Side Slipping

[Tolka]

When I side slip a Cessna 172 for a cross wind landing I notice that the indicated airspeed reduces. This is not surprising. I have been wondering how much of this is due to the extra drag in the configuration and how much, if any, is due to instrument error due to the change in the airflow around the pitot tube and static ports. Any thoughts?

Tolka

[Andy_RR]

When I side slip a Cessna 172 for a cross wind landing I notice that the indicated airspeed reduces. This is not surprising. I have been wondering how much of this is due to the extra drag in the configuration and how much, if any, is due to instrument error due to the change in the airflow around the pitot tube and static ports. Any thoughts?
Tolka


I've never flown a 172 - does it have multiple static ports, or like a 152 only on one side?

If so, it's most probably because the static port is measuring a component of the dynamic pressure and the pitot will be measuring only a component of the total dynamic pressure, thus the pressure differential is lower => lowered IAS.

If you sideslip the other direction the indicated airspeed should increase (if it's only got one static port on one side).

A

[High Wing Drifter]

Or possibly that the pitot is receiving fewer air molecules due to its angle to the relative wind.

[Flap40]

Firstly check your POH for YOUR 172. There used to be a restriction that prohibited the use of flap while sideslipping in the 172. Yours might be different but check!

[Gingerbread Man]

I'm fairly sure the C172s I have flown only have one static vent on the port side.

[bookworm]

I have been wondering how much of this is due to the extra drag in the configuration and how much, if any, is due to instrument error due to the change in the airflow around the pitot tube and static ports.

An interesting question. Take a GPS and get someone to check out what happens to the groundspeed. That should differentiate between instrument error and the real airspeed change.

[Mike Cross]

IIRC the prohibition only applies with 40 deg of flap set due to the blanketing effect on the tailplane. Those Cessnas with max 30 deg flap don't have the restriction. However don't trust me, look it up in the POH.

ASI's normally underread in a slip, by how much will vary according to installation and the placement of the pitot and static vents.

One explanation is courtesy of Mr Bernoulli. Passing an airflow across the open end of a tube will reduce the pressure of the air in the tube, when you slip the pressure reduces and hence the ASI reading.

A sideslip also leads to the nose dropping. If you don't apply a bit of backward pressure you'll find you come out going faster than you went in. Try it at a safe height, checking the height on entry and exit until you find the attitude that gives you the exit speed you want.

[FlyingForFun]

Certainly the C172s I used to fly had only 30 degrees of flap, but had a warning (not an outright prohibition) to avoid slipping with any amount of flap. (It's not possible to completely avoid slipping, or else how would you land in a cross-wind?)

An interesting idea would be to compare the change in IAS when slipping to the left vs slipping to the right, and see how this fits in with the position of the pitot and static ports. I don't know what would happen, I've never tried it, but please do, and let us know!

FFF
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[High Wing Drifter]

Is the flap restriction due to instability when slipping or because of pitot/static errors?

[nzmarty]

if you are side slipping during a crosswind landing, you are doing something wrong. side slipping is flying out of balance, in a crosswind landing you are (well should be!) in balance until the flare.

have a think about the dynamics of side slipping to the left in a 172 with 30 deg of flap for a minute. left wing has full lift, including that generated by the flap. the right wing on the other hand, has the inner wing shielded by the fuselage, the part of the wing that creates the most lift (inner, where the flap is) is receiving poor flow, buffeted airflow, and only the outer wing is generating 'clean' lift.

left rudder is applied, and you're out of balance.

i don't think you're too far away from the lower edge of the envelope here, maybe that's why the POH says no sideslips with flap?

and anyway, with 40 deg of flap, why do you need to sideslip? a poorly set up approach is the only reason, and a 172 will dive pretty steep with full flap anyway -they're not a piper cub with no flap.

[Mike Cross]

Mmmm....

You appear to know something the professionals (http://www.fly-safely.org/story.asp?id=79) don't.
Crosswind Landing Techniques
There are four commonly used crosswind landing techniques (side-slip, crab to touchdown, de-crab in the flare, and a combination of crab and side slip), the technique selected by the pilot can significantly affect the crosswind capability. For instance when using the side-slip (or wing down) technique, the bank angle required to maintain runway alignment increases in proportion to the crosswind component: therefore, in higher crosswinds there is a risk of an engine or wing striking the ground on landing.

[Mike Cross]

The following from this page (http://www.whittsflying.com/Page5.64C-172%20Techniques.htm)might be the reason.

Slips with Flaps
POHs 172
The owners manual for the 1967 Cessna 172 states on page 2-11
Normal landings are made power-off with any flap setting. Slips are prohibited in full flap approaches because of a downward pitch encountered under certain combinations of airspeed and sideslip angle.

The 1976 POH requires a placard which states "AVOID SLIPS WITH FLAPS EXTENDED"

Landing section for Normal landings states:
"Steep slips should be avoided with flap settings greater than 20 (degrees) due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings."
"The maximum allowable crosswind velocity is dependent upon pilot capability rather than airplane limitations. With average pilot technique, direct crosswinds of 15 MPH can be handled with safety." No max demonstrated crosswind speed is in the POH. The 1967 model has 40 degrees of flaps available.

