AERODYNAMIC BALANCE.
1. OVER HANG Over hang balance is a control surface area ahead of the hinge line that is distributed along the control surface. All aerodynamic balance must be considered with respect to the elevator hinge moments due to angle of attack, or floating tendency, and the hinge moments due to elevator deflection, or restoring tendency. Aerodynamic balance such as over hang balance tends to reduce floating tendency, due to the chord wise pressure distribution, and as a result increases the stick free stability. The hinge moments due to elevator deflection, or restoring tendency, is also reduced by overhang balance.
2. HANDLEY HANDLEY PAGE (inset hinge balance) The hinge is set back so that the air striking the surface in front of the hinge causes a force which tends to make the control move over still further. This partially balanced the effect of the air which strikes the rear portion. This is effective. It must not be overdone. Over balancing is dangerous since it may remove all feeling of control from the pilot. When control surface is set at any angle the CP on the surface is well forward of the centre of the area. It at any angle the CP is in front of the hinge will tend to take the control out of pilot hand. Usually 1/5 of the surface in front of the hinge
3. FRIESE The hinge line of frie se aileron is always at or below the wing’s lower surface. With the hinge point below the wing surface, an arc drawn from the hinge point to be tangent to the wing upper surface penetrates the wing lower surface some distance ahead of the hinge line, thus establishing an overhang balance. The gap between aileron and wing can be made as narrow as desired by describing another arc slightly larger than the first. The narrow wing-toaileron gap reduces airflow from high pressure wing under surface to the lower pressure wing upper surface, reducing drag.
FLIGHT CONTROL BALANCING AND RIGGING 1. Flight control balancing The purpose of flight control balancing is to prevent flutter for the moment of the flight control. The flight control balancing can be done two ways. They are fully powered controls or via mass dampers. Fully powered controls allow the hydraulic actuators to serve as dampers but sometimes they must have larger size compare to the normal size of damper it should have. 2. Typical control surface condition 1.
Vibration a.
Excess and unwanted aircraft vibration can cause discomfort and fatigue for the flight crew and passengers. The service life of expensive avionics and instruments is shortened considerably when the equipment is operated in a high vibration environment. Excess vibration can also shorten the life of expensive engine components such as vacuum pumps, exhaust systems, engine ducting and many other items.
b.
Cause Aerodynamics, mechanical malfunctions, and external factors such as atmospheric turbulence can cause airplane vibration. All vibrations have associated frequencies and magnitudes that may be readily detected or barely perceptible to the flight crew and passengers. For some vibrations, such as those associated with engine operation, the flight crew has dedicated instrumentation to measure magnitude. Other vibrations are detected by sight, sound, or feel and may depend on flight crew experience for analysis.
c.
Normal vibration Each airplane has a unique signature of normal vibration. This is a consequence of mass distribution and structural stiffness that result in vibration modes at certain frequencies.
d.
Abnormal vibration. The most easily identified abnormal vibration is that which has a sudden onset and may be accompanied by noise. The vibration may be intermittent or steady with a distinct frequency, or it may be a more random buffet type.
2.
3.
Roughness a.
In order for the aircraft’s control surface to have the smooth air flow during flight, it must have a smooth and fair surface.
b.
The most significant effects of surface finish appear as qualitative differences in the stall characteristics of the wing.
c.
The rough flight control surface can affected the aerodynamic efficiency of the aircraft. It can affect the aircraft’s lift and can increase the frictional drag.
d.
There are several factors that cause the roughness of the flight control surfaces. i.
The flight control surface getting hit by an FOD.
ii.
Scratch or paint strip.
iii.
The flight control surface does not have proper surface finish.
Misalignment
During aircraft production, the relationships between manual flight control inputs and an aircraft's resultant control surface movements must be accurately calibrated for both conventional mechanical, hydro-mechanical and fly-by-wire flight controls. The control surfaces must move precisely as the aircraft design engineers intended, so that the aircraft responds safely and predictably to the pilot's or flight control system's direction. The misalignment happened when the control surface movement calibration between these three system are not calibrate according to its manual.
