AUXILIARY OR TRIM CONTROLS

TRIM TABSTrim tabs are small movable portions of the trailing edge of a control surface. These tabs are controlled from the cockpit to alter the camber of the surface and create an aerodynamic force that will hold the control surface deflected.

Trim tabs on the trailing edge of control surfaces can be adjusted to provide an aerodynamic force to hold the surface in

a desired position.

Trim tabs may be installed on any of the primary control surfaces. If only one tab is used, it is normally on the elevator, to permit adjustment of the tail load so the airplane can be flown hands-off at any given airspeed. The airplane speed is set with the control wheel, and then the trim tab is adjusted until the airspeed can be maintained without exerting force on the wheel.

BALANCE TABSThe control forces may be excessively high in some airplanes, and in order to decrease them, the manu-facturer may use a balance tab. This tab is located in the same place as a trim tab. In many installations, one tab serves both functions. The basic difference is that the control rod for the balance tab is connected to the fixed surface on the same side as the horn on the tab. If the control surface is deflected upward, the connecting linkage will pull the tab down. When the tab moves in the direction opposite that of the control surface, it will create an aerodynamic force that aids the movement of the surface.

Balance tabs provide a means for the airflow across the surface to provide a "power-assist" to reduce high control forces.

If the linkage between the tab and the fixed surface is adjustable from the cockpit, the tab will act as a combination trim and balance tab. It can be adjusted to any desired deflection to trim the airplane for a steady flight condition. Any time the control surface is deflected, the tab will move in the opposite direc-tion and ease the load on the pilot.

ANTI-SERVO TABSAll-movable horizontal tail surfaces do not have a fixed stabilizer in front of them, and the location of their pivot point makes them extremely sensitive. To decrease this sensitivity, an anti-servo tab may be installed on the trailing edge. This tab works in the same manner as the balance tab except that it moves in the opposite direction. The fixed end of the linkage is on the opposite side of the surface from the horn on the tab, and when the trailing edge of the stabilator moves up, the linkage forces the trailing edge of the tab up. When the stabilator moves down, the tab also moves down. The fixed end of the linkage may be attached to a jackscrew so the tab may be used as a trim tab as well as an anti-servo tab.

An anti-servo tab attempts to streamline the control surface and is used to make an all-moving horizontal tail surface less sensitive by opposing the force exerted by the pilot.

SERVO TABSLarge aircraft are usually equipped with a power-operated irreversible flight control system. In these systems, the control surfaces are operated by hydraulic actuators controlled by valves moved by the control yoke and rudder pedals. An artificial feel system gives the pilot resistance that is proportional to the flight loads on the surfaces. Control forces are too great for the pilot to manually move the surfaces. In the event of a hydraulic system failure, they are controlled with servo tabs, in a process known as manual reversion. In the manual mode of operation, the flight control column moves the tab on the control surface, and aerodynamic forces caused by the deflected tab move the main control surface.

Servo tabs provide a force to assist the pilot in moving a primary control surface of a large aircraft in the event of a hydraulic system failure.

SPRING TABSAnother device for aiding the pilot of high-speed aircraft is the spring tab. The control horn is free to pivot on the hinge axis of the surface, but it is restrained by a spring. For normal operation when control forces are light, the spring is not compressed. The horn acts as though it were rigidly attached to the surface. At high airspeeds when the control forces are too high for the pilot to operate properly, the spring collapses and the control horn deflects the tab in the direction to produce an aerodynamic force that aids the pilot in moving the surface.

A spring tab is another means for assisting the pilot of a high-speed airplane to overcome high control forces.

GROUND-ADJUSTABLE TABSMany small airplanes have a non-moveable metal trim tab on the rudder. This tab is bent in one direction or the other on the ground to apply a trim force to the rudder. The correct displacement is determined by trial-and-error until the pilot reports that the airplane is no longer skidding left or right during normal cruising flight.

A ground-adjustable tab is used on the rudder of many small airplanes to correct for a tendency to fly with the fuselage slightly misaligned with the relative wind.

ADJUSTABLE STABILIZERRather than using a movable tab on the trailing edge of the elevator, many airplanes pivot the horizontal stabilizer about its rear spar, and mount its leading edge on a jackscrew that is controllable from the cockpit. On smaller airplanes, the jackscrew is cable-operated from a trim crank, and on larger airplanes it is motor driven. The trimming effect of the adjustable stabilizer is the same as that obtained from a trim tab.

