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

[IdahoRenegade]

Every "front line" US WWII fighter from about '43 on that I can think of got a 4 bladed "paddle" or high activity prop. My understanding is that this was for a couple of reasons, someone please correct me if I'm wrong. First, engine power increased dramatically throughout the war. Increasing the surface area of the blade made it possible to "bite" more air and better utilize the increased available power. In addition, high altitude, low air density operation made these props even more important at high altitudes. Bob Johnson's book Thunderbolt told of the performance boost this made on that plane. Swept area of the prop can be increased by increasing it's diameter (which has an obvious practical limit) or by increasing the number of blades (3 to 4).

The P-47, 51, F4U and Hellcat (I believe) were all refitted to 4 blade paddle props. I don't recall the 39 or 40 ever getting them, but they are a different animal. Why was not the same change made to the '38? I know the K model (2 protos) was fitted with a 3-blade high activity prop, and saw a significant performance increase. However, this was larger in diameter than the standard windmill, requiring different reduction gearing, which in turn drove changes to the structure and cowling. It seems like it would have been much more direct just to fit a 4 blade prop (perhaps off the Mustang, but adding a LH version) to it, with the same diameter, reduction gearing and engine. Was there any particular reason that wasn't possible or practical?

 

[gumbyk]

Besides that fact that you're suddenly producing a prop for one aircraft (the LH version that you speak of). I think that having two engines may have something to do with it, there might simply have been no need for it when each engine is running at a lower power setting for the cruise.

 

[GrauGeist]

I don't recall the 39 or 40 ever getting them, but they are a different animal.

The P-39F/J/K and P-39Q-21 through Q-25 used a hydraulic Aeroproducts four blade prop.

 

[tomo pauk]

Even the 3-blade wide-chord would've improved the already good RoC on the P-38, the diameter was already generous to start with. Perhaps also add a few mph to the top speed.
Why no wide-blade prop on the P-38? Guess we'd never know.

 

[grampi]

I had always wondered the same thing about the F7F Tigercat too...

 

[FLYBOYJ]

Aside from developing maximum efficiency based on torque and engine HP (something calculated by engineers) a 4 blade prop also adds weight. Ask someone who has picked up a blade removed a prop hub of a WW2 fighter

 

[Shortround6]

In the case of the Tigercat it was using 13'2" props to begin with. Prop blade design was always evolving and since the engines in the Tigercat were two speed engines but not 2 stage and were rated at 1700hp at 16,000ft military power compared to the 1800hp at 23,000ft that the engine in the F4U-4 was rated at perhaps the engineers got it right and an extra blade or wider blades were not needed. Please note that at altitudes of around 20,000ft and above a Late model P-38 could make 1600hp in combat power settings.
Using oversize props at low altitude can actually cut speed/performance (not to mention weight penalty )so it is always a bit of a balancing act.

Please note that prop blade dimensions are rather hard to estimate from photographs. Also the width of a prop blade (cord) is going to appear different on different diameter propellers. A small diameter propeller that looks like it has wide (long cord) blades may actually have the same or similar width blades (cord) to larger diameter propeller even though they look different due to the different aspect ratio of the blades.

 

[pbehn]

Isnt having a tricycle/tail dragger landing gear a factor in deciding which prop to use?

 

[FLYBOYJ]

Isnt having a tricycle/tail dragger landing gear a factor in deciding which prop to use?

This depends on a whole lot of factors. Aircraft use, hp, engine RPM, torque, I could go on and on.

 

[pbehn]

This depends on a whole lot of factors. Aircraft use, hp, engine RPM, torque, I could go on and on.

I can see that FB but in simple terms the fewer blades the better however with a tail dragger increasing prop diameter starts to increase the chances of it hitting the ground.

 

[FLYBOYJ]

I can see that FB but in simple terms the fewer blades the better however with a tail dragger increasing prop diameter starts to increase the chances of it hitting the ground.

By increasing prop diameter, it doesn't mean you're increasing efficiency. Propeller Performance: An introduction, by EPI Inc.

 

[Shortround6]

In the interest of maximum efficiency, some 37-40hp engines in 1930s were fitted with single blade props (with counter weight) on Early Piper/Taylorcraft/Areocna airplanes.
No prop is going to be the best prop 100% of the time so the designer/users tried to pick the best prop for the intended use of the aircraft averaged over multiple conditions or biased toward one aspect or another, better take-off vs high altitude or other trade-off.

 

[FLYBOYJ]

 

[Shortround6]

Thank you.

We have another thread going about long range fighters.
A large prop with many blades or broad cord blades may be the best for climb or high altitude work.
However while cruising (even high speed cruise) it may be transmitting 50-70% of the "peak" power of the engine and the larger surface area prop will have more drag and lower efficiency in cruise.
DO you want that last few percent in climb/rate or top speed at 25,000ft and give up a few dozen miles in operational radius? or go for the range/radius and give up a bit on peak performance.

