P-38 Lightning-why no 4-blade paddle prop?

[FLYBOYJ]

Both of those aircraft had multiple sources of production from a fairly early period of the war. That would allow them to convert one line/facility over while maintaining production at the other. Just speculation on my part but that might be a reason the WPB was more inclined to allow major changes to those aircraft.

Production changes (even major mods) were usually pretty transparent.

 

[IdahoRenegade]

Production changes (even major mods) were usually pretty transparent.

Lockheed estimated 2-3 weeks to incorporate the K model changes (Bodie), and that assumes Allison could have delivered the engines as promised. The War Production Board wasn't willing to accept that loss of production. Given the timeframe (Q2 of 1943) that makes some degree of sense. AT THAT TIME the Lightning was our only high performance, long range fighter. The Thunderbolts were still very range limited, and it would be another 6+ months before a P-51B would even fly a combat mission. Disappointing that it was never in production, but given the priority at the time, it makes some degree of sense. Still, once the 51B was available, it doesn't seem like it would have been a major sacrifice to cut it in.

When all is said and done, the shortcoming of the P-38 in 1943 was not propulsive power, but other issues - low critical Mach number, only one generator, faulty cockpit heating, issues with engine intake; all of this required the immediate attention. The situation was not helped with only one production source, indeed.

I suspect that was the bigger issue. Addressing those issues was a much bigger priority, and they weren't addressed until the J-25-LO and L models in early '44.

From the link:

There were still other modifications that were necessary. The Hamilton Standard props required a spinner of greater diameter, and the thrust line was slightly higher as well. This in turn, required that new cowlings be manufactured to properly blend the spinners into the engine nacelles. These were hand made and the fit was less than perfect. The new propellers necessitated a change to the reduction gear ratio. The Curtiss Electric props had a normal ratio of 2.00 to 1. The ratio was changed to 2.36 to 1.

Flight tests were conducted from late February through the end of April 1943. Performance was better than hoped for. Maximum speed at critical altitude (29,600 ft) was 432 mph (Military Power). At 40,000 feet, the "K" zipped along at a speed that was 40 mph faster than the current production P-38J could attain at this same height. Maximum speed in War Emergency Power, at critical altitude, was expected to exceed 450 mph. The increase in ceiling was just as remarkable. Flown to 45,000 ft on an extremely hot and humid day, Lockheed engineers predicted a "standard day" service ceiling in excess of 48,000 ft! Improvement of the cowling fit and the elimination of the heavy coat of paint would have gained even more performance. Due to the added efficiency of the new propellers, range was expected to increase by 10 to 15 %. Lockheed appeared to have a world-beater on their hands.

The plane, now designated the P-38K-1-LO was flown to Elgin Field for evaluation by the USAAF. Flown against the P-51B and the P-47D, this Lightning proved to be vastly superior to both in every category of measured performance. What astounded the evaluation team was the incredible rate of climb demonstrated by the P-38K. From a standing start on the runway, the aircraft could take off and climb to 20,000 feet in 5 minutes flat! The "K", fully loaded, had an initial rate of climb of 4,800 fpm in Military Power. In War Emergency Power, over 5,000 fpm was predicted.​

Maybe some wishful thinking as far as performance was concerned, but sure would have been nice to see.

 

[FLYBOYJ]

Lockheed estimated 2-3 weeks to incorporate the K model changes (Bodie), and that assumes Allison could have delivered the engines as promised. The War Production Board wasn't willing to accept that loss of production. Given the timeframe (Q2 of 1943) that makes some degree of sense. AT THAT TIME the Lightning was our only high performance, long range fighter. The Thunderbolts were still very range limited, and it would be another 6+ months before a P-51B would even fly a combat mission. Disappointing that it was never in production, but given the priority at the time, it makes some degree of sense. Still, once the 51B was available, it doesn't seem like it would have been a major sacrifice to cut it in.

This decision has always been arguable. 2 or 3 weeks worth of a production stoppage during that period would have meant about 80 aircraft not being delivered. In hindsight what would have been gained by the P-38K would have possibly eclipsed the production stoppage. I know people who were working at Lockheed during this period, it was always felt the reasoning for not building the P-38K was political

 

[IdahoRenegade]

This decision has always been arguable. 2 or 3 weeks worth of a production stoppage during that period would have meant about 80 aircraft not being delivered. In hindsight what would have been gained by the P-38K would have possibly eclipsed the production stoppage. I know people who were working at Lockheed during this period, it was always felt the reasoning for not building the P-38K was political

Tough call no doubt. General Kenney was screaming for every P-38 he could get in the SWPA as were MTO commanders. Through mid-43 it's questionable that either theater ever had over 80 flyable Lightnings at one time.

 

[drgondog]

Lockheed claimed, and Bodie repeated the performance claims above, but the flight tests at Eglin showed that the P-38K-1 actual performance at 500 pounds under combat load clean was 'marginally' better than a stock P-38L-5 and nowhere near the claims for climb which were slightly above the P-38J at the same weight.

It should be noted that the tests had concluded on the XP-51F and G which had better than the 'projected' P-38K and far better than the actual flight test results. And far cheaper for the P-51H production price agreement.

