# Aircraft Design Questions and Answers – Stick-Free Stability

This set of Aircraft Design Multiple Choice Questions & Answers (MCQs) focuses on “Stick-Free Stability”.

1. Which of the following is incorrect?
a) Uncambered wing lift curve is not same as that of the Cambered
b) Stick free and stick fixed are same
c) Lift is not always same as weight
d) Thrust required is not always same power

Explanation: Wing lift curve is different for uncambered wing and Cambered wing. Stick free and stick fixed phenomena are not same. Lift is not always same as weight. During cruise lift and Weight will be equal. Thrust required is calculated based on drag of the system.

2. Which of the following is correct?
a) Elevator can float in stick free condition
b) Drag is always same as weight
d) Stability is same for every aircraft

Explanation: Stick free condition allows floating of elevator. Power loading is defined as the ratio of weight of the aircraft to the power produced. Stability of aircraft depends on the design requirements. Military aircraft is less stable whereas commercial are more stable.

3. Stick free stability permits ____________
a) elevator to float
b) elevator to be fixed
c) fixed rigid body
d) lift and drag ratio

Explanation: In stick free, we allow elevator to float as angle of attack or relative wind changes. In stick fixed we will deflect elevator at some angle and then fix it at that angle. Stick free stability depends upon number of factors such as hinge moment etc.

4. Stick fixed and stick free are similar.
a) True
b) False

Explanation: No, stick free and stick fixed are not similar. Floating of elevator is permitted in stick free however, in stick fixed elevator is not permitted to float. In stick fixed, some desired elevator deflection is given as required. At that position, elevator is fixed and not allowed to float.

5. Which of the following is correct?
a) Aircraft static longitudinal stability will be dependent upon the arrangement of different components
b) Lift is always same as weight
c) Static stability is similar to dynamic stability
d) All the aircrafts are statistically stable

Explanation: Static longitudinal stability of aircraft will be dependent upon the arrangement of different components such as wing placements, tail location etc. At cruise condition or at trim position lift will be same as weight. Static stability and dynamic stability are different. System can be statically stable but that doesn’t mean that the system is dynamically stable as well.
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6. Stick force is function of ___________
a) hinge moment
b) lofting
c) fuel air ratio
d) pressure ratio of inlet

Explanation: Stick force is function of hinge moment. Lofting is a mathematical modelling used to define different cross section or geometry. Fuel air ratio is related to aircraft engine. Pressure ratio of inlet will be dependent of the type of inlet.

7. Which of the following is correct for hinge moment?
a) Hinge moment coefficient is function of tail angle of attack
b) Hinge moment is not dependent on the tail angle
c) Downwash is used to reduces angle of attack at tail
d) Lifting property of the airfoil is same as a sphere

Explanation: Hinge moment coefficient is function of number of factors such as tail angle of attack, tail setting angle, etc. Due to downwash the angle of attack at tail will be more. Lifting property of an airfoil and sphere will be different. Airfoil is 2d shape and sphere is 3d shape.

8. Hinge moment coefficient depends upon ________
a) Tail AOA, elevator deflection, etc
b) Only tail angle
c) Only wing angle
d) Lift to drag ratio of fuselage

Explanation: Hinge moment coefficient depends upon number of factors such as elevator deflection, angle of attack at tail, tail setting angle etc. Lift to drag ratio is defined as aerodynamic efficiency of the aircraft. Fuselage is primary drag producing member of the aircraft. Lift to drag ratio of the fuselage will be less.

9. Negative stick force gradient is used to provide ___________
a) speed stability
b) thrust
c) propulsive force
d) gliding ratio

Explanation: Negative stick force gradient is used to provide speed stability. Thrust is a propulsive force. Thrust is generated by aircraft engine. Propulsive force is a typical force which is responsible for driving the aircraft or a system in forward direction typically. Gliding ratio is related to glider.

10. Find the degree of elevator float. Consider tail AOA is 2° and CHδe= -0.005, CHɑt=-0.0052.
a) 2.08 deg
b) 4
c) 12
d) 2.119

Explanation: Given, tail AOA ɑ = 2°, CHδe = -0.005, CHɑt=-0.0052
Elevator float = -(CHɑt / CHδe)*ɑ = -(-0.0052/-0.005)*2 = -2.08°.
Considering only magnitude. Hence, 2.08°.

11. Determine the value of tail angle of attack if, elevator floats down by 1.8 degree. Consider CHδe as 0.003 and CHɑt as -0.006.
a) 0.9
b) 2.5
c) 3.4
d) 1.8

Explanation: Given, elevator float e = 1.8°. CHδe = 0.003, CHɑt=-0.006.
Now, tail angle of attack = – e* CHδe / CHɑt = -1.8*0.003/-0.006 = 0.9°.

12. Find the value of CHδe if tail angle of attack is 3 deg and CHɑt is -0.004. Consider elevator floats up by 2 degree.
a) -0.006
b) -7
c) 1.223
d) 1.225

Explanation: Given, tail AOA ɑ=3°, elevator float e =-2° (upward deflection is negative), CHɑt = -0.004
Now, CHδe = – ɑ * CHɑt / e = -3*-0.004/-2 = -0.006.

13. Stick fixed neutral point is defined at 40% of chord. Choose appropriate location of Stick free neutral point.
a) 0.39
b) 0.8
c) 0.65
d) 0.5

Explanation: Typically, stick free neutral point is located at 2-5% ahead of stick fixed neutral point.
Stick free neutral point = 2 to 5 % ahead of stick fixed = 0.02*0.4 to 0.05*0.4 ahead of stick fixed = 0.008 to 0.1 ahead of stick fixed neutral point
Hence, neutral point location of stick free = 0.4-(0.008 to 0.1) = 0.392 to 0.39.
Hence, Correct answer from the given options Will be 0.39.

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