Drag is the resistance of an object to moving through the air.
There are two main types of drag:
INDUCED DRAG
Induced drag is a by-product of producing lift.
Induced drag decreases with velocity.
It increases with weight and altitude (for a given speed)
It increases whenever the AOA increases.
It is related to the design of the wing and can be reduced by high aspect ratios and winglets.
PARASITE DRAG
All objects feel parasite drag when moving through a fluid.
Parasite drag increases with velocity.
It decreases with altitude.
It is not affected by changes in AOA.
There are three types of parasite drag:
- Skin Friction: friction between air and surface of object
- Form Drag: drag resulting from air moving around a particular shape (i.e. is it streamlined?)
- Interference Drag: flows over two joined surfaces interfere with each other (i.e. wing and fuselage)
Reference Turbine Pilot's Flight Manual Chapter 15
The SPEED OF SOUND in the atmosphere is solely dependent upon air temperature.
Temp (K) = Temp (C) + 273
Speed of sound (kts) = 39 x √ Temp (K)
A MACH METER is an instrument containing an airspeed capsule and an altitude capsule, which calculates the ratio between the aircraft's TAS and the local speed of sound. The same Mach number can occur at markedly different indicated air speeds.
Airflow can be classified as SUBSONIC, TRANSONIC, or SUPERSONIC, dependent on how it's speed compares to the speed of sound. Transonic flight usually ranges from 0.75 to 1.20 M. Within this regime there is both subsonic and supersonic airflow.
CRITICAL MACH NUMBER is the highest flight speed possible without supersonic airflow over any part of the aircraft.
Air is compressible. When an aircraft flies at slow airspeeds, the surrounding air molecules have time to 'get out of the way'. As the vehicle's speed increases, air molecules begin to 'pile up' ahead of the aircraft, increasing air pressure, density, and temperature in that region... a powerful compression wave, or SHOCK WAVE, forms at the boundary between the disturbed and undisturbed air.
I found this helpful diagram on the Wikipedia page about Compressible Flow.
As airflow passes through a shock wave:
- pressure increases
- temperature increases
- velocity decreases
LIMITING MACH NUMBER is the maximum operating speed of an aircraft in relation to the speed of sound.
Formation of a shock wave on the upper surface of a wing results in the center of pressure moving backward causing a pitch down. This can lead to an unstable condition called MACH TUCK at speeds higher than Mmo.
In high speed aircraft, SWEEPBACK design is utilized to reduce drag and increase the Critical Mach Number. An undesirable effect of sweepback design in a jet aircraft is "Dutch Roll," which is the tendency of an aircraft to roll whenever it yaws. Key characteristics of a swept wing design:
- high speed operations
- poor slow speed handling
- long range performance
- increased lateral stability
