OEI - Why does the Balance Ball show a slip?

[Jet_A_Knight]

I don't fly jets so this refers to multi engine, prop driven aircraft....

With an engine inoperative, and with 5 degrees AoB into the live engine, the aircraft is said to be 'n balance, but the ball shows a slip.

Can someone please explain the reason why the ball shows a slip toward the live engine?

I was once told that this is an indication of a rudder sideways force into the live engine - how accurate is that?

Cheers

[flyer4life]

Hi,

It's because you are banked towards the live engine but not actually turning. So the only force on the ball is gravity, and the ball therefore sits in the lowest point of the glass arc.

When flown correctly, you are in a zero-slip condition despite what the ball says. It's just a limitation of the instrument when in asymmetric flight. A true slip indicator such as a yaw string would show zero-slip correctly.

flyer

[Tinstaafl]

You'll find that 2-3deg is the AoB that will give you zero slip. Anymore than that will have the a/c flying not quite aligned with the relative airflow, increasing drag.

5 deg bank is a certification limit, set so that the a/c manufacturer can't specify an very low Vmc that would require unreasonable handling methods. It's not a performance related number in of itself. The more bank the manufacturer can use to determine Vmc the slower the a/c can fly before loss of heading control (I'm any other limiting factor eg Vs). The manufacturer wants a low Vmc because some of the other limiting certification V's can be affected by it.

The ball will be out because it's only a simple inclinometer. No different to the bubble on a builder's spirit level. If you fly with one wing low AND don't have some other horizontal force acting on the a/c (such as occurs in a turn) to push the ball a bit sideways then the ball has to fall to the lowest part of the tube. Tube is bent into a curve so the lowest part of it isn't at one end as would be the case if a straight tubed spirit level was used.

[RatherBeFlying]

Tape length of thick yarn to middle of windscreen.

Mark Center with piece of tape or whatever won't attack the plexiglass.

Check against ball in symmetrical flight and adjust as required.

Then try with one engine throttled down at safe height and airspeed and find what works best.

[Jet_A_Knight]

Tinny, I seem to remember that from when you trained me. Somehow, recently during a renewal, I got thrown the wobbly about the sideways force.

Excessive Professional deference I think it's called!

[Keith.Williams.]

The JAR POF exam often includes the twin to this question:

" Following a single engine failure, you use rudder to arrest the yaw and hold the nose towards the live engine, You then sideslip down the original track using ailerons to keep the wings level. What willl the slip indicator show?" ( I have added extra words to make it more inelligible than the JAR question really is)

Once again it will tell lies. This time you will be slipping but the ball will be central.

The moral of the story is if you trust your slip indicator following a single engine failure, don't do the JAR POF exam.

[safetypee]

The slip ball is more accurately described as a lateral accelerometer, thus it is a measure of aircraft lateral acceleration due to the forces acting on the aircraft (pendulum effect).

Take care not to confuse acceleration with a measure of aerodynamic sideslip as indicated by ‘a piece of string across the windshield’ in the relative airflow.

Following an engine failure it is important to use both rudder, and bank towards the live engine. Use of rudder provides a yawing moment that balances the moment from the thrust of the live engine. Using roll by banking towards the live engine reduces the aerodynamic force required from the rudder (reduced rudder foot loads) and reduces drag.

Thus for the JAA question: with wings level and only using rudder, the aircraft will have aerodynamic sideslip (piece of string over your right shoulder), but the lateral accelerometer (ball) will be centered – no slip indicated.

When correctly applying bank towards the live engine the ball is displaced a small amount (1 ball) towards the live engine i.e. in the direction of roll, and the string should be centred.

References: Extract from FAA/CAA PSM+ICR Turboprop training CD, ‘asymmetric flight’ and ‘Asymmetric flight at low airspeeds' by Tim Allen, BAE SYSTEMS safety magazine ‘JETSETS’ Issue 4; Jan 1997

I am requesting authority to attach the refs above to a future post

[4dogs]

Keith,

I thought that the "balance" ball was telling the truth in the OEI case: when wings level/ball central, it is telling you that the rudder force is balanced by the lateral forces generated by the sideslip and when flying straight, ie bank/ball into the live engine, the rudder force is unbalanced in the plane of the wings but is balanced in the horizontal plane by the lateral component of the slightly banked lift (TR) vector.

