GregP
Major
And a jock strap!
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Is that why the P-39 pilots were wearing shorts in all those phots?
See the best part of the conversation:
IOW - drive shaft specific to the P-39 is never mentioned.
Allison was trying to figure out what to do with all the 90 degree gearboxes and shafts left over from the Navy Airship project and was offering them on clearance sale.Kelly Johnson did have two types in his P-38 concept sketches, that had two shaft driven props, ine on each wing.
One concept was a tractor and the other was a pusher.
"Scrapped"… I so hate that word. You can't keep everything, but, damn, I know some old boys that'll give it a shot and eventually find a use for it.Probably scrapped.
A couple of notes from the deeper darker recesses of my brain. First, wartime research showed that the blade tips weren't necessarily the limiting thing for prop speeds. The rotational velocity at the root end of the blade was obviously lower, but that area had a round cross-section rather than a thin taper, and so the air flowing around the root could reach "critical Mach number" before the tips did, depending on forward airspeed, altitude, temperature, and pressure conditions. That's one of the reasons why cuffs were put around those sections, besides helping pull in more air for engine cooling. Critical Mach number is also determinable for the aircraft itself, based on simple surface contours like the curve of a canopy or especially the airfoil(s) used in the wing and tail surfaces. Once you reach that, the drag rises so sharply that you can add substantial increments of engine power without seeing much payoff in added speed, due to the big drag increases involved. I can vividly remember in the 1980s when my group at McDonnell Douglas was tasked with defining a follow-on variant to the AV-8B that was then entering service. We looked at all the ways we could increase thrust to improve vertical takeoff and landing capabilities, and assumed that there would be corresponding increases in maximum horizontal speed as well. But there weren't. Our head Aero guy showed us the drag curves, and proved to us that we could literally double or triple the amount of thrust from some imaginary impossible engine, and it would all be eaten up by drag in level flight, so we could never come close to having a supersonic dash capability. This was an exaggerated case due to AV-8B's supercritical wing, but in general it has applicability to all sorts of aircraft, whether they have props or jets or rockets.Why not? The Soviet Bear used BIG contraprops and they were at almost an absurd pitch angle. That aircraft, clean, could hit 575 mph. If the Merlin made more power, then they could run the propeller at higher pitch.
At 20,00 feet, the speed of sound is 706.9 mph or 1,036.787 ft/s on a standard day.
Say you are in P-51D mustang (11 foot diameter prop) going 450 mph true airspeed, turning 3,000 rpm, with a prop reduction gear of 0.5, making Omega equal 157.0796 radians per minute.
Your forward velocity is 660 ft/s ([450*5280]/[60*60]). Your radial velocity is 495 ft/s (V = omega r, with r = 5.5 ft). Square them both, add them up, and take the square root to get tip velocity, which turns out to be 825 ft/s., or Mach 0.7957. You could hit 514 mph before your prop tip speed reaches Mach 0.87, which is nearly ideal, assuming you have the power to turn the engine at 3,000 rpm with the prop pitch set to give 514 mph. Since the prop has an efficiency (assume 0.90 or so), it would likely have to be pitched to give 560 mph or so, making it even harder to turn, but that is just power. We are assuming a more powerful Merlin, V-1710, V-3420, etc., so let's say we have the power.
Let's say we go 450 mph with a Merlin making 1,490 hp. Using the old tried and true formula for new top speed with new power, assuming no increase in drag, I get that we'd need 2,220 hp to go 514 mph. That's possible.
A couple of notes from the deeper darker recesses of my brain. First, wartime research showed that the blade tips weren't necessarily the limiting thing for prop speeds. The rotational velocity at the root end of the blade was obviously lower, but that area had a round cross-section rather than a thin taper, and so the air flowing around the root could reach "critical Mach number" before the tips did, depending on forward airspeed, altitude, temperature, and pressure conditions. That's one of the reasons why cuffs were put around those sections, besides helping pull in more air for engine cooling. Critical Mach number is also determinable for the aircraft itself, based on simple surface contours like the curve of a canopy or especially the airfoil(s) used in the wing and tail surfaces. Once you reach that, the drag rises so sharply that you can add substantial increments of engine power without seeing much payoff in added speed, due to the big drag increases involved. I can vividly remember in the 1980s when my group at McDonnell Douglas was tasked with defining a follow-on variant to the AV-8B that was then entering service. We looked at all the ways we could increase thrust to improve vertical takeoff and landing capabilities, and assumed that there would be corresponding increases in maximum horizontal speed as well. But there weren't. Our head Aero guy showed us the drag curves, and proved to us that we could literally double or triple the amount of thrust from some imaginary impossible engine, and it would all be eaten up by drag in level flight, so we could never come close to having a supersonic dash capability. This was an exaggerated case due to AV-8B's supercritical wing, but in general it has applicability to all sorts of aircraft, whether they have props or jets or rockets.