 

[GregP]

Also, the standard formulas for predicting performance gains with a change in HP and drag don't support the claimed performance gains either. And they are very close to being spot-on for most changes in HP/drag when the actual values are known.

We don't, of course, know the dag change for the P-38K, but we DO know the change in HP and the drag can be estimated quite well. The change in HP coupled with a reasonable estimate of the drag indicate a slight increase in performance, but nothing like the claimed performance changes.

 

[FLYBOYJ]

BECAUSE KELLY JOHNSON SAID SO. THAT'S WHY.

 

[drgondog]

That is why the P-38K engine (-F15) had a modified gear ration of 2.36 to 1 ---> so the 13'6" Curtis Prop would work.

 

[pbehn]

That is why the P-38K engine (-F15) had a modified gear ration of 2.36 to 1 ---> so the 13'6" Curtis Prop would work.

Easy to focus on 4 blade props versus 3 blade, the P38 prop at 13'6" was bigger than the P51 4 blade paddle and not concerned with prop strikes.

 

[Shortround6]

BECAUSE KELLY JOHNSON SAID SO. THAT'S WHY.

See post #23 in this thread.

 

[pbehn]

See post #23 in this thread.

With the P51 was the 4 blades paddle prop to increase maximum performance or to improve economy at cruise speed (or both)? For an escort fighter covering miles and staying aloft with less fuel was just as valid as maximum speed.

 

[FLYBOYJ]

I agree.

Bigger prop more air is caught. Its the last inches that do the most.

Bigger prop means more weight and more mass for the engine or gear box to move. It's not that simple.

 

[Shortround6]

Probably both.

I would note that the P-38 with it's 11.6ft props had about 14% more disc area than a P-51 with a 3 bladed prop on an Allison engine.

And to add to what FlyboyJ said, the 4 bladed prop on a Mustang weighed about 70lbs more than the 3 blade on the P-38.

 

[tomo pauk]

Everyone was attaching ever better/heavier/more efficient props to their aircraft as the engine power grew, so I don't think it was much of a problem for the P-38 to get a 4-blade prop, the blades being preferably of wide chord.
With that said, P-38 have had more acute problems than choice of the props - just 1 generator per A/C, faulty heating, low mach limit, messy cockpit, low rate of roll. Granted, some of those problems were adressed, if a bit too late.
The biggest of it's problems (not tied to the design) was lack of another source for mass production.

 

[FLYBOYJ]

Everyone was attaching ever better/heavier/more efficient props to their aircraft as the engine power grew, so I don't think it was much of a problem for the P-38 to get a 4-blade prop, the blades being preferably of wide chord.
With that said, P-38 have had more acute problems than choice of the props - just 1 generator per A/C, faulty heating, low mach limit, messy cockpit, low rate of roll. Granted, some of those problems were adressed, if a bit too late.
The biggest of it's problems (not tied to the design) was lack of another source for mass production.

The P-38's cockpit was no more messy than any other twin engine aircraft of the day.

 

[tomo pauk]

The P-38's cockpit was no more messy than any other twin engine aircraft of the day.

Many features were not related to the number of engines - I'll just post stuff from 'America's hundred thousand', pg. 164:
"One pilot with considerable P-38 experience said: The cockpit had gotten lousier and lousier (with succesive models), and you could see fewer gages"
"At 1944 gathering of fighter pilots 55 percent rated the P-38 cockpit worst in arangement of many fighter types present, and noted it (P-38) had the least convinient landing gear and flap controls of all."

 

[FLYBOYJ]

Many features were not related to the number of engines - I'll just post stuff from 'America's hundred thousand', pg. 164:
"One pilot with considerable P-38 experience said: The cockpit had gotten lousier and lousier (with succesive models), and you could see fewer gages"
"At 1944 gathering of fighter pilots 55 percent rated the P-38 cockpit worst in arangement of many fighter types present, and noted it (P-38) had the least convinient landing gear and flap controls of all."

Yep - the old Col Rau letter - fully aware of all this but if you really look into this you'll find that most of the negatives written about the P-38 cockpit was done so by pilots with little or no multi engine experience, very swayed and biased towards their easier to fly single engine machines (and I can't blame them). Look at photos of the P-38 cockpit and compare them to the cockpit of the A-20, B-25 and even the Beaufighter.

 

[FLYBOYJ]

Landing gear levers on the P-38 and P-51 were on the same side of the cockpit;

 

[tomo pauk]

Yep - the old Col Rau letter - fully aware of all this but if you really look into this you'll find that most of the negatives written about the P-38 cockpit was done so by pilots with little or no multi engine experience, very swayed and biased towards their easier to fly single engine machines (and I can't blame them). Look at photos of the P-38 cockpit and compare them to the cockpit of the A-20, B-25 and even the Beaufighter.
...

Thanks for the feedback, Joe.
Still - none of this was written by Col. Rau. Granted, having two engines to cater for will add compexity in operation vs. P-51s and 47s.

 

[FLYBOYJ]

Thanks for the feedback, Joe.
Still - none of this was written by Col. Rau. Granted, having two engines to cater for will add compexity in operation vs. P-51s and 47s.

You're right but he was one of the first ones to complain about the P-38 and many of his sentiments were reflected in that report.