# Aerodynamics Questions and Answers – Helicopter Rotor and Notations

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Helicopter Rotor and Notations”.

1. What would an efficient vertical flight mean?
b) High lift
c) Less drag
d) Less pressure difference

Explanation: In a helicopter, rotor provides thrust force in the vertical direction in order to counter its weight. For an efficient vertical flight, the power loading has to be low which means that the ratio of rotor power required to the rotor thrust should be less. This is achieved by low disk loading i.e. The ratio of rotor thrust to the rotor disk area is minimum.

2. Why does helicopter have huge rotors?
a) Induced power is inversely proportional to rotor radius
b) Induced power is directly proportional to rotor radius

Explanation: Induced power loading is proportional to square root of the rotor disk loading and inversely proportional to the rotor radius for a rotary wing in hover. Therefore, it has a large disk region of large diameter rotors for a given gross helicopter weight.

b) Helicopter
c) Airplane
d) Cyclocopters

Explanation: The use of high disk loading rotors for lifting compromises vertical flight capability, both in terms of power and reduced hover endurance. Of all the VTOL aircraft, helicopter has the lowest disk loading and thus has the most powerful vertical flight capability.

4. What kind of motion is produced by the hinged blade which is lying in the plane of rotor disk?
a) Flap motion
b) Lag motion
c) Yaw motion
d) Roll motion

Explanation: The hinged blade allows rigid body rotation about the hinge point which is acted upon by restoring moment due to centrifugal force acting on the rotating blade. For the hinge that lies in the plane of the rotor disk, there’s an out-of-plane deflection of the blades produced. This is known as flap motion. When the deflection of blade along vertical hinge arises, it is known as lag motion.

5. How are the aerodynamic forces of the rotor controlled?
a) Feathering motion
b) Lag motion
c) Flap motion
d) Yaw motion

Explanation: The aerodynamic forces of the rotor is controlled using the pitch motion which helps in altering the angle of attack of the rotor blade. The change of blade pitch angle is known as feathering motion. This is achieved using hinge or bearing but for rotors without hinge, it is accomplished by the pitch moment about the torsional rigidity region at the rotor blade root.

6. What is the rotor whose blades are attached to the hub with hinges called?
a) Teetering rotor
b) Articulated rotor
c) Hinge rotor
d) Flap and lag rotor

Explanation: There are various arrangements of rotor based on how the blades are arranged to the hub for flap and lag motion. In case of articulated rotor, the rotor blades are attached to the hub with flap and lag hinges. This allows the blade to move independently with each blade that can either flap, lag, feather or lead.

7. What values of aspect ratio is generally used for helicopter rotor blades?
a) Zero
b) Low
c) High
d) One

Explanation: The ratio of its span to its mean chord is the aspect ratio of a wing. It is equal to the square of the wingspan divided by the area of the wing. Higher is the aspect ratio, more narrow is the wing and vice versa. A higher aspect ratio wing is generally preferred for helicopters as it reduces the tip vortices thus reducing drag.

8. Teetering rotors have blade attached to hub without hinges which is attached to the shaft by the gimbal.
a) True
b) False

Explanation: Teetering rotor is one of the rotor classifications where the blades form one continuous structure in the teetering or seesaw configuration that is connected to the shaft with a single flap hinge. It doesn’t have any hinge lag. If the blades without any hinges is attached to the hub which is further attached to the shaft using gimbal, it is known as gimbal rotor.

9. Why are twin rotor configuration used?
a) To generate more lift
b) To counter torques
c) To generate pitching motion
d) To generate yaw motion

Explanation: A twin rotor configuration is used in a helicopter to counter the torque generated by the main rotor. The rotors are counterrotating with equal size and loading. This leads to no net yaw motion which can be detrimental.

10. What are the components of rotor?
a) Mast
b) Hub
d) Shaft

Explanation: The helicopter is an essential component that provides the vertical lift. The main rotor generates the lift whereas the tail rotor counteracts the torque thus eliminating yaw-motion. The rotor system consists of the hub on which all the blades are attached, it also has the mast. Together these three components make up the entire rotor system. The rotor system in turn is attached to the shaft through which the power is delivered.

11. What is the azimuth angle of the rotor blade for a constant rotational speed?
a) 0
b) Ω
c) Ωt
d) 2Ωt

Explanation: The azimuth angle for a rotor blade is given by the angle between the downstream direction and the blade span axis. This angle is measured in the direction of the rotation of the blade. It is zero in the downstream direction and Ωt for a constant rotational speed.

12. What is the formula to compute the moment of inertia of the rotor blade about center of rotation?
a) Ib=$$\int _0 ^R$$mr2dr
b) Ib=$$\int _0 ^R$$mr3dr
c) Ib=$$\int _0 ^R \frac {m}{r^2}$$dr
d) Ib=$$\int _0 ^R \frac {m}{r^3}$$dr

Explanation: Moment of inertia or rotational inertia of the rotor blade about the center of rotation is given by:
Ib=$$\int _0 ^R$$mr2dr
Where, m is the blade’s mass per unit length which is a function of r
r is radial location of the blade measured from center (r=0) to the tip (r=R)

13. What is the value of linear twist rate in general?
a) Zero
b) Positive
c) Negative
d) Infinite

Explanation: The rotor blade of the helicopter is twisted along the length of the blade. The linear twist rate θtw is given by the difference between tip pitch and the root pitch, where the pitch is the angle of the blade. This value is mostly negative.

14. What is blade lock number?
a) Ratio of inertial and aerodynamic force
b) Ratio of aerodynamic and inertial force
c) Ratio of viscous force and aerodynamic force
d) Ratio of inertial force and viscous force

Explanation: The rotor blade’s lock number is given as the ratio of the aerodynamic and inertial force. The formula is:
γ=$$\frac {\rho acR^4}{I_b}$$
Where, γ is the blade lock number
a is blade section two-dimensional lift curve slope
c is the chord length
Ib is the moment of inertia of the blade

Sanfoundry Global Education & Learning Series – Aerodynamics.

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