Building The U. Maine Trainer
- Reinforcement -
- Reinforcement -
Now that all the major components of the aircraft have been cut from stock, it is possible to start putting them together to form a flying craft.
Unfortunately the material, building foam, from which these parts were cut is not very strong. If the parts were simply glued together as is, the aircraft will eventually break. It probably would break in flight but it will certainly break on landing. Somehow the foam needs to be made much tougher. Fortunately there is an easy technology which can be used to do this job. It is called glass reinforced plastics. Often referred to as "fiberglassing" or GRP (Glass Reinforced Plastics).
In the picture above fiberglass tape, the roll can be seen on the table, is being cut into thinner strips. The thinner strips will be used to reinforce the rudder and horizontal stabilizers.
Longer full width strips have already been cut for the wings. These strips will be cut down the middle so that one strip can be epoxied to the top of a wing, and the other can be epoxied to the bottom side of a wing.
As seen above, there are two strips per part. Note that the strips are cut just a bit short. This is so that there will not be any overhang of the ends of the parts. The parts will not be sealed. This means that end faces can promote peeling. If the glass does not go all the way to an edge, it is less likely that it will peel. Edges are often stress points, and promote separation of stiff things, like fiberglass locked in epoxy, from not so stiff things like foam. A way to avoid this is to not bring the fiberglass all the way out to the tips of the surface. This is true for the rudder and elevator as well.
There is less concern about this on the inner edges. In the case of the wing, the inner faces will be glued together, and wrapped with fiberglass. On the rudder the lower edge will be epoxied to the fuselage. In this way there will be no exposed edges to promote peeling.
If there is an exposed edge, a simple way help reduce the likelihood of separation is to cote the faces, in the immediate area with some epoxy. This should help seal the area, and reduce the chances of peeling. Also try to round the edge as best you can. Sharp edges also promote peeling.
There is one other part that needs some fiberglass. That part is the fuselage.
If you look closely in the above photo, the fuselage can be seen under some 1.5 oz. fiberglass cloth, just after it was cut from a larger piece of stock.
In the next photo the rough cut cloth is trimmed to follow the contour of the fuselage.
The idea is to from a "U" across the bottom of the fuselage, for the full length of the fuselage. The "U" need not be deep. It could go up the sides of the fuselage only a centimeter or two (0.5 to 1.0 inch). In this case it was desired to make the fuselage stiffer, so the glass was brought up almost to the full height of the fuselage. Obviously a full wrap of the fuselage would make the fuselage very stiff. The price you pay is heavier weight. One should be hesitant to add weight to an airframe.
There is one more part that needs to be made before epoxying begins. That part is the firewall.
In the above photo a piece of 1/4 inch Lauan plywood was used. A better choice would have been 1/8 inch or more hobby grade plywood. However, the Lauan was all that was on hand at the time. In the past Lauan plywood has been available at most building supply stores, although that may not be true these days. Again the better choice is clear 1/8 Birch or Poplar plywood from a hobby supplier. If needs be 1/4 inch plywood could be used but it is overkill for this application. All you are trying to do is provide something to fasten the motor to.
As in this case, being stuck with Luana, 1/4 in ply is best. Lauan is a soft material. It needs to be thicker so that screws, when used, have enough material to bite.
It would be better to bolt the motor to the firewall. But bolts were not available, and the installation will be blind. That means there will not be access to the back side of the firewall, so bolting is more difficult. With more work, access to the back side of the firewall can be made. But we are trying to keep things simple.
Above is a view of the bond between the firewall and the front face of the fuselage. Care was taken to be sure the faces were flush, so that the epoxy (regular 5 or 6 minute cure epoxy) would bond over a maximum area. The fuselage fiberglass will overlap the wood to help distribute the propeller load. The firewall plywood was cut a millimeter or two larger than needed. After the epoxy cured the edges were sanded flush.
Now that all the major components are prepped for the epoxy... it's time to get to it.
We will be using WestSystem epoxy.
The WestSystem epoxies are good quality products, used in a variety of industries. They are not cheap, and can be hard to find. Most quality marine suppliers should carry WestSystem products. On line there is CST or Aircraftspruce. I'm sure there are other vendors. The nice thing about CST is that they are geared toward small scale aircraft. The online sources CST, and Aircraftspruce also carry other manufacturers of epoxy, most of which are more expensive than WestSystem.
The reason why WestSystem, PRO-SET or MGS epoxies are used is because these products can be tailored to the application. Off the shelf hardware store 5 minute epoxy is too thick and awkward to work with. These "professional" products have enough work time, and are thin enough to provide quality bonding to most any surface. They can also be tailored to produce different surface qualities.
Since these higher quality epoxies are more expensive, although worth it in my opinion, it is best to be fully ready to apply them, so that you don't waste them.
It isn't that garden variety 5 minute epoxy isn't any good. Quite the contrary. Up to now, and for quick in the field repairs, such products are the go-to solution. Such epoxy was used to glue the firewall to the fuselage. The problem with it is its short work time, and it is WAY too thick for use in GRP laminates (layers of fiberglass, and other materials), and construction.
But lets get on with the job at hand... we need to mix some WestSystem 105 resin, and 205 hardener...
We will be using the WestSystem 105 resin with 205 hardener. These products are mixed 5:1. That's 5 parts resin to one part hardener. This means that if you measure out one part 205 hardener, the total volume once mixed with the correct amount of 105 resin, will be SIX times the amount of hardener. The following pictures illustrates this.
