Moderator: Mike Everman
Mike Everman wrote:Still, this is a sweet piece of design for manufacturing!
Mike Everman wrote:BTW,
The path length for the intake, acoustically corrected, of 7.3 should be 1/3 of the over-all acoustic length, or 21.9", for a duct length of roughly 21.5.
This 1/3 is important, and IMHO, the 1/8 is not, and well, if I may, just wrong.
My recent studies point to changes in diameter (to include the intake's entry to the CC) should fall on odd factors of the 21.9" length, enforcing the odd modes (pressure nodes) of the duct by making points of changing velocity. So your pinch point needs to be where? I'll have to think about that. L/5 sounds good, though some would say that the velocities are low there, so this location is less important than elsewhere.
Likewise, the L/5 for intake length is a good rule for a starting point; more germain on the intake is the aspect ratio (L/d) of it, though keeping the L/3 for your path length.
Not easy to make adjustable if you follow, but means that for every addition or subtraction from the intake length you want to make, there is an adjustment to the tail to preserve L/3 acoustic ratio. This of course makes the pinch in the wrong place, so it's good that it may not be so important! ;-)
Mike Everman wrote:Maybe we should have a little contest between the hard core builders here, for "best" (whatever THAT means) implementation of a straight piper at 22" long. Any takers? I'd love to take a stab, and we'd get more data if they all go on the same test stand. I'm hoping by year end to have real-time SFC for small motors, though for an RC model, "full military" thrust capability might be one category of "best", SFC be damned. Sounds fun to me, anyway, even if I do think they will all be slightly anemic, someone will come up with a new take on it, and shine (by a few ounces) for this eclectic group. We'd see annulars, multi-intakes, snorkelers; what fun would that be? ;-)
Mike Everman wrote:My comments at this point are anecdotal. Length tolerances I'm sure are a pale second to diameters chosen. I think your L/8 works OK because it is so close to L/7 or L/9.
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