# 7.1.2: Hypotheses

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Consider also the following hypotheses:

##### Hypothesis 7.1 Flat Earth model:

The Earth can be considered flat, non rotating, and approximate inertial reference frame. Figure 7.1: Wind axes reference frame.

##### Hypothesis 7.2 Constant gravity

The acceleration due to gravity in atmosphere flight of an aircraft can be considered constant ($$g = 9.81[m/s^2]$$) and perpendicular to the surface of Earth.

##### Hypothesis 7.3 Moving Atmosphere

Wind is taken into account. Vertical component is neglected due its low influence. Only kinematic effects are considered, i.e., dynamic effects of wind are also neglected due its low influence.

##### Hypothesis 7.4 6-DOF model

The aircraft is considered as a rigid solid with six degrees of freedom, i.e., all dynamic effects associated to elastic deformations, to degrees of freedom of articulated subsystems (flaps, ailerons, etc.), or to the kinetic momentum of rotating subsystems (fans, compressors, etc ), are neglected.

##### Hypothesis 7.5 Point mass model

The translational equations are uncoupled from the rotational equations by assuming that the airplane rotational rates are small and that control surface deflections do not affect forces. This leads to consider a 3 Degree Of Freedom (DOF) dynamic model that describes the point variable-mass motion of the aircraft.

##### Hypothesis 7.6 Fixed engines

We assume the aircraft is a conventional jet airplane with fixed engines.

##### Hypothesis 7.7 Variable mass

The aircraft is modeled as variable mass particle.

##### Hypothesis 7.8 Forces acting on an aircraft

The external actions acting on an aircraft can be decomposed, without loss of generality, into propulsive, aerodynamic, and gravitational.

##### Hypothesis 7.9 Symmetric flight

We assume the aircraft has a plane of symmetry, and that the aircraft flies in symmetric flight, i.e., all forces act on the center of gravity and the thrust and the aerodynamic forces lay on the plane of symmetry.

##### Hypothesis 7.10 Small thrust angle of attack

We assume the thrust angle of attack is small.

7.1.2: Hypotheses is shared under a CC BY-SA 3.0 license and was authored, remixed, and/or curated by Manuel Soler Arnedo via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.