Source: 2016 Q1 Beartracks, Mike Swain
I want to offer this disclaimer, prior to this excursion into wingtip making my composite experience was limited to patching a few screw holes in boats over the years. My objective is not to save money or time by building my own wingtips; I want to learn and hopefully end up with something unique. I wanted Hoerner style wingtips, primarily because I like the way they look, but there are some purported performance advantages over rounded. The Hoerner shape also lends itself to housing the Bob Archer style VOR antennas I made.
Stuff You’ll Need:
- MDF to make 2 full size airfoil forms
- Enough foam to make 2 wingtips the thickness you require
- Fiberglass Cloth & Resin (Epoxy or Vinylester)
- QuickFair or filler of your making
- Lightweight spackle or Polyurethane+Microballoons
- Sandpaper – I suggest 80 – 120 – 220
- Nichrome wire – 22 gauge
- Foil Tape, min 1.5 inch wide
- Power source for hotwire – Battery Charger, etc.
- High Build Primer
To start, I rough cut a piece of MDF and set it up against the outboard edge of my wing and traced the airfoil with a sharpie. I cut to the line using a band saw and jig saw. I then used a random orbital sander to finish until the line was just gone.
My plan was to mount the tips flush, not overlap, so I reduced the form by another .0625″ to allow for the fiberglass layup thickness. I drew a line .0625 in from the edge by using a scrap piece of aluminum the correct thickness and dragging it around the form with a sharpie. I then sanded the form down to the new line. Because I need two identical forms to hotwire the foam, I traced the completed form onto another piece of MDF and rough cut. Using a few screws, I screwed the two pieces of MDF together and used a flush cut router bit to make an exact copy of the first form.
The two forms will sandwich the foam and the hotwire will glide along the forms resulting in a airfoil shaped block of foam. The two forms need to be exactly in line with each other during this process. I am sure there are many ways to ensure the forms are inline, but I simply match drilled the forms with a couple of 3/8 guide holes while the forms were screwed together which threaded rod will go through lat-er in the process.
I made my wingtips 10″ wide, primarily because a single 4’x8′ sheet of 2″ foam supplied enough material. But you can make them any width you want, just glue together enough foam.
Using a framing square, I laid out lines on my workbench to match the distance between the 3/8 guide holes in the forms. The width of my wingtips are 10″, so the layout is 10″ + 1.5″ to account for the 2 MDF forms. I then made 4 supports from some scrap 1×2, 2 for the front and 2 for the rear. The pairs were match drilled and long enough to suspend the forms and foam in the air over my bench. I drilled the 4 sup-ports with a Kreg jig I use for wood joinery, but it’s perfect for attaching wood at right angles. You’ll also want to ensure the ends mounting down to your work surface are square, I cut mine using a chop saw.
I used one of the airfoil forms to trace the airfoil 5 times on my 4’x8′ sheet of 2″ thick polystyrene foam I picked up at Home Depot. I also drilled the foam through the 3/8 guide holes while I was tracing the airfoil. I rough cut the sheet into 5 pieces using the tracings in the previous step. I used an old hand saw I had laying around.
The foam I purchased had foil on 1 side and plastic on the other. I peeled this off so I could glue the 2″ pieces together to get 10″ total. I used 3M 78, which is a foam-safe spray adhesive. If you use a different glue, test it on some scrap foam, because many of the solvents in glue will eat polystyrene, especially the spray adhesives.
To glue everything together, I ran 2 threaded rods up through the guide holes on one of the forms so the rods were sticking up in the air. I slipped the first piece of foam down over the rods then sprayed the inside end with adhesive along with the subsequent joining end of the next piece. Rinse and repeat until you get to the last piece. You don’t want adhesive on the outside ends be-tween the forms and foam, we just want to glue the pieces of foam together to get our desired thickness. I slipped the second form onto the rods and snugged the whole assembly together while the adhesive dried. I went for tight but I didn’t want to crush the foam.
Once the adhesive is set, you are ready to hotwire. I set up the assembly using the 4 supports screwed down to my workbench, so the forms and foam are suspended over my bench.
I made a hotwire tool with a scrap piece of 1×2 and some scrap fuel strap (.025 4130 bent at 90 degrees) It needs to be wide enough for the width of wingtip you are building. You want the wire to glide along the two MDF forms. I purchased the Nichrome wire from Amazon. The wire needs to be as tight as possible. For a power source I used a car charger. It seems the newer digital chargers are smarter than the old dumb transformer types, so I used an old 12V UPS battery to trick my charger into putting out the juice.
Once you have your hotwire tool ready, it’s time to cut some foam. The idea is to have the wire glide along the top of your forms at a nice slow steady pace. Not too fast to drag the wire, but not too slow where you burn excessive foam. You can fix most imperfections when you fill the foam, but try to get a clean cut. Do one side, then flip the whole assembly in your supports and do the other side. Take it out of the fixture and you now have an airfoil shaped chunk of foam the thickness of your planned wingtip.
