dave d

Some training books I'm reading seem to consider slips necessary for
emergency landings, but not something you should use on "normal landings"
or "to correct a mis-judged appoach". I don't have flaps, so we have to
slip
if the power is already off. I try hard to get the glideslope correct
without
needing to slip, but if I called it wrong turning to final, why not?

What is the danger? The wings are banked, does load factor apply? Is stall
speed increased same way as a coordinated turn of the same bank? Should'nt
be much with the small bank angles I use. What about IAS? Our static port
is right underneath the pitot out on the wing. If the static port is on the
fuselage
does anyone notice variations in airspeed?


Dave D

brian burger

I have on Cessna 152 & 172. The static port is on the side of the engine
cowl, forward of the pilot's door, and if you're in a left forward slip -
left wing forward - the ASI will over-read because of ram air pressure
against the static port. I think the usual increase is about 10 knots.

I don't *think* stall speed would go up in a slip, but I won't/can't say
for sure. The last several flights I've done have all been with
first-time, nervous passengers - not good flights to test a/c performance on!
<grin> (or even to do ordinary things like slips that seem 'wrong' to
non-pilots...)

Brian - PP-ASEL/Night -

borislav deianov

The more you practice emergency landings, the higher your chance of
being able to execute one correctly for real. Why not practice on
*every* landing? This means you should fly your emergency approach
like a normal pattern - downwind, base and final. It also means you
should fly your normal pattern like an emergency approach - with the
power off. And if slips are necessary for emergency landings, they
*should* be used on normal ones, too.


If you overshoot - indeed, why not? You have other options (e.g.
S-turns or flying a speed slower or higher than best glide) but the
slip is much easier and safer. But what if you undershoot? On a normal
landing, you can add power, but what do you do if the engine is out?
You have no options at all.

A much better plan is to always try to need *some* slip, but not a
maximum-effort one. This still allows you to increase the slip if you
overshoot, but it also allows you to take the slip out if you
undershoot. In this respect, slips have a big advantage over flaps for
controlling the glide path - there's no penalty for removing a slip,
even at low altitude.


There isn't any. In fact, a slip is a very stall-resistant maneuver,
more so than coordinated flight. You fly a Citabria, right? Try it at
altitude some time, I bet you won't be able to get a stall break in a
maximum effort slip, even with the stick all the way back.

At a recent soaring seminar, Carl Herold (do a Google search for him)
even suggested that the base-to-final turn should always be flown in a
slip, to protect against a gust converting your nice coordinated turn
into an inadvertent skid.


No, because your flight path is straight.


Regardless of where the static port is, you can't trust the ASI in a
slip. It's just never been calibrated that way. One way to deal with
this is to note your pitch attitude before you begin the slip and just
keep it the same while in the slip.

Regards,
Boris

gary drescher

Regardless of flight path, the vertical component of lift is still
attenuated by a factor of cos(bank angle). So for a given angle of attack
(such as the critical angle, for example), steady-state flight requires an
increase in airspeed to boost the vertical component of lift so that it
equals the plane's weight again. (As always, when you maintain a given
angle of attack, the plane automatically adjusts its airspeed to produce
steady-state flight.) But as you say, the stall-speed difference is fairly
small (7.5% at 30 degrees, because lift is proportionate to the square of airspeed, and sqrt(1/cos(30))=1.075).


Slipping steepens the flight path (that's the reason for doing it, in this
context), so if you want to maintain the same angle of attack, you have to
let the plane pitch downward by the same amount that your flight path
diverts downward. If instead you maintain the same pitch, then your angle
of attack is greater. You might want the angle of attack to increase a
little (to reverse the small airspeed increase due to the slip), but not
necessarily by as much as it would take to maintain the same pitch
angle--that could produce a stall.

You can maintain your angle of attack by trimming for it before entering the
slip, and then just accepting the new pitch angle. You can confirm that the
angle of attack is the same by noting that the aim point (the point in the
scenery that is not moving up or down on your windshield) stays at the same
height on your windshield (the aim point will become a different point in
the scenery after the slip, but it should still be at the same height on
your windshield). You can judge angle of attack that way because the aim
point is straight ahead in the direction of flight, and the angle of attack
is the angle between the wing's chord line (which varies with the plane's
pitch) and the direction of flight. --Gary

greg esres

<<The wings are banked, does load factor apply? >>

No, since you have no vertical acceleration.

There are two issues:

1) In a slip, some of the lift of the a/c is being provided by the
fuselage, which has an angle of attack with the relative wind, due to
your rudder application. Some aerobatic airplanes can engage in
knife-edge flight, with all the lift coming from the fuselage.

This would tend to reduce the AOA needed by the wings, lowering stall
speed.

2) The relative wind is coming over the wings at an angle, but only
the component of the relative wind parallel to the chordline provides
lift, so some of the relative wind is "wasted."

This would tend to increase the AOA need by the wings, increasing
stall speed.

The authorities that speak to this subject maintain that it's harder
to stall the airplane in a slip, implying that factor #1 above is more
significant, but I don't know if they actually have data to prove
this.

gary drescher

Isn't there a third factor--the increase in airspeed or AOA required by the
tilting of the lift vector? Even though the flight path is straight, the
vertical component of lift still attenuates by cos(bankAngle). --Gary

cecil e. chapman

Make slips your 'friend'. Just a splendid 'tool' to have in you pilot
toolbox. I instantly enjoyed forward slips as a student pilot (even early
on, when stalls still spooked me a little).

