Sideslip as a manoeuvre

[Admin]

8.5 Sideslip as a manoeuvre

 

Types of sideslip vary in degree — from inadvertently flying cross-controlled in the cruise (i.e. one wing slightly low and compensating with opposite rudder) to a fully-fledged cross-controlled turn where the aircraft is steeply banked in a descending turn with full opposite rudder applied. All sideslips reflect uncoordinated flight and result in increased drag.

 

Note: in aerodynamic terms, any time it is evident that the aircraft's longitudinal axis is at an angle to its flight path (in plan view) then the aircraft is sideslipping (i.e. its motion has a lateral component), and the angle between the flight path and the axis is the sideslip angle. Aerodynamicists don't generally distinguish between sideslip and 'slip' or 'skid', but many pilots use 'slip' as the general term, 'skid' to describe slipping away from the centre of a turn and 'sideslip' to describe a particular type of height-loss manoeuvre.

 

The sideslipping manoeuvre is only for the pilot who has a very good feel for their aircraft because, among other things, the ASI will most likely be providing a false airspeed indication. High sideslip angles combined with high aoa must be avoided. There seem to be as many definitions of the types of slip as there are exponents of sideslip techniques, but

 

the safe execution of all sideslips requires adequate instruction and continuing practice

 

. Here are some types:

 

The straight or steady-state sideslip approach to landing

 

The helmet and goggles crowd who, very sensibly, like to fly biplanes and other open-cockpit aircraft not equipped with flaps, need a manoeuvre for use on the landing approach to a short strip that enables them to lose height quickly without increasing airspeed and which provides a good view of the landing area. The answer has long been the cross-controlled steady state sideslip; a manoeuvre designed to lose height over a short distance, dumping the potential energy of height by converting it to drag turbulence rather than kinetic energy. Such sideslips may also be a requirement when executing a

 

forced landing

 

, and the same type of slipping approach may also be necessary for those aircraft where, in a normal approach, the pilot's view of the runway is obstructed by the nose.

 

Once established on the approach descent path at the correct airspeed, the aircraft is banked with sufficient opposite (top) rudder applied to stop the directional stability yawing the nose into the relative airflow and thus turning. Slight additional backward pressure on the control column may be needed to keep the nose from dropping too far. The aircraft sideslips in a moderate to steep bank with the fuselage angled across the flight path, giving the pilot a very good view of the landing area. The greatly increased drag, from the exposure of the fuselage side or 'keel' surfaces to the oncoming airflow, enables an increased angle of descent without an increase in the approach airspeed. The execution of a sideslip to a landing varies from aircraft to aircraft and it may not work particularly well where there is a lack of keel surface — an open-frame aircraft like the Breezy, for example.

 

The sink rate is controlled by aileron and power is held constant, usually at idle/low power, and the sideslip must be eased off before the

 

flare and touchdown

 

. When recovering, care must be taken to coordinate relaxation of the back-pressure, leveling of the wings and straightening of the rudder — otherwise the aircraft may do its own thing or stall, particularly in turbulent conditions.

 

The straight sideslip is limited by the maximum rudder authority available; there will be a bank angle beyond which full opposite rudder will not stop the aircraft from turning.

 

Although this manoeuvre usually comes under the proprietorship of the 'stick and rudder' people, the use of the sideslip, by the captain of a Boeing 767, undoubtedly saved the lives of many people in an extraordinary incident that occurred in 1983 when, due to a train of errors — as are most accidents/incidents — an Air Canada 767 ran out of fuel at 41 000 feet. The captain subsequently glided the aircraft to a safe landing on an out-of-service runway, which was being used for a drag racing event at the time. The aircraft was sideslipped through several thousand feet to lose excess height on the approach. For more information about this magnificent demonstration of airmanship (following an execrable demonstration of preflight procedure by many people; keep the old adage in mind — "proper pre-flight procedure precludes poor performance"!) google the phrase 'Gimli glider'.

 

The sideslipping turn

 

Slipping whilst turning is a manoeuvre often used in non-flap equipped aerobatic aircraft where it is desirable to perform a curving landing approach. This is also a useful emergency manoeuvre if it is necessary to increase the sink rate during a turn — such as the turn onto final approach in a

 

forced landing

 

when an overshoot of the landing site is apparent. It is just a sideslip where insufficient top rudder is applied to stop the aircraft turning while slipping. The rate of turn and the rate of sink are controlled by the amount of bank and the amount of rudder but it is an

 

uncoordinated descending turn

 

. Dangerously high descent rates are achieved if the bank angle applied exceeds the full rudder authority.

