Slipping in crosswind approach and IAS

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Fly79

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fly79
Hi,

Scenario: I am approaching on a C172 with, say 65kt approach speed, and I have crosswind. I am approaching in a slip, with wing low into the wind.

If my ASI is indicating 65kt on the final (without slipping), how will the indication change, when:

a) wind is from the left, ie. I press right rudder to keep nose aligned with rwy and left wing dropped into the wind

vs.

b) wind is from the right, ie. I press left rudder to keep nose aligned with rwy and right wing dropped into the wind

Is there any difference?

Also:

Let's say I am showing 65kt indicated, and still in crab (ie no slip yet). Then I decide to drop wing and slip the rest of the final. How much should I take into account the change of ASI indication, to still keep flying the 65kt? (or any other speed, for that matter)


-fly
 
This won't be too scientific, but here it goes.

Any misalignment of the pitot tube to relative wind will cause some loss of ram air pressure that should be reflected in a slightly slower airspeed indication. The other thing to consider is when you are in a slip (or skid), you have exposed the airplane to increased drag which will make your actual speed decay. This will of course vary by degree of deflection.

In my opinion, this is part of the reason we use 1.3 Vso as it provides a margin above stall to allow for deviations in target speed and airspeed errors such as side slip.

You could consider going up to a safe altitude and experimenting with ASI indications with varying degrees of slide slip. If you calibrated with ground speed you might be able to determine how much of the ASI drop is real (from the drag) or just misalignment of the pitot tube.

My 2 cents. Let me know if I'm off guys.
 
iflyblog.com said:
In my opinion, this is part of the reason we use 1.3 Vso as it provides a margin above stall to allow for deviations in target speed and airspeed errors such as side slip. .
Click to expand...

Something similar to this was recently stated in another thread on slipping and I thought then that "the thinking" was backwards and still do.

If you're slipping on final, either for crosswind correction or to lose altitude, and you're maintaining your IAS at 1.3 Vso then your safety margin is increased, not decreased, because your ASI will be reading SLOWER than your actual speed.
 
Good question, one i've asked before. the answer is that the difference is negligible
 
My take:

If the pitot tube isn't going straight into the wind then it will read lower than actual. Except the wings aren't going straight into the wind either, so it's appropriate to fly faster.

How much faster? Well fast enough to bring your indicated speed back up to where it's supposed to be (in this case 65 kts).

I look at the pitot tube as a molecule counter. It takes so many molecules to make the guage (ASI) indicate a certian speed. If it takes 100 kts worth of air molecules going over the wing to fly then it takes 100 kts worth of air molecules banging into the pitot tube. If the tube is mis-aligned you get fewer and have to go faster to get the same reading. If the tube is at a high altitude then you have to go faster to get the same reading too.

You just want what you need. If the ASI is showing what you need then the wings are getting what they need. I've had people suggest we fly faster because we're at a high altitude airport. Wrong answer. You already ARE flying faster to get the same indication on your molecule counter (ASI). Adding to that in thin air is exactly what you DON'T want to do.
 
Just remember to get the nose down when you go into the slip to compensate for the increased drag.
 
What Renjamin said -- there is a difference when you're slipped, but unless you're talking about something like a 60-knot direct crosswind, the difference won't be significant when you're just slipping for a crosswind.

Where you will see a difference is in a slip to lose altitude where you have the rudder fully deflected so the airplane is significantly yawed. However, if you just hold your pitch attitude and ignore airspeed, everything will be fine.
 
I don't know about the 172C, but in general, it depends.

It depends on you pitot tube and your static source(s). It does not depend on which way the wind is blowing, but in some aircraft it depends on which way you slip. And, obviously, how hard you slip.

A Pitot tube is sensitive to the angle betwen the tube and the direction of the air flow. There are some designs that are less sensitive than the good old chunk of 1/4 inch tube pointing forwards. But, in general, the larger the angle between the tube and the air flow, the lower the Pitot pressure.

The static ports are also sensitive to air flow past their location. Many (but not all) aircraft have two ports to average out the effects of yaw, but even with two ports the average pressure at those two points may (will) not match the actual static pressure under all conditions.

For a minor cross wind correction the error is often not that large. But, again, depending on the airplane, once you get the pedal to the floor, the error in IAS vs. actual AS can get large.

Example: In a Cessna 120 I could get down to nearly zero IAS in a pedal to the floor slip to the left, but a slip to the right would result in a smaller change in IAS (don't recall how much exactly - it was years ago)
 
dell30rb said:
Good question, one i've asked before. the answer is that the difference is negligible
Click to expand...

Hey Ren, you may notice that if you slip the Cub with left rudder that the ASI will read less than if you slip with right rudder. Might need full rudder to maximize this effect. Airspeed doesn't seem to change much in a right rudder slip, but the indication drops more with the other rudder. J-3's don't have an exterior static connection - the ASI static is just vented to the back of the panel, so it's purely a difference in the airflow to the pitot tube. Since the pitot is on the left, I would assume it gets blocked or disturbed when you yaw the airplane to the left, since this puts the fuselage more between the relative wind and the pitot tube.
 
