There are three axes around which an aeroplane is controlled. These axes pass through the aeroplane’s centre of gravity (CG), or the point where the aeroplane’s weight is considered to be concentrated. An aeroplane that is stable requires little pilot attention after it is trimmed for a certain airspeed and power setting. An aircraft must be stable around all three axes to be considered safe.
Yaw is about the vertical axis and is controlled by the rudder. The normal axis passes through the aircraft’s centre of gravity perpendicular to the two other axes. Movement about the normal axis is called yaw. The secondary effect of yaw is roll. Stability about the normal axis is known as directional stability because it is concerned with stability in the directional or yawing plane. The rudder is hinged to the rear of the fin and is connected to the rudder bar so that when the right pedal is pushed forwards, the rudder moves to the right. The effect is to alter the aerofoil section of the fin and rudder combination to provide an aerodynamic force, which moves the rear of the aircraft to the left. The pilot sees this as a movement to the right.
Roll is about the longitudinal axis and is controlled by the ailerons. The longitudinal axis runs fore and aft through the aircraft’s centre of gravity. Movement about the longitudinal axis is known as roll. The secondary effect of roll is yaw. Stability about the longitudinal axis is known as lateral stability because it is concerned with movement in the lateral or rolling plane. The ailerons are moveable surfaces hinged to the rear spar of the wing and form part of the trailing edge. They are connected to the control column/stick so that when it is moved to the left, the port aileron rises and the starboard aileron goes down. As a result, lift decreases on the port wing and increases on the starboard wing, which causes a rolling motion to the left.
Pitch is about the lateral axis and is controlled by the elevators. The lateral axis passes through the centre of gravity across the aircraft from one side to the other, from wingtip to wingtip. Movement about the lateral axis is called pitching. Stability about the lateral axis is called longitudinal stability because it is concerned with stability in the longitudinal or pitching plane. The elevator is a symmetrical surface conventionally attached to the horizontal stabiliser. It is connected to the control column so that forward movement of the control column moves the elevator down. The effect is to change the tail plane / elevator section into a cambered aerofoil, which supplies an upwards aerodynamic force on the tail of the aircraft, thus pitching its nose down (and vice versa).
Ailerons (roll) and rudder (yaw) are completely interlinked. In fact, the secondary effect to using the rudders alone (yaw) is to roll the aircraft, meaning the aircraft will yaw and then roll. This is due to the outside wing travelling faster than the inside wing. If we now refer back to the lift formula, what happens when airspeed is increased? Lift is increased, meaning the lift produced on the outside wing is greater than the lift produced on the inside wing, and the aircraft generates roll.
The secondary effect of using the ailerons alone (roll) is to yaw the aircraft, meaning the aircraft will roll and then yaw. When the aircraft is rolled, the airflow strikes the rudder and fuselage at a more direct angle, therefore yawing the aircraft.
The effectiveness of the aircraft controls is completely related to the speed of the airflow over the control surfaces. At high airspeeds, airflow over the control surfaces is high, meaning the effectiveness of the controls is high—the controls are very responsive. At low airspeeds, the airflow over the control surfaces is low, meaning the effectiveness of the controls is low—the controls are not as responsive. That said, at low airspeed and high power settings, the rudder and elevator will still be very effective, while the ailerons will not be. This is due to the slipstream created by the propeller throwing air over these control surfaces.
As discussed, a secondary effect is present when a turn is initiated. The down-going aileron creates drag; therefore, a yaw is initiated in the opposite direction of the intended roll as the roll commences. This effect is termed aileron drag and, while sometimes unnoticeable at normal speeds, is liable to become pronounced—if not critical—at large angles of attack and low speeds, particularly if a wing has dropped and an attempt was made to restore level flight by the use of ailerons. In order to retain control of an aeroplane at slow speeds, the down-going aileron should increase the lift but not the drag, while the up-going aileron should decrease the lift and increase the drag. Many devices have been developed in an attempt to solve this problem; some of these devices are briefly described below.
Frise aileron: This aileron, which is of the balanced type, is shaped so that when it is moved downwards, it forms a surface continuous with the main plane, giving an increase in lift but little drag. When the aileron is moved upwards, the front portion of the aileron projects below the main plane, causing turbulent airflow and increased drag.
Differential aileron: The up-going aileron moves through a larger angle than the downward-moving aileron, thus giving greater drag and decreased lift to the wing.
Spoilers: On some aircraft, spoilers in the form of flat plates protruding into the airflow are used to increase the drag of the down-going wing. These can be used in a differential fashion to roll the aircraft.
If the control surfaces are hinged at their leading edge and allowed to trail from this position in flight, the forces required to change the angle of deflection would be prohibitive. Aerodynamic ‘flutter’ could also be experienced. To assist the pilot to move the controls in absence of power or power-assisted controls, and to prevent flutter, some degree of balancing is required. We consider the two types of balance: aerodynamic and mass balancing.
