Basic Aerodynamics& Theory of Flight

ByAhmad Ahsan

•“Any surface, such as a wing, propeller, rudder, or even a trim tab, which provides aerodynamic force when it interacts with a moving stream of air.” FAA•The mean camber line is a line drawn midway between the upper and lower surfaces •The chord line is a straight line connecting the leading and trailing edges of the airfoil.

The Airfoil

The angle of attack is the angle between the chord line and the average relative wind.

Greater angle of attack creates more lift (up to a point).

Angle of Attack

Four Forces of Flight

DRAG

WEIGHT

THRUST

LIFT

Variation of lift• In steady level flight, the lift force must always be equal and opposite to

the aircraft weight. • In landing and take-off where the speed is low, a large CL value is required.• As the flight speed increases, the lift coefficient required reduces.• The pilot controls the lift coefficient value primarily by altering the angle ofattack of the aircraft. The angle of attack must be gradually reduced as theflight speed increases.• Most aircraft are designed to fly in a near level attitude at cruise, and must

therefore adopt a nose-up attitude on landing and take-off.• In modern aircraft, the high lift coefficient required for landing is normally

produced by means of flaps which increases the camber and area of wing.• Flaps allow for approach at higher angle of attack with lower speed.

Stall• A stall occurs when the smooth airflow over the airplane’s wing is

disrupted, and the lift degenerates rapidly.• This is caused when the wing exceeds its critical angle of attack. • Stall can occur at any airspeed, in any attitude, with any power setting.• Most stalls result in some loss of altitude during recovery.• At a certain point, the lift starts to fall off. This effect is known as stalling.• The fall-off of lift may occur quite sharply and quickly• A sudden loss in lift can obviously have disastrous consequences • The stalling characteristics of an aircraft wing depend not only on the

aerofoil section shape, but also on the wing geometry• The resultant upward force (L) at increasing angles of attack acts more or

less at right angles to the surface, so drag is produced.• Stall affects controllability of the aircraft. > Combat Aircraft / Missiles

Stall

Stall

Stall

Flow Separation• Said to occur when the airflow around the airfoil is no longer streamlined

but turbulent and separates resulting in reduced lift.• The main difficulty of flight in separated flow is one of stability and control.• The lift, drag, and most importantly, the position of the centre of lift, all

vary rapidly. • To overcome this problem, the aircraft may need artificial stability in the

form of automatic control system. • Recent combat aircraft have demonstrated controlled flight at angles of

attack of more than 70°.• Even though it may be possible to control the aircraft in the stalled

condition, the instability of the separated flow may still cause structural• problems due to excessive buffeting.

Other Methods• Conical Lift Generation (Concorde)• On aircraft with straight un-swept wings, flow separation results in a poor

ratio of lift to drag. • If the wings are swept back at a sharp angle, the separated flow will roll up

into a pair of stable cone-shaped vortices. • This type of separated vortex flow represents an alternative method of lift

generation. • The air speed in the vortex is high,

and so the pressure is low. Thus, lift is produced because the low pressure on the upper surface is now produced mainly due to the vortex motion above it.

Other Methods• Using engine thrust (Harrier , F35 JSF – VTOL, STOVL)• Terms VTOL, STOL• Gas turbine engines are capable of producing a maximum thrust of more

than twenty times their own weight. • It is therefore possible to dispense with wing generated lift, and use engine

thrust instead, by directing the jet downwards.• Intermediate nozzle positions can be used for STOL.

> Advantage : Flexibility Of usage in different environments

> Disadvantage:Using engine thrust to produce lift is extremely inefficient, as it requires fifteen to twenty times more thrust than needed for conventional aircraft.

> no need of runways or proper airfield in wartime

> In vertical motion, hover and transition stages, the aircraft cannot be stabilized or controlled fully.

> High thrust means faster speed and easier maneuverability using thrust aid from the nozzle

> The aircraft is very vulnerable tofailures in the propulsion and stability system during vertical flight

Other Methods• Rotary Wings (Helicopter)• Helicopter rotor blades are long

rotating wings. The blades are mounted on an engine-driven shaft.

