Tail feathers:
Again you have a number of options here all of which work well. 

You can assemble the tail exactly as per the GWS instructions and it will work just fine. 

I make the following deviations from the GWS plans:  I first cut about 1/8" off of the back of the horizontal stabilizer and the front edge of the elevator.  I then laminate a piece of carbon fiber tube or bamboo stick across the area I just removed to reinforce it and bring it back to its original size. 
Wings:
The Beaver main wing is "under cambered" meaning it is concave on the underside.  Under cambered wings provide lots of lift with minimal power, which is why the Beaver is such a docile slow flyer…. However this also means the Beaver is very susceptible to wind especially with the light foam construction.  The GWS instructions on assembling the wings note that they should be put together with 10 degrees dihedral.  Dihedral is the "V" shape the wings have when the 2 halves are put together; too little dihedral and your plane will be less stable and will not turn as smoothly… Too much dihedral not only looks funny but can also have a negative impact on flight characteristics.  Not to worry though as the Beaver is very forgiving and anything from 3 degrees to 10 degrees seems to work fine. To set your dihedral place one wing on a flat surface then lift one wing tip up based on the measurement in the table below to set the desired dihedral … remember you can go less than the 10 degrees noted without fear.  I regularly use 5 degrees for my builds.  A simple method for estimating stock dihedral is to use the supplied wing saddle as a reference and simply move one wing up until the saddle sits nicely on both wings.

Click on the above images for a larger image


Once you have the dihedral set its time to join the two wing halves; general consensus is that the double-coated tape supplied by GWS is of no use to do this and that the best option is as follows:
·
-Clean mating surfaces with alcohol before gluing

-Place a piece of waxed paper on your bench top so you don't glue your wings to the bench

-Poke some holes in the two mating surfaces with a toothpick to allow adhesive to flow in and provide a stronger bond

-Use 2 part epoxy and micro balloons to adhere the two halves (micro balloons is a filler for 2 part epoxy which allows you to use less epoxy which is a good thing because epoxy can get heavy.  Some modelers use baking soda instead of the balloons however the soda only thickens the epoxy and doesn't lighten it as the micro balloons would)

-One other glue option is a polyurethane adhesive such as Pro Bond or Gorilla Glue.  Pro Bond can be a little harder to work with and requires moisture to set.  Simply mix some Pro Bond with water in a disposable container and feed it into the joint with a toothpick or Popsicle stick or moisten the contact surfaces with a damp sponge, apply the Pro Bond and push them together.  Place a piece of clear packing tape over the bond as the glue will expand and if not contained its tough to sand down smooth afterwards
NOTE: let the wing cure completely before handling!

Once the glue has cured you can apply the wing saddle using 3m spray adhesive, which is a contact cement.  Spray the saddle liberally then place it on the wings and press it down gently to get glue on both surfaces.  Immediately pull the saddle back off and let the adhesive cure for 15 minutes and then place the saddle carefully back on the wing and press into place.  Another option is to use a 1" strip of very thin styrene to cover the joint and dress up the top of the wing however this is only an option if you will not be using elastics to mount your wing (more on this later).

Now you can apply the strut mounts to the underside of the wing.  Again clean the mounting areas with alcohol first and then poke some small holes using a toothpick to make sure the adhesive soaks in.  Another point that is generally agreed upon is to turn the strut mounts both on the wing and on the fuse 90% to what GWS suggests in their instructions.  I use 2-part epoxy to adhere the parts and then as a reinforcement I take a 1.5" square piece of paper towel and cut a slit in it large enough to fit down over the strut mount.  Place the towel over the mount and then soak it with epoxy and push it in nice and snug and smooth around the mount; this will give some added insurance that your joint remains strong.

Modifications to reinforce that floppy wing:
There are a couple of different things that you can do to reinforce the somewhat floppy wing on the Beaver.  Many people have flown the Beaver with no wing modifications and as long as you fly it slow and only do scale maneuvers (no loops, power dives or rolls) and avoid crashing the wing seems to perform adequately.  Aggressive flying with the stock wing can cause it to flap and ultimately fail in extreme circumstances.   If you are a beginner or know you are going to fly this plane at or near its limits some or all of the following modifications are in order:

-GWS suggests placing an "X" of tape on the bottom of the wing which seems to help somewhat

-Many people tape or glue a piece of carbon fiber to the leading and or trailing edge of the wing.  In my experience the key is to place it right on the edge of the wing as opposed to under the wing or it can impact flight characteristics…. The thinner the CF the better

-GWS is now including a wing strut modification instruction (LINK) on the CD they include with the more recent Beaver kits.  This mod involves using an extra piece of bamboo strut material to add a strut running to the trailing edge of the wing

Wing struts:
The wing struts can be assemble in a number of different ways however one thing seems to remain constant:  if you are using the provided plastic clips and bamboo as struts you should
consider drilling a hole through the collar of the clip and the bamboo and then run a piece of wire or a straight pin through the hole pinning the two together.  Use a dab of CA to hold
everything together and you have some extra insurance that your struts will remain attached.

