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CG location effect on asymmetric yaw

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CG location effect on asymmetric yaw

Old 28th Dec 2020, 15:48
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CG location effect on asymmetric yaw

Does the magnitude of asymmetric yaw resulting from a failure vary between a FWD CG and AFT CG or is it not dependent on CG location at all?

Some sources say fwd/aft cg makes no difference and all that matters is distance between thrust line & longitudinal axis. Other sources however say there is a difference and a FWD CG causes less asymmetric yaw upon failure due to being closer to the engines.

Thanks

*I am not talking about CG location's effect on rudder effectiveness, purely its effect on asymmetric yaw.
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Old 29th Dec 2020, 07:27
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So long as we reasonably can assume that the lateral cg is constant, there is no thrust vector offset from the longitudinal axis, and that steering tyre loads don't change with a change to NW load, the yawing moment will depend on thrust and thrust offset. Longitudinal CG should have no effect.

I don't suppose that your source suggested just how the CG might cause an effect ?
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Old 29th Dec 2020, 08:51
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I was silently hoping you would appear, John
The other source was someone claiming they had selected the answer "CG FWD reduces asymmetric yaw" in an EASA ATPL exam and it appeared as correct.

Thanks for the clarification

Last edited by A320LGW; 29th Dec 2020 at 09:39.
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Old 29th Dec 2020, 09:55
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“On speed, into the trees”
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Old 29th Dec 2020, 10:22
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It seems to me that fwd lateral CG should reduce Vmcg coz of the rudder's longer effective lever arm. But experience tells me that this is for the laboratory, in the real world it will disappear into the static. Think engine trim, density altitude inaccuracies, tight wheel bearings, slight brake drag, slightly underinflated tyres.
So I say yes but you won't ever see it in the real world. If my thinking is off base JT, please correct my thinking here.
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Old 29th Dec 2020, 11:21
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On the ground more weight on the nose wheel will reduce the swing of an engine out, is that what they meant?

On most Airbus types you apply forward stick pressure (half or full with tail or x-wind) during the take off roll to increase the nose wheel load so forward CofG can only help I would have thought.

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Old 29th Dec 2020, 12:09
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Well it's certainly some food for thought. I admit that initially when I was posed the question by the individual I agreed with them and my own response was FWD lateral CG would reduce the asymmetric yaw. I was thinking surely if the CG was say over the wings (very close to underslung engines) and if it was way AFT by the exit door (for argument's sake) that these 2 varying distances from the engine would have some difference in the resulting 'swing' upon failure. This appeared as correct to them in the exam so we thought it was all kosher, though it should be added it wouldn't be entirely unlike EASA to have some wayward 'correct' answers.

Being me I then researched and followed it up and found a document which stated thrust line to longitudinal axis was the only factor in asymmetric yaw along with thrust setting and lateral stability.

Regarding nosewheel load, I can see where you're coming from and it seems to make sense. The question could perhaps be a bit clearer as to whether it's referring to an inflight scenario or on ground.
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Old 29th Dec 2020, 13:08
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There may be a clue from the question wording; 'CG' would relate more to the airborne case than on the ground.

AC 25-7 may add insight, (page 104 onwards), but I suspect no answer because the wording relates to the most adverse CG, which the manufacturer has to identify in the first instance.

The simple note below may help; cg is considered, but this involves rudder.
https://pdf4pro.com/cdn/asymmetric-f...org-1a9243.pdf
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Old 29th Dec 2020, 19:49
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It seems to me that fwd lateral CG should reduce Vmcg coz of the rudder's longer effective lever arm.

Absolutely, but this was excluded by the OP in the question. Additionally, we need to keep in mind that, while the effective Vmcg on the day will vary, the certification Vmcg, which is the basis for the AFM stuff, is a single number representing a reasonably worst case scenario.

Think engine trim, density altitude inaccuracies, tight wheel bearings, slight brake drag, slightly underinflated tyres.

Concur, but these aren't really relevant in a simple comparison as one wouldn't expect any of them to change, say, from one takeoff to the next in a short time frame ?

On the ground more weight on the nose wheel will reduce the swing of an engine out, is that what they meant?

Indeed, which led to my including that caveat.. As safetypee observes, there is a disjoint between the typical real world day (which should be conservative to some extent) and certification practice.

However, for real world situations, the load on the nosewheel is a small fraction of the aircraft's gross weight so a small change in load should result in a much smaller change in tyre/surface coefficient which, I suspect, will tend to get lost a bit in the face of other, larger forces. Safetypee, being of the FT persuasion, probably is better placed than I to speak to that consideration.

Caveat - the nosewheel load (and reject braking) goes up significantly in the case of nosewheel brakes (thinking B727).

I was thinking surely if the CG was say over the wings (very close to underslung engines) and if it was way AFT by the exit door (for argument's sake) that these 2 varying distances from the engine would have some difference in the resulting 'swing' upon failure.

A common misconception. We are thinking about moments and the consideration is the perpendicular distance to the force being considered in respect of the moment. The picture in the following link may help a little Moment of a Force | Engineering Mechanics Review at MATHalino As you observe, you found a reference which said the same thing. Probably a bit simplistic to suggest that the thrust offset is the only consideration but, certainly, it is the main driver, I suggest.
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Old 30th Dec 2020, 09:20
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What if the thrust line is not parallell to the a/c hartline? If the engines have toe-in or -out, then there is an additional vector. Then the position of c/g will have effect (slightly).
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Old 30th Dec 2020, 09:54
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RTQ.