In the above picture a cup is be marked for how far to fill it. Two other cups will be marked in the same way, for a total of three cups.
One cup will be used exclusively for the hardener.
The other two will be used exclusively for the resin.
In the above photo the two cups for the 105 resin are about to be filled. Why two cups? Because it is harder to loose count. During a typical build session, there are several folks who need help building their planes. As questions get shot out at random, it is easy to find yourself wondering "... was that the third or the forth resin cup...???". With two cups you can pour two sets, then add one.
On the fly, this simply works.
No doubt there are better ways of doing this. One would be to measure the height needed on the walls of the cups and memorize those numbers.
With cups like these, with slanted walls, do not be tempted to simply measure up 5x the distance in the 205 hardener cup. You will end up with too much resin. If the walls were straight, not conic sections, simply measuring would work. However, in this case because the diameter changes with height, the surface area increases with height. This means more material is needed to create the same change in height. The difference is slight but INCREASES the greater the difference in height. In our case that is a factor of five times. It is better to be accurate (reasonably so) when you measure these materials. Their ratio can greatly effect the quality of the end result.
The idea, with two sets of cups, is to always keep the two parts of the epoxy separate until you mix them in another container. In this way, if you need to make more you can reuse these "measuring cups". Just make sure not to mix them up. Sometimes it is hard to tell the difference between them.
Always use a separate mixing cup. Mix the materials thoroughly, and scrape as much from each cup as you can. Within reason.
In the photo above the mixing cup is shown above one of the wings, as a base coat of epoxy is applied. A piece of cardboard was used as a stir stick. In this case it doubled as an applicator.
The best thing to do is read the manufacturer's instructions. They tell you how much you will need to cover X, Y or Z surface area, with or without thus and such cloth. In our case we have someone who has already done this particular job a few times, and he knows how much to use.
It is better to waste some epoxy rather than run out. If you don't have enough you will likely be forced to rip out what you did, and start all over again.
DO NOT MIX SO MUCH THAT YOU CAN'T USE IT FAST ENOUGH!!!
Even if you know exactly what you want to do, it may take too much time to do it. Epoxy has a specific working time. Some working times are longer. WestSystem's 206 hardener has a longer working time than their 205 hardener. If a particular job requires a good deal of application, alignment or clamping time, see if you can do it in steps or use a slower hardener.
There is no harm in rehearsing. Start a stop watch and see how much time it takes to work with the part, the fiberglass, and some imaginary epoxy. If you don't have a healthy working time margin, do the job in steps.
In our case the amounts of 105/205 were enough to work the fiberglass reinforcement onto the wings, rudder, and horizontal stabilizer. There was NOT enough material or curing time to do the fuselage. That had to be done later.
Above the epoxy is being spread so that it will wet the entire surface below the fiberglass strip, which will then be placed upon it...
With a large enough wetted surface, the wing reinforcement glass is placed on the wing. Next the glass is tamped down onto the wetted surface...
Small additional amounts of epoxy may be needed to fully wet the glass. The idea is to have only enough epoxy to wet the fabric, and the surface it lies upon. This helps keep the weight down. Remember that there is not a whole lot of strength in the epoxy itself. It is quite brittle, although not as brittle as polyester resin. Its primary function is as a sticky bonding agent, which can work its way into semi porous surfaces, like fiberglass, wood, foam...etc. Once it works its way in, and hardens, the result is an integrated form fitting structure, whose combined strength is far greater than its constituent parts.
If you haven't already, cut some wax paper to cover the entire bottom surface of the wing. Use as many pieces of wax paper as necessary. The idea is to protect the epoxy/glass/foam sandwich, and to allow the assembly to be placed on other surfaces without sticking. Use a generous amount of wax paper. Epoxy can migrate to unexpected places...
With the wax paper in place flip the wing over and apply reinforcement to the other side... Now do the same to the other side...
The choice of the location of these glass strips is not random, although it is approximate. They are located near the typical center of lift, along the wing profile. This is not a fixed location. It moves about with the angle of attack. For this trainer it's close enough.
Don't forget the wax paper... Why wax paper? Epoxy does not stick to wax very well. If some wax paper does get stuck it can easily be sanded away.
Waxes are often used as mold release agents, although there are also specifically marketed mold release agents.
You have another half of the wing to do... so get busy. If you were smart you would have mixed enough epoxy for both wings, and had everything, glass strips, wax paper... etc, ready to go for one batch of epoxy. If you were REALLY smart you'd have all these materials ready for the horizontal, and vertical stabilizers as well...
Here's the same sort of steps for the stabilizers...
Now all you have to do is put some weight on the surfaces, and let the epoxy cure. In a pinch phone books, or text books can be used. Text books are good for the planer surfaces of the vertical, and horizontal stabilizers. The phone books are good for curved wing surfaces. There are other alternatives which will be covered later.
As you can see above, once the epoxy has cured (a few hours are best), the wax paper should peel off easily...
Above a compound miter saw is used to bevel cut the wing tip. While not strictly necessary some folks find this ascetically pleasing.
The same type of saw makes squaring up the inner wing ends easy. It also can be used to put a dihedral bevel (often only an angular degree or two. on one or both of the inner wing ends.
A dihedral bevel is important, if a dihedral is to be used. Structural integrity demands as few voids as possible. This means bubbles in fiberglass laminates, and gaps between parts should be minimized. Voids in the structure introduce stress points. Stress points promote micro cracks. Micro cracks promote cracks. Cracks promote structure failure.
Now it's time for main wing assembly..