Stability of Airplane & Wingtip Vortices ( Dynamic, Static, Directional Lateral)

Pilot training

Stability of Airplane & Wingtip Vortices ( Dynamic, Static, Directional Lateral)

Flight-with-JC 2020. 5. 13. 14:44

Static Stability

Let's start with static stability. Static stability is the initial tendency of an aircraft to return to its original position when it's disturbed.

There are three kinds of static stability:

Positive
Neutral
Negative

Positive Static Stability

An aircraft that has positive static stability tends to return to its original attitude when it's disturbed. Let's say you're flying an aircraft, you hit some turbulence, and the nose pitches up. Immediately after that happens, the nose lowers and returns to its original attitude. That's an example of positive static stability, and it's something you'd see flying an airplane like a Cessna 172.

Neutral static stability

An aircraft that has neutral static stability tends to stay in its new attitude when it's disturbed. For example, if you hit turbulence and your nose pitches up 5 degrees, and then immediately after that it stays at 5 degrees nose up, your airplane has neutral static stability.

Negative static stability

Finally, an aircraft that has negative static stability tends to continue moving away from its original attitude when it's disturbed. For example, if you hit turbulence and your nose pitches up, and then immediately continues pitching up, you're airplane has negative static stability. For most aircraft, this is a very undesirable thing.

Dynamic Stability

Now that you have static stability down, let's go over the really fun one: dynamic stability. Dynamic stability is how an airplane responds over time to a disturbance. And it's probably no surprise that there are three kinds of dynamic stability as well:

Positive
Neutral
Negative

Positive Dynamic Stability

Aircraft with positive dynamic stability have oscillations that dampen out over time. The Cessna 172 is a great example. If your 172 is trimmed for level flight, and you pull back on the yoke and then let go, the nose will immediately start pitching down. Depending on how much you pitched up initially, the nose will pitch down slightly nose low, and then, over time, pitch nose up again, but less than your initial control input. Over time, the pitching will stop, and your 172 will be back to its original attitude.

Neutral dynamic stability

Aircraft with neutral dynamic stability have oscillations that never dampen out. As you can see in the diagram below, if you pitch up a trimmed, neutrally dynamic stable aircraft, it will pitch nose low, then nose high again, and the oscillations will continue, in theory, forever.

Negative dynamic stability

Aircraft with negative dynamic stability have oscillations that get worse over time. The diagram below pretty much sums it up. Over time, the pitch oscillations get more and more amplified.

Stability in an aircraft affects two areas significantly:

Controllability: The capability of an aircraft to respond to the pilot’s control, especially with regard to flight path and attitude. It is the quality of the aircraft’s response to the pilot’s control application when maneuvering the aircraft, regardless of its stability characteristics.
Maneuverability: The quality of an aircraft that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers.

LONGITUDINAL STABILITY

Longitudinal stability is pitch stability, or stability around the lateral axis of the airplane.
It is dependent upon three factors

Location of the wing with respect to the CG
Location of the horizontal tail surfaces with respect to the CG
Area of the tail surfaces

Longitudinal stability

If the moments are initially balanced and the plane noses up, the wing moments and tail moments change so that the forces restore the moment.
Similarly, if the aircraft is nose down, the resulting change in moments brings the nose back up.

The center of lift has a tendency to change its fore and aft positions with a change in the AOA

CL moves forward with a high AOA, and moves aft with a low AOA.

The CL is usually to the rear of the CG so that the plane is nose heavy and requires that there will be a slight downward force on the horizontal stabilizer to keep the nose from continually pitching down.

The horizontal stabilizer is set in a negative AOA.
Horizontal stabilizer may be level when the plane is in level flight, there is a downwash of air from the wings which strikes the top of the stabilizer and produces a downward pressure.
The faster the plane, the greater the downwash.
When the plane goes slower, the downwash is less, so the plane descends and increases the airspeed which then makes the plane have a more downward force on the stabilizer to make the nose climb again.

A similar effect is noted upon closing or opening the throttle.

LATERAL STABILITY

Lateral stability is stability around the longitudinal axis, or roll stability.

Lateral stability is achieved through (1) dihedral, (2) sweepback, (3) keel effect, and (4) proper distribution of weight.

Dihedral

When you add dihedral, you add lateral stability when your aircraft rolls left or right. Here's how it works: let's say you're flying along and a wind gust hits your plane, rolling it to the right. When your wings have dihedral, two things happen:

1) First, your airplane starts slipping to the right, which means the relative wind is no longer approaching directly head-on to the aircraft, and instead is approaching slightly from the right. This means that there is a component of the relative wind that is acting inboard against the right-wing.

Second, because the relative wind has the inboard component, and because the wings are tilted up slightly, a portion of the relative wind strikes the underside of the low wing, pushing it back up toward the wings level. What's really happening here is the low wing is flying at a higher AOA and producing more lift.

