Stalls in slips on landing

[Guest TOSGcentral]

Good scenarios well put Motz.

 

To them I could add misjudging a wind gradient leading you to a semi stalled condition and also a bad bouncing situation on an enthusiastic tail dragger.

 

The common denominator to all is close proximity to the ground and you really have little choice but to use rudder to retain lateral and directional control. As a rule of thumb I would say anything within a wing span of the ground and you could have serious general control problems so must use what works.

 

Certainly I would sooner use the rudder in that circumstance than move the stick forward if very close to the ground and drive the machine on still semi stalled with a wing still down!

 

On the other hand if you were in the classic low and slow turn to try and get lined up and you got a wing drop then side slipping the aircraft (which is what you are doing when using the rudder in that manner) could well result in you not getting in at all and going through the downwind fence somewhere between stall and approach speed. Not nice when you are going be to the first one on the scene of the accident!

 

Regarding your 'Fox experience. Sure types differ and there may be a very large difference between what you can do rapidly and quickly well into the incipient spin situation to what you MUST do in a developed spin recovery.

 

For example I did my most valuable spin training on a Chipmunk and it did not have the anti-spin strakes fitted. From initiation of full spin recovery you had to sit there with full opposite rudder and stick hard against the front stops and wait for a couple more turns or so - filling in time by reviewing your rosary!

 

Aye

 

Tony

 

 

[motzartmerv]

Ok..Exellent Tony..thankyou..I am starting to see some serious holes in the training that we all recieve on RAA machines.. I was taught and im sure many others were aswell that if a wing starts to drop then we immediatly pick it up with rudder, but as tony has demonstarted, thats probably not the best course of action..And certainly height seems to be the key factor in which tehniqe to use.. So the question is how do we relate these findings to students..i only ask because im starting my instructor rating next week.. How would we train ppl to judge which technique is to be used in which scenario?? We can't just teach them to always pick it up with rudder because at certain times this could be detrimental to their health..

 

I have another problem.. An acft that has the wings out of level say in a left hand turn and approaches the stal, doesn't the high wing (right wing i a left turn) stall first becasue of its higher AoA, thus returning the wings to level and probably unstalling due to its reduced AoA??...again, i will be practising this tomorrow to see for myself but i invite any opinions ...

 

cheers

 

 

[Guest TOSGcentral]

Motz my friend you will have to do something about your theory or your visualisation! J

 

You have your turning case totally back to front.

 

I want you to visualise an ice cream cone that we have cut a fair bit from off the bottom – or one of those cone shaped noodle containers would equally do. You will agree that the upper and lower rims are different diameters?

 

OK, now draw a vertical line down the side of the cone and imagine that this is the aircraft’s wing (we will ignore the fuselage for this exercise). The entire cone represents the path the wings take in the air in a 360 degree turn.

 

You can see that in a turn the upper wing is describing a larger radius of turn in the sky than the lower – which is turning more ‘tightly’.

 

So by definition the upper wing tip is travelling through the air faster than the lower wing tip! On the other hand the lower wing tip is turning tighter.

 

The upper wing is therefore (relative to the lower wing) travelling at a higher airspeed and a shallower angle of attack. The lower wing tip in comparison is travelling at a lower airspeed and a higher angle of attack due to the tighter radius of turn!

 

The lift between the two wings maintains equilibrium and if the aircraft is above stalling speed then lateral damping is present and will keep the aircraft nicely banked with little effort on the part of the pilot – providing the bank is not that excessive that you enter the spiral instability regime.

 

This is the basic scenario for the classic stall/spin situation from a low and slow turn. Take a turn to the left. The lower wing is already travelling slower and at a higher A of A than the upper wing. Approach the stall, and particularly start attempting to force the aircraft around the turn with left rudder because it is not turning fast enough and the ground is unusually close etc etc so a shallow bank angle feels more comfortable!

