Playing in a different key
General mountain dulcimer or music discussions
Nigel, others have provided some good information here, and I hesitate to add to the mix. You know what they say about too many cooks . . .
The most common keys in old timey and bluegrass jams are C, D, G, and A.
With your dulcimer tuned DAd or DAA, you are obviously in the key of D. You can put your capo at the third fret and you will be in G. You can put the capo at the fourth fret and play in A. In both those cases, the fingering youhave already learned will work fine, but you will simply be in a different key.
For C, tune your dulcimer one note down to CGc of CGG. Once again, everything you already learned for DAd or DAA will work except that you will now be playing in C.
Paul is correct that you can tune DGD to play in the key of G, and many people do that for certain songs. But the fingering will be different from what you are used to.
As Carey and Strumelia have explained, since the dulcimer is diatonic, the capo is more compicated than it is on chromatic instruments such as the guitar. You cannot simply put it on the first or second frets to change keys as you will also be changing modes. There are uses for the capo at those keys, but the fingering you already know will not work.
However, if you want to play your diatonic dulcimer in multi-instrument jams, you should get used to tuning to CGc or CGG and using the capo at the third and fourth frets. Here is a Bing Futch video where he teaches the fiddle tune Hangman's Reel in D and then demonstrates how to play it in G and A using the capo. (And if you listen during the closing credits you will hear a "minor" version of the song which is the same fingering but the capo at the first fret.)
It is true that the law only guarantees that you can bring your instrument on board if it fits in the overhead compartment. However, that is more than is guaranteed for other carry-ons which are limited in size.
At a dulcimer festival this past summer I asked Aaron O'Rourke how he traveled with his dulcimers. He uses a David Beede dulcimer with a radiused fretboard whose precise curve is customized and a Clemmer banjammer that has also been fitted with a custom bridge and nut to bring the strings closer together. Neither is easily replaceable, in other words. He puts them in one of those double gig bags which he then puts into one of those huge hard plastic (SKB?) containers made for a set of golf clubs. He surrounds the gig bag with clothes, towels, and other items to make sure it can't move. Since the airlines are used to those large golf bag containers, they seem to handle it OK, though sometimes it costs extra due to its size.