Could you have designed a better P-39?

[krieghund]

Okay, think I get NACA's idea now
An issue of how would anyone succeed to mount intercooler there, is a new ball game.



While 2 cannons 4 HMGs as a SE fighter armament do sound like a good idea, the proposal has some shortcomings IMO. Ammo count, for example - we can stick to, say 100-120 shells or 250-300 HMG rounds per barrel. That way our SE fighter lugs around about same armament ammo weight as P-38, P-47, Typhoon, Tempest. Nice if one has at least 2000 HP on board, not that nice if there is only 1200 HP. Or, we can reduce the ammo count to save weight - US pilots would've hated us, and the weight of armament is still there.
We can delete a pair of HMGs, and the armament ammo weight is like at P-51D, Hellcat, Corsair - but our plane still lacks 300-400 HP if we want it to be competitive.
There are other minor things to consider, like need to purchase another 10000 cannons from UK, 3 guns (= cannon + 2 guns) are easier to install maintain than 6, less drag (3 openings less), single gun heater can heat all armament, almost no impact to accuracy if one gun jams, central battery is not susceptible to wing flex, less inertia - better roll...

My proposal cuts weight (P-39 was able to out-climb any contemporary US single-engined fighter anyway), while enabling all 3 guns to be fired simultaneously (similar bullet paths).

I think 5 x .50 cal in the nose would be sufficient.

 

[tomo pauk]

Sufficient indeed.
The minor issue would've been the change of CoG.
5 HMGs with at least 200 rpg yields 300 lbs to be expanded some 2-3 ft away from CoG, vs. 180 lbs that real P-39s had for 37mm 2 HMGs. It's 210 lbs for belt fed Hisso (120 rds) + HMGs.

 

[krieghund]

Sufficient indeed.
The minor issue would've been the change of CoG.
5 HMGs with at least 200 rpg yields 300 lbs to be expanded some 2-3 ft away from CoG, vs. 180 lbs that real P-39s had for 37mm 2 HMGs. It's 210 lbs for belt fed Hisso (120 rds) + HMGs.

What was the weight of the P39C with 1x37, 2x.50 2 x.30?

 

[tomo pauk]

Don't know; I was referring to the expandable weight - ie. ammo weight in this case.

 

[davparlr]

Air flow can be a funny thing. back in the late 60s General Motors had a number of divisions competing with various "super" cars. Mid sized cars (for the time) with big engines and every one had a "ram" air option of some sort. Pontiac and Buick has two small 'scoops' about half way out the hood. Chevrolet had a rear facing trap door inlet at the base of the windshield. Oldsmobile had two BIG scoops (14 in X 2in If memory serves) under the front bumper. The Oldsmobile set up worked best (biggest scoops and in a high pressure area) the Chevrolet worked next best, the base of the wind shield being a high pressure area. Pontiac setup worked hardly at all while the Buick 'scoops" being very low and in the middle of the hood were actually in a low pressure area and sucked air out from under the hood/intake area. granted these are at much less than aircraft speeds but the air flow along a fuselage is not constant.

I had to laugh when I thought of the aerodynamics of the the late 60s early 70s cars. While I love those cars and I still think they are some of the best looking cars ever (I had a 71 Cutlass S), I suspect there is turbulent airflow all over the place. All aerodynamic theories go out the door, Olds with the inlet closer to clean, laminar flow would be best. Chevy had all the turbulent airflow pile up at where the hood meets the windshield, which is also where cars took air in for the air vents since the mid 50s. The hood definitely had low pressure, and also boundary layer issues (low). If they had stuck up like a supercharged engine, it would probably had worked.

I would think that putting the intake behind the canopy has got to affect the airflow into it somewhat.

If the airflow is laminar, there should be no problem. However, the airflow could be perturbed by gaps and protrusions and possibly by stall caused by the cockpit, all causing turbulent airflow. I suspect stall would not be problem at the low speed it would occur. I am sure Bell understood this and tested the inlet for the conditions needed. They did appear to have a raised the inlet out of the boundary layer conditions, surprisingly, something Lockheed failed to do on the XP-80. The P-63, which updated issues with the P-39, appears to have a similar inlet, so apparently they uncovered no problems with this particular design.

 

[Shortround6]

I think the P-39 made the best of a bad situation. Stick the air intake up behind the canopy and take what you can get or try sticking it out to the side and then looping it up and over in the down draft carburetor (which might have more drag than the behind the canopy intake) or get really tricky and try to duct the intake air from the wing roots or nose past the pilot and engine.

If essentially the same engine installed in a P-39 gives a different (lower) FTH than the when installed in a P-40 under ram conditions there has to be an explanation somewhere. If both planes are going roughly the same speed (with in a few %) then one is doing a better job of managing the intake air (RAM) than the other.

 

[tyrodtom]

What way did they have to determine exactly how much horsepower a engine was developing installed in a aircraft at altitude ?

Was it just calculation from the dyno results, with corrections throw in for alltitute, and a little bit of swag thrown in as to how much ram air and popeller efficiency added to or took away from the total ?