 

So in the normal progression of events the inner wing in the turn (in this case the left wing) will stall first and drop rapidly. The aircraft slips left and natural yaw stability leaps in and yaws the aircraft left (and therefore nose downwards from the abruptly increased bank angle). You may not be spinning and probably are not but that is totally academic! You are pointing straight at the ground, accelerating and have insufficient height to regain normal attitude – you are also bloody frightened and in a state of initial shockbecause this is not an academic exercise at height – you are trying to land and suddenly the aircraft has done something frightful!

 

This is a terrible state of affairs and is bad enough in the most benign of aircraft (because they can all do it) but if you have something ‘sharp’then it can be absolutely terrifying and IS going to be fatal! For example, the Slingsby Dart 17 will roll its 17 metre wing instantly inverted and then point straight at the ground immediately afterwards on spin entry. Not something you want to be doing at 300’!

 

The situation may however not work this way. We will take the same left hand stall in a turn scenario and put ourselves in a Thruster with a 582 engine that has a very good overall power to weight ratio and quite a bit of torque (I am using this not because the Thruster is any more difficult than anything else but because I have frequent experience with the situation).

 

So there you are in a modest left hand turn and stall the aircraft with a reasonable amount of power present. The Thruster will then be over-ridden by the engine torque because of the low energy airframe state but high energy engine state and roll to the right ‘over the top’ and against the turn. If this alarms you and you go to full power before you have the attitude under control then the process just continues happening but with more enthusiasm!

 

Certainly know your aircraft and know your principles of flight soundly and most of all stay current. Many people do not get a tick against any of those boxes but it is all very easy.

 

Adopt the Safe Speed Near the Ground technique and DO NOT fly your aircraft below 1.5 Vs (one and a half times the stalling speed) when within 800’ of the ground. You cannot possible come to any harm and all of this I am expounding is just twaddle from an aging ex-instructor.

 

Aye

 

Tony

 

 

[motzartmerv]

Yes Tony.. I just went for a drive and was thinking about this problem and realised i had it back to front.. The outside wing would be travelling faster..hence the tendancy for overbank in a climbing turn and underbank in a descending turn..I was getting mindblock thinking about all this theory..:;)3:

 

Anyway, thankyou for your help and input..It is greatly appreciated..I am going to study the ppl theory concurrent with the instructor rating..and obtain the ppl soon (fingers crossed)..

 

cheers and thanx again Tony

 

 

[Jabiru Phil]

pduthoit

 

Hi all,

 

I like other members must be getting value from this thread and the cross conversation between a member and (ex) instructor. Excellent reading.

 

I would like to mention that I have a book that was given me that has ALL well explained and indexed chapters with diagrams. I call it the bible! I have also read most BFK books and refer to them occasionally also, but the "STICK and RUDDER" hardback wins every time. I can follow the above queries by refernce to the current topic posted.

 

Worth the purchase for ALL pilots.

 

cheers,

 

Phil.

 

 

[Guest thomaswb]

OK, now draw a vertical line down the side of the cone and imagine that this is the aircraft’s wing (we will ignore the fuselage for this exercise). The entire cone represents the path the wings take in the air in a 360 degree turn.

 

You can see that in a turn the upper wing is describing a larger radius of turn in the sky than the lower – which is turning more ‘tightly’.

 

I have heard this explanation so many times, and it never seemed right to me. So I did the sums, based on a typical scenario in my Jabiru: 90 knots, rate 1 turn. The centre of lift on the outside wing is going 0.5% faster than the centre of lift on the inside wing. That is so small you would have to be a very sensitive pilot to notice it. The rolling moment is less than that caused by me flying without a passenger.

 

So it seems to me that all this theory is irrelevant, and there is no tendency of the inside wing to drop-- or if there is, it is caused by some factor OTHER than differential wing speed.

