FIXED WING AIRCARFTS

AIRCRAFT DESIGN

- BLESSON VARGHESE

BASIC PARTS OF AIRCRAFT

- IMAGE BY NASA

FIXED WING AIRCRAFTS

• The basic forces acting on an aircraft are : Lift Weight Thrust Drag

AXIS SYSTEM IN AIRCRAFTThe basic axes of an aircraft are shown below:• Pitch axis• Roll axis• Roll axis

WING TERMINOLOGY

-IMAGE BY NASA

SOME BASIC WING SHAPES

-IMAGE BY NASA

LIFT vs ANGLE OF ATTACK• The curve shows the variation of lift with AOA for

different Aspect ratios(AR) of wing.

Airfoil• The shape of cross section of the wing of an aircraft is known

as airfoil or aerofoil.

• The amount of lift generated by the wing depends on the shape of the airfoil.

• The different types of airfoils and their uses are as follows: 1. Flat bottom section This type of wing has high lift and is common in trainer planes.

2. Fully symmetrical This is ideal for aerobatic type of aircrafts. Most of the lift is generated by the

angle of incidence of wing to the flight path.

3. Semi-symmetrical It lies between flat bottom and fully symmetrical

type with less drag. It is used in sport type aircraft. 4. Under – Cambered This type airfoil produces great lift even at low

speeds , but generates drag at high speed.

LIFT vs DRAG• Lift can be increased by increasing the angle of

attack(AOA) ,but at the same time ,the drag also tends to increase.

• The variations of lift and drag with AOA is shown in below.

STALL• When the angle of attack is increased over a limit, the lift falls

suddenly to a very low value due to excessive drag as shown in the curve above. This phenomenon is known as stall.

• Because of this lift can be increased by raising AOA only till a limit. Hence it is necessary to look on some special device to generate more lift. We will consider this later.

LIFT vs ANGLE OF ATTACK• The curve below shows the variation of lift with AOA

for different types of airfoils.

• From the curve above it is clear that AOA can be increased only up to a certain limit. Further increase in AOA results in stall.

• Therefore lift is increased using a set of special devices called high lift devices . This process is known as lift augmentations.

eg: - Flaps - Slats - Slots - Vortex generators - Winglets

AIM-TO INCTREASE PAYLOAD FRACTION• Payload fraction of an aircraft is defined as the ratio

of weight of the payload carried to the weight of the aircraft.

• In order to increase the payload fraction , the lift has to increased considerably, drag has to be reduced and thrust has to be increased.

• Use of T-tail stabilizers provides maneuverability at low speed.

• A small dihedral on the wing can increase roll stability of the aircraft. T-tail in Globe master carrier aircraft

TECHNIQUES TO INCREASE LIFT AT LOW SPEED

Airfoil• Rounded leading edge.• Sharp trailing edge.• Thickness to chord ratio(T/C) should be high.• Thickness must be well forward.• Use of cambered airfoil provides lift at low speed.

Note: At high speed cambered wing generates more drag than flat bottomed airfoil.

Wing configuration• To produce more lift at low speed a rectangular wing

configuration with more span is required.• Use of sweep back wing can reduce lift at low speed. • Reduction in chord length may reduce drag.

Position of wing• If the position of wing root aligns with the thrust line, the drag

on wings will be the least.

Stability• Adding a small dihedral angle to the wing improves

the roll stability. Note: Increase in dihedral angle tends to reduce lift.• Positioning wing above CG provides positive stability

to the aircraft. E.g : shoulder wing , high wing. • A T- tail stabilizer improves the stability and control

at low speed since it is away from the path of flow of disturbed air from the wings.

EFFECT OF CG POSITIONS • There are mainly three CG configurations : - Forward CG - Aft CG - Backward CG

1.Forward CG:• In this configuration the CG is set in front of the normal point

(lift line). It is also called as nose heavy configuration.• As the nose in this aircraft is heavier, a counter force should act on the tail, downward to compensate it. Therefore these aircrafts usually have inverted tail.

• The aircrafts with forward CG have high pitch stability.• Forward CG is most commonly used in low speed

aircrafts since the drag on tail increases at high speed, due to increase in downward force on the tail.

