Forward slipping with flaps down - low wing

[Fastglas]

I was told a few weeks back by a CFII that I shouldn't be using a forward slip with full flaps on final to shed altitude. Said that since I fly a low wing aircraft, the airspeed indicated is prone to error and especially so with full flaps. In retrospect I should have asked about side slipping as well.

Since then, I've become a bit more conscious about it and stopped using it other than in a clean configuration, but rarely do I need it unless I'm on short final and I'm configured dirty.

What say you?

 

[RoscoeT]

You appear new here - welcome! Now....you have restarted another classic worn out, beaten-to-death, never-ending debate. You may want to use the search function for this one. There's plenty. Here's a tip - ignore most of the "I was taught" statements.

 

[Hank S]

My plane slips well with full flaps, but I also have two static ports. What does the POH of the plane say? Mine doesn't mention it. You will want to be aware of your fuel levels when slipping, so that when fuel level is low you won't push the remaining fuel away from the port. In other words, slip towards the full tank or go around and try again.

 

[Fastglas]

You appear new here - welcome! Now....you have restarted another classic worn out, beaten-to-death, never-ending debate. You may want to use the search function for this one. There's plenty. Here's a tip - ignore most of the "I was taught" statements.

Thanks!

I tried. Spent about ten mins and gave up. Too much work. Besides, what's the point of a forum if we can't beat up on old topics over and over again.

Time for round 1,475,891!!!

 

[Fastglas]

My plane slips well with full flaps, but I also have two static ports. What does the POH of the plane say? Mine doesn't mention it. You will want to be aware of your fuel levels when slipping, so that when fuel level is low you won't push the remaining fuel away from the port. In other words, slip towards the full tank or go around and try again.

I have two as well. POH says don't slip with less than 10 gallons in the main tank. But no specific reference to configuration.

Anyone know what the RV experimentals have in their books about slipping dirty?

 

[Capt. Geoffrey Thorpe]

Air speed errors in a slip will happen no matter what the flap setting.

As far as the airplane is concerned, there is no difference between a side slip and a forward sip.

Some models of 172's have a statement something like avoid slips with full flaps in the POH because of an interaction with the elevator. But, that seems to be the exception.

What is in the POH of a homebuilt RV would be whatever the individual builder put in there. Vans may provide a generic book, but there is no rule against changing it.

 

[RoscoeT]

Anyone know what the RV experimentals have in their books about slipping dirty?

Nothing in the flight manuals to my knowledge. RVs don't slip all that great, but nothing special happens when you slip w/ full flaps. Same with pretty much all airplanes as you may learn.

 

[Fastglas]

Works for me. I guess I am putting it back in the bag of tricks.

Thanks all.

 

[EdFred]

Tell that CFII to quit instructing.

 

[kyleb]

Anyone know what the RV experimentals have in their books about slipping dirty?

Books? There are basically no books.

But I have 900+ hours in my RV-6 and got factory transition training from Mike Segar, who has thousands and thousands of RV hours.

When I was doing my transition training, I was high on final on one approach and asked Mike "How does this thing slip?". His response was "Try it and find out...". No problem.

They slip just fine with and without flaps (full or partial). I slip mine any time I need to steepen my approach. The biggest downside (IMO) is that rudder forces are high relative to the elevator and aileron, so you really have to stand on the rudder.

What you should do is go to altitude and practice slips to see if your particular airplane has any surprising tendencies and to establish a low IAS limit for slipping. Maybe your ASI is off, maybe your pitot/static system is off, maybe your IAS is off in a slip. Or maybe it is dead accurate. Go slip the airplane and find out.

I'm comfortable slipping mine at an indicated 1.2-1.3 VSO with full flaps all the way to the landing flare, but your experience may be different.

 

[JeffDG]

Slipping with full flaps is pretty routine in low wings.

Like some have mentioned, there are some issues with some models of C-172s, but I've never heard of a prohibition or negative recommendation in any low wing.

The ASI will be off...yep. So what? The situation where you're slipping with flaps should almost always be a quick descent at near Vfe, not right at Vso

 

[Pilawt]

I was told a few weeks back by a CFII that I shouldn't be using a forward slip with full flaps on final to shed altitude. Said that since I fly a low wing aircraft, the airspeed indicated is prone to error and especially so with full flaps.

Tell that CFII to quit instructing.

Or maybe the CFII could learn to fly by reference to the airplane's attitude on final instead of slavishly staring at the not-always-reliable ASI, and then he can begin to instruct.

 

[EppyGA]

I slip my Warrior with full flaps. It hasn't fallen from the sky yet.

 

[RoscoeT]

The situation where you're slipping with flaps should almost always be a quick descent at near Vfe, not right at Vso

Disagree. This implies that pilots need to tack on extra speed in a slip. Not so. Vfe is significantly over approach speed in most planes. Pilots should learn how to fly their airplanes slipping down final at no more than 1.3Vso, no matter the flap configuration. Pilots should also learn to not rely solely on the ASI.

