Ch. 5 Aerodynamics Of Flight Flashcards
The force produced by the power plant/propeller. It overcomes the force of drag.
Thrust
A rearward, retarding force caused by disruption of airflow by the wing, fuselage, and other protruding objects.
Drag
A force that is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the relative wind through the center of lift and perpendicular to the lateral axis.
Lift
A force that pulls the aircraft downward because of the force of gravity.
Weight
In straight, level, and unaccelerated flight all upward components of forces equals:
The sum of all downward components of forces.
In steady, level, unaccelerated flight all forward components of forces equals:
Sum of all backward components of forces.
The acute angle between the chord line of the airfoil and the direction of the relative wind
Angle of attack
The speed regimes of flight can be grouped in three categories. What are they?
- Low-speed flight
- Cruising flight
- High-speed flight
What is the maximum AOA, where lift begins to diminish rapidly? Also known as the stalling angle of attack.
Cl-max critical angle of attack (critical AOA)
Lift is proportional to the square of the aircraft’s:
Velocity
What is the formula for lift?
L=Clp(V^2)S0.5
What is the amount of lift generated by a wing or airfoil compared to its drag?
Lift-to-drag ratio (L/D)
How would one calculate the lift-to-drag ratio?
Cl/Cd
What is the drag formula?
D=Cdp(V^2)S0.5
If flying at L/Dmax what will be at a minimum?
Total drag is at a minimum.
What are the two basic types of drag?
Parasite and induced drag.
What are the three types of parasite drag?
- Form drag
- Interference drag
- Skin friction
This drag includes the displacement of air by the aircraft, turbulence generated in the airstream, or a hindrance of air moving over the surface of the aircraft and airfoil. What is it?
Parasite drag
Portion of parasite drag generated by the aircraft due to its shape and airflow around it.
Form drag
Portion of parasite drag that come from intersection of airstreams that creates eddy currents, turbulence, or restricts smooth airflow. What is it?
Interference drag
When is the most interference drag observed?
When two surfaces meet at perpendicular angles.
Portion of parasite drag that is the aerodynamic resistance due to the contact of moving air with the surface of the aircraft. What is it?
Skin friction drag
Each layer of molecules above the surface moves slightly faster until the molecules are moving at the velocity of the air moving around the aircraft. What is this speed called?
Free-stream velocity
What is the area between the wing and the free-stream velocity level?
The boundary layer
How thick is the boundary layer?
About as wide as a playing card.
What gives any object an “effective” shape that is usually slightly different from the physical shape?
The boundary layer
What is the condition when the boundary layer has separated from the body?
The airfoil has stalled.
When the wing is producing lift, what is the source of induced drag?
Wingtip vortices
How do wingtip vortices create induced drag?
The downwash from the vortices points the relative wind downward. Lift is always perpendicular to the relative wind. Therefore, the lift vector is pointed rearwards slightly.
What is the horizontal component of lift that acts rearwards?
Induced drag
The amount of induced drag varies inversely with the squad of the:
Airspeed
Parasite drag increases as the square of the:
Airspeed
The point at which all weight of the aircraft is concentrated.
Center of gravity (CG)
When CG is forward of CP, there is a natural tendency for the aircraft to pitch
Nose down
If CP is forward of CG there is a tendency for the aircraft to pitch
Nose up
How are wingtip vortices created?
The spanwise movement of air from the high pressure bottom of wing to low pressure top of wing results in “spillage” over the tips, setting up the vortex.
The intensity or strength of vortices will be greatest when the airplane is:
Heavy, clean, and slow because AOA will be greatest.
How should you avoid someone else’s wake on takeoff?
Rotate prior to their rotation point.
How should you avoid someone else’s wake when landing?
Touchdown after their touchdown point.
How far should you stay behind another aircraft in flight to avoid their wake?
1000’
How far should you be from a hovering helicopter to avoid the effects of it’s downwash?
At least three rotor disc diameters.
Wind is an important factor in avoiding wake turbulence because wingtip vortices drift:
With the wind at the speed of the wind.
If you’re unsure of the other aircraft’s takeoff or landing point how long should you wait to takeoff or land?
