Forward Slip Question

Discussion in 'Pilot Training' started by 1000RR, Jan 18, 2021.

  1. 1000RR

    1000RR Pre-Flight

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    The plane is a 1956 Skyhawk C172... Operators Manual states "Slips are prohibited in full flap approach because of a downward pitch encountered under certain combinations of airspeed and sideslip angle."

    This C172 has 4 positions on the flaps 10/20/30/40 degrees. For full flap landing, we use 30 (we don't go to 40). I had two notches of flaps in on final (20) and was high and decided to do a slip. Was a good landing but on short final (after the slip), my CFI looked down at the flaps as I pulled it to 30 degrees (once I was closer/over the trees) and said "you should not be doing slips with flaps". I responded that I read the POH and it stated not to do slips with full flaps (quoted above). He was curious and surprised so I sent a copy of the page from the POH to him, we'll follow back up later this week.

    What's the thoughts/consensus here?

    Thanks in advance.

    Fly safe!
     
  2. David Megginson

    David Megginson Cleared for Takeoff

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    I've never worn one myself, but I've heard that slips with flaps make things easier in the bathroom.
     
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  3. Greg Bockelman

    Greg Bockelman Touchdown! Greaser!

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    My thoughts are that your CFI is wrong.
     
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  4. Lindberg

    Lindberg En-Route

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    About reading and following the POH? I think you'll find the consensus is that it's a good thing.
     
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  5. Daleandee

    Daleandee Cleared for Takeoff

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  6. Cluemeister

    Cluemeister Line Up and Wait

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    Enough about airplanes, can't we get back to the reason for POA? By that I mean underwear styles?
     
  7. midwestpa24

    midwestpa24 En-Route

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    I'll throw this out there to stir the pot.

    If you are in a 172 at idle and 40 degrees of flaps aren't getting the job done, you are way too high! That thing will come down like an elevator without a slip!

    That being said forward slips are a great tool in the toolbox. I make sure to get all of my students comfortable using them, because it can be unnerving and unnatural at first to be crossed up like that at low altitude.
     
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  8. RyanB

    RyanB Administrator Management Council Member PoA Supporter

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    It’s just the concern that slips at full flaps (40deg) could blank out some airflow over the tail and create a buffet. The 172M that I flew was this way. Just don’t slip with more than 30deg and you’ll be okay.
     
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  9. FastEddieB

    FastEddieB Touchdown! Greaser!

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    Well, then by definition you’re not doing full flap landings.

    No doubt your instructor has his reasons. My mantra has always been “Maximum flaps as consistent with conditions”. Given the Law of Primacy, I’d want my students to get used to 40° flaps from the beginning - I’ve seen too many pilots shy away from full flap landings because they never got comfortable with them in the first place. Even on a 10,000’ runway, landing at the slowest possible speed and the least inertia is still a worthy goal. Flaps 40° won’t make a huge difference over flaps 30°, but every little bit helps.

    But my way is not the only way, so go with what your instructor says. But it’s OK to question his reasons - a good instructor won’t mind.
     
  10. TommyG

    TommyG Pattern Altitude

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    You can do a skip with full flaps.

    your CFI should be having you do slips with zero flaps, and full flaps. That’s how one learns, through experience.
     
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  11. 1000RR

    1000RR Pre-Flight

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    Thanks for all the responses, it's great to hear different thoughts on all the topics new pilots wonder about.

    @FastEddieB - agree, we're really not doing "full" flap landings. I'll revisit the reasoning for that, but for some reason I thought it had to do with too many folks early on (before Cessna went to only 3 positions on the flaps) were getting in trouble with go-arounds with 4 notches in (40 degrees). But I could be making that up - I'll ask him.

    @TommyG the first time we did a slip - I was kinda like WTH?! Nothing seemed right about it. Then we did some more and I felt a little better. My first time performing a slip on my own (soloing), no-flap slip was descending to pattern altitude on the 45 to the downwind and I didn't plan my descent properly (or at all). I decided to do a slip instead of a 360 as my visual would be better to the crosswind and downwind the whole time plus I wanted to give it a whirl. It was very cool - ended up really liking it. So with the slip I mentioned in the first post, felt pretty much the same, had the same result, and with the good landing - I was quite pleased.
     
  12. midwestpa24

    midwestpa24 En-Route

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    I can vouch for that, because I nearly did it. When I was a young lad student pilot, I was doing touch and goes in the rental 150 that had 40 degrees of flaps. In my haste to do the "go" portion of the touch and go, I either missed raising the flaps or the switch didn't lock in the up position. Either way I attempted to take off with 40 degrees of flaps in the little underpower 150. Got into ground effect just fine, but wouldn't climb or accelerate out of it. I was off the end of the runway pretty quickly, 5-10 feet in the air.

