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Correct OPTION is (a) HELIX

Best EXPLANATION: On examining the vortex system of the AIRSCREW blade, we see that the trailing vortex which is formed at the TIP of the blade traces out helix as the airscrew advances and rotates, the trailing vortex takes a helical form.

The correct option is (d) Generate DRAG

Best 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.

Correct answer is (a) Feathering motion

Easiest 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.

The correct choice is (a) LOW power loading

To elaborate: 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.

Correct choice is (a) Φ=tan^-1\(\FRAC {V_0}{V_E}\)

To explain: According to blade element theory, When the propeller rotates and advances, the tip traces out helix. Along with this the trailing vortex also traces helix. This angle is measured between the direction of the flow and plane of rotation and is COMPUTED USING the formula:

Φ=tan^-1\(\frac {V_0}{V_E}\)

Where, V0 is the FORWARD airspeed of the aircraft

VE is the effective resultant velocity.

The correct answer is (a) Plot of power coefficient as a function of thrust coefficient

To EXPLAIN I WOULD say: Plot of power coefficient \(\frac {C_p}{σ}\) as a function of thrust coefficient \(\frac {C_T}{σ}\) is known as ROTOR polar. There is no loss of profile power for an ideal rotor and there is minimum induced loss. Due to profile power loss, the rotor polar for a real rotor is at an offset compared to the ideal polar, and power increases faster with thrust coefficient due to the larger induced power.

Correct ANSWER is (d) Account for varying BLADE geometry

Easy explanation: There are several REASONS why blade element theory is preferred over momentum theory. It can account for varying blade geometry, allows torque estimation, allows non-linearities for example lift CURVE to be modelled. The other points are some of the disadvantages.

Right ANSWER is (b) Two-dimensional

The explanation is: The essence of blade element theory is to divide the blade into numerous segments known as blade ELEMENTS. These are CONSIDERED to be independent and not influencing the FLOW over other elements. Thus, it is treated as a two-dimensional airfoil whose aerodynamic forces are computed based on local flow conditions at that particular element instead of the ENTIRE airfoil.

Correct answer is (c) w=2v

Easy explanation: The momentum equation for the ROTOR in climb condition is given by:

T=\(\dot {m}\)(V+w)-\(\dot {m}\)V=\(\dot {m}\)w

And the energy equation for climb condition rotor is given by:

T(V+v)=\(\frac {1}{2}\dot {m}\)(V+w)^2–\(\frac {1}{2}\dot {m}\)(V)^2=\(\frac {1}{2}\dot {m}\)w(w+2V)

On rearranging and eliminating T/\(\dot {m}\) term, we get w=2v which is similar to the condition for hovering helicopter. Thus, the induced velocity in the wake region is TWICE that of the velocity at the rotor DISK.

The CORRECT choice is (d) Unsymmetrical airfoil

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.

The correct answer is (b) 100-500 \(\frac {N}{m^2}\)

For explanation I would say: BASED on the MOMENTUM THEOREM for the hovering helicopter, the disk loading must be of the minimum VALUE to have a low value of power to thrust. Disk loading value generally ranges between 100-500 \(\frac {N}{m^2}\) for the best PERFORMANCE.

The correct choice is (a) True

Explanation: Based on the RELATION between induced power PER unit thrust and the disk LOADING, \(\frac {P}{T}=\sqrt {\frac {T}{2\rho A}}\) we notice that in order to have a small value of P/T which is a requirement for HIGH hovering efficiency, we need to have a small value of T/A which is known as disk loading.

The CORRECT answer is (c) Ωt

For 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.

Correct answer is (d) W.J.M. Rankine

To explain: The momentum theory was DEVELOPED by W.J.M. RANKING initially for MARINE PROPELLERS in the year 1865 along with R.E. FROUDE in the same year. This theory was later extended in 1920 by A.Betz who incorporated the slipstream rotation.

Right answer is (a) Thin actuator DISK

Explanation: For conduction momentum theory analysis of the rotor, it is modeled as an actuator disk. It is considered to be thin circular disk of negligible thickness which can SUPPORT pressure difference and accelerate AIR. This WORKS as an approximate.

Right option is (B) When solidity is MUCH lesser than 1

Best explanation: In order to make sure that the blade element theory is applicable, there are certain conditions that have to be met. First being that the spacing to the CHORD ratio should be high.

\(\frac {s}{C}\) >> !

And solidity should be much lesser than 1. Solidity is DEFINED by:

σ=\(\frac {Bc}{πr}\)

Where, B is number of blades

C is the chord length

R is the radius

Right answer is (b) Twice

The best I can explain: If we consider the ANGULAR VELOCITY of FLOW in plane of the airscrew blade as bΩ, and the angular velocity behind the blade as indicated by the BOUND vortices as +βΩ, then the angular velocity of the flow behind the airscrew is given by:

ω=(b+β)Ω=2bΩ

This value is twice that of the angular velocity in plane of the airscrew.

