C172 Rate of Descent / Sideslipping
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C172 Rate of Descent / Sideslipping
I'm currently in the latter stages of my PPL training and have been having some thoughts about the rate of descent of a Cessna 172SP in varying configurations. My main question is this - Say I'm on the final leg of a PFL, with 20 degrees of flaps, and I realise I'm too high. Will my rate of descent be higher by keeping 20 degrees of flaps and sideslipping, or should I go to 30 degrees of flaps?
My POH states sideslipping is not permitted at full (30 degree) flaps. I believe this is due to the airflow creating a lack of elevator authority. Is this only really an issue when close to the ground, in the round-out and/or flare? I have no intention of trying sideslipping with full flaps but it would be nice to know if it would be an option if I find myself too high in a real engine failure situation.
Thanks in advance,
Stu
My POH states sideslipping is not permitted at full (30 degree) flaps. I believe this is due to the airflow creating a lack of elevator authority. Is this only really an issue when close to the ground, in the round-out and/or flare? I have no intention of trying sideslipping with full flaps but it would be nice to know if it would be an option if I find myself too high in a real engine failure situation.
Thanks in advance,
Stu
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That's a good one! I think (and this is my opinion) leave 20 degrees of flap and sideslip, as you have the option to come out of the slip and slow the descent again. Once the flaps are down then it is it! You're absolutely committed (I know you're committed anyway - but you take away options)
Once you have corrected the height you can then resume normal service, so to speak... and drop 30 for touchdown.
Once you have corrected the height you can then resume normal service, so to speak... and drop 30 for touchdown.
Stability problems can happen at any height, so the only things special about the approach in terms of not flying a full flap sideslip, are (a) it's the only place you'e likely to, and (b) you have much less height to deal with any resultant loss of control.
Also, if you've not practiced something already, during a genuine emergency isn't really the time or place to experiment!
My advice if you're over-high is either to pick a different field, or to perform an S-turn or two whilst keeping your landing field in sight. (Or turn base later, which would be my general preference, but that to some extent depends upon how you fly your PFLs.)
G
Also, if you've not practiced something already, during a genuine emergency isn't really the time or place to experiment!
My advice if you're over-high is either to pick a different field, or to perform an S-turn or two whilst keeping your landing field in sight. (Or turn base later, which would be my general preference, but that to some extent depends upon how you fly your PFLs.)
G
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descent
Another way is to increase your ROD by increasing the IAS. Putting the nose down. It is pretty effective.
There is one very important note to make though. You will have to move your aim point back to a place where you can start the round out to bleed off the speed. No stability issues either.
Before anyone pipes up. I AM NOT talking about 150 knots in the descent. An increase of 10-15 knots can have give you that advantage.
Also. It is good fun and a good way of building judgment on things like glide approaches.
If you have never done it before, try it with an instructor as the round out/flare may happen a little quicker. Nose wheel and all that.......
There is one very important note to make though. You will have to move your aim point back to a place where you can start the round out to bleed off the speed. No stability issues either.
Before anyone pipes up. I AM NOT talking about 150 knots in the descent. An increase of 10-15 knots can have give you that advantage.
Also. It is good fun and a good way of building judgment on things like glide approaches.
If you have never done it before, try it with an instructor as the round out/flare may happen a little quicker. Nose wheel and all that.......
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Either slip it or lower the nose and do a few S-turns, or both. You'd be amazed how quickly you can lose altitude without overshooting. Also, if you dump the drag flap and point it at the floor you may gain airspeed, but you'll lose the height very quickly and all it takes is a good tug on the yoke to bleed the excess speed off again.
Last edited by Shunter; 2nd Feb 2009 at 19:00. Reason: wine
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A reduction in airspeed will also help. The normal glide speed quoted for an aircraft is actually the best glide (ie maximum range) speed. Slow down a bit to Vmp (which conveniently equates to the minimum rate of sink speed) and you will stay in the air longer but cover less ground. Of course, this speed is closer to Vs but in reality for a 172 the difference between the speeds concerned is rather academic.
Having slipped a 172SP with full flap, I would also consider this course of action.
