Aspect Ratio for Rectangular Wing Aspect Ratio= Span ÷ Chord
Aspect Ratio for Tapered Wing Aspect Ratio = Span ÷ Mean Chord
Aspect Ratio for Delta Wing Aspect Ratio = Span² ÷ Wing Area
Thicker Aerofoil is used in slow speed Aircraft & heavy Aircraft.
Thinner Aerofoil is used in fast/high speed & light Aircraft.
Nose drops after stalling because the Tail plane effectiveness reduces as it comes under the influence of disturbed airflow because of buffet of the wings.
Drag is Minimum at O0 Angle of Attack.
Lift/Drag Ratio is best at 4o Angle of Attack.
Best Lift/Drag Ratio speed is higher then Vmd.
Airframe Efficiency = is the determination of airframe to produce lift at varying speeds, and produce least drag.
Airframe Efficiency = TRUE AIR SPEED ÷ DRAG
Finess Ratio = is the ratio of a aerofoil fineness from chord to the thickness of the aerofoil.
Finess Ratio = CHORD ÷ THICKNESS
Induced Drag is Inversely proportional to Finess Ratio.
LIFT
Lift = CL × ½ Rho v2 × s
Where-
CL = Coefficient of Lift that depends upon
- Shape of Aerofoil.
- Angle of Attack.
Rho = Air density
V = Air Speed
S = Surface Area
DRAG
Drag = CD × ½ Rho v2 × s
Where-
CD = Coefficient of Drag that depends upon
- Shape of Aerofoil.
- Angle of Attack.
Rho = Air density
V = Air Speed
S = Surface Area
At Mach 0.6 all flow is Subsonic.
At Mach 0.8 air flow over airfoil is Subsonic & Supersonic ahead.
At Mach 1.0 Supersonic flow increases, air flow behind the aerofoil is Shockwave, supersonic flow is ahead of the A/c.
At Mach 1.10 All air flow over the aerofoil is Supersonic, some flow is still Subsonic.
At Mach 2.0 Bow wave develops, all area over the airfoil becomes Supersonic.
Unless bow is attached to leading edge (which occurs only if the leading edge is very sharp) there will still be a small area of Sub sonic flow at the Stagnation point between bow & the leading edge.
Ground Effect-
A/c finds difficulty in climbing after T/o. or A/c tends to settle down after T/o. With in ground effect you feel nose heavy and back pressure on the control column is required because with in ground effect downwash of the tail plane decreases, therefore tail plane effectiveness decreases, this decreases down load being produced by the fail plane to become airborne.
A/c floats on landing. A/c gets airborne early due to the Ground Effect with in ground effect (up to one Wing Span of the A/c) there are no Wing Tip vortices, because air does not get a chance to go around .
Therefore, there is no Induced Drag in the Ground Effect.
Snaking – A/c yawing from side to side.
Propoising – A/c pitching up & down.
Rolling/Banking – About/Around Longitudinal axis and On/Along Lateral axis.
Pitching – About/Around Lateral axis On/Along Longitudinal axis.
Yawing – Abort /Around/On/Along Vertical axis.
Lift – Cl ½ ℓ v2s
Where
Cl- Co-efficient of lift, which depends on: –
(i) Shape of airfoil.
(ii) Angle of attack to the airflow.
ℓ- Rho, – density of air (unit of density is equal to 32.1739 lbs of mass per cubic foot under standard conditions of gravity.
V – Velocity of the airflow (lift and drag vary according to the square of velocity).
S – Surface area of the aerofoil.
Co-efficient may be regarded as a no of multiplier as a variable quantity in calculating the magnitude of physical property.
Co-efficient is dimension less, they are absolute no’s which are not associated with length, mass or time.
Co-efficient of lift is a function of the aerofoil shape and the angle of attack.
For a given aerofoil shape the co-efficient of lift varles with the angle of attack.
Lift always act perpendicular/900 to the relative airflow or flight path.
Drag always acts parallel to the relative airflow as diction of motion of the wing.
Weight always acts vertically downward from the pivot point of C.G. of the aircraft to the Earth, in level flight and as the angle of attack of the aircraft changes, the C.G. pivot point changes as well.
Mean Camber Line (MCL) – Mean Camber Line is an imaginary line joining leading edge and the trailing edge equidistant from the upper surface and the lower surface.
Mean Camber Line divides the Aerofoil into Two equal parts.
In a symmetrical wing-
Chord Line- Chord line is an imaginary line, which is a straight-line tangent to the lower surface at two points. Chord line is an arbitrary reference line from which measurements are taken. It is the straight line between the leading edge and the trailing edge.
Chord Line divides the Aerofoil into Two equal parts only in a Symmetrical wing. In Asymmetrical wing it divides the Aerofoil into Two unequal parts.
In a asymmetrical wing-
At 3° to 4° angle of Incidence lift increases up to 24 times than Drag.
Angle of Incidence- is an acute angle between the longitudinal axis of the a/c and chord of the wing. This angle is positive when leading edge is at a higher angle than the trailing edge.
This angle is negative when leading edge is at a lower angle than the trailing edge.
This angle is fixed and is set during the manufacture of the a/c.
Angle of attack- is on acute angle between the chord of the aerofoil and relative wind. It can be increased and decreased by changing the pitch angle.
Aspect Ratio = SPAN/ CHORD
High Aspect Ratio causes-
(i) Less Wing Tip Vertices
(ii) Less Induced Drag
(iii) Increases Lift.
Aspect Ratio for a tapered wing a/c = SPAN2/WING AREA
Aspect Ratio = Area/ Chard2
The angle of attack of a wing directly controls the distribution of the positive and the negative pressure acting on the wing.
Optimum angle of attack is the angle at which you get maximum Lift/Drag ratio, or we can say that the angle at which you get least drag for maximum lift.
Wing tip Vortices – Air flowing above the wing causes high velocity and low press and below the wing causes low velocity and high press. High press air under the wing will try to spill over on the top of the wing through the wing tips. High press air when try to spill over on the top of the wing through the wing tips, cause in forming of eddies or wing tip vortices at the wing tips causing the stream line flow to form vortices which are like little rotating whirlpools.
Wing tip vortices are clockwise on the left wing and anti-clockwise on the right wing.