Stay alive

[3 green's]

morning all

i have some questions related to the post -

1)should we not be refering to a slip string as opposed to a yaw string?

2)for maximum rate of climb (OEI) would we "fly" the aircraft diffrently if there was no requirement to "maintain heading"

3) in a multi-engine piston twin with counterrotating props and both engins operating and developing the same power and in a "balanced level turn" is it correct to say the aircraft is not yawing?

4)given the need to minimise drag / power required for best (OEI)single engine climb performance ( with constant heading) - DOES ZERO SIDSLIP ALWAYS PRODUCE THE MINIMUM DRAG / POWER REQUIRED ?

5)I am starting to write a technical report on this issue for a intructor upgrade...does anybody have an good book titles, websites, articles they can recomend on (OEI) climb performance

thanks

[john_tullamarine]

.. at the risk of offending some of the purists ...

(1) aerodynamicists and piklots mean two different things when they talk about yaw .. I wouldn't sweat it too much, though. Probably "slip" or "skid" is more meaningfully appropriate for the pilot community

(2) however you arrange it, you need to get more grunt (throttle), less drag (nil slip/skid), and less power required for a given climb/descent rate (suitable speed). So far as balance is concerned (which is the thrust of your question, I suspect), the aeroplane needs to be manipulated so that there is no slip/skid and the flight path appears to be steady to the pilot with constant body angle and airspeed. The climb performance will deteriorate in a turn due to the increased load factor (similar to a level climb at higher gross weight)

(3) re (1), the aircraft is yawing from the pilot's perspective but there is no aerodynamic yaw of any significance if the flight path is constrained to have no significant slip/skid

(4) not so simple as just a matter of looking at one parameter. Certainly no slip/skid will improve performance.

(5) suggest that you have a look at the websites in the tech log sticky and also run some searches on OEI operations in tech log and flight test forums.

Looking at the original intent of this thread, it is important that pilots understand that the ball is reacting to the net force and really doesn't know a great deal about what the aircraft is doing. .. works pretty well in normal flight but needs to be interpreted with care OEI as it can, and will, tell fibs if you give it a chance .. hence the proliferation of slip or yaw strings in aircraft where performance needs to be optimised.

[Keith.Williams.]

4 Dogs,

In both the wings level and the slight bank towards live engine cases the lateral forces are in balance. With the nose yawed just enough towards the live engine the aircraft will sidelsip down its original track. Its lateral stability will cause it to try to roll away from this, but this can be prevented by using the ailerons to hold the wings. In this situation the aircarft down-track sideslip will increase until all forces are in balance. The ball will then be central indicating zero slip. But the aircraft will be in a constant velocity sideslip down the track.

If on the other hand, the rudder is used to hold the nose on the original heading, the rudder force will cause sideslip away from the live engine. This is usually balanced by banking slightly towards the live engine. This tilts the lift force so that rudder force and horizontal lift component are equal. In this situation the ball will be out towards the live engine but the aircraft will not be slipping.

So in both cases the lateral forces are balanced. In one there is sideslip while in the other there is not. If you interpret the ball as indicating sideslip, it tells lies in both cases. But if you interpret the ball as telling you whether lateral forces are in balance, it tells lies in the slight bank case only.

[john_tullamarine]

Keith,

You may have confused me here.

If the forces are in equilibrium, then the ball reverts to a simple pendulous indicator giving us some measure of the net force vector direction ... in the bank-into-the-live-engine scenario, the ball will slide toward the lower wing .. as we expect in the normal OEI climb out with wing down.

As I see it, it is a simple exercise ...

(a) after the initial failure dynamics are sorted out, in the wings level, constant heading case there is a remaining lateral fin force toward the dead engine resulting in a small lateral acceleration which is, in turn, shortly balanced by a opposing lateral fuselage force. The end result is a modest steady velocity toward the dead engine resulting in a small sideslip.

(b) to get rid of the undesired sideslip, one needs to generate a lateral motion toward the operating engine while maintaining heading... the only practical way to do this is to bank sufficiently toward the live engine to develop a sideslip sufficient to "cancel" the existing sideslip toward the dead engine ... at which stage we end up with nil slip, constant heading and the ball out a bit, typically half a ball, toward the live engine. The actual forces will be complicated by the need to balance the yawing moment due to the fin/lift vector couple.

