Airfield Models - Installing Robart Hinge Points

Join Wing Panels of a Flying Model Airplane

May 03, 2015

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Back to Joining Wing Panels


Airfield Models ( Multiple Wing Panels

It is important that the wings are the correct shape but minor variances across the panel are insignificant.  By that I mean if the sheeting is a hair thicker in one area on one panel than the same area of the matching panel then you'll never notice a difference in flight.

You could map out a grid on the wing and take measurements with a micrometer if you're extremely obsessive and wanted to take the time to do it.

What is important is that the outline of the wing is the same for matching panels.  Especially important is that the leading edge shape is correct which is where we're going to start.

Important!  This wing does not have a dihedral brace.  If the wing you are building does have a dihedral brace you should always insert it into both panels while fitting them together.

Check the panels against each other to see where they must be sanded to match. Always sand the entire panel, not just the area that doesn't match.

For example, this photo clearly shows the leading edge is not the same on both panels.  We could just sand the leading edge in the root area until the panels match, but the edge is probably wrong along the entire length of the panel.

Use a long sanding block, such as a T-Bar, to sand along the entire leading edge.

Again, work slowly and check your work frequently.  Be sure to carefully align the wings on the building board as if you were going to glue them together.

The better the panels match, the easier it is to apply the finish. This photo was taken immediately after the one above.  It looks like the leading edges match well but nothing has changed from the photo above.

Note that the sheeting on one panel is slightly higher than the sheeting on the other panel.

While this could be left alone because it won't cause problems from an aerodynamic point of view, it will make it more difficult to finish the wing.

If the panels are off significantly then there is no question that you should spend some time on them using a sanding block.

Use good sanding blocks and work on specific areas until both panels are a close match. I spent over an hour working on the two panels individually (taken apart) to bring them to a close match.  Remember, we aren't actually working on just the center, but the entire wing.

I frequently fit the panels back together to check my progress.

Before the panels are actually glued together, I dry fit them and work out a jigging system to ensure everything is ok and that the panels will stay in the correct position while the glue dries.

I already did this several times when I was sanding the wing root in the previous installment of this article.

A good match is achieved with sanding blocks, elbow grease, time and patience. The panels are a very close match, the leading edges are the correct shape and I'm a happy guy.
Dry fit the panels by jigging them on the board with the tips propped up to the correct dihedral angle. The magnet shown here is propping up the wing panel.  You can prop up the panel anywhere you like as long as the tip is the correct height off the board.

Another magnet is used for the opposite wing panel.  It is in an exact mirror image location as the magnet for this panel.

Do whatever it takes to ensure the panels are aligned perfectly and will not shift while the glue dries. Joining the wing panels properly is extremely important.  I could just prop up the tips to the correct height, eyeball the center sections and leave it at that.

I don't care for this practice because on more than one occasion I've had the panels shift while the epoxy cured.  The only fix was to saw the panels back apart.

It's better just to do it right in the first place by jigging the panels so that they stay in the correct position until the glue is dry.

This wing is a rectangular planform so a straightedge at the leading or trailing edge will ensure the wings align properly.

Some instructions indicate that one panel should be flat on the board and the other panel is propped up.  While that works in theory, what can sometimes happen in practice is the propped up panel slides down which creates a strange center section.  As long as the panel is propped up to the correct height and the panels aren't twisted, you can use the method if you prefer.

My preference is to prop up both panels.  I think it gives a better end result.

At this point you can permanently join the wing panels if you are satisfied that everything is correct and that you have a way to keep the panels aligned while the glue dries.  Put a piece of wax paper under the center of the wing so the wing isn't glued to the board or the plan.

Do not join wings using fast-setting glues such as cyanoacrylate or 5-minute epoxy

I use 30 minute or slower epoxy to join wing panels.  Having time is something I especially appreciate when an operation is critical.  Straight wings fall under this category.

Smear a thin coat of epoxy on both root ribs.  Take some time to work it into the grain.  A thick coat of glue is not necessary or desirable.  Butt-joins are inherently weak and more glue won't improve the situation, but will make sanding much more difficult later.

If the wing has a dihedral brace then you can join the panels in your hands and then put them on the board.

Coat both sides, the top and the bottom of one half of  brace.  It is a good idea to also coat whatever the brace contacts inside the wing panel if you can reach it.

Slide the brace into one panel and then coat the other half.  Slide the second panel on the brace.  Pay attention to the brace to ensure it doesn't slide too far into the first panel.  Push the panels together firmly.  Wipe up excess glue using paper towels and alcohol.  You'll be glad you did this.

If the wing does not have a dihedral brace then you'll have to join the panels on the board unless you have a helper to hold the panels together while you wipe up excess glue.  Ensure that there isn't too much glue on the root ribs because it will squeeze out from the bottom of the joint where you can't get to it to wipe it off.

Put all the jigging back in place after the panels are on the board.  In the photo above you can see two magnetic upright fixtures placed at the spar ends.  These fixtures are applying clamping pressure to the wing center section.  You can also use heavy weights such as cinder blocks, car batteries or whatever you happen to have.

Measure everything immediately.  Ensure the two panels are not twisted.  Check that the wing tips are propped to the correct dihedral.  If you find that you simply can't get everything aligned properly then it is best to take everything apart and wipe off all the epoxy using alcohol.  If this happened then you skipped one or more steps of preparation.

Assuming that everything is correct, leave the wing to cure thoroughly before removing it from the board.  If the wing does not have glass cloth wrapped around the center then all you need to do is some final touch-up sanding and the join is finished.



Setting the Foundation for Properly Aligned Wing Panels
Fiberglassing the Center of a Wing

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Copyright 2004 Paul K. Johnson