The ailerons, elevator and rudder, are attached to the aircraft’s wing, horizontal and vertical stabilisers by hinges. At certain speeds and deflections, flutter is sometimes experienced. Manufacturers attempt to prevent flutter of the control surfaces by mass balancing. Mass balancing is achieved by fixing internal or external weights to the leading edge of the control surface so that the centre of gravity (of the control surface) is brought closer to the hinge line.
This is an unstable vibration/oscillation of a control surface and the control surface structure. It is caused by the interaction of aerodynamic and inertia forces in the elastic properties of the control. Flutter is overcome by mass balancing where the centre of gravity lies close to or forward of the hinge line, reducing control surface flutter.
Flexural Aileron Flutter
The aileron lags behind the rest of the wing in up/down movements caused by turbulence because its centre of gravity is behind the hinge line. This situation can become self-generating because the lagging movement of the aileron magnifies the movement caused by using flap.
Torsional Aileron Flutter
If the wing twists about its lateral axis (i.e., nose-up or nose-down), aileron oscillation can occur again because the centre of gravity is behind the hinge line.
Overcoming Aileron Flutter
Flutter is overcome by moving the control surface’s centre of gravity closer to (or in some cases ahead of) the hinge line. This is achieved by mass balancing.
Mass balance on a Cessna 172R, used to bring the CG closer to the hinge line (Image used courtesy of Robert McConnel)
Aerodynamic balance is achieved in several ways (inset hinge, horn balance), all of which decrease the force required to move the control surface in flight. Thus, aerodynamic balancing of the control surfaces is used to assist the pilot move the control surfaces (especially large surfaces) at high speed. The control surface must be aerodynamically balanced to reduce the hinge moments when the control surface is deflected. Aerodynamic balancing is generally achieved by hinging the control surface about a line set back from the leading edge (of the control surface). The effort required to move the control surface is determined by the aerodynamic force acting through the centre of pressure of the control surface multiplied by the distance from the hinge line. This force is known as the hinge moment of the control surface. The smaller the hinge moment, the less the effort required to move the control surface.
Overbalancing (as shown above) can occur if the centre of pressure of a control surface is positioned too close to the hinge line. Any movement of the control surface may result in the centre of pressure moving ahead of the hinge line. This will remove all aerodynamic feel of the controls from the pilot. Overbalance can be detected by the pilot as a decrease in the progressive force required to move the control surface, instead of an increase, for any given airspeed. This could result in the control surface moving on its own accord to full deflection. The common ways to achieve aerodynamic balancing are by:
We have already discussed balance tabs, so we will have a look at inset hinges and horn balances.
Inset hinges are a type of aerodynamic balancing that brings the centre of pressure (of the control surface) closer to the hinge line.
A horn balance is a design feature of aerodynamic balancing where a portion of a control surface lies in front of the hinge line and protrudes into the airflow. This has a similar effect to that of an inset hinge. An unshielded horn balance is where the horn balance extends to the leading edge and, thus, is directly in line with the relative airflow. A shielded horn balance is where the horn balance extends part way to the leading edge and is shielded from the relative airflow by the vertical stabiliser.
Elevator horn balance as seen on a C172 (Image used courtesy of Robert McConnel)
Rudder horn balance as seen on a C172 (Image used courtesy of Robert McConnel)
With aerodynamic balancing, as soon as the control is moved, the air striking the portion of the control surface in front of the hinge tends to assist the pilot in moving the control further. Therefore, the control surface partially balances the effect of the air striking that part of the aerofoil to the rear of the hinge. The extent to which a control is balanced must be carefully regulated, as overbalancing can dampen the feel of the control pressures. This manner of balancing can be applied in several different forms. In each of the above methods, balance is obtained by placing a portion of the control surface in front of the hinge.
A tab is a small-hinged surface forming part of the trailing edge of a primary control surface. If an aircraft is out of trim and requires a constant pull or push force on the control wheel (or rudder pedal) to maintain straight and level flight, this will soon tire the pilot. A tab can be used to trim out the holding forces and ease the task of the pilot. Thus, trim tabs have been developed to reduce the pilot’s physical workload and, when trimmed correctly, the aeroplane will hold the pilot’s selected attitude. The power of a trim tab varies with speed in the same way that the control surfaces do. At high speeds, small movements of the trim tab have an immediate effect. At low speeds, large tab deflections may be required to trim the aircraft. To be effective, trim tabs should be able to reduce control forces to zero over the entire speed range.
Trim tabs operate by creating small aerodynamic force acting near the trailing edge of a control surface. This force is used to hold the control surface at a desired angle of deflection.
An aircraft is tail heavy and requires a constant push force to maintain level flight. If the elevator trim is moved upwards, the result is a downward load on the elevator trailing edge, which moves the elevator down.
There are a number of types of trim tabs available. We will look at a few now.
A fixed trim tab is a small metal tab attached to the rear of a control surface. A fixed trim tab is used on ailerons on some aircraft (e.g., Pa38) and usually used on the rudders of single-engine aircraft to offset the slipstream effect; have a look at the Cessna 172R.