3.
Flight Control Flutters
A phenomenon whereby the elevator or aileron control surface begins to oscillate violently in flight. This can sometimes cause the surface to break away from the aircraft and cause a crash. There are many reasons for this, but the most common are excessive hinge gap or excessive "slop" in the pushrod connections and control horns. One of the most common causes of flutter is loose linkage between the servo and control surface. This is easily corrected but often overlooked or ignored. It only takes a moment to jiggle the control surfaces by hand to see how much movement is there and determine its origin. Servo gears get worn, clevises get worn, and control horns get worn. Not if, buy when the surface flutters aircraft will likely crash if immediate action hasn't taken place. If there is a low-pitched buzzing sound, reduce throttle and land immediately. Flutter can occur at any time on any control surface if the conditions are correct. All of the controls are subject to flutter but the typical pecking order is: ailerons, elevator then rudder.
4.
Methods of Aerodynamic Balance
The primary function of aerodynamic control surface balancing method is to reduce the force required to operate the control. The most common method employed to achieve this aim are hinge balance, horn balance, internal balance, balance tab, antibalance tab, spring tab, servo tab. Internal Balance The aerodynamic balance of control surface can be achieved internally without incurring any additional drag. The leading edge of the control surface of an internal balance is shaped such that it has a projection called balance panel, which is connected by a flexible diaphragm within a sealed chamber to a fixed part of the aeroplane structure, usually the spar and vented to atmosphere. The diaphragm divides the sealed chamber in two
Balance Tab
A balance tab is a small ancillary hinged flap attached to the trailing edge of a primary control surface. Its purpose is to reduce the force required to operate the control surface at a given airspeed by deflecting in the opposite direction to that of the primary control surface. Although the balance tab is relatively small when compared with the primary control surface it is a considerable distance from the hinge-line of the primary control surface and therefore produces a relatively large opposing moment. Its operation slightly reduces the effectiveness of the primary control surface and causes a small drag penalty.
Spring Tab At low airspeeds, for some aeroplanes, aerodynamic balancing is not considered necessary, however, as the airspeed increases it progressively becomes a necessity because of the increasing aerodynamic load on the main control surface. Towards the leading edge of the primary control top surface is pivoted a lever. The top of the lever is connected to the control column and the balance tab is connected to the same lever just above the pivot. Thus, movement of the control column is transmitted to the primary control surface but does not directly operate it. To ensure that the balance tab is only effective when it is required, the operating lever of the balance tab system is pivoted through a spring mounted on the elevator main control surface. This is the ‘sprin g tab’. When the elevators are operated with no aerodynamic load, the control column movement is fed directly to the primary control surface with no deflection of the balance tab so that there is no geometric movement between them. However, when there is an aerodynamic load on the primary control surface (at higher speeds), one side of the spring is compressed and changes the geometric position of the balance tab relative to the main elevator control surface. The spring causes that tab to move in the opposite direction to the main control surface at an ever-increasing angle, which is directly proportional to the increasing airspeed. Thus, the spring tab assists
Anti balance Tab
Because the hinge moment on some aeroplanes is too small, often as the result of the centre of pressure being too close to the hinge-line of the control surface, it is too easy to deflect the control surface against the aerodynamic load. Consequently, there is little control column load and there is a lack of feel to the controls. This could lead to excessive deflection of the control surface and result in serious overstressing of the airframe. To counteract these characteristics and assist the pilot to use the control surfaces correctly it is necessary to have antibalance tabs fitted. These operate in the opposite sense to the balance tab by deflecting the antibalance tab in the same direction as that of the primary control surface and increase the aerodynamic loading.
5.
