Why crabbing is correct and side slipping isn't (was Flying Technique Question of the Day

Dan,

You've got it right, at least the way that most of us are taught. But
let me clarify something that we aren't typically taught.
Aerodynamically speaking, there is no difference between a forward and
a side slip. One is used during final for crosswind landings in order
to align the fuselage with the runway throughout the final leg of you
pattern. However, it is, in term of how you use the controls, exactly
the same as any other slip. The difference is that you are executing
the slip along with a crab. That is, if you exit the slip, you will
have a crab angle that aligns your track with the runway.

The only reason I bring this up is that I've seen a thread that
suggests that there is a difference between a forward and side slip.
That the forward slip (by tilting the lift vector) somehow compensates
for a crosswind. Of course, it does not. Only crab angle compensates
for crosswind with respect to the ground. The slip simply aligns the
fuselage with the runway, at the cost of some glide performance.

This technique is very useful for smooth crosswind landings in high
wing taildraggers, where there is an advantage to setting one main
gear on the ground rather than both at once. It is not nearly so
applicable to gliders, though many of us were taught to compensate for
crosswinds this way. It has some minor advantages... the upwind wing
is "low" at touchdown, less finesse is required with the rudder... but
also has some disadvantages... the controls are crossed, the airspeed
indicator is unreliable, the glider will not achieve maximum
performance if sink or turbulence is encountered, there is some risk
of touching a wingtip if the flair is not properly executed.

But most important, many pilots seem not to realize that they are, in
fact, using the slip not to compensate for crosswind, but to align the
nose with the runway in order to gain a more "normal" view of the
approach. That is its only real value. Giving a slip different names
based on its application only perpetuates the confusion. And I guess
that's my point.

To sum up:

A slip is aerodynamically the same, regardless of wind.

A slip always increases drag.

A slip can be used to align the nose with the runway during a
crosswind approach (match heading with track), at the cost of
additional drag.

A tangent to the orignal thread, more concerned with semantics than
application, but I thought I'd turn the discussion in this direction.

Cheers.

On 10 Sep 2003 05:07:08 -0700, (Chris
OCallaghan) wrote:


Aerodynamically speaking, there is no difference between a forward and
a side slip.

Indeed. The simple truth that there is only one slip has been
obfuscated in every training publication in English since the
Wright Brothers.

It is the only maneuver in all of flight training in which the
reason for doing the maneuver is presented as part of the
maneuver itself.

It is a prime source of confusion for students, and any flight
instructor who presents slips as more than one maneuver should
be embarrassed.

-Pat

(Chris OCallaghan) wrote in message . com...
Dan,

You've got it right, at least the way that most of us are taught. But
let me clarify something that we aren't typically taught.
Aerodynamically speaking, there is no difference between a forward and
a side slip.

I agree, I thought that was implied in my post, ALTHOUGH, for
touchdown, the slip angle is typically small, and for glidslope
control I usually have full rudder application and a severe slip
angle. I would ALMOST call it a different manouver, just because of
the huge amounts of drag you can produce in, say, a Pawnee. Just a
question of degree.

The only reason I bring this up is that I've seen a thread that
suggests that there is a difference between a forward and side slip.
That the forward slip (by tilting the lift vector) somehow compensates
for a crosswind. Of course, it does not.

Seems to me that statement is true. If you hold fusalage alignment
with rudder, and your upwind wing low, you really are "tilting your
lift vector to cancel out the effect of the crosswind" What's wrong
with that statement?

This technique is very useful for smooth crosswind landings in high
wing taildraggers, where there is an advantage to setting one main
gear on the ground rather than both at once.
Or in strong gusty crosswinds, and yes, low wing tail draggers, and
gliders.

It is not nearly so
applicable to gliders, though many of us were taught to compensate for
crosswinds this way.

I think it applies well to everything that doesn't have turbine
engines hanging under the wings, gliders included.

It has some minor advantages... the upwind wing
is "low" at touchdown, less finesse is required with the rudder... but
also has some disadvantages... the controls are crossed, the airspeed
indicator is unreliable, the glider will not achieve maximum
performance if sink or turbulence is encountered, there is some risk
of touching a wingtip if the flair is not properly executed.

So, you're looking at your airspeed indicator at touchdown? And a
crosswind correction makes it unreliable? The only wingtip I ever
touched was the Pawnee with the droop tips on takeoff. Never on a
landing, & never in a glider, and in the strongest crosswinds you'd
ever care to try.

But most important, many pilots seem not to realize that they are, in
fact, using the slip not to compensate for crosswind, but to align the
nose with the runway in order to gain a more "normal" view of the
approach. That is its only real value. Giving a slip different names
based on its application only perpetuates the confusion. And I guess
that's my point.

