Forward Slips

[TEXAN AVIATOR]

Are ailerons, rudder, or both used for directional control? A lengthy response is encouraged.

Fly safe,
TA

 

[BoDEAN]

Not lengthly, but I would say rudder.

Actually, kick rudder to the side, then lower the wing. Actually after reading up on it, BOTH aileron and rudder will maintain directional control (in one form or another).

 

[roundout]

with a forward slip you want as much as possible of the fuselage exposed to the relative wind to add drag. the easiest way to do this is to smoothly put in full rudder while simultaneously adding some opposite aileron, but only enough to keep the airplane following your desired track over the ground. if you keep the rudder all the way to the floor you get plenty of directional control by using the ailerons. if you're descending too fast or if you're losing your ground track, you can always kick out some of the slip, too.

 

[TEXAN AVIATOR]

Actually, kick rudder to the side, then lower the wing. Actually after reading up on it, BOTH aileron and rudder will maintain directional control (in one form or another).

That's what I thought, thanks.

 

[willbav8r]

Rudder to the floor, aileron into the slip, pitch for desired airspeed. VSI points at the F/O's feet

Is the following correct?

Forward slip = maintain ground track (i.e. on final, aligned with runway centerline)

Sideslip = change in ground track (i.e. on base to final with strong crosswind)

 

[roundout]

you know, this is one of the reasons i love aviation.

forward slip - nose pointed to the side
sideslip - nose pointed forward

 

[Tinstaafl]

Aerodynamically they're the same. The only difference is the desired reference point.

Sideslip: Heading reference point remains with the 'old' track. Track made good deviates laterally.

If a x-wind is present then there may be no lateral movement over the ground, depending on wind strength vs amount of sideslip. We're all familiar with this from the 'wing down' x-wind landing technique


Forward slip: New track made good remains with the 'old' track. Heading deviates from the old track.

You'd also be familiar with this if you've ever used a slip to lose height while still maintaining the runway centreline.

 

[midlifeflyer]

Is the following correct?

Forward slip = maintain ground track (i.e. on final, aligned with runway centerline)

Sideslip = change in ground track (i.e. on base to final with strong crosswind)

Problem with that is that in both cases you are maintaining a desired ground track.

I prefer to think of them in terms of their purpose. A forward slip is used to lose altitude while a sideslip is used for crosswinds.

If you're doing an "altitude loss slip" on final you still want to maintain the ground track and not laterally away from the runway. And the "crosswind slip's" entire purpose is to maintain both ground track =and= longitudinal alignment with the runway.

(And, of course, as Tinstaafl said, they are aerodynamically identical)

 

[willbav8r]

Ah, but I can use partial rudder and aileron for a gentler forward slip to lose height (nose remains on centerline).

Or, I can use full rudder and more aileron for an aggressive sideslip to lose height (nose off centerline).

All whilst maintaining a straight track for a final (or indeed against a crosswind). The example I am using here is in a Citabria.

 

[Singlecoil]

This one always confuses folks. They are not aerodynamically the same; some aircraft prohibit forward slips, but not side slips. Think of what is happening to the wings in the two different slips. In a forward slip, the low wing is pointed toward the runway, which can force fuel to the extreme edge of the tank (not just the wing-tip, but toward the leading edge as well), which may cause fuel starvation.

Forward slip: you are moving forward through the airmass. (high on final, use forward slip to lose altitude)

Side slip: you are moving sideways through the airmass. (cross-wind landing technique)

Some airlines are even confused by this, maybe flx757 can point out to someone where he works that the company MD-80 manual instructs the pilots to land in a forward slip. I wouldn't want to be the first one to do that.

 

[midlifeflyer]

some aircraft prohibit forward slips, but not side slips.

Can you name one?

Think of what is happening to the wings in the two different slips. In a forward slip, the low wing is pointed toward the runway

And, being pointed to a point on the ground affects the aerodynamic factors by...?

Seem to me that in both cases, the aircraft is being turned sideways (at least partially) into the relative wind.

 

[Singlecoil]

Cessna 172? Doesn't it have a 30 second forward slip limit? I think the DC-6 prohibits them, but that memory is from jumpseating on one many years ago. Mar? ASquared?

 

[midlifeflyer]

Cessna 172? Doesn't it have a 30 second forward slip limit? I think the DC-6 prohibits them, but that memory is from jumpseating on one many years ago. Mar? ASquared?

I don't have the POH in front of me, but I don't think that the C172 recommendation to avoid slips with full flaps differentiates between forward and side slips.

 

[Tinstaafl]

Not correct, Singlecoil.

Aerodynamically a slip is a slip. The a/c's flight within an airmass is a self contained system ie it has no external reference such as to the ground. If it was otherwise then we could choose any arbitrary frame of reference we liked, no matter how silly, and each choice would lead to a plethora of different aerodynamic reactions. Planet Mars, perhaps? Or the flat-top semi trailer on Intersate 1?

Even though the a/c might still be maintaining some desired ground track, the flight path will have some sideways component wrt the a/c's longitudinal axis. All the pilot has done in a forward slip is ensure that the now longitudinally offset direction of flight through the airmass corresponds to the originally desired ground path.

One could also turn it around & say that for a 'sideslip', the pilot has chosen a new desired ground track & is now ensuring the longitudinally offset flight path matches it.

As far as the a/c is concerned, the ground track is irrelevent. Only the airflow over the airframe counts.

 

[Mmmmmm Burritos]

I don't have the POH in front of me, but I don't think that the C172 recommendation to avoid slips with full flaps differentiates between forward and side slips.

Correct. It's a recommendation to avoid slips with the flaps extended. If it was a limitation, how can you do a cross wind landing?

The slip will impose side loads onto the flaps, which isn't adviseable with the flap linkage system Cessna uses. It'll lead to stress cracks in the flaps themselves and the bracket mounts in the wing. I've seen them both, actually had the flap motor pop the breaker when on base I selected 20 degrees, which caused a crack where the flap track bracket mounts to the wing.