From The Aeronautics File By Max Feil

Topic #4: The Downwind Turn. Gusts and Gradients

So far I hope I have convinced you that if the wind is
Introduction blowing evenly, then your airplane cannot feel it or its
The topic this time is one which is sure to stir up opinions. direction. But this is not the end of the story since no wind
People I’ve heard discuss the dreaded “downwind turn” ever blows evenly. The two main effects found when the
seem evenly divided into two camps. On one hand are those wind blows are gusts and gradients.
who are sure that their airplane flies differently upwind than
downwind, and that when the wind is strong it loses altitude Wind creates turbulence and vortices which are felt as gusts.
noticeably after a turn downwind. On the other hand are Fortunately these are generally random in nature and their
those who think the first group are quite foolish, and that an effects tend to cancel each other out over time. For this
airplane never feels which direction the wind is blowing. reason gusts generally do not affect an airplane any
Try as they might, people from the two groups rarely differently no matter which direction it is flying. Gusts are
convince each other of their own viewpoint. However as we still important since on a very windy day your airplane may
will see, both groups have good reason to believe what they pitch, roll, or lose altitude without warning. On such a day
do. it’s a good idea to land with a slightly higher airspeed than
usual.
Inertial Reference Frames
The most common argument by those who worry about A second and much more important effect is wind gradient.
turning downwind is that the momentum of the airplane will Wind speed may increase dramatically as you move up from
be small after the turn and that the airplane will feel a ground level. The biggest increase is in the first few hundred
tailwind until its momentum has a chance to increase. For feet, exactly where model airplanes tend to fly. Here we can
example, say the wind speed is equal to the plane’s airspeed. see that the ground really does have an effect on the wind -
It will seem to be hovering when flying upwind and after a by friction it slows down the lower layers of the moving air
180 degree turn downwind it will surely be affected by the mass. On a windy day, this altitude related wind gradient
wind blowing to speed it up to twice its airspeed. can be quite large: 20 knots or more in the space of two or
three hundred feet.
This is not a sound argument, however. The way a physicist
might explain what’s wrong with the above paragraph is For the person convinced that the wind direction is felt by
that any point of view that moves at a constant speed can be their airplane, wind gradient explains some of the anomalies
adopted as an “inertial reference frame”. In this case if we they might be seeing. For example, imagine taking off with
take the uniformly moving mass of air as our reference a badly adjusted engine, so that there is barely enough
frame, the motion of anything within it is the same as if power to gain altitude. After making a turn and heading
there were no wind. Another example of an inertial downwind, things seem to get worse. This is because it is
reference frame is the earth. It is actually moving through actually harder to gain altitude when flying downwind.
the solar system, or even the universe, at millions of miles Gradient works against you and gives more and more of a
per hour. We do not notice this since we compare everything tailwind as the airplane climbs. Flying downwind is not all
to our local reference frame, the earth. Similarly, an airplane bad however. If the engine quits and you are still flying
in flight does not notice the ground since the air has no downwind, the gradient will help you. Altitude is lost more
attachment to it. In fact, if you were actually sitting in an slowly when gliding downwind since there is more and
airplane and the ground was obscured by clouds, you would more of a headwind as the airplane gets closer to the
not be able to determine the wind direction. Wind direction ground. The airplane’s ground speed may be quite high
is something humans use since we live on the ground, so we though, making landing difficult.
measure it with respect to the ground.
Another effect you may have seen which is purely due to
The R/C flyer is especially at a disadvantage when judging altitude-induced wind gradient is “weathercocking”. After
the effect of wind on their airplane since they stand on the pulling the plane vertical in a crosswind, if the wind is
ground and so measure everything with respect to the coming from the left then the airplane will not only move
“wrong” reference frame. This causes numerous optical visibly from left to right with the wind, but it will also yaw
illusions. The only time the two reference frames match is to the left (into the wind) as it follows the vertical up-line. If
when there is no wind at all. We could actually simulate a there was no gradient, the airplane would still move from
calm day when it was windy if we flew a model airplane left to right with the moving air mass, but it would not rotate
from the deck of an aircraft carrier. Even a very stiff breeze about the yaw axis. If we take the idea of weathercocking
could be matched by the ship’s engines. With the aircraft one step further, we can deduce that the plane will yaw to
carrier travelling at the same speed and direction as the the RIGHT (out of the wind) on the vertical downline, even
wind, the flight of a model airplane from its deck would be though the wind is still coming from the left. Observant
the same as if there was no wind. The only way for you (or fliers will have noticed this, perhaps when working on their
your airplane) to tell which way the wind was blowing stall turns. This “opposite yawing” on the downline is not
would be to look at the ocean. quite as pronounced since the plane is accelerating on the
way down, making the effect of an increasing sideways
wind component less noticeable.
Inside and outside loops into the wind seem snappier as ii) No matter what altitude an airplane is flying at, if it flies
well. This is part optical illusion, but wind gradient plays a at a constant height then turning upwind or downwind will
part making the loop entry quicker. Alternatively, loop entry make no difference. To somebody on the ground this may
when flying downwind can be more sluggish, however in not be obvious since observing from a different reference
the downwind case pulling out on the back side of the loop frame creates optical illusions.
is improved when compared with a loop into the wind.
iii) If an airplane is flying near the ground and gaining
Gradient can also have an effect on banked turns near the altitude, wind speed increases. This makes climbing into the
ground since the higher wing will be in a region of higher wind more effective, makes climbing downwind less
wind. This is most noticeable in gliders, which have long, effective, causes weathercocking, snappier loops, etc.
slender wings.
iv) If an airplane is flying near the ground and losing
Gradient is not always produced by altitude changes. When altitude, wind speed decreases. This makes gliding into the
slope soaring, the venturi effect causes wind speed to wind less effective, makes gliding downwind more
increase over the crest of the hill. If you fly too close to the effective, causes weathercocking out of the wind, etc.
hill and straight towards it, then this gradient will mean a
loss of airspeed, making turning out of danger more One final note: Turning in itself makes you lose energy and
difficult. can put you into a compromising position (e.g. higher stall
speed), so a decision to turn when you are in a tight spot
Summary may be a bad one even if you are in a position to take
We can summarize what I have said above by remembering advantage of wind gradient.
four main points:
Well, that’s it for now. Next time we’ll look at the following
i) If an airplane is flying at a very high altitude then wind question: “Do windmilling props produce more drag than a
direction will never have any effect on it since there will be stopped prop?” As with most of the topics I have dealt with,
no real altitude-related wind gradient. Model airplanes you’ll find there is no straight “yes or no” answer to this
rarely get this high. question either!