The intensity of wing tip vortices is directly proportional to the weight of the a/c and inversely proportional to the wingspan and speed of a/c.
The greater the angle of attack stronger will be the wing tip vortices.
A/c will have maximum strength of wing tip vortices during Take Off, climb and landing phase of the flight, due to high angle of attack.
Dihedral angle- is when the wing tips are higher than root.
It gives lateral stability excessive dihedral decreases lift and causes Dutch roll. It gets lateral stability due to pendulum effect.
Anhedral angle- is when wing tips are lower than roots.
It gives no stability, instead it decreases the lateral stability.
It gives more control effectiveness.
Sweep Back-Wing tips are farther back than root section of the wing. It gives lateral and directional stability and causes a higher critical mach no. Sweep back wing a/c experiences twisting of its wing tip at high speed.
If a straight wing aircraft is loaded (due to extra lift) it will bend, but usually not twist.
In Sweep back wing a/c the wing bends, twisting occurs which decreasing the angle of attack at the wing tip. This causes less lift, there by causing the center of lift to move inward and hence forward giving the a/c nose-up pitch (More G-load).
At the same angle of attack sweep back wing a/c produces less lift comparing to straight wing a/c, (Therefore, less G-loads).
Wash In- is a twist of the wing giving an increase of angle of incidence towards the wing tip.
Advantage:-
(i) Counteract the torque of the propeller.
(ii) Gives lateral stability.
(iii) Gives more lift.
Wash- out is a twist of the wing giving an decrease of angle of incidence towards the wing tip.
Advantage:-
(i) Less stalling effect at the wing tip, therefore at stall, ailerons are still effective. This occurs since ailerons are located at outboard of the wing, where the angle of incidence is less, and the ailerons do not come into the broken boundary layer caused by the stall, they remain effective
HINGE- is the point around which the control surface moves.
HINGE MOMENT- is the force required by the pilot to deflect the control surface by moving the control columns.
Hinge is always kept ahead of the C.G.
Control Flutter- is a high speed phenomenon, where control surface moves up and down about the mean position.
Mass is added to shift the C.G. closer to the hinge, but C.G. is still kept behind the Hinge.
Mass Balance- is a weight attached ahead of the Hinge.
Set back hinge/Inset hinge- The hinge point is pushed backwards as much as possible to be closer to C.G. but (hinge) is still kept ahead of C.G.
This is done to ensure that the Elevators are always in the down position, which gives a Nose Down moment of the Aircraft.
If the Elevator is allowed to go in the Up position, by keeping the Hinge behind the C.G, it will cause a Nose up moment, which is very un desirable for the flight
If the C.G. is behind the Hinge an Engine fails, it will give a Nose up moment, and the Pilot has to struggle to bring the Nose of the Aircraft down.
If the C.G. is ahead of the Hinge and an Engine fails, it will assist the Pilot to maintain the Aircraft nose down by exerting the Weight on the Tail plane due to C.G. location behind the Hinge.
TYPES OF TABS
Tabs are auxiliary (aero foils) or thin plates attached to trailing edge of the control surface for adjustments of flying control movements.
Movements of tab is related to the movement of the control surfaces or control column and not related to the loads on the control surfaces.
Spring tab – when the control column is moved, the control surfaces move, but the tab does not move.
Servo tab – when the control column is moved, the Control surface does not move, but the tab moves.
Balance tab – Moves in opposite direction of controls, It reduces the force required by the Pilot to move the controls, it is more suitable for flights at low speeds.
Centre of Gravity is always kept ahead of Centre of Pressure.
Aerodynamic Centre is a fixed point on the wing profile about which the lift moment co-efficient is constant and does not change with change in the Angle of Attack.
Aerodynamic centre depends upon thickness of wing but usually aerodynamic centre is normally 25% of the chord length behind leading edge.
Aerodynamic Centre moves forward with increase in wing thickness, or it becomes less than 25%.
Aerodynamic Centre moves backward with decrease in wing thickness, or it becomes more than 25%.
Therefore, C.G. is always kept ahead of C.P.
Tail plane is always set at a negative angle of incidence, because Tail plane always provides a downward lift to balance the pitching moments of Thrust and Drag couples and Lift and Weight couples.
Tail plane is affected by downwash of the wing, i.e., the airflow to the tail plane comes after meeting the main plane.
If angle of incidence of the tail plane is lower the angle of attack of tail plane becomes higher.
For stability C.G should always be ahead of the Neutral Point.
Distance between C.G and Neutral Point is static point/static margin.
When static margin increases the aircraft longitudinal stability increases.
If the weight located ahead of the C.G is removed the C.G. moves backward.
If the weight located behind of the C.G. is removed the C.G. moves forward.
If a weight is added ahead of the C.G. the C.G. moves forward.
If a weight is added behind the C.G. the C.G. moves backward.
Dorsal fin- cause wind cocking effect- dorsal fin provides directional stability at high angle of sideslip.
Dutch roll- A/c yaws and rolls simultaneously in opposite direction.
It occurs due to high lateral stability and less directional stability.
i.e., more Dihedral and less Fin area
As altitude increases the tendency of aircraft to get into Dutch Roll increases, because as the altitude increases the TAS increases, the restoring forces (fin area) remains the same.
In Dutch Roll yaw is more prominent than roll..
To correct the Dutch Roll- use ailerons or use Yaw dampers, but do not use Rudders, since the rudders will further worsen the situation by further increasing the amplitude of the oscillations.
Usage of rudder can cause a Pilot Induced Oscillations.
Pilot Induced Oscillations- this type of oscillations are produced when the pilot uses the incorrect use of the rudder or other controls the oscillations produced through this are very disastrous, he may unknowingly exceed the structural limit and cause an airframe failure.
Spiral Instability- A/c yaws and rolls simultaneously in opposite directions.
It occurs due to high directional stability and less lateral stability.
Example- A/c with Large Fin Area and less Dihedral Angle.
As altitude increases Spiral Instability decreases or
As altitude increases Spiral Stability increases.
At low altitude, Spiral Instability increases.
If A.R. increases wing tip vortices decreases Induced Drag decreases, since the wing tip vortices don’t get a chance to form due to big span.
If the speed of a/c is high that by the time wing tip vortices will come upwards the a/c would have moved forward therefore, Induced drag formed is less.
