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Building The U. Maine Trainer

- Assembling The Main Wing -

(last update: 15 Sep 2012)

The next thing to do is assemble the wing, so that it is an integrated unit.

Wing assembly will require:

What's a conformal weight?

It is a couple sandwich bags filled with lead bird shot. I would like to build these out of fabric, and then use the sandwich bags as protection from epoxy, or other glues. But I haven't managed to do that just yet. Your local outfitter shop should have bags of various types of shotgun load. These conformal weights use No. 6 shot. I bought a 25 lb (ll.34 kg) bag of it from a local gun shop.

Cut out two fiberglass cloth squares. These will be used to reinforce the center section of the wing. This is important because the center section of the wing is where all the lift of the wing is transfered to the fuselage. The fuselage is the most massive part of the aircraft. This means the majority of the forces are concentrated at the main wing/fuselage interface. This area needs to be strong enough to handle these forces.

Cutting out the wing center fiberglass reinforcement.

The dimensions of the squares are not critical. However, they should be sized such that there is 4 cm to 5 cm of glass extending from the leading, and trailing edge, out toward the wing tips. The excess glass will be wrapped around the leading, and trailing edges.

This should be more clear in the following photos...

Cleaning up the edge of a wing center fiberglass reinforcement.

Above an edge of one of the squares is being "cleaned up", by trimming a fraying edge. The cloth should be uniform in its weave whenever possible.

Rough location of center wing reinforcement patch.

In the above photo the upper center reinforcement patch is placed roughly where it will be glued, with respect to only one of the wing halves. The highlight indicates the portion of the patch that will be wrapped around the trailing edge, once the epoxy is applied, and the other wing half is in place.

There is a problem with this test fit. Can you spot it?

Location of center wing reinforcement patch.

Main wing center reinforcement patch looks like a generous fit... no?

Even people who "know what they are doing" make mistakes. Especially when they are "winging it"... ha ha ha.

Attaching the main wing trailing edges to the inner ends of the wing halves.

The photo above is a dead give away...

In the photo above the trailing edges of the wings are being glued to each half of the main wing. This is a good place for inexpensive "hardware store" 5 minute epoxy. Be sure to use the correct trailing edges, although there should be little difference between the left and right sides.

... and no... that isn't the problem...

Letting the trailing edges cure.

In the photo above we are using conformal weights to hold everything down while the 5 minute epoxy cures.

The problem involves the trailing edges of the wings.

Earlier the trailing edges were cut from the wings. The cut was made the full length of each wing. This was done because it is easy to do, and no one was thinking ahead.

The wing must be attached to the fuselage. Where it is attached to the fuselage, the ailerons cannot pivot. Thus there needs to be some amount of the full wing chord, on both sides of the fuselage, in order to attach the wing. Since the full length of each wing's trailing edge was cut, there is no full chord section. This makes attaching the wing to the fuselage more difficult.

This is why the full length ailerons were cut down, about 16 cm on each side, and the smaller sections (16 cm pieces) re-attached. This provides plenty of extra trailing edge. Shorter lengths can be used, to result in longer ailerons.

If the ailerons are too long they may bind on the fuselage or the method of attaching the wing. It all depends on how the wing is attached.

Be that as it may... lets see what "the problem" is...

Reinforcement patch too small.

Oooops... The reinforcement patches are too small.

They had been sized without the trailing edges attached... rookie mistake.

Let's go with it... it's only a trainer... if needs be we can always add a bit more cloth to protect the leading, and trailing edges.

But the first thing to do is glue the two wing roots together so that we can work with the wing as unit. Up to now each wing half has been treated separately.

Coat the wing roots with epoxy.

Apply some epoxy to each wing root. Here some of the high quality epoxy is being used. Alternatively the cheap 5 minute epoxy could have been used. The good stuff was used because we were about to reinforce the fuselage as well.

Be that as it may...

The goal here is to wet both wing roots with epoxy before joining them. Prior to doing this you should have laid out some wax paper for any excess epoxy at the wing root joint, and prepared a location on the bench where the wing assembly can cure undisturbed. Also needed would be sufficient conformal weighting, and something to prop up one wing tip to help set the dihedral. And a straight edge to align the leading edge of the wing.

I have to apologize at this point. The next steps for main wing assembly are poorly documented. Sometime in the future this will be corrected.

For the time being the following photos will have to do.

Main wing layup.

The above highlight shows some of the main wing layup. The leading edge is pointing to the left of the photo. A piece of angle aluminum (typical home building center or hardware store purchase) is used as a straight edge. A pile of fuselage reinforcement fiberglass obscures the center of the wing, and its joint. Over the joint is a pile of conformal weights, with a sheet of wax paper between them and the joint. At the upper edge of the photo highlight a conformal weight is holding the right wing tip down on the bench.

Main wing layup2.

In the photo above the center conformal weights and the left wing tip conformal weight can be seen on the right edge of the photo. Also there is a wood block holding the left wing tip above the bench. This block helps to keep the dihedral angle stable while the epoxy cures.

The angle aluminum is used to align the leading edges of the two halves of the wing. This main wing as a straight wing design. There is no sweep angle. The angle aluminum is there to help guide this alignment.

After the wing root cures enough to be moved the reinforcement patches can go on...

Main wing reinforcement application.

In the photo above the bottom side main wing reinforcement patch is being wetted onto the bottom side of the wing.

Once the bottom side patch is wetted onto the wing, flip it over. Wrap the bottom side leading edge excess cloth up onto the top side of the wing.

Top side main wing reinforcement application.

Then wet the top side patch onto the main wing. once it is all wet, drop a generously size sheet of wax paper over the top. You will likely have to cut a slit in the leading edge to the wax paper right on center. Otherwise the wax paper will not lie flat. A small piece of wax paper can be used to cover up any gap.

Light conformal weighting can help apply some pressure to force the glass to lie flat against the wing surfaces. For this to really work, since the wing is bent (has a dihedral) the bottom of the wing should lie on something soft. Old seat cushion foam maybe. Only use two or three conformal weights at most.

You don't have to do this extra weighting. You could just set the wing aside with the wax paper covering, and let it cure.

That's about it for the wing for now. Once the wing cures, it will be time to think about adding the ailerons, and servo control.

Once the wing cures, it will be time to think about adding the ailerons, and servo control.

However, there is quite a bit that needs to be done to the fuselage.

Reinforcing the fuselage