Chapter 3 : Airplane Stability and Control

All aircraft needs stability and the only way to stabilize an aircraft (if it cannot stabilize itself) is by its control surfaces. But first let us study the airplane’s axis of rotation. The airplane moves on it’s three dimensional axis (see Fig. 9a):

1) “X” axis or the longitudinal axis
2) “Y” axis or vertical axis
3) “Z” axis or lateral axis

Figure 9a : Three dimensional axis of rotation

The aileron controls the longitudinal axis. It serves to stabilize the aircraft by banking and steering left or right (see Fig. 9b). It is a common notion that the rudder in the vertical fin controls the steering of the aircraft. Yes its true, even the Wright brothers used the rudder on their first airplane until they discovered that the aileron is much better. If you have a model with just three channels (rudder, throttle and elevator) it will work just fine. It operates with similar principle but I would suggest that aileron is much better. I remember when I was flying a full size Cessna. It was my first flight and when the instructor let go of the controls and put me in charge, I used to experiment steering the aircraft by its rudder. He said,
“Hey, what are you doing?” I said, “I’m steering the aircraft.” Then he told me “Who told you to use the rudder? Use the aileron, you can’t use the rudder for turning.”

Figure 9b : Using aileron on banking and steering the aircraft

Maybe he is not aware that model aircraft can use rudder for turning. But I also don’t know why is it not possible in the full-scale aircraft. Not until I studied Aeronautical engineering that I discovered how those control surfaces and the viscosity of air aerodynamically affects the aircraft. Because the model is much lighter compared to full-size aircraft, rudder is quite effective in steering the aircraft. Unlike the full-size aircraft, because of the viscosity of air, it’s ineffective.

The aileron works by deflecting the air upwards or downwards. Since the airflow is disturbed, drag increases. But the upper and lower camber has two different functions so it will result in two different manners. Disturbing the airflow in the upper camber will create more drag compared to the lower camber. Why? Because the upper camber, as we’ve learned has lower pressure area that creates the lift. If the airflow is deflected upwards, the high-pressure region will leak to the low-pressure region, hence resulting in a loss of lift. Because of the leakage, turbulence will occur in the trailing edge until it reaches the upper camber. This causes more drag than deflecting the air downwards because the upper region, which is the low-pressure area, will not leak on the high-pressure area. Hence, turbulence is minimal (see Fig. 10).

This is why the airplane turns when banking. If you bank the airplane’s wing on the right, it will automatically turn right or vice versa. The drag created on the right wing causes the delay, which gives the left wing more speed.

Figure 10 : Aileron deflection of an airfoil

The primary purpose of the rudder is to stabilize the aircraft on its vertical axis. In model aircraft, rudder is utilized for steering. But what is amazing is when you deflect the rudder for example to the left (looking at the back end of the model)  the aircraft will turn left and the wing will bank to the left side or vice versa. The explanation is as you turn or rotate the aircraft to the left (along the vertical axis) the right wing travels faster than the left wing. Because the velocity of air is faster in the right wing, more lift is produced. Hence, the result is unbalanced lift that causes the aircraft to bank (see Fig.
9c).

Figure 9c : Rudder deflected to left for steering the aircraft

The elevator located on the tail end of the aircraft controls the lateral axis. Its main function is for take-off and landing of the aircraft. It stabilizes the up and down motion of the aircraft. The elevator pushes the tail down when deflected upwards or vice versa and increase the angle of attack of the wing so more lift is produced (see Fig. 9d).

Figure 9d : Elevator deflected upward

The location of the wing will also determine the stability of the aircraft. The most stable type is the high wing configuration on a typical monoplane. The pendulum stability of its wing gives it the natural stability because the weight is under the wing (Fig. 11a). The shoulder wing type is a little touchy because the weight is near the wing (Fig. 11b). The low wing type is the most sensitive to control because the weightis on the upper portion of the wing. That is why dihedral is used to add stability (see Fig. 11c).

Adding another set of wings can increase wing area. This configuration is called a biplane. The wings are decked together, one in the upper part of the fuselage the other on the bottom (see Fig. 11d). This type is quite common in the early days of aviation. In fact the first airplane flown by the Wright brothers the Kitty Hawk was a biplane. The only advantage is the longitudinal stability and drag is a major concern in this design due to the wire braces to support the wings. Triplanes fighters appeared in WWI and was not very popular it is because drag is also a major issue. That is why monoplanes are quite popular until this day because it produces the least drag.

Figure 11a : High wing monoplane

Figure 11b : Midwing monoplane

Figure 11c : Low wing monoplane

Figure 11d : Typical Biplane

Typical Triplane

Other forces that affects aircraft stability

There are other things to consider to stabilize the aircraft. One of them is the three degrees right thrust, which are necessary to stabilize the directional stability of the aircraft. The reason for this is the aircraft has a tendency to turn left when there is no three degrees right thrust (see Fig 12a). The pilot needs to trim the rudder to the right to counteract the left turn tendency. The engine torque against the propeller (see Fig 12b) causes this phenomenon. Most propellers turn in a counter-clockwise motion (front of the airplane). The opposite force is the engine torque, which is clockwise. So the aircraft has a tendency to bank along with the clockwise motion, which is banking to the left. A three degrees right thrust is needed to neutralize the aircraft to fly in a straight path.

Figure 12a : Top View of Model Aircraft Engine with 3 degrees Right Thrust

Figure 12b : Illustration of engine vs. propeller torque

What about the down thrust? It is used to counteract a natural tendency of the aircraft to pitch up or to nose up on a typical high wing monoplane. Since the thrust line is below the wing (see Fig. 13a), there is a tendency for the aircraft to pendulum on its neutral point (see Chapter 4). The engine literally pulls the fuselage up being the neutral point as the pivot point. A three degree down thrust is used to counteract this force to balance it aerodynamically (see Fig 13b). If the thrust line is along the neutral point, like in the mid-wing airplanes (see Fig. 11b) there is no need for down thrust.

Figure13a : Illustration of propeller thrust below the neutral point

Figure13b: Side view of model aircraft engine with 3 degrees down thrust

Landing gear design also has a destabilizing effect if not properly considered. Trainer type RC airplanes always have tricycle landing gear and a tail dragger is usually not recommended. This is not always explained in detail by other beginner books, I also didn’t realize before that this is also important to know for a beginner.

Whenever an aircraft lands, the main gear touches the ground first whether it’s a tail dragger or a tricycle type. Sometimes because of the wind direction an aircraft has to “crab”. Crabbing is a term used because the airplane flies side ways to counteract the wind perpendicular to the landing strip (see Fig 14a). If the aircraft flies straight without crabbing, the aircraft will deviate from its path (see Fig. 14b).

Figure14a: Aircraft crabbing to counteract wind direction

Figure14b: Aircraft deviates from straight path in a crosswind

Because of this, landing a tail dragger is not as easy as we thought. The entire weight of the aircraft is behind the main gear so the momentum is pushing the aircraft instead of pulling it (see Fig.15a). There is no inherent stability in tail dragger unlike the tricycle type. In a tricycle type, the weight is concentrated in front of the main landing gear (see Fig.15b). The momentum is pulling the aircraft so this is more stable because there is no tendency for the aircraft to tip over when landing and crabbing at the same time. The main gear will just drag and pull the aircraft to a straight path when the wheels strike the ground.

Figure15a : Tail Dragger in a Crosswind Landing

Figure15b: Tricycle Type in a Cross Wind Landing

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