One trick to getting the pin through the plastic and bamboo is to cut the head off and chuck the pin in your moto-tool.  Cut the tip off the pin with a pair of side cutters and the resulting bur can be used as a drill tip to drill a perfect size hole.  Simple CA the pin in, trim it flush and finish with a file.

Also shown in the picture is another popular aesthetic mod which is to slide a piece of streamlined aluminum tubing made by K&S
over the existing bamboo to dress it up.  Some take it a step farther and use Dubro threaded rod and end links inside the streamlined tubing, which allows for some adjustment of dihedral
One popular and highly recommended mod is Art's Struts as designed by Art Schmitz from RC groups and outlined here:

Make the struts out of 1/4 or 5/16th streamlined aluminum tubing, (4) 2-56 threaded ends from old push rods cut at 3/8" past the threads and put a slight bend in the smooth ends and lastly (4) of the smaller Dubro white nylon kwik-klips.  Now, draw a straight line a foot long on a piece of flat cardboard, which is part of the kit packing and mark off your hole-to-hole dimensions. Thread the nylon clips on to the threaded ends with about 4 threads showing through the clips then cut the tubing so that the threaded assemblies have a 1/4" amount of threads past each end of the tubing.  My tubing
length is 8 3/8".  Time to assemble the struts; dip each open end of the streamlined tube into alcohol about 1-1/2" and let it evaporate.  Wipe the ends with the slight bend in them with alcohol and place some wax paper over your jig line. Have a pair of pliers handy and mix some epoxy.  Use a toothpick to put a small amount of epoxy in each end of the streamlined tubing at the front of the streamlined cross section.  Put some epoxy on the bent ends of the assemblies and position them in each end with the bend facing aft making certain that everything is spaced correctly.  Use the pliers to crimp the rear of the streamlined tubing right up to the threaded ends  (mine are crimped 1/4" from each end).  The epoxy that 'squeezes out' is then cleaned off with some alcohol on a cloth and the assemblies are set aside to cure (the slight bend will not pull past the epoxy and the crimped end). Your Kwik-klip pins should be the correct hole-to-hole distance apart. Drill a 1/16th hole in the middle of the crimped metal at one end of each strut.  This hole will become the out board or wing end.  Clip your nice looking struts on to your mounted wing first, then to the fuselage. This alone will helps to straighten the leading edge of the wing.  Measure the angle between the wing trailing edge and the fuselage and adjust the individual strut lengths accordingly.
Here is where we deal with the trailing edge irregularities: Cut two sections of .030 to .040 music wire 6" in length, and put a Z bend in one end.  Hook each of the Z bends in the hard points already installed near the trailing edges of your wing. Now; one side at a time, swing the straight end of the wire up to the hole you drilled in the strut crimp.  Sight down the trailing edge and move the wire back or forward to straighten up the T.E. and mark that wire at the strut hole.  Carefully bend the wire at this point at a right angle bend that will pass through the hole. Use a small sized sliding keeper or a small wheel collar to retain it.  Repeat for the other side. I have found that the fiberglass packing tape does nothing for warps so I simply leave it off. The next, and last, thing to do is to provide for the rubber bands at the leading and trailing edges on each side.  I have cut sections out of the kit supplied plastic center section and glued them to the wing in the appropriate places; strips of 1/32 ply painted white also work to protect the wing. One further note: .It seems that setting the mounted wing and fuselage assembly out in the sun for a few hours helps normalize the manufacturing stresses and further relaxes the wing.
Alternatives to mounting wings with rubber bands:
Many people do not like the look of rubber bands across the top
of their wing. Some other wing mounting options are as follows:

Nylon bolts and nut:  Mount a piece of balsa or basswood across the inside of the fuse and then drill though the wood and the wing so the holes line up.  Glue your nylon nuts to the bottom of the wood in the fuse.  Install a tube that the new wing bolts will slide through into the holes in the wings.  The tubes should be cut to length so that the stress of tightening the bolts is completely on the tubes and not crushing the wings.

Magnets for wing mounting (no more rubber bands!):
Many people use small ¼" round rare earth magnets to hold on the wings.  To install them, locate 4 places where the wing contacts the fuse.  Carve out a little "hollow" for the magnets in both the wing and fuse and use 5-minute epoxy to hold them in place. After the epoxy sets you may want to put some clear packing tape over the magnets in case they happen to come loose.    This may not appear to be strong enough but when used in conjunction with the wing struts it should hold fine as long as you glue them securely.  One added bonus to this method is that in the even of a crash or a wing tip hitting the ground your wing simply pops off with little or no damage.