Many airplanes, including most jet transports, use an adjustable stabilizer to provide the required pitch trim forces.

HIGH LIFT DEVICESAn airplane is a series of engineering compromises. Designers must choose between stability and maneuverability, and between high cruising speed and low landing speed, as well as between high util ity and low cost. Lift-modifying devices give us some good compromises between high cruising speed and low landing speed, because they may be extended only when needed, then tucked away into the structure when not needed.

FLAPSPerhaps the most universal lift-modifying devices used on modern airplanes are flaps on the trailing edge of the wing. These surfaces change the cambers of the wing, increasing both lift and drag for any given angle of attack.

PLAIN FLAPSThese simple devices are merely sections of the trailing edge of the wing, inboard of the ailerons. They are about the same size as the aileron and are hinged so they can be deflected, usually in increments of 10, 25, and 40 degrees. Generally speaking, the effect of these flaps is minimal, and they are seldom found on modern airplanes.

SPLIT FLAPSThis is another design of flap that was used with a great deal of success in the past, but is seldom used today. On the extremely popular Douglas DC-3, a portion of the lower surface of the trailing edge of the wring from one aileron to the other, across the bottom of the fuselage, could be hinged down into the airstream. The lift change was similar to that produced by a plain flap, but it produced much more drag at low lift coefficients. This drag coeffi-cient changed very little with the angle of attack.

SLOTTED FLAPSThe most popular flap on airplanes today is the slotted flap. Variations of this design are used for small airplanes as well as for large ones. Slotted flaps increase the lift coefficient a good deal more than the simple flap. On small airplanes, the hinge is located below the lower surface of the flap, and when it is lowered, it forms a duct between the flap well in the wing and the leading edge of the flap.

When the flap is lowered all of the way and there is a tendency for the airflow to break away from its surface, air from the high-pressure area below the wing flows up through the slot and blows back over the top of the flap. This high energy flow on the surface pulls air down and prevents the flap stalling. It is not uncommon on large airplanes to have double and even triple-slotted flaps to allow the maximum increase in drag without the airflow over the flaps separating and destroying the lift they produce.

Triple-slotted flaps are used on many jet transports to balance the lift and drag necessary for reasonable takeoff and landing speeds with the requirements for high speed cruising flight.

FOWLER FLAPSFowler flaps are a type of slotted flap. The design of this wing flap not only changes the camber of the wing, it also increases the wing area. Instead of rotating down on a hinge, it slides backwards on tracks. In the first portion of its extension, it increases the drag very little, but increases the lift a great deal as it increases both the area and camber. As the extension continues, the flap deflects downward, and during the last portion of its travel, it increases the drag with little additional increase in lift.

LEADING EDGE DEVICES

SLOTSStalls occur when the angle of attack becomes so great that the energy in the air flowing over the wing can no longer pull air down to the surface. The boundary layer thickens and becomes turbulent and the airflow separates from the surface.

This separation can be delayed to a higher angle of attack by any means that increases the energy of the air flowing over the surface. One method used is a slot in the leading edge of the wing. This slot is simply a duct for air to flow from below the wing to the top where it is directed over the surface in a high-velocity stream. Slots are usually placed ahead of the aileron to keep the outer portion of the wing flying after the root has stalled. This keeps the aileron effective and provides lateral control during most of the stall.

A fixed slot ducts air from the lower surface to the upper surface of a wing at high angles of attack.

SLATSMany high-performance airplanes have a portion of the wing leading edge mounted on tracks so it can extend outward and create a duct to direct high-energy air down over the surface and delay separation to a very high angle of attack.

In many airplanes these slats are actuated by aerodynamic forces and are entirely automatic in their operation. As the angle of attack increases, the low pressure just behind the leading edge on top of the wing increases and pulls the slat out of the wing. When the slat moves out, it ducts the air from the high-pressure area below the wing to the upper surface and increases the velocity of the air in the boundary layer. When the angle of attack is lowered, air pressure on the slat moves it back into the wing where it has no effect on the airflow.