I've been surprised to learn how many pilots never do slips or practice them
to any degree. You talk to them about forward slips and they act as if you
had just suggested having a hot dog charcoal barbeque pit in your cabin.
There are a few (I've heard, anyways,,,, hearsay) POH's that prohibit them
with flaps. With the C172 POH they have this as an avoidance NOT a
prohibition; forward slips without flaps are fine, WITH flaps it is
suggested that the pilot try to 'avoid' them. Apparently there are two
'concerns' - One, that severe vibrations will develop in the deployed flaps
leading to possible flap damage,, the other concern often cited is that in
the forward slip attitude WITH flaps, the flaps reduce some of the airflow
over the elevator and horizontal stabilizer and might reduce elevator
authority to a significant degree. In most of the 152's the POH says go
ahead,,, flaps or no.. I can't tell ya about the Citabria, but Boris has
practically been living in those critters lately and I'm sure his advice is
just fine.

I asked my favorite CFI one time about forward slips in the C172 with flaps,
he said in an emergency do what you need to get down safely. He said at
most the flaps develop a significant vibration (at full flaps) that may make
you think the plane is falling apart, but one would be fine getting the
plane to the ground. He pointed out of course that the flaps may be
mechanically damaged, but should easily retain a functional constitution
long enough for the slip to a landing.


They truly are an invaluable aid - especially in an engine out/failure where
you HAVE to make your selected landing site - there IS no go-round without
power and overshooting is simply not an option. I was sure glad I had
forward slips in my 'tool chest' the day I lost engine power on a planned
'just for fun' cross-country and did an emergency descent to an airport that
was within gliding distance. I had plenty of altitude to manage (I had been
a 7500 feet) and purposely arrived 1000 feet above the traffic pattern flew
a 45 into the downwind and flew an otherwise 'normal pattern except on final
I kept myself high and then forward slipped all the way to the runway and
then kicked out the forward slip and landed at the airport. Kind of neat
little perk was that I heard the airport manager say over the CTAF (who had
been monitoring my progress since I called the emergency situation while
approaching the hills before Salinas) "Now THAT was 'textbook'!"

Slips (of all types) are your 'friend',,, be familiar with them... you won't
regret it!
--
--
=-----
Good Flights!

Cecil
PP-ASEL

Check out my personal flying adventures complete with pictures and text at:
http://www.bayareapilot.com

"I fly because it releases my mind from the tyranny of petty things."
- Antoine de Saint-Exupery -

"We who fly, do so for the love of flying. We are alive in the air with
this miracle that lies in our hands and beneath our feet"
- Cecil Day Lewis -

dave d

Thanks for the tips, guys. Some other questions:

You might think ram air into the static would balance ram in the pitot
giving lower airspeed indication. Maybe its more complicated 'cause the
pitot is screwed up too.

Our chief pilot said you can get up to 1500 fpm
on the VSI ( reading may be off due to these airflow effects), I'd like to
see if I can get
that much drop. Would that need pitching up as well?

The warnings against slips in my book (the heavy one that reads like a 9th
grade
social studies book), did smack of liability avoidance, and the general
dumbing
down that invades everything these days. Getting in and out of the slip is
the one thing I do OK. Judging glideslope from back at the turn to base
never
seems to work out, always high. My flares are early, and judging the sink
rate
is difficult while trying to keep everything going straight. My CFI did
warn me
about slowing up too much in a pretty aggressive slip (for me) once last
time.
Got me wondering about where the stall is in a slip.
I'll ask my CFI if we can do a power off stall in a slip next time. I've
heard its spin resistant (opposit rudder is already in), but can I really
pull the stick all the way back (no power)
and not stall? One more high airwork session (after a miserable pre-solo
phase check)
then its time to solo.

Gary, I agree that getting the correct pitch is more important than
calibrating the ASI.
I'll try and get a better feel for the "new aim point". Usually I just try
and get the numbers
back up on the windshield.

I think slips are the most fun. And I agree with Boris that without practice
it is useless to
expect you'll do it right in and emergency. But even my CFI, a crusty,
swaggering taildragger type,
cautioned me when I got too enthusiatic about slips with a DE in the room,
saying
"Of course, you know that stabilized approaches are best ...".

Dave

rmg1

Oh, how terrible. No, a slip is great for a "normal" landing. In some
planes a slip allows you to see in front just in case there is traffic
on the runway. In fast slick planes slips are necessary to adjust
speed in high traffic areas (I have a Mooney and very often slip to
work into traffic). I would rather have my students make every
approach a little high and use a small slip to take it out than to
risk coming up short and losing an engine. Of course a cross wind
landing is also a great time to slip.


-Robert, CFI

rmg1

Sorry, I wanted to say a few more things....

1) If you are worried about stalling in a slip, then you aren't doing
the slip right. A slip is not very useful near stall speed. The higher
your speed the more drag you'll get in the slip. A slip should always
include a pushnig forward on the yoke to push the nose down.
2) In a slip you are not "loading up" the wings, you are not pulling
G's the way you would if in a level turn of the same bank. Your stall
speed will not be increased by the G loading (since it should be near
1).
3) You should go out with your CFI and do stalls entered from a slip
configuration if you have any questions. The nice thing about
practicing stalls in a slip is tht the plane always rolls through
level, you don't get the extream bank angles that you do when
practicing stalls in a skid.

-Robert, CFI