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In article ,
Alan Dicey writes: Peter Stickney wrote: It's all a balancing act - but in ggeneral, you're best off going with the largest diameter propeller with the fewest number of blades that you can practically manage. i) I'm sure I remember seeing, years ago, a picture of a Noorduyn Norseman with a single-bladed prop. Since you seem to know what you are talking about (more than I do, anyway), what factors would drive a manufacturer to adopt such a radical solution? In a word, efficiency. Note that many of the model airplanes used in free-flight competitions, (Escpecially the rubber powered ones, where the judges issue you your engine (So many strands of Pirelli rubber, of some particular length) and "fuel" it up for you (So many turns of the rubber bands)) where getting the absolute most out of the limited omount of energy you've got means the difference between winning and losing, use very wide chord single-bladed propellers. The downside is that you need a fairly large diameter. That's not much of a problem in a hand-launched model airplane, but it doesn't work so well in Full Scale stuff. ii) Radical solutions such as the Unducted Fan proposals mooted a few years ago, had many curved blades - any idea what gain they were seeking that justified the loss in efficiency? In tha case, what they're trying to do is reduce the effects of the shockwaves that form on the propeller blades as they fly further and firther into the transonic region. It's not unlike sweeping a wing back to delay the Mach Number that the drag rise occurs at, and the magnitude of the drag rise. Above about Mach 0.65, the efficiency of a straight propeller drops off alarmingly. At typical airliner cruise speeds, (Mach 0.78-0.85) efficiency would be down around 60% at teh low end of the speed range, and 50% at the high end. That's not very useful at all - there are some measures that you can do to cut the tip speed down - for example, the Tu-95 Bear (Russian turboprop transonic bomber) uses a very high step-down gearing from the engines to the propellers - the props rotate at 750 RPM, vs, say, 1500 or so for that of a P-51, and a very clever variable pressure ratio compressor system in its engines that essentially "supercharges" them to deliver sea level power at 40,000'. (About 3 times what you'd get from a typical turboprop). The swept propeller blades supply efficiencies in the Mach 0.78-0.85 range of between 75% and 70%. Using many blades allows the diameter to be cut down from, say, 22 ft for our notional conventional propeller to 13 ft. This gives a lower airspeed at the propeller tip than a large diameter propeller, thus delaying the transonic effects. (Note that the entire propeller doesn't go transonic - the airspeed at the propeller blade is a product of the propeller's rotational speed, and teh forward speed of the airplane. The rotational speed of the propeller in ft/sec or m/sec increases as you move outward along the propeller blade. So, a propeller will start having supersonic flow appear at the tips, with the supersonic flow field moving inward as speed increases. A smaller diameter and a slower rotational speed are helpful in delaying the formation of these shock waves. (transonic/supersonic flow). You do lose efficiency in the lower speed ranges, but you get big gains at what your desired cruise speeds are. iii) How does this work with contraprops? On the face of it they must interfere with each other horribly, but they seem to fly quite well. Can you point me in the direction of some clues? A contraprop does lose some efficiency by placing one propeller behind the other, and it requires a more complex drive system. (Which gave fits on several early U.S. contraprop-equipped aircraft, most notably the XB-35 Flying Wing, where they never got the contraprops doped out, and the Hughes XF-11 recon machine (looked like a hyperthyroid P-38). which was lost on its first flight becasue the aft bladeset in one of the contraprops went into reverse pitch at low altitude. (This is the crash that nearly killed Howard Hughes, and led to his drug addiction (painkillers) and fear of infection.) What you gain is a greater ability for a propeller of a particular diameter to absorb power, adn the elimination of torque and P-factor (destabilization of the airframe due to the rotating airflow from the propeller affecting the airframe). P-Factor is a Big Deal, with a high-powered airplane. For example, with a P-51 or a Corsair, you have to be careful with throttle movement at low speeds, or on takeoff. If you jam the throttle to it too fast, you'll either swing off the runway or roll the airplane inverted. -- Pete Stickney A strong conviction that something must be done is the parent of many bad measures. -- Daniel Webster |
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