Risk of spinning from a sideslip?

[oscarisapc]

What is the risk of converting a badly judged sideslip into a spin? I have been practising sideslipping under instruction (so far from a safe height) but no-one seems to be able to answer this question to my satisfaction. In a sideslip, you have an airplane out of balanced flight, full rudder, a raised nose attitude and unreliable airspeed indication. Prime risk factors for a spin I would have thought and at circuit height, which is where most folk use a sideslip for real, unrecoverable. Nonetheless all the pundits who fly "real" airplanes advise using a slideslip from time to time, and it always comes out in advice on forced landings. Advice from anyone with experience would be welcome. Thanks.

[Pegpilot]

Hi Oscar

Having done loads of spinning this year on a gliding instructor's course, my twopennorth is as follows. A classic spin entry is an over-ruddered turn at low speed, whereas in the sideslip situation you describe, you have exactl the opposite - in other words, loads of out-of-turn rudder, so the configuration is inherently spin-resistant. The biggest problem is recovery from the sideslip just before the flare, assuming that you are sideslipping as a last resort to burn off energy and avoid an overshoot. As you recover, your ASI reading is, as you say, unreliable, and it is pretty easy to recover at the wrong attitude at a fairly critical moment and potentially stall in. This is one reason cited for sideslipping no longer being on the BGA pre-solo syllabus (whereas it was when I was learning 15 years ago). The other reason invoked by many for sideslipping coming off the syllabus is that modern gliders are so streamlined that offering up the fuselage side to the airflow does little to reduce drag, and therefore renders the sideslip pretty pointless. Doesn't mean to say that I don't still sideslip from time to time when an embarrassing overshoot a hundred yards past the launch point is on the cards !

[Mark1234]

As is often said, perhaps a good idea to discuss your concerns with the instructor, however, In my opinion the risk is minimal, however, ther usual caveat applies - don't go and risk your skin based on what I write!

Firstly your nose angle shouldn't be any higher than it would be for that phase of flight without the sideslip. You may however require a more rearward stick position to achieve that nose angle (elevator blanking)

As per my understanding of the aerodynamics, a spin doesn't just require out of balance flight, it requires one of the wings to be travelling faster, i.e. an increasing yaw angle, not a static, large yaw angle - what you need is one wing to stall before the other, hence the speed difference (and this is limited to straight wings, not swept).

Theory aside, in practice it's harder than people would have you believe to get most aeroplanes to spin (my experience is in a/c certified for spinning, so limited to those with reasonable spin characteristics). Most will give you an attention grabbing wing drop at the stall, but it requires persistent effort (or mishandling) to create a spin - if you relax the back pressure after the wing drop you'll just fly out of it. Further, most types I've flown don't want to spin once stalled - you have to be pushing yaw in at the right time (just around the stall) to get the spin.

I also conducted an experiment in a decathlon to test exactly this question - I went up to 5000ft, crossed it all up into the biggest sideslip I could create, and stalled it. It dropped clean and straight ahead. Again the caveat - I've been trained, signed off, and have spun a lot - don't tempt fate if you're not familiar spinning.

All that said, even a straight stall near the ground is bad news. Realistically though, your approach speed normally gives a pretty decent margin over the stall. Cues like stick position and pitch angle should complete the picture. If the stick keeps coming back, that's bad - if it stays forward of a certain point, the a/c cannot stall.

Frankly it's a wonderful tool to have in your box of tricks, but be comfortable with it, before you need it :)

Edit to add:
Pegpilot: The a/c will spin either way quite happily - in fact the usual reaction to stall in a turn is that the a/c flicks away from the turn (I don't know why, sorry!); it's just that the over-ruddered final turn is the classic way to set up a spin in a glider - especially if you're stretching the glide a bit, are low, slow, and those long wings seem awful close to the ground.. (Yes, also a glider pilot..)

When I did sideslipping in a glider it was hammered into me to memorise the pitch angle before sideslipping, and maintain the nose at, or slightly lower than that - speed would then be bang on when you straightened up. Complicated by use of the noise lever in a powered a/c, but the basic principle still applies. (Power+Pitch=Performance).

[maxred]

I do not like side slipping. I know plenty of pilots do it, but I would rather get the circuit/approach/final correct which in turn leads to a good landing (generally). I have witnessed some horror stories where pilots side slipped from great hight on final, in fact one just two weeks ago where I highly experienced aerobatic pliot, my friend who was joining me for lunch, saved a full ground loop by the skin of his teeth - quote 'should have gone around from that final':uhoh:

[BackPacker]

I know where Mark1234 is from and I fully agree with him. Just one more point:

In a sideslip, you have an airplane out of balanced flight, full rudder, a raised nose attitude and unreliable airspeed indication.

I would certainly hope that you do NOT have a raised nose attitude, at least not raised above the normal angle for your speed, power setting and phase of flight.

The typical reason for sideslipping is to wash off excess energy (altitude and/or speed) in the final approach phase, where flaps and/or airbrakes are not available or not sufficient. So you need to find another way to increase drag, and that's by using the full fuselage.

Drag is exponentially proportional to speed, so the most effective sideslip is one that is done at relatively high speed. So a sideslip just above the stall isn't nearly as effective as a sideslip performed at 1.3xVs or even higher. That should keep you well away from the stall and thus from the spin.

This is also the reason that you will want to LOWER the nose a bit in a sideslip. You've just greatly increased the drag and if you maintain the same attitude, you'll maintain the same glide slope, but with reducing speed. Not what you set out to achieve, normally.

Caveat here of course: the side loads on the fin & rudder in a sideslip are huge. You do not want to be much above your typical approach speed or risk breaking the plane. But I have to admit I do not know how to find/calculate the exact limit, or what normal certification limits are. I would not assume automatically that if you stay below Va, you can't break off the rudder. The aerobatic airplane I fly has a specific limit for flick rolls, (92 knots) and for sideslipping I would use the same limit. (Va is 127 knots in that aircraft, for comparison.)

