Stability 2

On my last post, we discussed about stability on 3 axes. (Longitudinal/Lateral/Directional)

Some may ask; as we travel with airlines, does it mean the more stable the aircraft, the better? Then, why don't we make aircraft to maximum stability?

Answer is, Not exactly. There's multiple concerns.

One of the concern would be Free Directional Oscillations (Dutch roll) and Spiral Instability.

Dutch roll

per FAA PHAK,"Dutch roll is a coupled lateral/directional oscillation that is usually dynamically stable but is unsafe in an aircraft because of the oscillatory nature. The damping of the oscillatory mode may be weak or strong depending on the properties of the particular aircraft."

FAA is Not the best explanation.

Dutch roll can be observed on aircraft designed with Massive lateral stability and a relatively weak directional stability.

Dutch roll is a series of out-of-phase turns, when the aircraft rolls in one direction and yaws in the other. (see diagram above.)

When aircraft designed with a relatively high lateral stability, which has tendency for continuous oscillation; aircraft manufacturer would installed yaw dampener to counteract dutch roll without any pilot input.

Key concept:

Lateral Stability > Directional stability

series of out-of-phase turns

Spiral Instability (spiral dive)

Spiral instability is a bit easier to visualize.

"Spiral instability exists when the static directional stability of the aircraft is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium."

When aircraft got disrupted by gust

1. Lateral equilibrium disrupted

2. sideslip introduced

3. strong directional stability tends to yaw the nose into the resultant relative wind

4. Due to this yaw, the outer wing travels faster than the inside wing

5. the outer wing generate greater lift and introduce further bank into the turn.

IF NO pilot input, the bank will be steeper and steeper.

As aircraft continue to banks (and overbank tendency), vertical component of lift will decrease; as the aircraft descent, it will accelerate rapidly.

If not pilot correction being made, the excess airspeed will generate extra lift; however, with the overbanking tendency, the lift generated is mostly contributed to horizontal component of lift; which only "tighten" the spirial.

Improper recovery from spiral instability leading to inflight structural failures has probably contributed to more fatalities in general aviation aircraft than any other factor.

Since the airspeed in the spiral condition builds up rapidly, the application of back elevator force to reduce this speed and to pull the nose up only “tightens the turn,” increasing the load factor.

The results of the prolonged uncontrolled spiral are inflight structural failure, crashing into the ground, or both.

Therefore, If improper recovery techniques being used, it could end up with catastrophic accident.

Correct recovery technique:

1. Close the throttle immediately.

2. Roll the wing level (avoid rolling and pulling up at the same time).

3. Ease out of dive by pulling up,

4. Apply power only after the airspeed has decreased to within normal range and positive rate of climb.

Key Concept: Directional Stability > Lateral Stability

Another confusing idea from FAA PHAK "Because it is more desirable for the aircraft to have “spiral instability” than Dutch roll tendencies, most aircraft are designed with that characteristic."

I also got confused when I first see this; as my understanding, spiral instability is more "dangerous". However, as I do my research, spiral instability is not as deadly, as long as pilot input and proper recovery technique being made.

Also, Spiral instability (dive) is more noticeable to pilot; to prompt the recovery; compare to dutch roll, it might not be as noticeable to the crew in front. However, continuous oscillation can cause discomfort to passengers at the back ( max amplitude oscillation over the tail section)