Thank you all for the thoughtful answers. Once again, I am very impressed by the quality of answers available here. Many thanks.
More of a question than a suggestion, but wouldn’t a couple of dozen spray cans of teflon based paint make a big difference?
IIRC, the Enola Gay (props to my namesake) had the engines modified by replacing the carburetors with fuel injection.
This is not correct. As I said above, a component of drag is induced drag - the drag created when lifting a mass. If you’ve ever seen ‘wingtip vortices’ coming off an airplane, that’s energy thrown off the airplane that has to come from somewhere. And it’s a LOT of energy, especially at very low speeds. The whole reason airplanes have the longest, skinniest wings possible is to reduce induced drag. It’s a direct function of lift, which in turn is a function of weight. Heavy airplanes go slower than light ones. However, the difference is not that dramatic, because induced drag is highest at low airspeeds, and at high airspeeds it’s quite low. Other drag increases with speed, so at high speed cruise parasitic and form drag predominate. Still, if you empty out an airplane, it will go faster. And in fact, pilots of returning bombers in WWII could go faster on the way back after dropping a bomb load if they needed to than they could when they were heavy with bombs.
In high altitude bombers, induced drag becomes very important, because they are actually travelling at a low indicated airspeed. That’s also why the U2 spy plane has such incredibly long, skinny wings. At the extreme altitudes it flys at, the indicated airspeed is just above its stalling speed, and induced drag is a major component of the total drag on the airplane. Long skinny wings help minimize that.
Not quite: Consolidated Vultee built the P81, the world’s first turboprop fighter, before the end of the war. It gave no performance boost to the fighter, but who knows the effect on a bomber?
Regarding piston fighters in level flight:
The P-47M, intended to chase buzz bombs, made it up to around 472 mph (although I am not sure it saw service before Germany capitulated).
The record holder for level flight by a piston engined plane remains (as far as I know) the non-production test bed P-47J with the radial replaced by a water-cooled monster that gave it a top speed of 504 mph.
A number of planes got closer to 550 in dives, (and the Jug would have certainly been among that group).
Yeah, but a P-47 was just a big engine and propeller with a tiny accessory package attached to it. (-:
Does this mean that for the same aircraft with the same airspeed, a heavier load means flying with a greater angle of attack, i.e. with the nose pitched higher? Because if it doesn’t then I’m very confused…
Yes. With more weight you need more lift - the two forces must exactly balance to keep the plane at a constant height. And if you can’t put in more power, you must use more of the power you do have to create the necessary lift, which means raising the nose a bit. Your plane’s maximum carrying capacity is reached when you can’t raise the nose any more without stalling.
matt, you’re correct, at least in the specific statement you make above. Sam Stone was saying that there was more to aerodynamic drag than NaturalBlondChap had naively said.
As Sam said, the various components of drag have different relationships with airspeed. Induced drag is higher at lower speeds, whereas the other types of drag are higher at higher speeds. That is counterintuive to somebody with only bicycle and car experience.
I have nothing more to add to Sam’s superb explanantion of aircraft design drag & ways to marginally improve a B29’s top speed. Now for the pilot’s point of view on speed in general …
For everybody: There are several different types of speed measured in aircraft. Not to mention the perennial confusion between knots & miles per hour, or even km/h. There are also several definitions of “maximum”.
These vagaries make meaningful comparisons less than obvious. Attempting to just compare one number with another is useless unless you know all 3 components: unit of measure, type of speed & engineering definition of the maximum.
There is “indicated airspeed”, “calibrated airspeed”, “equivalent airspeed” & “true airspeed”, or IAS, CAS, EAS & TAS. Then you add in wind effect to get “ground speed” or GS.
Back in the B-29 days, US aviation was just changing over from measuring everything in mph by default to measuring it in knots by default. The difference is 15%. 200 knots = 230 mph.
When the unit of measure is knots (worldwide std now except in Russia & a couple other places which use km/h) that’s abbreviated KIAS, KCAS, KAS & KTAS for “Knots Indicated …”
Depending on altitude & temperature, TAS can = IAS * 2.0 for the same exact motion through the sky.
So “300 mph” can be faster than “450 knots” if you’re at altitude and the 300 mph is IAS while the 450 knots is KTAS. And this is true even though 300 mph is slower than 300 knots when they’re both descibing the same kind of speed.
If your source doesn’t specify which type of speed they’re talking about and at what altitude & temperature it’s measured, you have no meaningful way to say which # is actually faster.
Turning to maxima …
There are several. The highest is “do not exceed speed”, the safe speed limit of the structure. Go faster than that and the breeze may start breaking things off the plane. For fast aircraft, that’s expressed in both IAS & in Mach and the relationship between those two numbers changes with altitude & temperature, one being the more restrictive limit down low & the other more restrictive up high.
Then there is a practical maxima which is how fast the plane can accelerate to in level flight with all engines at max power.
Then the max speed with all engines at the highest power setting safe for continuous use.
Then the max speed consistent with engine longevity or reasonable fuel consumption.
Somewhere below that number is the max speed for operation in turbulent air.
For a specific mission, say Saipan to Tokyo & return, there’s a max speed that is fuel-efficient enough to get there & back without running out of gas. The max efficiency speed is a lot slower than the max cruise speed, often 1/3rd slower.
Some aircraft have speed maxima which vary by weight or loading pattern.
High performance propeller aircraft often also have power limits that depend on altitude & temperature related to prop tip speed vs Mach 1. So the maximum safe power setting may limit the maximum continuous speed at some altitudes & temps and not at others.
So when somebody says “the maximum speed of an B-29 is XXX mph”, you have no idea which maximum they’re talking about.
All this is vastly more complicated than “my car tops out at 95 mph on more-or-less level ground.”
The drag curve of a typical aircraft of B29 shape is parabolic, almost exponential, at the high end. Once you get up against the knee in that curve, the power required to go meaningfuly faster is gigantic.
Just making up numbers … If a stock B29’s engines can push it to 300 KIAS in sustained level flight at typical cruise altitude, temp & weight, magically doubling the horsepower might get you to 315 KIAS. The curve is really that steep.
At the end and after WWII the Army Air Force had a test program to investigate control and other problems that had been encountered during high speed dives. P-47’s were used in that.
I plead ignorance rather than naivety,
explains it all rather well, (induced drag not my lack of knowledge)
It sounds silly, but a good wash and wax would’nt hurt. No paint - it only adds weight and collects dirt. That’s way later models of Army air force aircraft were ordered bare skinned - less weight, less fuel needed, slightly more speed, less maintenance - and less cost to begin with. Not all pilots and aircrew agreed, but it was generally assumed that the few miles per hour gained by having bare, sometimes waxed, aluminum were worth the disadvantages of being more visible.
A relative who flew in B-24’s out of Italy tells me the paint deletion was worth 300 pounds, didn’t really affect visibility compared to the dark brown the early models had, and got new planes delivered to the front just that much sooner. Lack of paint didn’t affect corrosion susceptibility either, since the life expectancy of a combat plane in Europe averaged just a few weeks anyway.
To expand a little on the answer, suppose we have an airfoil whose lift/drag charcteristic I’m looking at right now. To get 2000 lb of lift at 100 mph I need an angle of attack of 2[sup]o[/sup] If I increase the load so that I need 2500 lb of lift at 100 mph the angle of attack must be increased to 4[sup]o[/sup].