Chapter

12

Flight Mechanics

Introduction						365
Straight Horizontal Steady Flight						365
Tailplane and Elevator . . . . . . . . . . . . . . . .	. .	. .	. .	. . . .		. 366
Straight Steady Climb						368
Climb Angle . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. . . .		. 369
Effect of Weight, Altitude and Temperature.						369
Power-on Descent						370
Emergency Descent						371
Glide . . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. . . .	.	372
Rate of Descent in the Glide . . . . . . . . . . . . . .	. .	. .	. .	. . . .		. 374
Turning						374
Flight with Asymmetric Thrust						384
Summary of Minimum Control Speeds . . . . . . . . . .	. .	. .	. .	. . . .		. 395
Questions . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. . . .	.	398
Answers . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. . . .	.	404

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12 Flight Mechanics

Mechanics Flight 12

364

Flight Mechanics

Introduction

Flight Mechanics is the study of the forces acting on an aircraft in flight and the response of the aircraft to those forces. For an aircraft to be in steady (unaccelerated) flight, the following conditions must exist:

•the forces acting upward must exactly balance the forces acting downward,

•the forces acting forward must exactly balance the forces acting backward, and

•the sum of all moments must be zero.

This condition is known as equilibrium.

Straight Horizontal Steady Flight

In straight and level flight there are four forces acting on the aircraft: LIFT, WEIGHT, THRUST and DRAG, as shown in Figure 12.1.

Weight acts through the aircraft centre of gravity (CG), vertically downwards towards the centre of the earth. Alternatively, weight can be defined as acting parallel to the force of gravity.

Lift acts through the centre of pressure (CP), normal (at 90°) to the flight path.

For the purposes of this chapter (although not strictly true), thrust acts forwards, parallel to the flight path and drag acts backwards, parallel to the flight path.

AERODYNAMIC	L
DRAG

THRUST

REQUIRED

TO BALANCE

AERODYNAMIC DRAG

D	T FLIGHT PATH

W

Figure 12.1 Forces in level flight

12

Flight Mechanics 12

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12 Flight Mechanics

Mechanics Flight 12

For an aircraft to be in steady level flight, a condition of equilibrium must exist. This unaccelerated condition of flight is achieved with the aircraft trimmed with lift equal to weight and the throttles set for thrust to equal drag. It can be said that for level flight the opposing forces must be equal.

The L/D ratio of most modern aircraft is between 10 and 20 to 1. That is, lift is 10 to 20 times greater than drag.

The lines of action of thrust and drag lie very close together, so the moment of this couple is very small and can be neglected for this study. The position of the CP and CG are variable and under most conditions of level flight are not coincident. The CP moves forward with increasing angle of attack and the CG moves with reduction in fuel. Generally, the CP is forward of the CG at low speed, giving a nose-up pitching moment and behind the CG at high speed, giving a nose-down pitching moment.

Tailplane and Elevator

The function of the tailplane is to maintain equilibrium by supply the force necessary to counter any pitching moments arising from CP and CG movement. With the CP behind the CG during normal cruise, as illustrated in Figure 12.2, the tailplane must supply a downforce.

L	MOMENT DUE TO
	LIFT / WEIGHT COUPLE

TAIL

MOMENT

DT

TAIL

DOWN

FORCE

W

Figure 12.2 Tailplane maintains equilibrium

366