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

Greyman said:

With no automatic boost control - are manifold pressures limited (to a certain extent) only by the throttle discipline of the pilot?

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Even with automatic boost control most planes were limited timewise by the "throttle discipline of the pilot".
I believe that only the Germans put clockwork timers on the boost control that automatically returned the boost control to "normal" after a set period of time. Open to correction.
Most countries used some form of boost limiting (max boost the pilot could not exceed by throttle movement alone) except the United States but while it gave more flexibility it added greatly the workload. A Longnose P-40 could pick up 8in (4lbs) of boost dropping from 12,000ft to 6,000ft if he didn't throttle back.

That's what I was getting at re: Kevin J's post. Bell/Curtiss/Allison can tell the pilots that the maximum permissible boost is 42 inches all it wants - if the pilot says "Nope, it's 56 inches" then that's that. I've read many times that the Russians never took the American engine limitations seriously.

Main difference in the assessment of P-40's combat capabilities comes from that we and Allies had completely different exploitation of the aircraft. They use it as written in manuals, from letter to letter. We, as I said before, had a main rule is to take from the machine everything possible. How much "everything" is, it did not write in manuals, and even airplane designer didn't anticipate. This appears in combat. Everything said above goes for Airacobra, too. Have we flown them how Americans wrote it in the manual, we would all got shot down. It was a dud as the fighter aircraft on "birth" regime. On our regimes we had a equal combat with either MEs or FWs, but it would have meant 3-4 combats with subsequent engine change.
__-- General-Major Nikolay Gerasimovich Golodnikov

Kevin J said:

I thought war emergency only became available on the P-39K or did the Russians get there earlier on the D-2?

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Hello Kevin J,
The story of the P-39 is a very strange one for designations.
Very minor equipment changes were sufficient for a letter designation change.
The P-39D-2 and P-39K were contemporaries (mid 1942) and both used the same V-1710-63 engine.
The only real differences were some armour arrangements, the switch from a Curtiss Electric to a Aeroproducts Propeller and a possible change in cannon armament (depending on what source of documentation you wish to believe).
Another example is the P-39F which immediately followed P-39D production and was identical but for a different propeller as for D-2 to K.
Even the earlier P-39D, D-1, and F with the V-1710-35 engine had a War Emergency rating (51.0 inches Hg).
This was mentioned in the tests of the Airacobra against Koga's captured A6M2.

Attached is a letter from Allison regarding overboosting these early engines.
As I see it, but for the reduction gear arrangement, the V-1710-35 and -39 are close equivalents as are the -63 and -73.
Later engines were much less tolerant of high boost pressures.

- Ivan.

Ivan1GFP said:

Hello Kevin J,
The story of the P-39 is a very strange one for designations.
Very minor equipment changes were sufficient for a letter designation change.
The P-39D-2 and P-39K were contemporaries (mid 1942) and both used the same V-1710-63 engine.
The only real differences were some armour arrangements, the switch from a Curtiss Electric to a Aeroproducts Propeller and a possible change in cannon armament (depending on what source of documentation you wish to believe).
Another example is the P-39F which immediately followed P-39D production and was identical but for a different propeller as for D-2 to K.
Even the earlier P-39D, D-1, and F with the V-1710-35 engine had a War Emergency rating (51.0 inches Hg).
This was mentioned in the tests of the Airacobra against Koga's captured A6M2.

Attached is a letter from Allison regarding overboosting these early engines.
As I see it, but for the reduction gear arrangement, the V-1710-35 and -39 are close equivalents as are the -63 and -73.
Later engines were much less tolerant of high boost pressures.

- Ivan.

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Yes, seen that, but I thought the early Cobra engines were more like those of the Tomahawks and without the WEP option.

Kevin J said:

Yes, seen that, but I thought the early Cobra engines were more like those of the Tomahawks and without the WEP option.

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The point I was making was that these early Airacobra engines were more closely equivalent to the P-40D/E/K types.
The problem is that different sources don't all agree as to what the limitations were.

There was a change in the engines around Dec of 1941. They switched from using a shot peened crankshaft to one that was nitrided and shot peened with much greater fatigue life.
Somewhere over the winter of 1941/42 they also changed the casting method for the crankcase and cylinder blocks. There doesn't seem to be any change in engine designation for these changes and the new crankshaft could be put in an old engine. This is part of the reason that the engines in the D-2 and K s were rated at 1325hp for take-off.
For the earlier Ds limitations are going to be kind of up to the users. The USAAF didn't officially approve WEP ratings until the late fall of 1942 which is well after most of the early Ds had left the production line. There may have been twice as many (or more) planes with the new, tougher engines than with the old engines.