 

 

[motzartmerv]

Ask yourself this...Why do we get a wing drop in the first place? i'll have a crack at it, its becasue one wing or a section of a wing stalls before the other....and thats in straight level stalls... obviously theres a minute differance in the airflow over one wing and causes it to stall first...now that small drop may not be perceivable to the pilot but the science sure does feel it...even a differance of .5% is plenty when it comes to the science of a stalling wing..but more important is the angle of attack, and when we are only talking about 16 degrees to stall a wing then what %age of that would you call significant?? 1% diff in 16 deg is .16 deg...

 

my 2 cents

 

cheers

 

 

[slartibartfast]

Mine has been dropping the left wing on stall quite severely. Very exciting.

 

I fixed it this morning by putting a shim under the left wing drag spar, thus reducing the angle of attack of the left wing and putting it more in line with the right wing. Much better.

 

Probably only 5mm shim, but it made a lot of difference.

 

Great thread.

 

 

[Guest TOSGcentral]

OK Guys and Gals, I am not getting defensive here but I am trying to put in extremely simple terms, that are easy for the lowest common denominator of reader to get their minds around, that certain things definitely do happen and can kill you! So please do not escalate (at least my words) into an abstract session on theory

 

As a case in poiint: Over a two week time interval a few years ago, I watched news documentary footage on TV of a Piper Cub 'spinning in' out of what appeared to be a quite benign low banked turn. It was fatal.

 

A couple of weeks later it was the turn of a B52 eight engined bomber doing exactly the same thing - and that was awesome and killed the entire crew!

 

In neither case was anything (visually) out of the ordinary going on and so often I have heard 'it just dropped a wing and went in - nothing unusual happening'.

 

So for Thomas - you are quite correct. A pilot cannot pick that up! And there will be other factors involved - perhaps a bit of pressure on a rudder pedal, a bit of holding off of bank so the ailerons are involved, and an insidious creeping back of the stick to hold a reasonable looking attitude while you are sapping the thing of energy - until, collectively, it does stall one wing first and you do shed lateral damping.

 

For SlartiB (and I am not being snotty here mate) you were not correcting any angle of attack - you were equalising your wing incidence to even out the angle of attack!

 

I stuck that in because Wing Incidence, Angle of Attack and Relative airflow are significant conceptual challenges for students to differentiate between. Much as one of my pet hates is junior instructors referring to relative airflow as 'the wind' and hoplessly confusing their students.

 

T.

 

 

[Guest thomaswb]

Ask yourself this...Why do we get a wing drop in the first place? i'll have a crack at it, its becasue one wing or a section of a wing stalls before the other....and thats in straight level stalls... obviously theres a minute differance in the airflow over one wing and causes it to stall first...now that small drop may not be perceivable to the pilot but the science sure does feel it...even a differance of .5% is plenty when it comes to the science of a stalling wing..but more important is the angle of attack, and when we are only talking about 16 degrees to stall a wing then what %age of that would you call significant?? 1% diff in 16 deg is .16 deg...my 2 cents

cheers

That's what I'm saying-- that the wing speed differential is so small that it is around the same magnitude as many other factors: C of G to one side, slight foot weight on the rudder causing a slip, torque effect, propeller slipstream, fuel cap being on one side, etc, etc. And of course once the wing starts to drop, the increased AOA causes a cascade effect in that direction.

So to say that the inside wing drops in a stall is just as inaccurate as saying the pilot's-side wing drops (wing more heavily loaded, aileron always down slightly) or the right wing drops (right foot on rudder out of habit) or the left wing drops (insufficient rudder to balance propeller) and so on. The idea of a perfectly balanced plane is great for theory, but I've never seen one!

 

And the same calculations apply to climbing turns, descending turns etc. You have to know it for the theory exams, but it's irrelevant in the real world. And the theory of climbing/descending turns is wrong, but don't get me started on that...

 

The irony is that in the real world, not only is the theoretical effect swamped by real world effects, but it has no impact on the pilot's actions! In the real world, if a wing drops because of a stall, you reduce the back pressure on the stick and add power. If you are in the rudder camp, you add a bit of rudder on the bottom foot as well. That doesn't change if you are turning. Except in a full-power climbing turn, in which case you don't have any power to add.

