13.1 Theory of Flight Flashcards
Which controls provide lateral control about the longitudinal axis?
Ailerons
Which controls provide longitudinal control about the lateral axis?
Elevators
Which control provides directional control about the normal axis?
Rudder
Why are the control surfaces placed at the maximum distance from the centre of gravity?
Produce the greatest turning moment
What is meant by the term ‘instinctive’ in relation to control surface movement?
The pilot moves the cockpit control in the direction the aircraft is desired to fly
What are used to limit the angular range of movement of the control surfaces?
Mechanical stops at both the control surface and at the controls in the cockpit
The down going aileron has a greater angle of attack compared to the up going aileron.
What effect does this lead to? How can this be overcome?
More lift and drag on the down going aileron, aircraft will tend to yaw in the direction of the down going aileron (aileron drag or adverse aileron yaw)
Differential aileron
Frise type aileron
Slotted aileron
Spoilers (augment roll)
Wing tip washout
Describe a frise aileron.
Special section with hinges set back in the leading edge
The leading edge of the upgoing aileron protrudes below the wing, creating more drag
Thus, greater drag is produced on the aileron on the inner wing in the turn, helping to turn the aircraft in that direction
What is washout?
Reduction of wing incidence towards the wing tip
What movement of the elevators is necessary to pitch the aircraft nose up?
Elevator moving up
Stabilators provide stability, control and what else?
Trim
If a stabilator has an anti balance tab, what direction will the tab move in relation to the stabilator?
In the same direction as the stabilator trailing edge
How does an anti balance tab provide feel?
Increases the aerodynamic load which has to be overcome by the pilot
What is the primary advantage of a variable incidence stabiliser?
Having the elevator in continuation with the stabiliser ensures that drag is minimised
Full range of elevator travel is available
How would the rudder be positioned to achieve a yaw to the right?
Rudder will move right
How can sideslip or spinning be prevented when the rudder is operated?
Interconnecting the ailerons, therefore when the rudder is applied, the ailerons are automatically operated
What is the purpose of a rudder limiter?
At high speeds, large rudder movements cause large stresses on the aircraft
At lower speeds, large rudder movements are required for adequate yaw control
Rudder must be limited at high speeds, but free to move at low speeds
What does yaw damping prevent and how is this achieved?
Automatically moves the rudder approximately 3° either side of the neutral position fast enough to prevent Dutch rolling causing discomfort to the passengers
Why is the foreplane or canard mounted a larger angle of incidence than the mainplane?
Makes sure that the foreplane stalls before the mainplane so that the aircraft will not stall or spin
Describe an elevon.
Control surface on the trailing edge of delta wing aircraft
Acts both as elevators and ailerons
When acting together, acts as elevators
When acting in opposition, acts as ailerons
Which conventional controls would a ruddervator replace?
Rudder and elevator
How do leading edge slats increase lift at low speeds?
Slot produced is larger at the leading edge than at the rear
Airflow going through the slot from below wing to the top is accelerated and increases the velocity of the boundary layer above the wing
Delays separation of the airflow from the top surface of the wing
Increases the maximum coefficient of lift
How is deployment of the slat controlled?
From the cockpit using the flap selector handle
Move with the trailing edge flaps in the correct sequence
How do leading edge flaps increase lift?
Increases the leading edge camber
What additional advantage does a variable camber leading edge flap have over a Krueger flap?
Also functions as a slat
What happens to the centre of pressure as the extension flap is deployed? What affect does this have? How is it overcome?
The centre of pressure makes a large rearward movement
Causes the aircraft to pitch nose down
Smaller aircraft are fitted with a trim tab which moves down when the flap is lowered to prevent the nose down pitch
What 2 forms of protection are needed for high lift devices?
Asymmetry and very high flap loadings
What are flaperons?
Control surface that operate both as flaps or ailerons
Operate together as flaps
Operate differentially as ailerons
What is the difference between spoiler use on the ground and in the air?
