Wrathall Mark wrote:

If you look around commercial aviation, prop rpm's considered normal for
LyContinental's are the high side of the envelope. Turbines generally turn
their props below 2000 RPM (down to about 900 for the Tupelev Bear if you
have really long gear legs and 10K HP, and a Mach 0.7+ airframe.) The Shorts
Skyvan (look like the box an aircraft came in) flies at 150 kias with 2000
max prop rpm, to give a data point closer to homebuilding.

Although Tracy and Mike's point about minimizing risk (time) by providing a
propshaft at the same RPM and HP as a LyCon is valid, if you have to use a
reduction gearbox anyway, you then have the opportunity to optimise the
whole propulsion system rather than accept a limitation set by the trade off
involved in direct drive recips. The LyCon's have to turn that fast to
achieve a usable power to weight. A redrive free you to turn the prop and
engine at each's optimum RPM.

It strikes me Paul has a good point, that props are a quite well known
entity. Designing for 180hp at 2200 rpm (for example) is still well within
the body of known prop design experience, even if is not as common as
2700rpm, 180 hp for RV-6. The experience of most homebuilders, even of RV's,
seems to be that the prop will need at least one trip back to the shop to
fine tune the pitch before it is optimised. This is especially true of an
auto conversion, as the changes to intake and exhaust, with a bit of porting
and an aftermarket EFI means that you have no really idea of HP until you
operate it for a while. Most engines are not dynoed in their aircraft
configuration.

Ed (correct me if I am wrong), was over-propped intially, but has gained
significantly more HP through mods and the new engine build and is now right
up to design static RPM. The point is that props have to be optimised
through a couple of iterations, so the value of the 2700rpm experience base
is limited. As you have reached the prize and are actually flying at this
point, there is little penalty except a little more cash for having to
optimise this off design point low rpm prop.

Maybe the reaon so few have tried the 2.85:1 ratio, is that Tracy, as a
leader in this field, has so strongly opposed it??

Most RV's (the -3 to -6) were designed for 125-150hp, not the 180-200hp now
becomeing common, so you could accept a huge low speed effiency loss with a
180-200hp engine and still drag the thing off any runway a Cherokee could
use. Same goes for many homebuild designs.

The strongest argument in favour of lower RPM is noise reduction. Those who
have flown in Europe know what it is like to be held a ransom by a few
active anti -aviation %&$?$%&!!!!!!. GA is under pressure in many countries,
including parts of the US, noise is one of the biggest sticks the these
people beat us with.

Mark Wrathall

 
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