A century of phugoid approximations by S. Pradeep


By S. Pradeep

Airplane layout 1 (1998) 89—104

It is acknowledged in such a lot flight dynamics texts that the phugoid approximations offer terrible estimates whereas the quick interval approximations are actual. The survey during this paper unearths that there are a minimum of 5 approximations to the phugoid. the level of departure of every of those from the precise price is set for a pretty broad database representing quite a few airplane in several flight stipulations. it's chanced on that almost all of them are insufficient in predicting the phugoid features competently. still, approximations to the phugoid frequency that appear to have remained omitted are noticeable to be exemplary. nevertheless, no priceless approximation exists for the phugoid damping. With this history, a clean approximation for the phugoid mode is positioned forth herein. it truly is derived by way of equating the coefficients of the made of the fast interval equation (which has been proven to be very exact) and the phugoid equation (as but unknown) to the coefficients of the fourth-order attribute polynomial. the recent approximation is proven to be exact.

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When a helicopter is hovering, the tail rotor is operating in very disturbed airflow. As the helicopter achieves ETL, the tail rotor begins to generate much more thrust because of the less disturbed airflow. The helicopter reacts to the increased thrust by yawing. Therefore, as the helicopter achieves ETL, you must reduce tail rotor thrust by pedal input at about the same time that you need to make cyclic adjustments for lateral tracking, acceleration, and climb. Induced Flow As the rotor blades rotate, they generate what is called rotational relative wind.

A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft portion of the rotor blade. Therefore, the angle of attack is less and the induced flow is greater at the rear of the rotor disk. 2-22 Resultant Resultant Lift Lift Thrust Thrust Drag Drag Helicopter movement Resultant Figure 2-41. Forces acting on the helicopter during sideward flight. when maneuvering the helicopter sideways to avoid such hazards from happening. Refer to Chapter 11, Helicopter Hazards and Emergencies.

If the amount of lift is greater than the actual weight, the helicopter accelerates upwards until the lift force equals the weight gain altitude; if thrust is less than weight, the helicopter accelerates downward. When operating near the ground, the effects of the proximity to the surface change this response. The drag of a hovering helicopter is mainly induced drag incurred while the blades are producing lift. There is, however, some profile drag on the blades as they rotate through the air and a small amount of parasite drag from the non-lift-producing surfaces of the helicopter, such as the rotor hub, cowlings, and 2-13 landing gear.

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