M
mig-31bm
:
To answer a couple of your questions in a little more detail:
Why did the F4U Corsair need more fuel: The wing is the thing. (And I'm taking a shot at
drgondog
CLmax question).
Plane | Root chord | Tip Chord | CLmax | Cdmin |
Spitfire | NACA 2213 | NACA 2209.4 | ~1.7 | .006 |
F4U Corsair | NACA 23018 | NACA 23009 | ~1.6 | .007 |
Fw.190 | NACA 23015.3 | NACA 23009 | ~1.7 | .0065 |
P-47 | Republic S-3 11% | Republic S-3 11% | ~1.2 | .005 |
P-51 Mustang | NACA 45-100* (16.5%) | NACA 45-100* (11.2%) | ~1.2 | .004** |
Thanks to
Aeroweanie
for the base information; CLmax numbers for NACA profiles are from Theory of Wing Sections.
As I understand it, the Mustang wing profile started as a 45-100, but was 'massaged' to get the characteristics they wanted. (I'm not great at the higher math's they used). I'm using Reynolds numbers in the 6 x10^5 range (which is important; you get very different results when you drop number down to 5x10^5 <say for your 1/10 RC scale plane>).
My question for
drgondog
: Does the P-51 Mustang airfoil have what I consider the classic 'laminar bucket', where Cd is only very low for a narrow range of CL or is it more like the 22xx and 230xx airfoils which are a smooth curve.
So, the much lower wing drag during cruise means the Mustang needs less fuel (and that's not getting into fuselage and cooling drag). It all gets extremely complicated.
For why air cooled engines can't operate at as high of boost, I would have said there were a couple reasons.
1. RR developed an extremely efficient supercharger, which meant there was less temperature rise. RR also cools their mixture after both stages of compression (when its at its hottest, getting most benefit from intercooling), while P&W cools their mixture between 1st and 2nd stages. A hotter mixture is more susceptible to detonation.
2. Liquid cooling has the advantage short term of boiling. While, it only takes ~4.2 joules to heat 1g of water 1* C, it takes >2200 joules to boil 1g of water (already at 100*C). While, we don't want to boil all the cooling liquid, allowing a small fraction to boil allows for short term "over boost". (I hope I have my units correct, been a long time).
So, while the maximum boost pressure for liquid cooled is higher, the sustained pressures are much closer.
The mechanic supercharger in a WWII fighter aircraft takes a lot of power and is turning quite rapid, but the powerplant (engine and supercharger) needs to be compact.
For the 1st stage the power demands/speeds are reasonable so you can get away with only one layshaft (like in a car manual transmission)
However, for high ration the power/speed get large and little deflections of the shafts cause large issues, so RR used 2 lay shafts (like in a class 8 truck manual transmission)
For the 3 speed Griffon, they used 1 at 12 o'clock (low speed), 1 at 6 o'clock (medium speed) and 2 at 3 and 9 o'clock and the supercharger drive area was filled to the brim. In order to have more ratios, RR would have needed to make the engine longer. (my diagram has space between the input and output gears for clarity, in real life, they are as close as the manufacturing can cut them).
Then there is the shifting of the rations. Its not like your car where you can let off the gas to ease the shift, the shifts are happening at full power (what I would call powershifting, you push in the clutch, while pulling the shifter into next gear, never lifting off on the throttle). And that was a problem for the F4U-5 side winder - the load was so high, it broke things. So, rather than use clutches, P&W used more/less torque convertors. And "hydraulic connection" of torque convertor smoothed the acceleration of the auxiliary stage from neutral to low and from low to high (and back down again). Once the supercharger impeller had been accelerated, the connection speed was more/less constant. So, the R-2800-32W has a geared drive supercharger, it just has a fluid coupler for shifting, so it have the same jagged profile as the Merlin would.
Fun fact: For the 1st version of the R-2800-32, they put on 2 identical impellers in the auxiliary stage; and the P&W engineers couldn't figure out why it was so inefficient. Then, with red faces, they replaced the one impeller with one designed to turn the other direction and it worked as expected. One of those things, no one lets you forget.
For the XP-72, the auxiliary stage is actually a pump driving a motor with the motor running from ~1:1 to 6.375:1 (max) of the engine. Allowing smooth curve from ground level to 25k' (critical altitude). The challenge with a hydraulic drive - all the pressure you put into the oil to turn the supercharger creates
heat. And last thing we want is more heat in the engine. (Luckily the adiabatic compression of oil doesn't add nearly as much heat as compressing air. And you multiply the efficient of the pump but the efficiency of the motor to get the efficiency of the system...so a 90% efficient pump * a 90% efficient motor gets you an 81% efficient system and the other 19% is heat (In contrast the gears in the Merlin are about 95% efficient). (The engine stage of the R-4360 is running 6.08:1 of engine for reference)
So, as a designer, you need to decide is it better to have just one speed = simple and light, couple/three speeds (e.g. take off and mid climb) or smooth curve. Can your country make the required parts? If you are missing critical materials to make great gears and/or turbocharger or are other solutions attractive.