Fundamentals: yaw string and slip-skid ball

Fundamentals: yaw string and slip-skid ball

August 8, 2007 edition
Steve Seibel
www.aeroexperiments.org

Here are some fundamental concepts that will help the reader to understand some of the other ideas contained in this section of the Aeroexperiments website.

A "yaw string" is a tuft of yarn exposed to the airflow, mounted somewhere on the aircraft centerline, in the pilot's field of normal vision. When the free end of the yaw string deflects to the left, this shows that the relative wind (airflow) has a right-to-left component as it passes over the aircraft. This means that the nose of the aircraft is pointing to the left of the actual direction of the flight path and relative wind at any given moment. In relation to the actual direction of the flight path and relative wind at any given moment, we can say that the nose of the aircraft is "yawed" to the left. This situation can happen even if the flight path is curving to the right, and the nose of the aircraft is tracking from left to right in relation to the outside world (e.g. the aircraft's heading is moving from north to east to south and so on...). In fact this is exactly what we would expect to see in a steady, constant-banked right turn in a rudderless aircraft-- the yaw string would point toward the outside or high side of the turn, showing that the nose is pointing slightly toward the outside or high side of the turn in relation to the actual direction of the flight path and relative wind at any given moment. This is called a "slipping turn". As a general rule, a steady constant-banked turn in a rudderless aircraft does involve at least a slight amount of sideslip. This slip is caused by the fact that the outside wingtip experiences more airspeed--and thus more drag--than the inside wingtip. As a general rule, in an aircraft with a rudder the pilot will carry at least a slight touch of inside rudder to prevent this slip and keep the nose of the aircraft pointing directly into the relative wind.

Many of the ideas described in the Aeroexperiments website are based on observations made during many hours of flying hang gliders with a yaw string attached to a dowel rod projecting forward from the centerline of the base bar. A yaw string is not part of the normal instrumentation for a hang glider, because with no rudder, there is no real reason for a hang glider pilot to monitor sideslip.

A "slip-skid ball" is a ball in a curved glass tube. A "slip-skid bubble" is a bubble in a curved glass tube. The ball and bubble are redundant, and deflect in opposite directions-- when the ball moves left, the bubble moves right. Technically speaking, the ball (or bubble) indicate the ratio of sideways aerodynamic forces to upward aerodynamic forces, in the aircraft's own reference frame. When the ball is centered, there is no net sideways aerodynamic force is the aircraft's own reference frame. Normally this is desirable. After all the wing is the best way to generate aerodynamic force, and the wing's lift acts in the "upward" direction in the aircraft's own reference frame. When the relative wind hits the fuselage and vertical fin in a sideways (left-to-right) manner rather than straight on, this generates a sideways force. Normally this is not desirable--when the aircraft meets the airflow in this matter, it creates excess drag. When the nose of the aircraft points to the left of the actual direction of the of the flight path and relative wind at any given moment, so that the relative wind has a right-to-left component as it passes over the aircraft, then the yaw string deflects to the left, the airflow impacts against the right side of the fuselage and other surfaces of the aircraft, the aircraft experiences an acceleration toward the left, the slip-skid ball deflects to the right, and the slip-skid bubble deflects to the left. If the aircraft has minimal cross-sectional area as viewed from the side (as does a hang glider), the deflection of the ball (or bubble) will be small, for a given deflection of the yaw string.

If the aircraft has a rudder, applying left rudder will normally move the yaw string to the left and the slip-skid ball to the right (and the slip-skid bubble to the left). As the rudder yaws the aircraft to point to one side in relation to the actual direction of the relative wind, the relative wind impacts against the side of the fuselage and other surfaces of the aircraft, creating the aerodynamic sideforces that are registered by the slip-skid ball (or bubble). Normally the yaw string and the slip-skid ball (or bubble) are redundant. However this is only true so long as the only significant source of sideways aerodynamic force in the aircraft's own reference frame, is the impact of the sideways component in the relative wind against the fuselage, vertical fin, and other surfaces of the aircraft.

