Aerodynamics Flashcards
1. How does an airfoil generate lift?
air flows across a curved upper surface and is accelerated→ a low pressure region is formed; ⊥ to the relative wind → lift is the reaction
- The angle of attack of a helicopter rotor blade is defined as the angle between the:
blade’s chord line and the relative airflow.
- State the lilft formula!
FL = cL • ½ • ρ • v2 • S
- The lift coefficient of an airfoil section:
increases with an increase in angle of attack up to the stall.
- What is the magnus effect?
superposition of translational and rotational velocities of a rotating body (e.g. drum) with the result of pressure differences which cause a lift force
- The blade pitch angle of a rotor blade element is:
the angle between the chord line and the tip path plane.
- Which factors determine the magnitude and direction of the relative airflow in a still air hover?
Induced airflow velocity and rotational velocity of the blade element.
- The chord line of an airfoil section is the line:
drawn between the leading and the trailing edges.
- The center of pressure of an airfoil element:
is the point where the total aerodynamic force is acting.
- The centre of pressure of a symmetrical airfoil section is behind the leading edge approximately at the following % of the section chord:
25% (0,25)
- The force which acts at right angles to the relative airflow is.
lift
- The Centre of Pressure of an aerofoil section is:
the point on the chord line through which the resultant of al aerodynamic forces acts.
- The chord line of a blade section is:
a straight line from leading to trailing edge.
- The camber line of a symmetrical airfoil section is:
common with the chord line.
- In the case of a symmetrical aerofoil:
pitching moment varioations due to centre of pressure movement are small.
- Thickness/chord ratio of an aerofoil section is expressed in percentage of:
chord
- The resultant force from pressure envelopes around an aerofoil can be described as:
the total reaction
- That point where airflow leaves the surface of an aerofoil is known as:
the separation point
- A current requirement for the main rotor blade section is that:
changes in angle of attack produce minimum centre of pressure movement.
- The total rotor thrust is:
a component of total reaction acting at right angles of the aerodynamic forces on the blades, and perpendicular to the plane of rotation.
- State the drag formula!
FD = cD • ½ • ρ • v2 • S
- cL varies with:
angle of attack.
- What is the advantage a symmetrical aerofoil section as related to helicopter blade design?
The centre of pressure moves little in the normal angle of attack range
- An increase in angle of attack of a rotor blade would cause an increase in:
drag and lift forces.
- On a symmetrical blade element with a positive angle of attack lift is produced by:
airflow velocity increasing over upper surface giving decreased pressure and verlocity decreasing over lower surface giving increased pressure.
- Rotor blades profile drag is:
a component of total reaction the aerodynamic forces, acting parallel to the plane of rotation and backwards at 90 degrees to total rotor thrust.
- The amount of lift produced by a given helicopter rotor blade element is dependent upon:
angle of attack of the blade, the square of the air velocity relative to the blade element and the air density.
- The technical term “geometric twist” can be described as:
a reduction in blade angle towards the tip to give a more equal distribution of lift along the span.
- Rotor blade sections are designed so that the center of pressure:
is normally positioned close to the feathering axis to reduce control system loads.
- The term “washout” means:
that the used airfoil varies in design (f.e. thickness,camber) from blade root towards blade tip
- The in-ground-effect is caused by:
the airflow through the disc creating a divergent (spread out) duct with higher pressure beneath the rotor.
- The in-ground-effect on a hovering helicopter is greatest on:
level ground with no wind.
- What is the aerodynamic result when a vertical climb is initiated by raising the collective pitch? Explain by means of the blade element theory!
ϑ ↑ AoA ↑ cL↑ FL↑ FV ↑ Σ FV ↑ FT ↑ > FW → accelerated motion
- In a hovering helicopter, recirculated air at the main rotor blade tips will cause:
a reduction of lift.
- The effects of recirculation are at their worst:
close to building-type obstructions.
- In a constant speed vertical climb outside ground effect, if the effects of parasite drag on the helicopter fuselage are ignored:
total rotor thrust will equal aircraft weight.
- The “vortex ring state“ which may develop under conditions of a power-on descent at low forward airspeed is:
an unstable condition which may result in an uncontrolled rate of descent.
- In a free air hover how does Vi vary along the blade?
it is greater at the tip because of tip vortices.
x9. What are the preconditions for vortex ring state?
High rate of descend, no or low airspeed and driven rotor.
- Cyclic stick movement:
alters the tip path plane attitude.
