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-   -   CG location effect on asymmetric yaw (https://www.pprune.org/tech-log/637713-cg-location-effect-asymmetric-yaw.html)

A320LGW 28th Dec 2020 14:48

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.

john_tullamarine 29th Dec 2020 06:27

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 ?

A320LGW 29th Dec 2020 07:51

I was silently hoping you would appear, John :ok:
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

Gin Jockey 29th Dec 2020 08:55

“On speed, into the trees”

mustafagander 29th Dec 2020 09:22

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.

Max Angle 29th Dec 2020 10:21

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.


A320LGW 29th Dec 2020 11:09

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.

safetypee 29th Dec 2020 12:08

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

john_tullamarine 29th Dec 2020 18:49

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.

washoutt 30th Dec 2020 08:20

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).

Capt Pit Bull 30th Dec 2020 08:54

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)

john_tullamarine 30th Dec 2020 09:17

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.

washoutt 31st Dec 2020 09:07

Sorry John, misunderstood your words.

Owain Glyndwr 31st Dec 2020 10:34

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.


safetypee 31st Dec 2020 10:55

'It is just basic Physics,' or is that mechanics :ok:

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.

A320LGW 31st Dec 2020 11:42

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.

john_tullamarine 31st Dec 2020 19:00

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.


gums 31st Dec 2020 22:06

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.
===========================
Meanwhile, I wish all a prosperous new year and hope y'all had a cosmic Christmas.

safetypee 1st Jan 2021 06:50

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.


washoutt 1st Jan 2021 07:59

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?

gums 1st Jan 2021 15:54

Ya know, Wash, some of the yaw we experienced in that little jet ( A-37) might well have been gyroscopic precession. However, most of us concluded that it was the awesome increase in thrust from the original design ( hmmmm, thinking of a certain two motor jet with increased power and a different placement WRT the original airframe, but worst effect was in pitch, not yaw or roll).

For those not familiar, the original model had two engines that put out a little less than 1,000 ld thrust. The attack model engine put out 2800 lb !!! Yikes. So just one engine gave us 50% more power than both of the trainer engines. It's why we cruised on one engine a lot.

I am fascinated by the diverse contributions/opinions on the whole thing I see on this thread.


LOMCEVAK 2nd Jan 2021 15:41


Originally Posted by A320LGW (Post 10957773)
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

This is a very ambiguous question. In this context, precisely what is meant by 'yaw'? Is it yawing moment due to the thrust asymmetry ie. an instantaneous value with no yaw rate present? Is it the yaw rate open loop following an engine failure? Is it the moment which must be overcome with rudder in order to stabilise with constant heading? It would be far more meaningful if the question related to the effect of CG position upon Vmca and Vmcl (or equivalent parameters, however named). Vmcg is then a separate case due to the forces and moments from the landing gear.

As someone has already said, it would appear that whoever set the question did not fully understand the subject.

gums 2nd Jan 2021 18:46

In all fairness, the person posing the question may not be a Edwards or Pax River test pilot grad, or even have a college degree in aero, or may not have more than a few hundred hours in real planes. So understanding of all the "fine" points of the subject should be excused.

The question may have included the cee gee for some reason or other, maybe an incident of concern to the poster. My training 60 years ago was that in twin motor planes you had to be careful if operating on only one. Ya know, the old saw about rolling into the dead engine. Bob Hoover did it on his routine I saw in a P-38 to demo that skill and experience made a lotta difference on the outcomne.

As far as the question goes, seems like the fellow was asking about nose movement left or right when sitting upright in the front seat. So answers/contributions should be referenced to that, huh? Some here have gone thru gyrations that the other 95% never saw or even dreamed of. Those few folks can discuss with each other the very fine points of aero and touch and feel and control inputs that the other 95% who have not pulled 9 gees or did a tail slide or even had an engine failure won't need to use until that "one" day and situation. Those 95% of most pilots should just file the neat stuff away, and from personal experience, I can assert that the hints and clues come back real fast. It is why I rehearsed bad situations in my feeble brain while waiting for a haircut or hoping my wife came out of the doctor's office O.K. When the instant came, I just replayed my actions...... musta worked, 'cause I am here posting, heh heh

john_tullamarine 2nd Jan 2021 23:12

especially considering the pitch moment they exert. Seems to me that the yaw would be more evident than the pitch

Couple of additional things to consider. Especially at very low speed, high alpha, and with a fistful of thrust (did someone mention the miss from the flare ?) we can get two additional sources of unwanted excitement and which can impact adversely on pitch and yaw stability.

First, on props, the P-factor moves the net prop thrust position laterally. If this be outboard, it can worsen yawing considerations, whether singles or multis. Indeed, in a big, hairy, multi-thousand HP military bird with one motor, one could find a situation not unlike Vmc in a multi.

Second (props/jets), the high alpha results in a significant turning of the incoming airflow either into the prop disc or the jet's nacelle. Just like with a wing, this change in direction provides a change in momentum which gives you a vertical force (lift). In the case of the engine, this vertical force might be well out in front of the wing and provide a significant nose up pitching moment. Generally referred to as the prop/nacelle normal force. For piston to turboprop conversions, where the engine is pushed out further for static CG considerations, the effect can be sufficient that the pitch characteristics in the miss become unstable and the aircraft needs a SAS mod to fool the pilot into thinking things are still stable.

Obviously, if we consider slipping/skidding airflow, the pitch problem can become a yaw problem.


some of the yaw we experienced in that little jet ( A-37) might well have been gyroscopic precession

Sure could but consider the nacelle normal force as another option ?

most of us concluded that it was the awesome increase in thrust from the original design

Very relevant - consider piston to propjet conversions and jet engine growth. I suspect that some of the MAX's problem was associated with nacelle lip lift.

This is a very ambiguous question.

The joys of pilot theory training ....

It is why I rehearsed bad situations

.. and why many, who didn't, are no longer here to be able to offer thoughts on this and that.

LOMCEVAK 3rd Jan 2021 07:33


Originally Posted by gums (Post 10959317)
In all fairness, the person posing the question may not be a Edwards or Pax River test pilot grad, or even have a college degree in aero, or may not have more than a few hundred hours in real planes. So understanding of all the "fine" points of the subject should be

I agree that the depth of the question should be appropriate to the level of experience of the person taking the exam and the level of knowledge that they require but it must not, within any context, be ambiguous. The answers can have subtle differences if you wish but there must be just one clear answer. This is not the case with the question presented here.


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