Those are some weird statements. Is there anyone else out there that
believes the only real value in a slip is to give you a better view?
Does anyone else use their airspeed indicator during touchdown? In
anything?
(Shuttle excluded)

To sum up:

A slip is aerodynamically the same, regardless of wind.
TRUE.. All manouvers are the same regardless of wind, unless part of
the aircraft enters the wind gradiant (Very low turn in wind over
bushes)

A slip always increases drag.
TRUE

A slip can be used to align the nose with the runway during a
crosswind approach (match heading with track), at the cost of
additional drag.

TRUE. but if you don't do it, you'll be CRABBING off the edge of the
runway, and you just might roll the tire off the rim if you touch down
that way.

A tangent to the orignal thread, more concerned with semantics than
application, but I thought I'd turn the discussion in this direction.

Cheers.

I think you have the theory correct, just not the application.
-Dan

In article ,
says...
But most important, many pilots seem not to realize that they are, in
fact, using the slip not to compensate for crosswind, but to align the
nose with the runway in order to gain a more "normal" view of the
approach. That is its only real value. Giving a slip different names
based on its application only perpetuates the confusion. And I guess
that's my point.

Those are some weird statements. Is there anyone else out there that
believes the only real value in a slip is to give you a better view?
Does anyone else use their airspeed indicator during touchdown? In
anything?

I'm with Buck on this one: I use the slip to compensate for the cross-
wind, and to make it easier to touch down with the glider going in the
direction it's pointed (down the runway). The view of the runway isn't
improved any over a wings-level crabbing approach. I use attitude and
the feel of the glider, not the airspeed indicator, to maintain proper
airspeed, even though the airspeed indicator generally continues to
work properly in my glider.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

Buck & Eric,

go with what you know. If you are comfortable using a slip to align
the nose with the runway, it's a perfectly reasonable way to
compensate for crosswind. However, I must once again take exception to
the notion that tilting the lift vector compensates for crosswind.
This is simply wrong. Draw some pictures to work your way through the
problem.

Tilting the lift vector produces a turn, regardless of wind. The turn
will continue so long as the wings are banked. If, however, you use
opposite rudder to counteract the turn, you are creating a force with
the fuselage to balance the inward component of your lift. That is,
the forces are balanced... You will continue to fly straight. A slip
only increases drag by presenting more of the fuselage to the relative
wind. It is stable, straight line flight. An unbalanced force (like
tilting the lift vector) creates an acceleration, which means that
either your speed or direction changes.

Remember, that for an aircraft in the air, the wind is not a force.
Since the aircraft moves with the airmass, there is no wind. So
tilting a lift vector against "the wind" is meaningless. If the
airmass is moving with respect to the ground, you establish a desired
track across the ground by crabbing. (When was the last time you flew
x-country from point A to point B in a slip?)

The difficulty comes when we need to transition from the air to the
ground. The moment the wheels touch the ground, the wind becomes an
unbalanced force, and we need to make control inputs to deal with it.
There are two techniques. We know them both. But be clear, we
compensate for airmass movement using a crab, not a slip. Transition
to the ground is achieved by momentarily crossing the controls. I
prefer to do this during the flair. Others choose to initiate that
process after turning base. It's a matter of preference. I like my yaw
string straight and my airspeed indicator dependable when near the
ground. (Eric, note that your visual and aural cues become less
trustworthy when near the ground or when flying sideways.) Again, a
matter of choice. But let's get off this notion that a tilted wing
cancels out the effect of wind. It doesn't, at least, not while you
are in the air. To establish a track down the runway you are crabbing.
To align the fuselage parallel with the rundway, you are slipping. You
may initiate both simultaneously, but they are distinct actions and
serve very different purposes.

This is building block stuff, which is why I'm still beating it.

In article ,
says...
However, I must once again take exception to
the notion that tilting the lift vector compensates for crosswind.
This is simply wrong. Draw some pictures to work your way through the
problem.

I claim tilting the wing helps me compensate for the crosswind. I
don't know what is happening to the lift vector, since the wind is now
coming at the wing at an angle, and the dihedral and aileron
deflections will cause the left and right wings to have a different
lift distributions.

snip

If the
airmass is moving with respect to the ground, you establish a desired
track across the ground by crabbing.

I agree in general, but for the specific case where track=heading, we
don't call it crabbing; for example, if you are flying directly into
the wind.

When I use a slip to compensate for a crosswind during a landing, I'm
not "crabbing" because my heading and my ground track are aligned.

