Why were P-36s and Mohawks altitude-limited?

[OldGeezer]

I often hear that this or that aircraft was lackluster because it lacked the ability to fight at high altitudes due to the lack of the right supercharger/turbocharger equipment, but in reading Gerry Beauchamp's book "Mohawks Over Burma" I discovered another reason that I'd never suspected before, for the P-36 family. Apparently they lacked fuel booster pumps. The result, as described in a diary entries by one of the actual pilots, was a practical inability to go high:

"D. Parsons: ...we formed up while climbing steeply to the south, along the coast and levelled off at 17,000 ft. This, incidentally, was just about the operational ceiling of the Mohawk. Not because it had not the power, but at this height, with the ambient temperature of the fuel, with no electric boost pump but only a manual wobble pump, the fuel started to boil in the lines and it often needed quite a bit of wobbling just to keep the engine going."

The book is 40 years old and when it was written, many of the No. 5 and No. 155 Squadron pilots were still available to interview, or their families had their diaries to use as sources. I'm only halfway through but I'm learning a lot from the primary source materials. Just thought I'd share it here in case others found it interesting. Or maybe everyone already knew this, and I'm just late to the party as usual!

 

[MiTasol]

Vapour locking was a known problem on many aircraft but not recorded in many manuals.

One of the P-47 manuals "rectification" for the problem was to reduce altitude until the fuel supply stabilised.

 

[yulzari]

I often hear that this or that aircraft was lackluster because it lacked the ability to fight at high altitudes due to the lack of the right supercharger/turbocharger equipment, but in reading Gerry Beauchamp's book "Mohawks Over Burma" I discovered another reason that I'd never suspected before, for the P-36 family. Apparently they lacked fuel booster pumps. The result, as described in a diary entries by one of the actual pilots, was a practical inability to go high:

"D. Parsons: ...we formed up while climbing steeply to the south, along the coast and levelled off at 17,000 ft. This, incidentally, was just about the operational ceiling of the Mohawk. Not because it had not the power, but at this height, with the ambient temperature of the fuel, with no electric boost pump but only a manual wobble pump, the fuel started to boil in the lines and it often needed quite a bit of wobbling just to keep the engine going."

The book is 40 years old and when it was written, many of the No. 5 and No. 155 Squadron pilots were still available to interview, or their families had their diaries to use as sources. I'm only halfway through but I'm learning a lot from the primary source materials. Just thought I'd share it here in case others found it interesting. Or maybe everyone already knew this, and I'm just late to the party as usual!

How might this have been so?

The listed service ceiling of the standard P36 is usually given in excess of 30,000 feet. Surely the standard P36 must have been equipped to cope with the lowered boiling point of the fuel at such heights?

Yes the fuel on the Burmese front must have been at a high ambient temperature at take off but surely no worse than in the summer in the south of the USA. I do not doubt the first hand accounts above are correct but I confess I am puzzled as to why it should be so. If the listed service ceiling needed an electric boost pump why would the front line Mohawks not be so fitted?

 

[OldGeezer]

How might this have been so?

The listed service ceiling of the standard P36 is usually given in excess of 30,000 feet. Surely the standard P36 must have been equipped to cope with the lowered boiling point of the fuel at such heights?

Yes the fuel on the Burmese front must have been at a high ambient temperature at take off but surely no worse than in the summer in the south of the USA. I do not doubt the first hand accounts above are correct but I confess I am puzzled as to why it should be so. If the listed service ceiling needed an electric boost pump why would the front line Mohawks not be so fitted?

Reality always beats published numbers. The "standard P-36" was equipped to cope with the fuel pressure deficiencies at high altitude via the wobble pump and nothing else, for what reasons I don't know since (IIRC) the requirement against which it was submitted specified 32,700 ft ceiling. I'm guessing that the prototype, lacking military equipment (guns and armor for example) might have been able to struggle up that high, but when things were added to the production version performance would have suffered.

If you'll read Beauchamp's book (which is oddly organized but fascinating - you'd probably love it as much as I did) you'll see that this wobble pump business is a recurring theme, but it's something that the pilots seem to have taken in stride and not gotten too irritated with. And it describes another problem with trying to operate Mohawks at very high altitudes. Harold Nicholson, one of the pilots in 155 Squadron, describes a test flight that he made in one: "I and another pilot were sent up to find the absolute ceiling. We climbed up to 21,500 feet and at that height had practically no control. The plane just hung in the air with little response to the controls." So that's another thing I didn't know before, making me seriously wonder how the published numbers were arrived at. I need to find a copy of the P-36 pilot's manual...

 

[cammerjeff]

That's interesting, when I was in the CAP as a teenager we had a ex US Army DC Beaver we sometimes would fly, I remember the last flight I took in it we had a fuel issue of some sort, I don't remember what it was but after about 30 minutes into our flight the PW 985 started running rough, the Pilot (I was not and never have been a Pilot) told me to start working the wobble pump, and as long as I worked it the engine smoothed out, but if I stopped it would start running rough again in a few seconds. I can't imagine trying to fly the aircraft and continually work the fuel wobble pump, after a few minutes it was a whole lot of work!
I still don't understand the fuel pressure issue we were having, as the high mounted wing tanks should have just gravity fed the fuel to the engine. It was probably a partually clogged fuel filter. But I don't know that for sure.