I've decided the struts need to be
faired. After a lot of thinking, I decided on a foam rib approach
using a NACA streamline form, finess ratio of 4.500/1.145 or 3.93. I
got this by downloading the NACA 20 streamline shape data into
OpenOfficeCalc (freeware version of MS Excel) and rescaling it to get
something slightly over 1” thick. This gives me a rib profile that
has enough of a wall thickness (just a bit over 1/16”) to be able
to slip it over the 1.0” strut tube without tearing out. Here are
my notes on figuring out what foam to use;
Lowe's – Dow Styrofoam TM, Square
Edge Insulation board (blue)
comes in 0.55,0.78, 1.0, 1.5, 2.0, 2.5,
3.0, 4.0 thick boards 2X8 and 4X8 sheets
Home Depot - Owens Corning Formular 150 (pink)
Formular vs Square Edge compressive strength is stated at 15 vs 25psi so I'm choosing Square Edge. I'm guessing the fairing to be approx 60% of a
1x5x44 stick and there are 14 of them (7 per side)
.6x(1x5x44)x1 ft3/1728 in3 = 1.07 ft3
of foam for all the fairings
amounts to 1.69-1.59=0.1 lbs or 1.6 oz
difference between the two foams (and the Dow stuff is both cheaper
and lighter) I see no advantage in using Aircraft Spruce foam not to
mention it is extremely expensive.
At 1.59lbs x 1.07 ft3 = 1.7 lbs for
the fairings minus pi x 1” cut out of the middle which I'm
guessing to be 50% of the fairing profile area. This would leave
maybe 1.0 pound of material for the fairing plus whatever I end up
using to cover it (Monofilm) and the trailing edge strip which I
plan to use balsa for the initial proto.
As it turned out, I used a rib and beam structure which is significantly lighter in weight than what I figured on here. I never did weigh it out so do not know what the final fairing was.
The aluminum fairings (there were only
6 per side) weighed in at 34.3oz (2.14 lbs without the tape or
So, I expect roughly half the weight to
do foam fairings vs aluminum and also they should be less drag.
Dow Square Edge showed no problems at
225F from the fabric iron. Should tolerate all the model iron-on
films out there.
A bit of research on the internet
revealed Gorilla Glue to work well in gluing foam. I tried a sample
and saw the foam fail and not the glue-foam joint. Now for the
My idea uses ribs, leading edge and
trailing edge beams glued together with a polyurethane waterproof
glue marketed as Gorilla Glue and is widely available.
I decided to rough-cut a template from
plywood for the ribs and then sand it to the exact profile using a
vertical disc sander. The first step was to drill a 1.0” hole
through the plywood. I used cabinet grade since I will need to make
several hundred ribs. I then positioned the paper profile over the
1” hole as close as I could to center and traced out the pattern
onto the plywood. Next I rough cut to the outside of the profile
using a band saw and then carefully sanded up to the line using a
disc sander. I drilled a couple holes on each side of the template
and drove some smooth shank nails flush.
I bought a 1/2” router bit with a
bearing on the shaft end to ride against the plywood form mounted in
a cheap router table I borrowed. I had access to a hot wire set up
so didn't need to make my own. I did find that I wanted to replace
the nichrome wire which was too thick with a stainless steel fishing
leader I ordered from Cabela's. Too small of wire cuts too slow and
to large a wire transfers too much heat....trial and error.
Rough cut piece of half inch thick foam
mounted onto the template.
Starting the cut
First pass complete.
Trimming off the last bit on the
Next with the router
The finished hole
Complete rib removed from the template
THE INTER-RIB BEAMS
I arbitrarily chose a 3 3/8ths inch
spacing between ribs and fabricated both a trailing edge and leading
edge beam to form a complete structure for the film covering I used.
The leading edge beam was 5/8 thick and the trailing edge beam was 1
inch thick. This structure formed a strong enough frame to support
the monofilm covering I used and proved to be more durable than I had
anticipated for normal set-up and break-down wear and tear.
Here is the completed trailing edge
form lying on the hot knife table
In this shot the block of foam is in
the form held by shoving a hat pin into it through the hole I drilled
in the form. This helped to keep the foam from shifting while doing
In this shot, looking down onto the hot
knife table at the form, I have just finished the first cut.