[Pegpilot]

When I said "offering up the fuselage side of a modern glider does little to redice drag" I meant, of course, "INCREASE drag"

:O

[Mark1234]

Maxred - to an extent a fair point, get it right and never be shy to go around. However, getting the approach right is rather separate from sideslipping. The sideslip is a useful tool, and in some cases a necessary part of a correct approach (e.g. pitts!), not just something to rescue a bad approach. There should be no reason for sideslipping to lead to a groundloop.

It's a bit like flying upside down, do it enough and you might even like it :E

[FREDAcheck]

I'm not an instructor so don't take my views without a pinch of salt (and confirmation from a reliable source), but I've always understood a sideslip is safe, provided you don't let the speed fall. Keep well above normal stall speed, which generally means keeping the nose down, not up.

And sideslip means rudder opposed to ailerons: you're pressing the rudder on the high wing side, against the bank.

Top rudder: good
Bottom rudder: bad

[Mark1234]

Neither am I an instructor.. but I've probably spun more than most instructors ;) In fairness I should disclaim; I'm just a regular PPL, though I've had the fortune to fly with some very good pilots as instructors, done a lot of 'beyond ppl' training, and come through a more stick and rudder oriented route than most. I'm also (in case you didn't guess) in fairly regular aerobatic practice. [end disclaimer].

Fancy meeting you here BackPacker :} My understanding is that Va is the most limiting speed for any *single* full control deflection; JAR23 would seem to confirm this (pasted at the end) That's not a license to abuse the airframe, but Flicks/Snaps are a 'special case' as you're being pretty brutal with the rudder AND the elevator together. That said a sideslip is also a multi control deflection, though hopefully not as aggressive! I'd not be too paranoid about overstressing the a/c sideslipping.


JAR 23.441 Manoeuvring loads
(a) At speeds up to VA the vertical surfaces must be designed to withstand the following conditions. In computing the loads, the yawing velocity may be assumed to be zero:

(1) With the aeroplane in unaccelerated flight at zero yaw, it is assumed that the rudder control is suddenly displaced to the maximum deflection, as limited by the control stops or by limit pilot forces.

(2) With the rudder deflected as specified in sub-paragraph (1) of this paragraph, it is assumed that the aeroplane yaws to the overswing side-slip angle. In lieu of a rational analysis, an overswing angle equal to 1·5 times the static sideslip angle of sub-paragraph (3) of this paragraph may be assumed.

(3) A yaw angle of 15° with the rudder control maintained in the neutral position (except as limited by pilot strength).

(b) In addition for commuter category aeroplanes, the following manoeuvre must be considered at speeds from VA up to VD/MD. In computing the tail loads, the yawing velocity may be assumed to be zero; with the aeroplane yawed to the static sideslip angle corresponding to the maximum rudder deflection, as limited by the control surface stops or the maximum available booster effort, or the maximum pilot rudder force as specified by JAR 23.397 (b) at VA and 2/3 of the maximum pilot force specified by JAR 23.397(b) from VC/MC to VD/MD, with linear variations between VA and VC/MC, it is assumed that the rudder control is suddenly returned to neutral.

(c) The yaw angles specified in sub-paragraph (a) (3) of this paragraph may be reduced if the yaw angle chosen for a particular speed cannot be exceeded in -

(1) Steady slip conditions;

(2) Uncoordinated rolls from steep banks; or

(3) Sudden failure of the critical engine with delayed corrective action.

[Zulu Alpha]

Sideslipping is a very useful proceedure to have in your back pocket for any emergency landing. It allows you to carry a bit of speed/height and to lose it when you are sure you will make the field.

In many taildraggers it allows a good look forward. Practice it up high first as it does feel strange at first. Also make sure you can side slip both ways (left and right) as its best to have the nose into wind when you have a crosswind.

The nose will need to be lowered to keep the speed up.

Once you have mastered it you will wonder why you worried about it.

ZA

[RatherBeFlying]

ZA has it right in getting rid of excess energy that you have rightfully kept in your pocket.

In an emergency landing your aim point is well down the field. Once the field is made, it's nice to have a longer landing run or pick out the best touchdown spot.

In a normal landing, I like to have something extra in case the engine coughs or stops.

In earlier gliders, the spoilers were often not that powerful; so sideslips or higher speed to increase drag are often called for.

In the newer gliders, the handbook often cautions against full spoilers on touchdown; so, you have to reduce to half or so just before the flare -- or be just a few inches up as when you're picking the smoothest part of a field.

You need to come out of the sideslip a comfortable distance above the flare. As you become more attuned to the a/c, this altitude can get lower.

[mikehallam]

Most of the above appears to be experiences and general opinion.
So here's my penn'orth !

a) Flying into a strip(s) for well over 30 years in 65 to 100 h.p light a/c, side slip is a wonderful tool to help arrive at a safer speed & height to clear the obstacles, trees, fences etc. on short finals should the engine play up.
And THEN get rid of the all excess before straightening up with a hand ready on the throttle just in case you over cook it & need cushioning burst of full power.

b) I too was never sure about the safe low speed end to use. Right or wrong I do full rudder side slips at ias no slower than recommended finals speed. i.e ias stall plus minimum 20 %.

Seems to work though you may know better.

Mike Hallam.

[Miserlou]

The pitch attitude is flatter in a side slip. If you keep the same attitude in the slip the speed will be higher when you straighten out again. If you have done this to the groubd you will then have loads of speed, in ground effect, and float for miles.

Take a CD and draw an aeroplane planform on it. Then put it at a slight angle and turn it about the hole in the middle. There you go, a flatter attitude. If you turn it 90 degrees it will be level.

