craig bastin wrote:
>
> Fred and Frans, may I humbly refer you to the following link with regard to
> prop choice etc
> After going through this article I am opting for a smaller 4 blade prop,
> probably around 60 inches.
I know the article. I would love to have such a prop, but as clearly
explained, it only works well if it is carefully designed for a specific
engine-airplane combination. I will certainly keep an eye on it, and
maybe even volunteer to experiment with it once I'm flying.
I don't see the logic of going to a smaller 4 blade prop though. It is
true that with the eliptical design you can have more blades without
drag penalty, but again, this only applies to this eliptical design.
According to the article, most props have considerable drag at the tips,
and with each additional blade, you are increasing this drag. You are
effectively increasing the tip area, the place where the bigges losses
occur due to the spill over from high-pressure to low-pressure area.
Consider this: What would happen if you would cut the wings of your
Europa in half, and put another halved wing on top of your plane? (I
believed it is alled a bi-plane?). You would have the same wing area,
that's right. But you would get more losses, because you doubled the
rather lossy tip area. It is better to have two longer wings than to
have four shorter ones.
Look at Paul's props: He can afford to have multiple blades, because his
tips are so narrow. Four of these would still create less drag than two
of the conventional blade tips. For his props only, having more blades
is beneficial.
With conventional props, it is better to focus on overal blade efficiency.
According to the article, a very important gain comes from the high
twist near the root, converting this area into something usefull.
And that is precisely what my Woodcomp prop does: it has a very large
twist near the root, to allow the root portion of the prop to contribute
to the overall thrust. By minimizing the amount of blades, you minimize
the losses near the tips of the blades. Remember, the losses are highest
at the tips because of the higher speed in that area.
Also, compare this particular Woodcomp prop to the Ellipse prop. The
only thing missing here is the tip, which is way to wide. The root
portion has great similarities. The Woodcomp prop may be quite an
affordable compromise.
If you can somehow lay your hands on a (affordable) 4 blade, elliptical
prop, well, I would beg you to order one for me too.
--
Frans Veldman
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