Shortround6 said:

There was a change in the engines around Dec of 1941. They switched from using a shot peened crankshaft to one that was nitrided and shot peened with much greater fatigue life.
Somewhere over the winter of 1941/42 they also changed the casting method for the crankcase and cylinder blocks. There doesn't seem to be any change in engine designation for these changes and the new crankshaft could be put in an old engine. This is part of the reason that the engines in the D-2 and K s were rated at 1325hp for take-off.
For the earlier Ds limitations are going to be kind of up to the users. The USAAF didn't officially approve WEP ratings until the late fall of 1942 which is well after most of the early Ds had left the production line. There may have been twice as many (or more) planes with the new, tougher engines than with the old engines.

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The Take-Off rating of the V-1710-63 engines was 1325 HP, but there was a War Emergency rating that isn't in the SEFC, so I need to figure out where I saw this originally.
America's Hundred Thousand lists this as
1590 HP @ 3000 RPM @ 2500 feet at 61.0 inches Hg.

The fall of 1942 would match up pretty well with the various tests against Koga's A6M2 which began test flights in September, so if a WEP rating was approved by then, it would make sense the Airacobra in the test would be using it.

fubar57 said:

Documentation.....?

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Regarding the 175 Gallon drop tank, AHT also shows a range graph of the P-39 carrying as much as 295 Gallons of fuel with a note that all fuel above 120 Gallons was carried in a Center Line Drop Tank.

- Ivan.

tomo pauk said:

I really wonder when people will stop posting bogus figures on this fine forum.
Fw 190 from 1941 have had 1600 HP engine, that power being limited to 3 minutes. P-39 from mid-1944 have had 1420 HP (5 min limit). Talking about cubic inches is meaningless, it is thrust (enabled by power + prop combination, plus exhaust thrust) what propells aircraft, not cubic inches.

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Aint no replacement for displacement. The bigger the engine the more gallons per hour it burns. Hard to compare fuel loads for a plane that burns 200gph vs one that burns 100gph.

P-39 Expert said:

Aint no replacement for displacement. The bigger the engine the more gallons per hour it burns. Hard to compare fuel loads for a plane that burns 200gph vs one that burns 100gph.

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34 liters: DB 601E: 1200 PS at 4.9 km, 98.3 gph, 1350 PS at SL, 117.5 gph
27 liters: V-1650-1: 1240 HP at 11500 ft, 123 gph; 1120 HP at 18500 ft, 120 gph: 1300 HP at SL, 133gph
27 liters: V-1710-85: 1125 HP at 15500 ft, 138 gph; 1420 HP at 9000 ft, 170 gph.

Say again?

yet more unsubstantiated argument. I cant say much about planes, but for cars, the fastest and most powerful engines are not the largest.

For the time being, the Koenigsberg Agera RS is the most powerful production engine available (for cars). At 5.0 litres capacity the engine is large, but by no means the largest. It has a biturbo 5.0-liter unit that has 1,360 hp (1,014 kWThe fastest, soon to be boosted to over 1700 hp.

Fuel burn is going to proportional to the power being made, everything else being equal (which it won't be).
The larger engine could be expected to burn more fuel if it is turning the same RPM as the smaller engine and making more power. The V-1710 and the V-1650 were two of the few engines that turned at 3000 rpm. Most of the German V-12s turned at 2600-2800rpm (except for the older engines and the Jumo 213) and most radials ran at around 2700 rpm. There are exceptions. Then you get to the amount of boost used. Germans used low boost so they had a lower percentage of air going through the engine per rpm/.
The Allison in a P-39Q was using about 1.49 ata for military power, 1.69 ata for take-off and 1.9 ata for WEP.
Once you get to radial engine planes like the FW 190 the drag of the engine installation was a lot closer to the drag of a V-12 so the power (and fuel burn) needed at cruising speeds wasn't that far off, perhaps around 10%. Older radials were pretty bad. At high power the radials may have shifted to rich mixture sooner or used an even richer mixture than the V-12s at full power but then that level of power is only used for 15-20 minutes of a 1-2 hour flight (or longer).

parsifal said:

yet more unsubstantiated argument. I cant say much about planes, but for cars, the fastest and most powerful engines are not the largest.