Nothing more satisfying than the transition from slip to flare and three pointer.

[BackPacker]

Surely you sometimes need a bit of back pressure while you're actually in the slip?

This greatly depends on the airframe. Most light aircraft are constructed so that the tailplane generates a downforce. This helps to keep the aircraft speed stable. In a sideslip, part of the elevator may (note: may) be blanked by the rudder/fin. This reduces the "normal" downforce generated by the tailplane, leading to a pitch-down. To counter this you may have to apply backpressure.

Your best bet to determine how much backpressure is required, is to look out the window to the spot where you want to go. See if the aspect of the surrounding terrain changes to judge your approach angle. And use clues like wind noise, stiffening of controls, buffeting and so forth to determine if the speed is ok. You can't trust the ASI as an absolute indication of airspeed in a sideslip, but generally if the ASI shows an increase during the slip, you are indeed gaining speed and vice versa.

[Mark1234]

[edited as I'm starting to repeat my previous posts]

Miserlou, I'm afraid I disagree - you are right about the geometry, however, the angles of slip we are considering are fairly small (<30deg), minimising the geometric effect. Generally (and I'm sure there will be exceptions), the predominant effect is increased drag as the relative 'wind' swings and the effective 'frontal area' increases - form drag is increased. If everything else stays the same, you will slow down. If you haul the nose *up* at that point you're in a world of pain.

That's precisely WHY a slip is useful - it allows a steeper descent without increasing speed (and in certain types allows you to see where you're going, but I digress) Bit like flaps really - which is why it tends to be more common practice with a/c that don't have flaps..

[maxred]

Mark - you are correct, I left the thread after that post and just back. I should actually have inserted - 'on finals'. I am an aerobatic pilot, on taildraggers, and am fully experienced on spin/slip etc. I just do not like using it on finals, a personnal point I might add, which I thought was the subject on the original post.
I also do not think inexperienced/new PPL pilots should attempt this until full spin/slip awareness has been conducted. Should have lept incipient spin in the PPL course, however that would lead us to another thread.:ok:

[Cows getting bigger]

Backpacker, did you really mean to say?

You've just greatly increased the drag and if you maintain the same attitude, you'll maintain the same glide slope, but with reducing speed.

[Pilot DAR]

At the risk of not honouring the indepth posts here, this is much ado about nothing. I agree with Zulu and Ratherbe. With few execptions, there is not reason to not practice sideslips, and they are no more dangerous than co-ordinated flying. If you're at risk of spinning out of a sideslip, you probably weren't that safe straight and level, and I'd worry about that first!

I completley agree with a sideslip as an element of a forced approach for glide path control, and touchdown precision. Unlike extended flaps, you can undo and redo a sideslip in an instant if you choose to.

To entertain myself, I'll sideslip right through flare and touchdown, onto the ice of a frozen lake, in my C150. When there is no wheel friction, touching down going somewhat sideways is really not a problem! The object is to touch down on lnly one wheel, go along a short distance that way, and take off again, without ever touching the other wheel down. Do not attempt on surfaces with any friction!

When you have the finer points of sideslipping mastered, you'll find that the co-ordination you command can be applied in other useful situations. One of these is getting one wheel/ski/float off the surface below takeoff speed, and before the other, to reduce surface drag. Doing that is a very slight sideslip with lots of power, and requires good control. Also useful in strong crosswind takeoffs and landings.

Most Cessna 172's are not so great to slip with flaps out (placarded), the tail does get a bit blanked, as mentioned earlier, on approach between 65 and 55 knots. Similar to the way it handles with a door off. Not a terrifying ride, but it can get your attention.

Other important reason to sideslip: In very cold air, it will allow you to carry power, and reduce the shock cooling, or over cooling the engine during long, or poorly set up approaches.

Yes there are those perfect pilots who always set up their approaches perfectly, and never need to slip or go around - and there are the rest of us!

Pilot DAR

[austerwobbler]

I am not a high hours pilot but most of those are on tail wheel "Auster's and Cub's" and i side slip, not to save a bad approach but to land off a steeper approach angle or if you have to clear high trees or wires on the thresh hold.
I have always maintained a higher than normal approach speed in a side slip, because your pitto head, in straight flight, is pointing directly into the dynamic pressure going straight in , if you are side slipping your pitto head is at an angle to the dynamic pressure so is it actually reading true air speed ? ? ?. i am not an expert but it seems to work for me.

regards Austerwobbler " "and the wobble doesnt come from side sliping either" :ok:

[trex450]

While re-validating in a 152 a few years ago I got told by the instructor not to sideslip but to fly properly. I found this bizarre as it only showed his lack of comfort in light a/c. He was a jet jockey during the week. I have several thousand landings mainly on Islanders, and have slipped it just about any way possible. When operating in strong winds into short airstrips it is important to be able to slip well, at low level, to cope with rapid wind changes if you want to be able to get into the runway you are looking at. Indeed in the Islander I frequently used assymetric power to assist with the slip when countering crosswinds. Something that no-one seems to have mentioned here is that the wing down method for off setting a crosswind is itself a sideslip. My advice is simply don't do it if you are not comfortable but a sideslip is not just a method of recovering a bad approach, with practice you can fly the approach, touch down, ground roll and take off in a slip.

[BackPacker]

While re-validating in a 152 a few years ago I got told by the instructor not to sideslip but to fly properly. I found this bizarre as it only showed his lack of comfort in light a/c. He was a jet jockey during the week.

If he's a jet jockey, maybe thinking you were on the commercial track as well, then I can understand his comment. Side slipping is not very comfortable for the passengers so tends to be avoided in commercial operations.

(Cue the youtube videos of sideslipping large airliners...:ok:)

Backpacker, did you really mean to say?