3 minutes
As the wing encounters ground effect and is maintained at a constant AOA there is a:
Reduction in upwash, downwash, and wingtip vortices.
Due to the reduction of wingtip vortices in ground effect what will this do to drag?
Reduce total drag by reducing induced drag.
In the majority of cases, ground effect causes an increase in the local pressure at the static source, producing a:
Lower indication of airspeed and altitude.
When will the aircraft enter ground effect?
When within one wing span of the ground.
The axis that passes through the CG and parallels the nose and tail of the aircraft.
Longitudinal axis
The axis that passes through the CG and parallel line from wingtip to wingtip.
Lateral axis
The axis that is perpendicular to the longitudinal and lateral axis.
Vertical axis
Equal to the product of the force applied and the distance at which the force is applied.
Moment
Distance from a datum to the applied force.
Moment arm
The inherent quality of an aircraft to correct for conditions that may disturb its equilibrium and to return to or continue on the original flight path.
Stability
The initial tendency, or direction of movement, back to equilibrium.
Static stability
The initial tendency of the aircraft to return to the original state of equilibrium after being disturbed.
Positive static stability
The initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed.
Neutral static stability
The initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed.
Negative static stability
The aircrafts response over time when disturbed from a given pitch, yaw, or bank.
Dynamic stability
Over time, the motion of the displaced object decreases in amplitude and the object displaced returns toward the equilibrium state.
Positive dynamic stability
Once displaced, the displaced object neither decreases nor increases in amplitude.
Neutral dynamic stability
Over time, the motion of the displaced object increases and becomes more divergent.
Negative dynamic stability
The quality of the aircraft that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers.
Maneuverability
The capability of an aircraft to respond to the pilot’s control, especially with regard to flight path and attitude.
Controllability
The quality that makes an aircraft stable about its lateral axis.
Longitudinal stability
Static longitudinal stability is dependent upon what three factors?
- Location of wing with respect to CG
- Location of horizontal tail surfaces with respect to CG
- Area or size of tail surfaces
The Center of Lift (CL) tends to move where with an increase in AOA?
Forward
The Center of Lift (CL) tends to move where with a decrease in AOA?
Aft
Also known as the Center of Pressure (CP)
Center of Lift (CL)
The CG is usually in front of the CL, this causes the aircraft to be:
Nose heavy
Compensation for nose heaviness is provided by setting the horizontal stabilizer at a:
Negative AOA
Why does the nose drop after closing the throttle?
Downwash of the wings is reduced and the force on the tail is not enough to hold the horizontal stabilizer down.
Stability about the aircraft’s longitudinal axis.
Lateral stability
What are the four main design factors that make an aircraft laterally stable?
- Dihedral
- Sweepback
- Keel effect
- Weight distribution
Upward angle formed by wings
Dihedral
How does dihedral contribute to lateral stability?
In a sideslip the down-wing is at a greater AOA, thus creating more lift than the up-wing.
10 degrees of sweepback on a wing provides about how many effective degrees of dihedral?
1 degree
A high wing configuration can provide about how many effective degrees of dihedral over a low wing aircraft?
5 degrees over a low wing aircraft.
The fuselage behaving like a keel exerting a steadying influence on the aircraft laterally about the longitudinal axis.
Keel effect
Why do laterally stable aircraft have a greater portion of the keel area above the CG?
The combo of the aircraft’s weight and pressure of airflow against the upper portion of the keel area tends to roll the aircraft back to wings-level flight.
Stability about the aircraft’s vertical axis
Directional stability
What are the two primary contributors to directional stability?
- Area of the vertical fin
2. Area of fuselage aft of the CG
A minor improvement of directional stability may be obtained through:
Sweepback
How can sweepback improve directional stability?
As the aircraft yaws one way in flight the opposite wing will be more perpendicular with the relative wind. This will create more lift and drag on that wing. The drag will return the aircraft closer to it’s original heading.
A coupled lateral/directional oscillation
Dutch roll
What exists when the static directional stability of the aircraft is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium?
Spiral instability
How does spiral instability work?
As the aircraft enters a sideslip the strong directional stability yaws the nose before the dihedral corrects the down wing. As the up-wing is now yawed more into the turn it creates more lift, further causing a spiral downward.