 

Fishtailing

 

Fishtailing is a series of sideslips where the wings are held level in the approach attitude with (alternating) aileron while the aircraft is repeatedly yawed from side to side by applying alternate rudder; the increased drag increases the sink rate and is possibly used as an emergency measure if overshooting a forced landing. The manoeuvre is generally not recommended, because uncoordinated control use at low levels may lead to dangerous loss of control. Also, excessive alternating rudder reversals may overstress the fin.

 

Sideslip to a crosswind landing

 

In a sideslip to a crosswind landing, the aircraft is always banked with the into-wind wing down so that the sideslip can be smoothly decreased to a forward slip (below) before the roundout. Most aircraft tend to be slower in the slip, so the nose will need to be a bit lower than that needed to maintain the normal approach speed. A smoothly executed sideslip approach requires much practice, but displays considerable finesse to a ground observer.

 

The forward slip crosswind approach

 

A 'forward' slip is a moderate sideslip application designed only to compensate for crosswind during approach and landing. The slip can be applied throughout the final approach or just in the last stages, and it usually follows a full sideslip approach in crosswind conditions. The into-wind wing is lowered with sufficient bank so that the slip is exactly negating the crosswind drift, while opposite (top) rudder is applied to stop a turn developing and to align the aircraft's longitudinal axis with the flight path — and the runway centreline. If drifting off the path, just add or remove some aileron pressure and, at the same time, add or remove some rudder pressure to maintain direction. An approach speed 2–3 knots above normal is set up, the sink rate (which will be greater than usual because of the inclined lift vector) is controlled by the power setting, the into-wind main landing gear will touch down first and the aircraft is held straight with rudder by pivoting on that one wheel until ground speed has reduced to a safe level.

 

The forward slip is the particularly recommended technique for crosswind landings in high-wing taildragger aircraft. Incidently, a useful technique for a high-wing taildragger in a significant crosswind is to also perform the take-off run on one main wheel.

 

If there is any real difference between the straight sideslip and the forward slip it is just the amount of pressure applied to the controls. In a sideslip, the aileron pressure dictates the angle of descent and the rudder pressure dictates the amount the fuselage is deflected across the flight path. In a forward slip, the aileron pressure is just enough to compensate for the crosswind drift and thus maintain position on the extended runway line, and the rudder pressure just enough to keep the fuselage aligned with both the landing path and the flight path.

 

There is one manoeuvre for certified aerobatic aircraft that demonstrates what might be considered a reversal of all we have stated in this module. This is the 'four-point slow roll' or 'hesitation roll' where the aircraft is rolled through 360° in level flight around a point on the horizon, but the roll is paused for a second or two at each 90° point; i.e. when the wings are first vertical, when the aircraft is upside down, when the wings are again vertical and when the aircraft returns to normal attitude.

 

The roll is started (to the left) with normal aileron and a bit of left rudder, then as the roll progresses through the first 90° top (right) rudder is increasingly applied to negate the yaw, and also to hold the nose up. During the slight pause at the 90° position the aircraft is being held in a nose-up attitude by the rudder whilst the elevators are used to stop the nose wandering to the left or right across the horizon, and the ailerons are neutral. Some lift will be generated by the fuselage having an aoa because the nose is being held up. Then the roll is restarted until, at the 180° position, the aircraft is inverted and the nose is held up by a large forward movement of the control column and the aoa is negative; i.e. the

 

lift is being generated by a reversed aerofoil

 

. And so the roll continues. Of course, all the control movements involve gradual increase/decrease in pressures throughout the sequence.

 

Source:

 

http://www.recreationalflying.com/tutorials/groundschool/umodule8.html#sideslip

 

 

[Guernsey]

Thanks for the post Ian, very informative.

 

Alan.

 

 

[HeadInTheClouds]

To be honest I had no idea what a forward slip was until reading that... Thanks for the post!

 

 

[rdarby]

So a forward slip is the same as the cross controlled approach to landing in a cross wind?

 

 

[Tomo]

So a forward slip is the same as the cross controlled approach to landing in a cross wind?

Yes, you are only operating the controls to maintain the flight path down to the runway, whereas a sideslip is generally for descending/visual purposes so the aircraft is held sideways with rudder, and aileron is used to control angle of descent. From what I understand anyways...