Yes, but I might have phrased it a bit awkward. What I meant to say is if you're flying along a certain approach path, stable, with constant power and so forth, and all of a sudden you increase the drag by entering a sideslip, then something's gotta change. You either need to add power or steepen the approach angle, or you're going to lose speed.

Of course, normally the situation is the reverse. You're too high on approach and you want to steepen your descent angle without gaining extra speed. So you enter a sideslip (or forward slip, basically the same thing) and pitch down.

[oscarisapc]

Cheers folks - thanks for all the very helpful comments.

[Whopity]

Should have lept incipient spin in the PPL course,It is still in the JAA PPL syllabus!

[trex450]

If he's a jet jockey, maybe thinking you were on the commercial track as well, then I can understand his comment. Side slipping is not very comfortable for the passengers so tends to be avoided in commercial operations.

Backpacker, I can fully understand not slipping a 737 but the guy was instructing in 152's, he should have been comfortable with all methods of flying the aircraft, how his bum feels does not interest me. I never fly the whole approach in a sideslip, that would be untidy, but over my career passengers tend to be far more nervous about looking out of the window at the runway while approaching with a 30 degree offset than a few seconds of wing down just before and during the flare and touchdown.

[deltahotel]

If you feel uncomfortable side slipping don't do it! If you're going to do it, keep the speed up. The point is to increase drag and thus steepen the approach, so:

1. Have full flap
2. Apply rudder to generate drag
3. Use aileron to fly the ac where you want to (will be cross controlled)
4. Keep the speed up - best effect is just below the flap limit speed (or above in a real emergency and if needed)
5. Come out of the side slip smoothly and in good time - suggest practising at height to get familiar with time/height required

Good luck!

DH

[mikehallam]

Hang on a mo !

The whole object of side slipping is to steepen the approach (or descent) without gaining speed. The rudder & ailerons hold the a/c in a draggy attitude to obtain just that.

Lastly as above mentioned, on common types, viz. the Cessna 172, full flap is claimed to mask the rudder a bit and worse if side slipping, so here together in full may not be a suitable mix.

All based purely on my ancient 150/172 training way back and another 40 years on light PFA machines.

[ShyTorque]

One school of thought:

On some low wing aircraft, in a sideslip, especially where flap is used, the elevator on the "higher" side of the fuselage can become partly blanked.

The disturbed airflow from the flap can also mask stall buffet.

The "higher" wing may stall first due to fuselage blanking (pro-spin). You already have pro-spin (top) rudder in a sideslip and we all know that a stalling aircraft with flap is more likely to rapidly drop a wing ....

So a good idea not to let the airspeed get too low in this configuration.

But as always, read the manuals.

[BabyBear]

1. Have full flap

Careful there Deltahotel

Lastly as above mentioned, on common types, viz. the Cessna 172, full flap is claimed to mask the rudder a bit and worse if side slipping, so here together in full may not be a suitable mix.

Good point Mike, I was reading a 172 POH recently and slipping with full flap was forbidden.

[Ptkay]

What about strong side wind landings?

I use on my Socata Rallye always side slip to control the straight
approach instead of crab.

Rallye almost never spins and has very effective controls
(both rudder and elevator) so I never got concerned,
but maybe I am doing it wrong?

[Pilot DAR]

I was reading a 172 POH recently and slipping with full flap was forbidden

I have not seen the word "forbidden" used in aircraft placarding. I have seen "prohibited", though my recollection is that "prohibited" is not used on the slipping placard in some 172's (the very earliest ones did not have this warning).

I recall the placarding on the later 172's reading "Avoid sideslips with flaps extended". "Avoid" is not forbidding, it's a caution. It is my opinion that the aircraft would not pass the design requirements of CAR 3.118 if it were not safely controllable in a sideslip with full flaps. The wording of 3.118 is a little fuzzy with respect to the demonstration of slips with flaps extended, but I am certain that Cessna would not put the aircraft out with a handling defect of any consequence which is so close to normal maneuvering. When I have test flown 172's with mods, a full flap sideslip has been a part of my flight test, and I've never had a problem, I just approach with a little more caution.

Though it may seem trivial, the wording of placards and flight manuals is carefully chosen, and generally follows a convention. It is important that pilots interpret the terms with the meaning intended, to operate the aircraft as intended.

Pilot DAR

[deltahotel]

Apologies for the full flap ref - from my experience on low wing (Bulldog/Grob), for which it is fully relevant. The ultimate aim is to steepen the approach; the faster you go the more drag and the steeper the overall approach path will be. Yes there will be some kinetic energy to lose at the bottom, but the net effect is still a steeper approach.

A half way house if you don't fancy side slipping is to take full flap and fly just below the flap limit speed (or in a real forced landing and you have to make that field, a bit above). All the above applies - more drag, steeper overall approach path even with the excess kinetic energy taken into account.

DH

[BabyBear]

Enlightening, Pilot DAR, indeed I have to acknowledge that I cannot recall the exact wording and that I committed to memory not to sideslip with full flap, rather than the exact detail.

Unfortunately the aircraft in question, a 1979 172N, has gone to engineering. I will check it out on it's return.

I have actually been in the aircraft when it has been side slipped with full flap, I wasn't flying of course, and it did not seem too happy.

I will certainly pay more attention to the wording in future.

My point to Deltahotel remains, that one should not automatically select full flap prior to side slipping, without being aware of POH recs.

[flybymike]

I recall the placarding on the later 172's reading "Avoid sideslips with flaps extended".

A stupid placard since ordinarily, one would only wish to side slip after full flap allocation had already proved to be inadequate.

[BabyBear]

Note sure whether you are making a statement (which I can understand), flybymike, or telling me I am wrong?

[flybymike]

I have actually been in the aircraft when it has been side slipped with full flap, I wasn't flying of course, and it did not seem too happy.