The more dihedral an aircraft has, the more pronounced the effect becomes. But for most aircraft, they only have a few degrees of dihedral, which is just enough to keep the wings level during small disturbances, like turbulence, or bumping the flight controls in the cockpit.

Keel Effect

Keel effect depends upon the action of the relative wind on the side area of the airplane fuselage. In a slight slip, the fuselage provides a broad area upon which the relative wind will strike, forcing the fuselage to parallel the relative wind. This aids in producing lateral stability.

Sweepback

Sweepback is the angle at which the wings are slanted rearward from the root to the tip. The effect of sweepback in producing lateral stability is similar to that of dihedral, but not as pronounced. If one wing lowers in a slip, the angle of attack on the low wing increases, producing greater lift. This results in a tendency for the lower wing to rise, and return the airplane to level flight. Sweepback augments the dihedral to achieve lateral stability. Another reason for sweepback is to place the center of lift farther rearward, which affects longitudinal stability more than it does lateral stability.

DIRECTIONAL STABILITY

Directional stability is stability around the vertical or normal axis.

The area of the vertical fin and the sides of the fuselage aft of the CG are the prime contributors that make the aircraft act an arrow or pointing its nose into the relative wind.
To provide even more positive stability, a vertical fin is added.

If a plane is yawed to the right, the motion is stopped by the air that strikes the left side of the fin which resists the turning.
The restoring tendency is slightly slow to develop and will not return exactly to the original heading.

Sweepback wings, the center of pressure toward the rear

When the plane is yawed to one side, the opposite wing presents a longer leading edge perpendicular to the relative airflow which increases airspeed and drag. It Pulls the wing-back turning the aircraft back to its original position.

Dutch Roll

This is caused by stronger lateral stability compared to directional stability.
When a plane rolls to the right, the strong lateral stability rolls the plane back but the nose of the plane lags due to weaker directional stability.
It causes the nose to make figure eights on the horizon as a result of two oscillations.

Spiral Instability

Exists when the static directional stability of the plane is strong compared to the dihedral effect.
When the lateral equilibrium of the plane is disturbed by a gust of air, sideslip is introduced, the strong directional stability yaws the nose back while the dihedral lags. The outside wing will then travels forward faster than the inside wing so its lift will be greater. This produces an over banking tendency. The over banking continues if not corrected, and the strong directional stability that yaws the aircraft into the relative wind is forcing down the nose so this can create a steep spiral.

Wingtip Vortices - Spinning Air And Adding Drag

When an airfoil is flown at a positive AOA, a pressure differential exists between the upper and lower surface of the airfoil.
The pressure is lower in the upper half of the airfoil compared to the high pressure of the bottom half of the airfoil.
Air always moves from high pressure towards low pressure and the path with the least resistance is toward the airfoil tips, there is a spanwise movement of air from the bottom of the airfoil outward from the fuselage around the tips.
This flow of air results in spillage over the tips called a vortex.
As the air curls upwards around the tip, it combines with the downwash to form a fast spinning trailing vortex.
These vortices increase drag because of the energy spent in producing the turbulence.
A wing generates lift perpendicular to the relative wind. If you didn't have wingtip vortices, lift would point nearly straight up.
However, the wingtip vortices curve up and around the wingtips, pushing the air flowing over the wing downward. That angles your relative wind downward and tilts the lift vector backward.
This causes two problems - some of your lift is now pointing backward, adding to drag. And, you don't have as much lift pointing upward, countering weight.

Vortex Avoidance and Wake Turbulence

Vortex strength increases when the plane is heavy, slow and clean.

When following a larger aircraft on final approach, the key points the FAA recommends to avoid wake turbulence are:

Stay at or above the larger aircraft's final approach flight path.
Note the touchdown point, and land beyond it.

Avoiding wake turbulence on takeoff is a bit trickier, because larger aircraft often climb much faster than small GA airplanes. Here's what the FAA has to say on avoiding wake turbulence on takeoff:

Rotate prior to the point at which the preceding aircraft rotated.
Maneuver your aircraft to avoid the flight path of the preceding aircraft.
Waiting three minutes after the plane has taken off.
Avoid a path below and behind large planes.

Ground Effect

When an aircraft in flight comes within several feet of the surface, a change in the three dimensional flow pattern around the aircraft because the vertical component of the airflow around the wing is restricted by the surface.
This alters the wing’s upwash, downwash and wingtip vortices.
Ground effect is the due to interference about the ground.
As the wing encounters ground effect and is maintained at a constant AOA, there is consequent reduction in the upwash, downwash, and wingtip vortices.
Since there is fewer wingtip vortices due to ground effect, there is less drag and reduces the induced AOA.
A wing will require a lower AOA in the ground effect to produce the same Coefficient of lift.
Ground effect alters the thrust required because there is less induced drag which can cause the plane to be airborne at an indicated speed less than normally required.
A plane leaving ground effect will

Require an increase in AOA to maintain the coefficient of lift
Increased in induced drag and thrust required
A decrease in stability and a nose up change in moment
Experience a reduction in static source pressure and increase in indicated airspeed