The 1970 (C172K) model's POH (1970 model also had 40° flaps) has been changed to read "Slips should be avoided with flap settings greater than 30° ..." No required placard is noted in the Limitations section.

Placards are required by model:
According to the TCDS 3A12 (covering models 172 - 172S), regarding slips, the following On flap handle,
Models 172 through 172E:
"Avoid slips with flaps down."

Near flap indicator Models 172F (electric flaps) through 17271034, excluding 17270050):
"Avoid slips with flaps extended."

Tom,
The 172 manuals suggest, but do not restrict, not slipping the aircraft with more than 20 degrees of flaps. On that model doing so would occasionally induce "elevator oscillations." No such notation appears in the 182 POH, or the POH of any other model Cessna besides the 172, because no such elevator oscillations have shown up in full flap slips on other models. Probably has something to do with different fuselage lengths and tail sizes. Having intentionally gotten a 172 in this configuration in my flight instructor days, I did after much effort, get some elevator oscillation. No big deal, just some momentary change in stick force at the control wheel. Plane remains fully controllable, however, I can see where it would be disconcerting for the pilot, particularly if it occurred during the landing flare. But again this only applies to the 172.
John Frank, CPA Tech Rep mailto:[email protected]

[High Wing Drifter]

nzmarty,
All this slipping terminology! I perceive the general convention seems to be that a side slip is where you head the runway track to counter a cross wind, another name for a wing down down approach. A forward slip is where you fly more side on to increase drag and descent rates. For some reason it makes more sense to me the other way around but that is another story. I have noticed that some people to describe the other way around!! They are aerodynamically the same thing and seem to both fall under the name of side slip when not being specific about the application.
the inner wing shielded by the fuselage, the part of the wing that creates the most lift (inner, where the flap is) is receiving poor flow, buffeted airflow, and only the outer wing is generating 'clean' lift.
The text book generic high wing in a side slip theory is that windward wing (the one you call the outer), because of the high wing config will pool higher pressure air under the wing, whereas the leeward (the innerwing in your description) wing's underside will be shielded by by the fuselage so will be lower pressure and so will generate more lift. The result is that when side/forward slipping the windward wing will drop, making the aircraft unstable. Low wing are the opposite and so tend to want to recover from a side slip. That is the ATPL text's description, but like lots in the ATPL it is theoretical and the designers don't seem to let these problems find their way into production aircraft (as far as I can tell anyway!)

[nzmarty]

points taken, read and noted...

[Andy_RR]

The text book generic high wing in a side slip theory is that windward wing (the one you call the outer), because of the high wing config will pool higher pressure air under the wing, whereas the leeward (the innerwing in your description) wing's underside will be shielded by by the fuselage so will be lower pressure and so will generate more lift. The result is that when side/forward slipping the windward wing will drop, making the aircraft unstable.

I might have got your terminology confused, but isn't it the other way 'round? The windward wing of a high-wing configuration will generate more lift due to the higher pressure 'pooled air' under it.

A

[7gcbc]

what about bi-planes ? :uhoh:

[Mike Cross]

I suspect HWD is muddying the waters rather than clearing them.

A sideslip is a sideslip however you refer to it. Rudder is applied to yaw the fuselage to one side and aileron is applied to keep the wings level. Some elevator input is normally also required to stop the nose from dropping.

In the one-wing low landing scenarion the approach path is aligned with the runway centreline. The drift that would normally be caused by the wind is offset by banking the aircraft towards the wind direction, producing a horizontal component into wind and the resulting turning effect is cancelled by applying opposite rudder. It's still a sideslip but carried out without the wings level.

As far as roll effect goes the vertical position of the centre of lateral resistance in relation to the centre of lift will have some effect (it's above on a high wing and below on a low-wing, somewhere in the middle on a biplane).

There will be a spanwise element in the flow across the wing, effectively an increase in chord. While the wing in the lee of the fuselage will lose some airflow at the root the same will happen on the other wing as spanwise flow hits the fuselage causing a pressure build up that will disrupt the flow further out.

I find it's best not to ponder these things too much, just go out and practice at a safe height until it comes naturally.

The compniation of no flaps on the Luscombe, high trees on the approach and the downslope on 03 at Popham makes it an ideal place to refine the technique.

Mike

[High Wing Drifter]

Andy,
I might have got your terminology confused, but isn't it the other way 'round? The windward wing of a high-wing configuration will generate more lift due to the higher pressure 'pooled air' under it.
Yes, you're right. I got my terminology the wrong way round and then tracked direct to CONFUSED at FL600 without clearance. What a plonker :O

Nzmarty, take note old boy!

[shortstripper]

I find it's best not to ponder these things too much, just go out and practice at a safe height until it comes naturally.

Here, Here!

SS