My experience is that the 172 can be slipped perfectly "happily" with full flap, but that if an unbalanced stall is deliberately (or accidentally) induced, it will flick without any warning whatsoever. Such a stall and stick position however, is fairly difficult to achieve on any conventional approach and satisfactory speed management should present no undue danger.

[flybymike]

Note sure whether you are making a statement (which I can understand), flybymike, or telling me I am wrong?

Making a statement!

[BabyBear]

My experience is that the 172 can be slipped perfectly "happily" with full flap

So in your view; have I got it wrong, has Cessna got it wrong, or are you saying that despite what the POH states a 172 is happy being side slipped with full flap? If it's Cessna that have it wrong, why?

[flybymike]

I would not presume to argue with Cessna. I merely recount my experience!;)

[BackPacker]

Gents, before you start making definitive statements on this, followed by a slugfest: I seem to recall this same discussion a while ago and at the end they found out that the POH statement wrt. side slipping with full flaps in a 172 actually differs from model to model. But they only found that out after bruising a number of egos.

[Mark1234]

PilotDAR is (as usual) correct - it does say 'avoid' in any 172 POH I've looked at.

Slipping with full flap *is* useful, however, slips aren't only there for 'I've got full flap and I'm too high'

Cessna did arguably 'kinda' get it wrong - From what I've been lead to believe it was found at some point way after the a/c was in service that under certain circumstances there could be an undesireable pitch down due (I believe) to some flow interaction with the elevator - hence the 'avoid' phraseology. However it's (again apparently) not uncontrollable, just a little suprising.

I can confirm I never had a problem with sideslipping one before I noticed that bit in the POH (and no placard present)

Personally, given the era I rather suspect it's a bit of corporate covering of the rear. That said, I do avoid it where I can - but not to the extent of falling off the side of the runway in a crosswind :O

[Pilot DAR]

I shall quote from "Cessna, Wings for the World", one of my favourite books, written by former Cessna test pilot William Thompson,

page 41:

With the advent of 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 seatbelt if he was slow in checking the motion. For this reason a caution note was added 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"......Although not stated in the owner's manuals, we privately encouraged flight instructors to explore these effects at high altitude, and to pass the information to their students.......When the larger dorsal fin was adopted on the 1972 C-172L, this sideslip pitch phenomenon was eliminated, but the cautionary placard was retained.


I take what this book tells me about Cessnas to be "gospel", and refer to it frequently when planning test flights.

My 1958 C182 manual says: "...but slips are permissible with wing flaps extended if necessary"

My 1966 C185 maunal make no mention (though the airframe's aerodynamics are very similar to the 1958 182 (other than dorsal fin)

Pilot DAR

[Mark1234]

Good point on the various POH's...

As we're fixating on being high on approach, I might also venture a personal opinion about all this speeding up and slowing down stuff... you're asking for trouble! It's not something 'normal', and it will be hard to judge the slowdown - the theory works, but in practice you're most likely just increasing your chances of making a royal mess of things.

Additionally, you're most likely close(ish) in by the time you can tell sufficiently accurately, unlikely there's space or time to accel/decel.

Other than sideslipping, S turning may be an option, and if *grossly* high, an orbit (caution - don't turn your back on an outlanding field), or even, heaven forbid... a go around might be the best option (obviously not if the engine's died).

[flybymike]

PDAR, would be interested to learn whether any of your old Cessna aircraft bibles have any words of wisdom on sideslipping the C206? ( I fly one) There is nothing on the subject in my own POH

[Pilot DAR]

I agree with Mark, and disagree with those who suggest high speed slipping, or just flying with the flaps out (I'm thinking Cessna as I write this). As the drag increases geometrically with the speed, so does the strain on the flap tracks. Yeah, I know they are certified up to those speeds, I still "avoid" doing that to the plane. I have had a flap track break off in flight on a Cessna 180 (touch and go actually, but I only realized it in flight, when the flaps jammed at 20). I would really like to not ever repeat that! - So I don't abuse Cessna flaps. That said, I used to slip the C180 floatplane all the time to get into tight spots. You get lots of drag slipping with floats!

I like a stabilized approcah speed, which would be reasonable for the conditions no matter what drag configuration you chose. Then add the drag you want. I would not be purposefully speeding up or slowing down after that. If you've messed the approach up so badly that a normal speed full flap full sideslip won't get you to where you should be, there is just no excuse for not going around! If it's during an actual forced approach - you should have been practicing more!

Pilot DAR

[Piper.Classique]

Flap failures are not unknown, either. I have had a C150 flap motor fail with flaps 40 in a go-around, and a 152 flap failure at zero flaps. Guess which one is easier to deal with? ;)
I often land my super cub on a practice engine failure no flaps, just sideslip or s turn, and I can get it down with a fair degree of precision. I aim to enter and leave the sideslip at the same airspeed, which means a fairly nose high attitude during the slip. I wouldn't trust the ASI too much, depends where the static ports are, as well as the pitot.
A slipping turn will give an even greater rate of descent, and a fishtail yet more. Spinning off a sideslip is most unlikely, just think about the airflow over the wing and tail, and how much you are masking the upper wing. Buffetting in a sideslip is sometimes noticeable, and the fabric on a ragwing can flap around somewhat on the fuseage, as well as the disturbed airflow over the ailerons and tailplane.

[Miserlou]

Mark1234.
You say I am incorrect but introduce the phrase 'if everything else stays the same'.
The point is that everything else does not stay the same and that is the whole point of slipping. The flightpath becomes steeper and that maintains the speed with the increased drag.
The CD example demonstrates this very well, especially if you also introduce a little bank.
Take a look at your Pittses and Extras and Sukhois on approach. Generally descending in a three point attitude and slight side slip. The straightening up is pretty much all that is required in terms of flare if you've judged it right.