 

[Shortround6]

Some radial engines had torque meters built into the reduction gear box so direct readings of actual torque could be made and since HP = Torque x RPM ÷ 5252 it wasn't hard to know what those engines were doing. As far as inline engines ( and radials without torque meters) went they had the dyno results from the test chambers on the ground. They Knew the power at a certain RPM and manifold pressure. AS long as the RPM and manifold pressure were on spec then the engine put out the correct power. If the engine wasn't getting the correct or calculated manifold pressure at a given altitude for the prescribed rpm then something in the inlet system wasn't working as it was supposed to and needed investigation. The test chambers on the ground had air supplies that could be varied in both pressure and temperature to simulate higher altitude flight so there was a lot less WAG going on that you might think. Ram was easy to measure, If the engine without RAM (ground test) needed and got 44in of manifold pressure at certain air pressure that corresponded to say 12,000ft, just fly the plane at 12,000ft and then do a series of slow climbs until you no longer get 44in on the manifold pressure gauge. That is the altitude that ram will let you pull full power at in high speed flight. Ranm was not used to increase the rated power of an engine, it was used to get the rated power higher in altitude. At least until WER came along.

 

[vanir]

We want a Army fighter aircraft that works right now. (i.e. around 1940)

1. Turborchargers are still experimental for fighter aircraft during 1940.

Not exactly, metallurgy for impellers that can cope with more than 30,000rpm inside a hot exhaust pipe are still experimental in 1940, the issue is technological and/or industrial limitations and not a failure to come up with ideas to solve problems, no shortage of ideas ever because everybody wants to be king of the world.

2. The Allison engine is a dog at all but low altitude.

The Allison is a better engine than the Merlin from the F-series. The blower sizing was a doctrinal influence and not a technological one, and Allison performance under 11,000 feet is exactly the same as a low alt Merlin right down to the +15lbs two-minute WER. At this particular point the two engines are pretty much identical but the Allison is better built and tolerates lower fuel grades better. The medium alt Merlin is an interceptor engine and pursuit engines aren't interceptor engines, a Merlin 60-series should be compared with the P-38 turbocharged allisons and in this case the multiple stage blower is more reliable, but doesn't have the same development potential. When Packard started making Merlins a RR engineer toured the factory and decided to implement new bearing compositions and machining that Packard was doing with their Merlin 25, it was introduced into all British RR Merlin production because they improved the engine.

3. The RR Merlin engine is state of the art and arrangements are already being made for mass production in the USA. Britain is investing heavily into further performance improvments for this engine.

State of the art in 1936 mate. In 1941 state of the art is complicated multistage, multiple speed blowers, including your turbos.

Seems like a no brainer to me. Pay Packard to expand production capacity so they can produce RR Merlins for the P-39 as well as for the Lancaster bomber.

It might be interesting to put turbo Allisons in a Lanc but why would you put Merlin-25s in the P-39? You'd be better off putting a contemporary two-stage F7R or E9 motor, either will handle plenty of overboost easily and I see no problem with these motors.


Dave the real differences between the Merlin and Allison are very circumstantial and other than that they're just about the same motor, an inline V-12 of the second generation (the curtiss and kestrel are 1st generation, merlin, allison, jumo and hispano are the 2nd and all the others like klimov copied). Everybody was using the same tech, it was just nation-specific industry.

 

[Shortround6]

One might say that the Merlin, Allison, Jumo and DB engines were 3rd generation even. Hispano, Curtiss, Kestrel being second generation. All the WW I and WW I leftovers being 1st generation. But you are quite right, in the 1938-1940 period there was little to choose between the 4 engines themselves. And even later it was a mater of superchargers and not basic engine.
The USAAC did order the P-39 and P-40 without turbos (actually they didn't take the time to engineer installations that worked) because they estimated that such planes would be ready 1 year sooner than turbo equipped planes. The Army did have 13 YP-37s to play with. A P-36 wing and rear fuselage with a turbo Allison so they had some idea of the both the problems and capabilities of a turbo fighter.

 

[tomo pauk]

I think the P-39 made the best of a bad situation. Stick the air intake up behind the canopy and take what you can get or try sticking it out to the side and then looping it up and over in the down draft carburetor (which might have more drag than the behind the canopy intake) or get really tricky and try to duct the intake air from the wing roots or nose past the pilot and engine.

...

Perhaps something like this; the intake should enter the hull approximately at the same place it entered from inter-cooler of the XP-39?

 

[davparlr]

I think the P-39 made the best of a bad situation. Stick the air intake up behind the canopy and take what you can get or try sticking it out to the side and then looping it up and over in the down draft carburetor (which might have more drag than the behind the canopy intake) or get really tricky and try to duct the intake air from the wing roots or nose past the pilot and engine.

I think you are kinda right and kinda wrong. I do not think the inlet location is aerodynamically wrong. As you can see, both the B-2 and Tacit Blue both have engine inlet located in classically low pressure locations, on top of the wing and on top of a lifting body type fuselage. Both of these inlets are very efficient because they deal in a laminar flow slipstream. However the ducting could be problematic, being so short and sharply turned.

If essentially the same engine installed in a P-39 gives a different (lower) FTH than the when installed in a P-40 under ram conditions there has to be an explanation somewhere. If both planes are going roughly the same speed (with in a few %) then one is doing a better job of managing the intake air (RAM) than the other.

If the difference here is in the ram air design, I suspect the difference is in the ducting. The P-40 duct appears quite straight and long before it enters the turn to the carburetor. While this may increase friction loss, the divergent compression may be more efficient. The P-39 duct is short with a quick turn to the carburetor. This may cause losses. Also, duct design is complex and sophisticated aerodynamics which may have been in its infancy at this time, which could caused losses. It is really complex for supersonic airflow. The advent of stealth and the need for hidden engines and heat suppressed exhaust has driven ducting to an entirely new level of design.