EAAAirVenture OshkoshShopJoin

To slip or not to slip??

Posted By:
Mel Chapman
9
Posts
0
#1 Posted: 4/23/2011 20:58:13

Short final, a bit high, 2 notches of flaps already deployed. Is it safe to slip? I've had different flight instructors present different opinions on this. I have not found prohibitions in the POH's for the airplanes that I fly (typically C172/C152 & Cherokee 140s). Interested in more insight from this community on this topic? Thank you in advance for you input!



Bill Greenwood
Warbirds of America MemberYoung Eagles Pilot or Volunteer
121
Posts
24
#2 Posted: 4/23/2011 23:34:25

If I remember right, a 140 has 3 notches of flaps. If so, there is no reason to slip before you use full flaps. 

Most simple trainers allow slipping with full flaps.

But check and heed the manual to be sure. When you slip, don't get too slow as you are in cross control flight, but also don't allow the nose to drop so much that you pick up so much speed that you float too long in the flare.



Tony Johnstone
IAC MemberNAFI Member
61
Posts
15
#3 Posted: 4/24/2011 12:23:56

Mel- 

   There is absolutely no reason at all why you can't slip on final with or without flaps on most light GA aircraft.  Bill has a point, you may want to get full flaps down first, but there may be a reason (such as a strong crosswind) that you may want to land with less than full flaps, so go ahead and slip as necessary.

   There may be a prohibition against slipping with full flaps in some aircraft, so, obviously, don't do it in those types.  Nothing I've flown in the last 40+ years comes to mind.  The high-wing Cessna singles often carry a placard "Avoid slips with full flaps extended" or words to that effect.  That is NOT a prohibition although I have heard more than one instructor interpret it that way.  If you do a full-rudder deflection slip in a C172 or C182 with full flaps, you may experience a rather startling pitch oscillation, but the aircraft will immediately stop doing it if you release the slip inputs.  I don't think it is particularly dangerous but it can get your attention which I believe is the reason for the placard.  I believe it is due to the high-wing flaps blanking the airflow over the horizontal stabilizer.  This is obviously not an issue with a low-wing aircraft.

   In regard to the slip being a cross-controlled maneuver, this is obviously correct.  However, this is often misinterpreted to mean that if you get too slow in a slip, you will spin.  In fact, this is not likely to happen, you can certainly stall (which is a bad thing on final!) but you probably won't spin.   The reason is that, in a slip, the aircraft is rolling one way, but yawing the other.  If you look at the slip-skid ball and the turn coordinator, you can verify that they are deflected in the same direction (rolling left, ball to left thus yawing right).  The aircraft will usually spin in the direction of the yaw, but since it is rolling in the opposite direction the two forces are opposed.  I have demonstrated this many times in my Decathlon (at altitude!).  Put the airplane into a full-deflection slip and allow it to pitch up to a stall, it simply starts sinking at a rather dramatic rate but does NOT spin.  

   In a skidding turn, on the other hand, roll and yaw are both in the same direction, so if you stall in a skid you can (and probably will) spin.  The classic example is the base-to-final overshoot, with an attempt to rudder the airplane back to the runway causing a stall-spin at low altitude.

   Bill's point about pitch control is well-taken.  Remember, in a slip, the ASI may not be accurate to the pitot tube being sideways to the relative wind.  You need to maintain a constant pitch attitude into and out of the slip.  Most GA airplanes will tend to pitch up as you enter the slip, so you will need to add some forward elevator into the slip.  If you don't take this out as you roll out, you will pick up an extra 10 kts of airspeed right at the point you are trying to dissipate it.

   Probably more than you were looking to hear and I am sure some will disagree with some of the above, those are my opinions!   Go ahead and slip all you like, 

                           Cheers, Tony (MCFI-A)


   





Rick Robbins
Young Eagles Pilot or Volunteer
12
Posts
3
#4 Posted: 4/24/2011 15:45:23

Tony's reply is right on.

