A new direction for an old thread: Crosswind landings

The last few threads I initiated were preambles to a discussion I don't
think we've sufficiently explored. Those of you who keep track will
recall that I've asked first, whether we are sufficiently well-trained
(Dear Burt), and second, if some of the models we use to understand
flight couldn't stand improvement.

Let me preface this thread with an analogy. I think we can agree the
that the majority of drivers have little understanding of how their
vehicles work. They have learned that certain control movements result
in changes of direction and speed, but if asked to work their way
intellectually though the process, most would fall short of the level
of knowledge demanded of a pilot about his/her aircraft. And yet, the
vast majority of drivers manage to operate their vehicles successfully
(and safely).

My point is, just because we ask pilots to acquire more knowledge than
drivers, it isn't necessarily required to effectively pilot an
aircraft. And by extension, just because the pilot can control the
aircraft, doesn't necessarily mean that the intellectual models he uses
are accurate.

My object was to get us on a path where we could look more closely at
these models to discern where they might stand improvement.

Obviously, I have a very high opinion of the RAS!

That said, let's talk crosswind landings again. And to start the
discussion, one area of false intuition may result from the differing
nature of wind for an aircraft on the ground and one in the air. While
on the ground, the wind exerts a force on the aircraft. In the air, it
does not. This dichotomy becomes very important during a crosswind
landing, when we transition from being a part of the airmass to
becoming an object on the ground.

I've found that pilots can speak very clearly about the role of wind on
navigation when they are in cruising flight. But the closer they get to
the ground, I start to see control usage more appropriate for taxiing
than flying. Has our understanding of the effect of wind as viewed from
the ground infected our understanding of its effect on flight? And do
these become more obvious as we get approach the transition from flight
to taxiing? And perhaps, thus, much of the confusion pilots suffer over
side slips?

Asking pilots to describe the crosswind approach leads to a variety of
inaccurate language. Digging deeper will lead, almost inevitably, to
the conclusion that the wind is exerting a force on the glider, and
that the wing must be tilted in order to counteract that force (and the
rudder applied against the bank to keep the glider from turning). This
works, and even makes some sense. But the notion that the tilted lift
vector is compensating for wind drift is flawed. Useful, but flawed.

The Soaring Flight Manual (1999) says the following on the subject of
Crosswind Landings (page 14-15):

"The traffic pattern for crosswind landings is the same up to the final
approach using crab to maintain pattern alignment. In light to moderate
crosswinds, a wing-low sideslip or crab may be used on final to
maintain runway alignment. A strong crosswind usually REQUIRES a
COMBINATION of the two."
[the emphasis is mine]

This is an interesting mix of useful yet incorrect information. What
concerns me most is the implication that crabbig and side slipping are
additive. They are not. But if you deconstruct the implicit logic, you
are led to the conclusion that side slips counteract the force of the
wind. Otherwise, how can the effects be additive?

I suspect there is a visual/pschological effect that has crept its way
into the way we rationalize control use during the crosswind approach.
Consider it from another point of view. You adjust your path across the
ground by changing your direction. The most efficient way to do this is
by executing a coordinated turn. Once on the appropriate heading to
achieve a desired ground track, you fly wings level. If there is a
crosswind and your heading differs from your ground track, you are
crabbing. Just that simple. Whether you are 10 feet, 100 feet, or 1,000
feet above the ground.

The role of the side slip, then, since it is aerodynamically the same
as a forward slip, is solely to change the heading (but not the
direction) of the aircraft. The effect on flight path is exactly the
same as applying some spoiler. The advantage gained is that it brings
the landing gear more closely aligned to the aircraft's direction over
the ground, and thus reduces any sideloadings at touchdown. This is one
of two reasons to perform a slip during a crosswind landing: aligning
the gear with the direction of travel. The other, to steepen the glide.

So why might a pilot think that the side slip adds "additional force"
against the crosswind? Perhaps we are put ill at ease by a large crab
angle. Pointing the fuselage more directly down the runway makes the
approach look better (closer to "normal") and perhaps gives the
impression of additional control. But it doesn't add any force, and, in
fact, reduces the freedom of control.