If the aircraft has a rudder, the rudder itself can generate a significant aerodynamic sideforce even if the nose of the aircraft is pointing directly into the relative wind. This can only happen if the rudder is being used to counteract some other yaw torque (i.e. the increased drag experienced by the outboard wingtip during a turn, or the yaw torque created by an asymmetrical power setting in a twin-engined aircraft.) While the yaw string and slip-skid ball (or bubble) usually agree in their indications, in these specialized circumstances it is possible for the ball to be off center even if the yaw string is centered. Normally the aerodynamic sideforces generated by the rudder itself are small compared to the aerodynamic sideforces generated by the impact of the sideways component in the relative wind against the fuselage, vertical fin, etc. In special cases where this is not true, the yaw string and the slip-skid ball will not agree in their indications.

(As far as I'm aware, the only situation where this normally happens to a noticeable extent is when one engine fails in a twin-engine or multi-engine aircraft and the rudder must be strongly deflected just to overcome the resulting yaw torque and keep the nose of the aircraft pointing directly into the relative wind. In this case, if the yaw string is centered, the ball will deflect in the direction of the deflected rudder. If the yaw string is centered and the wings are level, the flight path will curve opposite the direction of the deflected rudder. The deflection of the ball and the curvature of the flight path both are caused by the aerodynamic sideforce from the deflected rudder. The most efficient way to create a linear flight path in this situation is not to deflect the rudder further-- this would work, but would be inefficient--the yaw string would not stay centered. Instead, the most efficient way to create a linear flight path in this situation is to only apply as much rudder deflection as is needed to center the yaw string, while banking the aircraft (toward the working engine, i.e. toward the deflected rudder) by whatever amount is needed to prevent the flight path from curving. The fundamental purpose of the bank in this case is not to help counteract the yaw torque arising from the asymmetry in thrust. Instead, the fundamental purpose of the bank is to counteract the aerodynamic sideforce from the deflected rudder.)

In the special case where the flight path is completely linear (not curving), the ball (or bubble) serves as a bank angle indicator. If the aircraft is not banked, the flight path cannot be linear unless no sideforces are present, in which case the ball (or bubble) must be centered.

In the special case where the wings are level and the airspeed is constant, the ball (or bubble) serves as a turn direction and rate indicator. A ball moves toward the outside of the turn (and a bubble moves toward the inside of the turn.) Again, if the aircraft is not banked, the flight path cannot be linear unless no sideforces are present, in which case the ball (or bubble) must be centered.

Elsewhere in this section of the Aeroexperiments website we'll describe a special case where the slip-skid ball deflects in the same direction as the rudder is deflected, not the opposite direction. In this unusual case, involving a hang glider with no fuselage or fixed vertical fin, the aerodynamic sideforce from the deflected rudder itself is actually stronger than the aerodynamic sideforce from the impact of the sideways airflow against the rest of the aircraft, so the ball moves in the opposite direction as usual. This also means that if the wings are forced to stay level, the flight path will curve in the opposite direction as usual-- away from the deflected rudder. This also means that to prevent the flight path from curving, the aircraft must be banked in the opposite direction as usual-- toward the deflected rudder. This is definitely an unusual case (but with some similarities to the asymmetric thrust case described above). Meanwhile the yaw string still moves in the normal direction-- in the same direction as the rudder is deflected.

In cases where the aerodynamic sideforce generated by the rudder itself are trivial in relation to the aerodynamic sideforce generated by the impact of the sideways component in the relative wind against the other parts of the aircraft, the yaw string and slip-skid ball (or bubble) will always give synchronized indications. Again, with most aircraft, this is the normal situation. Normally, the pilot uses the rudder as needed to center the yaw string and/or slip-skid ball, which keeps the nose of the aircraft pointing directly into the relative wind, i.e. aligned with the actual direction of the flight path at any given moment.

Copyright © 2004 aeroexperiments.org