The difficulty comes when we need to transition from the air to the
ground. The moment the wheels touch the ground, the wind becomes an
unbalanced force, and we need to make control inputs to deal with it.
There are two techniques. We know them both. But be clear, we
compensate for airmass movement using a crab, not a slip.

Perhaps this is where we are confusing each other. I say

Crab = ground track different from heading
Slip = fuselage not aligned with airflow

So, I can crab in coordinated flight or in a slip. All it takes to
have a crab is have track not equal to heading.

Transition
to the ground is achieved by momentarily crossing the controls. I
prefer to do this during the flair. Others choose to initiate that
process after turning base.

When using a slip on final to compensate for a crosswind, the controls
are crossed the whole distance, not momentarily. I'm sure you know
this, so maybe I didn't understand your statement correctly.

It's a matter of preference. I like my yaw
string straight and my airspeed indicator dependable when near the
ground. (Eric, note that your visual and aural cues become less
trustworthy when near the ground or when flying sideways.)

I do monitor my airspeed while on final, which remains accurate during
the slip (not all gliders are like this, of course). "Near the
ground", meaning a little before I flare, I don't monitor the yaw
string or the airspeed (regardless of slipping or flying coordinated),
but instead fly by attitude and how fast the ground is approaching.

Again, a
matter of choice. But let's get off this notion that a tilted wing
cancels out the effect of wind. It doesn't, at least, not while you
are in the air. To establish a track down the runway you are crabbing.

It's not crabbing if the heading and ground track are the same. It
won't be coordinated flight in a cross wind, but it's still not
crabbing.

To align the fuselage parallel with the rundway, you are slipping.

I can align the fuselage parallel to the runway easily while flying
coordinated (no slipping), but I will drift off the center line.

You
may initiate both simultaneously, but they are distinct actions and
serve very different purposes.

Crabbing isn't a flight maneuver, it's the result of flight maneuvers
and the wind; slipping is a flight maneuver which depends on the
pilot's inputs, and not the wind.


This is building block stuff, which is why I'm still beating it.

Ditto!

--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

(Chris OCallaghan) wrote in message . com...
Buck & Eric,

go with what you know. If you are comfortable using a slip to align
the nose with the runway, it's a perfectly reasonable way to
compensate for crosswind. However, I must once again take exception to
the notion that tilting the lift vector compensates for crosswind.
This is simply wrong. Draw some pictures to work your way through the
problem.
Tilting the lift vector produces a turn, regardless of wind. The turn
will continue so long as the wings are banked.

Wrong, wrong. You can't fly in a straight line with your wings banked?
How do you explain an axial roll? Some aircraft wil even fly straight
& level at 90deg bank angle. Knife edge.

An unbalanced force (like
tilting the lift vector) creates an acceleration, which means that
either your speed or direction changes.

Which is what you need to do in wind.
You better go draw some pictures yourself. Tilting the lift vector
does not produce a turn. It produces an acceleration in the direction
of the tilt, which is what you need to cancel your drift. Forget
forward speed for a moment. Picture a helicopter, hovering over the
runway, in a direct crosswind. Guess what he does to remain stationary
over the runway? um, tilts the lift vector into the wind. If he did
that in calm air, he would travel sideways at the speed that the
crosswind was. In fact, even in calm air, they must "tilt their lift
vector" slightly to compensate for the tail rotor blowing them
sideways...like a crosswind.

Remember, that for an aircraft in the air, the wind is not a force.
Since the aircraft moves with the airmass, there is no wind. So
tilting a lift vector against "the wind" is meaningless.
The difficulty comes when we need to transition from the air to the
ground. The moment the wheels touch the ground, the wind becomes an
unbalanced force,

Not if you are already in balance before you land, enough bank into
the wind so that your LIFT VECTOR keeps you from drifting, and enough
yaw so the wheels don't touch with a side load on them.

and we need to make control inputs to deal with it.
There are two techniques. We know them both.

Yeah, a wheel landing, or a full-stall landing.

But be clear, we
compensate for airmass movement using a crab, not a slip. Transition
to the ground is achieved by momentarily crossing the controls. I
prefer to do this during the flair. Others choose to initiate that
process after turning base. It's a matter of preference. I like my yaw
string straight and my airspeed indicator dependable when near the
ground. (Eric, note that your visual and aural cues become less
trustworthy when near the ground or when flying sideways.)

Wow, is that true Eric? That's a new one. I guess if you're staring at
your airspeed indicator, trying to time your last second manouver
during the flair.