    Fortunately my small hometown airport had a couple of miles of flat cattle ground off the end of that runway or I wouldn't be here today. Took me a few seconds checking engine power and other things before I noticed those big barn door flaps in my peripheral vision. Once I started milking those back in, the plane started to climb.

    Boy were those cows surprised though!
     
  13. Larry in TN

    Larry in TN Pattern Altitude

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    The airplane's AFM is controlling. Don't take a limitation from one particular Cessna and apply it to all Cessnas. The limitation isn't even universal in all C-172s. Some only say "Avoid slips with...", not "prohibited", and others don't have any slip limitation at all.
     
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  14. FastEddieB

    FastEddieB Touchdown! Greaser!

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    That’s a decent rationale. But with proper training on a go around, flaps 40° to 20° immediately should be automatic. Though midwestpa24’s “forgetting” is certainly not unique.

    I began teaching in C-150’s, and taught full 40°flap landings as the norm. When my flight school got some just-released C-152’s, I missed that last 10°of flaps and felt Cessna had “dumbed down” the plane to some extent. But I’m sure they had their reasons.
     
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  15. George Mohr

    George Mohr Line Up and Wait

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    There's a set of cessna year ranges that have a "no slips with full flaps" restriction, apparently.
     
  16. David Megginson

    David Megginson Cleared for Takeoff

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    Seriously, follow your POH. Instructors overgeneralise sometimes. Many (most?) aircraft models have no restricti
    Point taken, but in a forced landing, you want to keep extra altitude (if possible) until you're 100% sure the field is made and there are no hidden power lines about to pop into view, then you want to set it down at the minimum speed and shortest distance possible, which may require both flaps and a slip. It's not a bad idea to have practiced that combination from time to time, if not prohibited by your POH.
     
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  17. David Megginson

    David Megginson Cleared for Takeoff

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    That's one time when I'm grateful for that giant Johnson bar sticking up beside my knee in the PA-28. It may be primative compared to electric flaps, but it's hard to ignore its position.
     
  18. midwestpa24

    midwestpa24 En-Route

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    You'd think so, but tunnel vision is amazing. I instruct in Pipers mainly now, and I've had student miss the obvious.
     
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  19. Huckster79

    Huckster79 Pattern Altitude

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    I think the 150/152 began so “dumbed down” to begin with, in terms of all the engineered stability and such that one more minor step of it isn’t really changing the big pic. I get what your saying by all means though.. :)

    love my Johnson bar flaps! Pop em when ya wanna, ease em off by feel of What she is ready to get rid of...
     
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  20. aftCG

    aftCG Pre-takeoff checklist

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    This is an important point.
    OP: Your CFI was regurgitating tribal knowledge related to C172 slips with flaps. No doubt I did the same thing when I first became an instructor. Same thing goes for "we don't use 40 degrees of flaps", because of course Cessna removed that last notch.

    After being away from instruction for a while I took a reinstatement ride in a plane with 40 degrees of flaps, and when the DPE asked me about use of flaps (and slips) I fell into his trap and was handed my head. And rightly so.

    What does the book say?
    The OP's 1956 aircraft has a different airfoil, different tail, different tail cone, different engine - and come to think of it may share not even a single part number with an M model (except possibly the knob on the ashtray). Spend a rainy day with your POH and locate the specific wording.

    As stated by others above, you weren't using full flaps so you are not prohibited from doing slips. Just because 30 is the new 40 doesn't mean the restriction applies.

    Also, just because a flight school hammers the issue of not using 40 degrees doesn't mean you can't/shouldn't or that it is even unsafe. Practice go arounds until you're comfortable with use of full flaps and how to retract them safely. If it were genuinely unsafe there would be an AD and that position would be blocked.
     
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  21. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    The later 172's (the ones which still had 40 degrees of flaps) stated "avoid full deflection slips with full flaps" but it's not an absolute prohibition. It says a control oscillation may occur. Coming into IAD in a 172N on a regular basis I did a lot of high and fast approaches by necessity. I slipped hard with full flaps a lot. Finally one day I got the oscillation, the yoke started pumping in and out on its own. I'm yelling "there it is" but my wife has no clue that there's anything wrong. Releasing rudder pressure stopped it immediately and it wasn't a controllability issue (as the manual stated).

    The 40 notch went away to better the go-around performance during certification testing to get a bit more gross weight.
     