At Vmd- Velocity of minimum drag – drag is minimum.
At Vmd- Induced drag = Parasite drag
If the speed of a/c is high, the a/c is flying at a low angle of attack or the down wash at leading edge is less and up wash at trailing edge is also less. Therefore eddies formed at wing tip are also less.
With increase in angle of attack L/D ratio increases.
With increase in angle of attack Total drag also increases.
Drag is max at Stalling angle.
Stalling angle is also known as Critical Angle.
Usually L/D is usually maximum at 40 angles of attack.
L/D ratio indicates the Aerodynamic efficiency of the wing.
Two a/c having same Finess Ratio will have same Cd. but total drag will be more for a larger body because It will have greater value of S in the formulae Drag = Cd ½ ℓ V2 S.
Where S is Surface area.
FLYING CONTROLS
Flying Controls – All the 3 axis of a/c intersect at the centre of gravity and are perpendicular to each other.
Flying Controls –
(a) Elevators
(b) Ailerons
(c) Rudders
(d) Differential Spoilers
Elevators control pitch movement or movement on/ along longitudinal axis or movement about lateral axis.
Ailerons Control roll movement or movement on/ along lateral axis or movement about longitudinal axis.
Rudder Control yaw movements or movement on /along normal/ vertical axis.
Principle of Control Surfaces– Increase in camber increases lift because increase in camber increases the down wash of air flow, therefore lift increases.
Aileron Drag– Aileron drag is the drag created by the down going aileron as it moves in the area of high pressure area below the wing (air at the bottom of the wing has high pressure.)
The up going aileron produces less drag because it is moving into area of low pressure.
Adverse Yaw– yaw in the opposite direction of roll and is created by Aileron Drag.
To overcome Adverse Yaw -rudders are used.
Primary function of rudders is to overcome effects of Adverse Yaw.
Differential Ailerons- The down going ailerons move through a lesser angle and upgoing aileron moves through a greater angle.
Frise Ailerons- The upgoing aileron moves in such a way that a part of it moves on the under surface of the wing, thereby increasing drag on it, but the down going aileron moves in a normal way.
Control Coupling- Rudder and aileron are coupled. When a small roll is applied through control column the some side rudder is also applied. It is used in big transport category a/c to automatically apply rudder upto 10-15 bank.
Maximum movement of flying controls is limited by adjustable stops. Adjustable stops are provided in the cockpit and also at the flying control surfaces.
Aileron drag is more effective at low airspeeds and less at high airspeed. Because at low airspeed the control effectiveness is less, because the slipstream effect is less To cause the same change in roll greater movement of aileron is required, and therefore, aileron drag is more. Aileron Drag is more experienced at low speeds and less at high airspeeds. Because at low airspeed the control effectiveness is less, as the slipstream effect is less.
Therefore, to roll the a/c more ailerons deflection is required at low speeds. Therefore, aileron drag is more at low speed.
Effectiveness of controls depends upon slipstream effect or airspeed, higher the airspeed or slipstream, higher the control effectiveness.
Note- Effectiveness of elevators also depends upon C.G position.
A forward C. G will require a greater elevator movement to bring about the some change in attitude.
With forward C. G a/c is difficult to flare while landing, because the
(i) Air speed is less.
(ii) C.G is forward therefore requires more elevator movement.
Therefore wheel borrowing can occur in too forward C.G. positions, on landing.
In this condition the weight of the A/c is on nose wheel instead of main wheels. It can also occur on Take off.
Hinge is the point around which the control surface is moving up or down.
Hinge moment is the force required by the pilot in the cockpit to deflect control surfaces.
Therefore the force require by the pilot depends upon-
Hinge Moment = D × L
Where –
D- Dist between lift and hinge point.
L- Lift produced by the control surface.
To Balance Controls
(i) Mass balancing– Mass/weight is attached ahead of the wing.
This mass/weight shifts C.G. closer to hinge, but in any conditions C.G. is always kept behind the hinge point to allow control surfaces to be in down position without any external force required by the pilot.
Therefore hinge is always ahead of C.G.
Mass balance is used to reduce the force required by the pilot.
Mass balance also reduces control flutter.
Control Flutter– A high-speed phenomenon, where control surfaces move up and down about its mean position.
(ii) Aerodynamic Balance– We create aerodynamic forces in a manner that they help the pilot to move the control surfaces.
Aerodynamic Balance is attained by-
(a) Set back hinge/ Inset hinge– The hinge point is pushed backward as possible i.e. closer to C.G. but Hinge is still kept ahead of C.G.
(b) Internal Balance– In this the area between the tail plane and elevator is sealed and the tail plane and elevator are joined by a diaphragm. Now the high press at the leading edge of the elev. moves it up by moving the diaphragm up and therefore the trailing edge goes down and therefore the pilot’s force require decreases.
(c) Horn Balance– Horn is a part of control surface which projects ahead of hinge line.
Example- As control surface moves down (elevator moves down) the horn goes up assisting the control movement. Therefore the airflow from the up hitting the horn balance tends to push it back causing elevator to go more down.
Therefore air from below tends to push elevator up and above pushes the horn backwards both are acting opposite. In affect it decreases the pilot’s force required to move controls.
(d) System of Tabs– Tabs are auxiliary airfoils attached to the trailing edge of the control surfaces.
Types-
(i) Fixed Tabs can be adjusted only on the ground.
They cannot be moved in the air.
Tabs move in the opposite direction to the control surface.
(ii) Balance Tabs– Balance tabs have a mechanical linkage to the control column.
They move in the opposite direction to the control surfaces.
Movement of the tab is according to the movement of the control surfaces or control columns.
They are not of much use because we need tabs at high speed to reduce the high loads. At high speed because the control surface moves less and the tabs also moves, less although for even less deflection of controls the loads are higher at high speed.
(iii) Spring Tabs– Spring is attached to the tab which does not allow the tabs to move at low speeds or at less air loads. It only allows tab to move at high speeds or high air loads. Therefore in this only spring moves the tab.
During pre-flight check if control column is moved, control surfaces will move but tab will not move.
(iv) Servo Tabs– Tabs are connected to the control column, but control surface is not connected to the control column.
Depending upon the movement of tabs and airspeeds the control surfaces moves.
During pre- flight check if control column is moved the tab moves but control surfaces will not move.
