dakota123
Posts: 105
Joined: Wed Aug 30, 2006 11:03 pm

Crosswind Landing-- Slip Vs Stall Speed

Tue Jul 22, 2008 2:54 pm

Here's a question I've not gotten a satisfactory answer to despite asking several instructors over the years-- none of them ever seemed to be too sure of themselves, which makes me wonder if they were guessing...

When landing in a crosswind with controls crossed to maintain upwind wing-down but nose aligned with the runway, what happens to stall speed?

Thanks,

Mike
 
Ralgha
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Joined: Tue Nov 09, 1999 6:20 pm

RE: Crosswind Landing-- Slip Vs Stall Speed

Tue Jul 22, 2008 4:31 pm

Your indicated stall speed will increase, but only very slightly, mostly due to the angled wind hitting the pitot/static sensors. Not enough to worry about if you're flying the approach at the proper speed.
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soon7x7
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RE: Crosswind Landing-- Slip Vs Stall Speed

Tue Jul 22, 2008 4:59 pm

Watch your rate of decent, ...that will increase more than stall speed, your stall speed becomes critical in this configuration when flying high wing with full flaps as manufacturer does not recommend it...the downwind flaps effect erodes as the fuselage interferes with airflow, then your stall speed becomes big consideration...as stall spin is possible...if downwind flap washes out entirely.
 
Buzz
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RE: Crosswind Landing-- Slip Vs Stall Speed

Wed Jul 23, 2008 1:52 pm

Hi Dakota123, Buzz here. I'm thinking it depends what airplane you're flying.
I'm fond of Champs and Cubs, they don't come with flaps (well, the Super Cubs do) so we slip to dump excess altitude most of the time. And Crosswind landings are always in a slip for me, when landing a taildragger you -really- need to keep the runway,prop,tail all in a line. If you crab when you touch down you'll ground loop and end up where the nose was pointing (after an exciting ride).

So... in an Aeronca Champ the airspeed indicator gets inaccurate, but that's not a big problem on final (yeah, right.) because my eyes are outside. I'll start final approach at 60-65 mph, keeping extra altitude (engine failure in the pattern, can I at least get inside the airport fence?)
On short final, I'll do a forward slip to dump the excess altitude. Sometimes I've seen more or less 45 mph (Vs is 38) and feel a lot of buffeting if I'm at full travel on the rudder and stick. But it only takes about a wingspan of altitude loss to recover once you neutralize the controls. A few times I've kept my approach speed high (75 mph -almost cruise speed) and slipped, target speed over the fence is 55 mph. That's worked also.

On the Champs and Cubs, if I were to stall in a slip, the wing unloads. So the aileron input neutralizes, but the rudder input is still quite strong. So you end up with the wings leveling and you wonder why you've got all that yaw... many tailwheel pilots become sensitive with our feet. If I were to stall with a Skid... that's how you start a spin. (another topic, everybody should learn how)

The Cessna's POH admonishes the pilot not to slip with flaps deployed - under some circumstances the nose is supposed to bobble up and down. I'm thinking it's also hard on the flap attach brackets (thinking as an aircraft mechanic)

Notice on the C-150 series aircraft, the Static port is on the left forward fuselage. If you were to left slip, then the static port is now in an area of high (impact) pressure instead of neutral pressure. On the Champ it's on a tube out on the strut, and has 3 or 4 microscopic holes to try and get a neutral reading.

One factor in not paying strict attention to the airspeed is that I've got my eyes outside looking for other traffic, looking to see what the wind is doing to me. I'm flying trimmed, using attitude, and control pressures to deal with my airspeed. In a slip, I do have to hold the controls there - the Champs and Cubs really want to return to neutral and whatever speed they were trimmed for.
And all this might not apply to a modern, high tech airplane.

g'day
 
Ralgha
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Joined: Tue Nov 09, 1999 6:20 pm

RE: Crosswind Landing-- Slip Vs Stall Speed

Wed Jul 23, 2008 6:17 pm



Quoting Soon7x7 (Reply 2):
Watch your rate of decent, ...that will increase more than stall speed, your stall speed becomes critical in this configuration when flying high wing with full flaps as manufacturer does not recommend it...the downwind flaps effect erodes as the fuselage interferes with airflow, then your stall speed becomes big consideration...as stall spin is possible...if downwind flap washes out entirely.