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  22. texasclouds

    texasclouds Cleared for Takeoff

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    FYI, you will get a better descent rate keeping the plane slow with flaps vs pointing the nose down. Such as in the 172sp we normally approach final with 65 kts and flaps 20. If you are high, flaps 30, power idle, airspeed 60kts will result in an elevator-like descent. Now if you push the nose down in the same configuration and airspeed is like 80kts, you will find that the descent rate is much lower and you also end up too fast crossing the threshold. More airspeed means more lift.
     
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  23. Clip4

    Clip4 Final Approach

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    Slips prohibited with 40 degrees in you model, other 172s with only 30 degree flaps advise avoid slips with flaps extended. Practice slips zero flaps and be prepared to answer why you are doing this on the practical test. A lot of examiners do the slip to landing in a 172 as an equipment failure (flap motor).
     
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  24. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    If you allow the airspeed to build while slipping you're kind of defeating the purpose.
     
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  25. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    It's not tied to the loss of 40 degrees flaps. From the initial 172 up to the F it said prohibited. From the G to the K, it said avoid (because of pitch down). From the 1972 172L model which had the bigger dorsal fin, it said "should be avoided" due to "oscillations not affecting controllability." '

    The 40 degree flap didn't go away on the stock aircraft until after production restarted in 1980 with the P model. Some STCs artificially limited it in earlier models.
     
    Last edited: Jan 19, 2021
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  26. 1000RR

    1000RR Pre-Flight

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    @texasclouds, this was a little higher than that would allow. Generally speaking, I seem to tend to stay on the high side for my approach... but nothing crazy. If I'm landing with a PAPI, it is usually 3 white, 1 red to just having the 4 whites and no red (but barely). If I'm [a bit] higher than that, I always pull to idle and let the plane reacquire the glide slope I'm after (assuming I'm a bit high and a slight reduction in throttle won't do it). This particular instance was higher than that. So that's why I opted for the slip.
     
    Last edited: Jan 20, 2021
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  27. 1000RR

    1000RR Pre-Flight

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    When I was doing the slip, airspeed wasn't building... but the plane was dropping like a rock.
     
  28. 1000RR

    1000RR Pre-Flight

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    Thanks for the tip. My DPE for my practical happens to also be the owner of this particular plane - so he will know it quite well.
     
  29. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    That's why you do them.

    You want to have fun, fly in a Navion sometime with full (37 degree) flap out and the gear down . It comes down at a scary angle. Slipping doesn't add too much because the the side of the airplane doesn't add that much more drag (comparatively).
     
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  30. Lindberg

    Lindberg En-Route

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    If your DPE is a good one, he'll be more impressed if you know how to use the different tools available to you and you just do what needs to be done, rather than just doing what your CFI told you to do.

    The POH says flaps 40* for a short-field landing. So when doing a short field use flaps 40. Otherwise it is "as desired," so did what you desire, but be able to explain why. A good reason not to use full flaps for normal landings is strong gusts or crosswinds. Also, it's much easier to put in and take out a slip for small adjustments, especially with a long runway. Landing with full flaps "always" could really screw you in an emergency.
     
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  31. 1000RR

    1000RR Pre-Flight

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    For sure. I drill my CFI quite a bit for the practicality of most everything we do. I want to know the 'why' behind it, not be a monkey (...monkey see monkey do). He's been real good about all that.
     
  32. David Megginson

    David Megginson Cleared for Takeoff

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    True, but with the caveat that if your plane has only a single pitot and static port (as most trainers do), your ASI might not be entirely accurate in a slip. In my PA-28, I usually add 5 knots to my approach speed when I'm slipping, just to be safe (even though I have a pitot-static blade out on the wing, so I don't have to worry about the fuselage interfering), then shed it instantly by arresting the extra descent when I return to coordinated flight. The altitude loss still more than makes up for the extra airspeed.
     
  33. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Don't chase the airspeed indicator. Get into a steady-state pitch for the approach speed you want. Maintain that pitch while slipping. Airspeed doesn't control stalling, angle of attack does.
     
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  34. David Megginson

    David Megginson Cleared for Takeoff

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    Good advice not to chase the ASI. Your pitch attitude, as you mention, is primary, but in the early part of the approach (above, say, 200 ft AGL), you should still be glancing at the ASI from time to time to verify that your pitch is still giving you the intended result, and that the airspeed isn't slowly creeping down on you.

    Before anyone else jumps in, yes, it is entirely possible to land without looking at the ASI at all (I've landed 3 times with a failed ASI, relying just on control feel, and it was a 100% non-event). But it's still better to keep it in your scan (VFR or IFR) when it's working. We've all caught ourselves a bit slower than expected on approaches before.
     