 

[JeffDG]

Disagree. This implies that pilots need to tack on extra speed in a slip. Not so. Vfe is significantly over approach speed in most planes. Pilots should learn how to fly their airplanes slipping down final at no more than 1.3Vso, no matter the flap configuration. Pilots should also learn to not rely solely on the ASI.

Fair 'nuff.

I was referring to the situation where it's most used...you're high and/or fast on final. While in the slip, you're not in any danger of hitting stall.

 

[poadeleted20]

Unless your plane has a explicit prohibition either in the POH or on your panel regarding slipping with full flaps, there's no earthly reason not to do it, and plenty of good reasons for doing it, especially when making landings over obstructions. I'd suggest going back to that instructor and asking the reason for that suggestion.

As for airspeed errors, yes, you need to ignore the likely-erroneous airspeed indications while in a big slip, but if you keep the aircraft's pitch attitude in the right place, airspeed will remain where it should. And the airplane cannot tell the difference between a forward slip and a side slip -- aerodynamically, they're the same, and the only difference is how the pilot is managing the maneuver with respect to the ground. So, the effect of the slipped condition on airspeed indications will be the same in either case -- just keep the nose where it belongs, and you'll be fine.

 

[nosehair]

To the OP : This may have nothing to do with it, but it might.
You use the term CFII, which means Instrument Instructor.
Some Instructors present themselves as Instrument Instructors only because they don't like all that slipping and stalling and steep turning like Airplane CFIs might like more and will be more experienced in those areas.

 

[James331]

Tell that CFII to quit instructing.

That.

I've slipped all sorts of planes, low wing, high wing, mid wing, piston and turbines.

Given enough hours a slip WILL save your butt, don't be afraid of them and practice them often.

 

[Skip Miller]

The only rule of thumb I use is that if one tank is very low, I will use the fuller tank. It is possible to unport the tank that is on the same side as the uncaged ball. That has caused some pilots to lose the engine and land short.

-Skip

 

[Fastglas]

To the OP : This may have nothing to do with it, but it might.

You use the term CFII, which means Instrument Instructor.

Some Instructors present themselves as Instrument Instructors only because they don't like all that slipping and stalling and steep turning like Airplane CFIs might like more and will be more experienced in those areas.

You might have something there. We never flew anything more than a stabilized approach and standard turns. I did my instrument through him and my private from another. He was a crusty old fellow. He liked to talk about flying in the good old days and talked about my plane in general terms based on the type of wing. On a fairly recent flight buzzing around for IFR currency, we landed visual -- and he certainly didn't encourage me to continue the practice of forward slipping on final with full flaps.

 

[Checkout_my_Six]

unporting fuel in a low fuel tank might be a consideration....but other than that, slips away.

 

[FastEddieB]

I like to come in high enough that I need some tiny amount of slipping to fine tune my approaches, even with full flaps - which I virtually always use.

Exemplified in this video. At :50 in I begin a power off approach with a little slipping for that "fine tuning" I referenced:

http://youtu.be/zCjjm_Hrmgg

 

[brian]]

The DA20 I soloed in was a slippery little thing - SUPER FUN. It was easy to come in a little high or a little fast and the flaps don't seem to add much (if any) drag. But it was fun to go "elevator down" and slip that little aircraft right where you needed it.

Listen to "Check_my_six". My -35 has a limitation to slip no more than 30 seconds. Not sure why one would need to slip a Bonanza though - the flap are pretty much like pressing the "Lobby" button in an elevator. (Of course I have the same opinion for the C172).

 

[Checkout_my_Six]

Listen to "Check_my_six". My -35 has a limitation to slip no more than 30 seconds. Not sure why one would need to slip a Bonanza though - the flap are pretty much like pressing the "Lobby" button in an elevator. (Of course I have the same opinion for the C172).

then again....if the engine is out....who really cares if fuel is unported.

 

[Sac Arrow]

Some 172's are placarded to "avoid slips with full flaps." I think because of rudder buffeting. But I've done it in those planes and haven't experienced anything negative as a result.

 

[CharlieTango]

With sight tube fuel gauges I can see if a wing is too low on fuel for slipping. If one wing looks dry I keep the other wing high enough to avoid unporting it too.

 

[flyingron]

Yeah the real slick fuselages just don't have a whole lot of sail area to turn into the wind compared to the cherokees or skyhawks or even my Navion (which is such a pig turned crosswind that it's hard to taxi it on the ground).

 

[Stewartb]

Some 172's are placarded to "avoid slips with full flaps." I think because of rudder buffeting. But I've done it in those planes and haven't experienced anything negative as a result.

