Re: Fuselage length, CG and decalage?

From: Tapio Linkosalo <tapio.linkosalo_at_iki.fi>
Date: Sun, 05 Apr 2009 21:19:07 +0300

Ok, had this morning a chance to fly in a large, 12m (36ft) hall, and
did some measurements of the model afterwards. I had shortened the
fuselage by 100mm, so that my overall fuselage length is now 800mm. The
nose is about 50mm. My current (first attempt for VP) prop is a bit
heavy at 400mg, so the CG is forward, and thus based on previous
attempts I tried to move the CG back with a 35mm spacer at the front of
the rubber. My best flight with a 200mm loop of rubber, wound to 22gcm
at start, and no back-off, was 13 minutes, with the prop pitch going too
low and model prop-breaking from about 5 meters at the end of the
flight. I should increase the minimum pitch for better results, as there
was plenty of turns left after the flight...

The model has CG at 80% and about 5 degrees of incidence (plus 3.5 for
wing, minus 1.5 for tail). The prop pitch for start was around 600 at
high pitch (except at 120mm radius it was almost 800 - so it looks like
my prop jig is no good) while low pitch was around 300 for most, but
only 200 at tip, and 500 for the 120mm diameter. Prop pitch was high
until 10g*cm pitch, and met the bottom stop at 4g*cm. The rubber would
to 1200 turns drops below 10 g*cm at 1000 turns, and down to 4 g*cm at
200 turns. So there should be a few turn left.

The model at the current weights 1.7 grams. The first attempt for VP is
a bit too heavy, as is the wing, that failed under first flights, and
which I had to reinforce with boron _after_ covering. But I was quite
hapy with the trim setup, the wing twisted for high warps for the
initial launch, but quickly settled for reasonable twist, and the
resulting climb trim was quite OK. Hanging on the prop, with tailplane
barely keeping the nose from pitching up, but in good conditions the
climb was ok, no tailslides. So I guess I'm not satisfied enough of the
setup and details, so that I can build a couple of new models, knowing
what CG to aim for. And need to build better props.

Any comments on rubber dimensions, torques, prop pitch etc.? Am I even
in the ballpark?

Thx,


-Tapio-
Received on Sun Apr 05 2009 - 11:22:25 CEST