Correct Answer : Induced power is inversely 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.
Correct Answer : Ratio of aerodynamic and inertial force
Explanation : The rotor blade’s lock number is given as the ratio of the aerodynamic and inertial force. The formula is:Where, γ is the blade lock numbera is blade section two-dimensional lift curve slopec is the chord lengthR is the radial location to the blade tipIb is the moment of inertia of the blade
Correct Answer : Negative
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.
Correct Answer : Ωt
Explaination : 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 : 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.
Correct Answer : Helicopter
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.
Correct Answer : To generate pitching 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.
Correct Answer : High
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.
Correct Answer : Feathering 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.
Correct Answer : Flap 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.
Correct Answer : Articulated 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.
Correct Answer : Mechanical constraints
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.
Correct Answer : Avoid ground instability
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.
Correct Answer : High critical Mach number
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.
Correct Answer : 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.
Correct Answer : Two blades attached to hub without flap/hinge
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.
Correct Answer : Figure of merit
Correct Answer : Generate drag
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 : 1/rev
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.
Correct Answer : W.J.M. Rankine
Explanation : 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.
Correct Answer : 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.
Correct Answer : High power by thrust ratio
Explanation : According to the momentum theory, the induced power by thrust ratio is given by: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.
Correct Answer : w=2v
Correct Answer : Climb velocity
Explanation : The momentum equation for the helicopter in flight condition is given by:T=m?(V+w)-m?VT=m?wWhere, V is the climb velocityw is the induced velocity in the wake regionT is the thrustm? is mass fluxOn observing the equation, we see that the momentum equation is independent of the climb velocity.
Correct Answer : Non-uniform loading over the disk
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.
Correct Answer : Predict performance of airscrew airfoil
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 : Helix
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.
Correct Answer : Twice
Explanation : 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=2bΩThis value is twice that of the angular velocity in plane of the airscrew.
Correct Answer : Two-dimensional
Explanation : 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 : Account for varying blade geometry
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.
Correct Answer : 0.05-0.07
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:Where, V is the climb velocity which is zero for hoveringv is the induced velocityΩ is the rate of rotation of the bladeR is the radiusThe value of inflow ratio ranges between 0.05 and 0.07 for hovering.
Correct Answer : Plot of power coefficient as a function of thrust coefficient
Explanation : Plot of power coefficient Cp/σ as a function of thrust coefficient CT/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.
