XP-39 II - The Groundhog Day Thread

[Peter Gunn]

Got my "Luftwaffe Test Pilot" from Amazon yesterday, good stuff. there is a section on the P-39, some quick quotes from the German test pilot Hans-Werner Lerche:

"From the performance point of view the Airacobra with its 1200hp Allison engine was not of much interest to us, but we were rather intrigued by the construction of this fighter ... and the 37mm cannon."

"I would like to add that in the concluding report about the airframe and the flying characteristics of the Airacobra there was nothing really exciting to be mentioned."

The section on the Airacobra states they had the correct captured Allied fuel (not sure if he's totally correct there but benefit of the doubt) and he does make the statement "The Airacobra carried a comparatively large amount of fuel for a fighter..."

Also, "The Airacobra did not have the reputation of being exactly a terrific aircraft..."

He does say he was impressed by the design and workmanship of the aircraft and compliments the tricycle landing gear and good instrumentation and thought the red pins on the fuselage and wings to show the landing gear down and locked rather ingenious.

In the back of the book he has a chart of all the planes he flew and the performance figures he got out of them.

For the P-39D:
Weight: 7845 lbs
Speed: 335 mph @ 5,000 feet
Climb: 1 min 54 sec to 5,000 feet

His claims for the P-51B WITHOUT correct fuel, or a bit of a mixture with German fuel:
Weight: 11,200 lb
Speed: 440 mph @ 30,000 feet
Climb: 1 min 48 sec to 10,000 feet

I thought it might be interesting to see what the "Other side" had to say about it. I only included the Mustang numbers because reasons.

Did you really expect ME to NOT to post Mustang info at the drop of a hat?

 

[davparlr]

"Circa 1940, the US had a population of 132 million while the UK had just 48 million."

Certainly a greater ratio of cars to people in the USA in 1940.
26.5 million on the roads for the USA.
1.34 for Great Britain.

1940 British and European Car Spotters Guide

Between 1921 and 1935 over 36 million cars had been produced in the USA, and that includes 6 years of the Great Depression. In 1935, there were about 34 million household consisting of an average of about 4 people per household. It is not unreasonable to assume that, in 1935, a significant number of American households had an automobile, all of which, I'm sure, required a significant amount of maintenance in some form or other.

 

[FLYBOYJ]

23% MAC would be 130.1072 inch
80.64 inch * 0.23 = 18.5472 inch from LE MAC
18.5472 + 111.56 LE MAC = 130.1072 inch

My Daughter would be screaming at me for significant figures right about now!

- Ivan.

LOL - that makes more sense

 

[XBe02Drvr]

P-39 Expert - If you understand any of this feel free to chime in.

I don't think he's caught on yet that CG is not a fixed location. Most of my flying students were reluctant to wrap their heads around that idea, even after reading the W&B chapter in the student pilot manual.

 

[XBe02Drvr]

Why would the IFF radio be important to the AAF and not the Russians? I don't know.

Russian radar may have been a little more primitive than in the West, and may have lacked the IFF transmitter and receiver.

 

[Elmas]

Sorry for my quick translation, made from a page or two found on the internet, the whole book is on the way to my bookshelf. For those who want to know more the book is Italian and English.
The P39 Aircobra in service in the ICAF: a robust but dangerous aircraft
Extract from
Marco Mattioli - P39 Airacobra in Italian Service - IBN Editore