Balance Limit
The position of the centre of gravity along its longitudinal axis affects the stability of the airplane. There are forward and aft limits established by the aircraft design engineers beyond which the C.G. should not be located for flight. These limits are set to assure that sufficient elevator deflection is available for all phases of flight. If the C.G. is too far forward, the airplane will be nose heavy, if too far aft, tail heavy. An airplane whose centre of gravity is too far aft may be dangerously unstable and will possess abnormal stall and spin characteristics. Recovery may be difficult if not impossible because the pilot is running out of elevator control. It is, therefore, the pilot's responsibility when loading an airplane to see that the C.G. lies within the recommended limits. If the C.G. is too far forward, the airplane will be nose heavy, if too far aft, tail heavy. An airplane whose centre of gravity is too far aft may be dangerously unstable and will possess abnormal stall and spin characteristics. Recovery may be difficult if not impossible because the pilot is running out of elevator control. It is, therefore, the pilot’s responsibility when loading an airplane to see that the C.G. lies within the recommended limits.
6.
Flight Controls Condition
a.
Over-balance
Should the trailing edge of the control surface rise some position above a horizontal plane, an overbalance condition is apparent. Typical overbalance condition essential for good results in high-performance aircraft.
b.
Under-balance
Chord angles down at trailing edge. If the control surface assumes a trailing-edgelow attitude, it is statically underbalanced and a tail-heavy condition exists. The static under balance, or tail heavy condition first described, is the least desirable as it may result in unpredictable flight performance.
c.
Neutral balance
If the control surface remains in a level (horizontal) attitude, it is said to be statically 100% balanced and its center of gravity (cg) is co-located with the hinge axis. 100% balanced to a level attitude should consistently give good results.
7.
State of Balance (Flight Controls)
Static Longitudinal Stability Static longitudinal stability is a measure of the tendency of an airplane to maintain its trimmed angle of attack in 1g flight. More strictly speaking, it is a measure of the initial pitch response of an airplane to a disturbance in angle of attack. Following a disturbance, a statically stable airplane tends to return to the angle of attack for which it is trimmed.
Speed Stability Speed stability is a measure of the control force required to hold the airplane at airspeed other than the trimmed airspeed. The throttles are fixed at the trimmed thrust setting to eliminate pitching moment changes due to thrust. Speed stability is measured by trimming the airplane in level flight, with throttles fixed at the trimmed condition, then slowly varying airspeed with control column input.
Maneuverings Stability Maneuvering stability is related to static longitudinal stability. It is a measure of the Longitudinal stability tendencies of the airplane in other than 1g flight, and it accounts for the effects of pitch rate aerodynamic damping during maneuvering, as in the recovery from a Pitch upset. The column Force generates a pitching moment through the elevators, or stabilizer in some airplanes, that is eventually balanced by the damping moment created by the horizontal tail and the moment due to the change in angle of attack.
Static Longitudinal Stability and Speed Stability
Maneuverings Stability
8.
Calculations of changes to balance weight
By determining the weight of the empty airplane and adding the weight of everything loaded on the airplane, a total weight can be determined. This is quite simple; but to distribute this weight in such a manner that the entire mass of the loaded airplane is balanced around a point (CG), which must be located within specified limits, presents a greater problem, particularly if the basic principles of weight and balance are not understood. The point where the airplane will balance can be determined by locating the center of gravity, which is, as stated in the definitions of terms, the imaginary point where all the weight is concentrated. To provide the necessary balance between longitudinal stability and elevator control, the center of gravity is usually located slightly forward of the center of lift. This loading condition causes a nose-down tendency in flight, which is desirable during flight at a high angle of attack and slow speeds. A safe zone within which the balance point (CG) must fall is called the CG range. The extremities of the range are called the forward CG limits and aft CG limits. These limits are usually specified in inches, along the longitudinal axis of the airplane, measured from a datum reference. The datum is an arbitrary point, established by airplane designers, which may vary in location between different airplanes.
ASSIGNMENT SEMESTER JANUARY 2012 AERODYNAMIC BALANCE & FLIGHT CONTROL BALANCING AND RIGGING
AIRCRAFT FLIGHT CONTROL AAB30303
Prepared by: HUZAIFAH BIN MOHD ZAINUDIN
53259209208
6 BACHELOR 1
Prepared for: SIR ABDULLAH BIN MOHD NOOR
UNIVERSITI KUALA LUMPUR MALAYSIAN INSTITUTE OF AVIATION TECHNOLOGY
