Stall speed and ISA in a slip

[Mak]

I got into the habit of slipping on final to loose altitude. It's fun for a start and I think it gives me more options should the engine quit: I can get the a/c straight and should make it to the threshold with plenty of room to spare. In some a/c it dramatically improves forward visibility too.

It is all very easy with PA28s and C152s which can loose speed very quickly, so I'd slip at a relatively high speed (5~10kts above normal approach speed) and loose that speed as soon as I straighten up.

I just got checked out in a 182 and found it a bit more complicated, mostly due, I think to the greater speed and weight. I don't think I'll be slipping a C182 as often as I do a PA28 but if I need to I'm curious to find out how people judge slipping speed relative to the stall, specially considering the ASI will be misreading anyway.

Thanks.
Mak

[Genghis the Engineer]

Two answers to that.

One is keep your pitch attitude the same or slightly more nose-down, and you should stay well away from the stall.

The second is that if your ASI is misreading in a sideslip, the static is blocked on one side.

G

[javelin]

I would check the POH carefully, some of the Cessna's are not supposed to be slipped with flap, not that you mention that, but I thought I would add it.

IMHO, the slip is an excellent maneouvre in aeroplanes without flaps. When you have them, they are a hint that they should be used instead of slipping to control and lose height on final.

[LowNSlow]

Genghis surely the ASI is less accurate (admittedly not by a huge margin) during a steep side slip due to the airflow not going directly down the pitot tube? For a side slip to be effective, by definition, there will be a large amount of yaw taking place.

I tend to be nose down and feeling the aeroplane's reactions rather than looking at the ASI at this point anyway. The ASI is virtually ignored until I'm approaching the hedge. Maybe this is because I invariably come in high and sideslip to the threshold.

javelin most aeroplanes that you would take into strips that require sideslipping as a skill will perform the action without a problem. The exception being certain models of the C172 which will tuck their noses down when sideslipped with full (40deg) of flap applied. Allegedly. Possibly. Maybe.

[Flyin'Dutch']

LnS,

You are correct. For the slip to be meaningful your slipangle has to be considerate and therefore the pitot will not lined up with the airflow.

In a good slip in a glider/cub you can slip so much that you can see an airspeed of less than zero.

FD

[Feck]

It's all about PEC error.

Return the rudder to central fairly quickly from a sideslip, and watch what the ASI does. If it rises by 5 kt, then your ASI is under-reading in the slip by roughly that amount. Next time you want to sideslip at 50kt for example, fly at 50kt, enter the sideslip and hold an indicated 45 kt. And vice versa if it over-reads.

That's what they teach you at ETPS, anyway. But like Genghis says, if you keep the pitch attitude the same and don't let the IAS decrease once you're slipping you shouldn't go far wrong.

[Flyin'Dutch']

When you fly the slip you do not adjust the speed by reference to the ASI!

You fly the slip by flying the attitude.

If you chase the ASI you will invariably go too fast which makes it difficult to stay in the correct attitude (in pitch, yaw and roll) and you will be way too fast when you come out the slip.

This will then mean you are in a high energy situation close to the ground and a crap landing will follow.

To master the slip the pre-requisite is that you have an experienced instructor or tutor who knows what they are talking about explain the theory and show you how it is done.

Once you have the basics under control you go up and practice slips, left and right, high up along line features until the cows come home and can do them without even having to think about what the inputs are which are required.

You then go and practice with your mentor the slip before landing and progress from there onwards.

FD

[Genghis the Engineer]

Technically correct that the pitot may give errors in sideslip, but in my experience misreading ASIs in sideslip are almost always down to static.

Easy way to check, keeping pitch attitude the same, sideslip both ways - if you get the same ASI reading each way (but different to the same pitch attitude without sideslip) any errors are probably pitot, if it's different, it's probably asymmetric static.

G

[nosehair]

Cross-Control Stalls, Babes, that's how you do it. All this back-and-forth about the ASI has it's validity, but, in general, the ASI readings can't be counted on. You should go to altitude and get into a slip and bring the nose up just enough to stall, or get close enough to feel your airplane's control response, and notice your ASI reading. This will show you the nose-down attitude and speed to hold to stay at a safe margin while slipping on final. You should pay special attention to the sound and feel of the airplane controls as you increase pitch and decrease speed to the stall or impending stall.