• As they move through the air, they generate lift in the same way as a fixedwing. • The advantage over a fixed-wing aircraft is that the rest of the aircraft does not need to move relative to the air, and it can therefore hover. Rotor blades

may be hinged within limits.• Autogyro (Ultralight)

Other Methods• Rotary Wings (Helicopter)• Helicopter rotor blades are long

rotating wings. The blades are mounted on an engine-driven shaft.

• As they move through the air, they generate lift in the same way as a fixedwing. • The advantage over a fixed-wing aircraft is that the rest of the aircraft does not need to move relative to the air, and it can therefore hover. Rotor blades

may be hinged within limits.• Autogyro (Ultralight)

ByAhmad Ahsan

The Wings

The Wings• The ratio of the overall wing span (length) to the average chord (width) is

known as its aspect ratio.• Simple experiments confirmed that high aspect ratio wings produced a

better ratio of lift to drag than short ones for flight at subsonic speeds.

Lift Generation by Wings• The wing produces a circulatory effect; behaving like a vortex• English engineer F. W. Lanchester reasoned that if a wing or lifting surface

acts like a vortex • A theory of vortex behaviour indicated that a vortex could only persist if it

either terminated in a wall at each end, or formed a closed ring• More lift =

strong vortices• Danger behind

large aircraft • Turbulence• Flow downward and

outward. • Bernoulli’s

Lift Generation by Wings• From Bernoulli’s equation, we can relate pressure and speed of the air. • The air speed in the centre of the vortex is high and the pressure is low. • The low pressure at the centre is accompanied by a low temperature. • This causes any water vapour in the air tends to condense and become

visible in the centre of the trailing vortex lines, • The vapour trails frequently seen behind high-flying aircraft are normally

formed by condensation of the water vapour from the engine exhausts, and

The Wings• Below the wing, pressure is higher than the surrounding atmosphere, so

the air flows outwards towards the tips. • On the upper surface, the pressure is low, and the air flows inwards.• This results in a twisting motion in the air as it leaves the trailing edge.• Near each wing tip, the air forms into a well defined concentrated vortex,

but a rotational tendency or vorticity occurs all along the trailing edge.• Further downstream, all of the vorticity collects into the pair of

concentrated trailing vortices.• Wingtip vortices are circular patterns of rotating air left behind a wing as it

generates lift

Vortices• Wingtip vortices are circular patterns of rotating air left behind a wing as

it generates lift• The wing's main purpose in life is to produce a pressure difference

between the top and bottom surfaces. The contours of the wing force air to accelerate over the top surface, dropping pressure relative to the bottom, and providing a net upward force on the airplane, allowing it to fly.

• At the wingtips, high-pressure air on the bottom spills over to the top surface, swirling around in a horizontal vortex at each wingtip. The vortex influences the air travelling over to the wing, pushing it down and reducing the lift.

Vortices• Suppose that the tip of the wing curved up for the last few feet

instead. There would still be some pressure difference between the outboard and inboard sides of the wingtip (or winglet), but since the vertical section itself isn't producing lift, it would be less than in the winglet-free case.

• Wingtip vorticies are less intense and further away from the main, lifting, section of the wing when winglets are present, boosting wing lift and allowing an airplane to carry more payload further for the same size wings.

• the airplane now has to carry two surfaces that weigh something and add some drag. The optimum size winglet is that which properly balances the drag reduction frommoving tip vortices away from the wings with the drag increase from the extra surface area and the fuel penalty.

Vorticity and Horseshoe System• The wing-bound vortex, together with the trailing vortices, form a kind of horseshoe shape, and this is sometimes called the horseshoe vortex system. It

forms three sides of the predicted closed ring. The circuit is completed by the starting vortex.

• A strong starting vortex is formed when the aircraft rotates at take-off.• More vorticity is produced and left behind

when the aircraft produces an increase in wing circulation.

• An additional starting vortex is formed, when an aircraft starts to pull out of a dive.

• The counterpart of starting vorticity is stopping vorticity, which rotates opposite and is shed every time the circulation is reduced,landing.

The End