For the time being, the Koenigsberg Agera RS is the most powerful production engine available (for cars). At 5.0 litres capacity the engine is large, but by no means the largest. It has a biturbo 5.0-liter unit that has 1,360 hp (1,014 kWThe fastest, soon to be boosted to over 1700 hp.

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That is KoenigSEGG Agera.
I am actually in pretty good agreement with your general idea, but the problem with an analogy of street-able and production cars is that they must all fit on the existing roads. There isn't the same issue with getting an aeroplane in the air. The sky is a pretty big place. If you are looking for the absolute fastest piston powered cars, then you might consider the ones that have been run at Bonneville. Those DO tend to have very large displacement engines but are certainly not production street cars.
I also thought about bringing up comparisons of auto engines that I have some personal experience with, but this is already getting way off topic.

- Ivan.

Ivan1GFP said:

If you are looking for the absolute fastest piston powered cars, then you might consider the ones that have been run at Bonneville. Those DO tend to have very large displacement engines but are certainly not production street cars.

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And often more than one.

The land speed record for a piston powered car is 439mph. For a turbine wheel riven car it is 458mph.

https://www.fia.com/file/51532/download?token=KPX0lkyA

Hopefully we can get back to the original topic of the Airacobra versus German (or other contemporary) fighters.

Here is how I would characterize the Airacobra:
It was a very slick aircraft aerodynamically at least for profile drag.
There were no protruding radiators though it did have a small belly bump.
Its choice of airfoil probably hurt its maneuverability to some extent.
(Root: NACA 0015, Tip: NACA 23009)
The symmetrical airfoil tends to have a lower than typical Coefficient of Lift (about 1.3) but the NACA 23000 series airfoil at the tip is a bit higher lift and one of the more common airfoils used in fighters of this era.
NACA Report L-602 - Flying Qualities of P-39D-1 states that max CL is about 1.4.
This is in contrast to the stall speeds listed in Operating Manual for P-39Q-1:
(105 MPH Clean, 90 MPH Flaps Down)
With a stall speed of 105 MPH, even at a weight of 7800 pounds which is about what it would weigh with full fuel and ammunition but no external stores, the calculated CL is only about 1.30.
Perhaps there are a couple MPH in rounding errors?

As Shortround6 pointed out in another thread, the Mid-engine arrangement cost some extra weight in the Airacobra because of additional strength needed between the engine and propeller. There was probably a bit more than that for the separate oil reservoir and pumps needed for the remote reduction gear behind the propeller.
The Tricycle gear made for great ground handling and visibility and easy take-offs and landings, but also cost some extra weight about what a smaller tail wheel might have cost.

The hidden radiators and oil coolers were good from a drag standpoint but not so good from a cooling standpoint.
The Airacobra would tend to overheat in prolonged ground running or in hot climates.
In flight tests, it also had a tendency to overheat.

In the air, there was the CoG movement issue which actually seemed to be more a result of firing off the ammunition for the .50 Cal Cowl Guns than the Cannon ammunition. This has already been discussed at length.
Apparently it was possible to make the Airacobra "Tumble" by flying vertically until airspeed dropped to zero and with proper control inputs, but it was a fairly violent maneuver even if it was planned (from Pilot accounts found in Crowood book about P-39).

Other handling characteristics were a very sensitive Elevator (from NACA L-602) and a rather mediocre roll rate which was about 85 degrees / second max. Perhaps the Russians were able to increase this a bit by deleting the wing guns.

Stall characteristics were very good with a tendency to mush at the stall and the wing tips stalled last for good lateral control.

Speed of a late model Airacobra (P-39Q) was quite good. With the Wing Gun Pods removed, it was probably very close to a 400 MPH aircraft.

Thoughts?
- Ivan.

Ivan1GFP said:

Apparently it was possible to make the Airacobra "Tumble" by flying vertically until airspeed dropped to zero and with proper control inputs, but it was a fairly violent maneuver even if it was planned

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DUHH! That's a tail slide, which looks and feels like a tumble and is a violent maneuver in ANY airplane, though not a true tumble as in a Lomcevak or a roll divergence departure in an F4-A4-F101-F104 class jet. Not a good idea in any civil airplane except an all-out acro machine.
Cheers,
Wes

XBe02Drvr said:

DUHH! That's a tail slide, which looks and feels like a tumble and is a violent maneuver in ANY airplane, though not a true tumble as in a Lomcevak or a roll divergence departure in an F4-A4-F101-F104 class jet. Not a good idea in any civil airplane except an all-out acro machine.
Cheers,
Wes

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Hello XBe02Drvr,

Eeek! Jets again!

