Programmed Testing Questions and Answers

In Chapters 1 and 2 we have examined concepts which are of considerable importance in themselves and ensure further successful study of helicopter aerodynamics. We shall present some questions and answers to test the readers’ knowledge of this information.

The objective is to select the most complete and correct answer from three or four possibilities. Some of the answers given are completely incorrect, most of the answers are simply incomplete.

Question 1. Definition and purpose of blade geometric twist.

Answer 1. Geometric twist involves variation of the incidence angles of the blade elements. Twist is provided to distribute the loads uniformly over the blade and increase main rotor thrust.

Answer 2. Geometric twist involves variation of the blade element inci­dence angles along the main rotor radius. The root elements have larger incidence angles,.and the tip elements have smaller angles. Twist gives the blade elements angles of attack close to the optimal values and increases the main rotor thrust by 5-7%. Twist results in more uniform loading on the individual blade elements and delays flow separation from the tip portion of the blade.

Answer 3. Geometric twist is the difference between the incidence angles at the root and tip sections of the blade. Twist provides minimal incidence angles at the root elements and maximal angles at the tip elements. This is necessary to obtain higher rotor efficiency, increase thrust, and achieve more uniform loading on the different parts of the blade.

Question 2. Main rotor operating regime coefficient. /37

Answer 1. The main rotor operating regime coefficient is the dimension­less number p, equal to the ratio of the helicopter flight speed to the blade tip induced velocity

V

Answer 2. The main rotor operating regime coefficient is the number p, equal to the ratio of the projection of the flight velocity on the main rotor hub axis to the blade tip circumferential velocity

V sin A

JJ* rj ‘ •

‘ ID/?

Answer 3. The main rotor operating regime coefficient is the number p, equal to the ratio of the helicopter flight speed to the blade tip angular velocity

■л = —

* o> ■

Answer 4. The operating regime coefficient is the number p, equal to the ratio of the projection of the helicopter flight speed on the main rotor hub plane of rotation to the blade tip circumferential velocity

V cos A ^ ” <»R

Question 3. What is the connection between the operating regime coefficient and the main rotor operating regime?

Answer 1. The larger y, the larger the main rotor induced velocity and the closer its operating regime approaches the axial flow regime.

Answer 2. If у = 0, this indicates the axial flow regime. The larger y, the more effectively the properties of the axial flow regime manifest themselves.

Answer 3. Increase of the coefficient у indicates increase of the main rotor angle of attack and approach of its operating regime to the axial flow regime.

Question A. What is the connection between the main rotor angle of attack and its operating regime?

Answer 1. The main rotor angle of attack is the angle between the flight velocity vector and the hub rotation plane. In the axial flow regime, the main rotor angle of attack A = + 90°, in the inclined flow regime А Ф + 90°.

Answer 2. The main rotor angle of attack is the angle between the flight velocity vector and the hub axis. If the main rotor angle of attack A = 90°, the rotor is operating in the axial flow regime. However, if A ^ 90°, it is operating in the inclined flow regime.

Answer 3. The main rotor angle of attack is the angle between the plane of rotation of the main rotor and the vector of the undisturbed flow approach­ing the rotor. For A = 0° the inclined flow regime is present; for А ф 90° the flow regime is axial.

Question 5. What is main rotor thrust, and on what does it depend?

Answer 1. Main rotor thrust is the aerodynamic force which arises during rotor rotation as a result of the difference of the air pressure on the rotor blades

7*=– CT Z7 -— /Л

The thrust depends on the rotor area thrust coefficient, flight speed, and air density. The thrust coefficient depends on rotor rpm and blade element pitch.

Answer 2. Main rotor thrust is the aerodynamic force directed along the main rotor axis and formed as a result of the difference of the air pressures below and above the rotor

T = CTF-^-W<»-

The thrust depends on the thrust coefficient, main rotor area or radius, air density, and main rotor rpm. The thrust coefficient depends on the pitch.

Answer 3. Main rotor thrust is the aerodynamic force which arises as a result of the difference of the air pressure below and above the rotor

T = Cr5-|-u>/?/? or 7 = 2?FV].

The main rotor thrust depends on the thrust coefficient, area swept by the rotor, rotor pitch, and rotor rpm. The thrust increases with increase of the pitch and rpm.

Question 6. What is the main rotor reactive torque, what does it depend on, and how does it act?

Answer 1. The reactive torque is the torque opposing rotor rotation

M = Q, Rk
г Чэ

It retards rotor rotation and yaws the helicopter opposite the direction of rotation. The reactive torque depends on the rotor rpm, air density, rotor pitch, and flight speed.

Answer 2. The reactive torque is the moment of the forces of resistance to rotation about the hub axis. It is defined by the formula

Mr=VQk*

It depends on the rpm, pitch, air density, surface condition and flight speed. It retards rotor rotation and yaws the helicopter opposite the direction of rotor rotation.

Answer 3. Reactive torque is the moment of the forces of resistance to rotation, directed opposite the rotor direction of rotation, retarding rotor rotation and yawing the helicopter opposite the direction of rotation. It depends on the flight speed, rpm, and air density

Mr = 2Qbrk.

Question 7. Power required to rotate the main rotor and the constant rpm conditions.

Answer 1. The power required to turn the main rotor depends on the rpm, pitch, flight speed, and air density

If N = N the rpm is constant; if N > N the rpm increases, sup req sup req

Answer 2. The power used to overcome the reactive torque depends on the rotor pitch, rpm, and flight speed

= M ш.
r

		the rpm
increases

Answer 3. The power required to turn the main rotor and overcome the retarding action of the reactive torque depends on the main rotor thrust, rpm air density, and flight speed

When N = N the rpm is constant; when N > N the rpm increases sup req sup req

Question 8. What is the rotor blade element angle of attack and how is it changed?

Answer 1. The blade element angle of attack is the angle between the blade chord and the resultant velocity vector. It depends on the blade ele­ment pitch, induced velocity, and helicopter flight speed. The larger the induced velocity, the lower the angle of attack. The larger the vertical climbing velocity, the lower the angle of attack.

Answer 2. The blade element angle of attack is the angle between the blade element chord and the resultant velocity vector. It depends on the flight speed and induced flow downwash angle. With increase of the induced velocity, the angle of attack increases, with increase of the flight velocity it decreases.

Answer 3. The blade element angle of attack is the angle between the chord and the circumferential velocity vector. It depends on the pitch and helicopter flight speed. With increase of the vertical descent velocity, the angle of attack increases. With increase of the vertical climbing velocity, the angle of attack decreases.