It is just basic Physics,

End of discussion.

(Washout has a point, however in the ATPLs only factors mentioned in the question are to be considered)
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Old 30th Dec 2020, 10:17
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If the engines have toe-in or -out, then there is an additional vector.

Indeed, which is why I included that caveat. Not so much an additional vector, rather the vector components need to be considered. The effect could be reasonably significant between the forward and aft limits.
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Old 31st Dec 2020, 10:07
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Sorry John, misunderstood your words.
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Old 31st Dec 2020, 11:34
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John
However, for real world situations, the load on the nosewheel is a small fraction of the aircraft's gross weight so a small change in load should result in a much smaller change in tyre/surface coefficient which, I suspect, will tend to get lost a bit in the face of other, larger forces

The tyre/surface coefficient won't change appreciably, but the ability to resist yaw is coefficient times NW reaction, and a change in nosewheel load from (say) 4% to 6% TOM is a big increase in NW yawing moment potential even though both are relatively small fractions of gross weight.There has to be a reason for recommending forward stick during TO run.

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Old 31st Dec 2020, 11:55
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'It is just basic Physics,' or is that mechanics

RTQ,
But what is the question; if taken from an EASA examination then what wording might constrain situational thoughts, and if so, to what purpose.

Knowing the relationship between yaw and CG appears to have little practical value.
However, in actual conditions the need to control the aircraft with rudder, which could be influenced by CG, then crews should have this knowledge - and of many other operational factors depending on the situation. e.g. stick fwd, crosswind, runway width / surface condition.
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Old 31st Dec 2020, 12:42
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How will CG affect asymmetric yaw?
A CG FWD will increase yaw
B CG AFT will reduce yaw
C It is not dependent on CG
D CG FWD will reduce yaw

As an aside, could yaw damping be relevant? Would it be accurate to say the damping moment created by the vertical stabiliser depends on CG position and damping increases with FWD CG (due to the arm) therefore asymmetric yaw decreases? Could this be a reason for them to say D is the correct answer as opposed to C? It seems to me to be a jump from what is standard as a basic MEP has no yaw damper but I can't think of much else.
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Old 31st Dec 2020, 20:00
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The tyre/surface coefficient won't change appreciably, but the ability to resist yaw is coefficient times NW reaction

OG, wonderful to see you in the thread, good sir, and a Merry Christmas and Happy New Year to you and yours. I should have been a little more expansive in my comment to add clarity, I guess - concur with your thoughts .. and the reason for NWS requirements for Vmcg tests, of course.

How will CG affect asymmetric yaw?

The actual question and its inferences have changed, somewhat, the thoughts of the original post. The question now doesn't appear to be trying to exclude considerations of rudder forces and appears to be considering the yaw magnitude rather than the initial yawing moment ? I suspect damping is not a consideration in respect to the question's detail as we don't consider any delay in rudder application in the event of a failure, ie the expectation is that pilot rudder response with be instinctive and without any certification driven delay. Further, as you observe, unless the question were to quarantine consideration to aircraft so equipped, the majority of Types don't have any YD kit in their toolbox.

With knowledge of the specific question, I would now include consideration of pilot rudder input and we can discard (a) and (b) without too much head scratching. (c), I suggest, was relevant to your original question description but, with consideration of rudder input now can be discarded. That leaves (d) and it looks to be a reasonable option to me.

I will be interested in reading, and certainly will defer to, safetypee's observations. For those who don't know who he and OG might be, they both are extremely well-placed to be heard on this sort of question, the former especially from the practical, and the latter from the theoretical, sides of the house.

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Old 31st Dec 2020, 23:06
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Great thread to get away from the 737 stuff, although I started the RTS one here.

The whole thing about the yaw due to motor and CG is very complicated. It all comes down to moments and the aero surfaces to keep the pointy end fwd.

It would seem that using "toe in" on some planes with so-called centerline thrust would be a good idea. I don't know how to help yaw with motors on the wing out there 10 or 15 meters, especially considering the pitch moment they exert. Seems to me that the yaw would be more evident than the pitch. Oh well....

I flew one neat jet that FAA certified me for "centerline thrust", but that sucker would kill you if you were on one motor and tried TOGA without getting a gob of rudder in before cramming the one throttle full fwd. It was our first loss, and we had another a few years later for a go around on one motor. Cessna actually angled the engines in toward the average ceegee, but that motor was much more than the aero folks figured.

Great thread and I hope to learn sonme stuff.
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Meanwhile, I wish all a prosperous new year and hope y'all had a cosmic Christmas.
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Old 1st Jan 2021, 07:50
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JT, with fast fading memories (except of those 'testing' visits to the grass) there is little more to add technically to your views (D).

However, interest remains as to why the question is asked in examinations, why contextual ambiguity - ground/air, fin damping, rudder control, ….
To what objective.
What are pilots expected to recall as a result of the question, and most importantly does this mask what is really important in everyday operation.

A cynical view would entertain that those setting the question did not have the same understanding as reflected in this discussion.

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Old 1st Jan 2021, 08:59
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One thing I miss in the discussion is the gyroscopic effect due to engine rotation on yaw in flight. I was triggered by Gums remark on the need for rudder input in the case of TOGA with centerline thrust. Any thoughts on that?
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