1 - Aerodynamics Flashcards
Forces/Aerofoil
What are the forces acting on an aircraft in flight?
Drag, thrust, lift, and weight.
When thrust and drag are in equilibrium, an aircraft will maintain
a steady speed.
For an aircraft to accelerate, thrust must exceed the
value of drag. vVhen lift and weight are in equilibrium, an aircraft will
maintain a steady, level attitude.
For an aircraft to climb, lift must exceed the weight of the aircraft.
In a banked turn, weight is a constant, but lift is lost due to the effective reduction in wing span. Therefore, to maintain altitude in a
banked turn, the lift value needs to be restored by increasing speed and/or the angle of attack.
Forces/Aerofoil
What produces the maximum glide range?
A maximum lift-drag ratio.
Obtained by the aircraft being flown at its
optimal angle of attack and corresponding minimum drag speed (VIMD) , produces an aircraft’s maximum glide range.
Forces/Aerofoil
What is the effect of weight on the glide range?
The glide range does not vary with weight.
Because the glide range is proportional to the lift-drag ratio. Which does not vary with weight.
Provided that the aircraft
is flown at its optimal angle of attack and speed for that weight.
Therefore, if a heavy aircraft were flown at the correct angle of attack
and speed, it would glide the same distance as a lighter aircraft.
However, the heavier aircraft would have a higher airspeed than the lighter aircraft, and therefore, although it would glide the same distance,it would take less time to do so.
Forces/Aerofoil
What is rate of climb/descent?
Rate of climb/descent is the vertical component of the velocity of an aircraft.
Also determines the time it will take to either climb or descend from a given height.
It is normally expressed in terms of feet per minute.
Forces/Aerofoil
What is the effect of weight on rate of descent?
The heavier the aircraft, the greater its rate of descent.
This is so because a heavy aircraft will fly at a higher airspeed for a given angle of attack, and so its rate of descent will be increased.
(See Qs: How does weight
affect an aircraft’s flight profile descent point? page 18; Why does an
aircraft descend quicker when it is lighter? page 283.)
Forces/Aerofoil
What is an aerofoil?
An aerofoil is a body that gives a large lift force compared with its drag when set at a small angle of attack to a moving air stream.
e.g., aircraft wings, tail planes, rudders, and propellers.
Forces/Aerofoil
What is an aerofoil chord line?
The chord line is a straight line from the leading edge to the trailing edge of an aerofoil.
Forces/Aerofoil
What is the mean chord line?
The mean chord line is the wing area divided by the wing span.
(sometimes referred to as the standard mean chord).
Forces/Aerofoil
What is the mean chamber line?
The mean chamber line is a line from the leading edge to the trailing edge equidistante on the upper and lower surfaces of an aerofoil.
Forces/Aerofoil
What is the angle of incidence?
The angle of incidence is the angle between the aerofoil’s chord line and the aircraft’s longitudinal datum.
It is a fixed angle for a wing but may be variable for a tailplane.
(It is sometimes called rigging incidence.)
Forces/Aerofoil
What is angle of attack?
Angle of attack is the angle between the chord line of an aerofoil and the relative airflow.
Forces/Aerofoil
What is washout on a wing?
Washout is a decrease in the angle of incidence from the wing root to the tip.
This compensates for the early stall due to the higher levels of loading experienced at the wing tips.
Forces/Aerofoil
What is dihedral?
Dihedral is the upward inclination of a wing from the root to the tip.
Forces/Aerofoil
What is anhedral?
Anhedral is the downward inclination of a wing from the root to the tip.
Lift
What is lift?
Lift is the phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil.
Note: An aerofoil is cambered on its topside and flat on its bottom side.
Therefore, the airflow over the top of the aerofoil has to travel further and thus faster than the airflow below the aerofoil.
This causes the pressure below the aerofoil to be greater than above, creating a pressure difference, which results in an upward lift force.
Lift
What if the formula for lift?
1/2R + V2 + S + CL
1/2R = half the value of the air density V2 = airflow velocity squared S = wing span area CL = coefficient of lift
The combined values of these properties determine the amount of lift
produced.
Lift
What is coefficient of lift (CL)?
Coefficient of lift (CL ) is the lifting ability of a particular wing.
It depends on both the shape of the wing section (fixed design feature) and the angle of attack.
Lift
Describe center of pressure.
The center of pressure is represented as a single point acting on the wing chord line at a right angle to the relative airflow, through which the wing’s lifting force is produced.
The position of the center of pressure is not a fixed point but depends on the distribution of pressure along the chord, which itself depends on the angle of attack.
Thus, for a greater angle of attack, the point of highest suction (highest air pressure value) moves toward the leading edge.
The distribution of pressure and center of pressure point thus will be further forward the higher the angle of attack and further aft the lower the angle of attack.
Lift
Describe the lift-weight pitching moments.
If the forces of lift and weight are not acting through the same point (line), then they will set up a moment causing either a nose-up or nose-down
pitch depending on whether the lift is acting in front of or behind the center of gravity point.
Note: A center of gravity forward of the center of pressure has a nose-down pitching moment.
A center of gravity aft of the center of pressure has
a nose-up pitching moment.
The center of pressure moves if the angle of attack changes, and the center of gravity moves as the weight changes (mainly due to fuel being
used).
Therefore, their positions will vary during a flight.
Lift
Describe aspect ratio.
Aspect ratio is the ratio of the wing’s span to its geometric chord, e.g., 4:1.
High aspect ratio = high lift (gliders)
Low aspect ratio = lower lift but capable of higher speeds
Lift
During what phase of flight is lift the greatest?
In general, the takeoff.
Note: Lift is caused by a pressure difference above and below the wing, and the size of the difference determines the amount of lift produced.
(See Q: What is lift? page 3.)
The difference in pressure experienced is affected by the functions of
lift, which are
1. Configuration (flap setting)
2. Speed of airflow over the wing
3. Angle of attack (which is optimized during the takeoff stage offlight)
plus
4. Air density
Lift
What is direct lift control?
The elevator/stabilizer provides the direct lift control.
The elevator and stabiliser are aerofoils that by their positions create an upward or downward balancing force that controls the direct lift force from the main aerofoils (wings), thus determining the attitude of the aircraft around the lateral axis.
Lift
What are high lift devices?
The following devices increase the lift force produced by the wings:
- Trailing edge flaps (Fowler flaps) increase lift at lower angles of deflection.
- Leading edge flaps (Krueger flaps) and slats increase lift by creating a longer wing chord line, chamber, and area.
- Slots (boundary layer control) prevent/delays the separation of the airflow boundary layer and therefore produce an increase in the coefficient of lift maximum.
Drag
What is drag?
Drag is the resistance to motion of an object (aircraft) through the air.
There are 2 types of Drag.
Profile + Induced drag = Total Drag