On moving the control column tab will move but the control surface does not move. Therefore they lack effectiveness at low air speeds.
Full and free movement of control column check (done during vital actions checks) are done as a part of on external checks and not as Vital action checks from cockpit. Because the movement of control column will only move the tabs and not the control surface.
(v) Anti-balance Tabs– Anti- balance tabs move in the same directions as the flying controls. They spoil the balance. They are provided in the a/c that are over balanced and are provided to give a better feel of controls to the pilot. They increase force required by the pilot to move controls.
(vi) Trim Tabs– Trim tabs has separate control in the cockpit. They can be deflected to any desired position. They can be mechanically or electrically operated.
Hydraulic Controls – Hyd. fluid operated controls.
(i) Hydraulic Assisted/Power Assisted Controls– The hydraulic pressure helps the pilot to move the controls, pilot has the feels of controls.
(ii) Power/Hydraulic Operated Controls– Pilot sends a command to the hydraulic system to move the controls surfaces.
– Controls are moved by hydraulic actuators as pumps.
– In this pilot does not have the feel of controls. Therefore an artificial feel is provided. This feel is proportional to the airspeed of the a/c or dynamic pressure.
– It is proportional to the amount of deflection of the control surfaces.
These types are irreversible type controls.
Example- Controls can be moved from cockpit, but outside gusts/ air load cannot move the controls.)
Jack Stall – At very high Mack No., due to the formation of shock waves, near the Jacks. (Actuators that moves the controls) Because of adverse pressure distribution rear the jacks especially in ailerons, and other surfaces, the controls may get jammed.
In Fly by Wire System – Airbus a/c the pilot’s commands to the hydraulic system are sent to in the form of electrical pulses and not by mechanical cables etc as in the case of older system.
Rudder Boost System – Rudder boost system helps the pilot to maintain direction, in case of an engine failure by sensing the difference between the Torque of engines.
When one engine of a multi engine a/c fails it automatically applies the opposite rudder by correct amount and maintains the direction in case of a engine failure, thereby reducing the pilot’s workload in case of a engine failure.
Longitudinal Stab
Factors
(i) Wings of the a/c or moment of C.P.
– If the a/c nose pitches up, C.P. moves forward and lift increases but C.G. remains there only and therefore L/W couple increases this will give a nose down pitching movement.
Therefore by convention C.G is always kept ahead of C.P.
Aerodynamic center is a fixed pt on the wing profile about which the lift moment co-efficient is constant and does not change with change in angle of attack.
Aerodynamic center depends upon thickness of wing but aerodynamic center is normally 25% of the chord length behind the leading edge.
Aerodynamic centre moves forward only with increase in the wing thickness or it becomes less than 25%.
C.G. is always kept ahead of Aerodynamic center and C.P.
Tail Plane– Tail plane affects longitudinal stability maximum.
Tail plane provides a downward lift to balance the pitching moments of T/D couples and L/W couples. Tail plane is affected by downwash of the wing. i.e. The airflow to the tailplane is different from main plane because of air deflected from Main plane.
Tail plane generally has a lower angle of incidence than Main plane.
Airflow to the Tailplane generally comes from top.
Longitudinal Dihedral- difference between angle of incidence between Tail plane and Main plane.
Lift provided by Tail plane depends on-
(i) Angle of incidence of tail plane (long Dihedral) because on decreasing the angle of incidence of tail plane higher is the angle of attack of tail plane.
(ii) Shape of the tail plane- Tail plane is generally a symmetrical aerofoil section in most of the a/c.
(iii) Surface Area/ Size of tail plane-
All (I), (ii) and (iii) affects Longitudinal stability also.
– Position of C.G. affects a/c Longitudinal stability.
– A forward C.G increases Longitudinal stability, because distance between tail plane and C.G increases.
– A rearward C.G decreases Longitudinal stability, because dist between tail plane and C.G increases, In this case Maneuverability/Controllability of a/c increases.
– Neutral Point- Neutral point is that point at which stability is neutral. If we continue to take the C.G backward the stability decreases and a point will come where the stability become neutral.
For stability C.G. has to be ahead of neutral point.
The dist between C.G and neutral point is called Static point/Static margin.
If you increase the Static margin stability increases.
Therefore, for every a/c forward and rearward limits of C.G are fixed.
C.G. Forward, stability increases, stalling speed increases, controllability decreases, because distance between tail plane and C.G. increases.
A/c is difficult to flare on landing.
A/c may wheel barrow (weight on nose wheel) on landing
C.G Rearward stability decreases, stalling speed decreases and controllability increases, because distance between tail plane and C.G decreases. A/c is more stable at high speeds and less stable at low speeds.
Variable Incidence Tail plane- There is a lever arm in the cockpit by which angle of incidence of tail plane can be changed.
It increases the C.G. range.
To exceed rearward limit of C.G. we increase the angle of incidence and vice-versa.
It reduces the Trim drag.
Trim Drag- is the drag produced by the trimmers and flying controls when they are deflected into airstream.
Any change in the trimmer position requires a change in the power setting also to compensate for the trim drag.
For a/c nose pitch up- Decreasing angle of incidence, by adjusting the tail lever arm position in low position.
– It facilitates the full elevator movement.
Because if we change the angle of incidence of tail plane, we can fly the a/c with elevator in a neutral position.
Effects of high Mach No. (when flying above Critical Mach No.) on longitudinal stability- At very high Mach No. longitudinal stability decreases because of the formation of shock waves.
The air behind the shock wave is turbulent and boundary layer thickens and when this layer strikes the tail plane it’s effectiveness reduces.
Under these conditions a stabilator tail plane or an all moving tail plane is more effective than conventional ones.
Lateral Stability– Stability in roll or stability about the longitudinal axis. It is provided by: –
- Wing Dihedral– A Dihedral wing when sideslips the lower wing/ down going wing meets the relative airflow at a higher Angle of Attack and therefore increases Lift on the lower Wing, thereby it attains wing level position.
- High Wing Aircraft– High wing Aircraft are more Laterally stable due to Pendulous effect. But because they are very stable to provide Lateral control or control effectiveness of ailerons etc they are provided with Anhedral to reduce Lateral Stability.
- Sweep Back– If a Sweep Back wing sideslip the effective span of down going wing increases and its effective Chord decreases.