Correct choice is (a) True

Easiest EXPLANATION: When the rotor solidity is too low, HIGH ANGLE of attack is required to achieve the lift. This LEADS to an increase in profile drag. Therefore the rotor should always have solidity which is as low as possible.

Right CHOICE is (b) False

Easy explanation: There are THREE planes that can be considered- ONE immediately ahead of the plane, in plane of the blade and one immediately behind the blade. In case of the plane which is immediately ahead of the blade, the ANGULAR VELOCITY is zero resulting in bound and trailing vortices canceling out each other.

The correct choice is (b) Non-uniform loading over the disk

Easy explanation: There are VARIOUS assumptions made while formulating the momentum theory for the helicopter in hover and climb CONDITION. These are-the BLADES are MODELED to be a thin circular actuator disk, there is uniform loading over the disk, the slipstream is WELL defined, the rotational velocity in the slipstream is neglected and there is uniform induced velocity.

Right CHOICE is (c) High critical Mach NUMBER

The 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.

The correct option is (B) False

To explain: While formulizing momentum theory, the rotor blade is modeled as an actuator disk which is only an approximation. The flow FIELD in an actuator is assumed to be steady whereas in real life it isn’t. Hence, the actual induced POWER loss is higher than the calculated value from the momentum theory DUE to the presence of UNSTEADY vortices.

The correct ANSWER is (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.

The correct choice is (c) w=2v

To explain: The momentum equation for the rotor is given by:

T=\(\dot {m}\)w

Which can be REARRANGED as:

\(\frac {T}{\dot {m}}\)=w (equation 1)

And the ENERGY equation is given by:

TV=\(\frac {1}{2}\dot {m}\)

Which is rearranges as:

\(\frac {T}{\dot {m}}=\frac {1}{2}\)vw^2 (equation 2)

On EQUATING equation 1 and 2 we get w=2v

Thus, the induced velocity in the wake region is TWICE that of the velocity at the rotor disk.

The correct answer is (b) 0.05-0.07

For explanation: Inflow ratio is a nondimensional quantity that is used to compare the results from different rotor blades. The formula for inflow ratio is given by:

λ=\(\frac {V+v}{ΩR}\)

Where, V is the climb velocity which is ZERO for hovering

v is the induced velocity

Ω is the rate of ROTATION of the blade

R is the radius

The value of inflow ratio RANGES between 0.05 and 0.07 for hovering.

The correct CHOICE is (a) High power by thrust RATIO

The best explanation: According to the momentum THEORY, the induced power by thrust ratio is given by:

\(\frac {P}{T}=\sqrt {\frac {T}{2\rho A}}\)

Thus, for low inflow velocity and low induced power loss, air must be accelerated through the rotor by virtue of pressure difference. For improving HOVERING efficiency, the value of P/T must be small thus improving fuel efficiency and decreasing ENGINE’s weight.

The CORRECT answer is (a) Avoid ground instability

The best I can EXPLAIN: 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.

The correct choice is (c) Negative

To elaborate: 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.

Correct choice is (a) FLAP motion

Easiest 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.

The correct answer is (a) Induced POWER is inversely proportional to rotor radius

To EXPLAIN: 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.

Correct option is (b) False

The best 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.

Correct option is (c) Figure of merit

The 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.

Correct OPTION is (b) Ratio of aerodynamic and INERTIAL force

The best 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

R is the radial location to the blade tip

Ib is the moment of inertia of the blade

Correct choice is (c) High

Best 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.

The CORRECT choice is (a) True

The best I can explain: 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.

Right answer is (b) vh=\(\sqrt {\frac {T}{2\rho A}}\)

Best explanation: Momentum EQUATION helps in GIVING us a relation between the thrust of the rotor and the induced velocity. It is GIVEN by:

T=2ρAvh^2

Which is rearranged to obtain the induced velocity:

vh=\(\sqrt {\frac {T}{2\rho A}}\)

Where, T is the TOTAL thrust

A is area of actuator DISK

vh is induced velocity at rotor disk.

The CORRECT answer is (d) Shaft

To explain I would say: 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.

The CORRECT choice is (c) To generate pitching MOTION

For 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.

The correct answer is (a) Two blades attached to hub without flap/hinge

The best I can explain: 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.

Correct choice is (b) Articulated rotor

To elaborate: 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.

Correct option is (b) Helicopter

To EXPLAIN: 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.

The correct choice is (a) Predict performance of airscrew airfoil

For explanation: The blade ELEMENT theory is USED to predict the performance of an airscrew blade which is a type of LIFTING airfoil used in helicopters. It can be REPLACED by a single hypothetical bound VORTEX which sheds vortex from the tip of the blade.

Correct ANSWER is (a) Climb velocity

For explanation: The MOMENTUM equation for the helicopter in flight CONDITION is given by:

T=\(\dot {m}\)(V+w)-\(\dot {m}\)V

T=\(\dot {m}\)w

Where, V is the climb velocity

w is the induced velocity in the wake region

T is the thrust

\(\dot {m}\) is mass flux

On observing the equation, we see that the momentum equation is independent of the climb velocity.