Downwind turns ARE different

Downwind turns ARE "different"!

September 16, 2006 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

The "Brain teasers for those who believe that downwind turns are different" need the following DISCLAIMER: when an aircraft is climbing or descending through a GRADIENT, or being hit by sharp-edged GUSTS, then the wind direction certainly DOES affect the aircraft's performance. 

 

(By a "wind gradient" we mean a situation where the wind velocity increases with altitude.  There is often a very strong wind gradient within 100' or so of the earth's surface, because the wind is slowed by friction with the ground). 

 

A wind gradient will rob an aircraft of energy when a climbing, downwind turn is made near the ground. A wind gradient also will rob an aircraft of energy when a descending, upwind turn is made during the ground.  Actually, the fact that the aircraft is turning is not the key factor here--an aircraft will suffer a loss of energy whenever it is climbing though a tailwind gradient, or descending through a headwind gradient. 

 

The wind direction can also affect a pilot's behavior--when a pilot is consciously or unconsciously modifying the aircraft's bank angle and airspeed to make the flight path follow some preconceived radius of curvature and forward velocity, IN RELATION TO THE GROUND, rather than judging his flight entirely by the airspeed indicator, the sound of the airflow, and the feel of the controls, then the wind direction certainly does matter, and the pilot risks stalling the aircraft during a downwind turn.  A conscious or unconscious desire to make the flight path follow a fixed radius of curvature and forward velocity, in relation to the ground, will tempt the pilot to slow down too much during a downwind turn, and this is most likely to be a problem when the pilot is maneuvering in relation to an interesting target (such as a desired landing spot) at low altitude. 

 

So downwind turns CAN be trouble.  But in the absence of gradients and gusts, the aircraft has no way of "knowing" or "feeling" which way the wind is blowing, and for a given airspeed and bank angle, is no closer to the stall angle-of-attack when flying downwind than when flying upwind. 

 

A note to those who would say "the pilot of an RC model airplane has no choice but to judge the aircraft's motion in relation to the ground, not in relation to the airmass": I disagree.  The plane's responsiveness, especially in roll, gives strong cues as to the airspeed.  Allowing the aircraft to fly in a given pitch trim condition will keep the airspeed constant so long as the aircraft has some degree of positive pitch stability, and is not passing through shears and gradients.  By this I don't mean that the stick should be left in the neutral position and the aircraft flown with the trim, but rather that the pilot should have a good sense of where the stick is in the pitch direction and should move the stick with the deliberate intention of changing the airspeed rather than to try to hold a constant groundspeed.  A similar situation applies to the roll axis: I fly RC gliders often in strong winds, and usually make a practice of using a roughly constant bank angle during 180-degree and 360-degree turning maneuvers, even though this does not produce a circular path in relation to the ground with a defined center-of-radius that is stationary with respect to the ground.  In other words, I fly in a manner that is well harmonized to the fact that the aircraft's dynamics operate with respect to the airmass, not with respect to the ground.  Of course, the aircraft's dynamics will continue to operate with respect to the airmass regardless of how I fly, but if I try to hold the groundspeed constant or if I try to make the airplane follow a perfectly circular path of curvature with respect to the ground, my pitch control inputs might force the aircraft to stall during a downwind turn, or I might find myself forced into using very high bank angles on the "downwind" part of the turn, all for no good reason.  Spelled out in this manner, this all sounds like a rather constricted way of flying, but what I really mean to say is that even during radical aerobatic maneuvers and even when the pilot's feet are planted firmly on the ground, the "feel" of the controls and the responsiveness of the aircraft are very important cues and are products of the aircraft's movement with respect to the airmass, not with respect to the ground.  A pilot could be led to ignore these important cues if he becomes overly concerned with the aircraft's motion with respect to the ground--though a lack of concern for the location of the ground can only be taken so far in all forms of aviation!

 

Although the "downwind turn" is often thought of as a menace, a downwind turn can also add energy to an aircraft.  Of course we are now turning our attention to the subject of how to use a WIND GRADIENT to our benefit.  The whole idea of the most common form of DYNAMIC SOARING is to fly within a wind gradient in such a way that the aircraft (or bird) is alternating between an ascending flight path pointed into the wind, and a descending flight path pointed away from the wind.  Northern Harriers and other raptors that hunt low over the ground can often be seen to execute a small "zoom" climb when turning upwind and a small dive when turning downwind, and this extracts energy from the wind gradient.  RC model sailplane pilots use this technique to achieve groundspeeds of near 200 mph while performing a series of very-high-G looping ovals immediately downwind of a ridgeline, where is a very strong wind gradient, but no "lift".  The oval flight pattern can be sustained indefinitely--or until the aircraft overstresses and explodes.  This energy is being extracted entirely from the wind gradient--the net vertical velocity of the airmass that glider is flying in is probably negative (downward) rather than positive (upward).  We examine DYNAMIC SOARING in more depth in this article. 

 

Soaring pilots sometimes find that after several circles, a glider ends up falling out of the downwind side of a thermal column.  Does this suggest that the glider is "feeling" the wind?  No--actually it suggests that the thermal column itself is "feeling" the wind.  If the thermal column is slanted in the downwind direction--as is often the case--then a glider's sink rate (in relation to the surrounding air) will eventually make it fall out of the downwind wall of the thermal.  If the thermal column takes on a purely vertical orientation as it drifts over the landscape in the manner of a free balloon, then a glider will not tend to fall out of the downwind side.

 

 

For more, see these related articles on the Aeroexperiments website:

Brain teasers for those who believe that downwind turns are "different"--i.e. that an aircraft can "feel" the wind direction in flight

Mathematics of circles in wind

The never-ending myth of the "dangerous downwind turn"

 

 

And for more still more, see these articles from the "Ask J and D" feature of the "DJAerotech" website:

Downwind -- debunking the myth of the dangerous downwind turn

Wind_plane -- more on the above topic, with some interesting notes on wind shear

 

 

And for more still more, see these articles:

Challenging the wind by Martin Hepperle-- an interesting little article on the best strategy for flying in wind during a pylon race

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