POF Flashcards
What is the ratio of a wing’s span and mean chord known as?
Aspect ratio
What is an aircraft’s angle of incidence? (rigging angle)
The angle between the chord line of the wing where the wing is mounted to the fuselage, and a reference axis along the fuselage (directional of minimum drag OR longitudinal axis).
Applying Bernoulli’s Principle what happens to the local air pressure when the airflow speeds up over a wing’s surface?
The pressure on the surfaces of the wing will be lower above than below, this pressure difference results in an upwards lifting force.
What happens to the streamlines when the AoA of an aerofoil increases beyond the critical angle?
The lift coefficient decreases.
Streamlines become chaotic resulting in turbulent flow and a dramatic loss of lift.
In the lift equation what features of the aerofoil does CL take account of?
- Angle of Attack
- The aerofoil section (incl camber & LE radius)
- Wing plan form (aspect ratio)
- Wing surface condition
As temperature increases what happens to air density hence lift?
Air density reduces hence lift reduces.
What is the main variable that influences CL?
v
Free stream velocity
Lift proportional to v2
What will happen to the CL and the critical AoA with increased camber?
CL increases
critical AoA decreases
Once boundary separation occurs at a high AoA what will the aircraft experience?
Stall
dramatic loss of lift
typically 15degrees
With a swept wing, which part of the wing is likely to stall first?
The wing tip.
How can wing tip stall be prevented/minimised by design?
Washout (lower angle of incidence at tip)
Wing fences (redirect sideways flow rearwards)
Notches
Vortex generators
Modified aerofoil sections
Stall strips
What does wing span-wise flow refer to?
A bi-product of the pressure differential between upper and lower surfaces of a wing, the flow near the wing tips tends to curl around the tips from high to low. This generates ‘span-wise’ flow from tip to root across upper surface and root to tip across the lower surface.
What does span-wise flow cause?
Trailing vorteces are created at each wing tip.
Increased pressure differential at higher AoAs increases ‘leakage’ at wing tip causing the vortex to become detached and move inboard. This can result in a marked decrease in response from outboard ailerons.
What is the purpose of winglets on modern aircraft?
Reduce span-wise flow hence reducing wing tip vortex generation.
What does the deployement of flaps do to the CL CD and stall speed (VS)?
Extending flaps:
increases the camber therefore increasing CL,
increases CD
and reduces stalling speed.
How can the energy in the boundary layer over the flaps be increased to delay the onset of separation?
Vortex generators
Blow flaps
Leading edge lift devices: fixed slot, movable slat, LE flap.
What are the two components of total drag?
Lift dependent drag
Zero lift drag
What are the three types of zero lift (parasitic) drag?
Form drag
Skin friction drag
Interference drag
What is the main cause of lift induced drag?
Wing tip vortices
Span wise flow combines with chord wise flow = change of speed and direction.
As an aircraft accelerates what happens to zero lift drag and lift dependent drag?
Increase in IAS
Zero lift drag increases
Lift dependent drag decreases
Why is positive stability desirable in an aircraft?
Positive stability indicates a return towards the position prior to disturbance.
What is the main aircraft feature that provides directional static stability?
A suitably sized tail fin and its distance from the cg.
(yawing left & right)
To guarantee positive longitudinal stability what should be kept within the design limits?
The position of the cg.
(nose pitch up & down)
How might lateral stability be achieved?
Design to increase lift in down going wing
Increase counteracting forces produced by dihedral, wing sweep and general config of wing, fuselage and fin.
(disturbance in roll)
What causes Dutch Roll?
Strong lateral stability in relation to directional stability (ie poor yawing damping), with yaw leading to bigger roll divergence.
How can Dutch Roll be countered?
Yaw damping systems automatically apply rudder to minimise out of phase yaw and roll.
Increase Dutch Roll tendances:
Aircraft wings placed well above cg
Swept wings
Dihedral wings
What is the definition of VX?
Best angle of Climb
VX
(greatest altitude gain for shortest horizontal distance)
What is the definition of VY?
Best Rate of Climb
VY
(greatet altitude in shortest time)
What speed is coincident with the point where excess thrust is greatest?
VMD =minimum drag
and VX = best angle of climb (for a jet)
What speed is coincident with the point where excess power is greatest?
VY = Best rate of climb
Is VX higher in a prop or jet aircraft?
VX is higher in a jet aircraft.
How are VX and VY changed by altitude?
- increasing altitude increases Vx and decreases VY*
- ???*
How do EAS, CAS, TAS and Mach numbers vary with altitude?
When TAS is constant, altitude increasing:
EAS decreases (CAS corrected for compressibility)
- *CAS decreases** (IAS corrected for pos. and instrument error)
- *Mach increases** (TAS / speed of sound)
How is the absolute ceiling found?
The altitude at which climb performance falls close to zero and a steady climb can no longer be maintained.
The point at which VX and VY coincide.
How is the service ceiling for a light propeller driven aircraft found?
The maximum operationally usuable altitude of an aircraft.
100ft per minute with all engines operating for a light aircraft.
How would an increased T/O mass affect your VX and VY?
Reduced climb performance therefore VX and VY speeds increase with increased mass.
What affect does the deployement of flaps have on Vx and VY?
Reduced climb performance therefore VX and VY speeds reduced.
How does wind affect a climb limited T/O?
Rate of climb VY not affected by wind.
Headwind decreases distance travelled over ground, however speed still constant. Therefore VX also unaffected by wind.
