Helicopter Flight Information
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Helicopter Flight Information |
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The Lift Equation and What it Means to You Let's take a look at it: L=CL ½R S V² What it means: L (Lift) = CL (Coefficient of Lift, Angle of Attack, pilot adjustable) ½R (R(roe)=Density Altitude, adjustable only by God, you have what you have) S (surface area of airfoil, fixed and unchangeable, as Chuck Meager says, "unless you hit a tree") V² (velocity squared, should be fixed, but there are some variables) CL - If the pilot desires more altitude or faster flight he/she applies more collective increasing the angle of attack. ½R - We have learned since the beginning of flight training that cooler temperatures, low altitudes, and drier days are the pilots friend, but we have no control over it. We are where we are and we have what we have. S - The surface area of the airfoil as designed by the engineers. Many times students have asked why the engineers don't do this or that. The bottom line is that they must strike a balance between aircraft capabilities and cost objectives. Operators want the cheapest operation costs overall while being able to conduct the operations safely. This always means the smallest possible aircraft doing the maximum possible job. V² - Velocity of the air over the airfoil squared. In an airplane, this is achieved via airspeed; in a helicopter this is achieved via rotor rpm. Rotor rpm is an extremely important item that the pilot does indeed have control over. The two major factors affecting rotor rpm are throttle or engine power and disk loading. The pilot must always ensure that the throttle is sufficiently set to maintain rotor rpm in the top of the green; not more than the top of the green since this is a limit which if exceeded can cause serious damage to the rotor system as well as the engine. An interesting fact is that rotor root stress increases at the square of the increase in rotor rpm. In a turbine helicopter such as the Bell 206, there is a beep switch to fine tune the rpm at the top of the green. Provided that maximum power is not exceeded, throttle alone will control rpm. If maximum power is reached, the pilot must then limit collective application to maintain rotor rpm at the top of the green. If the pilot exceeds the maximum allowable collective application, rotor rpm will begin to decay and the aircraft will sink since lift is lost at a very high rate. The fact that lift is lost at a very high rate as velocity of the air decreases is a very important issue. As rotor rpm decays lift is lost for three important reasons; 1) Lift is lost due to the decrease in the velocity of the air over the airfoil. 2) The effective area of the disk also decreases (coning). 3) Blade tip vortices increase in size. As these vortices increase in size, the area of the blade producing lift decreases. As you can see, there is a multi-fold decrease in lift which if not corrected will result in an alarming sink rate of the aircraft.
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