Couldn't agree more with many of the comments about 2M (with 1 or 2
minor exceptions). I like this class for a lot of reasons, most of which
have been articulated, and have tried quite a few of the kits that are
out there. IMHO the best ones were the 2M Super-V, the 2M Duck and the
Laser 2M. However, it's probably about time for an update to some of
these designs. So a thought or two as grist for the design guys:
We tend to think of the 2M class as a small open class ship. So the open
class design tendencies are usually used. This is not necessarily a good
idea. Because of it's size, a 2M is not going to be able to roam as far
or as high as an open class ship - it's simply harder to see at
distance. So, like DLG, it's going to be more of a close-in ship.
Consequently, it probably should tend to better minimum sink and max L/D
rather than having great 'legs' for roaming all over the sky. This tends
to drive the design towards lower aspect ratios. Due to the span limit,
a high aspect ratio 2M may start to run into Re problems.
Carrying more wing area will allow a lower wing loading with modest
attention to building techniques. This should also help with the launch
as more area would help to carry the tow weight better.
The airfoils often used for 2M are not particularly well optimized for
this class. If you run X-Foil on a number of the popular 2M sections,
they really aren't that great below ~ 80,000. It's probably heretical
to say this but the MH32 is not a great 2M TD airfoil. For that matter,
neither are the SA or SD sections. Again, a look at those sections in
X-Foil will indicate that in the lower Re regime, they become 'draggy'
in the middle of the bucket. This compromises their ability to run
effectively.
So a couple of suggestions;
- Look to lower aspect ratio designs. An evaluation discussed in RCSD a
few years back suggests the 8 to 11 range is about right. You can't just
change AR arbitrarily. You also have to account for the total weight and
wing loading change when you tweak AR. When you do that, AR really isn't
a huge factor for a 2M in this general range,
- Sort through the Drela sections, many of them are much better in the
intermediate speed range than any of the sections commonly used for 2M,
- If you're going with a V-tail, size the surfaces correctly. Biggest
mistake I've seen (and flown!) with some of the euro designs is too
small a tail area and an inappropriate V-angle. My impression is that
most of these are converted 2M slopers that some distributors wants to
sell as TD ships,
- Work a bit on keeping the weight in the 30oz range. A ballast box is
great for the windy days but it's tough to add lightness.
Back to the original list (Super-V, Duck and Laser). They all tend to
run with more wing area than some 2M designs. And the tail surfaces
(especially for the Super-V) are sized appropriately. A really nice
update would be to change out the sections to something like the AG3X
series, maybe tweak the AR into the 9'ish range and use current
construction techniques to bring the weight down into the 30oz range or
less.
Sorry for the digression. Back to the cave to cut some wings and try it
out.
Great NATS. Sorry I couldn't stay for more events. Thanks Marc, Jim and
all the CDs, helpers and LSF guys who made it a terrific meet.
- Dave R
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