Here’s another technique related question.
I’m wondering how one is supposed to trill the (all-open) c# with the (middle) d.
I suspect it may be doable with a Meyerish flute with the extra high keys, or when you have a C foot. In my case it’s a regular D foot, though. It’s ugly 
On my flutes OOO XXX still plays an in-tune C# so I do a two finger trill of that and OXX XXX. It isn’t the greatest, but it works!
Likewise trilling Cnat to D I do a three finger trill of: OXX OOO to OXX XXX.
Hi
You can use the “low C#” key an octave higher which is effective especially in quick passages. (If you have an eight key)
H
The Rub circa 1987 http://www.box.net/shared/o13ofj007h Mazurka, Nip & Tuck (*Holmes) & Single Slip, The Hump (*Matthews) - Hawkes Eb Pratten type, serial 9714.
H&Mc Blackwood prototype D, Reel in D http://www.box.net/shared/evf8qpuoj6
OK well, that’s pretty much as I thought.
As stated I have no C foot. I had tried Sigurthr’s approach but that was pretty awful It sounded rather “flapping” because of the octave transition. Anyway, no biggie, just wonderin’.
(I’m working on the Air from Suite No. 3 in D from J.S. Bach. Nice practice piece for the higher notes, and for baroque fingering. The trill is only on the second last note.)
I checked the trill chart in my copy of “Method For The One Keyed Flute” by Janice Bolan to confirm the fingering that Sigurther gives above. If you are digging into the Baroque repertoire, this is a great book to get you going. One of the things about the ornamentation from that period is that they don’t always sound “right” to our modern ears that have grown up in the world of equal temperament. Many of the trills are audibly “wider” than we are used to hearing now.
This trill also can start to sound better if you really focus on breath support while playing it.
Clinton
Just another support for trilling L2&3 on the vented middle D for a C#-D trill - this is the orthodox 8-key fingering as well as that for baroque flute, and it is actually pretty well in tune and strong/even on most flutes and whistles - the C# is only very marginally flattened by the R hand fingers remaining closed. In my experience it works very well - no “flapping” or over-the-break difficulty in getting both notes to sound properly, nor does it matter much which way you approach it (first note D or first note C#). You probably just need to practice good, clean finger co-ordination on it - it shouldn’t need embouchure gymnastics.
If by any chance you want the trill the octave above, play the normal high D (oxx ooo,) and trill R 1 (oxx xoo, is the “standard high” C# fingering anyway and usually bang on in tune whereas the open C# at the top of the 2nd 8ve will be flat on most instruments as well as a harder trill).
Ah, so I have to practice 
Noted.
I agree that it doesn’t require embouchure gymnastics. Both sounds pop in by themselves, albeit a little “late” compared to, say, a b/c# trill. I guess that’s parallel to being unable to slide c#/d like you can b/c# – i.e. there are no “inbetween” states that make the trill sound more natural. But I can see how practice makes (more) perfect, indeed.
Update: there may be hope I’ve figured out the difference between a mordent and a trill, and I believe this particular note to be a mordent. That’s just a single alteration between the notes – should be doable 
Thanks also for the book reference; looks interesting. Currently I’m just constantly looking out for the next piece to to “one-up” my level, one piece at the time. I am considering taking some lessons to get some of the basics more solidly in, though. I know of both one Irish and one Baroque teacher in my greater area; the first I’ll meet in a workshop in February, the second, we’ll see.