A variation on the above method is to use only 4 magnets; 2 on the trailing edge of the wing and 2 recessed in the top of the fuse to match.  To hold the front of the wing down glue a locating dowel on the center of the wing (use either a piece of Carbon Fiber rod or a piece of bamboo stick) and mount a piece of lite-ply in the front of the wing opening with a hole to locate the dowel.  The advantage to this method is that the wing will still pop off on a hard landing but its more securely located during flight.  This is the method preferred by the author as shown below on some images of his Beaver modeled after C-FIUU
Other wing modifications to consider:
Myself and others are having great success with the Hobby Zone Super Cub wing from Horizon Hobbies.  Its nearly 48" long which is very close to correct scale for the GWS beaver plus it is a full profile flat bottom wing.  The chord is slightly to wide but that can either be overlooked or you can cut it down to suit your tastes.  More information on the Modified beaver build pages.
Some people are covering the bottom of their wing to give it a flat bottom.  NOTE: This looks good but requires higher speeds for lift
One popular mod is to install a scale flat bottom wing
cut from  a solid piece of foam again this looks great but
requires more  speed and generally the addition of
ailerons as well.  These  scale flat bottom wings are
available from RC groups member  "Spithungry"  as seen
in the image to the right.
Tracon, a member of RC Groups actually pieced together 2 sets of GWS wings to make a more scale 46" wing for his beaver by "butt joining the pieces; the only down side is that the plane must be flown in a scale fashion or the wings tend to flutter.

There are a number of other methods to make your own more scale sized wing posted on RC groups; here are a couple of them:

By "Grejen" from the groups, using poster board
(depron or celfoam would work also) with a carbon fiber tube taped to the LE.
1. Cut two 24" X 5.5" panels. Soaked and peeled and soaked and peeled all the paper off of both sides of the poster-board (note paper peeling is only required for poster-board) then curved them around a rolling pin to get an airfoil of sorts. Carefully slice and sand a 1.5" deep taper off of the TE (trailing edge) underside to get a sharp TE shape. Taped / glued the 2 pieces together in the middle and also taped a piece of poster board behind them to cover the gap thus making a 48" wing.  Create new strut mounts from scrap aluminum or simply use the GWS ones and measure carefully before using epoxy to mount them in place.

2. Add a 3/16 " carbon fiber tube to the LE (leading edge) either full span or just to span the length of joint.  This step will alleviate any drooping issues.

3. Constructed a bolt down system for appearance sake and because the elastic bands kept pushing the wing back.

4. Final step was to cover the length of the TE with packaging tape as it was thin and slightly ragged

End result is that the plane remains very manageable but not as 'parkie' as the stock wing.  The plane prefers to fly a bit faster despite having the same total area. You may need to switch to a 10X8 to keep the speed up.

Another wing build from RC groups by "TeamTEOR":
This is a lightly modified Clark Y foil wing designed to be used with ailerons, so it has no dihedral. The wing cores were cut out on a CNC hotwire machine including the slot for a carbon fiber rod. For carbon, two Midwest Products Stock #5724 rods .210" OD x .132" ID x 24" (5.3mm x 3.4mm x 610mm for you metric guys) were used. The carbon rods were joined in the middle using a 8" piece of 1/4" brass tubing K&S Stock# 1149. (Editors note:  you can make your own foam hotwire cutter easily and inexpensively… just do a google or search on RC groups).

The two wing halves were joined with epoxy and then covered with packing tape or your choice of low temp coating such as Ultra or Econocote.
For the rudder I use 3/8- 1/2" x 1" strips cut from the disc material inside a 3.5" floppy disc as hinges.  I then carefully cut slots in the vertical stab and the rudder and push the disc material into the slots and adhere with CA … this will also work on the elevator however you cannot glue the elevators to the stabilizer until after they are mounted on the plane or they will not fit through the slot in the fuselage.

Other popular alternatives for reinforcement are as follows:

Some people place a U shaped piece of bent wire on the bottom of the elevator for added strength

Another method is to replace the horizontal stab and elevator with 3/32 balsa.  The balsa can be covered with a scrap of Monocote or you could just as easily paint it. No perceptible weight difference from the foam and much stiffer. Some thin CA wicked into the end-grain will help keep it from splitting if you don't decide to cover.

When gluing tail-feathers don't go overboard with glue, as you may need to remove them if they become damaged.  On the horizontal stab there isn't a lot of stress so a spot of white glue at the front and rear top and bottom will hold it nicely.  The vertical stab gets 'full coverage' at the glue joint, as it tends to see more stress especially on hard landings where the plane may flip over.
For hinges on the horizontal stabilizer I prefer to use 3M Blenderm tape  (which can be found at most drug stores) and is commonly used in Hospitals to tape in your IV needle.   I tape For hinges on the horizontal stabilizer I prefer to use 3M Blenderm tape  (which can be found at most drug stores) and is commonly used in Hospitals to tape in your IV needle.   I tape only the top surface,   as taping both top and bottom would make the hinge too stiff.  the top surface,   as taping both top and bottom would make the hinge too stiff.
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