Dutch Roll testing
 

Question for the house.

I'm doing a bit of work on some mods to a high wing twin-turboprop. The company schedules call up aileron-doublets as well as rudder doublets for DR tests. The type seems to have pretty standard characteristics - can anybody suggest any reason (other than perhaps some over-enthusiasm at some point) why somebody should do both tests? My experience is that the only time One schedules roll-doublets for DR investigation is when the aircraft has inter-linked roll and yaw mechanisms, and then I wouldn't conduct a rudder doublet.

G

Genghis.

Happy New Year (probably the most significant bit of this post&#8230 ;)

I have hung back from replying hoping that somebody had something of real import to put to you. Now it has got to the point that I am afraid you would sooner have any reply than none!

You say the “company schedules” call for the aileron doublet. Just what schedule are we talking about? A flight test schedule submitted to a licensing authority with a view to certification a mod?

 If so, then the following (in no particular order) may have all occurred to you.

 Perhaps the writer is not experienced in these matters.

 Perhaps the writer is very experienced in these matters and has been caught out by some unusual aircraft in the past

 Perhaps the aircraft is known for possessing a large adverse aileron yaw term

 In the grand scheme of things it won’t take a moment to do so what the hell

 If the aircraft has split controls it may handle in quite an unusual way when the ailerons are used one at a time. (I don’t know what certification requirements have to be met in what is an emergency condition – but I have vivid memories of doing a full rudder sideslip with the ailerons disconnected in one current twin engined turbo prop mil/civil transport at cruise speed only to find that I could then abandon all the controls and they stayed just where they were…ummmm)

JF

Thanks for your thoughts John, yes it's a company schedule for CAA approval. I've been working on it since I posted, and I think now that it's a case of slavish reproduction of the code without full understanding.

T'would appear that the old BCARs D and K, gave a rather vague reference to clamping two of the three primary controls and then doing a doublet on the other. I suspect that's probably something passed down through various revisions before finally being eliminated by JAR/FAR 25 & 23.

One does tend to see this on certification reports and schedules by people without a full understanding - a tendency to write schedules around the standard rather than the aeroplane. However, the incredibly vague wording re: handling in any of the civil standards is probably paying your and my mortages, so I shouldn't complain too loudly.

Happy new year to you as well.

G

Interesting thread on dutch roll testing. I wonder if going back to basics will shed any light on the discussion. I thought that DR was driven by sideslip since direction response is a second order system, and lateral response a first order system. With that in mind you could trigger a DR response via any mechanism which creates sideslip. These include rudder doublet, release from SHSS, turbulence or aileron inputs (if AAY is bad).

I thought that the test fraternity settled on a rudder doublet as the chosen DR test as it allowed a nice DR response with AoB variations about wings level, thus negating any spiral stability effects. Easy to fly, easy to count and time. However not the only means to excite.

Am I missing something here?

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Precisely, that was why I was surprised to see both.

The only time personally that I've ever used anything but rudder doublets to excite DR is when an aircraft has no rudder. In view of that (and a very conventional control system) I shall go back to mother, and just do rudder doublets until somebody gives me a good reason to do otherwise.

G