It’s effective Aspect Ratio = SPAN/ CHORD increases and it produces more Lift and Aircraft returns to its level position.
- High Keel Surface – Vertical surfaces of Aircraft behind the C.G. and a high keel surface is a keel surface, which is above C.G.
Because when Aircraft is in sideslip the Relative Airflow coming from the direction of the Sideslip strikes the keel surface and therefore Aircraft returns to its original position.
Effects of high Mach No. on Lateral Stability:-
- Lateral Stability Decreases At High Mach No.’s Because– A very minute differences in the construction of the wing will change the Lift and Drag being produced by either wing.
They are not easily visible to the naked eye but at high Mach No. These differences can be experienced, as they affect lateral stability.
- At Low Mach No. we use trimmers to maintain Lateral balance but on crossing the Critical Mach no, due to the formation of Shock Waves the trimmer becomes in-effective and therefore lateral stability is lost.
Directional Stability
1. Fin/Vertical Stabilizer– If nose goes to the right the fin goes to the left and relative airflow is set from the left.
This relative airflow from the left strikes the Fin area and prevents the tail from going to the left.
This is also called- WIND COCKING EFFECT
If fin area increases Directional Stability increases.
- Dorsal Fin– Dorsal Fin provides Directional Stability at high Angles of Sideslip.
In a normal Fin at high Angles of Sideslips the Fin is almost horizontal and to maintain height as pilot continues to increase its Angle of Attack. It ultimately reaches stalling angle and thereafter it stalls losing all directional stability.
But in case of Dorsal Fin at high angles of sideslips the chord of fin increases and its Aspect Ratio decreases and therefore Lift produced decreases or stalling angle increases and therefore preventing it from stalling.
Dutch Roll – Dutch roll is a condition of Dynamic instability. In this Aircraft rolls and yaws in opposite directions simultaneously. Roll is more prominent than yaw. Dutch roll occurs in Aircraft having large Lateral stability and less Directional stability.
Or Aircraft having more Dihedral Angle and less Fin area.
Dutch Roll can be induced by:-
- Excessive/Incorrect use of Rudders.
- Turbulence.
- In advertent Lateral roll.
Dutch roll can be corrected by using the Ailerons against the upcoming wing.
Example- The wing which is coming up, we try to keep it down with the help of Ailerons, Or by using yaw dampers.
(Yaw Dampers) – Gyro devices fitted in the fin, work along with Autopilot.
– Aircraft’s tendency to get into Dutch Roll increases with increase in Altitude. Because of increasing TAS with increase in the Altitude the effect of restoring forces decreases.
With increasing in Altitude the forward speed (TAS increases) but restoring forces (Fin size) remains the same.
Therefore the Angle made by this force with Fin decreases and therefore restoring moment decreases.
SPIRAL INSTABILITY
Spiral Instability – Spiral instability takes place in Aircraft’s having large Fin areas and a small Dihedral Angle.
Example- Aircraft’s which are more Directionally stable and less Laterally stable.
– It can be corrected by using opposite Rudder. Aircraft is in a nose down attitude and the turn becomes tighter and tighter with each turn.
Example- If the left Rudder is given Aircraft yaws to the left, this yaw causes a Roll to the left, this roll further causes a yaw to the left. The Aircraft will sideslip and the Relative Airflow will be set up from the left side which will hit the Fin from the left side, the Fin will go up and the nose will go down and Aircraft continues to yaw and roll towards left.
The Spiral Instability decreases with increase in Altitude or Aircraft becomes more Spirally Stable with increase in Altitude.
CLIMB
Lift < Weight Weight is more than Lift.
Thrust > Drag Thrust is more than Drag.
Only during a maneuver Lift is more than Weight, otherwise always either Lift = Weight or Lift is less than Weight.
Any body moving along an inclined plane, It’s weight gets divided into two, one acting down slope and 2nd acting parallel to the plane.
– Rate of climb- height gained in a unit time.
– Angle of climb- height gained in a given distance.
– Climb Gradient- Slope along which Aircraft climbs. It depends upon TAS and ROC.
– Flight Path Gradient- is taken by Angle of Climb because it depends upon Ground Speed and ROC.
– Climb gradient is not affected by Wind velocity.
– Flight Path Gradient is affected by Wind velocity.
Aircraft Climbs, because of Excess Power, not because of Lift.
Best ROC is given by a speed at which difference between power required to maintain level flight and the power available is maximum.
We Climb at higher speed.
- Airspeed- Airspeed decreases climb increases.
- Weight- Weight decreases ROC increases, weight increases ROC decreases, Angle of Climb remains the same.
- Temperature- Temperature decreases, power output of Engine decreases and Power required to maintain level flight increases therefore ROC decreases.
- Altitude– Altitude increases, Power output of Engine decreases, Power required increases therefore ROC decreases.
Altitude increases ROC decreases, at one Altitude ROC decreases 100 ft/min and is known as Service Ceiling.
ROC decreases to 0 ft/Minute and is known as Absolute Ceiling.
With increases in Altitude, Power output of Engine decreases, therefore difference between speed ranges at which you can maintain level flight decreases.
i.e. Maximum speed decreases and Minimum speed increases.
At Absolute Ceiling there is only one speed at which you can maintain level flight.
- Wind Velocity does not affect ROC. It affects Angle of Climb.
In Head wind Angle of Climb increases.
In Tail wind Angle of Climb decreases.
ROC Head wind or Tail wind remains same.
Wind velocity affects Range
Tail wind increases Range.
Head wind decreases Range.
Endurance (Time taken) remains the same.
- High Density Altitude– Density Altitude is that Altitude at which the prevailing Density occurs.
It depends upon
(i) Pressure Altitude.
(ii) Temperature.
If Temperature increases, Density Altitude is above Pressure Altitude.
If Temperature = Standard ISA than Density Altitude = Pressure Altitude
If Temperature > Standard ISA than Density Altitude > Pressure Altitude
If Temperature < Standard ISA than Density Altitude < Pressure Altitude
A high Density Altitude decreases ROC because high Density Altitude means either high Temperature or a high Altitude.
- Flaps – Flaps increases power required, therefore excess power available decreases (Extra Power available decreases) therefore ROC decreases.