I know one direction this thread will follow... a perfectly reasonable
one, but let me color it a little: is it appropriate that we should ask
fledgling pilots to handle the controls differently in the riskiest
flight environments? We are taught, rehearsed, and tested on our
abilities to maintain coordinated flight. Then, under the most trying
conditions, we are asked to apply counter intuitive control movements
at low altitudes, many of them based on a false impression of the
forces acting on the aircraft and the effects control usage has on
balancing those "external" forces. It should be mantra with us all,
that when things are going bad, the first thing we should do is return
to and/or maintain coordinated flight. Understanding the foundations of
crosswind navigation are critical to helping all pilots fly more
safely. If you are confused about what keeps the aircraft tracking down
the runway, you may find yourself making control inputs against upsets
that increase your risk of loss of control.

Shouldn't every pilot know that when things are going bad on final, you
should return to a wings level, coordinated crab, where your ASI is
accurate, you have full control available, and you are exercising those
skills which your training has made most instinctive. Once you've
sorted out the upset, then you can return to your "runway alignment"
slip.

Lately I've been testing an exercise, one I tried years ago with
several students (with good results) but never formalized: I would have
pilots establish their final leg with a crab. Then I would have them
enter and recover from a slip, descibing is utility in aligning the
gear with the runway, noting its increased drag. This reinforced the
role of the slip, the role of the crab angle, and the necessity to exit
the slip if you needed to extend your glide. Two distinctly different
maneuvers meant to achieve different ends, applied together to ease the
tranistion from flight to taxi.

You might want to run through the Building Models thread where I've
tried to address the false dichotomy of side and foward slips. Viewing
a side slip as two distinct maneuvers... a turn and a slip, migh help
to put my last paragraph in context.

I'm hoping you'll poke holes, and not take too much offense when I
return your efforts in kind. That's the point of this thread.

I don't necesarily disagree with your analysis...if
I understand it. But the idea of waiting to kick out
of a full crab to a full slip(or however you want to
word this), does not seem to me any improvement of
using a combination of both...particularly in strong
crosswind conditions.

Personally I think one should fly whatever he/she is
most comfortable with...because rarely are short final
approaches the same in extreme crosswind conditions.
And I think for the most part...these strong crosswinds
are not seen much during training...rather we self-teach
ourselves the technique that works best for us. Personally
my preference is to not practice new techniques when
I have something that seems to keep me on the runway
when the crosswind is 20+.

On the flip side...I see some instructrion manuals
that recomend no crabbing whatsoever.

Stewart Kissel wrote:

On the flip side...I see some instructrion manuals
that recomend no crabbing whatsoever.

Makes crosswind landings interesting in a 20+ meter ship.

Stefan

OK I'll take the bait.
Just how does one deal with a crosswind in a 25m ship? I have some guesses
but no experience. 15m does not seem to be much of a problem--lots of
experience there.

--
Hartley Falbaum
"Stefan" wrote in message
...
Stewart Kissel wrote:

On the flip side...I see some instructrion manuals
that recomend no crabbing whatsoever.

Makes crosswind landings interesting in a 20+ meter ship.

Stefan

HL Falbaum wrote:

OK I'll take the bait.
Just how does one deal with a crosswind in a 25m ship?

I can't tell you what "one" does. But I know what I do: I simply crab
into the roundout and then use the rudder to align, carefully keeping
the wings level with the ailerons. Inertia combined with the slippery
fuselage is plenty enough to hold the glider on track during the short
flare. In fact, I never even thought of not keeping the wings level near
the ground, even in a 15 meter glider.

Stefan

"HL Falbaum" wrote in message
...
OK I'll take the bait.
Just how does one deal with a crosswind in a 25m ship? I have some guesses
but no experience. 15m does not seem to be much of a problem--lots of
experience there.

--
Hartley Falbaum


No experience with a 25 meter ship, but in a 23 meter Stemme S10-VT,
crosswinds aren't a problem. Ground handling and taxiing is possible in 35
knots, as evidenced by the four Stemmes that had to make multiple taxi turns
to launch out of Cedar City, Utah while on a safari 4 years ago. Though I've
not done so, I'm told the factory has taken off and landed in up to 25 knots
crosswind.

The Stemme's relatively low dihedral (.75 degrees), high conventional gear,
and shoulder mounted wing, make for good wing-tip ground clearance. Plus,
most all glass gliders have a slippery side profile compared to power
aircraft, so it doesn't take a lot of a slip bank angle to get a fair amount
of sideways movement to counter crosswind drift.