Tailwind reduces angle of climb.
What is the max range?
Cover the greatest distance for the fuel carried.
What speed do you fly at for max range in a jet?
Best range speed.
1.32x VMD
What speed do you fly at for best speed in a prop?
At minimum drag speed = VMD
What is max endurance?
Fuel used at lowest possible rate for greatest time airborne.
What is the speed for max endurance in a prop aircraft?
Minimum power speed = VMP
What is the speed for max endurance in a jet?
Minimum drag speed = VMD
What happens to VMD at increasing altitude?
IAS constant therefore VMD increases
What speed should a glide be flown up?
Minimum drag speed VMD
What are the initial actions prior to executing an OEI drift down?
Set max continuous power/thrust on the operating engine(s)
whilst countering adverse yaw with rudder
then trimming and disconnecting the auto throttle system where applicable
In a light twin aircraft decelerate to blue line speed VYSE
Which is the surface of a propeller blade on which the RAF impinges?
The face of the blade
What is the propellers blade angle or pitch?
The angle between the chord line of the blade and the rotational velocity.
The sum of the helix angle and the angle of attack of the blade.
How does rotaional velocity vary with propeller radius?
It increases with radius
What is the approximate optimum angle of attack for a propeller blade?
4 degrees
What is the helix angle?
The resultant path made by the prop through the airflow.
Difference between prop blade angle and AoA
How does pitch vary angle the length of a propeller blade?
Blade angle reduces towards the tip
What component of total reaction opposes propeller rotation?
Torque
How is propeller efficiency determined?
Prop efficiency= Power output / Power input
What will be the affect of changing airspeed on a fixed pitch propeller?
Increase in airspeed reduces thrust to zero
What is the efficiency of a propeller at full power when the aircraft is stationary?
Zero
What is the main limitation of a fixed pitch propeller?
Only efficienct at one speed
What propeller pitch is more efficient at high TAS?
Coarse
What limits the maximum airspeed of a propeller driven aircraft?
Blade tips begin to go supersonic which induces high drag and mechanical issues
How are the efficiency limitations of fixed pitch propellers overcome?
Variable pitch propellers coarsen with increased TAS to maintain optimum AoA
What is the purpose of a constant speed unit (CSU)?
Maintain constant RPM
If prop RPM falls CSU moves prop to finer pitch,
if prop RPM rises CSU moves prop to coarser pitch.
If power is totally removed what will a constant speed propeller do?
A total reduction in power will result in the CSU selecting full fine pitch pitch, which will result in a windmilling propeller.
What is windmilling?
If the propeller fails to feather or is set to fine pitch at high TAS, the blade angle of attack becomes negative and the total reaction produced will mainly be drag plus torque that drives the propeller and accelerates rotation.
What is the purpose of feathering?
Reduces drag and prevents damage to hub.
About 85 degrees (coarse)
How is reverse thrust achieved using a propeller?
Propeller pitch set to negative angle on the axis of rotation, produced with high engine power applied.
Why is one engine critical?
As most light twin aircraft have clockwise rotating props this tends to move the thrust lines to starboard.
Thus the starboard engine thrust line has a greater moment arm to the aircraft CoG therefore there is assymetric yaw produced.
Therefore the critical engine is on the left, loss of this engine would generate much greater asymmetric forces.
What is P factor?
Asymmetric blade effect at positive angles of attack the down going propeller blade produces more thrust.
What is VMCA?
And what are the assumptions?
The minimum speed, whilst in the air, that directional control can be maintained with one engine inoperative.
Assumptions:
Critical engine inoperative and feathered
Operating engine/prop at T/O settings
Trimmed for T/O and full rudder applied
Flaps at T/O
Maximum of 5 degrees of bank towards live engine
Most unfavourable weight and CoG
What is VMCG?
Minimum control speed ground
the minimum speed under take-off power conditions at which it is possible to recover using only rudder and without reducing power on the live engine.
What is VSSE?
The minimum speed at which intentional engine failures are to be performed.
What is V1?
The takeoff decision speed by which any decision to abort or reject the T/O must be made.
When airbourne how might you identify the failed engine?
Yaw to the dead side
Rudder input on the live side
Indication
Crew member
Visual
What is the main cause of aerodynamic heating at high Mach numbers?
Compression of air, increases pressure which increases temperature (GAS LAW)
At low airspeeds the gasses in the atmosphere can be considered as…?
Incompressible fluid
What factor affects the local speed of sound in the air?
Proportional to square root of ambient air temperature
How does the speed of sound vary with altitude?
Follows same trend as temp therefore decreases to tropopause then contstant them increases after stratopause.
What phenomenon is caused by rearward movement of CoP at transonic speeds?
Causes pitch down moment known as Mach Tuck
What does the mach number represent?
V (TAS)
c (speed of sound)
When and where do shockwaves first form on the aircraft?
When ML reaches Mach 1.0
Occurs first on the the upper surface of the wing then the lower surface and eventually they meet at the TE.
What is the speed at which shockwaves first form on the aircraft?
The MFS (free stream mach) at the point when the first shockwave forms is known as MCRIT
What effect does the formation of shockwaves have on drag?
Increases CD
What causes ‘Shock Stall’?
Boundary layer flow behind shock waves separates
What does the term Coffin Corner refer to?
the region of flight where a fast fixed wing aircraft’s stall speed is near critical mach number.
Where high and low stall speeds intersect.
