BRAIN TEASERS FOR THOSE WHO BELIEVE THAT DOWNWIND TURNS ARE "DIFFERENT"
September 14 2006 edition
steve at aeroexperiments.org
If you believe that downwind turns are "different"
from upwind turns, or if you think that a pilot can "feel" the direction of the wind, or that an aircraft tends to "weathervane" to point into the external, meteorological wind, then you'll enjoy these brain teasers:
Brain teaser #1:
1.) You are flying indoors.
In an immense, enclosed room.
The walls and floor and ceiling are black. You've launched off a
platform near the ceiling and are practicing turns, stalls, stalls from turns,
etc. There is no evidence of any air
movement in the room. Does your aircraft behave differently when flying in any
2.) Sunrise. You
realize that what you thought were black walls, are clear glass panels. The
room is actually the enclosed gondola of an enormous balloon. As you look down at the newly visible earth,
you see that the ground is passing by very swiftly far below. The balloon is in
a stiff south wind, and is being blown northward over the land. Now does your aircraft fly differently in
any particular direction, within the closed room? Is it now more dangerous to
turn downwind (toward the north) than upwind (toward the south)? Just because the sun came up and now you can now see the ground? What if you close your eyes? Can you "feel" the wind by the way the aircraft responds when flying in different directions?
3.) You notice that
each of the transparent walls of this enormous, enclosed room has several large
windows. Someone comes and opens all these
windows. But no air blows in through them.
Likewise the flags that decorate the outside of the gondola hang limp.
Anyone who has ever been in a balloon will recognize this to be true, and the
explanation for this is simple: the balloon is moving freely with the airmass
without resistance, and so the balloon's velocity is constant, and so acceleration is zero, and so net force also must be
zero: the wind cannot be "pushing" on the balloon in any way. Since the windows are now open the airmass
in the room is now the same as the airmass outside. Now does your aircraft fly
differently in any particular direction? Is it more dangerous to turn downwind
(to the north) than upwind (to the south)?
4.) The balloon is too heavy and needs to shed some
weight. Someone hits a button and all
of the walls get jettisoned. The floor, ceiling, and corner pillars are
all that is left of the "room".
Again, no air is blowing through the "room". Now is a downwind
turn (to the north) somehow "different" than an upwind turn (toward
5.) You fly out of one of the missing walls and into the
clear blue sky. Now is a downwind turn
any "different" than an upwind turn? Is it easier to stall when
turning downwind than when turning upwind?
(P.S. Part 3 of brain teaser #1 brings to mind another old
puzzle: if a fly takes wing within an enclosed aircraft, do the wings of the
aircraft no longer need to support his weight?
What if a window in the cabin is open?
What if the fly is buzzing around the cockpit of an old open-cockpit
biplane? What if the fly flies out of
the open window (or out over the side of the open cockpit) and then flies along
in formation with the aircraft? What if
he positions himself directly over one of the wings? At what point as the fly approached the window (if any) did the
aircraft stop "feeling" the weight of the fly?)
Brain teaser #2:
We are flying in still air over the San Andreas fault.
Suddenly the block on the west side of the fault starts sliding rapidly
northward. (Devastation is breaking out
below). As we fly from across the fault
from east to west in the still, uniform, airmass, we suddenly find ourselves flying in a north wind in relation to the land immediately
below. Does this affect the way the
aircraft flies? When we are on the west
side of the fault line, are we in more danger of stalling during a "downwind"
turn (toward the north) than during an "upwind" turn (toward the
Brain teaser #3:
Aliens arrive. After
consulting with Art Bell, they decide to use their advanced engineering prowess
to abruptly halt the earth's rotation.
You are piloting an airliner at 30,000' over the equator, and the
effects of this little disturbance have not yet propagated to your
altitude--the layer of the atmosphere surrounding your aircraft is still
rotating at a normal rate. From your
perspective, the ground has suddenly started moving toward the west at 1,038
mph. Relative to the ground, you are
now flying in a 1,038 mph west wind.
Does this have any affect on the way that the plane flies? Are "downwind" turns (toward the
east) now different than "upwind" turns (toward the west)?
Brain teaser #4:
You are in still air. Looking straight down, you see a train
driving south at 60 mph. You decide
that the train constitutes the "surface" of the earth for the few
seconds that you are overflying it. As you overfly the train, you are in a 60mph
south wind, in relation to the "surface". Does this affect the way your aircraft flies? If you close your eyes and fly in circles
over the train, will the "feel" of the aircraft tell you which
direction the wind is blowing, i.e. which direction the train is
travelling? Is there a greater danger
of stalling when you are flying "downwind" (flying toward the north), or when you are performing a "downwind" turn (flying toward the north), than when you are flying "upwind" (flying toward the south), or when you are performing an "upwind" turn (turning toward the south)?
(Extra credit for hang glider pilots: do you have to
"flare" your glider differently when landing on top of the southbound
train with the nose of your glider pointing south, than when you land on top of
the southbound train with your nose pointing north? Obviously answer is "yes"--landing with a 60mph
tailwind would be disastrous--but why?
Does it have to do with the behavior of your glider in relation to the
air? Or does it only relate to the fact
that you are trying to minimize your glider's groundspeed at the instant that
your feet touch the ground? If you were
practicing flares at high altitude, aiming for a given profile in the airspeed
and sink rate with no concern for ground track and groundspeed, could you tell
when you were over the train by the way the glider felt when it flared?)
Brain teaser #5:
This one also applies to those who believe that an aircraft
flies differently in "lift" (rising air) than in "sink"
Let's ignore the earth's surface, and take the sun as our
reference point. In relation to the sun, the earth's atmosphere (as well as the
rest of the earth) is moving at 66,674 mph.
If we are near the equator, the direction of motion of the atmosphere (as well as the rest of the
earth) is (roughly speaking) toward the west at noon, toward the east at midnight, straight up at
sunrise, and straight down at sunset. So we have an east wind at noon, a west
wind at midnight, an updraft at sunrise, and a downdraft at sunset. (Don't confuse yourself by factoring in the
earth's rotation around its axis, which is a mere 1,038 mph at the
equator). Bearing this incredible wind
velocity in mind, does an aircraft fly differently when turning to the west at
noon, then when turning to the west at midnight? Does an aircraft fly
differently in the sunrise updraft than in the sunset downdraft?
For more, see these related articles on the Aeroexperiments website:
Downwind turns ARE "different"!--this is a bit of a "disclaimer" for the "Brain teasers for those who believe that downwind turns are 'different'"!
Mathematics of circles in wind
The never-ending myth of the "dangerous downwind turn"
And for 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 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