Edited to include another example as there seem to be aerobatic pilots present.
Starting a slow roll. As the aircraft begins to bank rudder is introduced to LIFT the nose to maintain flightpath(altitude and heading).
If the altitude is to be maintained, the nose must be higher than in level flight.
QED.

[Stephen Furner]

I checked the Cessna POH for my 1967 F172H and it is quite clear that side slipping must not be carried out with full flap. It simply states in the “LANDING” section “Slips are prohibited in full flap approaches because of a downward pitch encountered under certain combinations of airspeed and sideslip angle”. Full flap for this particular model is 40 degrees.

While I don’t slip with flaps myself - since with 40 degrees of flap available I’ve never had problem that’s needed it and like many others here in the UK was taught not to slip 172s with any flap lowered - I assume that this means at lower flap angles slipping is allowed. However, I have seen later models that have a maximum flap setting of 30 degrees placarded that they must not be slipped with the flaps down.

[gpn01]

Rallye almost never spins and has very effective controls
(both rudder and elevator) so I never got concerned, but maybe I am doing it wrong?

This may be my naivity on the use of sideslips in powered aircraft (and the Rallye in particular) but the comments of "Almost never spins" and "has very effective controls" when describing the elevator, strikes me as a worrying combination!

[horizon flyer]

All Cessna C177 Cardinals are cleared for side slipping with full flaps as they are limited to 30. As advised keep the speed up and well over 1000fpm down can be acheived.

Only DANGER is DONT open up the engine before removing the slip, or a 177 will roll on its back so fast it will kill you. Dont know if any other Cessnas do this.

This has happened in a sight see situation with crossed controls, flying round a point on the ground, trying to keep it in sight.

I have never needed to slip a 172, 40 of flap, closed throtle and the nose well down for speed, required for the flare with that much hanging in the breeze, has always worked.

[Mark1234]

Stephen Furner - that's very interesting.. we seem to have proved the point that different POH's say different things. I also agree that the 40degree flaps are supremely effective.

Miserlou, I'm thinking quite hard and trying to see your angle here - I would suggest that the increase in drag does not it's self *cause* the aircraft to descend (how would it?). Drag acts parallel to the airstream, so can only slow the aeroplane. You certainly don't want the aircraft to slow at this point.. Now we may also reduce lift slightly by banking, but it would take very little pitch up to restore that, and we're back to gaining nothing, other than slowing down. You're back to being able to (well, HAVING to) fly a steeper path at the same speed due to the *effect* of increased drag.

I honestly believe you're swapping cause and effect. Actually, I think we're both oversimplifying, it's undoubtedly a mix of factors and heinously complicated aerodynamics. But I don't believe you're nose high in a sideslip where you wouldn't be at the same speed without one.

As for your examples, it just so happens that I've spent a chunk of today circuit bashing in..... a Pitts of all things (Sorry :O) A bad example in many ways as the purpose of the Pitts slip is not to increase the descent rate; it does an excellent impression of a homesick brick unaided - but to allow the poor fool holding the stick to see where the runway went..

Now, I'm only learning the type, however:

- When you roll off the final turn and into a slip, you have 2 choices. Either you move the loudness lever significantly forward, OR you let the nose drop (quite a lot). It's a draggy beast at the best of times, so that's more marked than most aircraft. If you do neither, the ASI unwinds dramatically and the chap in the back gets quite excitable.

- We slip down, but most definitely NOT in a 3 point attitude. That would be rather risky (i.e. too slow). We slip nose down with quite a bit of power, then straighten at about 10ft , closing the throttle and transition to the 3 point attitude (a flare in english) What you describe is a short field landing technique. That I have also done, and wouldn't go near in a Pitts. It's back on the drag curve stuff, and uses the fan at the front to help the wing, ground effect to help the flare, and a relatively firm arrival. It's also leaving pretty small margins all round.

Lastly, to the slow roll example; top rudder is only introduced from about 45-60 degrees, which would be a rather extreme slideslip for most pilots. But to be honest, I'm not sure what you're proving - you've traded a pair of nice winglike things for the side of a fuselage - unsuprisingly it's not very good at generating lift, hence the AOA has to be rather high. And it creates a heap of drag, slows down and falls from the sky. It would be a mistake to interpolate anything approaching a straight line between the 0 and 90 degree orientations.

[AeroAdz]

The way I see it is this:

Without getting too into detail, stalls and spins only really occur when there is significant angle of attack on one or both wings. The only way to get up to this angle of attack is to pull back on the yoke/stick significantly. In fact I once read somewhere that angle of attack is directly linked to stick position, which makes sense. I've pulled aerobatic aircraft around loops and steep turns playing with stick position, and you can easily feel that the pre-stall buffet occurs at exactly the same spot.

On finals, you're not going to be getting anywhere near this amount of angle of attack unless you're really trying to reel in your speed and ignore any stall warnings. Trim malfunctions or other control surface failures could also do it too I guess.

[BackPacker]

In fact I once read somewhere that angle of attack is directly linked to stick position, which makes sense.

AA, in general, you're right. Although trim position has a very slight impact too due to the aerodynamic effect of the trim tab itself. And CofG of course.

But in a sideslip part of the elevator may be blanked by the rudder/fin. So the elevator will lose some of its effectiveness and that means that stall is achieved at a different stick position. If that's more forward or backwards will depend on the airframe and the way the aircraft is balanced.

The other issue is that a sideslip generates the same aerodynamic warnings (different wind noise, buffet) that normally warn you about an imminent stall. And even the stall warner itself might be affected by the changed airflow around the fuselage. For good or for bad, and that may also depend on which way you're slipping.

[Mike Cross]

Risk of spinning from a sideslip?
On a point of pedantry a spin is not what happens when a wing drops, it results from a failure to initiate the proper recovery action. The oft-quoted "spin off the final turn" never gets the chance to develop into a spin as terra firma intervenes.