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  35. aftCG

    aftCG Pre-takeoff checklist

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    As Ron and Dave say above, you shouldn't be looking at your airspeed indicator in a slip. Watch the nose of the plane and don't let it go up or down.

    And I don't slip "sometimes". My plane doesn't have flaps
     
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  36. David Megginson

    David Megginson Cleared for Takeoff

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    Absolutely — pitch is primary, and ASI is secondary. But you should still be glancing at your ASI briefly from time to time to verify that the pitch you're holding is still giving you the intended result, and that your airspeed isn't creeping down imperceptibly. Final approach is the place most pilots die, and they're not all stupid or inexperienced pilots.

    I keep the ASI in my VFR scan until I'm getting reasonably close to the ground, then transition entirely to looking outside the window and control feel for the roundout, flare, and touchdown.
     
  37. Pilawt

    Pilawt Final Approach

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    It’s not a myth, but it’s not a big deal, either.

    Here’s what Bill Thompson, former Manager of Flight Test & Aerodynamics at Cessna, had to say about the issue of slipping with full flaps in the 172 (Cessna — Wings for The World, by William D. Thompson, Maverick Press, 1991, p. 41):

    With the advent of the large slotted flaps in the C-170, C-180, and C-172 we encountered a nose down pitch in forward slips with the wing flaps deflected. In some cases it was severe enough to lift the pilot against his seat belt if he was slow in checking the motion. For this reason a caution note was placed in most of the owner’s manuals under “Landings” reading “Slips should be avoided with flap settings greater than 30 deg. due to a downward pitch encountered under certain combinations of airspeed, side-slip angle, and center of gravity loadings”. Since wing-low drift correction in crosswind landings is normally performed with a minimum flap setting (for better rudder control) this limitation did not apply to that maneuver. The cause of the pitching motion is the transition of a strong wing downwash over the tail in straight flight to a lessened downwash angle over part of the horizontal tail caused by the influence of a relative “upwash increment” from the upturned aileron in slipping flight. Although not stated in the owner’s manuals, we privately encouraged flight instructors to explore these effects at high altitude, and to pass on the information to their students. This phenomenon was elusive and sometimes hard to duplicate, but it was thought that a pilot should be aware of its existence and know how to counteract it if it occurs close to the ground.
    The larger dorsal fin introduced on the 1972 C-172L apparently eliminated the issue.

    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.

    Flap slip.jpg


    There is a separate, unrelated phenomenon that Thompson described in newer models in full-flap slips: “a mild pitch ‘pumping’ motion resulting from flap outboard-end vortex impingement on the horizontal tail at some combinations of side-slip angle, power, and airspeed.” This is entirely benign and common with other high-wing airplanes. My Sport Cub did the same thing.

    So although the 172L’s larger dorsal solved the pitch-down issue, they kept the cautionary note in the POH because of the latter phenomenon.

    Unfortunately Cessna contributed to the “end of the world” fear of slips with flaps, by not explaining the phenomenon in the manuals; and in fact, many earlier C-172 manuals expressly said that slips with full flap were prohibited. I rummaged through my collection of old Cessna owners manuals:

    1958 C-172: “prohibited”
    1959 C-175: “prohibited”
    1966 C-172F: “prohibited”
    1972 C-172L (first year of the big dorsal): “should be avoided”​

    The manuals for these older models have been revised since then, and in fact the TCDS and new POHs no longer carry the prohibition against slips with flaps, or even discourage the practice. The POH for the current C-172S indicates that it's no biggie:

    Steep slips with flap settings greater than 20° can cause a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings.

    But a lot of us old-timers read the scary language in the old manuals back then and that's what we remember. But now, when tower clears me for a short approach with a close-in base leg, I don't hesitate to slip with full 40° flap in my 172N all the way down to short final.

    Ask your CFI if he has ever tried slips with full flaps in a 172, even at altitude, just to see what happens. I bet I know his answer.
     
    Last edited: Jan 19, 2021
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  38. kgruber

    kgruber En-Route

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    You needed to time your flare to happen when the "porpoise" was on the upswing!
     
  39. Dan Thomas

    Dan Thomas Final Approach

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    Those who think a 172 descends like an elevator at idle and 40 flaps have never slipped a Champ or Citabria at max rudder. Or a Jodel D11. Those airplanes will make a 172 at 40 flap feel like a sailplane.
     
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  40. Dana

    Dana Pattern Altitude

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    As I understood it, the reason was to meet the certification requirements... the 152 couldn't make the required climb performance with full flaps. I'm not sure the 150 could either, but the rules may have changed.