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