Single Engine Ceiling– In a multi-engine Aircraft if an Engine fails during Cruise, Aircraft won’t be able to maintain Altitude, Aircraft will have to descent to a lower Altitude.
It is always approached from top: –
It is also called Drift Down Altitude.
Drift down Altitude depends on (i) wt of Aircraft, (ii) Temperature conditions.
Drift Down Time – Time taken to descent to drift down Altitude is called Drift Down time, and the horizontal dist traveled during this time is called Drift Down distance.
Ground Effect – Ground effect is an upward cushioning affect.
(a) Aircraft feels while flying close to Earth.
(b) This is due to the Air trapped between the wings and the surface of the Earth.
It is present up to an ht of a wingspan.
It is perceptible up to an ht of ½ Wingspan.
Ground effect is more in a low wing Aircraft then a high wing Aircraft effects.
- Aircraft tends to get air borne at a speed less then normal Take off speed.
- With in ground effect Aircraft will require a lower angle of attack for producing same lift as compared to outside of ground effect.
- With in ground effect wing tip vortices don’t from therefore causes less Induced Drag.
- With in ground affect the tail plane effectiveness decreases due to decrease in downwash of tail plane. Therefore Aircraft feels nose heavy and a backward pressure (Elevator up) is require on the stick. (Trim up)
- Aircraft experience difficult in climbing after Take off. Aircraft tends to settle back on ground once it is out of ground effect.
(a) Because as we move out of ground effect the induced drag increases and we require more power, that power is not available and therefore Aircraft cannot climb easily out of ground effect.
(b) Secondly outside the ground effect lift produced is less, therefore we increase angle of attack to increase lift. Speed decreases further and to maintain speed or level we need more power, but because no more extra power is available a/c sinks further.
The only way to correct is to push the a/c nose down, decrease attitude of Aircraft and fly parallel to the ground to increase airspeed and get necessary lift.
Therefore this also tells why nose is pushed forward to increase speed after Take off.
- Aircraft floats during landing. Floating is common during Take off as well as landing. Ground effect is advantage for short field Take off.
Stagnation point is the point at which airflow separates.
Pressure at stagnation point is below atmospheric pressure.
It moves forward as angle of attack decreases.
It moves backwards as angle of attack increases.
The angle of attack indicator senses the changing position of stagnation point by means of a sensing unit, installed on wing slightly below the leading edge.
As angle of attack decreases, indicator moves to left (Red) or slow zone.
As angle of attack decreases, indicator moves to right (Green) or fast zone.
A spring loaded vane moves up & down as the stagnation point changes position & relay this information to Cockpit display.
The indicator display both current stagnation & also the point at which a/c will stall.
Angle of attack indicator does not relate to airspace & can therefore give a continuous reason of margin above shall whatever the flight attitude.
Stick Fixed Stability – If Elevator (or other controls) is kept in a fixed neutral position, it increases Stability. A/c having this stability is known as Stick Fixed Stability.
Stick Free Stability – If Elevator (or other controls) is allowed to float free, they will move from their neutral position by outside disturbance & the total Stability surface will decrease.
The stability of an A/c with free floating surface is known as Stick Free Stability.
Flaps decreases stalling angle and decreases stalling speed, whereas Snow, frost & ice on aerofoil decreases stalling angle and decreases stalling speed.
FLYING CONTROLS
Primary (1) Aileron (2) Elevator (3) Rudder (4) Differential Spoilers
Principles of flying controls are increase in camber increases lift.
Aileron Drag – drag experienced by down going aileron as it goes into the area of high pressure.
Aileron drag – causes the A/c to yaw in the opposite direction of roll, it is also known as Adverse yaw.
Adverse yaw- is max at low airspeeds & high angle of attack.
(At high angle of attack there is more decrease in pressure at top & a increase in higher pressure at the bottom of the wing).
Effectiveness of control depends upon airspeed & deflection of flying controls. At low speeds greater angle deflection of aileron is required.
Therefore aileron at slow speed travels more into area of high pressure below the wing, which increases aileron drag.
To overcome Adverse yaw-
1) Differential Aileron- Up going aileron moves a higher angle than the down going aileron.
2) Frise Aileron- down going aileron produces a smooth surface at the top & bottom of the wing, but leading edge of up going aileron protrudes the undersurface of the wing, increases drag on the leading edge area of the up going wing and assists the controls deflection.
Modern A/c has a combination of (1) & (2).
Aileron Reversal- A/c rolls in the opposite direction to the intended roll. It happens at high speeds in sweep back wing A/c.
It is a high speed phenomenon & is more prominent in Sweep Back Wing A/c.
Example- you want to roll to the left, left aileron goes up, trailing edge of wing produces less lift as compared to leading edge. This less lift causes the Centre of lift to move forward, wing twists up, angle of incidence increases & angle of attack increases & lift increases, A/c rolls to the right instead of left.
To over come it we use-
(1) Differential Spoilers – Spoiler is a device which decreases lift. How much the spoiler moves depends upon deflection of ailerons.
Example- you want to turn left, left spoilers will move up. Right spoilers don’t move.
Therefore lift on left wing decreases positively & causes roll.
(2) Use of high speed & low speed ailerons.
Back Lash – Too loose control cables will cause floating of controls during flight.
Controls will lag in movement. Controls will lag in movement due to back cash.
Turn Buckles – are provided to adjust the tension of the control cables.
Too tight control cables will cause increasing the friction of the controls.
Temperature compensator is present in the control system to automatically compensate for changes in the temperature.
Aileron cables are kept loose, which will cause them to float during flight.
CONTROL BALANCING
To decrease the force required by the pilot control balancing is done.
Hinge Moment = Lift × Distance
Hinge Moment is the force required by the Pilot to move the controls.
(1) Mass balance- A mass is attached ahead of the hinge, it shifts c.g. forward but c.g. is still kept behind the hinge point.
(Control flutter – occurs at high speed when controls flutter)
Mass balance decreases control flutter.
A mass/weight is attached ahead of the hinge, to decrease force required by the pilot.
(2) Aerodynamic balance
Aerodynamic forces are used to decrease force required by pilot to move controls.
(a) Horn Balance
Horn is the part of control surfaces which projects ahead of the hinge line.
(b) nsets/Set back hinge
Hinge is taken back closer to c.g.
System of Tabs
Tabs are auxiliary aerofoils attached to the trailing edge of the wing.