They were preoccupied with the V-1710.Why didn't Allison work harder on developing it? Did it have some serious flaw? (Hard to believe because the V-1710 seemed to be more reliable and easier to build than the Merlin. I'm beginning to think that the Allison got a really bad rap.) Like the P-38, a 1942 XB-42 COULD have gotten one (or two) good turbochargers... Of course the XB-42 might have been even better if it was a tractor ala the R2Y than a pusher...
I stumbled across the prop hub/cuff thing while looking for something else, so naturally I didn't bookmark it and I can't find it again now. But the 1949 classic "Principles of Aerodynamics" also mentions the phenomenon, though without the full math treatment: "Compressibility shock frequently occurs on the blunt shank of the propeller. This may be delayed by using a cuff, which consists of an airfoil-shaped sheet that surrounds the shank and reduces its per cent thickness. The cuff is also sometimes used to increase cooling air flow through the nacelle."Interesting concept. Not too sure the roots will actually DO that but, if they DID, it would surely be the "angle that pissed in your powder box" and would gum up the calculations. I am under the distinct impression that the cuffs were put there to help with both carburetor intake and cooling airflow. Never even considered that the blade root might be going supersonic, even in aerodynamics classes 50+ years ago.
Interesting concept. Not too sure the roots will actually DO that but, if they DID, it would surely be the "angle that pissed in your powder box" and would gum up the calculations. I am under the distinct impression that the cuffs were put there to help with both carburetor intake and cooling airflow. Never even considered that the blade root might be going supersonic, even in aerodynamics classes 50+ years ago.
When making such statements you need to elaborate on better at what.V-1710 was a decent engine, Merlin was far better
I stumbled across the prop hub/cuff thing while looking for something else, so naturally I didn't bookmark it and I can't find it again now. But the 1949 classic "Principles of Aerodynamics" also mentions the phenomenon, though without the full math treatment: "Compressibility shock frequently occurs on the blunt shank of the propeller. This may be delayed by using a cuff, which consists of an airfoil-shaped sheet that surrounds the shank and reduces its per cent thickness. The cuff is also sometimes used to increase cooling air flow through the nacelle."
(my bullet points)When making such statements you need to elaborate on better at what.
(1)The Merlin was the superior high altitude engine, and that's about the end of the story.
(2)The Allison had less parts count, better fuel economy, more rugged, was the engine of choice for the low altitude role, so much so there were those who wanted the Allison P-51 to remain in production alongside the Merlin version.
(3)So a statement saying "The Merlin was a decent engine, V-1710 was far better" is equally correct. Context, context.
That is an interesting thought and it can be extended backwards.I hope this doesn't constitute hijacking this thread, but I'd be curious if anyone has any insight into how the XB-39 (V-3420-17 powered B-29) compared to the XB-44 (R-4360-33 powered B-29)? I would think the comparison would be informative even if the V-3420 and R-4360 were on different developmental timelines, simply for the comparison between inline (and water cooled) vs. radial (and air cooled).
(my bullet points)
So I'll elaborate.
(1) Story just begins with one engine - Merlin in this case - being better (much better from mid-1940 on) at higher altitudes. Bar in the Eastern Front, all the ww2 air fighting was about who has the fighters that are better at high altitudes. Trick was also that it was much easier to improve the low-alt abilities of a high-alt engine, than the vice-versa. BTW - Merlin was just as good at low altitudes as the V-1710.
(2) Pilots and mission planers cared much more about the actual capabilities of an engine, than what they cared about the parts count. More rugged - do you have any statistics to back up the claim; how well was the V-1710 (or any other liquid-cooled engine) with it's coolant circuit leaking, or with it's oil system wrecked? Better fuel economy was achievable when enemy was not around so flying at 5000 ft and at 200 mph was viable. Once it was required to cruise at 300 mph at 25000 ft, the V-1710 was no better than Merlin, with caveat that it took a turbocharger to actually cruise at these altitudes and speed. Or the 2-stage version that was more than a year in service after a 2-stage Merlin, it was not as good, and it was a bigger engine that initiated problems of how to retrofit it easily on the existing aircraft.
British engine of choice for low altitudes was Merlin of different supercharger diameters and drive ratios. Non-turbo 1-stage V-1710s (= vast majority in use) being useful just for lower altitudes was a bug, not a feature. There was far more of those that cheered once Merlin was installed on the P-51 than those that clamored for more V-1710-powered P-51s. The former was slapping Luftwaffe above Germany proper, not the later.
(3) That statement is not equally correct. Or correct at all.