Some airplanes have slats operated by either hydraulic or electric actuators, and the slats are extended automatically when the trailing edge flaps are lowered. These slats prevent the airflow breaking away from the upper surface when the flaps increase the camber of the wing. Flaps that are used with slats are usually slotted. They duct high-energy air over the deflected flap sections so the airflow will not break away from their surface.

Slats extend out of the leading edge of the wing at high angles of attack and serve the same function as a slot. They may be actuated automatically by aerodynamic forces or mechanically operated, usually in conjunction with the trailing edge flaps.

LEADING EDGE FLAPSThe leading edge of some wings may be deflected downward to increase their camber. These leading edge flaps are usually electrically or hydraulically actuated and are used in conjunction with the trailing edge flaps.

STALL STRIPSIt is important that a wing stall at the root first so the ailerons will still be able to provide lateral control throughout the stall. If the wing does not have this characteristic naturally, it can be given it by installing small triangular stall strips on the leading edge of the wing in the root area. When the angle of attack is increased enough for a stall to occur, the strips provide enough air disturbance to hasten the stall on the section of wing behind them. This loss of lift will usually cause the nose of the airplane to drop while the outer portion of the wing is still flying and the ailerons are still effective. It also causes the disturbed air to buffet the horizontal tail surfaces, thus providing the pilot with a feeling in the controls of the impending stall.

A small triangular stall strip may be installed on the leading edge of the wing near the wing root to cause area behind it to stall first.

SPECIAL WING TIPSAir flowing over the top of a wing creates a low pressure, while the air passing below the wing has been slowed down somewhat and its pressure has increased. This difference in pressure causes air to spill over the wing tip and create vortices that effectively kill some of the lift and create drag, especially at high angles of attack and low airspeed. The wing tip vortex will continue to expand in the form of a giant cone and trails far behind the airplane. At this point it ceases to be a wing tip vortex and becomes wake turbulence, which can be dangerous to any aircraft.

A wing tip vortex develops as a result of air flowing around the tip due to pressure differences. All airplanes generate them. These vortices are strongest when the airplane is flying slowly at high angles of attack and can be very dangerous to other airplanes.

There are a number of methods that have been used to reduce the effects of wing-tip vortices. Some manufacturers install fuel tanks on the wing tips that serve the triple function of increasing the range of the airplane, distributing the weight over a greater portion of the wing and preventing the air spilling over the wing tip. Smaller airplanes that do not use tip tanks may have tip plates installed on the tip. These plates have the same shape as the airfoil but are larger and prevent the air spilling over the tips.

Tip tanks minimize the amount of high pressure air spilling over from the high-pressure area beneath the wing to the low-pressure area above it.

Far less drastic than tip tanks or tip plates are specially shaped wing tips. Some wing tips have a special droop and a square trailing edge to tighten the vortices and spin them away so they will not contaminate the upper surface so much.

Drooped wing tips are a simpler method of reducing losses from wing tip vortices.

WINGLETSAnother popular method of controlling, or reducing, wing tip vortices is by the use of winglets. Used principally on high-speed airplanes, they also allow for drag reduction and better airflow control.

Winglets actually recover some of the energy that would be lost to wing tip vortices and not only reduce the vortex strength but also reduce the total drag on the airplane.

CANARD SURFACESAny aircraft that has the equivalent of two lifting surfaces, instead of the conventional horizontal stabilizer that provides a down load, can be classified as a canard. The canard is the forward surface, and frequently is also a control surface.

In a conventional airplane, the wing stalls, aileron control is lost, the CG shifts forward, then speed builds up and control is regained. During this sequence of events there is always the chance that lateral control will also be lost, particularly when the airplane is in a turn, causing an accidental spin. With a canard configuration, the sequence changes somewhat. The canard stalls first; the nose drops and speed builds back up. The canard regains full lift and the nose comes back up. The CG never changed, and full aileron control is available at all times. This virtually eliminates the chance of an inadvertent stall/spin accident.

T-TAILSMany aircraft today use the T-tail configuration. Although somewhat heavier, this arrangement has several desirable characteristics. The stabilizer is moved away from the disturbed airflow of the wing, rudder effectiveness is improved because of the cap on its end. Many jet aircraft use this configuration because it allows the engines to be mounted on the aft fuselage. Spin recovery may be improved because of better airflow with the stabilizer moved higher and out of the turbulence of the wing and fuselage.