The OTHER Kevin in San Diego <skiddz "AT" adelphia "DOT" net> wrote:
| >Right at the ground, there is no wind, thanks to the drag of the
| >ground. And up at a large altitude, you'll have the full effect of
| >the wind, but as you get lower, the wind speed will decrease.
| Not entirely true. Near the ground, friction reduces Coriolis Effect,
| it doesn't eliminate wind altogether. Sure, the surface may slow it
| down a bit, but it can also increase it. (Look up Katabatic Winds)
I didn't say near the ground. I said *right at the ground*
If you look at your ceiling fan, the blades are probably dirty.
Common sense tells you that the air would be rushing past there pretty
quickly and so dust couldn't accumulate there -- but there it is. The
reason is that there's a small film of air that is barely moving
(compared to the fan blade) and that catches dirt.
| You should try changing the oil on my truck in my driveway. There's
| ALWAYS a good breeze blowing as I live at the bottom of a valley.
| Unless I put up some sort of wind barrier, the oil goes everywhere but
| the drain pan.
Yes, but that's a foot above the ground -- not a fraction of a
milimeter. Of course, you don't fly your plane's wing down to a
fraction of a milimeter from the ground, so it's a moot point, and not
something I'm going to argue about.
| >This gradient causes at least two problems close to the ground --
| >1) when you're landing upwind, you lose airspeed as you descend. This
| >is generally a good thing, as it helps `suck' the plane into the
| >ground and prevent balooning, but it does mean you don't want to come
| >down for a landing right at your stall speed.
| Actually, you lose groundspeed.
Yes, I'm aware of that. That's the obvious reason why you land
upwind, but it's not the only one. I'm talking about something more
than that, something that's less obvious.
Do you have Martin Simon's `Model Aircraft Dynamics' ? If so, read
section 4.14 `The wind gradient' -- that's what I'm referring to.
| Airspeed is the speed the aircraft "feels" as it moves through the
| relative wind. If I have a 20 knot headwind right off the nose and
| I'm indicating 70 knots, my groundspeed will be about 50 knots.
Of course, and this is well known, but it's not the entire story..
However, as you lose altitiude, the wind speed generally slows (due to
the gradient), which will generally reduce your airspeed, which will
reduce lift unless you correct for it.
I've had this discussion before -- even in the same context! I even
typed out a paragraph of Mr. Simons' book there -- you can read it
there, if you wish.
... maybe that'll help explain what I'm talking about.
| I asked a student pilot a couple months ago, "Can you fly a helicotper
| backwards at 25 knots on a calm day?" He answered, "No."
Why would he say that? I can fly my R/C helicopter backwards, with or
without wind. And I'm not even very skilled at helicopter flight!
| >2) if you're doing a steep turn at low altitude, the lower wing will
| >be in air with a lower wind speed than the upper wing, due to this
| >gradient. The effect is especially high with gliders with long wings.
| >In any event, if you're flying upwind at a low altitude and enter a
| >tight bank, the airspeed over the high wing will be higher than that
| >over the lower wing, which generally means that it'll create more
| >lift. This will tend to pull the plane into an even tighter bank and
| >has probably caused the death of more than one full scale glider pilot
| >(since they're low and don't have enough altitude to recover from a
| >nearly 90 degree bank.)
| Has nothing to do with the wind. Once you're aloft, the aircraft
| moves along with the airmass and unless it's really gusty, the
| airplane doesn't care what the wind is doing. All it knows it that
| it's flying at some airspeed and attitude.
You're ignoring the wind gradient. Figure 4.15 in Martin Simons' book
covers exactly this case. Here's the caption --
Figure 4.15 Turning into the wind near the ground is dangerous. The
lower wing enters the slow moving airstream and loses lift. The
higher wing enters the faster aistream and gains lift. The result is
a strong tendancy to increase the angle of the bank.
| The death spiral you describe is a pitch problem, not a bank problem.
I wasn't talking about a spiral at all, though what I'm talking about
does sometimes result in death, especially to glider pilots making
sharp turns near the ground.
I also don't think this is what happened to the B-52 model -- it
looked like it had plenty of altitude, at least at first, which would
make the wind gradient very small. But of course, I've only seen the
video like everybody else, so I don't have any special insights about
what really happened.
| >Of course, this is all about steady winds -- if you have gusts of
| >wind, or lulls in the wind, they'll certainly affect your airspeed.
| True, but again, once aloft, the aircraft doesn't "feel" the wind. It
| only moves along with the airmass it's flying in.
Of course, but your airplane certainly does `feel' *changes*
wind, be them due to changes in the wind direction, gusts or a wind
gradient. (And wind sheer is just an extreme form of wind gradient,
though I was talking specifically about a gradient caused by getting
closer to the ground.)
Doug McLaren, email@example.com Save the whales! Collect the whole set!