PS. S-turns are very effective but you are bringing in the variable of increased wing loading and therefore increased stall speed. There is a real temptation to 'pull' a bit when aiming for a field. Fancy doing a quick bit of maths at a couple of hundred feet?
Having slipped a 172SP with full flap, I would also consider this course of action.
PS. S-turns are very effective but you are bringing in the variable of increased wing loading and therefore increased stall speed. There is a real temptation to 'pull' a bit when aiming for a field. Fancy doing a quick bit of maths at a couple of hundred feet?
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Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails. The best way to loose hight is to slow it down, select full flap then put the nose down while monitoring the flap limit speed. It comes down very nicely and quickly. Sometime a bit of 'faith' is required as you bring the nose up to loose the speed and the runway dissapears from view, but as soon as the flaps are down, push the nose over and you'll be fine.
At the risk of being jumped on from a great height by some, there is no reason why having selected full flap (30deg) you cannot raise them again to reduce overall drag if you are getting too low. I do not mean in the last 50ft above the round, but drag flap is exactly that, more drag than lift so the 'overall' result will be less drag albeit with a slight sink initially. Try it at a safe height and see.
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Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails.
On a PFL to a field, your aim point should be 1/2 to 1/3 into the field. Once you have that absolutely made in a C-172 adding flap past 20 degrees brings you in steeper and gives you more field to land in.
My last C-172 checkout included a number of surprise simulated engine failures from downwind to a short runway. I like to do it short and steep and would end up at 40 flap and pointing the thing at the end of the runway.
The roundout of course has to be done not too low and not too high -- and you do need to carry enough airspeed to have enough elevator control to raise the nose when you need to.
My last C-172 checkout included a number of surprise simulated engine failures from downwind to a short runway. I like to do it short and steep and would end up at 40 flap and pointing the thing at the end of the runway.
The roundout of course has to be done not too low and not too high -- and you do need to carry enough airspeed to have enough elevator control to raise the nose when you need to.
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IS, Good question and kudos for asking this while you are still learning for your PPL. May I suggest the following things:
1. Stay within the POH. If the POH says don't sideslip with full flaps, then don't. Some test pilot somewhere found a reason for that restriction and that should be good enough for you. And note that not all 172s have this restriction: it really depends on the exact model.
2. Go up with an instructor and simply try out the different techniques. For every trick, write down the vertical speed and the horizontal speed you achieved and do a few calculations afterwards to see which was most effective. For horizontal speed, don't trust the ASI, since it may show serious errors in a sideslip (both due to the pitot tube not being aligned with the airflow, and the static port not being perpendicular to the airflow). Instead, use GPS groundspeed, corrected for wind, or fly directly towards or away from a DME and use that readout, corrected for wind.
Personally I have sideslipped a PA-28, with full flaps, at just below flap limiting speed (top of the white arc) and it's extremely effective - I guess we achieved a glide angle of almost 45 degrees.
The other thing you'll want to find out is if you can place the rudder at the stops, and still have enough aileron authority left to make reasonable turns in both directions.
Oh, and you've got to learn to push the nose down instinctively if you do this sort of thing. Sideslipping just above stall speed (1.3Vs0 or approach speed comes to mind) is a recipe for a perfect spin entry.
1. Stay within the POH. If the POH says don't sideslip with full flaps, then don't. Some test pilot somewhere found a reason for that restriction and that should be good enough for you. And note that not all 172s have this restriction: it really depends on the exact model.
2. Go up with an instructor and simply try out the different techniques. For every trick, write down the vertical speed and the horizontal speed you achieved and do a few calculations afterwards to see which was most effective. For horizontal speed, don't trust the ASI, since it may show serious errors in a sideslip (both due to the pitot tube not being aligned with the airflow, and the static port not being perpendicular to the airflow). Instead, use GPS groundspeed, corrected for wind, or fly directly towards or away from a DME and use that readout, corrected for wind.
Personally I have sideslipped a PA-28, with full flaps, at just below flap limiting speed (top of the white arc) and it's extremely effective - I guess we achieved a glide angle of almost 45 degrees.
