# Aircraft Performance Questions and Answers – Equations of Motion for Performance

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This set of Aircraft Performance Interview Questions and Answers for freshers focuses on “Equations of Motion for Performance”.

1. Which of the following is a correct equation?
a) Fa+Fp+Fg=Fl
b) Fa+Fp+Fg=Fl
c) Fa+Fp+Fg=Fl
d) Fa+Fp+Fg=Fl

Explanation: The correct equation is Fa+Fp+Fg=Ft where Fa=aerodynamic forces, Fp=propulsive forces, Fg=gravitational forces and Fl=inertial forces. The statement says that the system of forces containing gravitational forces, aerodynamic forces and propulsive forces results in the inertial forces acting on the aircraft.

2. The system of forces acting on the aircraft are propulsive forces, aerodynamic forces, gravitational forces and these result in inertial forces.
a) True
b) False

Explanation: The system of forces acting on the aircraft are propulsive forces, aerodynamic forces, gravitational forces and these result in inertial forces. From Newton’s law the equation becomes Fa+Fp+Fg=Ft where Fa=aerodynamic forces, Fp=propulsive forces, Fg=gravitational forces and Fl=inertial forces.

3. What is the basic assumption taken in formulating equations of motion?
a) All the engines will operate at equal gross thrust
b) All the engines do not operate at equal gross thrust
c) All the engines will operate at equal gross drag
d) All the engines do not operate at equal gross drag

Explanation: The basic assumption taken in formulating equations of motion is all the engines will operate at equal gross thrust. There are other assumptions also taken in the case of convectional aircraft there are:

• Rate of change of mass is neglected
• Aircraft is in symmetric flight
• Gross thrust acts in aircraft body axis
• Total net thrust is T cosα-DM
• Thrust component is small compared to lift component.
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4. Which of the following is not the additional assumption taken in the derivation of the equations of motion?
a) Gross thrust acts in aircraft body axis
b) Thrust component is small compared to lift component
c) Aircraft is in symmetric flight
d) Rate of change of mass is neglected

Explanation: The aircraft is symmetrical flight during the derivation of equations of motion.The other assumptions taken are:

• Rate of change of mass is neglected
• Aircraft is in symmetric flight
• Gross thrust acts in aircraft body axis
• Total net thrust is T cosα-DM
• Thrust component is small compared to lift component.

5. The total thrust force assumed in the derivation of equations of motion is ___________
a) FN=T cosα+DM
b) FN=T cosα-DM
c) T cosα=DM-FN
d) DM=T cosα+FN

Explanation: The total thrust force assumed in the derivation of equations of motion is FN=T cosα-DM where FN=net thrust force, T is thrust force, cosα is lift component and DM is drag force acting downwards.

6. In static performance the acceleration of the flight is zero.
a) True
b) False

Explanation: In static performance the acceleration of the flight is zero. In this case the resulting conditions are proved to be as:

• Thrust is equal to drag and
• Lift is equal to weight of the aircraft.

7. Excess thrust in the longitudinal equations of motion is given by ____________
a) FN+D
b) FN-D
c) D-FN
d) FN*D

Explanation: Excess thrust in the longitudinal equations of motion is given by FN-D where FN is net thrust force and D is drag force. This excess thrust provides the increase in potential energy and kinetic energy.

8. If the aircraft has a power-producing engine which have driven propellers then the power is to be converted to thrust to formulate the equations of motion.
a) True
b) False

Explanation: If the aircraft has a power-producing engine which have driven propellers then the power is to be converted to thrust to formulate the equations of motion. The conversion is given as FN=ṁ(Vi-V) where FN is net thrust force, ṁ is mass flow rate and Vi, V are velocities.

9. What is the function of excess thrust in longitudinal equations of motion?
a) Decrease the potential energy and kinetic energy
b) Increase the potential energy and kinetic energy
c) Decrease the potential energy and increase kinetic energy
d) Increase the potential energy and decrease kinetic energy

Explanation: The function of excess thrust in longitudinal equations of motion is to increase the potential energy and kinetic energy. Excess thrust in the longitudinal equations of motion is given by FN-D where FN is net thrust force and D is drag force.

10. The aircraft is symmetrical flight during the derivation of equations of motion.
a) True
b) False

Explanation: The aircraft is symmetrical flightduring the derivation of equations of motion. The other assumptions taken during the derivation of equations of motionare:

• Rate of change of mass is neglected
• Aircraft is in symmetric flight
• Gross thrust acts in aircraft body axis
• Total net thrust is T cosα-DM
• Thrust component is small compared to lift component.

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