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
Mark makes some good points although the examples of low rpm props are on
planes where the huge diameter leaves no choice in the matter. The lack of
success for the 2.85 - 1 drive & prop combination can not be blamed on 'lack
of trying'. I know of many who tried and got less than expected results,
even after investing a lot of money optimizing props. David Atkins started
with this ratio and even tried a large diameter CS prop (in ground
adjustable only mode I think). Last time I checked, he was using the
2.176 - 1 drive and I think it speaks volumes when someone with his
experience makes this change. Jump in here David or Kathy if I have any
details wrong.
Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com
There are plenty of examples of small diameter low RPM props. The Cessna
Caravan comes to mind relative to its 650 HP or more. Note the paddle blade
shape indicating the tips are not near supersonic. As Mark points out
small diameter low RPM props are mandatory in Europe for noise reasons.
Atkins was trying to figure out why his IVO did not work. No doubt he went
off in the wrong direction. If he had contacted Don Bates Don would have
straighten him out and sent him in the right direction as Don was one of
the first to quantify the problems with IVO props.
Paul Lamar
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