 

 

[Ballpoint 246niner]

A very under utilized manouvre but one that requires practice and finesse to gain the effective result. It does place high loads on the vertical stabiliser and attachment points so this should be taken into account when being utilised and shouldn't be seen as an instant fix to a poorly planned approach in normal cct operations.

 

Get your instructor to demonstrate the various flight phases where crossed controls can be used to your advantage....

 

 

[robinsm]

Side slipping to slide into a limited landing area or short runway is a great way to steepen your approach without increasing your speed. Fun to practice and a valuable tool in your arsenal. Not much point having it demonstrated unless you are going to practice it. Great to wash off height on country airstrips or difficult landing areas.. Forward slip works great in heavy crosswinds and eases the strain on the landing gear as there is very little lateral thrust on a straight line, one wheel first, landing. Great tools to have in your skills arsenal and fun to practice.

 

Cheers

 

Maynard

 

 

[Admin]

When I was going for my RAAus Cert, the examiner told me to side slip the Gazelle in which I did but apparently I was too gentle in the side slip because when we came back for a full stop, on final he said keep it up high and level. As we got closer without any descent I was wondering what he was up to as it was definitely going to be a go-around because there was no way I was going to get her down without massive speed.

 

Next minute he says to me "I have the plane"...and then said "Now this is how you side slip"...crikey, I almost fell out of the Gazelle, the angle was so great I thought I was on a carnival ride but hey, he was the examiner so he must know what he was doing...we literally just dropped out of the sky...he then said "You have the plane"...ummm...aarrr...quick, straighten up, calm down, and fly the aircraft but low and behold, we were exactly at the right height coming in just over the threshold where I could just beautifully place the Gazelle gently down on the strip and roll out...1,000ft just wiped off in what felt a couple of seconds.

 

I hope that I would never be in a position where I would have to try such a manoeuvre but I became more aware on the limitations of the Gazelle and it was then that I fell in love with that aircraft...as we know each aircraft have their own limitations especially in manoeuvres such as a slide slip

 

 

[HeadInTheClouds]

A sideslip with full rudder in the foxbat also makes it feel like you're going to go flying out the door, good fun

 

 

[facthunter]

I've found some pilot's don't really like the manoeuver as it has a lot of side load and you are pressured to the side. When I trained it was a standard manoeuver and a lot of aircraft didn't have flaps, so we all got on with it and enjoyed it.

 

It needs to be taught properly and practiced and is a good skill to have even though a few aeroplanes can't/ don't do it well. IF you have really effective flaps or a very draggy plane there are other techniques that work nearly as well.

 

With a thruster or a drifter, you will do nearly as well to just do a steep nose down, power off approach . they are so draggy that you will get a sink rate which is high enough for most situations. just allow a bit of height and distance for the flare.

 

With the Cesna's full flap position, you can use almost the same technique, although I understand that the real full flap position is not available on some.

 

The most common error is to just do a flat skid.

 

The article describes the technique well in saying to bank the plane and stop the turn with rudder. This has the bank as a priority and essential part of it. The rudder effectiveness is the limiting factor. Don't do the slip at too high a speed. This where the higher airframe loads occur. Just carry your normal margins above stall for the approach conditions and as you straighten out you may have to lower the nose slightly to keep descending. Do all your initial practicing at a safe height and make sure that your pitch management is precise and there are not a lot of speed fluctuations indicating an unstable approach would result, if you were doing it near the ground. Since you are trying to get the steepest possible approach path there would be no purpose in having any power on either...Nev

 

 

[eightyknots]

Kyle Communications took this little video and there is a lovely sideslip at 1:40 minutes!

 

See:

 

 

 

[farri]

A very under utilized manouvre quote]Not by me.

 

Frank.

[Yenn]

The great advantage of slipping on the approach is that it is more variable than flaps and unlike a flapped approach, a go around poses no problems, get rid of the slip and you are ready to go.

 

 

[Gnarly Gnu]

I enjoy side slipping too, didn't realise there were these different types till I read this interesting and helpful article. On engine failure training I found it's a nice backup, allows you to keep some extra height as a safety margin till you are quite close to the landing spot chosen & happy with it then loose the excess height rapidly.

