This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Blades and Propellers”.
1. Why is the flap hinge offset in articulated rotor?
a) Design constraints
b) Mechanical constraints
c) Aerodynamics constraints
d) Vibrational constraints
View Answer
Explanation: In an articulated rotor, the blades are attached to the hub along with lag hinges and flaps. They are usually offset because of mechanical constraints. For the shaft to transmit torque to the rotor blades, the flap hinge has to be offset by some amount.
2. What is the use of mechanical lag damper?
a) Avoid ground instability
b) Dampen the hub vibration
c) Dampen the resonance in rotor blades
d) Absorb instabilities in hinges and flap
View Answer
Explanation: Articulated rotor makes use of lag hinge and with that it is essential to use a mechanical damper because there is mechanical instability which is generated due to the coupled motion of rotor lag and hub in-plane displacement. This is known as the ground resonance.
3. Which of these conditions is essential for a rotor airfoil when the helicopter has high forward speed?
a) Low critical Mach number
b) Low drag divergence
c) High critical Mach number
d) High angle of attack
View Answer
Explanation: When the helicopter moves at high forward speeds, the front tip of the rotor blade usually has a very high Mach number. For this reason, it is essential for the rotor blade airfoil to have a high critical Mach number so that it doesn’t reach stall conditions very fast.
4. Which of these is not suitable for helicopter blade?
a) Moderately hick airfoil
b) Thin airfoil
c) Small pitching moment
d) Unsymmetrical airfoil
View Answer
Explanation: The rotor blade for a helicopter has small pitching moment. Apart from this, it is moderately thick and symmetrical which remains same throughout the wing span because it is easy to construct. The reason for symmetrical airfoil is that it yields zero pitching moment. The most common airfoil used in helicopter blade is NACA 0012.
5. Which of these options is a feature of teetering rotor blade?
a) Two blades attached to hub without flap/hinge
b) Three blades attached to hub with only flaps
c) Two blades attached to hub with single flap hinge
d) Four blades attached directly to hub
View Answer
Explanation: Teetering rotor is a type of rotor in which two blades are directly attached to the hub rigidly without any flaps or hinge unlike articulated rotor which makes use of both. The two blades make one complete structure which flaps relative to the shaft.
6. What is the flap frequency of a gimballed rotor?
a) 1/rev
b) 2/rev
c) 0.5/rev
d) 10/rev
View Answer
Explanation: Gimbaled rotor is a multi-blade helicopter rotor unlike teetering rotor which has only two blades. It is characterized by flap hinge which lies at exactly the rotational center which yields a flap frequency of 1/rev.
7. Which of these is not the purpose of rotor blade?
a) Generate lift
b) Counter weight
c) Provide thrust
d) Generate drag
View Answer
Explanation: The rotor system of the helicopter comprises of two or more blades whose main aim to generate the lift, counter or support the heavy weight of the helicopter while hovering, and provide thrust which counters the drag generated as a consequence of lift generation.
8. Helicopter blades have large diameter.
a) True
b) False
View Answer
Explanation: One of the main functions of helicopter is to hover. This leads to higher power requirements due to high thrust force generated. For this, large diameter rotor blades are used as it results in lower downwash for higher thrust.
9. Which of these is an important parameter for selecting the rotor blade based on aerodynamic efficiency?
a) Solidity ratio
b) Structural ratio
c) Figure of merit
d) Hovering ratio
View Answer
Explanation: Figure of merit is one of an essential measure for choosing the rotor based on the aerodynamic efficiency. It is given by the formula:
M=\(\frac {1}{Κ+\frac {3}{4}\frac {c_{d_0}/\overline {c}_l}{\lambda _h}}\)
Where, Κ is an empirical factor which accounts for losses in the rotor
cl is mean lift coefficient
cd0 is profile drag
High value of the figure of merit is achieved when the airfoil has low drag for high lift coefficient.
10. For forward flight, the airfoil of the blade have fixed stall characteristics.
a) True
b) False
View Answer
Explanation: When the helicopter is in forward motion, when the blade rotates, stall occurs periodically. Therefore the airfoils have unsteady stall characteristics. Good dynamics are determined by good stall characteristics.
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