>Also, regarding the Goldberg Super Chipmunk, I went back and actually
>read the whole desciption (grin). Seems it's aimed at the intermediate
>flyer, and has a 64" SEMI-symmetrical wing so it ought to have
>reasonable low speed handling. I'd be very interested in hearing
>comments from anyone who has any first hand knowledge about this
>particular kit. Will I need any special servos? I've never built a
>plane this large.
Don: I'm about two-thirds done building that same CGM Super Chipmunk, and
you'll be happy (I suspect) to know that the wing is actually fully symmetric
(you can flip any pair of ribs and they'll match perfectly). The description
in Tower's catalog is definitely in error (wouldn't be the first time!)
I'd been thinking about building one of these for a while but was put off by
the ad. Fortunately a kit showed up in my local so I had a chance to check it
out quite closely and confirmed that the wing is symmetric...
The low-speed handling (assuming this is correct) is likely due to
(a) moderate wing loading (range should be somewhere between 20 and 26 oz/sq
ft - 23oz/sq ft if you hit the 7lb middle of the expected weight range),
(b) the verging-on-high-aspect ratio wing - not as extreme as the Kaos wing,
but getting there, and (c) a rather inordinate amount of dihedral.
I'm not looking for the utmost in low-speed stability, so I've taken half that
dihedral out so it closely matches the dihedral used on the wing of my
UltraSport-60 (which is fairly stable as it is).
Also, the plane is designed to use standard-size servos, so no problems there.
I'm also using an OS 4-stroke - a .91FS - so I'm facing some of the issues
you'll need to work out if you go ahead with this. Basically, I compared the
length (and weight) of this engine with the (antique) K&B .61 2-cycle that the
plans are built around, and found that I'd need to move the firewall back at
least 1/2" to get the cowl nose ring to end up in the right place. This ought
to help with balancing the plane as well, and the back of the TruTurn spinner
will be 1/8" in front of the cowl when it's all done.
Other changes I'm making:
- the kit comes with a lot of ABS (cringe). Between the cowl, wheel pants,
wing tips, belly fairing, and***pit details (!) there's almost as much
plastic as a Lanier Stinger (yucko!) Sooo...I've replaced the cowl and pants
with R/C City glass, and I carved my own balsa wingtips and belly fairings
(I'll probably keep the plastic***pit interior ;^) The result looks sharp
and I won't have to deal with painting all those plastic bits (and that would
melt if I tried to put film over them).
- I've installed flaps roughly following the plans using a central servo in
the wing, but I've installed dual aileron servos in the wing (you can *just*
squeeze standard size servos between ribs 8 & 9 if you lay them on their sides
and work out the mounting). If you follow the CGM aileron design (central
servo, LONG pushrods and bellcranks) I think you'd end up with significant
linkage slop leading to deadly flutter, not to mention sluggish response.
- I completely sheeted the top-side of the wing - as I want to follow the Art
Scholl trim scheme but don't like painting over film, and would rather not use
fabric (or glass) and paint either - too much weight involved. The trim scheme
used on the top-side of the wing is "busy" with stripes, too busy to reliably
piece the film together on my glass benchtop and then pull it over the wing,
as I usually do, but with the full sheeting under the base white film layer I
can apply the red striping with an iron and have something to press against
(not possible with open bays).
- Instead of laminating together the lite ply firewall per the plans (almost
impossible to come out truly flat, plus the quality of the ply in my kit
really sucked), I used 1/4 AC grade ply, and beefed up the reinforcement to
survive the engine that'll be dragging this 'Munk around. Might not be
necessary but it makes me feel more comfortable.
- generally made sure that any critical high-stress areas are "ruggedized", in
particular the gear mounts, wing bolt blocks, firewall, and the fuse area
immediately behind the wing bolt block (which looked frail to me). I glassed
6" of the wing center as well. I don't mind adding a bit of weight when it
comes to preservation of the aircraft on our less-than-manicured
runways, and I don't want to worry about parts flying off the first time I
throw my 'Munk into a *** snap ;^)
The instructions are pretty good - easy enough to follow and well
illustrated. Read them a few times over before diving in. Full size
plans - hand-drawn, unfortunately, folded (BOO!) and with some detail
obscured, and some technical data not provided - like incidences and thrust
angles. Material quality varied: excellent balsa, decent hardware, but the
worst lite ply I've ever seen (really *** stuff). Take plenty of time
dry-fitted the fuse together and you'll end up with it nice and straight.
Whoops! Just noticed how long this has gotten (I'll put a warning in the
title!) but I hope you (and others) find it helpful. Taking a glance at the
finished wing and half-built fuse, I figure another couple of weeks before
it'll be ready to cover. If anything turns up along the way I'll let you know.
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