Risk of spinning from a sideslip?

[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]

Originally Posted by Ptkay

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]

Side slip dangers

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 ) 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:

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 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 (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

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]

Originally Posted by Mike Cross

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!

Originally Posted by Mike Cross

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.

Originally Posted by miserlou

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

See above..

Originally Posted by miserlou

You're getting closer to the truth, beginning to understand.

If you mean understanding that it's futile to try to argue.. yes I shall shut up now - I should probably have buttoned it after my first couple of posts

[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.