On 22 June 1944, ten. col. Fanali, Magg. Martinoli and ten. Alessandro Mettimano transported the three P-39s to Campo Vesuvio for the training of the pilots of the 12th Grp. On 24 June, another flight department of the 4th Stm, the 9th Grp, gave its MC.205s to the 21st Grp of the 51st Stm. On 7 July 1944 the Command of the 4th Stm moved from Nuova to Campo Vesuvio, followed by the 10th Grp Caccia, which from the Nuova base reached the Vesuvian runway, after having delivered its MC.205s to the 155th Grp of the 51st Stm on July 9th. On July 18, 1944 the 9th Grp of the 4th Stm moved from Lecce to Campo Vesuvio, completing the deployment of the Stormo on that base. In three months of intense training on the P-39N, 77 pilots of the 4th STM carried out 1,702 flights for 1,000 flight hours: there were eleven accidents, all caused by the bad characteristics of the airport or by sudden engine failures, which caused three pilots dead and several others injured, including two serious ones. The first of the three pilots who died was s. ten. Armando Moresi, who crashed on July 20, 1944, while carrying out a training flight paired with the ten. Giorgio Bentolaso: Moresi was an excellent aerobatic pilot, and he had tried to loop with his Airacobra, losing control; the Italian pilot, instead of launching himself, had tried to bring the P-39 to the ground. The death of Moresi convinced ten. Bortolaso to refrain from superfluous acrobatics. Bertolaso, recalls that when P39s were delivered to the pilots of the 4th STM, the American chief pilot, a major whose name he does not remember, made only one recommendation to the Italians: "P-39 is a wonderful aircraft , but don't do stunts; I flew more than 500 hours with him, but if I am still alive I owe it to the fact that I never let myself be tempted by aerobatics. "For many of the Italian pilots, convinced that a good fighter pilot must also be a good aerobatic pilot, these sensible words sounded like a challenge, and for this reason some of them lost their lives. The P-39 was an aircraft designed for ground assault, but not for flying stunts; in this regard, ten. Bertolaso (now General) still remembers: "the stabilizer of the Airacobra was very low, more or less at the same level as the wing; because of the way the weights were arranged, it was very close to him. When you pulled a lot, the stabilizer went into the shadow of the wing, and stalled before the wing itself, so the aircraft began to rotate through its transverse axis, and there was no maneuver capable of taking it out of this aerodynamic situation ( as demonstrated by the Moresi accident). Someone, we don't know how, happened to get out, but most of them were saved by parachute ". Ten. Bertolaso recalls that another inconvenience that made the "Cobra" unsuitable for acrobatic flight was the engine: "While our engines and those of the Macchi 202 and 205, the Daimler Benz DB6O1 and DB605, were fueled by injection, with a hydraulically driven compressor, the Cobra's Allison V-1710 had a carburettor with a mechanically driven compressor. This fundamental difference meant that the Daimter Benz had a greater elasticity of operation and stability of the mixture. The Allison, on the other hand, with its carburetor, during the acrobatic maneuvers and especially in the sustained reverse, suffered from considerable impoverishment of the mixture, which often caused considerable backfires - the famous spitting of the engine - which for the complex compressor represented a very violent shock. Its gears broke, and the pieces were swallowed by the engine, smashing you heads. I personally had this terrible experience, during a very low-altitude tonneau: I had a flash back and all hell broke loose. Fortunately the engine, albeit with some broken pistons, smoking and trumpeting, brought me in sight of the field, but I had to land with the cart retracted, because I didn't have time to extract it. In normal conditions, the engine was very good ". The other two pilots of the 40 Strm. fallen in air accidents were the sergeant major Teresio Martinoli, ace of the axes of the Regia Aeronautica in the Second World War with 22 victories, crashed during take-off on 25 August 1944, and decorated with the Gold Medal for Military Valor (MOVM), and S.ten. Guerniero Silvestrini, who crashed due to engine failure during take-off on 27 September 1944, and was killed by the explosion of the auxiliary tank which hit the ground. According to the ten. Bertotaso, who witnessed the death of Silvestrini, the accident occurred due to a strong overheating of the engine, due to the fact that before take off, Silvestrini remained stationary for too long with the engine running at the bottom of the runway. Among the injured pilots were Mar. Martelli (21 July), ten. Clauser (August 3), Ser. Novetto and ten. Rizzitelli (7 September), s. ten. Stoppani (9 September) and ten. Zanioto (September 14). The accident that occurred on ten Rizzitelli was singular: the engine of his P-39 crashed due to the volcanic dust, but Rizzitelli managed to land without landing gear at very high speed; in the collision, the Airacobra lost its entire nose and the pilot lost his shoes, remaining unharmed and barefoot.

 

[SaparotRob]

I pulled some other charts from what I can find on the internet. It seems that the P-39N used a %of MAC for it's CG envelope, the P-39Q uses inches aft of a datum line which I believe is at the nose of the aircraft. I see similarities between both models and can probably "interpulate" a C/G range. I have more info but let's see what our friend comes up with.

Please tell me what is "MAC"?

 

[BiffF15]

Please tell me what is "MAC"?

Mean_Aerodynamic_Chord_(MAC)

 

[Elmas]

 

[FLYBOYJ]

So based on Ivan's calculations (Thanks again Ivan, I know your math is better than mine!!) we have the "station" C/G range of 130.1072 - 136.5584. The MAC range is 23 - 31%.

