Looking for a connection between pitch inputs and sideslips in sailplanes and airplanes--overview

Aerial experiments: Looking for a connection between pitch inputs and sideslips in sailplanes and airplanes--overview

July 12 2005 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

Airplane and sailplane pilots are usually taught that the rudder is the control surface that serves to keep the nose of the aircraft directly aligned with the actual direction of the flight path and airflow (relative wind) at any given moment, preventing the aircraft from slipping or skidding.  And conversely, they are also taught that the rudder can be intentionally used to yaw the nose out of alignment with the actual direction of the flight path and airflow (relative wind) at any given moment, creating an intentional slip or skid.  In most modern flight training manuals for airplanes or sailplanes, there is no suggestion that a pilot's pitch control inputs will promote or prevent a slip or a skid.

 

However, in some of the older training literature for airplane and sailplane pilots, the authors do contend that if a pilot rolls the aircraft from wings-level into a bank without making an adequate aft movement of the control stick or yoke to increase the wing's angle-of-attack, this will make the airplane or sailplane "sideslip" toward the earth.  Authors that have made this argument include Wolfgang Langewiesche in his classic physics-for-pilots book "Stick and Rudder" (1944 and 1972), and USAF Major General Neil Van Sickle in "Modern Airmanship" (3rd edition, 1966). 

 

The idea that a pilot's pitch inputs can promote or inhibit a sideslip is also very widespread among hang glider and trike pilots, and may be found throughout the hang-gliding training literature.

 

I've carried out some experiments in airplanes and sailplanes to examine this idea.  In all the experiments that I'm about to describe, I kept my feet off the rudder pedals.

 

In a variety of sailplanes, I carefully watched the yaw string while entering a turn, and while flying in a constant-banked turn, and while exiting a turn.  I found that as I was rolling the glider into a left bank, the yaw string deflected toward the right.  This showed that the nose of the glider was yawing to the right in relation to the actual direction of the flight path and airflow (relative wind) at any given moment.  In other words, the glider was slipping.  The amount of deflection of the yaw string--i.e. the amount of sideslip--was strongly related to the roll rate.  The faster I rolled the glider into the turn, the more the yaw string deflected to the side.  I saw roughly the same amount of deflection of the yaw string--i.e. roughly the same amount of sideslip--regardless of whether I moved the control stick aft enough to hold the airspeed constant as I rolled the glider into the turn, or I made no movement of the control stick in the fore-and-aft-direction (which caused the flight path to curve downward and the nose to drop and the airspeed to rise), or I pushed the control bar forward as I rolled the glider into the turn (which caused the flight path to curve sharply downward and the nose to drop sharply and the airspeed to rise rapidly). 

 

In other words, I found nothing that supported the idea that a glider will "sideslip" excessively if the pilot fails to make an adequate pitch "coordination" input as he rolls the glider into a turn.

 

In a constant-banked left turn, I found that the yaw string deflected slightly toward the right of the centered position.  This showed that the nose of the glider was yawed to point slightly to the right of the actual direction of the flight path and airflow (relative wind) at any given moment.  In other words, the glider was showing just a few degrees of sideslip--much less than occurred while I rolled the glider into the turn.   I saw roughly the same deflection of the yaw string--i.e. roughly the same amount of sideslip--regardless of whether I was making pitch inputs that caused the glider to fly at a slow, constant, airspeed, or caused the glider to fly at a fast, constant, airspeed, or caused the glider to pitch up and rapidly bleed off airspeed, or caused the glider to pitch down and rapidly gain airspeed.

 

In other words, I found that a sustained, constant-speed, diving turn involved no more sideslip than a low-speed, efficient turn. I also found that an accelerating, diving turn involved no more sideslip than a low-speed, efficient turn.

 

I found that as I was rolling the glider from a left bank back to wings-level, the yaw string deflected toward the left.  This showed that the nose of the glider was yawing to the left in relation to the actual direction of the flight path and airflow (relative wind) at any given moment.  In other words, the glider was skidding.  The amount of deflection of the yaw string--i.e. the amount of skid--was strongly related to the roll rate.  The faster I rolled the glider out of the turn, the more the yaw string deflected to the side.  I saw roughly the same amount of deflection of the yaw string--i.e. roughly the same amount of skid--regardless of whether I moved the control stick forward enough to hold the airspeed constant as I rolled the glider back to wings-level, or I made no movement of the control stick in the fore-and-aft-direction (which caused the flight path to curve upward and the nose to rise and the airspeed to drop), or I pulled the control stick aft as I rolled the glider out of the turn (which caused the flight path to curve sharply upward and the nose to rise sharply and the airspeed to bleed away rapidly). 

 

I also kept an eye on the slip-skid ball during these experiments.  The movements of the slip-skid ball were consistent with the movements of the yaw string (i.e. the slip-skid ball moved to the left whenever the yaw string moved to the right), and confirmed the conclusions that I drew from watching the yaw string.

 

I carried out similar experiments in 2 light airplanes.  Again, the results were similar.  Because of P-factor and other related effects, the results did vary somewhat according to the power setting, and also according to whether the aircraft was turning left or right.  The results given here are the overall average results from a variety of experiments involving both left and right turns.  For economy of language, we'll "frame" these overall, average results by using language appropriate to a left turn.  (These are the same results that we saw in the sailplane experiments, except that the primary instrument was a slip-skid ball rather than a yaw string). 

