Empennage Sizing Questions and Answers

Answer: A

Correct answer: Fit a stick pusher to prevent entry

Deep stall is treated by prevention rather than recovery, because a developed deep stall may be unrecoverable - the blanketed tailplane leaves the elevator without the authority to lower the nose.

The standard installation is a stall warning system with a stick shaker and, where the testing shows the hazard, a stick pusher that forces the nose down automatically before the critical angle is reached. Moving the centre of gravity aft would make matters worse, and neither rudder chord nor fin aspect ratio bears on a longitudinal problem.

Answer: B

Correct answer: The extra cabin or hold volume it also provides

Both options buy the same tail volume, so the choice turns on what else each brings. A fuselage extension adds usable length - another seat row, or more hold - which is revenue the tail area cannot generate.

The other claims do not hold. A fuselage plug is generally the more expensive change, since it disturbs systems routing and the structure; a 1.5 m barrel section has more wetted area than 15 per cent of a tailplane; and the extension moves the tail's mass further aft, shifting the centre of gravity more, not less.

Answer: C

The minimum control speed falls when the rudder can balance the asymmetric moment at a lower dynamic pressure, so more rudder power is the direct lever. Moving engines outboard increases the moment and makes the problem worse.

Answer: D

Correct answer: static margin

The static margin is

\[ K_n = \frac{x_{np} - x_{cg}}{\bar{c}} \]

and it is the standard measure of longitudinal static stability, positive when the centre of gravity lies forward of the neutral point. Transports typically require a minimum of around 5 per cent.

The tail volume coefficient is a geometric ratio of the tail to the wing; the aerodynamic centre is the point about which the pitching moment does not vary with angle of attack; and the neutral point is one end of the distance the static margin measures, not the distance itself.

Answer: A

Correct answer: tail arm

The tail arm is the distance between the wing's aerodynamic centre and the tail's, and it is the lever through which the tail's force becomes a moment about the centre of gravity.

It appears in the numerator of both tail volume coefficients, so a longer arm buys tail volume without tail area - the central trade in empennage sizing. The mean aerodynamic chord is a wing reference length, and the neutral point and static margin are longitudinal stability quantities rather than geometric distances between surfaces.

Answer: B

Correct answer: ruddervators

A V-tail's two surfaces carry ruddervators, which perform both the elevator's and the rudder's functions. Deflected together they pitch the aircraft; deflected differentially they yaw it.

A mixer in the control run combines the pilot's pitch and yaw inputs into the two surface commands, and the coupling that results is the configuration's main handling drawback. Elevons combine elevator and aileron on a tailless or delta wing, flaperons combine flap and aileron, and spoilerons use spoilers for roll - each a different pairing of functions.

Answer: C

Correct answer: dorsal fin

A dorsal fin is the fixed fillet extending the fin forward along the top of the fuselage. Its very low aspect ratio and highly swept leading edge shed a stable vortex that keeps the flow attached to the main fin far beyond its natural stall angle.

That preserves directional stability in a large sideslip and prevents rudder lock. A ventral fin does related work below the fuselage, staying clear of the wake at high angle of attack; a strake is a smaller vortex generator; and a fairing is a purely aerodynamic smoothing of a junction.

Answer: D

Correct answer: deep stall

Deep stall is the condition in which the wing's separated wake rises at extreme angle of attack and blankets a high-mounted tailplane, so the elevator has no clean air to work in and cannot lower the nose.

It is self-sustaining, the aircraft descending at high incidence with the wake still over the tail, and recovery may be impossible - which is why the treatment is prevention by stick shaker and pusher. Flutter is an aeroelastic instability, buffet is separated-flow vibration, and Dutch roll is a lateral-directional oscillation.

Answer: A

Correct answer: P

Read the tail volume column first to find which options are acceptable, then compare their areas. P, Q and S all reach about 0.95 to 0.98, while R at 0.79 falls short.

Among the three that qualify, P uses 24 m\(^2\) against Q's 30 and S's 34. It achieves that with the longest arm of the three, 17 m, which is the trade the table is built to illustrate: area buys tail volume expensively, while arm buys it cheaply wherever the fuselage length can be found.

Answer: B

Correct answer: Vertical values are roughly an order of magnitude below horizontal ones

Read the two columns together: 0.80 against 0.070, 1.05 against 0.085, 1.10 against 0.090, 0.55 against 0.040 - a factor of about twelve in every row.

The reason is the reference length rather than any weakness of the fin. \( V_H \) divides by the mean aerodynamic chord and \( V_V \) by the span, which is much larger, so the two are not comparable in magnitude. The table also refutes the fourth option, since both coefficients rise from the general aviation single to the wide-body.