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Old Nov 19, 2003, 02:03 AM
MX
Embedded Systems Engineer
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Grass Valley, CA
Joined Sep 2001
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AP Plane Design #2 - Comments?

After flying my first AP plane design for a while, I've designed to
improve on the design based on what I've learned. My goals
are to:

- Reduce weight further.
- Improve wind penetration.
- Add ailerons for roll control and flaps.
- Make the wings and tail detachable for transport.
- Make it look better to civilians interested in my AP.

To accomplish this, I'm going for balsa built-up wings and tail,
carbon fiber boom fuselage, solite covering, and ailerons. I also
have a plastic trunk from K-Mart that it's designed to fit in so I
can transport it while avoiding hanger rash.

Here's my drawings thus far:



Here's some shots of progress:

- Stack of ribs after sanding the profile and cutting stringer slots:



Airfoil is Selig 3021:



Wing partially assembled:



Tail Pieces:



(cont)
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Old Nov 19, 2003, 02:04 AM
MX
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Grass Valley, CA
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Brass Tubes that slide inside each other will hold wings together:



Tubes will slide through the keel (plywood/balsa sandwich).
Camera will mount below wing on keel:



Wings will come apart for transport:



Entire wing:



Nice and straight:



But it flexes too much:



The wing seems to have a lot of flex in torsion. If I grab one end,
I can twist it about 45 each way pretty easily. Any suggestions
to stiffen it up?? I added some balsa between ribs but it didn't
help a whole lot:



Will the covering help this?

Thanks for looking. Sorry for the lengthy post.

MX
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Old Nov 19, 2003, 03:23 AM
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Roseville, CA (Sacramento area)
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I bet the covering will help some. I wish I could handle balsa, it seems to be such a nice thing to build with (I am unfortunately allergic to it). That looks like a great start to a nice AP rig. How do you plan on keeping the wings together?
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Old Nov 19, 2003, 03:49 AM
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Fredrik W's Avatar
Stockholm, Sweden
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Quote:
Originally posted by MX
Any suggestions to stiffen it up??
You should add a D-box. That means that you cover the area from the trailing edge to the main spar with a sheet of balsa or thin plywood, both on lower and on top side of the wing. (A cross section of the wing will then look like a long D. ) That will stiffen up the wing a lot - and make it more crash resistant.

The covering will also add strenght and stability to the wing.

Your ribs looks very thin. That could be a problem, you will find out when you shrink the covering. I made a built up wing a month ago, when I shrunk the covering the ribs flexed and looked like S-urves. I had to remove the covering, stiff up every rib before I could cover the wing again.

I too is building a new AP-plane as we speek.


// Fredrik Wergeland
Sweden
www.flyingmodels.org
The Great Electric Motor Test
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Old Nov 19, 2003, 08:58 AM
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Near Austin
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Also glue shear webs in between your top and bottom spar.

CTF
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Old Nov 19, 2003, 09:49 AM
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AUBrian's Avatar
Sweetwater, TN
Joined Sep 2002
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Um....tex...in that last picture, he has shear webs ;o) I'd say since you're going for lightness, go ahead with what you've got and cover it. It always amazes me how much torsional rigidity that adds, and having just a touch of flexibility may help it bend in a crash instead of breaking.....
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Old Nov 19, 2003, 09:58 AM
KLH
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Hayward CA / Valley Springs CA USA
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As per Fredrik and CTF, If you do both the sheeting from the leading edge (top and bottom) to the main spar and the shear web (vertical grain) between the top and bottom spar to form a "D" tube or box section you will stiffen the wing up.
I've built up a lot of balsa wings of my own design over the years and the "D" section as described above is one of the easy ways of gaining torsional strength. The sheeting does not have to be very thick. On my smaller planes I use 1/32" contest grade and on my larger planes I use 1/16". There are several examples of this type of construction on my homepage if at www.kern.hanson1.com
I'm just about to try out a new AP plane thats very similar in design, but I got lazy and am using and old Wingo wing that I took most of the dihedral out of and add ailerons.
Good luck with it! Your workmanship appears to be outstanding!

Kern
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Last edited by KLH; Nov 19, 2003 at 10:01 AM.
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Old Nov 19, 2003, 11:06 AM
MX
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Quote:
Originally posted by Trikster
I bet the covering will help some. I wish I could handle balsa, it seems to be such a nice thing to build with (I am unfortunately allergic to it). That looks like a great start to a nice AP rig. How do you plan on keeping the wings together?
Thanks. The wings will first slide together using the brass tubes,
then I will have a hard block glued to inside of each first rib,
and blocks glued to the center keel. There will be a vertical hole
through each. So I would slide the wings in from the sides, then
put nylon wing holddown bolts in from top to bottom. This will
lock them in. By the way, the center two ribs on each wing are
1/8" ply.

MX
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Old Nov 19, 2003, 11:09 AM
MX
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Quote:
Your ribs looks very thin.
The ribs are 1/8" balsa. They seem to have enough stiffness
front to back.

MX
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Old Nov 19, 2003, 11:12 AM
MX
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Quote:
Originally posted by AUBrian
Um....tex...in that last picture, he has shear webs ;o) I'd say since you're going for lightness, go ahead with what you've got and cover it. It always amazes me how much torsional rigidity that adds, and having just a touch of flexibility may help it bend in a crash instead of breaking.....
I was concerned about there being too much flexibility in the wing
to the point that it would flex in flight, causing too much wash
out or wash in. Perhaps even causing problems like tip stalling
in wind and such.

MX
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Old Nov 19, 2003, 11:17 AM
MX
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I may try doing the D box by sheeting from leading edge to
trailing spar with 1/32" balsa, then do covering.

Thanks for the suggestions.


Another questions: do my ailerons look too small? They are
24" x 0.75".

Thanks,

MX
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Old Nov 19, 2003, 11:27 AM
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jgleigh's Avatar
Manassas, Virginia, United States
Joined May 2002
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If you are worried about torsional stiffness, I'd suggest you use monokote for the covering. It seems to add more stiffness than some of the other products, and DJ Aerotech specify it for their built-up balsa sailplanes. The transparent colors are reasonably light, especially the yellow.
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Old Nov 19, 2003, 11:55 AM
MX
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I wonder which would way more: sheet about 50% with 1/32"
balsa and cover in solite or cover all with monokote?

MX
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Old Nov 19, 2003, 12:59 PM
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Well, if you add 2 inches of 1/32 sheet to the top and bottom of the wing, that's 2*2*52/32 = 6.5 cubic inches of balsa. Fairly light balsa weighs in at say 10 lb/cuft, or a little less than 0.1 oz/cuin. So that would add about 0.6 ozs. I don't know the weight difference between solite and transparent monokote, but we're looking at 8*2*52 = 832 sqin = 5.8 sqft of wing area. Anyone?
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Old Nov 19, 2003, 01:08 PM
MX
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Grass Valley, CA
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I can't use transparent monokote. A couple other guys have
mentioned problems with the camera image being colored due
to sunlight going through the transparent wing. Unless I use
clear. I do have a bunch of doculam that came with my
MiniFlash. It's clear.

I think I'm going to cover one wing with solite and see if only
the covering will be enough to eliminate most torsion. I can
peel it back off if I need to. I'm really trying to cut down on
weight with this one.

MX
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