Think of propellers a bit like tires on a car. What works best on pavement (low altitude air) doesn't work so good on sand/mud (high altitude air) and vise versa.

 

[mikewint]

Learn something every day!! Would NEVER have imagined a one-blade prop

 

[GrauGeist]

I can see that FB but in simple terms the fewer blades the better however with a tail dragger increasing prop diameter starts to increase the chances of it hitting the ground.

The Curtiss SC-1 had a small diameter prop (10' 2") but still had good performance in spite of the drag imposed by it's float and outriders. It was powered by a Wright R-1820-62

Here's a photo of the SC-1 with it's fixed gear mounted to show just how small it's prop actually was:

 

[gumbyk]

The Curtiss SC-1 had a small diameter prop (10' 2") but still had good performance in spite of the drag imposed by it's float and outriders. It was powered by a Wright R-1820-62

Here's a photo of the SC-1 with it's fixed gear mounted to show just how small it's prop actually was:
View attachment 310232

But with the float fitted, there was much less clearance.

Also, its not uncommon for floatplanes to have a shorter prop - it helps take-off performance and reduces erosion of the blade due to the water spray.

 

[GrauGeist]

I considered showing a photo of the SC-1 with it's float, but the fixed gear photo is far better to illustrate just how small the props were.

 

[pbehn]

By increasing prop diameter, it doesn't mean you're increasing efficiency. Propeller Performance: An introduction, by EPI Inc.

Flyboy, I wasnt thinking in any particular direction but I was thinking in particular about two aircraft, firstly the Spitfire, without a complete re design of wings and landing gear the maximum prop diameter couldn't really be increased so it sprouted more blades. The second is the Corsair which was laid out as far as the wing shape and landing gear was concerned to have the biggest prop available yet still be able to land on a carrier.

There are a lot of well read people here, the Spitfire and Bf109 were designed in an era when 1000BHP was massive, in a short time designers were considering 2000BHP and the issues around what that meant. One, I believe, must have been how you transmit that power to the air, basically you must increase diameter or increase the number of blades, or both. I dont know if any readers have any info on the subject.

 

[GregP]

For propellers, you can increase 1) diameter, 2) number of blades, 3) blade chord, and 4) change the propeller airfoil. There isn't much else to do until you start looking at Q-tips and strange blade geometry, unless you look at the maybe cuffs around the blade hub. The max RPM is set when the tip goes supersonic (or close to it) and you start losing efficiency rapidly.

My take on the P-38 is that the Allison was rated at 1,425 HP (in the P-38J and later, anyway) with 1,600 HP at what would be WER. The propellers could handle that much HP and the turbocharger allowed the Allison to maintain more of its HP to a much higher level than simple single-stage supercharging did, but they never did run too much HP to the props at any altitude. The P-38 never really got a big shot of extra HP. Yes, it went from early Allison levels (1,050 HP) up to 1,425 HP / 1,600 HP, but the prop could handle that. Had the P-38 gotten an engine of maybe 2,250 HP, I think it would have needed some propeller attention, but since it never really did take a big HP jump, it wasn't necessary. Also, had the P-38 gotten a big HP jump, it would have been even easier to get to critical Mach number and have compressibility issues. That's the probable main reason it never DID get a lot more HP ... it was already pushing the critical Mach limit somewhat too often.

The Allison was rated at 3,000 rpm and had a 2 : 1 gear reduction. So prop rpm was 1,500 at 3,000 engine rpm. The P-38 had a prop diameter of 11 feet 6 inches. At 1,500 rpm, the tip was travelling at 615 mph or 903 feet per second. So at sea level the prop tip was already going Mach 0.808 on a standard day and the speed of sound drops with altitude. At 35,000 feet on a standard day, 3,000 engine rpm results in a tip speed of Mach 0.92. I'd think your want a prop tip redline of Mach 0.88 - 0.92 or so, and the P-38 was right there. For cruise, you'd probably want a tip velocity of some 315 - 350 mph or so, which translates into some 1,700 rpm or so at cruise. Overspeed to maybe 3,150 engine rpm would not be a big deal because you'd be in a dive and not really wanting more thrust. Hitting critical Mach number in a dive would be a MUCH larger concern than propeller efficiency ... at least to the pilot.

The calculations above are tangential velocity and do not take into account forward velocity. At 350 mph true airspeed at sea level and 3000 rpm, the tip speed counting forward velocity comes out to 1,039 feet per second, or Mach 0.928. At 35,000 feet on a standard day the tip speed at 3,000 rpm and 350 mph true airspeed would be Mach 1.067. I don't think they needed any more diameter. Maybe less.