(a) Fixed Tabs – are tabs that can only be adjusted on the ground, Tab moves in opposite direction to controls surface.
(b) Balance Tabs – are connected to the control column & they automatically moves in the opposite direction to the control surfaces.
These tabs are not of much use at higher speeds.
Balance tabs decreases force required by pilot to move controls.
(c) Spring tabs – are tabs that are connected to the control column.
The movement is related to the load on the stick.
If press on stick is more, tabs will move through a greater angle.
Therefore spring tabs move through a greater angle at high speeds and move through a lesser angle at the low airspeeds.
(d) Servo Tabs – are tabs that are directly connected to control columns but control surfaces are not connected to control columns.
Therefore when control column is moved, tab moves, depending upon airflow (speed) & movement of the tab moves control surfaces.
Servo Tabs are not much effective at low speeds.
Therefore they are of main use only at high speeds.
Servo Tabs during pre flight when Control Column is moved up controls surfaces don’t move, tabs move.
Spring tabs- during pre flight when Control Column is moved up control surfaces move, but tabs don’t move.
Trim Tab – has a separate trim control, it can be moved in any direction.
Anti balance Tab – provides a greater feel of controls to the pilot, they are used in overbalanced A/c to increase the force required to move controls.
This tabs move in the same direction as control surface.
Mass balance/horn balance/set back hinge decreases force required by pilot.
Servo Tab is not effective at low speeds.
Balance tab is not effective at high speeds.
Therefore Balance tabs can be said to be low speed tabs
Therefore Spring tabs can be said to be high speed tabs.
Control Balancing is the process of shifting C.G. ahead or taking hinge point backward.
Stability – is an inherent quality of A/c to return to its equilibrium position, even when the disturbing forces are removed.
Positive Stability
Neutral Stability
Unstable or Negative Stability
All three axis of A/c passes through c.g.
A/c pitches about/ around lateral axis on/ along longitudinal axis.
A/c rolls about/ around longitudinal axis on/ along lateral axis.
A/c yaws about/ around/ on/ along normal axis.
Therefore A/c roll, pitches & yaws about C.G.
For an A/c to fly it must have static stability.
Factors affecting Longitudinal Stability-
(1) Wings of A/c or movements of C.P, C.G is always kept ahead of C.P.
If nose pitches up –
C.P. moves forward, lift increases, weight remains same
This causes a nose down pitching moment & a/c returns to its original position.
Aerodynamic centre is a fixed point in the wing profile where the lift moment co-efficient remains constant & does not change with change in angle of attack.
C.G. is always kept ahead of C.P.
A.C. is generally 25 % behind the leading edge.
A.C moves forward with increase in wing thickness.
In thicker wing A.C is closer to the leading edge & vice-versa.
(2) Tail plane – is of Symmetrical shape and it gives negative lift stability depends upon-
(1) Shape of Tail plane.
(2) Size of Tail plane.
(3) Angle of incidence of Tail plane.
Long dihedral is difference of angle of incidence between Main plane & Tail plane.
Effectiveness of Tail plane depends upon position of Centre of Gravity.
Distance between Tail plane & Centre of Gravity is so high that even small changes in lift of Tail plane can cause large changes in pitching moment.
Forward C.G increases Stability Stalling speed increases.
Rearward C.G decreases Stability (Distance between Tail plane & C.G decreases) Stalling speed decreases.
Neutral point – is the point, where stability is neutral.
For Stability C.G should be ahead of Neutral point.
Static Margin – distance between C.G & Neutral point.
Greater the Static Margin, greater is the Stability in longitudinal axis.
Shock Stall- occurs at high mach no. higher than Critical Mach no due to formation of Shock waves, following will happen
1) It decreases longitudinal stability.
2) C.P moves backward
3) Tail plane effectiveness decreases
4) CL will decrease
5) A/c will get into a nose down attitude & start loosing height
6) Disturbed airflow behind the shock waves will cause high buffet on the hail plane giving an indications similar to stall (Shock stall).
LAT STABILITY
Dihedral increases Lateral Stab
Anhedral decreases lateral stability, increases control effectiveness.
High wing A/c is more laterally stable, due to Pendulum effect.
Sweep back increases lateral stability, decreases Mach no and decreases CL.
High Keel Surface is a Vertical surface of A/c behind C.G.
High Keel Surface increases lateral stability.
DIRECTIONAL STABILITY
Fin.gives sideways lift, which gives directional stability.
DYNAMIC STABILITY
Dynamic Stability is stability in oscillation.
Dynamic Stability – If amplitude of oscillations decreases with time.
Dynamics Instability – If amplitude of oscillations increases with time.
Neutral Dynamic Stability/Instability – If amplitude of oscillations remains same with the time.
DUTCH ROLL
Dutch Roll is a condition of Dynamic instability.
When A/c rolls & yaws simultaneously.
Roll is in opposite direction to yaw.
Causes
(1) Incorrect use of rudder
(2) Inadvertent lateral roll.
(3) Turbulence
Roll is more prominent then yaw.
To Correct
(1) Use aileron against the upcoming wing.
(2) Use Yaw Damper.
Dutch Roll increases with increase in altitude.
Dutch Roll occurs in A/c having large Dihedral, (More lateral Stability) & less Fin area (Less directional stability).
SPIRAL INSTABILITY
Spiral Instability occurs in A/c having larger fin area, (More directional stability) & less dihedral (less lateral stability).
Causes – Incorrect use of ailerons.
To Correct – Use rudders
Spiral Instability increases at low altitude.
OR spiral Stability increases as altitude increase.
Take off Run – is the time A/c starts rolling till the time A/c becomes airborne.
Take off Distance is the distance covered on ground from point A/c starts rolling to a point where A/c reaches a height of 35″ feet after becoming airborne.
If temperature increases, Take off Run increases & Take off distance increases.
Elevation increases, Take off Run increases & Take off distance increases.
Humidity increases, Take off Run increases & T/o distance increases.
Worst condition for Take off is
1) high temperature 2) High elevation. & 3) high humidity
Temperature increases density decreases the engine power output decreases & power required increases therefore cruising speed decreases, Fuel Consumption increases, AIR NM per gallons decreases, Range decreases.