There's a certain amount of generalisation going on here when behaviour is surely airframe related. While a flying brick may need the nose to be lowered you should not extrapolate that behaviour to all airframes. Entering a slip in the Luscombe automatically results in an increase in airspeed and a little back pressure is needed to prevent you exiting with excess speed just when you don't want it, in the flare.

The Luscombe has no flaps so a sideslip is the normal method of steepening the approach. In an aircraft with flaps you have the option of dumping more flap and in a glider you use the airbrakes. The need to steepen the approach is absolutely not an indication of a poor approach. Wind gradient is an example where you fly into slower moving air that is going to result in a close view of the far hedge if you don't introduce some more drag. Flying the aircraft in such a way that have to go to the extremes of what is available to you to adjust the descent is not particularly good airmanship, whether you're desperately trying to descend or whether you have to give it a fistful of power to avoid landing in the undershoot.

Mark said "I would suggest that the increase in drag does not it's self *cause* the aircraft to descend (how would it?)."
I suggest you get a Cessna to a safe height, set it into a glide below the flap limiting speed and introduce some drag by dumping full flap. If you can keep the rate of descent the same as it was with the flaps up for more than 30 seconds I'll buy you a pint.

In any case the slip does a lot more than introduce drag. You get spanwise flow that reduces lift. The flow obstruction caused by the fuselage disrupts the airflow near the root on the forward wing and blocks the airflow near the root of the rearward wing. It's a similar effect to using airbrakes to kill the lift over a section of the wing of a glider.

The limiting factor on a slip is usually rudder authority, you run out of it before you run out of aileron authority, which if you think about it is exactly what you want. The ailerons remain effective because they're out near the ends of the wing, away from the disrupted airflow.

I was returning to the field during a BFR in the US a few years ago when the instructor gave me a simulated engine faiilure in a 172. "Engine failure, what are you going to do now?" he said. When I reached for the flap switch he added "you also have an electrical failure". His timing was perfect, start of downwind on a long runway with no chance of getting to the far end. No easy option, except...... full rudder and a very long sideslip on to a cross runway at the end of which my leg was aching. Slipping a 172 is no problem.

[Miserlou]

Mark1234,
You're getting closer to the truth, beginning to understand.
It's a long time since I have flown a Pitts but have spent much time in Stampe and Decathlon. My technique for slow rolling is bringing the nose up with the rudder and elevator from the start. The nose should describe a circle with the straight and level start point at the bottom. The maximum rudder input is reached passing the first 90 degree point, then decreasing through the inverted and increasing to slightly less than the maximum at the 270 degree position.
Try practicing this by slowing the roll down to take 10 seconds or more. Things will become clearer.

Back to the slip. The point of slipping for this discussion is to increase the descent angle rather than view out.

The increased descent angle is the desired effect, the cause being the increased drag.
If you read back to your last post you will see you have written as much.
I learned this some 25 years ago in a glider when I maintained the same pitch angle in the slip as I had in the straight approach, recovering at low altitude. It was a long walk back to the start point because of the excess speed we had accumulated!

Get you model aeroplane out.

[hugh flung_dung]

oscarisapc (http://www.pprune.org/members/214651-oscarisapc):

The answer to your initial question is that a sideslip is a perfectly safe and very useful manoeuvre provided that you keep a safe speed, recover sufficiently early to effect a safe landing, and your aircraft does not have any type-specific restrictions.

Some considerations:

If you stall in a stable sideslip the upper wing will invariably stall first because it's part blanketed and therefore operating at a higher angle of attack, also the spanwise flow leads to thicker boundary layer at the tip (although I can't comment on how significant this latter effect is). The rate of roll can be significant and your instinctive untrained reaction might be to use aileron:eek: rather than moving the stick forward and preventing further yaw - many years ago a student did this to me whilst quite low on the approach and the recovery required my use of skills which my experience should have avoided:O (to paraphrase the old saying)
The indicated airspeed in a sideslip may be in error because of yaw across the pitot, or having a static port only on one side of the fuselage
The elevator effectiveness will be reduced due to blanketing so the nose may tend to drop
In theory an overly-aggressive entry into a sideslip could stall the fin and you *really* don't want to do this at low level. The only time I've seen it was in a glider, during a deliberate demo at height.Converting a turn onto final into a slipping turn and then smoothly into a slipped approach and then smoothly into a wing-down landing is a thing of beauty!

Advice to the wise: if you want to play-around with deliberate mishandling it's best to find an aeros instructor, an aerobatic aircraft and some height.

HFD

[Piper.Classique]

In theory an overly-aggressive entry into a sideslip could stall the fin and you *really* don't want to do this at low level. The only time I've seen it was in a glider, during a deliberate demo at height.
Out of interest, which type? Inquiring minds.......Just wondering if it was a Puchacz?

[RatherBeFlying]

Was once in the back seat of a C-172 while a student pilot friend was on approach. Being high he simply pointed the nose to the ground:eek:

Later he claimed that he remained below the flap limit speed,but somehow I never got around to flying with him again to verify that claim.

I carry some extra knots for the flare if power off in a C-172 with 40 flaps as the knots bleed off quite quickly with the nose up.

[hugh flung_dung]

Piper.Classique:
I thought it was a K21 but it may have been a Puchacz or a Grob Acro. It was about 25 years ago (with DerekP) but I recall being rather surprised, first by the odd feeling as the fin stalled and then at the rate at which the nose pitched down as we went sideways.
Definitely not something to do deliberately or accidentally when near the ground, and clearly not when above Va!

It's about 15 years since I last flew a glider but I remember the Puchacz for its enthusiasm for spinning.