 

This is one of those rare occasions where the real world is a lot simpler than the theory.

 

 

[Guest pelorus32]

And just to add to this some complexity. In a descending turn the inner wing will tend to stall first. In a climbing turn the outer wing will tend to stall first.

 

See this thread:

 

http://www.recreationalflying.com/forum/showthread.php?t=5017&highlight=kermode

 

Regards

 

Mike

 

 

[facthunter]

Slips & Stalls.

 

Sometime back it was mentioned that the airspeed will be erroneous. It does UNDER-READ. Reason;- The pitot head is at an angle to the airstream due to the slip. This provides an increased margin .

 

Someone said that you don't need to pay much attention to the airspeed. The opposite is true, especially initially till you are very used to the individual aircrafts characteristics, great attention should be paid to the stability of the airspeed. What should it be? Normal margin over stall for the approach is OK. The nose attitude is generally slightly HIGHER than normal. If you perform the slipping approach at higher airspeeds, it feels rotten, overloads the structure,(fin & rudder) and you don't achieve the steepest approach path (which is the aim of the exercise) and you will end up with excessive speed to wash off proir to landing, hence defeating some of the point of the exercise The lower wing will not be the one to stall first. The top wing will, and since the aircraft has to rotate through level to the opposite wing down, more warning is available and time to react. The recovery from a sideslip is simple, you allow the rudder to centralise, level the wings and lower the nose slightly. This manoeuver, as I have observed so far, is one of the least understood and poorly executed that I am aware of.. I question the need for it in aircraft such as Bantams, Thrusters and Drifters etc. as an overspeed power off dive aiming just a little short of the normal touchdown point achieves almost the same result. but as a complete pilot concept, why not learn it properly so that it is understood for ever? Nev..

 

 

[Yenn]

From all the above posts, I am still of the same opinion that you can pick up a falling wing with rudder. I have done it many times, most recently during my BFR, when I was in an aeroplane I had never flown before. A Jabiru J160. I was told to do a stall and did so then kept picking up whichever wing dropped with rudder. When the instructor said i had lost 200' I had to explain that I was enjoying myself. I have done the same in Cessna's and a Thruster.

 

 

[Guest Juliette Lima]

Interesting discussion....Like others,I flew gliders for many years in strong thermal turbulence, often at 45 degrees bank on the edge of stall to get maximum lift.

 

It often happened that a gust would want to turn the glider on its head and in so doing...big stall....Particularly so in my Slingsby Dart 15m (a joy to fly in)

 

Try to correct with top rudder only, and all hell broke loose in severe thermals.....the really simple AND CORRECT solution was, and is, immediately release back pressure on stick THEN apply top or opposite rudder. For glider pilots this reaction is instantaneous and second nature.

 

In strong thermals, this correction might be necessary many times in the rapid climb from 2000ft to 10000ft.....so miss those magic rides.

 

There is a very well respected guy around Cessnock/Maitland who teaches advanced flying techniques/aerobatics , Phil Unicombe (apologies if spelling is incorrect)...Phil does courses and has written a terrific article on this topic...in Australian Flying about eighteen months ago....In short, what Tony has written is spot on, and mirrors Phil's article and training. Phil demonstrates stalls in all extreme aerobatic situations and in every instance...Stick forward then opposite rudder,where necessary, for immediate recovery and little loss in height.

 

Cheers

 

JL

 

 

[Guest Juliette Lima]

Hello again,

 

Just found the article referred to in my previous post,

 

Phil Unicomb of Action Aviation is the man (but then again so also is Tony...thankfully for these forums)..the article is called Understanding the Stall , published in March/April 2006....Essential reading for those intent on continuous improvement.

 

Cheers

 

JL

 

 

[bilby54]

Quote:

 

I question the need for it in aircraft such as Bantams, Thrusters and Drifters etc. as an overspeed power off dive aiming just a little short of the normal touchdown point achieves almost the same result.

