..Couldn't have said it better
...thank you.
A little more gas on the fire here: Believe it or not, if there is a 10 knot wind blowing near the ground, there may well be 20 or more knots, within the next 20 feet up! My downwind passes, in the field, will
INDICATE a much higher airspeed than my upwind passes!
In discussions about this seemingly unbelievable phenomenon with other graying, very experienced ag pilots, we still scratch our collective heads
We kinda, sorta concluded that the upper third of our rather large airplanes (8,000 lb. gross) and with 50' wingspans, have the upper part of the fuselage in the faster air, while the lower part and the pitot tube is in the slower air???
....and my 496/groundspeed, agrees!
Go figure.
Best,
This is explainable, in that you may have actually been doing a bit of "dynamic soaring" in the wind gradient you describe. It's similar to how the wandering albatross exploits the energy that can be extracted from the wind gradient - the difference in wind speed caused by frictional slowing of the wind very near the surface, compared to the higher wind speed a bit higher up over the ocean. Your maneuvers in Ag flying, using your example, where your upwind pass is close to the ground in the slower air moving 10mph caused by the wind gradient, followed by your zoom climb while still going upwind into the faster moving 20mph air up higher, followed by your 180 deg. turn, and then a descent while traveling downwind back into slower moving 10mph air near the surface, are a close approximation to part of what the Albatross does to soar thousands of miles over the ocean without flapping at all. The bird gains energy when climbing
into the faster moving air while going upwind in the zoom climb, and again gains energy again in the dive when going downwind and entering the slower moving air near the ground. You also gained some energy in these maneuvers. Of course the energy that can be extracted from the 10 mph difference in the wind speed you described is not going to enable your heavily loaded Ag plane to "Soar", but you did actually gain some airspeed, which is because you gained some "total energy". This gain in total energy can be taken or traded off by us in different ways. When climbing into a wind gradient on take-off going upwind, most would take it as a better than normal climb rate. Of course it only lasts as long as while you're actually transitioning through the layer where the wind speed is changing. Once you reach the altitude layer where the wind speed is constant again, the effect is gone, although you still have the extra energy you
gained in the form of some extra altitude you wouldn't otherwise have at that instant. When flying into a the slower moving air while diving towards the ground going downwind, you tend to take the gain in energy as an increase in airspeed, which is what you noticed. This gain in airspeed in this case is not related to being in ground effect, it is related to the gain in energy from transitioning through the wind gradient from the faster air up high to the slower moving air down low as you descended through it at the beginning of your downwind pass.
Radio control glider pilots have become very
adept at doing this type of dynamic soaring. They make loops diving in an out of the slow moving air in the lee of a ridge going downwind, and zoom up into going upwind into the faster air on the upwind side of the ridge. With each loop, they gain more energy and have
gotten their gliders over 400mph in recent years doing this. Full scale glider
pilots mainly only dream of being able to soar like this, although a few have
claimed to do similar maneuvers to the albatross across a wind gradient found at higher altitudes, such as near temperature inversions, which can provide
wind gradients at altitudes that are safer in which to operate full scale gliders. Ingo Renner, former world soaring champion was one of the first to
write about a soaring flight using dynamic soaring.
The only problem with Ag flying is that on the other end of the field, where
you're going downwind near the surface, and do a zoom climb up into the faster air while going downwind, followed by a 180 deg turn and a dive back down to the surface going upwind, you actually are going to lose some energy in those maneuvers. The Albatross doesn't do that. After diving going downwind near the surface, he makes a 3g zooming 180 deg turn into the wind but then
immediately turns downwind again and dives right away back towards the surface going downwind, instead of diving straight into the
wind like you would have to do spraying crops. So you're gaining energy 1/2 the time, and losing energy the other 1/2 of the time in a wind gradient like you described. I think if Steve Smith our resident aerodynamicist was reading this thread, he'd probably agree with what I'm trying to say.
http://en.wikipedia.org/wiki/Dynamic_soaring
http://esoaring.com/albatros_presentation_esa.pdf
http://www.youtube.com/watch%3Fv%3DOix6sHKzOLU&sa=U&ei=LS9NT_L5OamfiALE-b3BDw&ved=0CBsQtwIwAw&usg=AFQjCNEz6FFQ_esbeN5PIWrnPIWWRbU7Sw