Bjorn’s Corner: Yaw stability, Part 3.

By Bjorn Fehrm

March 08, 2019, ©. Leeham News: Yaw stability, which we discussed last week, has in itself no tricky dynamic problems. It’s a matter of making the vertical stabilizer large enough to have enough weathercock moment on the aircraft.

The tricky stuff comes through the interaction of yaw stability and roll stability.

Figure 1. The surfaces affecting yaw stability on an aircraft. Source: Leeham Co.

Roll stability

Roll stability is about how prone the aircraft is to level its wings after an upset in roll. To achieve roll stability one designs the aircraft to get an increased lift on the wing which is dropping.

When a wing drops, the lift of the aircraft is no longer pointing straight up. It’s tilted to the side of the wing drop. This makes the aircraft slide towards the dropped wing. This sideslip increases the lift of the dropped wing and the aircraft gradually regains wings level again.

Dependent on the design of the aircraft this increase of the lift on the dropped wing can be weak or strong. Design principles which increase the lift are:

  • High wing instead of low wing
  • Wing dihedral (wings are angled up towards the tips) instead of level wings or wing anhedral (wings are angled down).
  • Swept wings instead of straight or forward swept wings.

High wing means the aircraft’s center of gravity tends to force wings level. In addition, the fuselage blocks a part of the high wing during sideslip. It decreases the lift of this wing and increases roll stability.

An aircraft with dihedral will get a higher Angle of Attack (AoA) from the dropped wing from a sideslip, and the the leeward wing will get a lower AoA. This creates an asymmetrical lift which straightens the roll.

As a swept wing aircraft sideslips the windward wing gets a lower sweep angle and by it a wider effective span, Figure 2.

Figure 2. Swept wing and yaw/sideslip. Source; Google images.

The leeward wing has a shorter span measured against the air stream. This creates a rolling moment to straighten the wings.

The stability in roll is nice to have as the aircraft doesn’t start hanging on a wing and wander off course when flying with the hands off the stick. But an aircraft which is stiff in roll will experience a nasty second effect, Dutch Roll.

Dutch Roll

The Dutch Roll is an uncomfortable side effect for an aircraft which has too strong roll stability combined with a too weak yaw stability. As the aircraft sideslips, it quickly rolls up and goes past wings level. This starts a sideslip in the other direction.

After one or two sideslips the aircraft is moving from side to side like the Dutch ice-skaters on their Canals, Figure 3. Therefore this movement came to be called Dutch Roll.

Figure 3. An aircraft starts a Dutch Roll movement. Source: Google images.

The sideslip and dipping of the wings are not too uncomfortable. But the combination with wagging of the tail is. It’s a problem for airliners as passengers in the rear of the aircraft get seasick from the movement.

Typically the low wing airliner of today have wings which are angled upwards to give ground clearance for the engines, increasing roll stiffness. In addition, to lower transonic drag, the wings are swept and this increases roll stiffness further.

The result is an aircraft which is Dutch rolling. To counter the Dutch Roll, all business and commercial aircraft have a yaw damper which keeps the aircraft from yawing when experiencing a short disturbance, by it stopping the Dutch roll.

A weak Roll stiffness will create spiral instability, however. This we will discuss in the next Corner.


9 Comments on “Bjorn’s Corner: Yaw stability, Part 3.

  1. “This makes the aircraft slide towards the dropped wing. This sideslip increases the lift of the dropped wing and the aircraft gradually regains wings level again.”

    So when a wing drops, an aircraft yaws/slips the tail towards the side of the dropped wing. Is that because the horizontal stab is pressing down on the tail? Otherwise, it seems like if lift and weight were balanced, there would be no yaw and the aircraft would just drift off centerline following the direction of the tilt.

    • Hi Ted,

      the aircraft sideslips towards the dropped wing. The sideslip will cause yaw to develop as the Vertical tail gets a horizontal angle of attack from the sideslip. So it’s a sideslip and combined yaw. If the correcting rolling moment is strong the aircraft will roll up and then to the other side which causes a sideslip to this side which causes a yaw to this side. If this moment is badly damped it develops into a Dutch roll. It’s easy to see if an aircraft Dutch rolls, you look at the wingtips. If they do a slow circle movement the aircraft has a Dutch roll going. Almost all airliners don’t, but one can see a mild circle movement sometimes.

      • Yes, I didn’t even realize sideslip was different than yaw, so I was a little lost. In Figure 3, the third aircraft down, I would think the aircraft would move laterally or ‘sideslip’ to the right. So I don’t get why it has moved to the left of centerline.

  2. Very informative article as usual Bjorn.

    Your Figure 2. here has really helped me understand yaw stability a lot better.

    The United 585, USAir 427, and Eastwind 517 issues now make a lot more sense to me. I see now why a sustained maximum deflection of the rudder would cause the aircraft to roll over.

    American 587 complete loss of rudder shows the importance of training, human factors and ergonomics.

  3. Thanks Born.

    I flew in the rear of a 747-100 lo man years ago.

    Definitely could feel the roll. It was not realty bad but enough to understand that it means.

  4. Another MAX goes down shortly after take off ,its hard not to speculate.

    • Depending on the moves by the FAA, one might expect that the 2nd MAX accident will result in a worldwide grounding.

  5. 737 MAX with Ethiopian Airlines has crashed, no survivors

    Reported problems on takeoff.

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