I have been flying for awhile.  I like to slip.  It is good to for altitude control also little slips will put you on the center line.  I learned to fly in Aeroncas (no flaps) and Cessna 140s (worthless flaps).  If you are a little nervious find an instructor that likes them and practice.  I have been at some mountain airports where 40 of flaps were needed along with a healthy side slip to get around things poking out of the ground.  Some of my students are amazed to find that on a practice engine out, with full flaps we are too high.  They want to go around, this is where a good slip will place them right in the landing zone. Learnig to be comfortable with slips save you bacon someday.



Frank Giger
Homebuilder or Craftsman
117
Posts
33
#5 Posted: 4/26/2011 02:30:30

I'm just impressed you're consistent enough in the pattern not to have to slip all the dang time like this pilot "friend" of mine. 

However, on a nice long runway with a plane that lands in a very short distance, I'll take a bit too much altitude that needs to be bled off than the alternative.

And my experience is that when in a slip on final (not on a turn), stalls are pretty straight forward affairs that are easily dealt with with no real spin threat.  And that's all I gots to say about that.



Doug Drummond
Young Eagles Pilot or Volunteer
9
Posts
0
#6 Posted: 4/28/2011 21:18:18

I'm an 850 hour private instrument pilot.  Once landing in a  severe crosswind, I had full flaps and the rudder pedal all the way to the floor in a C-172.  I was finishing my IFR rating and was real current and had an experienced instructor on board.  As we turned final I told her I would make one attempt to see if the crosswind was too bad and use another runway if it was.  I also said that it was a good thing we had two experienced pilots on board. 

Well I did get the pitch osculation, which apparently is due to the tip vortex ruffling the tail feathers, but it was a very slight and symetrical pitch wiggle.  Just wiggled up and down a little bit.

 

Doug Drummond, International Flying Club (K-DPA)

 

 



Doug Drummond, International Flying Club PP-ASEL-IA
Richard Warner
Homebuilder or CraftsmanAirVenture Volunteer
32
Posts
2
#7 Posted: 4/28/2011 21:50:05

I agree completely with Tony.  I've been flying for 58 years now and there is absolutely no reason not to slip if you need or want to.  Also, it won't spin while in a slip but if you stall, it comes down like a manhole cover on edge.



Tony Henrie
Homebuilder or Craftsman
1
Post
1
#8 Posted: 5/2/2011 07:43:19
Frank Giger wrote:

 


And my experience is that when in a slip on final (not on a turn), stalls are pretty straight forward affairs that are easily dealt with with no real spin threat.  And that's all I gots to say about that.

 

  The thing that gets some new pilots (and low-time instructors as well) is that they are taught by low-time instructors (yes, many instructors were taught by low-time instructors) that slipping on a high-wing aircraft is dangerous.  If things are just right, a full-flap slip can disrupt the airflow over the elevator and reduce elevator authority, however, as one poster above noted, as soon as the slip inputs are released that goes away.  I once had an Instrument instructor who nearly had a fit when I slipped a 172 on final.  I got another instructor.

The poster quoted above indicated that a slip shouldn't be conducted in a turn.  Actually, a slip IS a turn, just input cross-controlled so as to keep the aircraft tracking in a straight line.  Starting a slip on base, while turning to final is no more dangerous than slipping in a straight line.  The only difference in slipping "in a turn" and turning is that in a turn your rudder goes in the direction of the turn and in a slip it goes against the turn.

I think this is what confuses and catches some new pilots and instructors.  If you are turning base to final, to slip you hit "top" rudder, in other words, if you are turning left base to final, you would punch right rudder as the plane banks left.   That instantly puts you in a proper cross-controlled slip.  If you hit left rudder, you will put the aircraft into a skid and possibly spin the plane, deadly when low and slow on final.  I have been with fairly high-time pilots who attempted to practice a "slip" by using left rudder after straightening out after the left base to final turn.  Much easier to simply hit top rudder and start the slip with the turn if you already realize you are high.  If you are doing a shortened pattern (ie "Navy" pattern) in which your base to final is simply a long rounded turn from downwind, you will simply start hitting top rudder at the point in the turn at which you deem it necessary to lose more altitude without gaining airspeed.  Just don't try a right slip in a left turn, because that's skidding - bad.

