Newsletter ~ September 2010 Enrollment Special

While going through flight training have you been confused with the L/D Max charts? Wondered just what does this mean to me? Been confused with the way the graph lines cross? What does it all mean? Well it actually means quite a lot and is the basis for the performance of your aircraft.

If you recall, there are two descending lines on the graph, one descending from the left and one from the right that cross as they descend. The Y axis (vertical) of the graph shows drag and the X axis (horizontal) shows speed. So...with these two opposite descending lines you can see that as "something" increases or decreases in speed, the total drag goes up or down. What is the "something"? The line descending left to right is induced drag and one descending right to left is parasite drag. These are both critical to the performance of your aircraft and are present on every aircraft. To understand each think of them this way: Induced drag means something has been "induced" or made to happen. What is an aircraft made to do? To fly. How? Lift generated by airflow over the wings. Lift is "induced" by airflow over the wings. A wing generates lift in two ways: airflow over the wing generating a pressure differential - Bernoulli's principle and also the downward deflection of the air creating an opposite upward force - Newton's third law. Keeping these principles in mind you can see that the faster an aircraft moves through the air the greater the lift generated - more airflow greater pressure differential and greater downward deflection of the air leading to greater upward force. Conversely, the slower the airspeed, the less lift generated. So induced drag increases with a decrease in airspeed. In other words as the wing has to "work" harder to generate lift the drag increases rapidly. This is also why to maintain altitude, the angle of attack must increase as the airspeed is reduced. So...induced drag increases as speed is reduced.

The other curve, going right to left is parasite drag and just as the name suggests is all of the drag created by parts of the aircraft that contribute nothing to lift. They are "parasites" This could be the fuselage, windows, antennas etc. As these parts are in the airstream, the greater the airflow (speed) the more drag they generate.

As these two curves are equal but opposite, the point at which they meet is the point of minimum drag for a known airspeed. This is the best glide speed of your aircraft and is only available from this graph or in the POH. It is not shown on the airspeed indicator.

Ask your flight instructor to show you this graph and remember this is how the best glide speed is calculated and the basis of the performance of your aircraft.

Blue Skies,

Peter Swift, Chief Instructor