On the other hand, my ham-fisted handling and poor decision making (I
shouldn't have even tried it) attempt at launching an 18 meter ASH26E in 15
knots of crosswind at Minden, resulted only in a cloud of dust and a ground
loop. I'll add that Schleicher's main gear is hell for stout!

all the best,

bumper

Can't speak for 25 metres but I can for 20 metres and
it's crabbing. I fly an ASW 17 and the tips are not
that far from the ground, even when the wings are level,
and sideslipping (using bank) near the ground has never
appealed to me. My aim is to keep the wings level near
the ground and kick off the drift at the point of flare
but I have often wondered if it is a really good idea
to apply yaw at the same time as increasing the angle
of attack of the wing. Of course to conteract the further
effect of rudder opposite aileron may have to be applied
to keep the wings level but can be avoided by not being
too heavy footed with the rudder. The upside is I am
in ground effect when I do that.
Thi biggest problem with a large span (any) glider
is keeping it straight as the speed decreases and with
a really stong crosswind there comes a point where
overcoming the tendency to 'weathercock' into wind
is impossible. Perhaps that emphasises the importance
of staying within the published crosswind limits and
another good reason for stopping as soon after touchdown
as is safely possible.



At 15:00 20 February 2005, Hl Falbaum wrote:
OK I'll take the bait.
Just how does one deal with a crosswind in a 25m ship?
I have some guesses
but no experience. 15m does not seem to be much of
a problem--lots of
experience there.

--
Hartley Falbaum
'Stefan' wrote in message
...
Stewart Kissel wrote:

On the flip side...I see some instructrion manuals
that recomend no crabbing whatsoever.

Makes crosswind landings interesting in a 20+ meter
ship.

Stefan

I would counter that the only way to maintain a track is by crabbing. I
think we confuse the role of the slip. In order to track straight down
the runway in a crosswind, we must adjust the direction of the glider
so the sin of the angle equals the crosswind component. This is
accomplished by turning, our yaw string ultimatley revealing our path
through the air. If you recover from your side slip, you will point at
that angle while continuing to track down the runway. Side slip,
recover, side slip, recover. The side slip has nothing to do with your
direction... it simply changes your heading so you can land with the
wheels straight.

And thus my quibble with calling crabs and side slips additive.

The advantage of using a side slip for alignment is that it reduces
variables: once the slip is established, the pilot uses the controls as
normal (or very nearly so).

My intent here is not to discern which approach is better... to me they
are pretty much the same... with preference for when the alignment
correction is made... early on final or just before touchdown. Instead,
I want to point out that there are some published, formal notions that
appear at least to cause confusion and at worst are simply WRONG.

Someone come to the defense of the sages of sport, the authors of the
Soaring Flight Manual!! How are a crab and side slip additive?

In article ,
Don Johnstone wrote:

Can't speak for 25 metres but I can for 20 metres and
it's crabbing. I fly an ASW 17 and the tips are not
that far from the ground, even when the wings are level,
and sideslipping (using bank) near the ground has never
appealed to me. My aim is to keep the wings level near
the ground and kick off the drift at the point of flare
but I have often wondered if it is a really good idea
to apply yaw at the same time as increasing the angle
of attack of the wing. Of course to conteract the further
effect of rudder opposite aileron may have to be applied
to keep the wings level but can be avoided by not being
too heavy footed with the rudder.

If you want, you can side slip and when close to the ground use the
ailerons to level the wings. You are then in precisely the same
situation as if you crabbed and then kicked in rudder, except that
levelling the wings is probably easier and less critical than precisely
timing and judging a bootfull of rudder.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

Bruce Hoult wrote:

If you want, you can side slip and when close to the ground use the
ailerons to level the wings. You are then in precisely the same
situation as if you crabbed and then kicked in rudder, except that
levelling the wings is probably easier and less critical than precisely
timing and judging a bootfull of rudder.

I think both are equally hard/easy in timing. Levelling the wings and
yawing straight both start the aircraft moving sideways across the
runway/strip and require the same accuracy of timing.

Judging the required control input is different. You used the phrase
"use the ailerons to level the wings". Why didn't you say "use rudder
to yaw the glider straight"?

The colourful phrase you actually used - "a bootfull of rudder" - from
an instructor has probably caused more students to find crosswind
landings difficult than any other aspect of the manoeuvre. Do you teach
the final part of a slipped landing as "shove the stick over"?

In my experience, old multiengine pilots like to crab and use rudder.
Old single engine pilots like to slip and use aileron. Which technique
is used just reflects the tribe you give your allegiance to. Both of
them work well if taught properly.

GC