case one failed over some long shark and Japanese infested stretch of water were decided and appreciated assets) came under the attentions of front-line mechanics who improvised all sorts of custom gun arrangements for their pilots. I'm suggesting a P-39 ATL with this prop setup would lend itself, to an extent anyway, to the same treatment.

One drawback of getting rid of the standard puller-prop/engine combo in front was that engines in front, notably radial engines, could themselves serve as a component of pilot armor. As you note, shooting up a liquid cooled engine is bad news; the time to engine failure, even if the main works of the engine are untouched, is limited to minutes or less if the coolant system is disrupted. At least a P-38 had two of them. A pilot of a Lightning or an Airacobra both lacked protection in front that a Thunderbolt or Corsair pilot could take comfort in; air-cooled radials not only lacked fussy watercooling systems (one reason the USN decided to standardize on them) but could in some cases keep on operating (if with some inefficiency and risk of eventually seizing up) with some cylinders shot up completely! Relatively little additional armor had to go in front to protect pilots of such a plane. For that matter the forward Merlin of a P-51 would serve as front armor as well, but if it took shots of course being liquid-cooled the plane is going down. It's just that the pilot is not dead yet. (What his chances are bailing out over the Reich is another matter. Way better than some poor slob who has to bail out of the Pacific though!) Either the pilots of a Lightning or Airacobra had to go commando, without the usual level of armor protection an American airman desired (the OTL Airacobra at least had the prop arrangement to take some hits for him) or else extra weight of armor, in an unfortunate position way ahead of the center of lift at that, had to be added in.

Again with ingenious mechanics on the spot to improvise a custom nose job, P-38 and my ATL P-39 pilots can decide for themselves how Japanese they want to go, perhaps waiving the armor in favor of Moar Dakka to serve as dynamic "armor" on the "get him before he gets you!" proactive theory. Gotta balance the plane around center of mass anyway.

Now I guess by now you see how this Evil plan of mine relates to your dive-bomber modification.

1) We've eliminated the nose prop and moved props off to the side; the wing guns are gone already. (Unfortunately in their place is a section of drive shaft, a gear box, and a propeller on each side, so perhaps we have even less room for gas than before). Therefore, no trapeze for the bomb is required, it can be mounted snug on the centerline.

2) Now it should be possible to use variable pitch on the two propellers to serve as air brakes; just crank back the pitch and the fans are airbrakes instead of propellers! This would work better if the props were mounted as pushers behind the wing instead of tractors before it, but the latter is not only conventional, it seems like the obviously easier way to engineer the props with less structural weight and aerodynamic interference. Front props make the trailing control surfaces function normally; major redesign is needed to accommodate pushers. Pusher props however would clearly serve better as air brakes, pulling back on the shaft in tension and behind the line of lift rather than balancing it unstably pushing back ahead of the line of lift.

3) with the extra warload carried dead center with no offsetting trapeze, streamlining should be a bit better and so will aim be. A rocket mounted there instead would fire straight ahead whichever way the plane points

4) as above, a pilot with good mechanics with decent access to suitable materials can choose a wide variety of combinations of guns, cannon, and armor. Instead of one moderately heavy cannon and one machine gun to bug the defenders near his bomb target with, he can have two cannon, or a honking big single cannon, or an array of many machine guns, depending on his philosophy. If in fact one of these planes winds up in Europe and the plan is to go tank-hunting, a suitable whopping powerful cannon that can hurt a tank might be installed.

Well anyway, I can think of one plane design that was going to use side shafts to drive props on the sides from a central engine--but it was a post WWI scheme for a passenger/cargo plane, and IIRC the central engine was going to be a steam engine! Anyway it never flew.

I like the concept though. In a bigger airplane, another idea would be to install two engines. When both run, one drives one prop, the other drives the other. But given suitable gearing and clutches, it would be possible to shut down one or the other, and switch both props over to be driven by the single engine still running. Thus fuel economy could be realized by flying lower and slower. Then when approaching a crisis, the other engine is started up again, and the plane "switches gears" to connect its formerly shut down engine to its designated prop while the other one has full power available for its own single prop. Thus we can combine high power in a crisis (including takeoff and landing) with a simple efficient way to get cruise economy. If one engine fails there is a backup, too. Such a plane would be able to compete with Lightnings or Mosquitoes in combat (if the two engines have similar power anyway) and beat either at range and endurance.

