Assymetric-How much difference between heading and track?

[one dot right]

Can anyone tell me by how much your track will differ from your heading when assymetric? Obviously this will vary according to your speed/power setting but i just wondered if there was some rule of thumb to use when trying to maintain a track when climbing out after the obligatory V1 cut,etc.
All replies gratefully recieved.:ugh:

[Notso Fantastic]

I would say if one was climbing out enexpectedly assymetric, having gone through the drama of pushing correct rudder in and getting the machine roughly balanced out, you haven't got the mental capacity left to think about difference between heading and track due to engine out! The wind will be affecting you too. Just fly the plane, and if you are drifting to one side, correct for that. It is an irrelevant question!

[Yellow Sun]

Can anyone tell me by how much your track will differ from your heading when assymetric?

It will be exactly the same as the difference between track and heading when flying at the same speed under symmetrical power. That is providing you have applied the correct amount of rudder to maintain balanced flight.

YS

[Maximum]

Yellow Sun

In asymmetric flight you are sideslipping, so heading and track do differ.

One dot right

No rule of thumb, depends on aircraft type and flight regime. As NSF says, in practical terms it's not really relevant. Just control the aeroplane and when you can use whatever nav aids are available to sort out track if IMC.

[Yellow Sun]

In assymetric flight you are sideslipping, so heading and track do differ.

OK, So why is the slip indicator (ball) in the middle?

YS

[bookworm]

In assymetric flight you are sideslipping, so heading and track do differ.

The challenge for asymmetric flight is to use the appropriate combination of rudder and bank so that you do not slip.

If you fly wings level you will slip into to dead engine. If you use too much bank into the live engine, you will slip into the live engine. If you get the combination right, you will not slip.

(The position of the balance ball will not be in the middle in a no-slip condition with asymmetric power.)

[northwing]

Hot tip for passing exams:- Learn how to spell "asymmetric".

[Maximum]

bookworm

I agree, by applying the correct amount of bank you regain balanced flight, I was just trying to keep it simple to answer the original question.

Yellow Sun

Interestingly, as bookworm says, with the correct amount of bank applied, the ball will be slightly off. It's just operating within the limitations of its design, ie, seeking the lowest point of the guage, which because of the bank, is no longer between the marks.

All the above is more relevant (but not strictly limited to) small prop twins. In a typical modern jet transport aircraft, once the engine failure is under control, then the aircraft is flown wings level, using rudder as appropriate. The result will be a small sideslip.

[compressor stall]

on lighter types the ball's half a ball to the live engine on climb out.

[bookworm]

All the above is more relevant (but not strictly limited to) small prop twins. In a typical modern jet transport aircraft, once the engine failure is under control, then the aircraft is flown wings level, using rudder as appropriate. The result will be a small sideslip.

Interesting. I've never flown a jet, let alone a "modern jet transport aircraft". Why the difference? Why fly it wings level? Because you can? For the better balancing of the G&Ts in the back? Or are there other issues?

[Maximum]

bookworm

You're basically correct when you say "because you can". Excess thrust combined with a large rudder make this possible in simple terms. (That's not to say an engine failure at V1 at max weight doesn't present difficulties - but that's another thread). Also initially up to 5 degrees bank into the live engine may be required until speed etc is sorted.

With the large amounts of thrust available in a modern jet, and the large speed range plus configuration changes and subsequent thrust variation and therefore much variation in rudder input required, it can all get a bit messy. Very easy to start overcontrolling. So why complicate it further with bank? It just wouldn't be practical. Hence we fly wings level.

It keeps things simple. When the aircraft is properly trimmed out for a specific speed and thrust setting asymmetric, then with the aircraft steady on heading, the wings will be level and the ball will be slightly out towards the live engine. If the correct amount of rudder is applied, the control yoke will also be level. If too little rudder is applied, the control yoke will be turned towards the live engine to hold wings level. Too much rudder and it will be turned towards the dead engine to hold wings level.

[john_tullamarine]

Several considerations for transport jets ..

(a) best climb performance is somewhere around 2-3 degrees bank (similar for any multi) ... not all that easy to fly in a high workload situation ... most of us oldies have trouble enough seeing the ball, let alone guessing bank angles between the marked index positions ...

(b) the variation in climb performance for small bank angles is not overly great

(c) some gyro systems will re-erect to a false vertical in prolonged flight at small bank angles

(d) the reduction in manipulative workload while handflying, I suggest, is the main driver for flying wings level. If the boxes are flying, then there is no reason why the system can't go for the best attitude.

[bookworm]

Thanks for the interesting perspectives.

