What is drag?
Drag is the resistance to motion of an object (aircraft) through the air.
Deline the two major types ol drag and their speed relationship. Profile and induced drag = total drag Profile drag is also known as zero-lift drag and is comprised of
1. Form or pressure drag
2. Skin-friction drag
3. Interference drag
Profile drag increases directly with speed because the faster an aircraft moves through the air, the more air molecules (density) its surfaces encounter, and it is these molecules that resist the motion of the aircraft through the air. This is known as profile drag and is greatest at high speeds. 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, i.e.,
VIMD = minimum drag speed
Therefore, this speed also represents the best lift-drag ratio (best aerodynamic efficiency) that will provide the maximum endurance of the aircraft.
The drag curve for a piston/propeller aircraft
For a piston-engined propeller aircraft read straight-winged. It has a typical total drag curve comprised of a well-defined steep profile drag curve at high speeds. This is so because the wing is not designed for high speeds, and therefore, as speed increases, profile drag increases as a direct result. It also has a well-defined induced drag curve at low speeds.
This is so because the straight-winged aircraft has a higher CL value, and with induced drag being proportional to lift, the lower the speed, the greater is the angle of attack required to achieve the necessary lift, and therefore, the greater is the associated induced drag component. It also has a well-defined bottom VIMD (minimum drag speed) point and is capable of a lower stall speed than a jet. Flight below VIMD in a piston-engined aircraft is very well defined by the steep increase in the drag curve in flight as well as on paper. Speed is not stable below VIMD, and because of the steep increase of the curve below VIMD, it is very noticeable when you are below VIMD. That is, below VIMD, a decrease in speed leads to an increase in drag that causes a further decrease in speed.
The drag curve on a jet aircraft
The drag curve on a jet aircraft is the same as for a 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.
The pitching moment associated with the thrust-drag couple
If the forces of thrust and drag are not acting through the same point (line), then they will set up a moment causing either a nose-up or nosedown pitch depending on whether the thrust is acting above or below the dragline. Therefore, a change in thrust (increase or decrease) in straight and level flight can lead to a pitching tendency of the aircraft. Likewise, an increase or decrease in drag also can lead to a pitching tendency of the aircraft. For example, an increase in thrust on an aircraft with engines mounted under the wing, with a higher dragline, will cause a nose-up pitch as thrust is increased.
The following devices increase the drag penalty on an aircraft:
1. Trailing edge flaps (in high-drag/low-lift position)
2. Spoilers a. In fight detent, used as a speed brake b. On the ground, used as lift dumpers
3. Landing gear
4. Reverse thrust (ground use only)
5. Braking parachute