The weight and balance chart shown in the P-39N manual has 3 loading configurations (Max Fuel, Bomber and Normal load) at 29.%, 28.8% and 28.9% MAC. From what I see if you load your aircraft in any of those configurations you're good to go.

Now in the P-39Q flight manual I've seen, the same representation is made in inches from a datum point (which seems to be the tip of the nose). C/G data is omitted. ???

So if we use this information and go back to the original chart we can start removing items and see where they align within the W&B envelope. Mind you this is only for the horizontal C/G, I haven't even touch on the Vertical C/G.

 

[SaparotRob]

Mean_Aerodynamic_Chord_(MAC)

Thanks, I can Wikipedia the definition now.

 

[FLYBOYJ]

Thanks, I can Wikipedia the definition now.

Some manufacturers calculate weight and balance based on % of MAC, others use fuselage stations and the distance from a datum point.

 

[XBe02Drvr]

based on Ivan's calculations (Thanks again Ivan, I know your math is better than mine!!) we have the "station" C/G range of 130.1072 - 136.5584.

So, with a CG range of roughly 6.5 inches we have all three common loading examples clustered within 1 inch of the aft CG LIMIT, before ammunition is expended or fuel burned? This plane needs a helium generator back by the IFF that operates whenever the nose guns are fired to keep it out of Lomcevak Land! Easy, just wire it into the gun camera circuit.
"Don't give me a P39!"

 

[SaparotRob]

It'll tumble and roll. It'll dig a deep hole.

 

[FLYBOYJ]

So, with a CG range of roughly 6.5 inches we have all three common loading examples clustered within 1 inch of the aft CG LIMIT, before ammunition is expended or fuel burned? This plane needs a helium generator back by the IFF that operates whenever the nose guns are fired to keep it out of Lomcevak Land! Easy, just wire it into the gun camera circuit.
"Don't give me a P39!"

Here's one chart for a P-39N

 

[P-39 Expert]

Thank you for all the comments about what I understand and don't understand. I understand mean aerodynamic chord and CG being within certain limits of distance from the wing leading edge.

I also understand that the CG is also a fixed point on the plane, and that if an item is removed from one point on the horizontal axis of the plane then an adjustment must be made on another part of the plane in order to maintain CG. If the nose armor is removed then corresponding weight from the rear of the plane must be removed or moved to the CG. Or a different item must be substituted where the nose armor was. It's called balance.

I also understand, as should you, that the manufacturer was able to maintain the CG within acceptable limits when items of different weights were substituted or removed. Such as larger (heavier) propellers or different (lighter) cannons were substituted in the nose, or different radios were substituted or removed from the tail. Balance was maintained by the manufacturer through all these different equipment changes. Bell stated that the P-39M did not need the nose armor as ballast. And the Russians removed the IFF radio that weighed 120lbs from the tail cone of the plane yet did not remove anything from the nose to maintain balance. According to you all the Russian P-39s should not have been able to fly.

All of you are making much more of this than actually happened. The P-39 was by all accounts easy to operate, safer than most and capable of performing all combat maneuvers. It had good stalling characteristics, acceptable spin characteristics (which could only happen after a stall) and would not tumble when ballasted properly. Most all other fighter planes had some unfortunate characteristics. Such is the nature of high performance airplanes.

 

[Milosh]

Not sure if this will help, CG and Balance calculator.
Aircraft Center of Gravity Calculator

 

[DerAdlerIstGelandet]

Except CG is not a fixed point. It changes based on the distribution of weight in the aircraft. It changes in flight as the fuel is depleted.

i suggest reading...

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/12_phak_ch10.pdf

 

[P-39 Expert]

Russian radar may have been a little more primitive than in the West, and may have lacked the IFF transmitter and receiver.

It wasn't Russian radar, it was American radar supplied to the Russians under lend-lease. We supplied them with tactical radar sets beginning in April 1943. This allowed them to detect and intercept German attacks without flying standing patrols. I have read that they removed their IFF sets because they operated on different wavelengths than their own radio equipment, but I don't know that for sure. The IFF sets in the planes transmitted a signal for 15 seconds out of every minute and supposedly notified ground stations that the plane was friendly. Maybe we didn't supply them with the IFF receivers, I don't know. But they did remove the IFF transmitters from their planes.

 

[P-39 Expert]

Except CG is not a fixed point. It changes based on the distribution of weight in the aircraft. It changes in flight as the fuel is depleted.

i suggest reading...

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/12_phak_ch10.pdf

Not if the fuel is on the center of gravity as it should be.