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

[P-39 Expert]

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.

Agree with pretty much everything you said.

The "tumble" could be entered by some and not others after a contrived effort (nose ammo expended, climbing vertically to the stall, etc). A little knowledge could keep you out of a lot of trouble.

It would overheat on the ground if you taxied too long, but for normal operation with this type it wasn't a concern. Forward visibility during taxi was good enough to drive it like a car to the end of the runway, perform your mag and prop checks and take off.

Wouldn't cool to Army hot day cooling requirements during maximum climb but was tolerable for 5 minutes and cooled well at normal operations.

 

[Greg Boeser]

That long nose wheel strut was a bit fragile for rough fields though.

 

[P-39 Expert]

That long nose wheel strut was a bit fragile for rough fields though.

Yep, but most all AAF planes needed concrete or steel mat runways. Would like to se some of those fields that they called "rough".

 

[fubar57]

Airfields had grass runways in England for fighters for the most part

 

[P-39 Expert]

Airfields had grass runways in England for fighters for the most part

Which were probably a lot smoother than those at Port Moresby.

 

[Shortround6]

A lot of the British (and Continental) prewar grass fields were actually carefully prepared with attentioned paid to drainage, filling in low spots and rolling the field periodically.
If you can find it the 1938 "Jane's all the World's Aircraft" has a section on airfields/aerodromes with a number of pictures. I mention this edition as it was reprinted by Arco in 70s(?) and may be easier to find than an original.

Some airfield were actual circular for the flying surface with hangers and terminal/tower on one side, paved aprons in heavy traffic areas with grass flying surfaces.

 

[fubar57]

I do have that book

 

[XBe02Drvr]

Which were probably a lot smoother than those at Port Moresby.

What about those USSR fields? Betcha the Ivans did a lot of taxiing and TO/Ldg with the stick right back in their laps. They still thought it was one of the best fighters they had.
Cheers,
Wes

 

[Shortround6]

AS to the airfieldS at Port Moresby :

Port Moresby Airfield Complex - Wikipedia.

Please note that the Americans had used 7 mile Drome to stage through when they ferried B-17s to the Philippines in Sept of 1941.
The article is not clear as to exactly when the air strips got PSP (like what month) but from the descriptions the Port Moresby air strips were not crude or significantly lacking in engineering support.

 

[Milosh]

Berry Airfield (12 Mile Drome)

The airfield was constructed in early 1942, being completed on 15 May. The airfield had an 8-inch base of crushed rock and pit gravel for a single earth runway approximately 4,500' by 150.

40th and 80th FS USAAF with P-39s stationed there.

 

[Ivan1GFP]

What about those USSR fields? Betcha the Ivans did a lot of taxiing and TO/Ldg with the stick right back in their laps. They still thought it was one of the best fighters they had.
Cheers,
Wes

Hello XBe02Drvr,

Maybe Ivan thought it was one of their best fighters because other aircraft also had their own issues.

Хотя трехколесное шасси обеспечивало прекрасный обзор на рулении и взлете, и в принципе позволяло перемещаться по аэродрому с большой скоростью, не опасаясь капотирования, на неровных грунтовых аэродромах носовая стойка вибрировала и нередко ломалась, что вынуждало ограничивать скорость руления.

Although the three-wheeled chassis provided excellent visibility on taxiing and take-off, and in principle allowed to move around the airfield at high speed without fear of nosing, on uneven ground airfields the nose strut vibrated and often broke, which made it necessary to limit the taxi speed.

from:
Недостатки P-39 Аэрокобра

- Ivan.

 

[CORSNING]

America's Hundred Thousand lists this as
1590 HP @ 3000 RPM @ 2500 feet at 61.0 inches Hg.

Ivan, I have had a long day at work and am too tired to look this up.
Sir, if you do not mind, what page of AHT is this information on?
, Jeff

 

[CORSNING]

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

Maybe, but check this little bugger out.

 

[Ivan1GFP]

Hello Corsning,

Look on Page 191 of AHT.
I tried to do a scan of that page but I do not have scanning software on my laptop and GIMP seems to have issues with the scanner which is why the model designations are cropped. The engine variants are still readable, so this should be sufficient.
The original source I found had slightly different information on it.
Manifold Pressure was in the high 50's (maybe 57 or 59 inches Hg) and power was only 1550 HP.

- Ivan.

 

[CORSNING]

Thank you Ivan, I am just a wee bit tired tonight and appreciate the help.
, Jeff

 

[Kevin J]

Hello Corsning,

Look on Page 191 of AHT.
I tried to do a scan of that page but I do not have scanning software on my laptop and GIMP seems to have issues with the scanner which is why the model designations are cropped. The engine variants are still readable, so this should be sufficient.
The original source I found had slightly different information on it.
Manifold Pressure was in the high 50's (maybe 57 or 59 inches Hg) and power was only 1550 HP.

- Ivan.

View attachment 514526

That's pretty much what I understood from what I've read on the Russian sites. No WEP until the P-39D-2/K. So no more power than the Tomahawk had and that entered service in 1940.

 

[Ivan1GFP]

Hello Corsning,
No problem!
You caught me at just the right time. I am just packing for a trip.
In a couple hours I won't have access to my laptop or to all the books at home, so with just a cell phone for Internet access, I won't be posting anything for a couple days.

Hello Kevin J,
You presume this is the only reference there is. It most certainly is not.
It just happened to be the easiest place to find what I needed to address one specific point.
As Shortround6 and I were discussing earlier, by late 1942, there was a WEP specification even for the early V-1710-35.
You can see the Allison memo to address what the services were ALREADY doing in the field.
I am stating specifically that WEP was used by the P-39D or D-1 (not D-2, not K) that was tested against Koga's A6M2 starting September 1942.
At least that is what the books say.
One has to wonder why the K was not tested against A6M2 since it begin production in July.

You can of course believe what you will. There is also a Russian manual for the P-39 with the Allison E4 engine (V-1710-35) and I have translated parts of it and found some fairly cool stuff but haven't gone looking for the power settings.

- Ivan.

 

[pbehn]

Airfields had grass runways in England for fighters for the most part

Certainly true at the outbreak of the war, by the end of the war most were hard top, concrete at least. Building runways in UK was a huge civil project in ww2.

 

[Kevin J]

Hello Corsning,
No problem!
You caught me at just the right time. I am just packing for a trip.
In a couple hours I won't have access to my laptop or to all the books at home, so with just a cell phone for Internet access, I won't be posting anything for a couple days.

Hello Kevin J,
You presume this is the only reference there is. It most certainly is not.
It just happened to be the easiest place to find what I needed to address one specific point.
As Shortround6 and I were discussing earlier, by late 1942, there was a WEP specification even for the early V-1710-35.
You can see the Allison memo to address what the services were ALREADY doing in the field.
I am stating specifically that WEP was used by the P-39D or D-1 (not D-2, not K) that was tested against Koga's A6M2 starting September 1942.
At least that is what the books say.
One has to wonder why the K was not tested against A6M2 since it begin production in July.

You can of course believe what you will. There is also a Russian manual for the P-39 with the Allison E4 engine (V-1710-35) and I have translated parts of it and found some fairly cool stuff but haven't gone looking for the power settings.

- Ivan.

My understanding was that WEP was only allowed in the V-1710-39 of the P-40D/E from the beginning,not the V-1710-35/37 of the P-39C/D/D-1.

 

[fubar57]

Certainly true at the outbreak of the war, by the end of the war most were hard top, concrete at least. Building runways in UK was a huge civil project in ww2.

I read that the breakaway from grass was due to heavier bomber introduced though I can't recall the time frame