It's about 10 knots when slipping to lose altitude (i.e., full rudder) in a C172. It's going to be proportionally smaller than that with less rudder.

There is at least one error above. The ASI reads LOW in a right slip (rolled to the right), and HIGH in a left slip, in a 172. This is because of where the static port is.

If you have a GPS, you can use it to measure the effect, at least in constant winds. Keep the ground speed and course constant going into the slip, and see what the ASI reads compared to the coordinated configuration.

I haven't measured this on a Piper, but I'd expect the effect to be much smaller due to where the static port is on those.
 
I agree with above ...use your gps GS if you have it to measure this on the ASI. .when things feel "right" the GS should roughly equal Vapp in no wind conditions
 
I knew there was a reason I never approach in a slip. I always crab until just befere touchdown.
 
j1b3h0 said:
I knew there was a reason I never approach in a slip. I always crab until just befere touchdown.
Click to expand...

It's no big deal. If you are flying airplanes without flaps you either do it frequently or just drag it in under power. I prefer the former.

I find slipping easier than farting around with those flap things.
 
Capt. Geoffrey Thorpe said:
It's no big deal. If you are flying airplanes without flaps you either do it frequently or just drag it in under power. I prefer the former.
Click to expand...

Yep, me too. But these days everyone is taught to drag it in, which makes slipping moot...and also why so many pilots are uncomfortable with and unwilling to slip. Plus, most modern trainers slip like pigs compared to the tube-and-fabric tailwheel types. But they still work and can still be put to good use...if flying power off approaches. But if a plane has flaps, I will use all of them...plus slipping, since flaps also reduce float and lower stall speed, which slipping doesn't do.
 
I'm neither unable or unwilling to slip, I just think remaining in a crab until just before or during the flare is a more practical way. I suppose, however, an arguement can be made in favor of slipping for folks who would like slipping practice...in fact a good thing to practice in 10-12kt crosswind is flying the airplane a few feet off the ground while tracking the centerline in a slip.
 
Capt. Geoffrey Thorpe said:
.

The static ports are also sensitive to air flow past their location. Many (but not all) aircraft have two ports to average out the effects of yaw, but even with two ports the average pressure at those two points may (will) not match the actual static pressure under all conditions.

Example: In a Cessna 120 I could get down to nearly zero IAS in a pedal to the floor slip to the left, but a slip to the right would result in a smaller change in IAS (don't recall how much exactly - it was years ago)
Click to expand...


The 172s had their static port on the left forward fuselage, and the slip to the left changes the air direction from a flow past the port to a flow against it. Anytime static pressure increases, the ASI will decrease.

Slipping to the right can change it but only a tiny bit. It will depend on the shape of the flow on that left side, but there won't be any ram pressure against the port.

Dan
 
timwinters said:
Something similar to this was recently stated in another thread on slipping and I thought then that "the thinking" was backwards and still do.

If you're slipping on final, either for crosswind correction or to lose altitude, and you're maintaining your IAS at 1.3 Vso then your safety margin is increased, not decreased, because your ASI will be reading SLOWER than your actual speed.
Click to expand...
That's not guaranteed as it depends heavily on the performance of the static system. If the slip causes an area of low pressure at the static port(s) the ASI can read high. That said, if you start coordinated at 1.3 Vso and slip aggressively while maintaining your pitch attitude (horizon at the same level on windshield directly ahead of you) then you won't lose speed or flirt with a stall.
 
Hi all,

Interesting discussion and great replies. Most of the stuff was already in my mind when I wrote the opening post. But wanted to see what the consensus around this thing is, if one even exists.

Correct me, if I am wrong, but here are my current assumptions about it:

Slip to the right -> wind coming from right = left rudder, and right wing low:

C172 static port (it's on the LEFT side, forward fuselage) will get same amount or slightly less STATIC air pressure, thus causing the pressure difference (dynamic - static) to stay the same or just slightly increase => ASI will show about the same or slightly more than actual airspeed.

Slip to the left -> wind coming from left = right rudder, and left wing low:

C172 static port will get increased dynamic pressure, which causes the static pressure to increase. Thus the pressure differential (dynamic - static) becomes smaller, and ASI starts to indicate LESS than the actual airspeed.

==> right wing low, ASI shows about right
==> left wing low, ASI shows less than actual airspeed

Back to the original scenario with the 65kts as Vapp to start with. If I was to slip right, I wouldn't have to make any correction. Maybe increase airspeed a little, if anything. If I was to slip left, I could treat the ASI as showing a little less than what's real and accept maybe 60kt or so as indication (with ~65kts being the actual IAS).

Btw, I am thinking of situations where considerable wind is present, ie. 10-15 kt crosswind component.

- fly
 
MAKG1 said:
There is at least one error above. The ASI reads LOW in a right slip (rolled to the right), and HIGH in a left slip, in a 172. This is because of where the static port is.
Click to expand...

I actually think it's the other way round :confused:.