The other thing you'll want to find out is if you can place the rudder at the stops, and still have enough aileron authority left to make reasonable turns in both directions.
Oh, and you've got to learn to push the nose down instinctively if you do this sort of thing. Sideslipping just above stall speed (1.3Vs0 or approach speed comes to mind) is a recipe for a perfect spin entry.
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Personally I'd stick the extra flap in and possibly S turn. Not because I'm afraid of slipping; in a warrior I regularly sideslip - it's effective, and I'm comfortable with it, but the warrior doesn't have a limitation enshrined in the POH.
I have been lead to believe that the restriction noted in the 172 POH is because in the earlier days, a few suffered a full tail stall when so doing, and promptly nosed over completely - i.e. the flow over the tail can be sufficiently disturbed to cause a tail stall, not just a loss of effectiveness. I don't have a verifiable source for that though.
However, the 172 example I fly goes to 40degree flap, whereas yours seems to have 30 (later model). Both contain the same verbage about full flap slips not being advisable/permitted:
If I reduce to 30deg (less than full flap), can I slip where you can't? I doubt that..
Is the warning not necessary for yours? Maybe.
Where's the limit at which you can slip? Dunno...
So that's why I'd not slip - it's quite possibly corporate a$$ covering, but I'm not wagering my life on that - I'll avoid slips with flap extended in the 172 period. That said, every crosswind landing ends with a cross controlled, wing down arrival.. which looks remarkably like a sideslip
P.S. S turning shouldn't require much calculation - you're already well above Vs, just let the nose drop a bit in the turn, allow it to gain 5 kts or so, and don't go nuts with 60degree banks..
I have been lead to believe that the restriction noted in the 172 POH is because in the earlier days, a few suffered a full tail stall when so doing, and promptly nosed over completely - i.e. the flow over the tail can be sufficiently disturbed to cause a tail stall, not just a loss of effectiveness. I don't have a verifiable source for that though.
However, the 172 example I fly goes to 40degree flap, whereas yours seems to have 30 (later model). Both contain the same verbage about full flap slips not being advisable/permitted:
If I reduce to 30deg (less than full flap), can I slip where you can't? I doubt that..
Is the warning not necessary for yours? Maybe.
Where's the limit at which you can slip? Dunno...
So that's why I'd not slip - it's quite possibly corporate a$$ covering, but I'm not wagering my life on that - I'll avoid slips with flap extended in the 172 period. That said, every crosswind landing ends with a cross controlled, wing down arrival.. which looks remarkably like a sideslip
P.S. S turning shouldn't require much calculation - you're already well above Vs, just let the nose drop a bit in the turn, allow it to gain 5 kts or so, and don't go nuts with 60degree banks..
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Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails.
Irish Stu, Though I will suggest caution when slipping fully flapped 172's, to my knowledge (the older ones for sure), doing this is something to "Avoid", it is not prohibited. This stems simply from non ideal handling in pitch at certain speeds. Similarly, a door off on a 172 can induce this at about 65kts, and it just feels a bit spongey in the elevators. It certainly won't hurt you.
During flight test, all of this era of planes had to meet this requirement:
3.118 Directional and lateral stability
(a) Three-control airplanes.
(1) The static directional stability, as shown by the tendency to recover from a skid with rudder free, shall be positive for all flap positions and symmetrical power conditions for all speeds from 1.2 Vs1 up to the maximum permissible speed.
(2) The static lateral stability, as shown by the tendency to raise the low wing in a side-slip, for all flap positions and symmetrical power conditions, shall:
(i) Be positive at the maximum permissible speed.
(ii) Not be negative at a speed equal to 1.2 Vs1.
(3) In straight steady sideslips (unaccelerated forward slips) the aileron and rudder control movements and forces shall increase steadily, but not necessarily in constant proportion, as the angle of sideslip is increased; the rate of increase of the movements and forces shall lie between satisfactory limits up to sideslip angles considered appropriate to the operation of the type. At greater angles, up to that at which the full rudder control is employed or a rudder pedal force of 150 pounds is obtained, the rudder pedal forces shall not reverse and an increased rudder deflection shall produce increased angles of sideslip. Sufficient bank shall accompany sideslipping to indicate adequately any departure from a steady unyawed flight.
