PROP ART Whats most beautiful about the propeller is that its our path to the sky and the fact that a fan on steroids can get us in the air is nothing short of amazing. The unique characteristics of the propeller bring with them unique flight considerations. Each of the major left turning tendencies can be attributed to the propeller. They are: TORQUE - as seen from the cockpit, most modern propellers turn clockwise. That turning force has an equal and opposite reaction, which causes the airplane to want to roll left. PRECESSION - The fast spinning propeller acts like a gyroscope (imagine holding on to a spinning bicycle wheel). When the airplane pitches up, the force is applied 90 degrees ahead, causing the left yawing force. SLIPSTREAM - The turning propeller causes a spiraling airflow around the fuselage that eventually contacts the vertical stabilizer on the left side , pushing the tail to the right side and nose to the left. P-FACTOR - The easiest way to think of P-factor is to look at the profile of the propeller. As the arc comes down on the right, the angle of attack is higher than when it comes up on the left. That higher angle of attack means more lift, or pulling force, which results in a tendency to turn left. By far the most common propeller on training aircraft today is made of aluminum, with two blades and an integral hub. As engines get more powerful, their propellers go from two to three or four blades or even more and usually to a constant speed control mechanism with an oil filled hub and individual blades. Less powerful airplanes often use wooden propellers. FIXED PITCH VERSUS CONSTANT SPEED PROPELLERS General aviation aircraft commonly employ one or two types of propeller: the fixed pitch or constant speed type. Fixed pitch is self explanatory as the blade angle is fixed to a central hub, which is bolted directly to the engine. Power to the propeller is controlled by the throttle, which simply makes the engine and the blades turn faster or slower. In a constant speed propeller, the pitch of the blades continually changes to maintain a constant rpm, which the pilot sets through the propeller control. The individual blades of a constant speed propeller attach to a central hub that also houses a governor, a complex device that automatically adjusts blade angle through oil pressure, springs, and counterweights. Oil found leaking from a constant speed propeller hub during pre flight indicates a potential problem. Although more expensive, complex , and with a risk of failure, constant speed propellers are superior because they allow the pilot to set the optimal blade pitch for take off, climb , cruise and descent. A fixed pitch propeller, on the other hand, is optimized for one flight segment only. What is good for climb isnt good for cruise.
Posted on: Sat, 26 Apr 2014 07:27:28 +0000
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