The final elevators position causes a decrease in the lift force on the horizontal stabilizer.Moving up or down (both left and right simultaneously), the elevators are able to vary the vertical forces on the horizontal stabilizer making the aircraft climb or dive in a short term (assuming constant the other aircraft commands inputs) (Figure 2).įigure 1. Angle-of-attack and lift force, assuming level flightįigure 2. Elevator commands and longitudinal dynamics.Ĭonsidering our aircraft, flying straight and steadily with constant speed and altitude, if the elevators are moved upwards, we have the following sequence of events: More specifically, the longitudinal dynamics of the aircraft will be performed by the elevators, the small moving surfaces at the rear of the horizontal stabilizer. The basics of the aircraft longitudinal commands (upwards and downwards) will be provided by the balance of the forces generated by the main wing and the horizontal stabilizer (little wing at the rear of the fuselage).
A conventional aircraft, flying straight with no changes in its altitude, has wings with this magical shape such that, roughly speaking, the greater the angle between a centerline through the wing section and the wind direction (called angle-of-attack), the greater the magnitude of the supernatural force upwards, called lift (Figure 1). Let us start picking up some facts very well explained by the giants (Stevens And Lewis, John Anderson, Robert Nelson, etc). I’m assuming the reader has only some Mechanics background in the beginning, but the last section will require some Control Theory background, once that we will try to illustrate this puzzling concept called Non-Minimum-Phase (NMP) zero. This fact is well explained by the aeronautical engineering experts and I’ll try to build up this explanation throughout the following lines. There is a very subtle and intriguing fact (at least for flight mechanics geeks) when the aircraft is commanded “to climb up”: the very first response of the aircraft body, for a short time period, is in the other way round, i.e., it dives a very little bit before climbing up. Imagine a conventional aircraft, like the ones which are flying across the world carrying passengers, for example.
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