lesson 5 - building on the basics of lift - theory of flight

7/29/2019 Lesson 5 - Building on the Basics of Lift - Theory of Flight

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Basic Aerodynamics

& Theory of Flight

By

Ahmad Ahsan


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Lift - Revision According to Newtons Laws, Lift is the reaction that results from the

action of forcing air downward by the airfoil(Jeppesen)

It is the upwards acting force on an airfoil

NASA has established through experimentation that pressure on lower

surface of wing is higher than upper surface (hence pressure difference

exists)

Lift results from the higher pressure below the wings surface and lower

pressure above the wings surface.

Lift acts perpendicular to the relative wind


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Lift Production Magnus Effect Discovered by Heinrich Magnus in 1852

The effect was already observed by Newton in 1672.

The Magnus effect states that any object rotated in a fluid to produce a

vortex or circulation, will generate lift when placed in a stream of air.

It is possible to generate a very large amount of lift by using a rotating

Cylinder

But the mechanical complexities of such a system normally outweigh any

potential advantages.


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By

Ahmad Ahsan


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Relation to theory? It is often incorrectly thought, that the resultant force due to the pressure

acts at right angles to the plane of the wing.

However it has been observed that in addition to a perpendicular or normal

force component N, produced by the difference in pressure between upper

and lower surfaces, there is also a tangential component T, produced as a

consequence of the low pressure acting on the leading edge.

This ties in with theory which indicates

that as long as the flow follows the shape

of the aerofoil, the lift force should be at

right angles to the direction of the main

air stream, and not at right angles to the

plane of the wing.


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By

Ahmad Ahsan


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Stagnation The air following the dividing streamline slows down as it approaches the

wing.

If the wing is not swept, the air actually stops instantaneously on the

surface before dividing.

Because the particles are momentarily stagnant at this position, it is

known as a stagnation point


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Stagnation If the wing is swept, then only the component of flow at right angles to the

wing leading edge is stopped.

Stagnation pressure is the pressure at a stagnation position; a position

where there is no relative motion between the air and the surface.


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Variation of lift with AoA and camber The angle at which no lift is generated is known as the zero-lift angle.


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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 of

attack of the aircraft. The angle of attack must be gradually reduced as the

flight 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.


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Stall A stall occurs when the smooth airflow over the airplanes 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


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Stall


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Stall


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Stall


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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.


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Other Methods Conical Lift Generation (Concorde)

Using engine thrust (Harrier

VTOL)

Rotary Wings (Helicopter)

Autogyro (Ultralight)


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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.


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The 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. > Reduce

Bernoulli


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By

Ahmad Ahsan


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Span wise flow


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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


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The End

lesson 5 - building on the basics of lift - theory of flight

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