Now we will create the Hoerner shape. To do this, we will apply foil tape along the edge of the foam for the hotwire to ride on. On the lower inboard side of the wingtip, I made a line 1 inch from the edge. I carried this to about the center of the front of the air-foil. Now on the TOP of the OUTBOARD edge of the airfoil, I came down about .25″ inch from the edge. There is nothing hard and fast about .25. but this will determine the thickness of the outboard edge of the wingtip. If you want thicker, make the dimension bigger.(right)
To hotwire this, I plunged the hotwire into the foam until just making contact with the foil tape, then gently drug the wire trying my best to keep the wire square for a nice even pull. The video shows this in more detail.
Now you’ve got a funny looking airfoil-hoerner sculpture (above). The next step is to put the front radius on. From this point on it’s more up to you and how you want the final shape to be. I followed the convention outlined in the video (https://youtu.be/RoOh-fgI8oU) which is to use the width of the wingtip as the radius of the front curve, 10″ in my case. I am happy with the look and the way it came out. I used a piece of safety wire around a sharpie to draw the 10″ radius from the front inboard edge of the airfoil to outside edge. I then used my hotwire tool held vertical to the foam and rough cut the radius.
You will end up with a blunt edge on the front of the airfoil where you cut the radius. Use a rough file or rasp to blend this blunt edge into the existing airfoil. I went ahead and ever so slightly radiused the outboard edge of the wingtip to the point where the airfoil is the thickest. Seems to be about 1/3 of the way back from the front. This was done by eyeball until I got a pleasing shape.
I then used some 80 grit sandpaper to rough sand any ripples from hotwire and generally level the foam. I used some 120 to put a nice radius on the outboard edge of the wingtip. You don’t want any sharp edges since wet fiberglass will pull away if you try to layup over a sharper corner.
I then used some lightweight spackle to go over the form. Then a process of sand, fill, sand, and fill. You want a nice smooth surface under the layup. Any imperfection will transfer into the fiberglass. I read in the Bingelis book that Polyurethane mixed with Microballoons makes a good mix to cover foam forms. I had already used spackle before I found that tidbit, but you might want to explore that method. You’ll want to make sure the entire form is covered, because paint and certain resins will eat foam.
After getting things as smooth as possible, I put a couple of coats of rattle can spray paint. I used orange because I wanted to be able to see the form as I was laying up, to make sure there were no air bubbles.
To prep for layup, I waxed the painted form with 4 coats of good carnuba car wax I had in the garage. I then sprayed 3 coats of PVA mold release.
My layup schedule was 4 layers of 8.8 oz Bi-directional cloth from Aircraft Spruce (PN 01-00642). There was no science in that choice, just a guess. They seem to be pretty sturdy and came out around 7 lbs. each.
I used System 3 Silver Tip Epoxy as my laminating resin. It is a popular choice among home boat-builders. No amine blush and simple 2:1 mix ratio. It can also be post-cured at higher temps to improve the structural properties, which is a bonus for room-temp cured epoxies. Make sure your foam is 100% protected if you use Polyester or some other non-foam safe resin…..or all that hard work will melt away.
I will not go into detail on attachment, but I ended up using piano hinges. I didn’t want to have to undo a bunch of screws every year at annual. I followed the steps the RV guys have outlined. The only exception is I built an access panel on the top of the wingtip to access the hinge pins, since the Bearhawk’s tip rib extends all the way back, unlike the RV method of securing the hinge pins.
I followed this method: http://goo.gl/lRg0i2. I used .040 strip of 2024 as my spacer, since there is no cutting of these wingtips. Also, since there is no gelcoat on my homebuilt wingtips, they were translucent enough for me to mark the hinge and drill based on marking on the hinge.
Once I had the wingtip mounted, I used Quick Fair (a commercial fairing compound) to level and get a nice surface on the wingtips. This requires several thin coats and lots of sanding. I use a 3M semiflexible longboard. 80 grit for rough, then 120, then 220. Depending on the quality of finish you want will determine how much time is spent here.
I applied high build epoxy primer next, sanding between coats, until i got a perfect surface. Once you are happy, they are ready to be painted or further modified for lights, etc.
I will be making cutouts and fabricating lenses next. My landing, strobe and position lights will be housed in each wingtip. I hope to write up those steps in a future article.
Notice that the positioning of the elevator bellcrank is not arbitrary. The photo to the right shows an excerpt of the plans page 26. This depiction shows where the bellcrank should be in the neutral elevator position. When making elevator cables, be sure to establish this alignment. Incorrect alignment will cause higher cable tensions at deflected elevator positions. Also, Bob recommends using galvanized control cable for most Bearhawks, and not stainless cable. The stainless cable is still safe, but it has a less favorable fatigue resistance, and will likely wear out faster. An early hardware list specified stainless cable instead of galvanized, but that was not the best choice for planes that will not see service on floats in salt water.
