Lateral Stability Questions and Answers

Answer: A

Spiral instability follows from too little dihedral effect relative to directional stability. Adding dihedral strengthens the rolling response to sideslip and rebalances the two modes, whereas a larger fin would worsen the imbalance.

Answer: B

Lowering an aileron raises local incidence on a wing already near its limit, so it may stall rather than gain lift. Rudder and spoilers avoid the problem because neither increases the local angle of attack anywhere on the wing.

Answer: C

The dihedral effect rolls the aeroplane away from the sideslip and the aileron must resist it. The ratio of aileron to sideslip is therefore a direct flight test measure of how strong that rolling response actually is.

Answer: D

Correct answer: sideslip

Bank alone produces no restoring rolling moment: an aeroplane will sit at a bank angle indefinitely as far as roll is concerned.

What follows the bank is a sideslip, because the tilted lift vector no longer balances weight vertically and the aeroplane slides towards the lower wing. The dihedral effect acts on that sideslip, raising the lower wing's incidence and generating the restoring moment. The recovery is therefore indirect, which is why \( C_{l\beta} \) is defined with respect to sideslip rather than to bank.

Answer: A

Correct answer: dihedral

The dihedral effect is the rolling moment produced by a sideslip, written \( C_{l\beta} \), and it takes its name from the geometric dihedral that is its most visible source.

The name is a little misleading, since geometric dihedral is only one contributor: wing sweep, wing position on the fuselage, and the fin's side force acting above the rolling axis all add to it. Designers therefore speak of effective dihedral - the total, expressed as the geometric angle that would produce the same moment.

Answer: B

Correct answer: Anhedral

Anhedral droops the wings and so subtracts from the dihedral effect, which is what a high-mounted swept wing needs.

Such a wing has two large contributions already: sweep, which grows with lift coefficient, and the high wing position's fuselage interference. Together they can give more effective dihedral than is wanted, and the symptom is a poorly damped Dutch roll. Drooping the wing trims the total back, which is why aircraft such as the C-17 and the An-124 show pronounced anhedral.

Answer: C

Correct answer: Adverse

Adverse yaw is the nose's tendency to move away from the intended turn when ailerons are applied, caused by the extra induced drag on the wing whose aileron is lowered.

The word names the direction: adverse because it opposes the turn being commanded. The opposite case, favourable or proverse yaw, is what spoilers produce by adding drag on the descending wing. The remedy for adverse yaw is coordinating rudder, or aileron designs - differential and Frise - that add offsetting drag on the rising wing.

Answer: D

Correct answer: reversal

Aileron reversal is the condition in which the wing twists enough to cancel and then outweigh the lift change the aileron produces, so the aeroplane rolls opposite to the input.

The mechanism is aeroelastic: a down-going aileron loads the section well aft, twisting the wing leading-edge-down and reducing its incidence. Both effects grow with dynamic pressure, but the twist grows faster because torsional stiffness is fixed. Flutter is a dynamic instability, float is a surface trailing with the flow, and balance concerns hinge moments.

Answer: A

The sweep contribution depends on the difference in lift between the two wings, which grows as the whole wing works harder. A swept wing aeroplane therefore feels much more strongly stable in roll on the approach than in cruise.

Answer: B

Correct answer: It is slow and easily controlled, whereas a bad Dutch roll is not

The table shows the trade: strengthening the dihedral effect stabilises the spiral but spoils the Dutch roll, and strengthening directional stability does the reverse. Something must be given up.

What makes the spiral the acceptable sacrifice is its timescale. A divergent spiral takes tens of seconds to double its bank angle and a pilot corrects it with a glance at the instruments. A poorly damped Dutch roll oscillates in seconds, is tiring and can be difficult to arrest - and it is the one certification scrutinises most closely.