Overall, I think that it is quite misleading to view the ball as a slip indicator as that is not it functional purpose. In normal operations we make inferences regarding the slip situation based on the pendulous indications of the instrument but it still can only give an indication of where the net force vector might be.

Perhaps I could ask you to revisit your post to assist .... ? .. or have I misread it and find myself talking the same tale in misplaced ignorance ?

[safetypee]

3 green’s

1. Yaw is angular motion about the vertical axis, you stop yaw by use of rudder. The string is an indication of the relative airflow in the lateral plane (sideways); the angle between the string and the aircraft longitudinal axis is known Beta. This is the equivalent of Alpha in the pitch plane.

2. For maximum rate of climb fly the aircraft as recommended by the manufacturer (normally V2), there should be no difference in flying technique between straight and turning flight. Be aware of bank limitations as the engine out climb speeds are usually slower than normal, thus there is a reduced margin from stall. Flying the correct speed is also important for maintaining control of the aircraft.

3. With wings level and steady heading there should not be any yaw.
In a turn there is a component of ‘yaw’, the nose moves round the horison. If there is a small beta angle then rudder may be required. Many variations in this area depend on the aircraft type, auto stabilization, etc, etc.

4. Generally yes, but there are always exceptions. Some aircraft with roll spoilers etc accept small side slip angles (beta) and climb with less than optimum wing down or even with wings level. This option may require a slightly higher speed than theory predicts, but with no need for roll control input, the spoilers remain in which gives lower drag and easier handling.

5. Still looking for an appropriate heavy weight book, but meanwhile try FAA-H-8083-3 - AIRPLANE FLYING HANDBOOK chapt 14, not much, but a start. http://pages.prodigy.net/jedinein/acs/faappselbook.html

[john_tullamarine]

safetypee,

Be aware that V2 is somewhat below the best performance climb speed .. hence the regular use of overspeed V2 takeoffs where there is spare runway available.

Any of the standard aerodynamics undergraduate text books will be the place to start if you want to confuse yourself with some mathematics ...

[Keith.Williams.]

John, I suspect that we are actually saying the same thing.

Whenever the lateral forces are unbalanced the aircraft will accelerate towards or away from the live engine. So whenever the aircraft is in a steady velocity sideslip, these forces must be balanced.

In your case (a) with wings level, the aircraft will be slipping towards the dead engine but the ball will be central. My slightly modified version of this is when the nose is then yawed towards the live engine just enough to make the direction of the slip to be down the original track. Once again the aircraft will be slipping but the ball will be central. If we interpret the position of the ball as an indication of slip (as in the original question in this string) then the ball appears to be telling lies in both cases.

You case (b) with slight bank towards the live engine uses a lateral component of lift to balance rudder force, such that lateral forces (relative to the earth) are in balance. This time there is no slip but the ball is out towards the live engine. Once again if we interpret the ball as indicating slip, we find that the indication is incorrect.

I agree that the ball should not really be interpreted as an indication of slip, but the fact remains that this is exactly how far too many pilot use it. The JAR questions ask for the ball position and not whether the indications are true are false. But almost without exception students initially get these questions wrong, because they consider the ball to be a slip indicator.

[john_tullamarine]

ah .. thanks, Keith .. your last post is crystal clear to me ... pity we can't all have Concorde gear so that the slip is presented directly ....

[Dick Whittingham]

Once you are well above Vmc you can fly OEI pretty much as you wish, wings level or not. It might be reasonable to trim out wings level and heading constant, in which case you would have some sideslip on, and aileron trim to counteract the natural lateral stability.

However, the cases referred to in the JAA questions are for Vmc, and if you want minimum Vmc you will have to adopt the wing slightly down toward the live engine state. Any other condition is either using rudder to generate unecessary yaw forces or has placed limits on aileron authority or performance.

Keith's intermediate stage will not give you min Vmc.

Thus, in the two states postulated in the JAA questions you are either wings level but have sideslip on and are moving laterally and not going where you are pointing but the ball is in the middle or you have bank on, no sideslip, no yaw and no lateral movement but the ball is off toward the live engine.

The whole point of the exercise is for pilots to understand how to fly at Vmc in the most critical case.

Dick W

[Bellerophon]

john_tullamarine

…pity we can't all have Concorde gear so that the slip is presented directly…

Ball

http://www.concordesst.com/inside/cockpittour/flightcontrols/pictures/c8.jpg

Sideslip

http://www.concordesst.com/inside/cockpittour/flightcontrols/pictures/c10.jpg

Both photos courtesy of, and copyright to, Gordonroxburgh, from his ConcordeSST.Com website.