HFD

[Mark1234]

Mark said "I would suggest that the increase in drag does not it's self *cause* the aircraft to descend (how would it?)."
I suggest you get a Cessna to a safe height, set it into a glide below the flap limiting speed and introduce some drag by dumping full flap. If you can keep the rate of descent the same as it was with the flaps up for more than 30 seconds I'll buy you a pint.
Ok, I realised 2 things: 1) I'm being a pedant 2) I'm not explaining myself adequately.

ref 2) Picture the old Lift/Weight/Drag/Thrust diagram. Increase Drag, then there is only one logical conclusion - the aircraft will try to slow. One must either slow down, or counteract that by increasing thrust Thrust, or some increase in the component of weight acting opposite Drag. So, either descend, open the throttle, or just hold the nose up until it stalls.

However, I confess that I'm being pedantic. It's pretty much the same debate as to whether the elevator or throttle controls speed. I shall leave it!

There's a certain amount of generalisation going on here when behaviour is surely airframe related. While a flying brick may need the nose to be lowered you should not extrapolate that behaviour to all airframes. Entering a slip in the Luscombe automatically results in an increase in airspeed and a little back pressure is needed to prevent you exiting with excess speed just when you don't want it, in the flare.

Very true. Most a/c I've slipped require some back stick even with a slightly lower nose attitude. However I strongly suspect that while the stick comes back, you are not approaching 'nose high' in the luscombe either, which is the point I was arguing against.

The increased descent angle is the desired effect, the cause being the increased drag.
See above..

You're getting closer to the truth, beginning to understand.
If you mean understanding that it's futile to try to argue.. yes :ok: I shall shut up now - I should probably have buttoned it after my first couple of posts :oh:

[Miserlou]

Hi Mark.
Sorry if you feel I was being patronizing; that was certainly not the intention.

Discussing nose attitudes in a slip is a delicate matter especially when there is no aeroplane to jump into together to go see or without a pint between us and a beermat to use as a model;-)

The point which we have not defined is 'relative to what'.
But if you would like to picture an aircraft in level flight with 30 to 60 degrees of bank and a constant speed, I am sure you would agree that the power and the nose would be higher than before.
Without the addition of power a descent rate would be required to maintain airspeed. In this condition the nose may be lower than in straight and level flight but the datum (flightpath) has changed.

Best we leave it at that.

[Piper.Classique]

I thought it was a K21 but it may have been a Puchacz or a Grob Acro. It was about 25 years ago (with DerekP)

Ah, one of the world's truly great instructors. I suspect it was the Puch or the Grob. A Janus will do this trick, too, and a super falke. I was at Lasham around that time, and got to learn some interesting things from him, including just how far a Janus will glide from 700 feet. I wish I had a Puchacz now, a truly brilliant training aircraft. But I am in France, and the french won't buy anything made abroad if they can help it. :*

[RatherBeFlying]

Mention has been made that spinning from a sideslip is just about impossible, but the Puch may be a special case. Certainly it will happily spin out of a co-ordinated turn.

Derek Piggott in, I believe, Going Solo asserts that a glider will not spin or stall from a 45 degree turn because the airflow against the tailplane comes at an angle that does not allow it to generate sufficient downforce to stall the wing.

It takes some muscle, but the Puch does spin from a 45 degree banked turn with two aboard. Remember though that the Puch is aerobatic; so, is designed with more elevator authority than normal gliders.

[Mark1234]

Without the addition of power a descent rate would be required to maintain airspeed. In this condition the nose may be lower than in straight and level flight but the datum (flightpath) has changed.

Precisely :)

As I half suspected, we're not really disagreeing - as much a matter of understanding each other as the subject, which is why arguing on 'tinternet tends to become cyclic & futile :ugh:

On a slightly related note - if you're flying the decathlon anywhere around the south east (uk) would you mind dropping me an PM..

[Piper.Classique]

It takes some muscle, but the Puch does spin from a 45 degree banked turn with two aboard.

Wellllll.....It depends. On the CG position for a start. Well forward, I don't think it will. Near the aft limit, yes, it will, but without the lines on the canopy you can't really be sure that you have a genuine 45 degree turn. Also you do need to approach the stall rather more briskly than for a normal entry. As you say, it takes some muscle.

As to whether it will spin from a sideslip all I can say is that I never managed it.

[hugh flung_dung]

Ratherbeflying:
I can personally confirm that the Puchacz, Bulldog, T67A, T67M, Cap10b/c and a variety of others will all spin from a sideslip with only mild abuse. As I said before, it's a perfectly safe, simple and useful manoeuvre if flown correctly.

Piper.classique:
We may know each other. I was an instructor at Lasham on Friday evenings and Saturdays from roughly 1984 to 1994; I also flew the tugs on Wednesday evenings.
Last autumn I spent some time at Gap-Tallard and now see why Brits move to France:* ... an aviation paradise!

HFD

[Piper.Classique]

Piper.classique:
We may know each other. I was an instructor at Lasham on Friday evenings and Saturdays from roughly 1984 to 1994; I also flew the tugs on Wednesday evenings.

Yes, I think we do. I was instructing at Lasham at that time, tugging as well. I was working for Threes Counties in 1990 then moved to France.

[djpil]

I've learnt not to make very many general comments about spinning. In Australia, spin training approval is type specific and our (my) briefings stress that we're flying a Decathlon (usually for me these days, I have 6 examples to play with) and characteristics of other types may (will, in some types I mention) be different. Examples of some types may even behave differently eg different wing rigging in a Decathlon or a rebuilt leading edge on a Cessna 150.
We routinely sideslip the Decathlons on final so I show instructor trainees what happens if grossly mishandled. Plenty of warning, shaking and rattling usually. Sometimes a slow roll to level. Only one indicated a departure into the spin but fairly slowly and relaxing on the controls tamed it.
On the other hand they generally go very neatly into a spin from a skidded turn. Also from a mishandled roll off the top - easier to spin upright but they will go inverted if the student is really hamfisted.

PS Mark1234, I generally agree with what you say.