Really worth and rich explaining
Additional concerned, some ww2 aircraft has transmission or gearshift and other the connected directly to the prop such as in radial engine, does that related to torque convertion or just shift the engine crankshaft motion?
From your info, what is the max RPM allowed for P-51 mustang or A6M zero aircraft or any other good example you knwo
Also, when i camper, the hp for A6M zero is 940 hp from Nakajima Sakae 14 cylinder engine with 554 k/m as speed and Cessna 172 i see with 160 hp and 4 cylinder only and 226 km as speed
I am wondering, does this high hp was required at that time due to the limited of technology in other words, can i make by today same P-51 mustang or A6M zero aircraft with same performance with lower hp engine?
The quick answer is "Not Really". The more accurate answer is "That Depends" - which is the True Answer to all Aeronautical Questions - everything depends on context.
To explain it better, we need to start by looking at the relationship between Engine Horsepower and Propeller Thrust. To save typing, I'll paste in a snippet from a piece I did on Usenet (rec,aviation.military) many years ago:
Thrust: How much force is being exerted by the aircraft's powerplant to push it through the air.
This is a toughie. Reciprocating engines don't produce thrust directly, they produce power. (Which is defined as Torque * rotational Speed - don't sweat that). Power doesn't directly translate into thrust, so we'll have to do a bit of arithmetic:
1 HP = 550 ft-lb/sec. Now, thrust is lbf (pounds force), since we're on Earth, we can safely assume 1G - 32.2 ft/sec^2 and not sweat it) So, to get lbs out of a horsepower number, we divide by 550 ft/sec. (Hey ft/sec, that's speed!) so, if we do a bit more figuring, to get the speed part down, we end up with T (Thrust in lbs) = HP * 550/v (v in ft/sec). As you can see, at low speeds, we get bags of
thrust per horsepower. At high speeds, the thrust decreases.
Here's another table that shows this: (remember that 550 ft/sec = 375 mph)
Speed Thrust (1 HP) HP (1# of thrust)
100 mph 3.75# 0.266
200 mph 1.88# 0.533
300 mph 1.25# 0.800
400 mph 0.94# 1.067
500 mph 0.75# 1.333
As you can see, as speed goes up, thrust drops off alarmingly. But, of course, there's more to it. A reciprocating engine generates its thrust by turning a propeller, which is basically a set of rotating wings, which turn torque into thrust by moving a large volume of air from in front of the propeller disk to behind it. This of course, isn't 100% efficient. While the propeller can be considered a set of wings, it moves through a complicated airmass. The airspeed that a propeller's airfoil sees is defined by the rotational speed of the propeller, and by the forward motion of the airplane. There are also, of course, altitude effects. An airplane propeller, like a wing, stalls at low speeds, and has transonic problems at higher airspeeds. The efficiency of a propeller isn't onstant. At low speeds, it can be rather poor, and it drops off at high speeds. The altitude effects also mean that a given propeller setting is only most efficient at a particular combination of Torque, RPM, airspeed, and altitude. This led to problems in the 1930s, when airplanes with wide speed ranges were beginning to be developed, and supercharged engines, which produced their best power at higher altitudes, were introduced. As an example, the Boeing Monomail transport prototype, with a supercharged Pratt & Whitney Hornet engine, couldn't take off with its propeller set for the cruising design point of the airframe/engine combination -
the propeller efficiency was too low. When the propeller was set for takeoff performance, there was a hefty hit on cruise speed. This was resolved by producing variable pitch and constant speed propellers. Basically, the pitch change allows the peak efficiency to be maintained over a wide combination of engine power/ airspeed and altitude combinations. What this means for this analysis is that the
thrust produced for a particular horsepower isn't dependant on altitude.
Oh, yeah, there's one other factor as well. Because the combination of airplane airspeed and the propeller's rotational speed can get quite high, there's a loss of efficency as the propeller's blades approach the speed of sound. To get past that, the propeller shaft is geared down to keep the total speed low. The Mustang's V1650 engine had a gear ratio of 0.479. For every 1000 engine RPMs, the propeller turned
479.
The efficency of an airplane propeller is best referenced by the Advance Ratio, or 'J'. 'J' is defined as J = V(true airspeed) / n(rotational speed)* d (diameter). For a typical WW 2 fighter airplane propeller, the highest efficiencies are reached at Js between 1.5 and 3.5.
So, as we can see, as speed increases, partularly beyon 300 MPH, Available thrust drops off sharply. As other posters have noted, when you push an airplane through the air more quickly, the drag increases by the square of the speed - so, to double the speed, you need 4 times the thrust, triple it, you need 6 times - all things being equal.
But they aren't.
The airflow over the wing of an airplane increases in speed as it flows over the wing's surface. At higher speeds, the eccelerated airflow starts reaching the Speed of Sound. (Mach 1) When you reach that point, Shock Waves form over wings and tail (Their airfoils, too), and Drag increases sharply - as in a nearly vertical climb to over 10x the subsonic drag in the span of about 30 mph - 50 or so km / h. At the speed and altitude range of a late World War 2 fighter, that's roughly 500 mph True Airspeed. There's a reason why the "Superprops", like the P-51H, or the Spiteful, or whatever, had to put in a lot of effort, and a _LOT_ of Horsepower for a small increase in level flight performance.
Jets, on the other hand, are esentially Constant Thrust - which means the faster they go, the more Power they generate. That's why everybody essentially halted Piston Engined fighter development in the later part of 1944, (The airplanes developed around that time wree basically available at, or just after, the War's end) and all the attention, and most of the engine development, went into Jet Engines and Jets.
We have made some improvements in reducing drag in General Aviation airplanes - it's easier to produce optimized seamless shapes with modern composites, but not as much as you'd think. There are some small useful gains between, say, 120 and 300 mph.