Slip to the right (roll right) causes the LEFT side of the fuselage to be pointed even more away from the relative wind, causing LEFT side static port to get less air, increasing pressure differential ([dynamic+static from pitot tube] - static from static port) => increased differential causes bigger needle deflection and thus increase in ASI indication. It shows more airspeed than what's real. As in calm conditions, the Static port should get no dynamic pressure on it, there shouldn't be a big error in the right slip, where the port faces even more away from relative wind. Thus on right slips I gather the error is very small or almost non-existent.

Slip to the left (roll left) causes the static port to face the relative wind more than what happens in calm conditions. The reference static pressure thus increases, which makes the pressure differential smaller and ASI needle deflection smaller => ASI shows less than what's real.

- fly
 
Keep in mind that the only time it's necessary to keep track of this is when you're slipping to lose altitude, i.e., with full rudder input. When you do that, you present the side of the aircraft to the slipstream. When rolling right, it's the left side that gets it.
 
Without even looking at the ASI:

If you enter a slip and try to maintain the same descent rate you had before entering the slip your airspeed is going to decay. If you allow the descent rate to increase, as it will naturally do without a pitch input change from you, your speed will remain the same.

I basically agree with j1b3h0, on final approach you are still flying and there is no good reason that I can think of to perform an extended slip other than to shed unwanted altitude or prevent a nervous passenger from freaking out :yikes:

The efficient way is to fly the wind (crab) until it is time to align with the runway and land.
 
HOW DOES IT FEEL?

Once your on short final and have the field made dont bother looking at that crap.
 
I found it easier to enter the slip after turning final, and to fly in the slip all the way down. Instead of trying to figure out in the roundout / flare if the rudder will be enough for the crosswind, and what kind of bank I need to keep me centered. YMMV. I don't see how it would freak out any passengers. The plane is tracking straight ahead towards the runway, in a little bit of bank.

- fly
 
Fly79 said:
I found it easier to enter the slip after turning final, and to fly in the slip all the way down. Instead of trying to figure out in the roundout / flare if the rudder will be enough for the crosswind, and what kind of bank I need to keep me centered. YMMV. I don't see how it would freak out any passengers. The plane is tracking straight ahead towards the runway, in a little bit of bank.

- fly
Click to expand...

That's the great never-ending x-wind debate- slip or crab. I'm in the camp that feels slipping down final is unnecessary. If that's one's preference that's fine, but with experience, you will know the airplane's (and your own) capabilities for handling any given x-wind not simply based on what the ASOS is reporting, but also what kind of crab-angle you have as you fly down final. Plus, wind normally diminishes the closer you get to the runway surface. The x-wind you're experiencing at 700', a mile out on final, may not be what's happening at the runway level. Plus, with experience, it's really not much of a problem to simultaneously handle the roundout and slip inputs. It's not like you really must decide early on final whether the x-wind is too great. At any point during the roundout process you look out the window and see too much drift or misalignment, then it's go-around time.

But I don't feel like there's really much to this whole debate, technique-wise. It's still proper to touch down in a slip. The slip vs. crab debate will always rage on, but I don't think it's really an important debate to have. All it comes down to is that some pilots like to start the slip sooner than others. That doesn't seem like a major difference in technique to me. Either way, the actual landing itself is generally made the same way.
 
Starting the slip early is a good technique for determining the amount of cross-wind and whether you can comfortably manage it.

That said, however, some people are more sensitive to uncoordinated flight than others, and when you get into bigger airplanes where people are farther from the CG than they are in a four seater it can be uncomfortable for them to be in uncoordinated flight for an extended period of time.

The consensus I hear from senior CFIs and examiners is that they don't care which method a private pilot uses, but when they teach and test a commercial student they generally prefer the "crab and kick" method where the slip is entered in the last moments of the approach. There are of course exceptions if the winds are gnarly.
 
TMetzinger said:
Starting the slip early is a good technique for determining the amount of cross-wind and whether you can comfortably manage it.

That said, however, some people are more sensitive to uncoordinated flight than others, and when you get into bigger airplanes where people are farther from the CG than they are in a four seater it can be uncomfortable for them to be in uncoordinated flight for an extended period of time.

The consensus I hear from senior CFIs and examiners is that they don't care which method a private pilot uses, but when they teach and test a commercial student they generally prefer the "crab and kick" method where the slip is entered in the last moments of the approach. There are of course exceptions if the winds are gnarly.
Click to expand...

Personally, and this is just me, I crab for the crosswind and slip to get down to the numbers. And after I flare with my usual flair, it's just whatever it takes to keep the airplane lined up with the runway.
 
Flying the airplane in slip over the runway centerline, with enough power to keep it from touching down is the best way to develop crosswind skills. If/when the crosswind is too great...or your skills too weak, it's time to go around or find another runway. The problem with crosswinds (or rather, the mystery surounding them) is unless you get to practice the dickens out of them you'll never get good at them. So, practice them already!
 
I never pay that close attention to my ASI that far down final to notice, but you can assume that any misalignment of the pitot will cause a reduction in IAS.
 
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