Chapter 3 Flashcards
Bernoulli’s Principle
As the velocity of a fluid increases, its internal pressure decreases.
Venturi
Tube which is narrower in the middle than at the ends.
Airfoil
Any surface which provides aerodynamic force when it interacts with a moving stream of air.
Upwash
Deflection of oncoming airstream upward and over the wing.
Leading edge
Part of airfoil that meets the airflow first
Trailing edge
Portion of airfoil where airflow over upper surface and lower surface join
Downwash
Downward deflection of airstream as it passes over wing and past the trailing edge.
Angle of Attack
Angle formed by wing chord line and relative wind.
Relative Wind
Airflow which is parallel to and opposite the flight path of the airplane.
Camber
Characteristic curve of the airfoil’s upper and lower surfaces.
Chord Line
An imaginary straight line drawn through airfoil from leading edge to trailing edge.
Lift acts _____ to the relative wind, regardless of angle of attack.
Perpendicular.
Coefficient of Lift (C_L)
A way to measure lift as it relates to the angle of attack.
How is C_L determined?
Wind tunnel tests. Based on airfoil design and angle of attack.
Stall
Separation of airflow from the wing’s upper surface. Results in rapid decrease of lift.
Stalling or critical angle of attack
For a given airplane, a stall always occurs at the same angle regardless of airspeed, flight attitude, or weight.
First indications of stall in training airplane?
- mushy controls
- stall warning device
- slight buffering of airplane
What is the essential goal of recovering from a stall?
Restore smooth airflow by decreasing angle of attack to a point below the critical angle of attack.
C_LMax
Angle of attack that has the maximum amount of lift. After this point lift decreases rapidly.
Four main design considerations for wing design
1) wing planform
2) camber
3) aspect ratio
4) total wing area
Boundary layer
Thin layer of air next to the surface of an airfoil which shows a reduction in speed due to air’s viscosity.
The boundary layer can be described in two ways what are they?
Laminate or turbulent
Laminar flow
Begins near leading edge. Consists of smooth laminations of air sliding over one another.
Aspect Ratio
Span of wing tip to wing tip, divided by average chord. Higher the aspect ratio, the higher the efficiency of the wing.
One of primary factors in determining lift/drag characteristics. At a given angle of attack, a higher aspect ratio produces less drag for the same amount of lift.
Wing area
Total surface area of wings.
Planform
Shape of airplane’s wing when viewed from above or below.
Elliptical Wing
- Ideal for low speeds.
- Minimum drag for a given aspect ratio
- Difficult to construct
- Stall characteristics not as favorable as rectangular wing
Rectangular Wing
- not as efficient as elliptical wing
- tendency to stall first at wing root which provides adequate stall warning and aileron effectiveness
Tapered Wing
- decrease in drag and increase in lift which is most effective at high speeds
- highly tapered wing stalls first inboard of tip
What is a good compromise between rectangular and tapered wing?
Rectangular inboard followed by taper outboard. Good stall characteristics. Reduction in weight. Improves aspect ratio.
Sweptback wings
Efficient at high speeds. Low speed performance not so good.
Angle of incidence
The angle that the chord line is attached to the fuselage with. Typically slightly upward.
Wing twist or washout
Characteristic where wing tip has less angle of incidence than wing root. Gives better stall characteristics.
Why is stalling at the wing tips first at bad characteristic?
Reduced aileron effectiveness. May get to the point where it’s impossible to control airplane about its longitudinal axis.
Stall strips
Two metal strips attached to leading edge near fuselage. Disrupt airflow at higher angles of attack to cause wing area behind them to stall before wingtip does.
What is a benefit to using wing twist?
With the wing tip having a few degrees less angle of incidence when approaching high angles of attack the wing root will stall before the wing tip does, leaving control to the ailerons.
What is a by-product of lift?
Drag.
How can the pilot increase lift?
Change angle of attack or airspeed. You can also use flaps.
How does changing airspeed change lift?
Faster equals more lift. Lift is proportional to the square of airplane’s speed.
Flaps
Increase lifting efficiency of the wing and decrease stall speed.
Configuration
Refers to position of landing gear and flaps.
Clean configuration
Landing gear and flaps are up.
Lowering flaps does what to the chord line of the wing?
Increases it.
Plain Flap
Attached to wing by a hinge. Increases effective camber and changes wing’s chord line.
Split Flap
Hinged only to the lower portion of the wing increases lift but it produces greater drag because of the turbulent wake it causes.
Slotted Flap
Changes wing’s camber and chord line. It also allows a portion of high pressure air to travel through a slot. This increases velocity of airflow over the flap and provides additional lift.
The high energy air from the slot accelerates upper surface airflow and delays airflow separation to a higher angle of attack.
Fowler Flap
Attached to wing by track and roller system. Moves rearward and downward. Increases total wing area, camber, and chord line.
At what point on the flaps does the amount of lift vs drag switch from “high lift low drag” to “low lift high drag”?
Flaps at half position.
Vortex Generators
Small airfoil-like surfaces on wing which project vertically into airstream. Vortices are formed at the tip of these generators. These vortices add energy to boundary layer to prevent airflow separation. This reduces stall speeds and can increase takeoff and landing performance.
Parasitic drag
Caused by any aircraft surface which deflects or interferes with smooth airflow around the airplane.
There are three types: form drag, interference drag, and skin friction drag
Form Drag
Results from turbulent wake caused by separation of airflow from surface of structure.
Interference Drag
Occurs when varied currents of air over an airplane meet and interact. Placing two objects adjacent to one another may produce turbulence 50-200% greater than the parts tested separately. Ex. Wing and tail surface, landing gear struts, Wing struts.
Skin Friction Drag
Caused by roughness of airplane’s surfaces. A thin layer of air clings to these rough surfaces and creates small eddies which contribute to drag.
The combined affect of all parasitic drag varies how to airspeed?
Proportionally to the square of the airspeed.
How can skin friction drag be reduced?
Employing a glossy, flat finish to surfaces, and by eliminating protruding river heads, roughness, and other irregularities.
Induced Drag
Generated by airflow circulation around wing as it creates lift.
Wingtip Vortices
When the high pressure from the lower surface meets the low pressure from the upper surface of wing it creates vortex at trailing edge.
What effect to wingtip vortices have?
Deflect airstream downward in the vicinity of the wing, creating an increase in downwash. Therefore wing operates in an average relative wind, which is inclined downward and rearward near the wing.
How is induced drag related to airspeed?
Inversely proportional to the square of the speed.
L/D_Max
The point where drag is at a minimum and therefore lift to drag ratio is greatest.