Friday, August 15, 2008

The P-39

This week's post comes to you courtesy of the National Museum of the U.S. Air Force in Dayton OH. I just spent the week in Dayton and although I did not have time to visit the museum, I managed to dig these photos out of my archive. As I've said before, this is an amazing museum with a very large collection and you have to have plenty of time if you want to get the most out of it.

These pictures came from the WWII hangar.

Meet the P39 Airacobra. These were produced by Bell Aircraft Corporation beginning in 1939 and used in combat throughout WWII.

If it looks a bit odd for an aircraft of that era, there's a reason. Most propeller driven fighter aircraft were designed around an engine and prop in the nose of the aircraft. This design was different, it centered around an Oldsmobile T9 cannon which was actually made to fire through the center of the prop. This was a particularly large and bulky weapon and the desire to make it fire through the propeller made it necessary to find another location for the engine. The engineers at Bell settled on a mid-engine design.

That's what the P39 looks like on the inside. The engine sits behind the pilot and a driveshaft (which looks surprisingly thin to me) runs to the nose of the aircraft where it is joined to the propeller with a gearbox. This design gave it very different characteristics from the other fighter aircraft at the time. The location of the center of gravity made the aircraft nimble, with a roll rate comparable to or better than many of its contemporaries. This also led to a tendency to go into flat spins if there was not enough weight in the nose of the aircraft. (Aircraft which were not carrying ammunition had to be ballasted in order to be safely flown.) The plane also had to be equipped with unusual nose wheel landing gear system.

The engine location meant that the nose could have a very streamlined and efficient profile, but putting the engine compartment behind the fuselage meant that there was no room left for gas tanks. This meant that the fuel tanks had to be installed in the wings. This limited the fuel capacity (though it was often supplemented with drop tanks) meaning that the plane was typically limited to short range missions rather than long range patrols. The plane's operation was also limited because it was underpowered compared to many other aircraft. The single stage supercharger limited it to low altitude work, which is unusual considering that it was originally designed in response to a request for a high altitude interceptor aircraft. This was later resolved with the introduction of the P63 kingcobra that had a more powerful engine and different wing profile.

Pilots in the US and the UK typically did not find the aircraft very appealing. Aside from some complaints about having to fly with a high speed driveshaft between their legs, they found that the lack of power and high altitude performance severely handicapped their work. They were still quite effective as low altitude interceptors, which made them useful for quite some time. Unfortunately the tactics used in the western European theater did not allow the full potential of these planes to be explored. The US mostly used fighters for escorts on high altitude bombing runs, and the airacobra was not able to perform well at these altitudes.

It was the Russians who really used the plane to its full potential. They needed escorts for low altitude bombing runs and fighter bombers for ground attack. The airacobra shined in this role. Not having the engine in the nose meant that when the plane was making ground attacks, the engine was not up front and was therefore not very vulnerable to ground fire. The nose cannon also provided a very stable firing platform with an incredible amount of power. This made the airacobra an effective ground attack aircraft. It's commonly believed that the airacobra was used extensively as a tank killer, though some have questioned this as armor piercing rounds were not available for the P39’s cannon.

The Russians most certainly used this in fighter duty against the Germans in the air. This is where it developed a very good reputation as a fighter. Low altitude battles were the airacobra's forte because it had enough power to compete with other fighters and because of its considerable maneuverability.

In air combat altitude is quite important. It can be traded for speed and speed gives you the ability to escape from a dogfight. This fact has saved many fighter pilots from attacks by aircraft with superior maneuverability. But when you eliminate the ability to dive out of a fight, maneuverability becomes much more important and this is where the airacobra performed well.

It didn't hurt that it was incredibly well armed. The T9 cannon could fire a 1.3 lb projectile! Of course with ammunition of that size and weight, you couldn't carry very many rounds, or fire very fast, so the cannon was supplemented with either two or four machine guns mounted on the nose (In the conventional manner) and three machine guns mounted in each wing. Supposedly, some Russian pilots removed machine guns from the wings to improve the roll rate of the aircraft. They apparently felt that the nose guns were sufficient for their purposes. I can't blame them.

It's hard to think you need more firepower when you have one of these!



At 6:30 PM, Blogger Brigid said...

That's a GREAT Museum, only a few hours from where I live and worth the trip, though I haven't been in a while.

Great photos and dialog on what is there now. It's worth seeing for anyone that's interested in airpower or history.

At 10:36 AM, Blogger Mr. Engineering Johnson said...


I'm really hoping I'll have time to make it down there for the 2009 WW-I fly in. I'm no pilot, but I might get an old uniform out of mothballs and join in the festivities just the same.


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