Drag Flashcards
What is drag?
Drag is the resistance to motion of an object through the air.
Define two major types of drag and their speed relationship!
Profile (parasite) drag and induced drag. Profile drag is drag created through the shape, form, skin-friction and interference of the aircraft itself and it will increase as speed increases. Induced drag is lift dependant drag and is a by product of lift. Induced drag will decrease as speed increases. Profile drag and induced drag added together will give total drag.
At what speed is drag the least?
Drag is least at a speed where induced drag and profile (parasite) drag are equal. It is here that total drag is least. (see drag curve) It is also here where the best lift to drag ratio occurs and an aircraft flies at its best endurance speed.
Describe the differences between the two drag curves of typical piston engine aircraft and jet engine aircraft!
The curve of the jet engine aircraft is usually flatter as a result of the reduced profile and induced drag due to swept back wing designs. The swept back wing has poor lift capabilities at low speeds. Because of the flatter curve there is no major change in flight characteristics around VIMD. VIMD itself is higher as well due to the swept back wing having more efficiency against the profile drag.
Describe the pitching moment associated with the thrust-drag couple!
If the two vectors of thrust and drag are not in line, they will create a moment which can result in the nose of an aircraft either pitching up or down. An increase in thrust can therefore cause the nose to pitch up or down depending on the location of the engine and the drag vector.
What are some high drag devices?
- Flaps
- Spoilers
- Landing Gear
- Reverse Thrust
- Braking Parachute
What are wing tip vortices? What causes them to form?
Wing tip vortices are rotating swirls of air (turbulences) that originate at the tip of a wing and linger behind an aircraft as is moves forward. Span-wise flow of the upper and lower air of the wing, due to pressure differentials, causes rotation of the air at the wing tips. These vortices are the cause for induced drag and can also be a danger to following aircraft.
What are the effects of span-wise airflow over the wing?
- It creates wing tip vortices
- Reduced aileron efficiency
- Reverse span-wise airflow, increasing disturbed airflow on the wings upper surface, contributing to a wing tip stall.
What are the effects of wing tip vortices?
- They create induced drag and therefore reduce the endurance of an aircraft.
- They create a hazard to other aircraft following
- They change the direction of airflow on the tailplane of an aircraft reducing the aircraft longitudinal stability
How do you prevent span-wise airflow on a wing, especially a swept back wing?
Fences and vortex generators which direct the airflow over the wing’s upper surface perpendicular to the leading edge.
What is the purpose of vortex generators/fences?
Vortex generators and fences are methods of reducing span-wise flow across a wing and therefore reduce its effects such as reduce aileron controllability, creation of vortex generators and reverse span-wise flow. Vortex generators may also help maintain the overall laminar airflow over a wing for a greater section of the wing by creating small eddies/currents/turbulences which tend to stick to the surface of the wing for longer and drag smoother air along with them. This allows for greater lift creation of the wing.
What are winglets, and how do they work?
Winglets are aerodynamically efficient surfaces located at the wing tips. They are designed to reduce induced drag by reducing span wise flow and the creation of wing tip vortices.