- The rotor thrust is always:
perpendicular to the tip path plane.
- In level flight, as forward speed is increased, induced airflow velocity:
decreases and the component of the horizontal airflow through the disc increases.
- What can be noticed during transition from hover to forward flight (anti-clockwise rotor)?
significant climb without raising the collective pitch lever
- Translational lift becomes useful:
as airspeed reaches a value of approximately 20 kts.
- The coning angle is the angle:
between the longitudinal axis of the blade and the tip path plane.
- Transition to forward flight
causes a roll towards the advancing side.
- When the cyclic stick is pushed forward, a main rotor blade will reach its maximum blade pitch angle:
on the retreating side.
- If the collective pitch lever is raised during straight and level flight the helicopter will roll to the (1) because (2)
(1) advancing blade (2) the coning angle increases
- A “transition” in a helicopter is:
a change in the flight condition from or to hovering flight.
- The tip path plane is:
the path plane described by the blade tips during rotation and perpendicular to the axis of rotation.
- For a rotor which turn in an anti-clockwise direction seen from above, a sideways hover to the left with zero wind, the pilot will have the advancing blade:
in front of him.
- For a rotor which turns in a clockwise direction seen from above, in a sideways hover to the right, with zero wind, the pilot will see the retreating blade:
behind him.
- For a rotor which turns in a clockwise direction seen from above, in a backward hover, with zero wind, the pilot will see the retreating blade:
on his left.
- Reverse airflow is associated with:
flight at high forward speed and originates at the root of the retreating blade.
- If the collective pitch lever is lowered during straight and level flight the helicopter will pitch (1) because (2)
(1) down (2) of the dissymmetry of lift
- If the collective pitch lever is raised during straight and level flight the helicopter will roll to the (1) because (2)
(1) advancing blade (2) the coning angle increases
- When the cyclic stick is pushed forward, a main rotor blade will reach its maximum blade pitch angle:
on the retreating side.
- Retreating blade stall is most likely to occur at:
high forward speed, at high gross weight, high altitude and temperature.
- During an autorotation descent the maximum gliding distance will be obtained at:
speed greater than that associated with the minimum rate of descent.
- That point where airflow leaves the surface of an aerofoil is known as:
the separation point
- The purpose of the swept back tip region in some modern rotor blade designs is to:
improve high speed performance.
- What happens to the coning angle if rotor RPM decreases and collective pitch is constant?
It increases.
- How does rotor downwash affect a helicopter with a tail boom mounted horizontal stabilizer in a free air hover?
it will pitch nose up.
- Compared to a straight and level flight to perform a coordinated turn (same altitude and speed) the collective blade pitch angle (1) and power (2) must be:
(1) increased (2) increased.
- Which factors have an influence on the bank angle in turning flights?
velocity, curve radius, gravity
- What is the load factor?
Factor which refers to the extend of thrust that must be increased to hold altitude while turning.
- What sources of energy are available in case of a double engine failure?
Potential energy because of the height, kinetic energy because of airspeed and rotor RPM.
- In a normal forward autorotation descent, if the collective lever is raised by a small amount, the rotor RPM will (1) and the rate of descent will (2):
(1) decrease (2) decrease.
- What is the effect of an increasing airspeed on the region of driving blade elements during autorotation?
shifting of the driving blade elements to the retreating blade
- During an autorotation descent the maximum gliding distance will be obtained at:
speed greater than that associated with the minimum rate of descent.
- The “avoid“ areas in a height/velocity diagram (deadman’s curve) define the height/velocity combinations:
from which it is not possible to make a safe autorotation landing.
- A tail rotor is fitted to most helicopters to compensate for:
main rotor torque reaction and give directional control (yaw control).
- Yawing in a helicopter is the term used to define a rotation:
about the vertical axis.
- A helicopter having an anti-clockwise rotating main rotor (when seen from above), with power on, will have a natural tendency to drift:
to the right.
- Tail rotor drift is corrected by:
tilting the main rotor disc in the opposite direction to the drift.
- In hovering, for a single rotor helicopter whose main rotor turns clockwise from above, the thrust of the main rotor will be mainly vertical but with a slight orientation towards the:
right.
- With a tail rotor positioned lower than the main rotor a helicopter at the hover will:
fly left side low if main rotor rotates anti-clockwise viewed from above.
- During flight, an increase in main rotor torque will require:
an increase in tail rotor pitch.
- An anti-torque rotor is necessary on:
a single rotor helicopter.
