by Harold Green
Published in Midwest Flyer – December 2018/January 2019 issue
An experienced Designated Pilot Examiner (DPE) recently observed that a common thread he saw among applicants was that some were not properly recovering from stalls or unusual attitudes. He even admitted to being somewhat concerned about the outcome in a couple of cases. The obvious cause is improper use of controls to maintain controlled flight. The following is a basic review of the function of the control components on the average airplane; it is also an appeal to maintain coordination at all times. This discussion plows no new ground. Rather, it is an attempt to provide a slightly different perspective on control response and recovery procedures. The time-honored emphasis on airspeed, coordination and attitude does not change.
In the following discussion, it should always be remembered that whenever lift is created, so is drag, and as lift increases, so does drag. That is, the old TINSTAAFL or, “There Is No Such Thing As A Free Lunch” comes to the fore again.
We all know that the elevator controls the pitch of the airplane. Harking back to basic aerodynamics, remember that the airplane is suspended under the center of lift with the center of gravity ahead of the center of lift and the horizontal stabilizer aft providing a balancing force. Thus, the center of lift and the downward force on the stabilizer form one end of a teeter-totter with the center of lift being the fulcrum. Lift variation on the tail created by varying the angle of the elevator causes a change in pitch by changing the angle of the teeter-totter.
In straight and level flight, there is always a downward force on the tail keeping the airplane balanced. When the elevator is pivoted UP, it actually creates lift in the down direction on the tail. That happens when you apply backpressure on the pitch control, be it stick or control wheel. This causes the nose to rotate upward about the center of lift. Then, whether we stall or climb depends on the degree of pitch and the amount of power available. Hence the term Pitch + Power = Performance.
Of course, if the deflection of the elevator is negative, forward pressure is applied to the pitch control. If the elevator is deflected downward, lift is upward. In actuality, less downward lift on the tail is created, the nose pitches down and the speed increases. As the speed increases, the downward force on the horizontal stabilizer increases due to increased air flow tending to raise the nose again. Thus, in order to hold a specific angle of attack while descending, it is necessary to increase the pressure on the pitch control proportional to the airspeed increase to keep our teeter-totter balanced. In short, the plane wants to maintain the airspeed in order to achieve balance of the teeter-totter.
This then leads to another control element – the “elevator trim.” The elevator trim actually flies the elevator. Thus, when you want to relieve forward pressure, the trim is adjusted “upward” to cause the elevator to shift downward and replace the downward force you were holding. Obviously, this reverses if the need is to increase the pitch attitude. Remember, the trim tab actually moves opposite to the direction of the desired force change.
It might be enlightening to check the emergency section of your Pilots Operating Handbook (POH) and you may see that in the event of a flat nose wheel tire or a nose gear that is broken or won’t extend, the procedure calls for setting the pitch trim full down to reduce the load on the nose wheel. That’s because when you override the trim, it acts as an extension of the elevator and its effect is reversed since it just adds effective pitch to the elevator.
Remember, the trim force is a function of the airspeed over the elevator. Therefore, as speed is changed, it should be remembered that control pressure must be used to stabilize the air flow over the elevator before the trim can be adjusted properly for sustained level flight.
We will ignore the effect of aileron trim since the discussion is already complex enough and aileron trim is not as universally available as pitch trim.
Next, consider the ailerons. (Ignore spoilers today.) These little guys are way out on the end of the wings and flop up and down in opposite direction to each other in response to lateral movement of the control wheel or the stick to control the roll of the plane. The aileron that is down causes an increase in lift (and hence an increase in drag), and the one that is up causes a loss of lift (and hence a decrease in drag). The result is twofold. The plane rolls in the direction of the UP aileron due to the decreased lift, and the increased lift of the opposite, or DOWN aileron, and the plane yaws in the direction of the down aileron due to the increased drag associated with the increased lift coupled with decreased drag of the opposite aileron. Therefore, if you bank left without rudder application, the nose goes right before the horizontal component of lift makes the plane turn, and if you bank right, the nose goes left before you turn. In both cases, with shallow bank once established, the plane will continue in a gentle turn with little or no additional input on your part. The amount of bank that remains coordinated without rudder is related to the amount of dihedral of the wings. Steeper, and you have to take additional steps with the rudder and elevator.
The rudder is the vertical hinged thingy on the tail, which is connected to the pedals on the floor. Normally the rudder controls where the nose points. That’s referred to as the “yaw” of the plane. In the extreme, knife-edge flight where the plane is at a 90-degree bank, the rudder controls the pitch of the plane. Please note that this is still yaw. It’s just rotated by 90 degrees. However, in normal flight, the rudder does not contribute to lift. What it does do is counter the drag created by the ailerons or other non-symmetrical drag effects including P factor and torque compensation. The rudder is what keeps the little ball in the center of its cage. That is a fact often ignored by pilots, both experienced and novice. This little ball never seems to receive the attention it deserves. Failing to keep the ball in the cage leads to many accidents, sometimes even fatal. In fact, coordination, and hence the ball, is very important to this entire discussion since if the ball is not in its cage, the plane is not in coordinated flight.
