Remembering/Knowledge
Define static longitudinal stability in terms of the pitching moment coefficient (Cmα).
Cmα<0 (negative slope of Cm vs. α curve).
Explain why a forward CG enhances static longitudinal stability.
Increases the moment arm between CG and aerodynamic center, amplifying restoring moments.
An aircraft has a negative Cm0. What does this imply about its trim angle of attack?
Requires positive α to trim (Cm=0), typical for conventional tail-aft designs.
Analyze why a canard configuration can achieve trim but complicates static stability.
Canard lift adds to total lift, but stability requires careful CG-tail balancing.
Justify the use of relaxed static stability in modern fighter aircraft.
Improves maneuverability; stability is artificially restored via fly-by-wire.
What is the primary function of the horizontal tail in longitudinal stability?
To provide a restoring moment about the CG after an angle-of-attack disturbance.
How does elevator deflection affect the Cm vs. α curve?
Shifts the curve vertically (changes Cm0) without altering slope (Cmα).
Predict the effect of reducing horizontal tail area on the neutral point.
NP moves forward, reducing static margin (less inherent stability).
Compare the stick-force gradients of a stable vs. neutrally stable aircraft.
Stable aircraft has negative gradient (pull to slow down); neutral has zero gradient.
Design a test procedure to evaluate stick-free stability in a wind tunnel.
Measure elevator float angle vs. αα with free-floating hinges, record hinge moments.
Name the two requirements for an aircraft to achieve trimmed flight.
Describe the role of trim tabs in reducing control forces.
They deflect to neutralize hinge moments, allowing zero stick force at trim.
How would flap deployment affect longitudinal trim requirements?
Nose-down pitching moment increases, requiring more up-elevator to trim.
Why does stick-free stability typically require a larger static margin than stick-fixed?
Elevator float reduces tail effectiveness, necessitating more inherent stability.
Evaluate the trade-offs of using all-moving tails vs. conventional elevators.
All-moving tails reduce hinge moments but complicate control linkage.
What is the neutral point (NP)?
The CG location where Cmα=0 (boundary between stable/unstable).
Why does maneuver stability require more negative Cmα than static stability?
To counteract additional lift-induced pitching moments during maneuvers.
An aircraft exhibits elevator float in stick-free conditions. How does this impact stability?
Reduces effective tail contribution, degrading stick-free stability.
Discuss the impact of supersonic flight on Cmα and neutral point.
NP shifts aft due to AC movement, reducing Cmα (less stable or unstable).
A prototype exhibits poor maneuver stability. Recommend two fixes
(1) Move CG forward,
(2) Increase tail volume ratio (VH).
Identify the control surface responsible for adjusting trimmed angle of attack.
Elevator (or trim tab).
Contrast stick-fixed vs. stick-free stability.
Stick-fixed assumes fixed elevator; stick-free accounts for elevator float due to hinge moments
Propose a design change to improve maneuver stability without moving the CG.
Increase horizontal tail area or moment arm (VH).
Critique the statement: "An aircraft with Cmα=0 is uncontrollable."
False—it lacks inherent stability but can be controlled actively (e.g., FBW).
An aircraft’s stick forces are too heavy. Propose solutions.
(1) Add trim tabs,
(2) Reduce elevator chord,
(3) Use aerodynamic balancing