Aerodynamics Flashcards
What is the effect of weight on the glide range?
The glide range does not vary with weight, provided that the aircraft is flown at its optimal angle of attack and speed for that weight, because the glide range is proportional to the lift-drag ratio, which does not vary with 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.
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 to a moving airstream, e.g., aircraft wings, tailplanes, rudders, and propellers.
What is an aerofoil chord line?
The chord line is a straight line from the leading edge to the trailing edge of an aerofoil.
What is the mean chamberline?
The mean chamberline is a line from the leading edge to the trailing edge of equidistance on the upper and lower surfaces of an 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.
What is angle of attack?
Angle of attack is the angle between the chord line of an aerofoil and the relative airflow.
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.
What is dihedral?
Dihedral is the upward inclination of a wing from the root to the tip.
What is anhedral?
Anhedral is the downward inclination of a wing from the root to the tip.
How is lift produced?
Lift is the phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil.
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 farther 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.
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.
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.
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/delay the separation of the airflow boundary layer and therefore produce an increase in the coefficient of lift maximum.
What is drag and define the two major types of drag and their speed relationship?
Drag is the resistance to motion of an object (aircraft) through the air.
Profile(parasite) and induced drag = total drag.
Profile drag increases directly with speed because the faster an aircraft moves through the air the greater these molecules that resist the motion of the aircraft through the air.
Induced drag is caused by creating lift with a high angle of attack that exposes more of the
aircraft’s surface to the relative airflow and is associated with wing-tip vortices. A function of lift is speed, and therefore, induced drag is indirectly related to speed, or rather the lack of speed. Thus induced drag is greatest at lower speeds due to the high angles of attack required to maintain the necessary lift. Induced drag reduces as speed increases because the lower angles of incidence associated with higher speeds create smaller wing-tip trailing vortices that have a lower value of energy loss.
Minimum drag speed (VIMD) is the speed at which induced and profile drag values are equal. It is also the speed that has the lowest total drag penalty.
Therefore, this speed also represents the best lift-drag ratio (best aerodynamic efficiency) that will provide the maximum endurance of the aircraft.
Describe the drag curve on a jet aircraft.
The drag curve on a jet aircraft is the same as for a piston aircraft in that it is comprised of induced drag, profile drag, and a VIMD speed, but its speed-to-drag relationship is different. This is so because the jet aircraft has swept wings, which are designed to achieve high cruise speeds, but as a consequence has poorer lift capabilities, especially at low speeds. Therefore, because profile drag is a function of speed and induced drag is proportional to lift, the drag values against speed are different on a jet/swept-winged aircraft.
The three main differences are
1. Flatter total drag curve because
a. Profile drag is reduced, especially against higher speeds.
b. Induced drag is reduced (flatter drag curve) because the swept wing has very poor lift qualities, especially at low speeds.
These factors combined give rise to a smaller total drag range against speed, which results in a flatter total drag curve.
2. The second difference is a consequence of the first because of the relative flatness of the drag curve, especially around VIMD. The jet aircraft does not produce any noticeable changes in flying qualities other than a vague lack of speed stability, unlike the piston-engined aircraft, in which there is a marked speed-drag difference. (Speed is unstable below VIMD, where an increase in thrust has a greater drag penalty for speed gained, thus with a net result of losing speed for a given increase in thrust.)
3. VIMD is a higher speed on a jet aircraft because the swept wing is more efficient against profile drag, and therefore, the minimum drag speed is typically a higher value.