The shape of the wing as viewed from directly above and deals with airflow in three dimensions.
Planform
Wingspan divided by average chord length
Aspect ratio
A decrease from wing root to wingtip in wing chord or wing thickness
Taper ratio
Rearward slant of a wing, horizontal tail, or other airfoil surface.
Sweepback
Why is the elliptical wing the ideal subsonic planform?
It provides fro a minimum of induced drag for a given aspect ratio.
Why is the rectangular wing/tapered wing favored for low cost, low speed airplanes?
Stalls at wing root before wingtip. Gives adequate stall warning, aileron effectiveness, and is usually stable.
What is the formula to calculate load factor?
Secant(bank angle) in degrees
What is the formula to calculate stall speed under increased load factor?
Vs*SQRT(L.F.)
A rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical AOA.
Stall
In a stall does the wing stop producing lift?
No. The wing doesn’t totally stop producing lift. Rather it cannot generate adequate lift to sustain level flight.
Why is it desirable to have the wing stall at the root first?
To maintain aileron effectiveness and controllability of the aircraft.
What are two ways wings are designed to stall at the root first?
- The wing is “twisted” so that the root is at a higher AOA than the wingtips.
- Installing stall strips on the first 20-25% of the wing’s leading edge.
Why do most training aircraft tend to nose down in a stall?
Due to the CG being forward of the CL.
What is the typical critical angle of attack angle?
16-20 degrees
What are three flight situations in which the critical AOA can be exceeded?
- Low speed
- High speed
- Turning
How can you exceed the critical AOA in low speed?
As the aircraft slows lift decreases and therefore the AOA must be increased. Eventually an AOA is reached that results in a stall.
How can you exceed the critical AOA in high speed?
In a dive at high speeds if the pilot pulls back on the yoke the momentum of the plane will keep it in its original flight path, yet the AOA is now very high.
Why is a contaminated wing so dangerous?
The boundary layer will now separate from the wing at a lower AOA than the critical AOA. That angle and the resulting stall airspeed is unknown.
Angle between the chord of the blade and the plane of rotation
Blade angle
The distance in inches, which the propeller would screw through the air in one revolution if there were no slippage.
Pitch
Every fixed-pitch propeller must be a compromise because it can be efficient at only a given combo of:
Airspeed and RPM
The power expended in producing thrust depends on:
The rate of air mass movement.
On average thrust constitutes approximately 80% of torque. Where happened to the other 20%?
Lost in friction and slippage.
Most efficient AOA for propellers varies from:
2-4 degrees
The actual blade angle necessary to maintain 2-4 degrees AOA on propellers varies with:
Forward speed of aircraft.
The benefit of a constant-speed propeller is:
The pilot is able to provide the most efficient AOA at all engine and aircraft speeds.
The power efficiency of any machine is the ratio of:
Useful power output to the actual power input.
The ratio of thrust horsepower to brake horsepower
Propeller efficiency
Difference between the geometric pitch of the propeller and its effective pitch
Propeller slip
Theoretical distance a propeller should advance in one revolution
Geometric pitch
Distance propeller actually advances in one revolution
Effective pitch
Why is a propeller’s blade twisted?
Keeps more nearly uniform thrust along length of propeller.
When maximum power and thrust are required, constant-speed propeller is at what blade angle or pitch?
Low propeller blade angle or pitch.
Why do you want the propeller at a smaller blade angle or pitch for developing maximum power?
Allows propeller to handle a smaller mass of air per revolution. This allows engine to turn at high RPM and convert maximum amount of fuel into heat energy in a given time.
RPM and slipstream velocity are high, with low aircraft speed, there is maximum thrust.
What are the left turning tendencies?
- Torque reaction
- Corkscrewing effect
- Gyroscopic action
- P-factor
Which of Newton’s laws of motion apply to torque reaction left turning tendency?
Newton’s third law: for every action there is an equal and opposite reaction.
How are modern aircraft designed to counteract the effect of torque reaction?
Designed with the engine offset.
What causes the corkscrew effect left turning tendency?
High-speed rotation of an aircraft propeller gives spiraling rotation to the slipstream.
