#18 Strut Fairings

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)

material density

• Aircraft Spruce blue foam (9.0 oz/16 oz/lb)/(0.5/12x2x4 ft3) = 1.69ft3
• Square Edge 33.85 oz/16 oz/lb/(2/12x2x4 ft3) = 1.59 lbs/ft3
• Formular 150 = 1.3 lbs/ft3

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 rivets)

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 fabrication process....

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.

THE RIBS

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 trailing edge

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 the cutting.

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 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.

THE DIAGONAL STRUT TRANSPORT CASES

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.