Well yes, but they expected you to use it!

Bellerophon always found flying OEI in zero slip even harder than zero bank! :D

Sadly, no longer a problem.

Regards

Bellerophon

[Keith.Williams.]

Dick,

The JAR questions to which I refer included no mention of any particular speed nor indeed any particular stage of flight. They frequently appear in POF papers and the condition specified is usually something along the lines of "if ailerons are used to keep the wings level following a single engine failure, what will the turn indicator and ball show"?

The reason most students are unable to answer this question is because they apply a faulted line of logic. This usually goes something like...."If I use the rudder to control yaw, the sideforce will cause me to slip away from the live engine. I would normally prevent this by banking slightly towards the live engine. But the question specifies wings level, so I will still be sideslipping. If I am sideslipping the ball must indicate a sideslip".

The error of course occurred in the final stage of the argument. The ball has no means of detecting sideslip and is not intended to indicate sideslip.

Making the arbitrary judegement that the question is concerned with achieving VMC adds nothing to the process. As we all know too well, the student must answer the question asked, not the one that they wish they had been asked. For this reason I must disagree with your comment that "The whole point of the exercise is for pilots to understand how to fly at Vmc in the most critical case. When students are taking their JAR exams the whole point of the exercise is to pass those exams by answerring the questions they have been asked.

Although the method chosen in dealling with an engine failure will affect the subsequent performance of the aircraft, such matters are not directly related to the original question. It is for this reason that I have ignored them in all of my posts in this string.

I have of course no objection if discussion subsequently moves on to the performance aspects of the matter, but it is better to avoid confusing the two.

[safetypee]

The whole point of the exercise is for pilots to understand how to fly at Vmc in the most critical case.
Surely the points that the JAA should be examining are those concerning the safety of flight. By all means demonstrate VMCA and the pitfalls of incorrect control during training, but for examination it is the positive aspects that should be instilled and tested.

Engine failure: detect heading change / ‘slip ball’ displacement, arrest the yaw with rudder.
Fly the correct speed (not VMCA). Identify the failure, (feather the prop), apply wing down as required by the flight manual. Carry out follow up actions.

Thus it would be more appropriate to examine aspects of this generic drill. Is it too much for us (the industry) to request that the JAA focus on the realistic and positive safety issues, or is my position somewhere in excess of 15 degrees right on Bellerophon’s beta gauge.

[Dick Whittingham]

Keith

Well, I have to disagree with you. Yes, we have to teach students to pass the exams, but must not concentrate on rote learning to the exclusion of knowledge. We don't, and our students pass just as well as any others.

I have checked our feedback, and I am fairly certain that there is only one question on this, and it says something like "after engine failure the aircraft continues in straight and level flight with wings level. What will happen to the ball and turn indicator?" This, as you can see, is a very simple question.

I admit that I was surprised to find that this is so. Our tests have examples of the second (banking) case, but these are in there to illuminate the cross connection with the JAR certification rules and the bank rate limit on Vmcl for example. Our tests have this because we teach this.

A problem with analysing feedback is that it is an imperfect science. I, personally think you are putting too much emphasis on criticism of the question bank. There are howlers, but by and large it is sensible, and the howlers are being worked out by challenge.

Dick W

[john_tullamarine]

ah .. the discussion is livening up a bit ...

For the benefit of any newchums ..

(a) it is very important to keep in mind that Vmc/Vmca is a certification animal and not a great deal of use in the real world (unless you get down too close to it .. in which case it becomes a major driver to pilot strategy)

(b) it is not feasible to play with the AFM Vmc in the real world due to thrust variations

(c) if the pilot finds him/herself at or close to Vmc, then the bank into the live engine becomes a necessity. Unless you have documented advice to the contrary, it is safe to presume that the AFM Vmc data is based on the 5 degree-into-the-live-engine trick.

(d) be VERY wary of bank near Vmc .. the real world Vmc is VERY sensitive to, and dependent on, the actual bank angle. Acknowledging the limitations of flight simulators, it is always an interesting exercise to give the student a Vmc limiting takeoff failure ... if the bank is not well controlled (and this usually means aggressively ..) the aircraft rapidly banks into the dead engine .. the real world Vmc goes up past the actual speed .. and the outcome is foregone. A bit of practice corrects the technique errors but the lesson for the student is driven home strongly and very convincingly.

