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.
Ground effect
The earth’s surface actually alters the three-dimensional airflow pattern around the airplane.
What does ground effect do?
Causes a reduction in wingtip vortices and a decrease in upwash and downwash. Induced drag decreases.
When the wing is at a height equal to its span the decline in induced drag is what?
1.4%
When the wing is at a height equal to one-fourth it’s span what is the decrease in induced drag?
24%
What could happen if you attempt to climb out of ground effect before reaching speed for normal climb?
Airplane could sink back to surface.
When landing how does ground effect affect you?
Makes airplane seem to float on the cushion of air beneath it. Most noticeable in low-wing aircraft because wings are closer to ground.
Stability
Characteristic of plane that causes it to return to equilibrium, or steady flight, after it’s disturbed.
Positive static stability
Initial tendency to return to the position from which it was displaced.
Positive dynamic stability
Aircraft returning to original position after disturbance over period of time through series of successively smaller oscillations.
Center of gravity
Where the three axes of the plane meet.
Longitudinal axis
Goes from nose to tail
Lateral axis
Goes through left side and out right.
Vertical
Goes through top to bottom.
Longitudinal stability
Involves the pitching motion or tendency of the aircraft to move about its lateral axis. An airplane which is longitudinally stable will tend to return to its trimmed angle of attack after displacement.
Center of pressure or center of lift.
Point along wing chord line where lift is considered to be concentrated.
Does the center of gravity move?
Yes. The pilot can choose distribution of weight in aircraft.
Does the center of pressure change?
Yes it changes with different flight attitudes. Forward with more angle of attack. Aft with angle decreasing.
CG range
Distance between forward or aft limits on center of gravity.
Adverse side effects of being nose heavy.
Longer take off distance and higher stalling speeds. Eventually elevator or stabilator won’t be able to raise nose.
Adverse effects of CG too far aft.
Less stable at all speeds. May be unable to recover from stall or spin.
Tail-down Force
Force caused by negative angle of attack on horizontal stabilizer. Counter acts the nose heaviness.
Why when you reduce power is there a nose down tendency of the plane?
The reduction of downwash from the wings and the propeller which reduces elevator effectiveness.
Increasing power causes nose up pitch. Why?
Increased downwash on the horizontal stabilizer which decreases contribution to longitudinal stability and causes nose to rise.
Thrustline
Determined by where the propeller is mounted and by general direction in which thrust acts.
In most light airplanes where is the thrustline?
Parallel to longitudinal axis and above CG.
By having thrust line slightly above CG and parallel to longitudinal axis how does that affect nose pitching tendencies with power?
If thrust is decreased, the pitching moment is reduced and nose heaviness tends to decrease.
An increase in thrust increases pitch moment and nose heaviness tends to increase.
By having the pitching tendencies reverse to the pitching tendencies from an increase or decrease in downwash, what does that do?
Minimizes the destabilizing effects of power changes and improves longitudinal stability.
Why is it important to maintain precise aircraft control during power-on approaches or go arounds?
Longitudinal stability may be reduced due to high power, low airspeed. There is increased downwash and decreased airspeed reduced overall stabilizing effect of horizontal stabilizer. Especially too if using flaps.
Lateral Stability
Stability about longitudinal axis.
Four most common design features that influence lateral stability.
- weight distribution
- dihedral
- sweepback
- keel effect
Dihedral
Upward angle of the airplane’s wings to the horizontal. Makes wings appear to make a V when looking from tail or nose.
How does dihedral help give lateral stability when the plane goes into an uncoordinated roll?
When the plane rolls one wing will be up and the other down. The plane will sideslip down towards lower wing. When this happens the lower wing will have greater angle of attack than the high wing, therefore producing more lift on the lower wing, and finally leveling plane back to where it was.
Why is there more dihedral on low wing aircraft?
The fuselage in front of the wing experiences a downwash and accounts for 3 to 4 degrees of negative dihedral in a sideslip. Therefore you have to make up for it with more dihedral in the wings.
Why is there less dihedral in high wing aircraft?
The fuselage in front of the wings experiences an upwash and will give a positive 2 to 3 degrees of positive dihedral in a sideslip. Therefore you need less dihedral in the wings.
The greater than 90 degree angle that is formed by the wings and longitudinal axis.
Swedback
Purpose of sweepback in high performance airplanes?
Maintain center of lift aft of CG and reduce wave drag when operating at speeds transonic or supersonic.
Main purpose of sweepback in training airplanes?
Improve lateral stability
How does sweepback aid in directional stability?
If plane yaws one way the more sweptback wing will have less drag than the less sweptback wing. Therefore helping straighten out plane.
Steadying influence exerted by the side area of fuselage and vertical stabilizer.
Keel effect
Stability about the vertical axis.
Directional stability
What is primary contributor to directional stability?
Vertical tail.
A combination of rolling/yawing oscillations either caused by your control input or wind gusts. They will usually happen when dihedral effects are more powerful than directional stability.