The completed beam ready to be put in
the finished pile.
The leading edge beams were made using
the leading edge profile and a form just like the trailing edge form.
This shot shows the strut with all the
ribs in place ready for the beams to be glued in.
I used small rubber bands to hold the
ribs against the beams while the glue set up. I learned later that this was not necessary and just added extra time to the fabrication process. The glue is tacky enough to easily hold the joints together during cure.
I used a flat 2X4 with a long piece of
180 grit self adhesive sandpaper as my sanding bed to even up any
topography problems that came up as a result of me not aligning the
beams and ribs exactly and also to make the trailing edge a straight
line. I used monofilm applied with a hot
iron exactly as in model plane covering.
This shot shows the fairing end covered.
Laying out the main sheet of monofilm
to begin the covering process.
Before heat shrinking
After heat shrinking.....magic!
One note on final heat shrinking.....
I consistently had the film "stick" to the strut tubing causing a series of puckers in the middle of each bay. I tried adding more ribs to decrease the distance between the bays with no noticeable effect. The only thing I was able to do to reduce this problem frequency and severity was to apply double stick tape onto the strut surface between bays and then dust the tape with talcum powder just prior to applying the film. Although this didn't completely eliminate the problem, it was significantly better and I felt well worth the extra step.
THE DIAGONAL STRUT TRANSPORT CASES
THE ALIGNMENT "WRENCH"
Once the fairing was completed it
became clear that the router template I made to produce all the ribs
had just the right size hole on it to thread the ribs onto the strut
without breaking them and produced enough friction that the fairing
can be rotated but will easily stay in position once I align it. I
made an alignment “wrench” to do this.
It took some trial and error to figure
out how much clearance I needed on each end of the fairing to clear
the cable swages. The main problem is how the plans had you rig it.
All of the cables are double swaged (for no clear reason) and the
main swage nearest the bracket was pulled so tight (there's a drawing
in the plans on this) so as to ride right up onto the bracket plate
making it very hard for it to rotate and also putting the second
swage a couple inches or more out onto the cable causing interference
with the fairings I'm trying to install. I had to provide nearly 4
inches of clear strut on the middle positions both leading and
trailing edge to keep those fairings from being ripped up on fold
down. Even with the extra clearance I fold both middle negative
wires to the other side of the bracket on tear down to avoid contact
with the fragile fairing. As it is I fabricated a foam block about 4
inches thick for the root end of the leading and trailing edge at the
lower wing panel bracket and glued velcro on the aluminum gusset and
the block to hold it in place while folding the panels down. These
blocks keep the wings from collapsing completely thus avoiding
smashing the inboard fairings against the ribs. I still have some
deformation but it's acceptable. This way I can leave the fairings
in flying alignment in the collapsed transport mode without having to
worry about having them move out of position by the wing ribs.
Next, I made two foam cases for each
set of diagonal struts to protect their fairings during transport
since it is so easy to tear the monofilm covering which is easy to
repair but a nusance I prefer to avoid.
Here are a few photos of the fairings installed in Darrell's glider which I am using to train on. Notice the two rear root struts are not faired. As mentioned above this is necessary in order to enter the cage and handle the glider while on the ground.
SOME FINAL NOTES ON HOW THE FAIRING WORKED OUT
I found right away that leaving both the root diagonals and rear root struts unfaired was best in the learning phase. After one is accomplished in handling the glider in a variety of launch conditions, then adding fairing to the diagonal should not present a real problem. Since you cannot grip the fairings (way to fragile) I can't really handle the glider without having access to the rear root struts to get into the center cage area and then lift the glider to attach the harness lifter shoulder straps I made. I therefore left these rear root struts unfaired. The fairings are amazingly durable. I've crashed them, flown with the root diagonals faired and gone through enough set-ups and take-downs to convince me they will hold up fine under normal flying. I do feel the diagonal storage cases were worth the effort to make since they are so exposed to damage when not in the airframe. I had to provide extra clearance on several of the strut ends to avoid damage during fold down and also at each quick disconnect position where you need extra space for your hand to get the negative wires attached.