 

As I rolled the airplane into a left bank, the slip-skid ball shifted toward the right, showing that the airplane was slipping.  The amount of sideslip was strongly related to the roll rate.  The faster I rolled the airplane into the turn, the more the slip-skid ball deflected to the side.  I saw roughly the same amount of sideslip regardless of whether I moved the control yoke aft enough to hold the airspeed constant as I rolled the airplane into the turn, or I made no movement of the control yoke in the fore-and-aft-direction (which caused the flight path to curve downward and the nose to drop and the airspeed to rise), or I pushed the control bar forward as I rolled the airplane into the turn (which caused the flight path to curve sharply downward and the nose to drop sharply and the airspeed to rise rapidly). 

 

Again, I found nothing to support the idea that an aircraft will "sideslip" excessively if the pilot fails to make an adequate pitch input "coordination" input as he rolls the aircraft into a turn.

 

In a constant-banked left turn, I found that the slip-skid ball deflected slightly toward the left of the centered position.  In other words, the airplane was showing just a few degrees of sideslip--much less than occurred while I rolled the airplane into the turn.   I saw roughly the same deflection of the slip-skid ball--i.e. roughly the same amount of sideslip--regardless of whether I was making pitch inputs that caused the aircraft to fly at a slow, constant, airspeed, or caused the aircraft to fly at a fast, constant, airspeed, or caused the aircraft to climb at a steady rate, or caused the aircraft to descend at a steady rate, or caused the aircraft to pitch up and rapidly bleed off airspeed, or caused the aircraft to pitch down and rapidly gain airspeed.

 

I found that as I rolled the airplane from a left bank back to wings-level, the slip-skid ball deflected toward the right.  This showed that the airplane was skidding.  The amount of skid was strongly related to the roll rate.  The faster I rolled the airplane out the turn, the more the slip-skid ball deflected to the side.  I saw roughly the same amount of skid regardless of whether I moved the control yoke forward enough to hold the airspeed constant as I rolled the aircraft back to wings-level, or I made no movement of the control yoke in the fore-and-aft-direction (which caused the flight path to curve upward and the nose to rise and the airspeed to drop), or I pulled the control yoke aft as I rolled the aircraft out of the turn (which caused the flight path to curve sharply upward and the nose to rise sharply and the airspeed to bleed away rapidly). 

 

Some of these experiments in sailplanes and airplanes involved very sharp forward movements of the control stick or yoke, which unloaded the wing all the way to the zero-lift angle-of-attack, putting the aircraft in a 0-G or "weightless" condition".  This was done while the aircraft was steeply banked.  In most cases this did not create a visible sideslip.  In some experiments involving exaggerated wingovers, where the airspeed was brought far below the normal 1-G stall speed as the aircraft "floated" over the top of the maneuver in a vertical bank with the control stick far forward and the wing at the 0-lift angle-of-attack and the pilot experiencing 0-G "weightlessness", the yaw string or slip-skid ball did show some sideslip.  I think that these unusual, exaggerated, cases, the airspeed was so low that the aircraft's inherent yaw stability or "weathervane effect" became relatively ineffective and failed to keep the nose of the aircraft aligned with the actual direction of the flight path and airflow (relative wind).  But in most cases, moving the control stick or control yoke forward--even to the point of 0-g weightlessness, and even when the aircraft was very steeply banked--did not seem to promote a sideslip. 

 

I submit that by moving the control stick or yoke aft while rolling into a turn, a pilot prevents the flight path from curving downward into an accelerating dive.  I submit that Langewiesche and Van Sickle were mistaken in their contention that this pitch "coordination" input has anything to do with preventing a sideslip.  Adverse yaw is the primary cause of sideslip as an airplane or sailplane enters a turn. Slips and skids are yaw-axis phenomena--they are caused by nothing more complicated than the nose of the aircraft being out of alignment (in the yaw axis) with the actual direction of the relative wind (i.e. with the actual direction of the aircraft's flight path through the airmass).

 

A few more words on adverse yaw: adverse yaw is known to have several different causes. In most aircraft, one of the most important causes of adverse yaw is the fact that the descending wing and the ascending wing each experience a twist in the local direction of the relative wind, compared to the direction of the relative wind experienced by the aircraft as a whole. This change in the direction of the local relative wind is caused by the rolling motion of the aircraft--because the descending wing is descending, its relative wind vector has more of an upward component than does the overall relative wind experienced by the whole aircraft. This change in the local direction of relative wind twists the descending wing's lift vector forward, creating a thrust component in relation to the reference frame of the whole aircraft. Conversely, because the ascending wing is rising, its relative wind has more of a downward component (or less of an upward component) than does the overall relative wind experienced by the whole aircraft. This change in the local direction of the relative wind twists the ascending wing's lift vector aftwards, creating a drag component in the relation to the reference frame of the whole aircraft. Naturally, these thrust and drag components create a yaw torque. Other significant causes of adverse yaw in most aircraft include the physical difference in the shape of the descending and ascending wings that arises (due to the deflection of the ailerons) when the pilot makes a roll control input. Another effect that is not negligible is an aircraft's rotational inertia in the yaw axis. If an aircraft had a control system that was designed to create zero yaw torque (no adverse or "proverse" yaw) when the pilot made a roll input, the pilot would still see some slip while rolling into a turn and some skid while rolling out a turn, due to the aircraft's rotational inertia in the yaw axis. (This last point is only true for bank angles up to 60 degrees-as the bank angle is increased beyond 60 degrees, the yaw rotation rate needs to decrease, rather than increase, so the aircraft's yaw rotational inertia will tend to create a skid rather than a slip).

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