Forward Slips with Flaps, Cessna... Is it Ok?

[Keith Matthews]

they're fine. there is a chance that with full flaps and the proper loading condition you could blank out some of the flow over the tail, creating an annoying slight buffet. I had this happen to me last week in a 172. the yoke just bobbed in and out slightly. flight control was not compromised.

your latest instructors dont understand the difference between avoid and prohibit. you're right, the forward slip is a great tool and I would recommend staying sharp on them. In a 40deg flap cessna with a forward slip, you can really make a nice, steep, safe approach.

Hey buddy, dig this. You’re at 1000’ with a student and the engine sucks a valve. You’re a glider, -1 minute from touch. You roll final in a c150 aerobat and you are dying to clear the power line at the approach end of farm field. You make it but the field is short. My response was 40 flaps and nearly drove that down wing into the dirt before I rolled level and stuffed it. Any problem with my choice? All theoretical until you’re a mayday ship with a ditch at the far end. KMANOUT

Welcome to the Can of Worms Hotline!

Anyway, welcome to the board -- you'll get lots of responses, but will still have to come to your own conclusions.

It's true that many Cessna POH's specifically warn against slips exceeding 30 seconds duration with flaps extended. The most logical reasons I've heard have been fuel unporting and tail blanketing (airflow disturbance causing loss of effectiveness of the horizontal stabilizer).

Yet, many of us have slipped C152s/172s/205s longer than 30 seconds and have lived to slip again.

I admit there are few manuevers more useful than a well-executed slip. On an older Cessna with 40 degrees of flaps, you can lose some serious altitude in a very short distance.

A valid question for folks who need to slip for longer than 30 seconds is "why?"

If you're that high on final, maybe you need to work on something other than slips.

to a firm landing and stop short of the irrigation ditch at the end of the field.

 

[MauleSkinner]

Hey buddy, dig this. You’re at 1000’ with a student and the engine sucks a valve. You’re a glider, -1 minute from touch. You roll final in a c150 aerobat and you are dying to clear the power line at the approach end of farm field. You make it but the field is short. My response was 40 flaps and nearly drove that down wing into the dirt before I rolled level and stuffed it. Any problem with my choice? All theoretical until you’re a mayday ship with a ditch at the far end. KMANOUT

Treat the power line like the door of a pay toilet…go under it.

 

[bflynn]

 

[MauleSkinner]

View attachment 99503

And didja notice how he picked a glider pilot/instructor/examiner as the person to whom he had to explain all about flying without power? Apparently he’s still hopped up on adrenaline from his first engine failure.

But I guess when someone joins just to resurrect a 12-year-old thread, they should be given some form of participation trophy.

 

[steingar]

Not only Cessnas. I have read that forward slips are to be avoided in long body Mooneys with full flaps.

 

[Witmo]

... If they were aerodynamically the same, I don't see how this could be. I realize that there is an area where they sort of overlap, especially in stronger xwinds where a side slip will require a lot of control input, but I don't think they are quite the same.

They are aerodynamically the same. Your mistaken belief doesn't change that fact.

 

[midlifeflyer]

Not only Cessnas. I have read that forward slips are to be avoided in long body Mooneys with full flaps.

There are a number of airplanes with POH admonitions to avoid extended slips due to potential fuel delivery issues, including the short and mid-body Mooneys and Cirrus. I haven't seen one in a long-body Mooney manual. But again, like the Cessna, no differentiation between the aerodynamically equivalent side and forward slip.

 

[midlifeflyer]

They are aerodynamically the same. Your mistaken belief doesn't change that fact.

I think what happens is this. People look at the track over the ground and see a difference but find it difficult to picture that the relative wind is the same.

 

[DFH65]

The point is mute in the older Cessnas with 40 degrees of flaps you could probably pull the flaps at pattern altitude over the numbers and still make it down on the runway. Elevator going down......

 

[geezer]

My last instructor when I was training for PPL only referred to slips.

After I had the PPL, I asked the Luscombe owner in the next tie down what the difference was in forward and side slips. He had been flying since the '30's in that taildragger, so must be good at such.

Simple, if you re lined up with the runway, inbound to land, and drop the left wing, hold heading with the rudder, you will slide to the left, in a side slip, and loose alignment with the runway.

If you are lined up with the runway, and the wind is from the left, drop the left wing and add right rudder to keep the plane parallel to the runway AND in line with it, you are in a forward slip. This is simply balancing the wind drift with the side component of the slip to produce a stable approach to the runway, with the airplane always pointed straight down that runway.

The FACT that you are in a side slip is hidden by the FACT of continuing straight to the runway, thus you see a forward slip.

None of my first 7 instructors referred to slips by category, they were simply "slips". 5 of those 7 were instructing in J3 Cubs, no flaps to confuse the issue. We simply slipped enough to stay aligned with the runway, and land.

The final 2 were in a 1960 Cessna 150, and again, the slips were to maintain alignment inbound, but flaps were used to achieve the desired descent. They would have been called forward slips. I did make some no flaps, slip to land in no wind conditions, and roll wings level just before flaring, to demonstrate that I could land without functioning flaps. Some here would call those side slips, I guess.

My instructors accepted any degree of slip without any concern, but reacted instantly to any skid.

 

[Tantalum]

C-172 are a disgraceful blight on the aviation world.

 

[Hang 4]

My last instructor when I was training for PPL only referred to slips.

After I had the PPL, I asked the Luscombe owner in the next tie down what the difference was in forward and side slips. He had been flying since the '30's in that taildragger, so must be good at such.

Simple, if you re lined up with the runway, inbound to land, and drop the left wing, hold heading with the rudder, you will slide to the left, in a side slip, and loose alignment with the runway.