(c) some gyro systems will re-erect to a false vertical in prolonged flight at small bank angles

They do? What sort of system would do that?

[CBLong]

Bookworm,

The air-driven gyro in a normal light aircraft AI needs to have some form of "vertical sensing" so that it can erect itself correctly when first starting The gyro's centre of gravity is below its gimbal's "rotational" centre (if you see what I mean) so that it hangs vertically when the gyro isn't spinning. So I guess it would make sense that, if held at a steady non-vertical angle for long enough, it would eventually decide that it *was* vertical...

(I'm no expert, by the way; it just happens that I was reading about this recently in the IMC Confuser - there's a lot more going on inside the AI than I'd thought!).

I once looked into the possibility of producing a cheap "bank angle" indicator to flog to sporting motorcyclists (they tend to rate each other's machismo according to the amount of lean they get round corners), but it seemed that keeping the system calibrated to true vertical during an hour-long ride, during which the bike might never be truly vertical for more than a second or two, would be a big problem....

Maximum,

I'm confused - you say:
When the aircraft is properly trimmed out for a specific speed and thrust setting asymmetric, then with the aircraft steady on heading, the wings will be level and the ball will be slightly out towards the live engine.

Surely the ball is just the simplest possible "vertical indicator"?? If the aircraft is in steady, unaccelerated, wings-level flight, how can the ball *not* be centred? It would be centred if both engines were running, would it not? How does the ball "know" that one engine has stopped...?

cbl.

[Maximum]

CBLong Surely the ball is just the simplest possible "vertical indicator"?? If the aircraft is in steady, unaccelerated, wings-level flight, how can the ball *not* be centred? It would be centred if both engines were running, would it not? How does the ball "know" that one engine has stopped...?

Because in the situation I have described, the aircraft is still slipping slightly. Try visualising an aircraft flying on one engine, where it's possible to level the wings with aileron, without applying rudder. The wings would be level but the heading would be changing towards the dead engine - you would be in a flat, out of balance turn (interestingly, a technique sometimes used by cropsprayers applying rudder to turn and just enough opposite stick to keep the wings level to go round an obstacle in the field).

Anyway, now imagine applying rudder to "kick" the ball into the middle if possible while reducing the aileron input to keep the wings level (remember the rudder will be helping to roll the aircraft as well now). Keep applying rudder until the heading is steady and check the wings are level. At this point the ball will still be slightly off. Think about the moment arm of the thrust vector with the engine out on the wing, and the vector of the balancing rudder force, and the point round which the aircraft yaws. You will see the aircraft heading and track will be slightly different - if you tried now to kick the ball further into the middle while still using aileron as necessary to keep the wings level, the aircraft would start turning towards the live engine, ie, you would have applied too much rudder. Phew.

[BOAC]

If brain capacity allows, ODR, try 5-8 degrees heading change (737). Less obviously for rear engines. It won't be right but it will help. As said elsewhere, keep an eye on NDB needles or whatever (ILS?) to make sure you are not heading into 'Indian country'

NB It can be important to try to maintain track in places like Salzburg 16 and Innsbruck.

[CBLong]

Maximum,

Sorry, I still can't see it. I'm not trying to be argumentative, honest! I'm a PPL with no multi-engine experience at all, so I'm just intrigued...

I can see why the aircraft would still be slipping slightly - if it weren't, there would be an unbalanced sideways force from the rudder. However, that doesn't seem strictly relevant - as far as the balance ball's concerned, the aircraft could be going backwards, but if (a) the wings are *exactly* level and (b) there are no other forces / accelerations involved (ie straight and level flight), then the balance ball *must* be centred, surely?

I would have thought that if the aircraft is slipping, even slightly, then the wings must be off level, abeit equally slightly - that's the definition of a slip. In the situation you initially described ("the ball will be slightly out towards the live engine"), would the AI indicate *exactly* wings level?

cbl.

[bookworm]

The air-driven gyro in a normal light aircraft AI needs to have some form of "vertical sensing" so that it can erect itself correctly when first starting The gyro's centre of gravity is below its gimbal's "rotational" centre (if you see what I mean) so that it hangs vertically when the gyro isn't spinning.

That's true as far as it goes. But when the aircraft is flying along in a straight line with an angle of bank on, the gyro remains vertical while the instrument rotates around it. Thus I don't see how the erection mechanism would affect it, even after prolonged flight in that state.

[CBLong]

Good point, bookworm... As I said, I can't explain the exact mechanism, but there's a lot more going on inside the AI than you might think, particularly to do with "pendulous vanes" and the like. The rotation of the earth over time might also be a factor...

