Chapter 5 Flashcards
What is the assumption made for the first part of chapter 5?
- Gliding flight (no prop)
- Controls fixed (stick-fixed).
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What does stick fixed mean?
The controls are not moved by the pilot throughout the phase of flight being investigated.
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What is the primary understanding/knowledge to be gained by deriving the longitudinal, stick-fixed, equations for static stability?
Moments about the airplanes Y-axis, through the c.g. and their variation with the airplane’s lift coefficient.
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Static stability demands that a _____ moment accompany an increase in lift.
nose down
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Static stability demands that a stalling moment (nose up) accompany a decrease in ________ from equilibrium.
Lift Coefficient
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The aerodynamic center of any subsonic airfoil is located close to _______.
The quarter chord point.
x_ac = c/4
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What do the following symbols mean?
1. i_w
2. i_t
3. \varepsilon
- Incidence angle of the wing in degrees
- Incidence angle of the tail in degrees
- Average angle of downwash behind the wing at the horizontal tail (HT)
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All forces in static stability are usually resolved into ________. Why?
Normal and chordwise forces.
Because it’s easier to take moments about the c.g. if the forces are orthogonal or parallel to each other.
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For equilibrium, the moment about the c.g. must be ____.
Zero
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What is the mathematical definition of tail efficiency?
The dynamic pressure on the tail divided by the dynamic pressure on the wing.
qt/q
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Why is tail efficiency less than unity for power-off flight?
Because of the loss of energy as the air interacts with parts of the wing wake and fuselage boundary layer.
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For the airplane to be stable, the slope of Cm vs CL must be _______.
Negative.
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dCN/dCL is usually ______ for the wing.
Zero.
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The derivative dCM_ac / dCL is______ by definition.
Zero.
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The wing contribution to stability is a function of _______.
The position of the c.g. wrt/ the wing aerodynamic center.
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xa/c is the same as ______.
x_cg - x_ac
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If the c.g. is ahead of the a.c. with respect to the mean geometric wing chord, the aircraft is ________.
Statically stable.
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If the c.g. is behind the a.c. with respect to the mean geometric wing chord, the aircraft is _________.
Statically unstable.
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The x_cg and x _ac values are usually measured _________.
With respect to the mean geometric chord of the wing.
The derivative dCN_tail / dCL is not the same as dCN/dCL for the wing because ______.
The effects of downwash due to the wing wake.
What do the terms a_w and a_t represent? What are their mathematical definitions?
Both are lift curve slopes for the wing and the tail.
a_w =( dCN/dCL)_wing
a_t = (dCN/dCL)_tail
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Describe a wing’s “vortex system.”
A bound vortex is located at the wing quarter chord, and two trailing edge vortices emanating from the wing tips.
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What is the major influence of the downwash behind the wing with respect to the vortex model of lift generation?
The trailing vortices are the dominant factor.
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The strength of the vortex system behind the wing is proportional to the ________.
Lift Coefficient
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