The Bearhawk Patrol in all of its glory was ready to go. I was as ready as I could be. I had been in the air a dozen times but this was my first week long trip. I have known Bob Barrows for most of my life. I started making fiberglass parts for the Bearhawks about 5 years ago. After working on other R&B Aircraft projects and learning more about the Bearhawks, I decided I wanted to fly them. First thing first, I will need a license and knowledge of flying. I can’t think of a better way to get things started than an adventure in a Bearhawk Patrol with Bob.
During Oshkosh this year I was wandering through the Bearhawk booth when Mark Goldberg suggested that he had “just what you need.” I’m mostly scratch building a Patrol but have purchased a few parts from Mark. When he told me that he had a Patrol pre-fit tubing kit that was available, it took all of 10 seconds to accept. I’m sure glad I did.
One thing that became apparent during the assembly is the HUGE number of clamps that were necessary to hold things in position before tacking. Some of the most useful clamps were these “EAA” welding clamps. They are cheap to make and very versatile for holding multiple tubes while getting ready to tack. I used 4 of them plus a lot of the common spring clamps.
First of all, a disclaimer about this article…I AM NOT AN EXPERT!!! I’m just a homebuilder like everyone else, muddling my way through a really big project. One of my criteria for a homebuilt is to put it on floats. That’s the genesis of all this research on the subject and the search for the answer to the somewhat elusive formula: To fin, or not to fin?
Also, if a landplane is converted to a float plane, the additional forward vertical surface area of the floats requires a balancing vertical surface area aft, either in the form of a ventral fin or vertical stabilizer fins on the horizontal stabilizer.” So now that we know that ventral fins are used on a variety of aircraft, not just float planes. So how does that apply specifically to an airplane on floats?
On the other hand Mike Carriere with the yellow Bearhawk with the monster IO-550 and large floats can barely put enough additional vertical area on the tail on to make it controllable. Mark Scott is an aeronautical engineer by profession and built a beautiful Bearhawk. Here’s what he has to say from some previous emails that I had saved: “As an aerodynamicist I have done some aircraft stability work. The need for additional area on an airplane is a function of prop size and float size. The fact than an O-360 Bearhawk with regular floats needs less ventral fin than an IO-550 on large floats makes sense. By looking at the Bearhawk I’m pretty sure it would benefit from some additional area. The FAA directional stability certification requirement is that an oscillatory decay from a rudder doublet pulse damp down to 1/10th amplitude in seven or less cycles. One could make a survey of float equipped aircraft out there and the size of their ventral fins. A plot could be made to figure out how much ventral fin area would probably work well on the Bearhawk. I think in the end pay an extra pound or two and make a good sized fin that looks right. That is what I am going to do when I put my plane on floats. I recommend making the largest reasonable-looking sized ventral fin. They are not difficult to make. If the directional stability feels really solid you could make another smaller one. Another trick is to install much stiffer rudder return springs. I think there is an STC for Pipers and some Cessnas for this modification.”– Mark Scott
I have been looking at the float installation on the Bearhawk quite a bit and ventral fin attach points were on the list of things to do. The Bearhawk that has flown on Clamar floats apparently has done well. For his installation, it looks like a ventral fin is unnecessary. But, if you choose to go with a different float, a fin may be necessary to compensate for the float size. I plan on EDO 2870s.
Perhaps the thing I like the most about aircraft is the ability to leave the city and arrive in the depths of the backcountry in next to no time. It creates the flexibility to plan trips in a way which would otherwise be impossible. In our case, we usually fly home. On this particular occasion, we had a week at our disposal and plans to hunt chamois and go trout fishing.
As we start to slow with basically no braking action, we strike a powerful crosswind gust over our left wing. It’s the prevailing wind shooting out of the watershed which drains at right angles to the airstrip. I’m alarmed as the aircraft lifts some weight off the gear, slews right, and begins to depart the airstrip. Applying full left rudder and brake, but no power, we skid sideways, the nose is point at least 15 degrees left to our track. My hand is tense on the throttle, the “get me out of here” button with 260 horses in waiting. The situation improves as momentum catches up with us, the wind eases, and our direction of travel becomes the same as our heading again. I work the rudder stop to stop, and even out the brake pressure. We slide to a stop with both wheels locked.
After some hours using the binoculars, interspersed with climbing uphill, we sighted what we were after, a family group of twelve chamois with large Buck in attendance. The European Chamois arrived in New Zealand in 1907 as a gift from the Austrian Emperor, Franz Joseph. They are considered a threat to native plants and hunting is encouraged year-round. There are no laws governing flying and hunting on the same day, in contrast to some places.