From this PDF (http://www1.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/f98ed9b5360eccdd862569dc00720c3f/$FILE/ChapIV_pg36-40.pdf) (the rest of the book is here (http://www1.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/F98ED9B5360ECCDD862569DC00720C3F?OpenDocument&Highlight=instrument%20flying%20handbook) ) :

------------------

Another group of errors, associated with the design and operating principles of the attitude indicator, are induced during normal operation of the instrument. A skidding turn moves the pendulous vanes from their vertical position, precessing the gyro toward the inside of the turn. After return of the aircraft to straight-and-level, coordinated flight, the miniature aircraft shows a turn in the direction opposite the skid. During a normal turn, movement of the vanes by centrifugal force causes precession of the gyro toward the inside of the turn.
Errors in both pitch and bank indications occur during normal coordinated turns. These errors are caused by the movement of the pendulous vanes by centrifugal force, resulting in the precession of the gyro toward the inside of the turn. The error is greatest in a 180° steep turn. If, for example, a 180° steep turn is made to the right and the aircraft is rolled out to straight-and-level flight by visual references, the miniature aircraft will show a slight climb and turn to the left. This precession error, normally 3° to 5°, is quickly corrected by the erecting mechanism. At the end of a 360° turn, the precession induced during the first 180° is canceled out by precession in the opposite direction during the second 180° of turn. The slight precession errors induced during the roll-out are corrected immediately by pendulous vane action.
Acceleration and deceleration also induce precession errors, depending upon the amount and extent of the force applied. During acceleration the horizon bar moves down, indicating a climb. Control applied to correct this indication will result in a pitch attitude lower than the instrument shows. The opposite error results from deceleration. Other errors, such as "transport precession" and "apparent precession," relate to rotation of the earth and are of importance to pilots and navigators concerned with high speed and long-range flight.

------------------

Interesting stuff!

cbl.

[bookworm]

I don't doubt that's true CBL, but that's all about acceleration errors, that is where an acceleration changes the alignment of the apparent vertical. In the asymmetric flight case, we're wing down but with no acceleration. I don't see why you would get errors, or to be more precise, I don't see why you would get errors that you wouldn't get in wings-level flight.

I was hoping that JT might come up with an interesting mechanism... :)

[john_tullamarine]

I should dig out some old 727 notes but my recollection is that the VG would re-erect to a false horizon if prolonged bank were to be maintained ... some head-scratching here but 7-8 degrees max for the problem to exist strikes a chord in the memory .. I will dig out the systems notes next week and check ...

[ITCZ]

john tulla says....

(a) best climb performance is somewhere around 2-3 degrees bank (similar for any multi) ...

Banking away from the horizontal will never give you better climb performance. You are tilting the lift vector away from the vertical.

The "five to the live, half ball slip" is a Controllability (=Vmca) technique.

Think of the sine v. the cosine of an angle of 2-5 degrees.

Tilting the lift vector a small amount (2-5 degrees) results in a LOSS of climb performance, but negligible in a practical sense (cosine of a small angle approaching 1). But it does result in a small but useful turning force into the live engines (sine of the same small angle), effectively lowering Vmca for the same power setting. A very useful technique in piston twins and light turboprops. Rent a Duchess and a skilled Multi instructor and see the difference in controllability approaching Vmca with (a) nil bank and (b) 'five to the live.'

This is not usually required in most transport aircraft due to the rudder power available.

Also, the Duchess, etc don't have roll spoilers.

SOP in the BAe146 with one or two inop on the same side is to fly wings level, and apply rudder on the 'live' side to level the control wheel. A small amount balance ball left or right as appropriate will be indicated.

This also ensures that the roll spoilers, activated at about 3 degrees of control wheel deflection, are not deployed (and therefore not adding drag during one or two inop abnormals) except when rolling into and out of a turn.

[Tinstaafl]

ITCZ,

I disagree with your assertion that wings level will give max performance (asymmetric case). If the wings are kept level there WILL be some amount of sideslip with a consequential rise in total drag.

Min. drag will occur at around 2-3 deg AoB and that's where best performance will be found. Is that slight gain in performance necessary? Most times not, expecially in performance 'guaranteed' a/c. Occasionally (or perhaps, more likely) in FAR23 or equivalent types it can well mean the difference between descending or not.

Yes, 5 deg AoB is a controllability consideration, and is one of the conditions set by the certification authority to set realistic boundaries to just how low a Vmc is achievable. A very low Vmc is desirable by the manufacturer because of V1/Vr etc gains.