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Re: [Phys-l] [tap-l] Wind Shear



Bernard Cleyet wrote:
Cross posted ...
On 2009, Jun 12, , at 09:12, Thomas J. Bauer wrote:

...a plane descending through slowing air can end up with an air speed less than its stall speed and ends up crashing to the ground. This is a good intro mechanics problem.

Tom Bauer
Wellesley College
It's reasonable to mention that when there is any wind at all, a descending aircraft descends through a wind that usually slows and backs [which means its direction rotates anti-clockwise] near the ground - a matter of ground friction. Then within a half wing span of the ground, an added lift component is available - called ground effect.

In the ordinary course of events, an aircraft flies in relation to the surrounding airmass, so a gradual wind change is not an issue. In fact light aircraft, with rather low areal wing loading are all too able to respond quickly to even rapid wind gusts - which makes the ride bumpy on Summers days.

Wing loading used on airliners is much higher, so they need to fly faster to support their weight. For this and several other reasons, a big airliner provides a much steadier ride, and is much less responsive to sharp edged gusts.

It used to be a popular misconception that turning in the pattern exposed an aircraft to changing winds, and could provoke stall conditions. While it is the case that a turning aircraft turns in its surrounding airmass, there is finally, some slight justification for this old myth, in that heavy aircraft are not free to accommodate to every wind change immediately, on account of their considerable inertia.
The down burst associated with a wind shear like the one that brought a Delta flight to grief on final approach at DFW 20 years or more ago is particularly deceptive - in that the initial perception is an increased headwind which provides unwanted lift over the desired glide slope, and may persuade the crew (or autopilot) to throttle back - next to penetrate a down flow with increased descent rate, then finally with the increasing tail wind, an even greater sink rate and decaying airspeed. ...All this accompanied by considerable turbulence. When such conditions are encountered, the desired response these days is to hold attitude and maximize thrust in order to escape the scene of the impending disaster.

The runway length used by airliners needs to be long enough to accommodate their acceleration to a relatively high speed such as 150 MPH, and also to accommodate
the capability of braking to a halt from the latest possible moment in the roll,
if some dangerous condition is detected. This is called a balanced field length.
Windspeeds are a less important factor in proportion to the takeoff speed of heavy aircraft, in comparison to runway slope, which is surprizingly effective in prolonging the required takeoff length.

For light aircraft, a small tailwind has a more important effect. Here, not just runway slope, but surface texture like wet grass (if this is the surface in use) provides a soft draggy footing, which has much the same effect as an uphill slope.
When runway length is in question, an aircraft may derive some benefit from lifting its weight off wheels early, and accelerating so far as possible in ground effect until a sufficient lift reserve is available to climb away. This may look like the aircraft is being held in contact, but that is usually unproductive.

Brian W