I have been wondering about something I read recently about drawing diapasons and flutes together, and hope that someone may be able to provide an explanation. Going through some ancient things in The Musical Times, I came across a letter to the editor in vol. 77 no. 1120 (June 1936), written by Henry Willis III in which he stated as follows at p. 543:
"It can be laid down in the most definite manner that Harmonic Flutes do not blend with Diapasons, that any open flutes are undesirable, but that correctly voiced Stopped Flutes are capable of successful combination. Why? The reason is simple. Open flutes have various upper partials present in varied order of prominence: many, when developed in a flute, are inimical to those of a Diapason, With a Stopped Flute the odd numbered partials only are present, and the blending power is correct." (Emphasis original.)
Now, the only Willis instrument of any size that I've ever had any opportunity to play had both a Claribel Flute 8' and Harmonic Flute 4' on the Great Organ, and I recall that the lack of blend was particularly noticeable if the Harmonic Flute were drawn. So HWIII certainly seems to have been spot-on in relation to his own firm's instruments.
On the other hand, not too long ago I had the opportunity to have a go on an acquaintance's H******** installation - surely I dare not even name the digital beast on a forum devoted to the pipe organ! - but it did at least provide something of a context to HWIII's comments insofar that it included a virtual reproduction of a French romantic instrument which has, on the Grand-Orgue, jeux de fond comprising a Bourdon 16', Montre 8', Flûte Harmonique 8', Bourdon 8', Gambe 8' and Prestant 4'; the Récit Expressif included a Diapason 8 as well as a family of Flûtes Harmoniques at 8', 4' and 2'. To my ears, virtually any combination of these seemed to blend well. Now, I realize that a French Montre is not an English Diapason, but it does cause me to wonder if the blend, or lack thereof, is strictly a matter of upper partials, or if there is in fact more to the question. Any comments?
By the way, in the same letter HWIII went on to state:
"Myself, I regard the presence of flutes on the Great Organ of a three-manual instrument with any pretence to an adequate scheme as out of place: on the Swell, Choir, and Solo, yes; on the Great, no. The Great Chorus should be a pure Diapason structure, capped, if the size of the instrument permits, by the Chorus Reed or reeds."
As I say, I have only limited direct experience of Willis instruments, but I have never come across one in all my recordings or in reading about them which had a Great Organ consisting solely of diapasons and reeds. Hmmm...