... and

(e) thanks to B for his picture-worth-a-thousand-words .....'

(f) as Dick suggests, for normal ops OEI, it is conventional and convenient to fly wings level and take a small performance hit due to whatever sideslip exists ... but note (d)

(g) having had a lengthy involvement in pilot theory training in years gone by .. I have to echo Dick's sentiments about pass versus education. If the knowledge is there, then the pass is a small additional effort to come to grips with the idiosyncratic nature of examinations .. but if the training has only concentrated on past papers, etc., then the pass might be there, but there is no depth to the knowledge ... the Industry and the examiners really ought to question what it is that the examinations are trying to do.

[Keith.Williams.]

Dick,

Nothing that I have contributed to this nor to any other pprune string has ever advocated rote learning. Indeed the entire focus of my contributions has invariably been a desire to provide explanations pitched at a level at which the typical student can handle them. A good many students who have previously studied at other schools come to me for help in preparing for examination resits. The vast majority subsequently comment that having spent time with me they not only know the WHAT, but also the WHY of the subjects covered. Such comments are not the product of rote learning.

The subjects of minimum control speeds and aircraft handling following single engine failure are a standard part of the JAR POF syllabus. I therefore deal with them in that manner. Before we get too wound up on what is or is not an appropriate aim for the JAR exams, we should remember that the advanced flying training carried out on completion of the written exams includes a period of theoretical and practical exercises which cover these subjects in great detail. You appear to be making the mistake of assuming that the ground school training is the only training. It is not.

When the time comes for providing guidance on examination technique, I remind the students of the advice most frequently voiced by the CAA examiners. "Answer the questions that have actually ben asked in the exam paper, and not those that you might wish to have been asked" Or to summarise "RTFQ".

The comments in my previous post related specifically to your assertion that, "However, the cases referred to in the JAA questions are for Vmc, and if you want minimum Vmc....". This is a classical case of failure to RTFQ. Any student faced with this question is quite simply wasting valuable examination time if he/she ponders the subject of Vmc. The question is actually about forces in flight and instrumentation, and should be addressed as such.

You comments "I, personally think you are putting too much emphasis on criticism of the question bank." is somewhat curious. I am not aware of having made any serious critiscism of the question banks in any of my posts to this string. I actually think that the questions to which we have referred are quite good in that they combine aspects of more than one subject and require the students to think.

Finally it might be worth looking again at my comments regarding the purpose of the student when taking the examination. From the moment a student walks into the examination room concepts such as learning or what should or should not be in the syllabus become irelevant. The sole objective of the student must be to pass that examination. Nothing else matters for the duration of that examination period.

[john_tullamarine]

... now that appears to be an entirely pragmatic approach to the whole thing ....

[alf5071h]

Notes on asymmetric flight here (http://www.pprune.org/forums/showthread.php?s=&postid=1099929#post1099929)

[safetypee]

The extract from the BAE SYSTEMS safety magazine JETSETS Asymmetric flight at low airspeed.pdf can be found here (http://uk.geocities.com/[email protected]/alf501h.htm) together with the Turboprop PSM+ICR files, thanks to alf5071h for hosting the file at his web site
Please note disclaimer from JETSETS magazine:
BAE SYSTEMS does not warrant the accuracy of the information contained in this document and accepts no liability for the misuse or misinterpretation of such information. It is published as flight safety information and does not supersede or replace any officially approved aircraft publication, nor does it constitute a warranty or condition of any contract. For definitive guidance, reference must always be made to Flight Manuals, Crew/Manufacturer's Operating Manuals and Maintenance Manuals as appropriate.
JETSETS is published solely for the purpose of improving flight safety and, unless copyright is indicated, articles may be reproduced providing that the source of material is acknowledged.

[newcrew]

hi there - am a bit confused

had a look at the notes on asymetric flight the diagrams (some) indicate that a "component of weight acts as a turning force" when a aircraft is flying with zero sideslip

can somebody please explain (thought weight always acted toward earths mass center through the center of gravity?

thanks

[alf5071h]

Several notes on asymmetric flight – revised pages Turboprob PSM+ICR and Asymmetric Flight (http://uk.geocities.com/[email protected]/alf5071h.htm)