434 Aircraft Performance

Rclimb = s, is obtained by summing the values obtained in the small steps of altitude gain. The fuel consumed for a climb, Fuelclimb = fuel, is obtained by summing the values obtained in the small steps of altitude gain.

Descent

A descent uses the same equations as for a climb except that the thrust is less than the drag; that is, the rate of descent (R/Daccl) is the opposite of the rate of climb. The rate of descent is expressed as follows:

Unlike in a climb, gravity assists a descent; therefore, it can be performed without any thrust (i.e., the engine is kept at an idle rating, producing zero thrust). However, passenger comfort and structural considerations require a controlled descent with the maximum rate limited to a certain value depending on the aircraft design. A controlled descent is carried out at a partal-throttle setting. To obtain the maximum range, an aircraft should ideally make its descent at the desired minimum rate. These adjustments entail varying the speed at each altitude. To ease the pilot’s workload, a descent is made at a constant Mach number; when the VEAS limit is reached, the aircraft adapts to a constant VEAS descent, similar to a climb. Special situations may occur, as follows:

1. For an unaccelerated descent, Equation 13.19 becomes:

At a higher altitude, the prescribed speed schedule for a descent is at a constant Mach number; therefore, the previous tropopause VTAS is constant and the descent is maintained in an unaccelerated flight.

This indicates that at a constant V(L/D), the R/Cdescent is the same for all weights. As in a climb, the other parameters of interest during a descent are range covered (Rdescent), fuel consumed (Fueldescent), and time taken (timedescent). There are no FAR requirements for the descent schedule. The descent rate is limited by the cabin-pressurization schedule for passenger comfort. FAR requirements are enforced during an approach and a landing. At high altitude, the inside cabin pressure is maintained as an approximate 8,000-ft altitude. Depending on the structure design, the differential pressure between the inside and the outside is maintained at

approximately 8.9 lb/in2.

Integrated performances for a climb to cruise altitude and a descent to sea level are computed, and the values for distance covered, time taken, and fuel consumed are estimated to obtain the aircraft payload range. Textbooks may be consulted for details of climb and descent performances.