December 18, 2021

Mount a Model Aircraft Wing to a Fuselage — Part II

This is the second part of Mounting a Wing to a Fuselage.

At this point the wing should be perfectly aligned with the fuselage. You should not even consider drilling the fuselage for the mounting bolts until you are absolutely satisfied that the wing is properly aligned.

After the holes are drilled it is very difficult to make coarse adjustments.

Mounting a Wing to a Fuselage - Part 1

They Can't Be Serious!

A piece of brass tubing that is notched to act as a hole saw. Many instructions tell you to glue the dowels at the leading edge of the wing that key into the fuselage into the wing during construction. The completed wing is placed in the wing saddle and you are somehow supposed to be able to mark the dowel locations on the fuselage former at the front of the wing saddle.

If you can draw around the dowels, you still have to guess where the center of the hole should be.

After completing that ridiculously inaccurate task, you are supposed to somehow fit a drill into the wing saddle area and drill the former for the dowels.

I can not think of a more tedious or less accurate way to get the job done. There usually is not room to fit a drill so you have to turn a drill bit by hand until it cuts through the plywood former.

I have a better way:

Do Not drill the wing for the dowels.
Use the plans to determine the dowel locations in the former. Drill the holes in the former before you glue the former into the fuselage.
Make a hole cutter from brass tubing that is the same size as the dowel. The leading edge of the wing is balsa, so a few notches on the tubing will cut the balsa easily. A piece of rubber tubing glued to the tubing makes it easier to grip. The one shown here is on the short side. A longer piece of tube will be easier to work with in most cases.

	With the wing held firmly in place, use the holes in the former as a guide and hand turn the hole-cutter into the leading edge of the wing. Cut one hole and then put in the first dowel to ensure the wing stays in place while cutting the second hole. I have never trusted a dowel glued only into the balsa leading edge to be strong enough. That is why I used plywood shear webs in the center section. The dowels go through the leading edge and into the shear webs.
	Put the dowels in place to ensure the wing will remain in alignment while drilling for the nylon bolts at the trailing edge. Do not glue the dowels in place until the aircraft has been finished. It is much easier to sand, cover and paint the model without the dowels in place.
	A 1/16" plywood plate is used to spread the force of the tightened wing bolts and prevent the wing bolts from crushing the balsa wing. The holes are spaced so that they will go into the center of the wing bolt blocks in the fuselage. You can either inset the plywood into the trailing edge as I will do here, or cover it separately and glue it on later. If you choose to go that route, then remove covering from the wing in the area where the plate will go. Cover the visible side of the plate and over-lap the covering to the bottom about 1/8" all the way around. Epoxy the plate in place.
	I use the plate as a template to mark the outline with a razor blade.
	The balsa wood is relieved and the plate is epoxied in place.
	Use the plate as a drill placement guide. The holes in this plate are 3/16". A #7 bit is actually the correct bit to use when tapping for 1/4" x 20 threads. I am not sure there is a difference in these bits, but 3/16" is close enough. The holes should be drilled so that the bolts are perpendicular to the outside of the wing. This allows the bolt heads to seat properly against the plywood plate. Drill through the wing and through the blocks in the fuselage.
	The holes in the wing bolt blocks ready to be tapped. The holes are well centered due to care taken when drilling the plywood plate before gluing it to the wing. Careful alignment of the plate when gluing it to the wing ensures the bolt holes will end up in the right place.
	I run the tap in and out of the holes several times to ensure the thread is cleanly cut. After I test-mount the wing I use liberal amounts of thin CA to harden and oil-proof the holes. Let the CA set up for a couple hours and then tap the holes again. I might repeat this step two or three times to get the threads really hard. When soaking wood with CA it does not always set up quickly. You do not want to get wood fibers glued in your taps, so it is best just to do something else while it sets up. Note that the wing bolt blocks will be flush with the bottom of the wing. This is important to prevent the bolts from flexing. It also allows the bolts to break cleanly in a crash. However, if this plane crashes, it will not matter if the bolts break or not — there will be a lot of damage.
	The holes in the wing still need to be enlarged. In this case the holes need to be 1/4". After drilling them to their finished size, harden the holes with thin CA. Again, allow the CA to cure for a couple of hours and then drill the holes a second time. Here the wing has been mounted in place using nylon bolts. You can see that the bolt heads are flush against the plywood plate.

Although there are a lot of steps required to mount the wing properly, it is really a matter of taking your time and taking careful measurements. The improved flight qualities of a straight aircraft will make the work pay off.

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Mounting a Model Aircraft Wing to the Fuselage
Build the Wing Tips

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