Fast arpeggios

[cocoazul]

Hi I’m new here and my English is not very good, I'll try to explain as best as possible.
I need help with fast ascending arpeggios in the right hand.
The problem is, for example:
when playing arpeggios quavers (Eighth notes), it sounds like triplets

I am currently studying Mendelssohn op 53 no 3. and I'm having this problem at the end of the piece, in the last seven measures (right hand)
the arpeggio is: D G ,  B-flat D G B-flat D G B-flat D, then descends
of course, That is because I’m accentuating with my thumb, but could someone tell me how to study
I hope you understand and help me
again, sorry  for my English, I used a little Google translator

[m1469]

There are 5 basic note-groupings, regardless of metre, which form music's basic rhythmic language, those are:

1.  One note per beat
2.  Two notes per beat
3.  Three notes per beat
4.  Four notes per beat
5.  Six notes per beat

We generally develop the first four fairly well throughout our study, whether by specific intention or subconsciously through repertoire study.  There is a physical and mental language that we become accustomed to as we translate these note-groupings into technique on the piano, and most commonly, we are thinking in terms of 16th notes = category 4, and when we think of or are taught about triadic arpeggios, we are thinking in category 3.

The figuration you are talking about is not either of those, but is in group 5.  In 6/8 time, we have to be able to structure our musical thoughts into two big beats per measure, and in the case of this piece, we need to be able to understand what a note-grouping of 6 sounds like and feels like (even if it includes a rest).

In order to understand the language of note-groupings in those 5 ways in a very fundamental way, I find it useful to practice chromatic scales in all 5 note-grouping styles, with the beat remaining constant throughout (starting in category 1-5).  When first developing the feel for how the notes are grouped, it can be helpful to accent the beat (the first note of every grouping).

Once you have a feel for where an accent falls and where it doesn't, you at least have a rhythmic structure to help map the sound around.  If you really grasp the rhythmic structure, that in itself should help, but then remember that there is a metre to be thinking about, as well.  In 6/8 time there are two big beats, beat #1 having more emphasis and "pull" than beat #2, so beat #2 won't include the same type of accent that beat #1 will.

You will want to be able to demonstrate the basic rhythmic structure slowly, first, and once you are really hearing it and feeling it as a whole group, you can speed it up.

[j_menz]

What m1469 said.

[cocoazul]

Thanks for your time and help
That will help me a lot