What kind of moment does torque reaction cause?
Rolling moment to left
What kind of moment does corkscrew effect cause?
Left yawing moment
The left yawing moment caused by movement of the airplane’s longitudinal axis with relation to the plane of rotation of the propeller is due to
Gyroscopic action
Gyroscopic action follows the:
Right hand rule.
When a force is applied to the plane of rotation of a spinning object the resulting force is felt:
90 degrees ahead of the applied force in the direction of rotation.
With the propeller spinning clockwise as viewed from the cockpit, which way will the tail have to be moved to cause a left turning tendency due to gyroscopic action?
Up
With the propeller spinning clockwise as viewed from the cockpit, which way will the tail have to be moved to cause a right turning tendency due to gyroscopic action?
Down
When the aircraft is flying with a high AOA, the “bite” of the downward moving blade is greater than the “bite” of the upward blade. This causes the center of thrust to move to the right on the propeller. What is this phenomenon?
P-factor
Any force applied to an aircraft to deflect its flight from a straight line produces a stress on it’s structure. The amount of this force is the:
Load factor
1.5 load limit load factor is called:
Factor of safety
What is the positive limit load factor of a normal category aircraft?
3.8 G’s
What is the negative limit load factor of a normal category aircraft?
-1.52 G’s
What is the positive limit load factor of a utility category aircraft?
4.4 G’s
What is the negative limit load factor of a utility category aircraft?
-1.76 G’s
What is the positive limit load factor of an acrobatic category aircraft?
6 G’s
What is the negative limit load factor of a acrobatic category aircraft?
-3 G’s
A safety factor of what is added to limit load factors?
50% of limit load factor
The speed below which the pilot can move a single flight control, one time, to it’s full deflection, for one axis of airplane rotation (pitch, roll, yaw) in smooth air without risk of damage to the airplane.
Maneuvering speed: Va
A force applied to an aircraft that causes a bending of the aircraft structure that does not return to the original shape.
Limit load
The load factor applied to the aircraft beyond the limit load and at which point the aircraft material experiences structural failure.
Ultimate load
Rate of turn formula degrees per second
ROT=1091*tan(B.A.)/airspeed
Radius of turn formula in feet
R= (V^2)/[11.26*tan(B.A.)]
Longer takeoff runs, shallower climbs, increased fuel consumption, slower cruise speeds, reduced range, increased stall speed, faster touchdown speed, and longer landing rolls are caused by:
A heavier gross weight
Why does having a more forward CG result in more drag and a higher stall speed?
Tail surface needs more down load, which adds to wing loading and total lift required from wing for altitude to maintain. This requires a higher AOA from the wing, creating more lift and higher stall speed.
What is a benefit to a more forward CG?
More stability due to the increased tail down force applied to maintain level flight.
How does having an aft CG improve cruise speeds, lower stall speed, reduce fuel consumption, etc.?
Less tail down force is needed. Therefore there is less wing loading and a lower AOA is needed to maintain level flight. With less AOA needed there is less drag and lower stall speed.
What is the negative aspect of having a more aft CG?
It can become easy to over control the airplane. Poorer stall recovery due to lack of rudder effectiveness from reduced lever arm to rudder. There may not be enough rudder to nose down the airplane and recover from a stall/spin.
What can happen if the CG is too far forward?
There may not be enough elevator effectiveness to pitch the nose up enough for takeoff or landing.
Why does an aircraft stall at a higher airspeed with a forward CG?
The stalling AOA is reached at a higher speed due to increased wing loading.
Why does an aircraft become less stable as the CG is moved rearward?
It causes a decrease in AOA. Therefore, the wing contribution to the aircraft’s stability is now decreased, while the tail contribution is still stabilizing.
The ratio of true airspeed of the aircraft to the speed of sound in the same atmospheric conditions
Mach number
The speed of an aircraft in which airflow over any part of the aircraft or structure under consideration first reaches (but does not exceed) Mach 1.0 is termed:
Critical Mach number or Mach Crit
Why does a sweepback wing design stall at the wingtips first?
The boundary layer tends to flow spanwise toward the tips and to separate near the leading edges.
