Transcript Slide 1 - 663 VGS

CGS Ground School
Principles Of Flight
Manoeuvres
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No Part of this presentation may be reproduced without the permission of the issuing authority.
The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.
Centripetal force
If a tennis ball is attached by a
string to a post and then swung
around, the string will pull tight.
The weight of the ball is pulling the
string tight, if the string was cut,
the ball would fly off on a tangent.
If the string remains intact, it
applies a pulling force to the ball
causing it to follow a circular path.
This pulling force by the string can
be considered as centripetal force.
For an aircraft in a turn, part of the
lifting force (when in a banked
attitude) acts as the string “pulling”
the aircraft around the centre of
the turn.
Load factor
As an aircraft turns it experiences ‘g’ force,
this increases the relative weight of the
aircraft.
This apparent increase in weight is known as
the load factor.
As the angle of bank increases to tighten the
turn, the load factor increases, this then
increases the stalling speed.
Consider the following example:
An aircraft which normally stalls at 40kt flying
at varying angles of bank (AoB).
Approx
AoB
30°
45°
60°
75°
Load
factor
1.2
1.35
2
3.86
40 x √1.2
= 43.8
40 x √1.35
= 46.5
40 x √2
= 56.6
40 x √3.86
= 78.6
Stalling
speed
If the speed is reduced but the load factor
increased, the aircraft will ‘g’ stall.
Looping forces
When an aircraft is in flight the weight is
always acting vertically downwards, as
the aircraft rolls or pitches the angle that
the lift acts to the aircraft remains the
same, however the angle between the
lift and the weight is constantly
changing.
Consider the following example of an
aircraft doing a loop around a constant
radius:
During the entry it pulls 4g, this is made
up of a 3g pull and the 1g of weight
acting vertically downwards and
opposite the pull.
At a 90° nose up attitude the g force
would reduce to 3g. This is purely due
to just a 3g pull. The 1g of weight does
not affect the vertical g force felt by the
aircraft and pilot.
3g
3g
1g
1g
Looping forces
When an aircraft is in flight the weight is
always acting vertically downwards, as
the aircraft rolls or pitches the angle that
the lift acts to the aircraft remains the
same, however the angle between the
lift and the weight is constantly
changing.
Consider the following example of an
aircraft doing a loop around a constant
radius:
When inverted, it will be 2g. This is due
to the 3g pull and the 1g of weight
acting in the same direction.
When in a 90° nose down attitude the g
force would increase to 3g. This is
purely due to just a 3g pull as when 90°
nose up.
1g
3g
1g
3g
3g
1g
Looping forces
When an aircraft is in flight the weight is
always acting vertically downwards, as
the aircraft rolls or pitches the angle that
the lift acts to the aircraft remains the
same, however the angle between the
lift and the weight is constantly
changing.
Consider the following example of an
aircraft doing a loop around a constant
radius:
During the recovery from the loop the
lifting force once again starts to oppose
the weight, as such the pilot must be
aware of the increased g force at the
bottom of the loop and the tendency for
the aircraft to accelerate due to the
extreme nose down attitude.
This combination of increased speed
and g force could lead to overstressing!
3g
3g
1g
1g
Manoeuvre envelopes
Load factor (g)
The previous slides showed
Viking T Mk 1
that if mishandled the aircraft
could stall at a much higher
6
speed than normal stalling
5
speed and could also be
4
3
overstressed if flown too
2
quickly and/or too much g
1
force pulled.
0
20 40 60 80 100 120
When an aircraft is designed
-1
it is given a manoeuvre
-2
-3
envelope which shows pilots
-4
a number of different
limitations.
The manoeuvre envelope is a graph showing load factor
against airspeed.
A line is drawn in to represent the stalling speed at various
load factors.
Then a line to represent the VNE (velocity never exceed).
And lines to represent maximum and minimum g force.
140 160
Airspeed (kt)
Manoeuvre envelopes
Load factor (g)
A line showing maximum
Viking T Mk 1
manoeuvring speed (VMAN)
can then be drawn on.
6
The green shaded areas
5
mean that full control
4
3
deflection may be used.
2
The orange shaded areas
1
mean no more than ⅓ control
0
20 40 60 80 100 120
deflection may be used.
-1
The operational g limits for
-2
-3
the Viking are significantly
-4
lower to reduce airframe
fatigue.
Therefore the area shaded in yellow is where flight is allowed
as per the aircraft’s RTS (Release To Service).
The area shaded in pink is impossible to fly in due to the
aircraft’s stalling speed.
The area in red is where structural damage will occur.
140 160
Airspeed (kt)
Manoeuvre envelopes
Load factor (g)
The manoeuvre envelope for
(850kg-908kg)
Vigilant T Mk 1 (AUM
up to 850kg)
the Vigilant is slightly simpler
due to the aircraft not being
6
permitted to fly below 0g.
5
The Vigilant has two
4
3
envelopes depending on
2
which weight band you are in.
1
This example is for an all up
0
20 40 60 80 100 120 140 160
mass (AUM) up to 850kg.
-1
Airspeed (kt)
If the aircraft weight is
-2
-3
increased in to the higher
-4
weight band (850kg – 908kg)
the manoeuvre envelope
becomes more restrictive.
The maximum g force reduces as does VMAN and VNE.
Airbrakes
The manoeuvre envelopes previously described are
for Vikings and Vigilants with the airbrakes closed.
In normal flight the lift is evenly distributed across the
whole span of the wing.
When the airbrakes are extended the lift distribution
changes which increases the stress on the outboard
section of the wing.
The Vigilant and Viking airbrakes are not speed limited
but for the reason shown below it is good airmanship
to use the airbrakes very carefully and expose the
airframe to as little fatigue as possible.
Winch launching
In normal flight the lift is distributed fairly evenly over
the wing.
When compared to the weight of various parts of the
aircraft, we can see that the weight distribution almost
mirrors the lift distribution meaning that each part of
the aircraft is lifting its own weight.
During the winch
launch the weight of
the winch cable acts
through the fuselage
which dramatically
alters the weight
distribution.
The pull of the winch also acts through
the fuselage, further distorting the
weight distribution.
Winch launching
As the pilot pulls to overcome the weight and pull of
the cable the angle of attack increases which
increases the amount of lift.
The lift increases over all parts of the wing meaning
that the distribution is still
fairly even.
The outboard section
of the wings is now
producing a
disproportionate
amount of lift for the
weight it is carrying
causing the wings to
bend upwards.
Due to the extra stresses and strains on
the airframe, VNE is reduced to 65kt
during the winch launch.
Flutter
We have already seen that flying too fast can
structurally damage an aircraft.
Another way that an aircraft structure can be damaged
is known as flutter.
Flutter is a condition where the trailing edge control
surface oscillates either side of its neutral position.
Although an aerofoil appears rigid, during flight it
bends and twists. This flexing of the structure is
known as aero-elasticity which can be transmitted
rearwards towards the control surface.
Flutter
If the control C of G is rearward of the hinge line, the
bending and twisting will create a control deflection
despite no control input.
This then creates bending and flexing in the opposite
sense which in turn deflects the control surface in the
opposite direction.
If left unchecked, the oscillations would continue and
quickly increase in amplitude leading to structural
failure.
This condition could also be caused/aggravated by an
aircraft having slack control cables.
THE END
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