• They can easily recover from stalls.• Smooth landing is difficult due to heavy nose (pancake

landing).• It is generally used in sport type aircraft and less

commonly used in passenger aircraft.• The tail plane does not produce any lift , since they are

used in inverted mode, generate downward force.• The amount of tail force at different speeds are shown

below:

The force diagram of a forward CG stability is shown :

LIFT DUE TO WING

CG

WEIGHT OF NOSE DOWNWARD TAIL FORCE

2. AFT CG:• As the weight acts in-between the wing and the tail plane, the

tail has to generate an upward lift in order to support the weight• Aircrafts with aft CG(CG behind the wing) are generally unstable.• These aircrafts can fly at higher speeds, as the drag on the tail is

the least.• The stall speed is relatively less .• Hence they can land smoothly at very low speed.• But this type of aircrafts are generally unstable.• Once they start stalling , it is impossible to recover and the

aircraft may ultimately end in a crash.• Aft CG configuration is commonly used in passenger flights due

to their high speed and other useful characteristics. • The figure below shows the aft CG positions in an aircraft.

CG

CG

Aft CG stability force diagram: LIFT DUE TO WING LIFT DUE TO TAIL PLANE

NET AIRCRAFT WEIGHT

• Spin Spin in an aircraft is influenced by the position of

centre of gravity(C.G.). If C.G. is well forward, less readily the aircraft will spin

and more readily it recovers form a spin. If C.G. is aft the body , then the aircraft fails to regain

from a spin induced , i.e. aircraft is said to have a negative dynamic stability.

Roll – Yaw • If the wing has dihedral , then there is a coupling

between roll and yaw , i.e ailerons not only causes roll but also yaw the aircraft in the direction of roll.

• If position of C.G. is well forward , then an induced roll can cause a yaw in the corresponding direction, i.e the nose yaws in the respective direction.

• Therefore the optimized condition is: LIFT

DRAG THRUST

WEIGHT

• FLAPS Flap is a device that is attached at the trailing edge of

the wing, that can be deployed when needed. They are used during take-off and landing to generate excessive lift at low speed. When deployed they protrude out of the trailing edge , increasing the surface area and camber of the wing .

Note: During take-off flap inclination (0-25) degree is used generally where lift is greater than drag. During landing flap inclination >25 degrees is employed produce more drag compared to lift.

HIGH LIFT DEVICES

SLATS

Like flaps , slats are also the devices fitted on the wings of an aircraft to enhance lift. But unlike flaps these are fitted on the leading edge of the wings. Slats may be either fixed or deployable as to meet the requirements. Usually in slow speed aircrafts

slats are fixed , but in high speed aircrafts slats are deployable as they contribute to drag at high speed.

Slats when deployed increases the camber of the wing and thus increases the maximum angle of attack of the wing. When slats are extended they create a slot in-between, the

combined effect of both reduce the stall speed of the aircraft.• Wing slats of some commercial aircrafts are shown below:

SLOTS Slot is an air gap between slat and the wing produced due to

the extension of slat. Latest research has found that wing slot has many advantages.

Some of them ate listed below:• Increases maximum angle attack• Reduce in stall speed• Short take-off runway• Steep landing• High stability at low speed• More maneuverable

• Slat and slot in a Cessna plane

• Some aircrafts have slots throughout the wing span while some others have partial slots on the outboard wing portion. This ensures that outboard portion of the wing remains unstalled at higher angles of attack than the inboard portion of the wing. This contributes to docile stall behavior and maintains aileron controls throughout stall.

WING WASHOUT

• It is nothing but increasing the angle of attack of wing root over that of the tip. This provides stability and to the wing tip , even when the inboard portion of the wing stalls.

• It is achieved by: Reducing AOA at wing tip. Reducing chord length at tip. Using vortex-generators.

WINGLETS• Winglets are the devices fixed at the wing tips to increase the

lift generated at tips. Winglets have opposite effects to washout. Hence they are also known as wash-in.

WINGLETS

• Installing winglets may cause spinning effect, hence to compensate the effect more washout is required.

VORTEX GENERATOR

• Vortex –generators are small rectangular or triangular sections attached to the leading edge of the wing surfaces to overcome the problem of boundary layer separation at high at high angle of attack.

• Benefits : - Increases maximum take-off and landing weight. - Shorter take-off run. - Lower stall speed. - Lower approach speed. - Higher angle of attack. - Increased stability at low speed. - Gentle stall characteristics.

-Increased rate of climb. - More effective control. - Easy and quick installation. VORTEX GENERATOR

WING DROOPING• Wing drooping is a technique where the entire

leading edge of the wing rotates downwards. Advantages: - Increase in camber providing high lift. - Reduction in stall speed.

FLAP

WING DROOP

GALLERY

A glider with long wings to generate sufficient lift

Winglet of a commercial aircraft

Leading edge root extension Flaps and slats deployed on landing

T-tail and Winglets to increase stability Nose drooping in a Concorde

A nitro engine used to propel RC plane

Sport aircraft at air show

THE END

- BY BLESSON