Slips with full flaps are prohibited in the 172 that I remember. The other Cessna's that I've owned say to avoid slips with flaps. Not full flaps, just flaps. Personally I don't get what the hoopla is about. I haven't needed to slip a plane in 20 years from being too high. To already have flaps out before I recognized I was too high? I can't even imagine that happening.

 

[flyingron]

Slips with full flaps are prohibited in the 172 that I remember.

Only some models. It was changed to "avoid" in other years as well.

I used to do full flap / full rudder deflection slips in the 172N we had all the time (it was my standard "keep your speed up / drop like a rock" approach into IAD). In all the time I did that, I only once got the pitch oscillation effect. It was kind of neat but I'm yelling "there it is!" at my wife and she didn't even notice it was going on.

 

[Pilawt]

Some 172's are placarded to "avoid slips with full flaps." I think because of rudder buffeting. But I've done it in those planes and haven't experienced anything negative as a result.

In early models, the horizontal stab would sometimes encounter the upwash from the upturned, pro-slip aileron, causing a sudden pitch-down. It was, in the words of a Cessna test pilot, "elusive and hard to duplicate," but easily controllable if the pilot was ready for it.

Compare coordinated flight with flaps down:

... with slipping flight with flaps down:

The longer dorsal fin of the 1972 C-172L apparently eliminated this issue.

There is also a different phenomenon in C-172s, a mild pitch oscillation in full-flap slips, common to other high-wing singles as well. This one is caused by the turbulent vortex off the outboard end of the flap hitting the horizontal stab.

 

[poadeleted20]

Some 172's are placarded to "avoid slips with full flaps." I think because of rudder buffeting.

It's actually a pitch oscillation, which (if it happens at all) stops the instant you kick it out of the slip.

But I've done it in those planes and haven't experienced anything negative as a result.

Me, too.

 

[Pilawt]

It's actually a pitch oscillation, which (if it happens at all) stops the instant you kick it out of the slip.

That's the latter phenomenon I described, a benign "pitch pumping" that feels like very low-frequency vibration in the yoke.

The former was a rare but definite pitch-down, described by a Cessna test pilot as in some cases "severe enough to lift the pilot against his seat belt if he was slow in checking the motion." It happened only rarely in pre-1972 models, and not at all in later ones with the larger dorsal fin.

 

[nosehair]

I haven't needed to slip a plane in 20 years from being too high. To already have flaps out before I recognized I was too high? I can't even imagine that happening.

Yes, you can Imagine an engine failure and you find yourself on final to a short field with full flaps and still a bit too high. You don't think that can happen to you?

 

[RoscoeT]

Yes, you can Imagine an engine failure and you find yourself on final to a short field with full flaps and still a bit too high. You don't think that can happen to you?

Nope, it's well-known that pilots who fly in Alaska are there because their **** is just too damn hot for anywhere else!

 

[Stewartb]

I said what I meant and I mean what I said. Maybe you guys find it important or entertaining to do full flap slips. I don't. If you have a problem with that? Deal with it.

 

[comanchepilot]

I've slipped all sorts of low wing airplanes to landing and even, God forbid - landed the upwind wheel and then landed . . .

Your poh will tell you if its a problem. Some airplanes have fuel unporting problems - some have limits for empennage air flow - others have no limits and you can slip them to the ground to your hearts content.

 

[dell30rb]

The only rule of thumb I use is that if one tank is very low, I will use the fuller tank. It is possible to unport the tank that is on the same side as the uncaged ball. That has caused some pilots to lose the engine and land short.



-Skip

If you're slipping to lose altitude, you are high anyway


Sent from my iPhone using Tapatalk

 

[EdFred]

If you're slipping to lose altitude, you are high anyway


Sent from my iPhone using Tapatalk

Yeah, but who says being high is wrong? There's a lot of places, like oh, all of them, where I'd rather be high than low. I don't know of any place where you have to land under something. There it would be bad to be high.

 

[Stewartb]

In early models, the horizontal stab would sometimes encounter the upwash from the upturned, pro-slip aileron, causing a sudden pitch-down. It was, in the words of a Cessna test pilot, "elusive and hard to duplicate," but easily controllable if the pilot was ready for it.

Compare coordinated flight with flaps down:

... with slipping flight with flaps down:

The longer dorsal fin of the 1972 C-172L apparently eliminated this issue.

There is also a different phenomenon in C-172s, a mild pitch oscillation in full-flap slips, common to other high-wing singles as well. This one is caused by the turbulent vortex off the outboard end of the flap hitting the horizontal stab.

FYI, in 1963 the horizontal tail span grew by 8". That was the same year that the rear window appeared.

 

[poadeleted20]

If you're slipping to lose altitude, you are high anyway

Or trying to land over a tall obstacle. Makes a big difference getting down over tall trees without landing halfway down a short runway.