Topic 1 Helicopter Limitations and Their Physics Study time (1.00) Content: 1. Balancing helicopter range performance 2. Speed Restrictions 3. Overload. - презентация

1 Topic 1 Helicopter Limitations and Their Physics Study time (1.00) Content: 1. Balancing helicopter range performance 2. Speed Restrictions 3. Overload Restrictions 4. Bank Angle Restrictions 5. Pitch angle restrictions 6. The angular of turn rate 7. Wind speed 8. Conclusion

2 WEIGHT LIMITATIONS Weight limitations are necessary to guarantee the structural integrity of the helicopter, as well as enabling you to predict helicopter performance accurately. Although aircraft manufacturers build in safety factors, you should never intentionally exceed the load limits for which a helicopter is certificated. Operating above a maximum weight could result in structural deformation or failure during flight if you encounter excessive load factors, strong wind gusts, or turbulence. 2

3 Helicopter Balancing Range Performance Balancing is controls process, during which a pilot seeking complete equilibrium by the control units, ie, all the forces and moments acting on the helicopter, are mutually balanced. Balancing deviations of control units and helicopter leverage vary with the speed, altitude, flight helicopter mass variants suspensions, etc., because of these factors depend on the value of the aerodynamic forces and moments included in the equilibrium equation. The allowable range in which the CG may fall is called the CG range. An out-of-balance loading condition a helicopter is difficult to control and also decreases maneuverability since cyclic control is less effective in the direction opposite to the CG location. Position of gravity center, extreme front and rear + 300/-95 mm 3

4 Balancing of the helicopter Ideally, a pilot should try to perfectly balance a helicopter so that the fuselage remains horizontal in hovering flight, with no cyclic pitch control needed except for wind correction. Since the fuselage acts as a pendulum suspended from the rotor, changing the center of gravity changes the angle at which the aircraft hangs from the rotor. When the center of gravity is directly under the rotor mast, the helicopter hangs horizontal; if the CG is too far forward of the mast, the helicopter hangs with its nose tilted down; if the CG is too far aft of the mast, the nose tilts up. 4

5 CG FORWARD OF FORWARD LIMIT You can recognize this condition when coming to a hover following a vertical takeoff. The helicopter will have a nose-low attitude, and you will need excessive rearward displacement of the cyclic control to maintain a hover in a no-wind condition. You should not continue flight in this condition, since you could rapidly run out of rearward cyclic control as you consume fuel. You also may find it impossible to decelerate sufficiently to bring the helicopter to a stop. In the event of engine failure and the resulting autorotation, you may not have enough cyclic control to flare properly for the landing. 5 A forward CG will not be as obvious when hovering into a strong wind, since less rearward cyclic displacement is required than when hovering with no wind. When determining whether a critical balance condition exists, it is essential to consider the wind velocity and its relation to the rearward displacement of the cyclic control. Extreme front position of gravity center creates maximal diving moment which can be safe compensated by cyclic controls pull up on all rang of flight speeds.

6 CG AFT OF AFT LIMIT The helicopter will have a tail-low attitude, and you will need excessive forward displacement of cyclic control to maintain a hover in a no-wind condition. If there is a wind, you need even greater forward cyclic. If flight is continued in this condition, you may find it impossible to fly in the upper allowable airspeed range due to inadequate forward cyclic authority to maintain a nose-low attitude. In addition, with an extreme aft CG, gusty or rough air could accelerate the helicopter to a speed faster than that produced with full forward cyclic control. 6 In this case, dissymmetry of lift and blade flapping could cause the rotor disc to tilt aft. With full forward cyclic control already applied, you might not be able to lower the rotor disc, resulting in possible loss of control, or the rotor blades striking the tailboom. Extreme rear position of gravity center creates maximal climbing moment which can be safe compensated by forward shift cyclic control to on all rang of flight speeds.

7 Speed Restrictions V min allowable V max allowable 7

8 Minimum Allowable Speed of a Helicopter H< m Н > m Unstable operation of УС-450 (airspeed indicator) Small reserves of engines power for maneuvering High vibration level Using all available power for maintaining constant flight height 8

9 Maximum Allowable Speed of a Helicopter H< m Н > m Occurrence of significant pressures into booster part of the control system from action of hinge moments on the blades of the main rotor Stall air flow from main rotor blade which influences to controllability deterioration and unintentional reaching of helicopter restrictions 9

10 Overload Restrictions n у max allowable =1,5 is limited due to restriction structural strength of a helicopter. n у min allowable =0,6 is limited due to stable operation of the hydraulic system. п х –is limited during acceleration only available power value, and during deceleration possibility of increasing rotor RPM more acceptable. n z =(0,15-0,2) n у is limited to tail part of helicopter strength 10

11 Bank Angle Restrictions γ max allowable – is limited due to maximum operating overload. The maximum allowable bank angles, depending on the altitude, degrees Note: 1. Accelerated bends and turns at altitudes m at normal takeoff weight or less on the flight speed km / h is allowed to perform with banks up to At altitudes up to 50 m above the terrain allowed the bank angle, the magnitude of equal altitude, but no more than the value specified in the table. Altitude, m Helicopter mass kg and less Helicopter mass more kg More

12 Pitch angle restrictions The maximum allowable pitch angle of ± 20 0 is limited to the possibility of output restrictions on the minimum allowable speed on rising figures, and during perform downward figures of output restrictions on maximum allowable speed. 12

13 The angular of turn rate Maximum turn rate at hovering is limited to 12 0 /sec because a helicopter has the small margin of directional (yaw) control, and in order to exclude the possibility of falling into a spontaneous rotation (autorotation). 13

14 Wind speed Spin up and Stopping main rotor TaxyTake-off and Landing The wind from front and side is restricted r because of the possibility of contact of the rotor blades with the tail boom. Wind back is limited on conditions of reliable start engines The wind is limited through the risk of a helicopter rollover Restrictions on the wind are the same as in the spin up (stop) of the main rotor 14

15 Reasons of side wind constraints: The risk of helicopter rollover on the ground after an increase in rotor thrust while taking a pitch blade during takeoff. Appearance of a sharp helicopter imbalance in the laterally and longitudinal directions during takeoff. Ensuring an adequate margin of longitudinal and lateral control during takeoff and landing, especially with MTOW. 15

16 The reasons for limiting tailwind: Instability of the position of the helicopter's tail to the wind hovering; Ensuring of adequate pull up longitudinal control margin with the maximum front helicopter centering; The risk of the ground hits by tail foot with maximum rear helicopter centering; Exhaust gases get into the engines entrance and as result reducing the available power. 16

17 Acceptable wind speed, m/sec Wind direction During spin up and stopping of main rotor During takeoff and landing Front20 Right side10 Left side1510 Tail810(5) –for a helicopter with armor protection 17

18 Conclusion Safe pilots are aware of the limitation of their helicopter. Never exceed for your helicopter limitation half a century of operating experience shows that Mi-8 is one of trustworthy helicopter of wold. 18

19 Zhukovsky curves 19

20 Influence of operational factors on available range altitudes and speeds 20

21 Questions? 21