Not all high wing airplanes have this recommendation, and the ones that do (that I am familiar with) are not for that reason. In a slip the flaps will disrupt the airflow over the elevator causing it to buffet, hence the recommendation against doing so.

In any case, that wasn't his question.
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BoeingOnFinal
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RE: Crosswind Landing-- Slip Vs Stall Speed

Wed Jul 23, 2008 10:23 pm

From the C-172 POH:

If flap settings greater than 20 degrees are used in sideslips with full rudder deflection, some elevator oscillation may be felt at normal approach speeds. However, this does not affect the control of the airplane.

It is recommended to use a lower flap setting when doing x-wind landings, if landing distance available allows it.
norwegianpilot.blogspot.com
 
Fly2HMO
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RE: Crosswind Landing-- Slip Vs Stall Speed

Wed Jul 23, 2008 11:04 pm



Quoting BoeingOnFinal (Reply 5):
From the C-172 POH:

What version of the 172 exactly? I can't find that mentioned in the 172S NAVIII POH, though I do remember reading somewhere that slips are not recommended, but didn't say why.
 
soon7x7
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RE: Crosswind Landing-- Slip Vs Stall Speed

Thu Jul 24, 2008 5:42 pm



Quoting FLY2HMO (Reply 6):

I have a Cessna publication that was sent to the location that I rent 172's from. It stated that slips in aggressive cross winds not recommended as the fuselage washes out the downwind flap at half to full flap settings creating the very high possibilty of causing a stall spin...It made complete sence to me as I also have high time in gliders and used slips all the time...so I never forgot reading this information...I just don't remember which document I read it in....and I recently moved and my life is still currently packed in boxes...j
 
Fly2HMO
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RE: Crosswind Landing-- Slip Vs Stall Speed

Fri Jul 25, 2008 2:44 am



Quoting Soon7x7 (Reply 7):
I have a Cessna publication...

Aahh gotcha. Wonder what it was. The current POH is very vague, it just says don't do it IIRC.
 
soon7x7
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RE: Crosswind Landing-- Slip Vs Stall Speed

Fri Jul 25, 2008 9:03 pm



Quoting FLY2HMO (Reply 8):

I know that I still have the piece but having just moved...(still have thirty boxes to triage)...for that very reason,(not found in Cessnas' POH) It stuck out in my mind of special not to do procedures!. It said nothing about burbling off the elevators, it pointed out flap effectiveness asymmetry...that could result in STALL SPIN. When I first started flying gliders, we trained in high wing Schweizer 2-33's. Forward slips were used daily with no worries as they do not have flaps,in many cases, we flew in conditions way outside the envelope of the airframe but we were a commercial operation and performed many flights daily in ALL kinds of weather (Legally). What I was able to do in gliders, I would not necessarily do in power planes as gliders are incredibly more forgiving. Responding to the question...any deviation from a trimmed, cleaned up airflow flight profile will result in deterioration of sink rate and slight increase in stall speed. Thats what 5,000 hours of non powered flight has taught me. Same rules apply to powered planes certainly along with other considerations...one last point to mention that in any light aircraft I have flown...during the cross wind landing or slip...the airspeed indicater becomes eratic ...(another reason that can cause stall spin in this configuration)
 
BoeingOnFinal
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Joined: Fri Apr 14, 2006 6:47 am

RE: Crosswind Landing-- Slip Vs Stall Speed

Fri Jul 25, 2008 9:18 pm



Quoting FLY2HMO (Reply 6):
What version of the 172 exactly?

That exact statement is from the C-172R, 2007 model with G-1000 fitted.

I also find the POH way to vague. The most important aspect of explaining why one should not do a certain type of maneuver, is to elaborate the reason. And that is just not done.
norwegianpilot.blogspot.com

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