Ground – Reduce wing lift on landing
Air – Assist large aileron control wheel commands, raised to assist the up going aileron
What governs movements of the spoiler’s when the speed brakes are selected?
Speedbrake lever in the cockpit, on the centre pedestal
What four conditions must be met to allow lift dump?
Speedbrake lever in the armed position
Aircraft weight on the undercarriage (through the air/ground sensing system)
All thrust levers in their idle positions
Aircraft wheels rotating (provides a time delay and ensures the aircraft is on the ground)
What happens to lift dump if the thrust levers are moved forwards after landing?
The speedbrake lever automatically moves to the down position
Spoilers retract
What device can be used to initiate a stall at the wing root rather than the tip?
Leading edge spoiler or stall wedge
What devices speed up the boundary layer and prevent separation?
Vortex generators
State the three methods that reduce the spanwise flow and create a chordwise vortex.
Wing fence
Vortilon
Saw tooth leading edge
State 3 flying faults.
Tendency of the aircraft to:
Yaw
Fly nose or tail heavy
One wing low
What direction will a trim tab move in relation to the flying control surface?
In the opposite direction
Describe the trim corrections being made if an aircraft is flying left wing low.
Trim tab on the left aileron is moved up by rotating the trim wheel to the right
What are the four methods of aerodynamic balance?
Horn balance (horn tip)
Inset hinge
Internal balance panels
Various types of tab
How does a balance tab automatically oppose control surface movement?
Hinged into the trailing edge of the control surface
Normally connected by a rod to a fixed part of the main surface structure
When the control surface moves in one direction, the tab automatically moves in the opposite direction
In a geared balance tab, what happens to tab movement as displacement increases?
Allows the tab to produce progressive anti–balance effect
What is the purpose of a servo tab?
Less effort is required to move the tab into the airflow than would be required to move the main control surface, which might be large
Allows pilot to control the tab directly
Aerodynamic force acting on the tab produces a turning moment about the control surface hinge
Makes the control surface move in the required direction until a position equilibrium is reached
Describe a spring tab?
Similar to servo tab
Direct control of control surface a low speed
At high speed, no control of control surface, force required overcomes spring pressure in the control system which allows control of tab
Tab produces a turning moment which moves the control surface to move in the required direction
Thus, there is direct control at low speed but servo tab control at higher speed
How is flutter prevented?
Fitting a weight in front of the control surface hinge (mass balancing)
What is the Mach number?
Ratio of the true speed of the aircraft to the speed of sound in the air through which the aircraft is flying
Mach number = true speed of aircraft/speed of sound locally
What happens to the local speed of sound as altitude increases and temperature decreases?
Increasing altitude – reduces the speed of sound
Decreasing temperature – reduces the speed of sound
What is the Critical Mach number?
Airflow over the wing reaches the speed of sound at the point of maximum camber
When and where will an incipient shock wave first appear?
When the aircraft is moving at about three quarters of the speed of sound
Represented by a line approximately at right angles to the surface of the aerofoil at the point of maximum camber
What happens to the supersonic airflow as it passes through the shockwave?
- Sudden rise in pressure, density and temperature
- Airflow is slowed down by the shock wave
- Boundary layer breaks away and becomes very turbulent (known as compressibility buffet)
- Rapid rise in drag
- Loss of lift and often a rearward shift of the centre of pressure
- Control surfaces become less effective
- Turbulent airflow behind the shock wave can cause severe buffeting, especially if the airflow strikes the tailplane
How does shock stall differ from low speed stall?
Occurs at small angles of attack
Define Subsonic, Transonic and Supersonic.
Subsonic – Mach number below 0.8, all airflow is subsonic
Transonic – Mach 0.8 to 1.2, some airflow is subsonic, some is supersonic
Supersonic – Mach number above 1.2, all airflow is supersonic
What 3 design features can raise the critical Mach number?