This philosophy I do one operational example of; the post-WWII British carrier based Gannet (I think it was mainly meant for ASW) which had two turboprop engines and a tractor contraprop. One engine or other could drive both elements of the contraprop for cruise economy, then it could fire up the other engine and each engine would drive one element. If it had had two props on the wings to the side driven by a shaft, with that same clutching arrangement, it would be exactly what I suggest for the Airacobra. And its pilot would have a clear view ahead, though in its role there was no need for a set of central guns.

I anticipate one objection to two side props driven by one engine--in turning, specifically yawing, with both set to the same pitch by a typical constant-speed sort of variable pitch prop, the one inside the turn is overpowered while the one outside is underpowered (since they are at different airspeeds) and would thus tend to impede the turn; there has to be some means of shifting more power to the outer prop, some kind of differential. Either the pilot has to operate it manually or some ingenious means of inferring the rate of turn and adjusting them automatically has to be installed. A differential as in a car would be a heavy and breakdown-prone extra piece of equipment.

Well, we already need some kind of "beta" control on the props to make them reverse thrust and serve as airbrakes anyway. I'd go so far as to suggest developing an alternative to the throttle, a kind of servomechanical "thrust control." Instead of gunning the throttle to cause the engine to start to turn faster, which causes the speed governor in the prop to raise pitch thus raising torque thereby limiting the engine RPM and holding it by this feedback in a narrow band, with the net effect of raising pitch thus converting the extra power into thrust, I'd have the pilot shove on a thrust control yoke that says in effect to the props "raise your pitch to give me more thrust!" The yoke does not move except under force of a stepping motor or solenoid that moves it back and forth as an indicator of the actual force on the propeller drive shaft; given that the pitch of the props does respond by rising and thus raising the force, the yoke then moves. So in effect it sort of acts like the throttle. The actual throttle though is a governor on the engine that seeks to maintain RPM; raising pitch on the prop naturally starts to slow it down so the governor gives the engine more throttle to maintain speed. A cam or set of cams to also take altitude into account indicates the RPM that best corresponds to the current throttle setting; the governor seeks and if this raises the RPM the thrust would tend to rise, which pulls the throttle stick forward, but the pilot doesn't want more thrust so he holds it place and this activates the "give me less thrust" switch that lowers the pitch, thus maintaining the thrust--and now the RPM rises more to shoot past best indicated so the throttle goes back, aiming now for a lower RPM. Lowering RPM would lower thrust but the pilot holds it steady so the pitch comes back up. Depending on the time frames of the various responses such a mechanism might turn into a nightmare of constantly surging thrust, frantically reversing pitch changes, and an engine laboring through unsteady RPM and fluctuating throttle settings. But with the timing right and some nuance in the sensors on the yoke--so that different pressures of the pilot's hand cause different rates of pitch variation--it would instead act pretty smoothly, similar to the standard "constant speed" mechanism anyway, to give the pilot exactly what he wants and stabilize the engine on the optimum RPM to do it.

So if the yoke has another dimension, in which there are really two solenoid controlled thrust indicator yoke shafts, if the pilot twists his hand he nudges one back and one forward, meaning one pitch goes down, the other up, and this alone can start to yaw the plane with no rudder action. With rudder and throttle properly coordinated, yaw maneuvers can be done right briskly.

On a dive bombing run of course, when the pilot turns over to dive and starts demanding braking, he is pulling back hard on the stick, and the pitch drops to zero--at some point before then thrust drops to null and the sticks swing back to neutral in unison, more or less anyway--any unwanted variation is controlled automatically by the pilot not putting up with twist and holding the thrust selectors steady. If he keeps pulling back, his props' pitch drops to zero and the stress meters observe negative thrust which shoves the thrust control assembly back past neutral. This drives the engine; the engine governor will have shut off all the fuel by this point but engine braking will absorb some energy. When the drag is right, the engine is being driven to shed much power, the props are dragging the plane back, which is to say up, ie slowing the fall. Dropping his bomb, the pilot pulls up and instinctively tries to gun the "throttle" which restores engine power. The thrust control thus indirectly drives the throttle via its governor.
 
Check! However I have an Evil Plan for the wings that would take up some of this freed up volume. Maybe not too much of it though.

Thank you Shevek23, I appreciate that you understood my ideas about a modified P-39 fighter/dive bomber. You are someone who reads postings carefully and comprehensively.

Not to quibble too much but just for the sake of accuracy I think if you take a closer look at fine detail cutaway drawings of OTL P-39s you will see you may have misinterpreted the design of the propellor drive shaft.