[Genghis the Engineer]

Please don't listen to the naughty boy children, Nosehair's advice is pro-spin, and likely to end in tears.

G

[stillin1]

Nosehair
Presuming you are aware of just how risky your advice was ~ t'would be a good idea in future you add a few health-warnings.
Like - tis the great way to depart the ac and possibly spin, you betta have a lot of height and take an experienced instructor along for the ride!
I ain't advocating the nanny state. I like people who can really use thier ac. But finding the limits by trial and ERROR is usually IMHO a less than satis way of doing it.
There are a majority of PPLs out there that should not even consider your idea without an experienced instructor aboard

Sits back and awaits incoming! :cool:

[c-bert]

In IMC, if you are unsure of your height AGL, just decend slowly until you feel you wheels brushing the tree tops - set you altimeter and away you go.

Anymore top tips?

[Genghis the Engineer]

Doesn't work c-bert, you need to accurately know the height of the treetops. Much better to do it over the sea, until you see splashes from the maingear up over the side window, set zero, and you know you've got QNH.

G

[c-bert]

Fair point, but with your method you need to find the sea first...with mine you can use any tree, bush or shrub. I'd like to think of the two methods could be used for QFE and QNH respectively. :p

[djpil]

Yes, take nosehair's advice and try approaching (up high) a stall in a slip. You will find that a slip is fairly resistant to spins. However, there are some associated traps for the unwary hence an instructor is a good idea. Not just any instructor - choose some-one familiar with the concepts in Rich Stowell's book - Emergency Maneuver Training.

[Final 3 Greens]

Hey C-Bert

Beware, lest the ground rises up and smites you :} :}

[Classic]

Am I the only one to consider sideslipping a bad idea? Deliberately increasing the stalling speed, destabilising the aircraft and taking off all your power producing errors in the ASI and altimeter and a high RoD close to the ground all seem to cry out NO to me!

The aim of every approach should be to put the aircraft in the right place at the right speed, correctly configured and with some power on. If you don't have those then you should Go Around!

It seems to me (imho) a little lazy when people get into the habit of putting themselves high and then gliding and/or sideslipping the last few hundred feet. It's rather like taking the car out of gear a few hundred metres from your parking spot and then coasting in; it's possible, but not very good practice, and in an aircraft it's creating extra risk!

I agree it is a skill which can be introduced with experience to possibly help out in the event of an actual forced landing, but even here, with plenty of practice of PFLs I think it unnecessary.

I'm not suggesting they should be banned or anything, but an inexperienced PPL or student might read through this thread and think they are an acceptable and regular method of putting an aircraft on the ground. Hopefully my comments might put sidelipping into perspective and start some debate on the negative aspects of the issue.

Unless, of course, you think different....

[c-bert]

Well I'm happy to admit to being a student PPL and I must admit it doesn't sound like a particularly good idea to me - see my previous (slightly sarcastic) comments.

I'm sure if people wanted to and they had the aircraft they could do away with the pattern completely and split S onto finals from directly overhead but equally, maybe not the best idea....:ok:

[nosehair]

I see that I have spoken out of turn. I did not realize there was a lack of training in this area of slips and cross-control stalls.
In my country, training and demonstrated proficiency in these maneuvers are required of PPL's and especially FI's.
But, having let the cat out of the bag, I feel I must explain my comments with a little more detail.
Let's take Classic's comment as an example. He says slipping is a bit like taking your car out of gear and coasting. Exactly! That is the idea. Imagine that, for some reason, perhaps to sneak in quietly, you want to do exactly that: cut off your engine and coast quietly into a spot. You must arrive exactly at that spot. If you cut the engine too soon, you will stop short. If you cut the engine too late, you will overshoot and have to apply your squealy brakes, announcing your arrival. So, you have to cut the engine at precisely the exact right moment. This is very hard to do. And the weight and speed would always have to be the same. But what if you can apply a little bit of braking without too much noise? then you can delay the engine cut a few moments, knowing you will arrive with a little overshoot which you can kill with a little braking. Wouldn't you do that? And the only variable in the car is speed and weight, but in the airplane, the big, big variable is wind. Wind will certainly change as certainly as your girlfriend's mind will change with not a whit of notice.
So, no matter how skilled you are at judgeing your "key" position, if you arrive there on speed and on altitude, there will come a day when you will still fall short of your spot, because the headwind will increase and push you back.
So, it is a good idea to always arrive at your "key" position with a little extra altitude and plan on "slipping" it off. Slipping instead of flaps at that point, because flaps cannot be retracted if you misjudge, but a slip can be immediately stopped if you think you are dropping short. Then when the field is assured, use flaps.
So, besides the plain, simple, long-established fact that slipping is a good idea, it is a required maneuver for a PPL license.
Practicing cross-control stalls is how we get used to the feel and sight and handling characteristics of a particular airplane so that we don't accidently stall it when slipping. Older airplanes sometimes did not have flaps, and slipping was the only way to lose altitude without increasing speed. Standard procedure.
If you have not been trained in slips and cross-control stalls, well, of course you should get a feel for it with an experienced instructor.