For a A/c only one airspeed gives best L/D speed/ratio speed. Weight increases best L/D ratio speed also increases.
Whether you fly at speed above or below best L/D speed, gliding angle will be Steep.
Gliding angle depends upon L/D ratio, which depends upon airspeed.
Best glide angle is same at all weights, best L/D, but speed is lower at low weights.
Weight does not affect gliding angle it only affects apparent gliding angle.
A Head wind increases angle of climb.
A Tail wind decreases angle of climb
Stressed skin – is the skin capable of carrying some of the air loads.
Monocoque type Fuselage – has a structure, which is like a egg shell having no supporting structure inside.
Semi-Monocoque – has a circular or oval rings in the frames.
They are joined together by longitudinal members called Longerons.
Stringers are also longitudinal members of the fuselage.
Spars are fitted in the wings to take some air stress.
FUSELAGE
Strut type / TRUSS type – has internal bracing or support with the help of a wielded tube structure rectangle in shape. These structures are called TRUSS.
GUSSET is a part of TRUSS structure to increase stress capability.
Ribs are longitudinal members of the wing. They run from leading edge to the trailing edge.
They give the Aerodynamic shape to the wing.
They are held together by the SPARS & STRINGERS
Cantilever wing has no external supporting structures.
Semi-Cantilever wing has external supporting structure known as STRUTS
Full throttle height is maximum altitude at which a given boost can be maintained.
Mean Aerodynamic chord = Area in sq. feet ÷ SPAN
Aspect Ratio for Tapered wing = SPAN ÷ Mean Chord
Stabilator Tail plane – is the one that has no tail plane & is a full moving tail plane.
Vso is stalling speed in ldg configuration at the most forward C.G.
Climb Gradient- is calculated by True Air Speed and ROC, it gives Rate of Climb.
Flight Path gradient – is calculated by Ground Speed & ROC, it gives angle of climb.
If ROC changes angle of climb also changes, but if angle of climb changes ROC may or may not change.
Gliding angle – is angle of flight path to the horizontal plane.
Apparent Gliding angle – is angle of flight path to a fixed point on the Earth.
Gliding Ratio = Horizontal district traveled/Height of A/c.
It is same as L/D ratio.
E.g. If L/D ratio is 6:1.
How much the A/c will travel forward if Height lost is 1nm (1 nm =6080 feet)
As altitude increases A/c maneuverability decreases.
Because of IAS/ TAS relationship.
1) As altitude increases thrust decreases, therefore maneuverability decreases.
2) As altitude increases Mach no increases.
Sweep back wing A/c has lower Coefficient of Lift as compared to straight wing A/c.
It has lower Coefficient of Drag as compared to straight wing A/c.
Vortex generators are located on the leading edge of the wing, they decreases Coefficient of Lift towards root and increases stalling angle.
Aileron Drag is high at low speeds, therefore Adverse Yaw is more at low speeds.
A High Aspect Ratio wing will stall at a lower angle of attack, therefore induced drag is low & wing tip vortices formed are less.
A low aspect ratio wing stalls at a higher angle of attack, therefore induced drag is high & Wing Tip Vortices are high.
Within Ground effect, the downward effect decreases & tail plane effectiveness decreases and A/c feels nose heavy.
Vmc – is Minimum controllable speed with critical engine inoperative, Propeller of failed engine wind milling or feathered if equipped with Auto-feather, wing flaps & cowl flaps on T/o position, landing gear retracted, maximum Take off weight, unfavorable C.G position & a bank of of not more than 5 degrees towards the good engine.
As weight decreases, VMC increases (actual).
C.G goes forward VMC decreases
C.G. goes rearward, VMC increases
If power on live engine decreases tendency of A/c to yaw & roll towards in operative engine also decreases, lowering VMC.
If power on live engine increases tendency of A/c to yaw & roll towards in operative engine also increases, increasing VMC.
As altitude increases, power output of engine decreases, lowering VMC.
Angle of climb is governed by Thrust.
Rate of climb is governed by Power.
A sweepback wing when sideslips, the down going wing gives more lift & vice versa & the effective aspect ratio of lower wing is more. The lower wing produces more lift & A/c return to the level flight.
In multi-engine a/c if one engine fails, apply opposite rudder & a bank of 5 dig can be given towards live engine & Turn and Slip Indicator ball will go towards live engine (Showing Slip).
Blue radial line indicates best rate of climb speed with one engine inoperative (Vyse).
When angle of attack of a symmetrical shaped aerofoil is changed the C.P. does not move.
It is necessary to increase elevator back pressure to maintain altitude during a steep turn to compensate for the loss of vertical component of light.
The ground speed of A/c landing at high elevation airfields will be higher than at sea level.
If angle of attack & other factors remain constant & airspeed increases to double, the lift produced at higher speeds will be 4 times (Four times).
If A/c airspeed becomes double in level flight the parasite Drag will become 4 times (Four times).
Light ½ & V2 SCL Speed Double 4 — 8
42 = 16
82 = 64
Therefore Lift will increase 4 times.
A/c flying above Critical Mach no experiences a Nose down pitch moment.
Humidity within the passenger cabin should be between 30%-70%.
Minimum radius of turn can be accomplished by best climb IAS.
Whenever ROC changes, AOC also changes.
Whenever AOC changes, ROC may or may not change.
Reynold’s numbers are the number given to the Aerofoil.
Generally higher the Reynold’s number, the greater the tendency of the Aerofoil to resist the Boundary layer separation.
Therefore, if Reynold’s number increases, Boundary layer separation delays & Coefficient of Lift maximum increases.
A sharp nose of A/c leading edge of wing (e.g. with small radius of curvature) may cause separation & a low value of CL max.
PHUGOID IS usually a long period, poorly damped motion involving large variation in the speed & height of a/c, but with negligible changes in load factor.
Fail operational – means a system in which one failure (or sometimes more) can occur, but leaves the overall system still functioning & without causing degradation of performance beyond the limits requirement for automatic landing & roll out (IN Automatic landing sequence system). This is also known as Fail Active & Fail Survival.
Fail – Passive /Fail soft means the ability of a system to withstand a failure without endangering passenger safety, & without excessive deviations from the flight path.
In the Area of Reverse Command to maintain a lower airspeed you need a greater power but when you increase the airspeed again you need more power to maintain a higher airspeed.