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FLY 'EM BETTER
Old Wives Tales or Non-Problem Problems

Jim Irwin, Vice President, Products

The customer was not a newcomer; he had been flying at the airport for years in a variety of aircraft. He came in complaining that his new autopilot was causing the aircraft to fly with one wing low. Must be something to it, this guy wasn't born yesterday. Or was he?

Most Avionics dealers and a lot of pilots have either heard this story or thought about it at one time or another.

The complaint that has been surfacing and resurfacing for years is that a customer's two axis autopilot flies the aircraft with one wing low. Often it takes a lot of discussion with the pilot to convince him/her that the problem is not in the autopilot, but is instead due to uncoordinated flight controls.

The answers to the flight conditions that bring about this complaint are very simple and straightforward. An aircraft can only be made to fly a straight course in a slight bank, i.e., with a wing down, when the rudder is offset. A rudder offset requires the ailerons to be displaced in the opposite direction causing a bank to offset the rudder input. All of which is to say, the aircraft is out of trim. When this occurs, the "ball" in the slip/skid indicator in the Turn and Bank or Turn Coordinator instrument, will be displaced opposite the displaced rudder. This displacement indicates directly the action required by the pilot to retrim the airplane for level flight. If the autopilot is a two-axis system, one that controls roll and pitch, it cannot be the source of the out of trim condition because it cannot displace the rudder. If the aircraft is flying with one wing low, check the rudder trim.

Another auto pilot complaint that comes up occasionally after a new autopilot installation is "when I engage the autopilot altitude hold mode, the airplane slows down 5-6 kts."-" I can't keep the airplane on the step with the autopilot on", or something similar. This one is sometimes very difficult to overcome, because it requires that the complainant give up a long held, and erroneous belief that stems from early flight training and hangar flying sessions. The belief is that you can cause an aircraft to fly at a higher airspeed than normal for the power setting used, simply by descending at a higher speed prior to leveling off for cruise. In this way the airplane is said to get "on the step" and will thereafter maintain a velocity several Kts. above the normal speed for the power applied.

The simplest explanation for why this view of the dynamics of flight is not true is found in reviewing the four forces of flight, which are, Thrust, Drag, Lift and Gravity. You know the answer. If thrust is greater than drag, the aircraft accelerates. If thrust is less than drag, the aircraft decelerates. When equilibrium exists between these two forces, the velocity is maintained. (Today we talk of energy management in the operation of airplanes, but the old principles still apply). In level flight, in clean air, a particular power setting will produce an airspeed that is dependent on the power and the drag of the airplane at that time. If you dive the aircraft slightly to pick up additional speed and return to the original level flight condition, the airplane will slow to the original speed, all original parameters being the same.

Pilots operate a good deal of the time using " indicated values" in aircraft operation, i.e. indicated altitude, indicated airspeed, and indicated manifold pressure and engine RPM. All the pressure values, or those associated with the measurement of air, are subject to all manners of variations that do not show up in the "indicated" values. These variations cause changes in engine power and airspeeds even when our cruise conditions appear to be generally the same from day to day. This may cause the pilot to look for an excuse for the performance change and blame the autopilot. Another event, probably occurring more often, is the aircraft trimmed for cruise flight and descending slightly just prior to engaging the altitude hold function. After engagement the aircraft does establish level flight and subsequently slows down to its equilibrium velocity. The pilot seeing the difference between the initial velocity at altitude hold engagement and that resulting after stabilization, decides that the autopilot has somehow caused the aircraft to slow down. Not so. The autopilot can not alter the power and drag relationship of the airplane and, therefore it can not alter or change the velocity of the aircraft without climbing or descending.

Unfortunately these questions come up over and over again with each generation of pilots. The answer to these questions is education about the dynamics of flight. Avionics dealers can't suggest that their customers go back to the books, but maybe you could provide a copy of this newsletter and it may get them started again. We all need to go back to the basics once in a while.