Again, a
matter of choice. But let's get off this notion that a tilted wing
cancels out the effect of wind. It doesn't, at least, not while you
are in the air.
This is building block stuff, which is why I'm still beating it.

http://www.mpaviation.com/lessn15.htm
Kicking out the yaw during the flair is fine if it works for you. I
use that technique in corporate-type stuff, being heavy, faster &
having a training wheel on the nose make it the preferred method. In
taildraggers & gliders & lighter aircraft more affected by the wind, I
prefer the line it up & lower the wing.
All I was trying to tell you is that if you bank the wing, you tilt
the lift vector. If you hold that bank for a mile final, lined up on
the centerline, you are still tilting it. Yes, it would cause you to
track towards the low wing, except that the crosswind is holding your
track straight. Call it what you want. My Dad taught me never try to
teach a pig to sing. It wastes your time, and annoys the pig.

-Buck Wild

Yikes! I feel like I'm trapped in an argument with Cliff Claven. There
is no winning, so why bother to continue the debate.

Alas, I've broken my own rule against discussing subjects on the ras
that have life-threatening consequences. My apologies for inviting
such an outwelling of misconceptions and fallacies. I hope they don't
ultimately hurt anyone.

My last word, go back to your primers and make up your own mind.

Apologies to all for my impatience. I've chatted with several pilots
on this subject. I've come to the conclusion that most pilots
misunderstand the control inputs they are making to initiate a side
slip as opposed to a forward slip. This has resulted in misconceptions
about the forces at work while initiating and during a side slip, to
wit, that the banked wing counteracts the effect of crosswind.

This is not the right forum for me to develop what is essentially a
new way to understand side slips. I've started a power point
presentation to codify my ideas. I'll develop this into an article and
submit it to Soaring and to AOPA. If you are interested in peer review
during development of the article, drop me a note at oscar_charlie at
msn.com. Don't respond unless you are willing to put some time and
effort into the process (a couple of hours). I'll need well reasoned,
thoughtful criticism. Priority to the first three respondents, then
the next three based on credentials.

The more I look at this problem, the more certain I am that we've
happened onto something really interesting during the course of this
thread. I jumped into a few books over the past 24 hours, and none
offered a full explanation of the dynamics of the side slip... rather
their treatments featured application, either implying reasons, or
glossing over them without carefully explicating the process of
initiating a turn from base to final during a crosswind approach.

Good stuff.

OC

(Chris OCallaghan) wrote in message . com...
Yikes! I feel like I'm trapped in an argument with Cliff Claven. There
is no winning, so why bother to continue the debate.

Alas, I've broken my own rule against discussing subjects on the ras
that have life-threatening consequences. My apologies for inviting
such an outwelling of misconceptions and fallacies. I hope they don't
ultimately hurt anyone.

My last word, go back to your primers and make up your own mind.

(Chris OCallaghan) wrote in message . com...
Dan,

You've got it right, at least the way that most of us are taught. But
let me clarify something that we aren't typically taught.
Aerodynamically speaking, there is no difference between a forward and
a side slip. One is used during final for crosswind landings in order
to align the fuselage with the runway throughout the final leg of you
pattern. However, it is, in term of how you use the controls, exactly
the same as any other slip. The difference is that you are executing
the slip along with a crab. That is, if you exit the slip, you will
have a crab angle that aligns your track with the runway.

The only reason I bring this up is that I've seen a thread that
suggests that there is a difference between a forward and side slip.
That the forward slip (by tilting the lift vector) somehow compensates
for a crosswind. Of course, it does not. Only crab angle compensates
for crosswind with respect to the ground.



This is where you are wrong. Slipping is just another way to go sideways.
Some people like to slip, some like to crab!!!!





The slip simply aligns the
fuselage with the runway, at the cost of some glide performance.

This technique is very useful for smooth crosswind landings in high
wing taildraggers, where there is an advantage to setting one main
gear on the ground rather than both at once. It is not nearly so
applicable to gliders, though many of us were taught to compensate for
crosswinds this way. It has some minor advantages... the upwind wing
is "low" at touchdown, less finesse is required with the rudder... but
also has some disadvantages... the controls are crossed, the airspeed
indicator is unreliable, the glider will not achieve maximum
performance if sink or turbulence is encountered, there is some risk
of touching a wingtip if the flair is not properly executed.

But most important, many pilots seem not to realize that they are, in
fact, using the slip not to compensate for crosswind, but to align the
nose with the runway in order to gain a more "normal" view of the
approach. That is its only real value. Giving a slip different names
based on its application only perpetuates the confusion. And I guess
that's my point.

To sum up:

A slip is aerodynamically the same, regardless of wind.

A slip always increases drag.

A slip can be used to align the nose with the runway during a
crosswind approach (match heading with track), at the cost of
additional drag.

A tangent to the orignal thread, more concerned with semantics than
application, but I thought I'd turn the discussion in this direction.

Cheers.