A couple things:  You should slip into the wind.  Slipping with the wind will still lose altitude, but you won't be able to stay on your approach, because you'll be slipping with the wind rather than into it.  Also, you must watch your airspeed and not get too slow in a slip.  Keep the nose pointed down.  Adjust the slip to stay on the line and descent rate you want, and adjust your attitude to keep your airspeed where you want it.  Remember, your stall speed will be higher in a slip than in straight and level flight.  The previous poster is right, when you stall in a slip, you will fall right out of the sky like a rock unless you have enough altitude left to straighten out and recover.

One final note about slipping.  One poster noted that slipping is an essential skill, particularly with regard to engine-out landings.  Absolutely correct.  Not only that, but on a dead-stick landing, you wouldn't want to extend your flaps until you are absolutely sure you have made the landing site.  That means YOU SHOULD BE HIGH on approach!  Once you start adding flaps you can't go back.  If you slip to lose that extra altitude, you can simply reduce the inputs to adjust your descent and you still have the option of flaps to reduce airspeed if necessary on short final.  With full-flaps and a full-slip, you can really lose altitude fast without gaining forward speed.  Important if your landing site is a short field off-airport with a fence at the end!

I love slipping and enjoy practicing it.  I don't have the statistics to back it up, but I'll bet there are more accidents that might have been avoided or minimized with proper slipping technique than have ever been caused by slipping.

Just my thoughts.



Bill Greenwood
Warbirds of America MemberYoung Eagles Pilot or Volunteer
121
Posts
24
#9 Posted: 5/2/2011 12:21:12

I learned slips as part of my initial private in a Piper 140, not hard and not much worry about stalling. 

The tip about being able to use it in an emergency engine out landing is good. I also use it sometimes in gliders to more accurately make a landing spot. It can also be useful in some planes thatdon't have any flaps, like a Champ or Cub, etc.

But, im my opinion slips normally should be an aid to landing, not the standard way to save a bad approach. Much better to make the approach correct in the first place, or if not go around and rejoin the pattern to make another correct approach and landing.

Nowadays I often fly more high performance planes with flaps and higher wing loading, and where I don;t want to take a chance on stalling on short final, so I don't slip often.

I use the flaps or full flaps to descend and slow down. I don't see the point of slipping when there are still flaps avialable.

From a safety standpoint it seems better to lower full flaps than to slip with partial or no flaps. Full flaps will usually lower the stall speed quite a bit, it;s 10k, from 72 to about 62 in one plane I fly. Slipping a clean plane to descend and get slow seems to give up some of the margin that flaps give you. 

I do use slips in crosswind landings, either alone or at the end of  a crab approach if wind is stronger.



Frank Giger
Homebuilder or Craftsman
117
Posts
33
#10 Posted: 5/17/2011 02:22:53 Modified: 5/17/2011 02:27:50

Tony wrote "The poster quoted above indicated that a slip shouldn't be conducted in a turn."

Actually, what I meant is that one shouldn't stall while in a slip during a turn, particularly on turn from base to final!

From what I understand, the resulting paperwork can be tremendous!


wink

 

Bill wrote "But, im my opinion slips normally should be an aid to landing, not the standard way to save a bad approach. Much better to make the approach correct in the first place, or if not go around and rejoin the pattern to make another correct approach and landing."

Quoted for truth.  Slipping is for tweaking a pretty good but not perfect approach, going around is for a botched one, IMHO.

Pride goeth before the crunch.

 



John Nealon
Homebuilder or Craftsman
3
Posts
1
#11 Posted: 6/25/2011 20:36:55

Some planes slip better then others. C172 is probably the worst, it just doesn't do much in the slip.  Something like a Cub or my favorite, a Beaver are a joy to slip. Very effective, very stable and very useful on some landings.  It's also a good technique to to know in the event of an engine out landing.   It allows you to keep your altitude (potential energy) until the last moment and then bleed it off safely. 



Jerry Rosie
Young Eagles Pilot or Volunteer
482
Posts
101
#12 Posted: 6/26/2011 07:53:08

John, I notice that you are apparently new to this forum.  Just wanted to welcome you and to note that the advice you have given since joining has been valuable.  Welcome aboard.



Cheers, Jerry NC22375 out of 07N