A fixed tab is not adjustable in flight and can only be adjusted by an engineer or qualified pilot on the ground. The correct positioning of a fixed tab is, therefore, determined by trial and error during test flights.
Trimming is also done on some light aircraft by adjusting the tension in springs that are connected directly to the pilot’s controls. Thus, an adjustment in the spring tension will bias the control cables/rods in any desired position.
Adjustable Trim Tab
Adjustable trim tabs are used to trim out any holding forces encountered in flight (i.e., a change in power or speed or a centre of gravity change due to a change in weight). Adjustable trim tabs are pilot operated, usually by hand wheels in the cockpit or electrically. The hand wheels always operate in the natural sense (i.e., holding forward pressure, trim hand wheel is moved forwards and vice versa). When the control surfaces move, the trim tab remains fixed in relation to the main surface. Thus, the trim tab only moves when the pilot moves the trim control. The tab effectiveness varies with speed; at high speeds, small movements of the trim tab have an immediate effect. At low speeds, large tab deflections may be required to trim the aircraft.
Balance or Geared Tabs
The purpose or effect of a balance tab is to balance or partially balance the aerodynamic load on the control surfaces, thus reducing the control wheel force (i.e., assisting the pilot). The balance tab is not controlled by the pilot, but the tab angle is changed automatically whenever the main control surface is moved.
Link Balance Tab (Lagging Tab)
A link-balance tab is linked to the main surface ahead of the control surface. When the control surface is moved, the tab (by virtue of the linkage) is moved in the opposite direction, through an amount proportional to the control surface deflection. Thus, the movement of the tab helps the control surface to move and reduces the stick force on the control column, assisting the pilot to move the control surface. Note that this is a rigid link of a fixed distance that can only be adjusted on the ground. Also note a link-balance tab moves only when the control surface is moved.
Anti-Balance Tab (Leading Tab)
As the pilot deflects the control surface, the centre of pressure moves forwards close to the hinge line, which causes the control surface to deflect even further, or possibly go to full deflection by itself (i.e., overbalance). An anti-balance tab is used to reduce the amount of aerodynamic balance and make the control heavier to move. The anti-balance tab is designed to move in the same direction as the main surface. This creates a force opposite to that of the main control surface movement, thus preventing movement of the control surface. Anti-balance tabs are often used to move large control surfaces such as full flying tail planes. Note the linkage is a fixed length that can only be adjusted on the ground.
A servo tab is operated directly by the control wheel or rudder bar. The control surface itself is not connected to the pilot’s controls. By connecting the tabs directly to the cockpit controls, the tab can be made to apply the hinge moment required to move the control surface. When the aircraft is stationary—movement of the control wheel or rudder pedals moves the tabs only—the control surface is unaffected. During flight (assume a serve tab is on the rudder surface), when right rudder is applied, the tab moves left, thus deflecting the control surface to the right. This creates a cambered surface between the fin and the rudder, producing an aerodynamic force that helps yaw the nose of the aircraft to the right. One of the disadvantages of a servo tab is that it lacks effectiveness at low speeds, since any tab loses its effect when deflected through an angle of more than 20°. The large control surface deflections required at low speeds need correspondingly large tab deflections and, therefore, this system is not always satisfactory. The spring tab is used to overcome this fault.
The spring tab is a modification of a balance or servo tab, in which a spring is incorporated in the linkage permitting the tab ‘gearing’ to be increased according to the applied control wheel forces. In a spring tab system, the cable from the control wheel is connected to one end of a spring, the other end of which is fixed to the control surface—the cable is also linked to the tab itself. A spring tab is designed to reduce large hinge moments and, therefore, control forces arising when the aircraft is at high speed (hinge moments are proportional to the square of the indicated airspeed). A spring tab has an advantage over a servo tab since:
Spring tabs are commonly used on ailerons and occasionally on the elevator and rudder. Certain spring tab installations, usually on elevators, are pre-loaded. This means that the spring is tensioned so that it does not begin to stretch until a fairly high control wheel force is used. The pilot could feel the effect of a pre-loaded spring tab by a sudden reduction in the force required when pulling out of a steep dive at high speed.
Aileron reversal is a dangerous offshoot of torsional aileron flutter. When the ailerons are moved, the down-going aileron, acting as an elevator on its wing tip, tends to twist the leading edge of the wing downwards about its torsional axis. At the same time, the up-going aileron has the opposite effect on the other wing. At low speeds, the rigidity of the wing is sufficient to prevent any distortion, but at higher speeds, the ailerons may be sufficiently powerful to distort the wings. The effect of aileron reversal is to decrease aileron effectiveness as the wings are twisted so as to oppose the rolling moment set up by the ailerons. There is a theoretical design speed well in excess of VNE at which the rolling moment drops to zero and beyond which the aircraft would behave in the opposite direction to that intended by the pilot.
Discuss the following questions in class now.