(4) Any short period oscillation occurring between stalling speed and maximum permissible speed shall be heavily damped with the primary controls (i) free and (ii) in a fixed position.
The 172 does meet the requirement, but is marginal.
I shall quote William Thompson, former Cessna test pilot, who writes in "Cessna, Wings for the World"...
...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 degrees due to a downward pitch encountered under certain combinations of airspeed, sideslip angle, and center of gravity loadings".
I very highly recommend that any Cessna pilot read that book, it is facinating!
Use caution, discuss with your instructor, and learn. Do not do "prohibited" things in planes. "Avoid" or "Use caution" are just that. Don't worry about flap tracks, just fly well maintained planes, and do walk around inspections as if you're about to hang in the air from the plane.
Be careful, but go and learn, that's what you're up there for.
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Go up with an instructor and simply try out the different techniques
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Quote:
Do you have a source for this claim? I am genuinely interested, as this is not noted in my POH (Hawk XP).
Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails.
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Many instructors coming out of sausage factories these days would not know how to sideslip. They were never taught it.
I was a PPL student with one such an instructor. At that time I regarded him highly, being the lowly student that I was.
Anyway, one day we ended up way too high on approach (my fault). Instructor said we were not going to make it and I told him I agreed, but would he mind if I tried anyway?
So I threw the PA-28 into a full-flaps, full sideslip at almost flap limiting speed and achieved the 45 degree slope I talked about earlier. We made the runway easily, even touching down in the usual spot and vacated halfway down.
Instructor didn't say anything about it afterwards.
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Stu
Thanks for asking this question, I'd been wondering about this myself having covered PFLs in the last two lessons. I've not been specifically taught about sideslipping but of course realise that it's an option to steepen the approach. The replies on this thread have been really useful and I'll be sure to discuss with my instructor.
Also I was not aware of the PoH advice on slipping with full flap.
PS I suspect you're training in the same place as I am
Gav.
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Thanks for asking this question, I'd been wondering about this myself having covered PFLs in the last two lessons. I've not been specifically taught about sideslipping but of course realise that it's an option to steepen the approach. The replies on this thread have been really useful and I'll be sure to discuss with my instructor.
Also I was not aware of the PoH advice on slipping with full flap.
PS I suspect you're training in the same place as I am
Gav.
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Don`t forget that 172s usually show oscillations in IAS/ALT/VSI in sideslips due to the position of the static vent and pressure head,You should get a steadier reading if you select`Alt static`,somewhere under the panel ? in both left/right sideslips.(may not apply to all models,but try it and see.
As has been suggested,`S`turns( never turn away from the runway) are probably easiest, keeping the aircraft balanced throughout,or use max flaps to descend to the correct g/path,and then reduce flap setting( at which point the instructor will fail the electrics!),With an `S` turn you can always cut it short if it looks tight.
As has been suggested,`S`turns( never turn away from the runway) are probably easiest, keeping the aircraft balanced throughout,or use max flaps to descend to the correct g/path,and then reduce flap setting( at which point the instructor will fail the electrics!),With an `S` turn you can always cut it short if it looks tight.
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Thanks for everyone's interesting replies and rest assured that I won't go trying anything silly on my next solo cross country.
Madgav - we do indeed train in the same place, we've spoke in the past! I'm only down on saturdays now so maybe we don't cross paths as much but I'll give you a shout if I see you.
Madgav - we do indeed train in the same place, we've spoke in the past! I'm only down on saturdays now so maybe we don't cross paths as much but I'll give you a shout if I see you.
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Also interesting - it appears I've been incorrectly informed regarding this..
PilotDAR - is this a repeatable scenario? I'd rather experience the tendancy with some space and get my eye in as it were, than have it just 'happen' one day on approach..
PilotDAR - is this a repeatable scenario? I'd rather experience the tendancy with some space and get my eye in as it were, than have it just 'happen' one day on approach..