Now consider an actual situation. Straight and level flight will be ignored since that is not interesting for our purposes.
Assume you are executing a steep turn while practicing for a check-ride. The private pilot level of a 45-degree bank was chosen by the FAA because during the turn you need to be aware of the coordination of the plane. The ball tends to slide out of its cage due to the effects of torque and P-factor . As a result, you must use rudder during the turn. The ball should stay in its cage. To complicate the issue further, the amount of rudder is different when turning left vs. turning right. The same is true of the ailerons. If in visual flight conditions, the secret is to be looking outside, holding the bank and pitch, and applying a combination of rudder and aileron to hold the attitude. Control forces vary between left and right to compensate for P-factor, torque, and adverse yaw. But, in any event it requires coordinated effort of rudder, elevator and aileron to keep that pesky little ball in its cage while holding altitude. Too much bottom rudder and you not only slip, you tend to lose altitude. Too much top rudder and you skid and tend to gain altitude. Too much elevator and you tend to climb…too little and you tend to dive. Changing any one affects the others. That is, it requires coordinated use of all except the trim to maintain level steep turns. If you get too tired, you can always use trim to alleviate the pitch force.
Now we go on to “unusual attitudes.” Typically, in an unusual attitude, the pitch and power are adjusted to correct the situation, but all too often rudder and aileron coordination are ignored. That’s fine, but in an uncoordinated airplane, unusual attitudes can be exacerbated. Therefore, stay coordinated in order to keep the dirty side down.
Next, “execute a stall.” Of course the first priority is to reduce the angle of attack to stop the stall. That means increased power and decreased pitch. That doesn’t happen instantaneously, so some other things also need to be done. During the stall a wing may drop due to a non-symmetrical stall of the two wings or perhaps the stall was induced during a turn. Today’s light airplanes will generally allow coordinated use of rudder and aileron to correct a wing drop. However, this can be abused. When a wing drops in the stall, it needs to be brought back. If we use ailerons, the aileron on the down wing will also move down attempting to raise that wing. This creates increased drag causing that wing to slow down and hence increasing the stall on that wing. That in turn, could cause the plane to roll inverted and enter a spin. That doesn’t happen too readily as modern airplanes are designed to stall from the wing root outward, hence reducing the leverage of unbalanced lift and effectively controlling the growth of the stall. However, as mentioned in a previous column, earlier design airplanes and highly maneuverable planes have wings designed for performance and don’t have this advantage.
This does not mean that with some effort, you can’t get into trouble. If, when performing the stall, for whatever reason, one wing stalls more, or earlier, than the other, there will be an unbalanced roll force and the more completely stalled wing will drop faster than the other. One way to create that situation is to be uncoordinated when entering the stall. The natural reaction is to apply aileron to raise the wing. This means the aileron on the down wing will deflect downward, thus increasing the drag on that wing, causing the plane to rotate slightly to that side. That increases the stall on that wing causing more roll action.
WHOOPS! Over you go. A better reaction is to use rudder to increase the velocity of the down wing, hence reducing or eliminating the stall imbalance condition. This will speed up the down wing increasing the lift and thereby raise the wing. Thus, if the right wing is down, use left rudder to compensate, and when you use aileron, use them in a coordinated fashion. This is a rare case where uncoordinated flight is okay, but it doesn’t last long.
Since modern airplanes are designed to stall from root outward, you can also use COORDINATED rudder and aileron to alleviate the situation. This does not mean that you can’t get into trouble using ailerons with modern airplanes. Note that the wings cannot be designed to eliminate a stall…they are just designed to control the development of the stall. A fully developed stall can result in some very dramatic maneuvers, spins being only one effect. At this point the pilot effectively becomes a test pilot. The pay doesn’t change, but the consequences can be extreme.
In all, this has been a pitch for more coordinated, knowledgeable, and effective use of the aircraft controls. Note that the physics described herein apply no matter the type, size or purpose of the airplane involved. Our goal is safer operations throughout the flight regime. Stall recovery still requires a reduction in the angle of attack and usually an increase in airspeed regardless. Controlling coordination throughout all aspects of flight, and understanding what the control forces do, could go a long way to reducing upsets and loss of control accidents. Have fun!
EDITOR’S NOTE: Harold Green is an Instrument and Multi-Engine Instrument Instructor (CFII, MEII) at Morey Airplane Company in Middleton, Wisconsin (C29). A flight instructor since 1976, Green was named “Flight Instructor of the Year” by the Federal Aviation Administration in 2011 and is a recipient of the “Wright Brothers Master Pilot Award.” Questions, comments and suggestions for future topics are welcomed via email at firstname.lastname@example.org, or by telephone at 608-836-1711 (www.MoreyAirport.com).
DISCLAIMER: The information contained in this column is the expressed opinion of the author only, and readers are advised to seek the advice of their personal flight instructor and others, and refer to the Federal Aviation Regulations, FAA Aeronautical Information Manual and instructional materials before attempting any procedures discussed herein.