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CG location effect on asymmetric yaw

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CG location effect on asymmetric yaw

Old 1st Jan 2021, 16:54
  #21 (permalink)  
 
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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.

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Old 2nd Jan 2021, 16:41
  #22 (permalink)  
 
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Originally Posted by A320LGW View Post
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.
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Old 2nd Jan 2021, 19:46
  #23 (permalink)  
 
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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

Last edited by gums; 2nd Jan 2021 at 23:04. Reason: corrections
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Old 3rd Jan 2021, 00:12
  #24 (permalink)  
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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.
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Old 3rd Jan 2021, 08:33
  #25 (permalink)  
 
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Originally Posted by gums View Post
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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