[slim_slag]

c-bert and classic,

You should be able to demonstrate a height losing forward-slip to landing to your PPL examiner. Side slips are generally used to land in crosswind conditions, and plenty of students also learn this technique as part of their training. If you do it properly you will be side-slipping when you are an inch off the ground. Side slips and forward slips are aerodynamically the same, and aren't dangerous if done properly (like everything I guess). You shouldn't be worried about them. And just to labour the point, it's skidding that causes stall/spin accidents in the pattern, you won't spin in a slip if you keep your nose even remotely pointed down.

Agree with what nosehair says..

In fact at my last BFR, the aerobatics instructor took us to altitude and all we did was cross controlled stalls, late recovery stalls and MCA work. It really beat me up, and I was very current on aerobatics at the time. We didn't spin but we were prepared for it to happen. Excellent practice.

[Classic]

Slim Slag,
You should be able to demonstrate a height losing forward-slip to landing to your PPL examiner.

As I mentioned in my post, there is a relevant use for high rate of descent sideslipping, ie during a PFL when needing to lose excess height to stop in a required space.

It is not a technique for either slipping 'quietly' into an airfield or as a common practice during a normal powered approach.

Side slips are generally used to land in crosswind conditions, and plenty of students also learn this technique as part of their training. The 'wing down' crosswind landing approach is different to the type of sideslipping I was describing: A small angle of bank with rudder to keep straight and power to fly maintain rate of descent is not what I was criticising. It is the few individuals I have flown with who cross control with no power at low altitude to produce a high rate of descent INSTEAD of achieving a stabilised approach.

it's skidding that causes stall/spin accidents in the pattern Side slipping causes accidents in the pattern as well as other reasons, including skidding.

In fact at my last BFR, the aerobatics instructor took us to altitude and all we did was cross controlled stalls, late recovery stalls and MCA work And good practice that is too, but practicing recovering from a mishandled approach is of little benefit if you fly a technique below the height from which it is recoverable. The best idea is to avoid sideslipping - it isn't necessary.

My aim was to highlight the fact it is not a recognised method of adjusting a normal approach, merely a technique to be employed in extremis when no other options are available, ie forced landing.

You shouldn't be worried about them. After flying F16s to 747s as well as 1500hours of light aircraft instruction and pleasure flying (still current), I'm not. But I'm worried that other people don't appreciate their risks.

[slim_slag]

Well classic, you asked for people who thought differenty, so now I throw it out onto the floor :)

[Lowtimer]

Classic said: It is not a technique for either slipping 'quietly' into an airfield or as a common practice during a normal powered approach.

How else are we to fly the many types with poor straight-ahead visibility, and/or no flaps? When flying a flapless Cub, Champ, pitts or a Yak-50/52 from the rear cockpit, or even a Tiger Moth, the sideslip is absolutely common practice for a normal approach. In the big-nosed types it gives you a decent view of where you're going, but on all the above types it also gives you a means of adjusting descent angle very precisely to meet a desired touchdown point, with the advantage (compared to relying on flaps) that you can safely flatten the descent by taking off some sideslip, whereas it may not be prudent to reduce the flap setting on short final.

[Milt]

Pitot Errors

Funny how most of you think that by pointing a pitot to one side gives more errors than pitching it up and down through much larger angles. What is the angle at a straight stall?

Most PEC comes from static pressure error.

