Owain,
Originally Posted by Owain Glyndwr
Well not entirely d'accord
I have really nothing against Davies formulation ! Thank you for quoting it here. The only thing I see in my method (perhaps Davies did it for himself but did not express it here) is to get quantification instead of global words which can be misunderstood by pilots who revendicate to be stupid...
My bold show the words I did quantify :
Originally Posted by Davies
The control of divergent dutch roll is not difficult so long as it is handled properly. Let us assume that your aeroplane develops a diverging dutch roll. The first thing to do is nothing - repeat nothing. Too many pilots have grabbed the aeroplane in a rush, done the wrong thing and made matters a lot worse. Don't worry about a few seconds delay because it won't get much worse in this time. Just watch the rolling motion and get the pattern fixed in your mind. Then, when you are good and ready, give one firm but gentle correction on the aileron control against the upcoming wing. Don't hold it on too long - just in and out - or you will spoil the effect. You have then, in one smooth controlled action, killed the biggest part of the roll. You will be left with a residual wiggle, which you can take out, still on ailerons alone, in your own time.
Don't attempt to correct the manoeuvre with rudder; as explained, the yaw is often suppressed and it is difficult to work out which way to apply the rudder at what particular moment and there is a good chance that the wrong rudder will be applied which will aggravate things very quickly. It is not difficult, however to apply the correct aileron control. Further, don't attempt to squash the dutch roll flat in one fell swoop but be content with taking out a big bite first time then sorting out the remainder next time
I am seeing too that Davies's method is a step by step method effectively as you said it it can be seen as on open loop action, mine is continuous in the closed loop.
When Davies says "watch the rolling motion", I do the same but quantifying. Despite Davies's lack of precision we both know that we must read in the beginning of the transient response the clue to start the correction before the transient part is finished. He does not suggest to the pilot to start asap the correction, but when he feel "good and ready". But I understand that prudent formulation and choice of Davies, so long as pilot don't want to reckon some precisions... :
I know very well that if a pilot decides to actively control a dutch roll using aileron then unless he properly adapts his internal transfer function to match the open loop dynamics of the airframe there is a risk that at some value of pilot gain the root locus of the closed loop system might cross over into the negative damping region.
Exactly ! Probably my teacher had a sudden thought to that risk at the moment he was speaking of root locus and decided him to say his sentence.
It seems that Chris's method on B727 is Davies's one like but less developped in the formulation I found in the Yaw damper thread ?
Why did Learjet not adopt Davies's method and stay wrong with the rudder use, and reversal use, and brutality use ?
Stopping the bad divergent oscillation is not all : probably your aircraft wants to play it again ! that divergence had a cause, and watching the motion quantatively helps to fly safe at home avoiding a new divergence with your jammed yaw damper...
Here I do disagree with you. The object of certification is to avoid any resonance not devise rules to tame it
The final object of certification is to do the aircraft safe for the wanted use. Today we have to increase air safety despite crashes statistics are getting better. All the crashes which can be avoided have to get progressively suppressed. Changing certification rules can perhaps help, and if it should help why not try ? Perhaps it could improve the speed of information from engineers to pilots ?
Writing operational rules may sometimes be included in certification. I remember when Concorde has been built some people said the bang should not be listen on the continent, so it was not possible to certify it. But thinking that the sound energy was decreasing with distance finaly it could be certified if respecting operational rules like not focalising turns, and respecting some distance of the coasts, and determining quantitatively point at which Concorde should start acceleration to M 1.