- Slimness
- Sweepback
- Supercritical wings
How can pitching control be maintained at Transonic speed?
Power operated all–moving or slab tailplane may be used
What is control reversal?
The force on the wing is very large, therefore an aileron moving up or down will tend to cause the wing to twist, which changes the angle of incidence of the wing, and therefore the angle of attack
This change in the angle of attack acts in opposition to what is required by the aileron movement
Describe Mach trim.
Frequent pitch changes to counteract ‘tuck under’
Automatically position an all moving tailplane or variable incidence tailplane
Fuel is also transferred from a front trim tank to a rear trim tank
Having a consistent cross sectional area will reduce wave drag. What is this known as?
Area rule
What causes aerodynamic heating at supersonic speeds?
Friction between the air and the surface of the aircraft
What happens to supersonic airflow in a diverging duct?
Increasing velocity
Decreasing pressure
Decreasing density
What happens to supersonic air flow in a converging duct?
Decreasing velocity
Increasing pressure
Increasing density
Name the three types of shockwave.
Oblique shock waves
Normal shockwave
Expansion wave
For a conventional blunt nosed aerofoil where will the centre of pressure be for supersonic flow?
At approximately 50% of the chord
What additional advantage does the OGEE Delta platform have over the Arrowhead platform?
The stalling angle is large and unlikely to be reached during normal flight
How can the ineffectiveness of the elevators be overcome at supersonic speeds?
Hydraulically operated all–flying or all–moving tailplane
What is a pitot intake?
Diverging duct
What type of shock waves are best for slowing down the airflow in a supersonic intake?
Oblique shockwaves
When is a foreplane known as a canard?
When it provides less than one quarter of the total aircraft lift
Why do propeller driven aircraft not tend to have leading edge high lift devices?
Lift of the wing is increased in that area affected by the propeller slipstream
How are Krueger flaps and variable camber flaps controlled?
Using the trailing edge flap selector handle
What is the undesirable feature of a split flap?
Sudden decrease in lift at the stall
When the extension or Fowler flap is lowered, what does this increase?
Both the area and the camber of the wing
What does adding a slat to an extension flap setup do?
Prevent the large rearward shift of the centre of pressure caused by extension flaps
What will the asymmetry protection system do?
Stops the movement of the high lift devices if the angular difference between them exceeds predetermined limits
What does the automatic system of flap load relief do?
Partially retract the flaps if they are lowered at too high a speed
Due to the speedbrake system, what does the loss of lift putting the aircraft’s weight on the undercarriage do?
- Increases directional control
- Improves braking
- Reduces the risk of aquaplaning on wet runways
What can cause flying faults?
Changes in weight and CofG position due to the consumption of fuel
Varying positions of passengers and cargo
Flight under asymmetric power conditions
Pitch changes caused by lowering the flaps
In a horn balance or horn tip, how does it assist the pilot in overcoming the aerodynamic forces of moving a control surface?
Horn in front of the hinge line ‘catches’ the airflow, to counteract the force of the airflow acting on the control surface behind the hinge
What might a balance tab also be called?
Lagging tab
What might an anti–balance tab also be called?
Leading tab
What instrument is used to display the speed of the aircraft in relation to the local speed of sound?
Machmeter
What is ‘tuck under’?
At transonic flights, downwash is eliminated
The tailplane angle of attack becomes positive and it now produces an upload
Causes the nose to pitch down
What is the speed of airflow behind a normal shockwave?
Subsonic
For a conventional blunt nosed aerofoil where will the centre of pressure be for subsonic flow?
Located at about 25% of the chord
How can rudder limiting be implemented?
Automatically reducing the hydraulic pressure being supplied to the rudder actuators as the speed of the aircraft increases
Using a ratio changer which reduces rudder travel in relation to rudder pedal travel with increase in forward speed
Using an airspeed sensing device which adjusts the position of the rudder travel stops to reduce the travel of the rudder as the aircraft speed increases