[LowNSlow]

milt I would agree that most errors come from a duff static system but I would say that there would be a greater error caused by yaw than there would be caused by pitch. If you point the nose up the aeroplane climbs maybe the pitot isn't pointing in exactly the direction of flight but generally it's pretty close. Same for pinting the nose down. In yaw however, the pitot head may be pointing 20-30 degrees off the line of flight which is quite significant.

Classic so how would you land a Tiger Moth? I have nothing like your level of experience but I fail to see what is dangerous about a properly executed side or forward slip to the threshold. I fly an ancient Auster with an equally ancient engine. I far prefer to be high and sideslip into the approach secure in the knowledge that if my engine did quit at the last moment I could safely reach the field and not be picking myself out of a pile of crumpled fabic in the undershoot.

[Sensible]

personally I prefer to set the aircraft 10kts above the approach speed, put the aircraft into a slip and then use my ears as the airspeed speed indicator. I have heard that the ASI is inaccurate in a slip and in any event prefer to keep looking outside the cockpit.

[Flyin'Dutch']

No doubt duff static ports will exacerbate the ASI readings in slip.

However the reason why it underreads in a slip is not primarily due to static port problems but because the pitot is not aligned with the airflow in a slip.

It is difficult to stall in a slip.

Of course slipping on finals is an extremely dangerous pastime and should only be done by demi-gods not any ol' ATPL/heavy metal driver.

Come on get real!

As others have said, with proper tuition and practise a slip is perfectly safe and useful.

FD

[Classic]

Some good points. I've never done tail wheel flying and it seems there is some validity to side slipping for those aircraft with lots of airframe in front of you. Thanks for that. Good debate means we all learn!

Still not keen on it in a tricycle type undercarriage,though.

[slim_slag]

Classic, I don't think it's got anything to do with the layout of your landing gear. Of course you don't want to be slipping a 747 on final, and it is a nose-dragger, but I am led to believe that's more to do with it having a swept wing and turbines than anything else.

I was once in the back of a 737 which came in on a very high base leg into SAN, and they slipped it in, not sure about that.....

To all you students. DON"T BE SCARED OF SLIPPING, even in a spamcan. You don't have to be an ace pilot to pass your PPL practical, but you really should be able to slip. Get your instructor to take you through the aerodynamics of uncoordinated flight and concentrate on the angles of attack of both wings, and what happens when you stall one. In a skid you will be flipped upside down before you know it, in a slip not so. Rule of thumb: You will not stall if you are slipping and your nose is pointing down towards the ground, like what is happening when you are on the way to landing.


PS I've even slipped a 172 with full flaps, naughty huh? :)

[BraceBrace]

Indeed, slipping should be no problem, I always take some 5-10 kts extra for safety.

To me, danger is present at the "slip exit". It's not the slip itself that is dangerous, but your "stable approach" might become "unstable" when aligning with the centreline.

If you are too high, you can try to slip it in without any danger. However, use commen sense to know how low you can go with the slip. There are pilots who can slip it up to the flare, I wouldn't try it. If I still need to slip at 200-100ft AGL, I'll leave it and call the go around. The reason why I like to do this is speed indeed. What is the speed of the aircraft? You don't really know except that it is too high. In an Archer on approach to a very short strip, every extra knot will give you a nice float up to the end of the strip... So keep in account the runway and its environment.

[shortstripper]

Take a look at http://www.pprune.com/forums/showthread.php?s=&threadid=128580&perpage=15&pagenumber=1

The subject was pretty well covered!

Personally I think forward and side-slipping are essential skills. Certainly, whilst sliping does affect stall speed, it does not raise it in the same way as flaps. By that I mean the drag can be taken off quickly and stall speed reduced rather than drag taken off but stall speed increased! Not unlike the difference between flaps and airbrakes in this particular case. Make sense? ...

Lets say you are trying to get into a short strip at minimum speed (to reduce landing distance) and steepest descent angle (to clear a row of trees in the undershoot). Without slipping you are looking at a full flap, back of the drag curve approach and a quite difficult job. If you find your descent rate is too high you'll have to add power ... or worse reduce flap! Adding power will most likely add speed (remember you are already trying to maintain highest descent angle) and reducing flap will increase stall speed with its associated dangers. However, if you slip off that last bit of the approach, you can constantly juggle the amount of slip and power to suit descent rate and/or angle. You can do this without affecting your airspeed or stall speed adversly. Makes the job much easier! :ok:

Ivan