Dutch roll
Associated with airplanes that have strong directional stability in comparison with lateral stability.
Spiral instability
Used to simulate the conditions and aircraft configuration you will most likely encounter during a normal landing approach.
Power-off stalls
Stalls that are normally encountered during takeoff, climb-out, and go-arounds when the pilot fails to maintain proper control due to premature flap retraction or excessive nose-high trim.
Power-on stalls
Used to help understand stalls at higher than normal stall speed.
Accelerated stalls
Stall that is most likely to occur when a pilot tries to compensate for overshooting a runway during a turn from base to final while on landing approach.
Crossed-control stall
Typical indication of a stall.
Mushy feeling in controls and less control effect as aircraft’s speed decreases.
In fixed-pitch propeller plane what is a second indication of a stall during power-on conditions?
Loss of engine rpm.
Normally caused by poor stall recovery technique, such as attempting flight prior to attaining sufficient flying speed.
Secondary stall
What is the first basic guideline to stall recovery?
Decrease angle of attack. Pushing forward on yoke.
What is the second basic guideline to stall recovery?
Smoothly apply maximum allowable power. Increase throttle to minimize altitude loss and increase airspeed.
Third basic guideline to stall recovery?
Adjust power as needed.
An aggravated stall which results in the airplane descending in a helical, or corkscrew, path.
Spin
Typical cause of spin.
Exceeding critical angle of attack while performing uncoordinated maneuver.
Spin with slightly nose down rolling and yawing motion in same direction.
Erect spin
Spin upside down with yaw and roll occurring in opposite directions.
Inverted spin
Spin that yaws about vertical axis with pitch attitude approximately level with horizon.
Flat spin
Portion of spin from time from the time of stall and rotation starts until spin is fully developed.
Incipient spin
After incipient stage when angular rotation rates, airspeed, and vertical speed are stabilized from turn to turn and flight path is close to vertical.
Fully developed spin
Final stage of spin and when application of anti-spin forces result in a slowing and/or eventual cessation of rotation coupled with a decrease in angle of attack below C_Lmax.
Spin recovery
Torque from the propeller causes what effect that is greatest at low air speeds, high power settings, and high angles of attack?
Left turning tendencies.
The turning propeller also exhibits characteristics of a gyroscope. The characteristic that produces a left turning tendency is?
Gyroscopic precession.
When descending blade of propeller takes a greater bite of air than the ascending blade causing what?
Asymmetrical thrust.
Known as P-factor
Asymmetrical thrust
Makes an airplane yaw about vertical axis to the left.
P-factor
What causes P-factor?
Higher angle of attack of descending blade vs. ascending blade of propeller.
Causes a change in airflow around the vertical stabilizer. Due to the direction of propeller rotation the resulting slipstream strikes the left side of vertical fin. Moves nose left.
Spiraling slipstream.
The angle of attack resulting in the least drag on your airplane gives the_____ and therefore the max gliding distance.
Maximum lift-to-drag ratio (L/D_max)
At a given weight, L/D_max will correspond to a certain airspeed. This important performance speed is called
Best glide speed.
If power failure occurs after takeoff, immediately do what?
Establish proper gliding attitude and airspeed.
Distance an airplane will travel forward, without power, in relation to altitude loss.
Glide ratio
Angle between actual glide path of airplane and horizon
Glide angle
Does weight affect glide ratio?
No.
True/False: A heavier plane that has the same glide ratio as another lighter plane will travel the same distances so as long as the proper air speeds are met.
True. The heavier plane will reach the ground sooner but will travel the same horizontal distance.
What component of lift causes airplane to turn?
Horizontal.
Horizontal component of lift creates force directed inward toward center of rotation
Centripetal force
Inertia tending to oppose centripetal force.
Centrifugal force.
Since induced drag is a by-product if lift, the outside wing in a turn also produces more drag than inside wing. This causes a yawing tendency toward outside of turn which is called
Adverse yaw.
Caused by additional lift on outside, or raised, wing. Outside wing is traveling faster than inside wing producing more lift and causing the plane to want to roll more.
Overbanking tendency.
Refers to amount of time it takes for an airplane to turn a specified number of degrees.
Rate of turn
Amount of horizontal distance an aircraft uses to complete a turn
Radius of turn.
Ratio of the load supported by airplane’s wings to the actual weight of aircraft and its contents.
Load factor
Load factor of airplane in cruising flight while not accelerating in any direction.
1
In turn you must compensate for apparent increase in weight and loss of vertical lift. How do you do this?
Increase angle of attack
Stall speeds increases in proportion to the ____ of the load factor?
Square root.
Stalls that occur with G-forces on an airplane are called
Accelerated stalls
The amount of stress, or load factor, that an airplane can withstand before structural damage or failure occurs
Limit load factor
Graphically depicts the limit load factors for the associated airplane at a variety of speeds.
V-g diagram
Represents maximum speed at which you can use full, abrupt control movement without overstressing the airframe.
Design maneuvering speed V_A