 

Hi Facthunter,

 

I just wonder why you would make a statement like that as maybe Boeing and Airbus have it all wrong in hanging all that metal out when they want to land?

 

Cheers,

 

B54

 

 

[facthunter]

Horses for courses.

 

The aircraft that I mentioned are samples of the type of aircraft that "run up against a brick wall of drag" when you try to make them go fast,even by as little as 25 knots. This can be utilised to achieve a steep approach. (use up the potential energy ), by operating the aircraft at an inefficient (excessive) speed rather than reconfiguring the aircraft, which is not available anyhow.

 

Since the minumum manoeuvre speed of the Boeing and Airbus clean is about 235 Kts AIS, and the typical glide angle (power at flight idle) is about 3 degrees, it becomes pretty obvious that to use the principals of newtonian physics that I am relying on here, to the more aerodynamically efficient designs , would not be appropriate. The bigger stuff has to be "dirtied up' and its lift coefficient altered to permit it to come over the fence and stop in the confines of the aerodrome. Tyre limit speeds are generally around 175 Kts also. Nev..

 

 

[bilby54]

Looking back at the original question, it was asked why the difference in IAS if the pitot is mounted on the side of the fuselage. The airspeed indicator reading low due to slip is only true if the pitot is mounted in clean air away from any airframe disturbance.

 

If the pitot is positioned on the side of the fuselage then the IAS will be different depending on which way the aircraft is slipped. Drag occurs due to the pressure difference between the into wind and out of wind parts of the aircraft in flight. When an aircraft is side slipped, it presents a large frontal area to the relative airflow that will slow it down. To maintain flight, it is necessary to lower the nose and then forward speed becomes less while the vertical speed increases..... great for getting in over the trees or a hill at the end of the runway.

 

If the pitot is positioned on the into wind side of the slip, the slight increase in pressure will cause the IAS to read higher than normal while if it is positioned out of wind in the the slip, it will read slightly lower. The airspeed indicator is a pressure indicating device that is calibrated to read airspeed.

 

I suggested that slips should be flown by attitude rather than relying purely on airspeed due to the dynamics of the slip. The stick is essentially pointed where we want to go and the aircraft is then 'balanced' with appropriate opposite rudder and the airspeed will remain in the safe approach region without the need to constantly check it as that can lead to constantly chasing it.

 

 

[poteroo]

The original Q was in regards to a STOL type of aircraft. Why would you need to do anything dramatic to it operationally - especially a passenger frightening forward slip?

 

cheers,

 

 

[farri]

Stalls in slips on landing.

 

Hi All,

 

Shouldn`t this thread be directed to a Zenith 701 instructor so as to hopefully get the correct information.

 

It`s fine to make everyone think about it however not everyone has the correct answers there by confusing the issue and as we all know each aircraft has it`s own performance characteristics.

 

Cheers,

 

farri.

 

 

[Yenn]

Is the windward side of the fuse really the high pressure side? If there is an obstruction such as the fuselage it may be that the flow of air on the windward side has to speed up, resulting in a lower pressure and on the other side would be a higher pressure resulting from lower speed air.

 

I wonder what happens, but on my plane the pitot is beneath the wing and would probably be incorrect with slips in either direction. When I need to slip to lose height the nose goes down for the same reason, which would tend to increase speed.

 

 

[farri]

Yenn,

 

If a sideslip is used to loose the greatest amount of height for the shortest distance traveled over the ground then the lowest airspeed possible is required therefore the nose needs to be held up to obtain the required speed,different in every aircraft.

 

I doubt that many airspeed indicators would read accuratly enough to simply work on the given figure,each pilot must know and understand the manouvre and their aircraft

 

thoroughly before atempting it near the ground.

 

we all know that it`s not just about figures,how many pilots have their airspeed indicators calibrated regularly.

 

The presure side of the fuselage is a good question.

 

Regards,

 

farri.