I KNOW what a Tailslide is and I know what a Hammerhead is. That is why I described it the way I did with the specific conditions that it took to actually make an Airacobra tumble.
Keep in mind that we are discussing an aeroplane that is fairly small for a fighter but very heavy in comparison to the aerobatic types we see today.
Combine that with a CoG that is far enough aft to make the aeroplane unstable and apparently the critter can be made to tumble.
One thing I remember is that the pilot was very specific about bracing the control stick or it would thrash about quite violently.
Such are the pilot accounts in the Crowood book. I can find the reference if you need but it may take a while.
Remember, this was not a civil aeroplane and I strongly doubt this maneuver was ever approved.

Keep in mind also that ANY aeroplane can fly a Tailslide, but the Airacobra was rather unique in its reputation (deserved or not) for tumbling.

- Ivan.

Ivan1GFP said:

I KNOW what a Tailslide is and I know what a Hammerhead is. That is why I described it the way I did with the specific conditions that it took to actually make an Airacobra tumble.

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I've ridden (not flown) through tail slides in the Pitts and the T-34; you can make them swap ends (violently!) either forwards or backwards depending on which way you brace the stick. But still they swap ends suddenly followed by a couple of oscillations as they accelerate downward, not entering a sustained end-over-end tumble as the(eeek!) jets do. There's a NAVAER training video circa 1967 on roll divergence that shows F4s and A4s doing divergence departures. Pretty amazing to see smoke streaming out of the intake ducts. If you have a reference for a sustained Cobra tumble, that would be interesting to see. Thanks.
Wes

I'm glad the Russians liked the plane, developed good tactics and found it effective. As far as over boosting and detonation, thins will have a lot to do with the ambient temperature. In Russia in the "cooler" season considerably more power could be developed than on a standard day. Engines in their design have some characteristics that allow more leeway than others. The R2800 (an engine that I Am familiar with was structurallyquite tolerant of high MP values but not at all tolerant of over RPM. The improvements going to the C models from the B models were many, but an extra 100 max RPM was achieved. Importantly the oil scavenge system was improved as several hundred HP were utilized in slinging oil around in the crankcase.

Not often appreciated was the power required to run the supercharger. For the R2800 with a 2 stage blower, high blower operation could subtract as much as 400 HP!

P-39 vs German fighters, here you go:

XBe02Drvr said:

I've ridden (not flown) through tail slides in the Pitts and the T-34; you can make them swap ends (violently!) either forwards or backwards depending on which way you brace the stick. But still they swap ends suddenly followed by a couple of oscillations as they accelerate downward, not entering a sustained end-over-end tumble as the(eeek!) jets do. There's a NAVAER training video circa 1967 on roll divergence that shows F4s and A4s doing divergence departures. Pretty amazing to see smoke streaming out of the intake ducts. If you have a reference for a sustained Cobra tumble, that would be interesting to see. Thanks.
Wes

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Hello XBe02Drvr,
Actually I have ridden through a Tailslide also (in a Cessna 152). We were doing multiple loops with a 90 degree roll between each and didn't have quite enough energy to finish. I think we ended up in one or two rotations of a spin after finishing the first two loops. Things happen in slow motion in a Cessna, so it wasn't bad at all.
I remember the Instructor telling me that you can tell if you have aligned your loop correctly if you feel the bump when flying through your own slipstream at the completion of the loop.

I never claimed it was a "sustained Cobra tumble", just a tumble.

Just to make sure we are on the same page:
I am stating that I was reading a pilot's description of how to make an Airacobra tumble.
This was in the Crowood Aviation Series book "Bell P-39 Airacobra" by Robert Dorr and Jerry Scutts (Page 44).
I believe the account to be plausible; The Airacobra has a reputation for tumbling though attempts to duplicate tend to vary in their accounts. Very few aeroplanes have this reputation.

You, on the other hand, do not believe the Airacobra could tumble. You believe that the pilots were actually experiencing a Tailslide and did not recognize that fact.

Eeek, Jets!
I do agree that roll divergence in its various forms is interesting but am choosing not to get into that discussion at the moment because it is somewhat irrelevant to the current issue.

- Ivan.