Daryl (and list),
Thank you for the comments on 2M design. Your input is extremely
valuable as both a designer and competitor. The stability issue is well put.
Just to clarify aspect ratio a bit, some design work was done 2 years
ago that suggests optimal AR for 2M is somewhere between 8 and about 11.
The range and boundaries are 'soft' depending on the pilot's
preferences. This was done on the basis of a fairly detailed polar
analysis which uses UIUC airfoil data or X-Foil results, a modestly
detailed description of the planform and some assumptions about
parasitic drag and fuselage wetted area.
Why such a broad range? Well, once a design is set, decreasing AR
increases wing area (and the associated tail volumes) which increases
the weight (but reduces the wing loading), increases induced drag,
reduces profile drag coefficients a bit due to Reynolds number, etc.
etc. It's an interesting interplay but one has to consider that weight
is not an arbitrary parameter but is tied to the planform once a general
construction method is adopted.
The span loading issues that you mentioned are not addressed in this
type of analysis. Nor are handling, turning, stability, etc. etc. So
there's a lot more to the picture once the general planform parameters
are established. Field experience and the pilot's preferences are the
deciding factors.
The other thing that comes into play (for 2M and smaller) is the flow
attachment on the airfoil. The Drela sections are just better designed
that way than just about anything that preceded them. You can compare
X-Foil with UIUC data for those sections where measurements have been
made. The result is generally gratifying. Extending that analysis to the
AG sections leads to the observation of separation bubbles in many of
the popular sections used for 2M. The bubble generally appears at modest
Cl values so the problem is not high drag at high Cl (low speeds) but in
the intermediate speeds (cruise). Consequently, airfoil choice for 2M
will be particularly sensitive in the mid-speed range (generally higher
than max L/D - about the range where you want to scoot to cover ground).
At these Cls, induced drag is not really an issue (AR is not terribly
important at lower Cl). Profile drag is the big issue and the non-Drela
sections are generally not great in this range. When you go to open
class, the chords and speeds go up enough that the problem tends to go away.
Bottom line, there are a lot of trade-offs. Input from guys who have
your level of field experience are invaluable to the design process.
Polars look real good on paper but if all it does is run great in a
straight line then it's not much use. However, a polar is a useful
starting point and the calculations are generally supported by what
little field data there is.
It's a little tough to go into too much more detail on RCSE. Also
probably not of general interest to everyone on the list. Is this topic
of sufficient interest to open a 2M forum on RCGroups where some more
detailed numbers, graphs etc. can be posted. Or does such a forum
already exist?
Thanks very much for the feedback Good luck with the girlfriends. In
1963, Jimmy Soul had a #1 hit with some interesting advice along those
lines.
- Dave R
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