On Tue 17 Aug, Dan Ashenfelter wrote:
> Tord:
> I have been reading the spar thread on the Zagi list and am
> still not clear on the location of the carbon spar. Are you placing the
> spar spanwise at the CG location?
No, rather not. I install the TWO spars parallell with the edges, one parallel
with the trailing edge, one parallell with the leading, as to give most strength
where it is needed - at the CG, approximately.
What the Zagi wing lacks (if flown hard) is both bending and torsional stiffness,
therefore the spars should be separated and the forward should be embedded in the
upper surface as the wing mainly have problem in compression. Here suitable CF
rods costs about $2 per meter - must be cheaper in the US! They should be thin
enough to be easy to bend, as we use them to prevent compression in normal flight
(especially true for the forward one), while the rear is there mainly to stop
fluttering (that was my prime goal from the beginning!) and it doesn't seem to
matter if that is installed in the upper surface or on the lower!
So cut a slit in the EPP leading edge (top!) just where the EPP meets the
styrofoam - sometimes it isn't glued very well, so you don't even have to cut!
Add some suitable glue in the slit - if there is no styrofoam where I put it
I use goop, or similar, if you end up exactly at the border between EPP and
Styrofoam you have to use Styrofoam-friendly glue, say hot glue, epoxy, or ...
Just a little to ensure that it stays put.
I insert the rod, and press in some glue on top! Doesn't have to be
perfectly evenly done! This I do after covering, but you can as well do
it before!
The rear spar is trickier/simpler, depending on the method you want to use.
I install these parallell to the trailing edge (so the two spars almost
meet at the tip), about 2" elevon. As the wing is much thinner here I
an not sure that cutting a slit is the best method, and making a perfect
slit in the tough Styrofoam isn't that easy, but can be done with a router,
I guess :-)! I've tried with a very sharp knife, but the result wasn't
100% (EPP cuts so much better!). So after having installed my first
TE rods on the top surface by cutting a trench for the rods and then
glue and tape them in before covering the wing, I decided to try
another method for my unpowered THL. I simply taped them to the wing's
lower surface, still about 2" from the TE, with ordinary Trick covering
tape. These I let overlap at the centre by about 2" and I ended them
about 4" from the tip of the wing (same for the leading edge rods, as
I thought that would be enough. The end result was an almost flush rod
in an area of the wing where the loads normally is downward, thus the
rod is in the right place, I think :-)!
At very, very hard launches it still flutters a bit, so if I would
do it again I would continue the rear rod, at least to the tip. The
powered THL has rods all the way and doesn't flutter one bit!
So, I see no practical difference if the rear rod is on the upper or lower
surface, and it doesn't seem worth the effort to bury the rear spar in the
foam. If you get hold of 1/8" thin walled tubes it might be a good idea to
install it as a trailing edge spar, just at the end of the wing proper.
Burying them in the foam would reduce the foam's strength, so I do not
recommend that!
None of these spars are necessary if you doesn't launch hard (say, if you
only hand launch the THL I doubt you can dive it so fast that it flutters -
but it might be possible - anyone done it?) or fly your Zagi 400 at a
pedestrian pace!
> Thanks in advance for any insight. I'm about to start building my Zagi
> 400 and want to get it right the first time.
I think you will!
Tord,
Sweden
--
If reply difficulties - use [EMAIL PROTECTED]
Tord S. Eriksson, Ovralidsg.25:5, S-422 47 Hisings Backa, Sweden
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED]