 

 

[Timold]

Hi all, I use the forward slip for crosswind landings every time in a C182 with full flap. Mostly all of my landings are on shortish bush strips and ii find this techniqe to be very effective in high cross winds. Tim

 

 

[Guest pookemon]

On Monday there was a massive thermal hanging around on base at YBLT. In the Gazelle you reduce the power (Eg. 3500rpm) and begin your descent when you turn base. On Monday I cut the power to 2000rpm, brought the speed back to 60kts and was still climbing. The result was that instead of turning final at 2100ft AMSL, I was actually closer to circuit height (2400ft AMSL). Left stick over and bags of right rudder and side slip all the way down to about 50ft AGL. My instructor taught side slips on about my 3rd flight as a way to correct a high approach. It's THE correct way to correct the high approach. You can descend in the Gazelle at 700ft/min while still plodding along at 60kts. He also taught the "Forward Slip" (I've never heard that term before) for crosswind approaches (aka wing down approach). It's handy to practise the side slips as part of your engine failure procedure, including turning in the slip (level the wings a bit or ease the rudder). It's better to be a bit high with a slip up your sleeve, than too low with a fence fast approaching.

 

 

[farri]

It's better to be a bit high with a slip up your sleeve, than too low with a fence fast approaching.

 

Frank.

 

 

[naremman]

WhenTiger Moths and flapless Austers were the contemporary aircraft slideslip was used to regulate the descent profile. With the introduction of the American lighties it went out of favour, with dire predictions of what loads were being imposed on the airframe.

 

The Victa Airtourer provides the opportunity to really explore the boundaries of sideslip. The handling notes OK sideslip with full flap up to 87 KIAS. With a central fuel cell unporting is not an issue. The rudder and flaperon authority are substantial. I think it is possible to pass a brick going down!! In a glide approach you can just about lob it into a bucket.

 

Peter Lloyd told me at a ALAC competition that they had to write the rules for the Forced Landing Competition to exclude Victas sideslipping, as they were creaming the event.

 

My experience in the Foxbat and Jab favour the Foxbat for sideslipping. I kept on trying to boot more rudder into the J160 until I came to the realization that I was on the stop, and that was it.

 

 

[HeadInTheClouds]

Sideslipping in the Foxbat is really effective!

 

When practicing glide approaches last time I did a slip with full right rudder. It came down like a rock so this would be damned useful in a forced landing situation. It was just a pity that I floated all the way down my imaginary 'runway' which my instructor said was available to use (Between the two white touchdown points) because with full flap it just doesn't want to land! Oh well, practice will help that.

 

Another time it was useful - Due to a runway change and also a 'brain snap' on my behalf I had it in my mind that we were on downwind and not base, so we ended up turning final at around 900'... Simple fix - a full sideslip and it comes down like an elevator!

 

I actually find them fun to do, however when I can eventually take passengers I think I will try to avoid it because they might not appreciate feeling like they are going to fall out the door...

 

I tend to think of a decent sideslip as changing the flight model from one of an aircraft, into that of a large and somewhat aerodynamic rock, and back to an aircraft again

 

EDIT: This video is relevant to put here:

 

 

 

[Tomo]

I slip in most of the landings here, only mild side slip though... by the way I was fairly hot on most approaches so I would touch down in front of the hangar...

 

Starts at around 2.15 I think -

 

 

 

[motzartmerv]

Its in the syllabus. To be issued a certificate you must demonstrate sideslipping. Aswel as a slipping turn through 90 deg's. If you havn't been taught or are not confident in this manoeuvre grab your instructor and go and learn it. Its could be a lifesaving skill. Highwings generally slip better then lowings and some aircraft shouldn't be slipped under certain circumstances, so read the POH and talk to an instructor. It is not a dangerous manoeuvre, but care should be taken. The controls are crossed but it is in an ANTI-SPIN configuration.

 

Another consideration is the IAS may change during a slip. This is due to a few factors and depends on type, but in aircraft with pitot tubes a long way out on the wing the airflow into the tube has to travel around a corner and experiences less dynamic pressure, so the tendancy is for the IAS to over read. In some jabirus I have seen it read 6-8 kts in error.

 

Cheers

 

Fly safe

 

 

[dazza 38]

Every pilot should be confident in slipping it in.Especially first thing in the morning.

 

 

[motzartmerv]

...lol..

[alf jessup]

Every pilot should be confident in slipping it in.Especially first thing in the morning.

Dazza,

 

What happens if your not a morning person, is it ok to slip it in the arvo or before lastlight???

 

Alf

 

 

[alf jessup]

I slip in most of the landings here, only mild side slip though... by the way I was fairly hot on most approaches so I would touch down in front of the hangar...Starts at around 2.15 I think -

 

Always impressive your video's young Thomas not to mention your flying skills