Your redesign of the P-39 is something you have put a lot of thought into alright. It's a shame we will
never know how it would have turned out. Whether it's abilities would have justified the weight and complexities is the question. But it is an interesting approach. As Just Leo said I'd would also love to see a drawing of your Super-Airacobra.
 
Belldivebomberx.png Not all the specs of the DBP-39EP fit on the same airplane, and it's a lot of trouble, weight and complexity just to fit a central store station which doesnt require a trapeze for bomb launching, and a Tiny Tim rocket, the only big rocket in the arsenal, still doesn't fit with the nose gear. I don't know if it could have been launched vertically anyway.
 
View attachment 280022 Not all the specs of the DBP-39EP fit on the same airplane, and it's a lot of trouble, weight and complexity just to fit a central store station which doesnt require a trapeze for bomb launching, and a Tiny Tim rocket, the only big rocket in the arsenal, still doesn't fit with the nose gear. I don't know if it could have been launched vertically anyway.


The DBP-39EP Supercobra. Just wow! This is turning into a fashion show with practicality vying with imagination.

But I think Shevek23s' idea could have been applied to a larger airframe like the P-63 Kingcobra and with a more powerful engine.
 
Thanks Leo, for drawing it, even if you had your doubts.

Would a transverse shaft and 2 gearboxes for 2 props really be much heavier than the OTL forward shaft to the single nose prop?

In order to get the same thrust and power loading, the two side props can be root 1/2 or just a tad over .7 the radius of the single nose prop. No one has pointed out that with the low wing, the ground clearance of wing-mounted props is perhaps a problem. Lowering the radius helps but perhaps not nearly enough.

Anyway with the lowered radius, the transverse shaft has to go from the centerline to the radius of the wing root on the fuselage, then provide for adequate clearance of the prop tip from the fuselage (which might only be a matter of inches) then the rest of the half-span accounts for the actual prop radius. If we multiply this by two, do we get a length from one gearbox to the other of much more than the length from engine to nose prop on the OTL model? Even if it is somewhat longer, the torque that needs to be delivered is split between two props, so the strength of the shaft necessary is cut in half, which should allow for some lightening. Then there is the fact that we need two gearboxes, but I doubt the OTL forward drive shaft was simply bolted to the prop; there would have been a gear box there too, and it has to take the full torque, while the two wing gearboxes of my ATL are each taking just half, so I suspect the shaft-gearing arrangement would come out very close to the OTL transmission, and perhaps actually come in lighter.

Another thing along those lines--shorter radius props can be driven to a higher RPM before their tips hit sonic speed limits, so the drive shaft which I believe is geared to very high RPM in either version (because the higher the shaft RPM the less torque it has to deliver to deliver a given power) doesn't have to be geared down as much as the single nose prop would require--lightening the side gearbox a bit.

I went overboard perhaps with my backward feedback based control system but it is a notion I've had on my mind for more than a decade so I beg everyone's pardon. I suppose with two standard type constant speed props, the possible anti-yaw factor I pointed to might not be more than a nuisance at worst, and if it is a thing, beta-control on the prop pitch is needed to turn them into airbrakes anyway. (And does that seem infeasible to anyone?) A knob on the throttle or perhaps something on the stick--but wait, yaw is managed with the rudder which is controlled by the pedals. A simple bias in proportion to the operation of the rudder might be just the thing. Then basic constant speed props, with the rudder biaser varying the target RPM setting and a brake lever to bias both to lower pitch might get the job done.

The only thing I don't like is the low ground clearance of the props due to the low mounting of the wing. If Bell had decided to do side props I suppose the wing would have been mounted higher on the fuselage and perhaps the wheels would have to go into the fuselage or something like that.
 
Imaginary aircraft always fly well until they become real. Developing it into hardware introduces a world of hurt. The Bell twin-prop is really too complex to work in the world. The FMA Pucara is the same aircraft without the gearboxes and shafts, with a pair of 18" diam. 300 lb Astazous substituting for all the hardware from engine to prop. A concept called the Gevers Genesis is a fine example of an alternate history aircraft, even more complex and ambitious than the Bell, a triphibian with twin pusher props that do all kinds of weird wonderful things. It's been in the design stage for 30 years, and the build stage for 3. Investors are welcome. I don't know if it's another Moller Skycar or not, but I don't have any spare capital myself. In the real world, hardware and concept failure is real and somewhat unpredictable, but designing complexity certainly increases the risk. I wonder how the Pucara would have done in WWII. You don't have to worry about component or concept failure for imaginary aircraft, or finding investors, so don't sweat the ground clearance on the props. That's OK. Torsional vibration is probably a bigger problem.
 
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