If you are lined up with the runway, and the wind is from the left, drop the left wing and add right rudder to keep the plane parallel to the runway AND in line with it, you are in a forward slip. This is simply balancing the wind drift with the side component of the slip to produce a stable approach to the runway, with the airplane always pointed straight down that runway.

The FACT that you are in a side slip is hidden by the FACT of continuing straight to the runway, thus you see a forward slip.

None of my first 7 instructors referred to slips by category, they were simply "slips". 5 of those 7 were instructing in J3 Cubs, no flaps to confuse the issue. We simply slipped enough to stay aligned with the runway, and land.

The final 2 were in a 1960 Cessna 150, and again, the slips were to maintain alignment inbound, but flaps were used to achieve the desired descent. They would have been called forward slips. I did make some no flaps, slip to land in no wind conditions, and roll wings level just before flaring, to demonstrate that I could land without functioning flaps. Some here would call those side slips, I guess.

My instructors accepted any degree of slip without any concern, but reacted instantly to any skid.

Aerodynamically, the plane would see no difference between the two.

 

[steingar]

There are a number of airplanes with POH admonitions to avoid extended slips due to potential fuel delivery issues, including the short and mid-body Mooneys and Cirrus. I haven't seen one in a long-body Mooney manual. But again, like the Cessna, no differentiation between the aerodynamically equivalent side and forward slip.

If you drain a tank dry you can have a rude surprise in my Mooney if you put it in a slip. The fuel collects the the wing root, you can unpost it on the downward wing. Not placarded in any way, nor is it mentioned in my POH. The limitation in the long body Mooneys is aerodynamic.

 

[bflynn]

Guys, just read back over the previous 3 pages. Henning and Ron Levy already explained all this.

 

[luvflyin]

Guys, just read back over the previous 3 pages. Henning and Ron Levy already explained all this.

?? Would I be guessing right that this Ron Levy dude is poadeleted20?

 

[bflynn]

yes

 

[midlifeflyer]

If you drain a tank dry you can have a rude surprise in my Mooney if you put it in a slip. The fuel collects the the wing root, you can unpost it on the downward wing. Not placarded in any way, nor is it mentioned in my POH. The limitation in the long body Mooneys is aerodynamic.

Yeah, I think the reason is the same in most low wings which have the caution. I was a bit surprised not to see it in the Ovation manual. It's obliquely mentioned in its recommendation to crab most if the way down for a crosswind landing.

 

[Witmo]

?? Would I be guessing right that this Ron Levy dude is poadeleted20?

I served with Ron Levy in the Air Force flying F-111s. Ron started out as an NFO/RIO in the Navy and ended up a WSO in the Air Force. He may have rubbed people the wrong way at times but he's an honorable guy who served his country. I'd say many of his detractors can't hold a candle to his aviation experience.

 

[BryanAV8R]

When I began initial flight training, before I was solo'ed, my instructor taught me forward-slips as another tool during approach to landing.

The training aircraft was a Cessna 152. When I later transitioned to a 1973 Cessna 172 for Instrument instruction, (though not a part of instrument instruction) forward-slips with flaps were still Ok per my instructor in this aircraft.

I didn't fly from mid-April 2006 'til August 2007, but when I did (at another flight school) I was told that Forward-Slips with Partial or particularly Full-Flaps applied, were unsafe in Cessna 172's.

***DELETIA***

Oh well, what are your thoughts/opinions on the subject?

Thinking that's a hold over from when 172/182/etc had max flap at 40 degrees; it was POSSIBLE (not probable, not every time) that if you cranked into a deep slip with the barn doors out that you would shade the elevator a bit and change the responsiveness of the flippers. They changed the max flap settings to 30 degrees somewhere post-M model, I believe, to minimize the possibility of issues at the edges of the envelope (aeronautical design by legal liability?)...but the "Avoid Slips with Flaps Extended" placard is next to the dial (or lever) still.

It says, in my 172K "Avoid Slips...." next to the degree gauge. I try and be on a stabilized approach, so, in the 5 years I've owned 79975, I think I've had to put in a mild forward slip 2 maybe 3 times to account for sloppy technique (hey, we're human, ok?)? If they were prohibited, then I think that handling a 172 or other high wing in a crosswind landing scenario would be a bit of a problem, no?

(Of course, you can avoid this by flying Pipers (my preference, but all that were available when I was getting back into ownership were 172s, so, a plane is a plane...), but that is a religious fight for another thread..)

 

[Pilawt]

Thinking that's a hold over from when 172/182/etc had max flap at 40 degrees; it was POSSIBLE (not probable, not every time) that if you cranked into a deep slip with the barn doors out that you would shade the elevator a bit and change the responsiveness of the flippers.

It's not that the tail is "shaded" or "blanked out". It's upwash from the wing-low aileron hitting the horizontal tail ("relative upwash", actually, compared to the strong downwash from the adjacent flap). These images illustrate the upwash from the upturned aileron encountering the horizontal stabilizer. In the first image the airplane is in coordinated flight; in the second it is in a slip to the left. In both images the airplane is tracking (in still air) toward the mountain peak in the distance.





See post #50 above for the full explanation, by a Cessna flight test engineer.

They changed the max flap settings to 30 degrees somewhere post-M model, I believe, to minimize the possibility of issues at the edges of the envelope (aeronautical design by legal liability?)...but the "Avoid Slips with Flaps Extended" placard is next to the dial (or lever) still.

172s had 40 degrees of flap through the 1980 model 172N. Flaps were limited to 30°on the 1981 Model 172P and thereafter, because the gross weight of that model was increased by 100 pounds, and the heavier airplane could not meet FAA balked-landing performance criteria with 40° of flap extended.