2003 Mirage Construction Notes

A short kit offered by Isthmus Model Company. The Mirage is Blaine Rawdon's original and innovative design, first published in RCM Magazine's February 1979, Issue.

Construction and modifications notes by - Michael "Augie" McKibben

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The Wing Parts

This photo shows the laser cut ribs from Isthmus Model Company. I have laid out all of the ribs and separated them by panel location. This helped to locate the ribs that I wanted to modify by adding spoilers and an improved wing mount.

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The Fuselage Parts

Perfectly cut fuselage parts, ready to assemble. Also shown are the plywood sides for the wing tip joiner boxes and the joiner blades.

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Modifying the Ribs

I decided to install spoilers on my Mirage. This photo details the ribs that I modified to accept a 11/4" x 5/16" spoiler blade. The recess must allow the spoiler blade to stand just above the surface of the ribs. Later, carefully sand the top surface of the spoiler blades to match the wing's airfoil.

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Modifying the Ribs Continued

Here are the modified spoiler bay ribs with their neighbors, in order of construction. I have added a 1/8" x 1/4" spar slot at the rear of the spoiler bays to accommodate a spruce sub-spar that will back up the spoiler bays. The sub-spar must be even with the airfoil surface.

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Modifying the Ribs Continued

This is a photo of ribs with the addition of a 1/8" hole towards the trailing edge. This allows me to install the tubes that will house the spoiler cords.

Also, I have increased the depth of the center section's main spar slots, top and bottom, allowing for .007 thick carbon fiber laminated to the bottom side of both spars. I used 220 grit self adhesive AR sandpaper, stuck to a piece of spar material to achieve the proper depth.

Note: You will need to make a duplicate of the two center ribs. This new rib will be split at the spar and a duplicate of the forward piece will need to be made as well. This will be installed later.

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The Spars

Here, the finished spars are ready for construction of the center section. The main spars are made from 3/8" x 1/8" x 48", good quality spruce laminated on one side, to a piece of 3/8" x .007 x 48" pre-preg carbon fiber. I used West Systems Epoxy and clamp my assembly to the table against a straight edge.

Note: Make sure that there are no knots or cracks in the spruce you select for these spars. All of the strength for the wing is designed around a strong center section.

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Strengthening the Center Section

The center section is going to have an improved wing mount design, replacing the original rubber band style mount. I wanted to strengthen the center of the wing to accommodate the new design. I beefed up the center shear web by using 1/8" birch plywood, laminated with slow cure epoxy, between 1/8" vertical grain balsa shear webs. I used slow cure epoxy to glue the new shear web to the top side of the bottom spar as well as to the adjacent ribs.

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Shear Webs

This photo shows the placement of ribs and shear webs. I used aliphatic resin to glue the ribs and shear webs to the top side of the bottom spar. Working from the inside out, glue rib, web, rib, web and so on. I increased the thickness of the shear webs incrementally from the center out. I used 3/8" vertical grain balsa from either side on the center bay, three bays out. I used 1/4" shear webs in the next three bays and finished off with 1/8" shear webs, stopping at the wing tip joiners.

Test fit the top spar. Make sure that it makes contact with all of the shear webs and is even with the top of the ribs. Sand with a piece spare scrap and sand paper until satisfied with the fit. When you have a good fit, use slow cure epoxy to glue the top spar in place and weigh it down until fully cured. Clean up all excess epoxy before it sets up.

Note: I like to pre-cut my webs to the correct width and use them to set the appropriate distances between the ribs. This is usually a more accurate way to set ribs. Relying on your plan set is problematic at best because paper expands and contracts with its environment.

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Joiner Box Construction

Using a piece of scrap plywood the same thickness as the joiner blade to fill the joiner box space. Use slow cure epoxy and clamp the joiner box side to the top and bottom spars. Use epoxy sparingly here. Too much epoxy oozing into the joiner box will make a good panel fit very difficult.

Note: The joiner box sides are angled on one end. Be sure that the angled end is toward the outer rib of each panel. This helps ensure the correct dihedral for the wing tips.

Use the same method as above for the tip panel joiner boxes. Note that the sides of the tip panel joiner box are made of 1/64" plywood. Use some scrap material on the outside to spread your clamp pressure evenly across the 1/64" plywood side as well as inside to insure the correct joiner box width.

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Spoiler Bay Construction

Here are two photos showing the spoiler bay detail. I positioned the spoilers just behind the main spar, one bay in from the outside edge of the main panel and backed it up with a spruce sub-spar. I stiffed the sides of the spoiler bay with 1/4" x 1/4" hard balsa. The balsa is also needed to give the Monokote™ a good contact surface. The spoiler cord tube is held in place under the sub-spar with a piece of scrap spruce with a recess sanded in its top to accept the tube.

Note: The spoiler actuation that I used has one long cord attached to one spoiler, traveling down the tube and exiting the bottom of the wing, through the main bulkhead and is looped around a ball joint mounted to a servo arm, then back around and through the main bulkhead, into the other spoiler cord tube, through that tube and attached to the spoiler on the other side of the wing. After I have the correct length, I wrap a little thread around the cord just behind the ball joint and tie a knot. A drop of CA will hold it in place.

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Wing Mounting Detail for the Center Section

This photo details the additional rib in the center bay, the installation of the turbulators, the top main spar and the bottom center sheeting.

  • Install the lead edge material, the bottom turbulators (only) and the bottom center sheeting.

  • Install some ofthe trailing edge material on top of the bottom sheeting, in front of the trailing edge, between the two center bay ribs and flush with their top edge.

  • Take your duplicate rib and split it at the spar joint. Make a duplicate of the piece that goes in front of the spar.

  • Remove the necessary material to fit the rear half of the new rib between the trailing edge material and the spar. Make sure you are centered and glue in place.

  • Install the two forward rib halves between the front of the spar and the lead edge. They should be 1/4" apart and centered. This void will accommodate the 1/4" hardwood dowel used as part of the wing mount.

Note: I suggest that you substitute the 1/8" x 1/8" balsa turbulators, in the center section only, with good quality spruce or even basswood. Use balsa for the tip panels per the plans. The success of the glider depends on light weight tip panels.

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Completed Center Sheeting

This photo shows the completed sheeting over the center of the main wing panel. This is the only sheeting for the entire wing. The tubes for the spoiler actuation cords run out of the bottom of the center sheeting. They exit either side of the center rib and just behind the spar, about 1/2".

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Joiner Box

This photo shows the completed joiner box for the center section. Note that end rib has not been opened to reveal the joiner box just yet. Do that after sanding the panels to just about finished and ready to cover. Be sure that you glue the end ribs in at the appropriate angle to ensure a good fit.Check this angle while sanding the ends and fine tune as necessary.

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Joiner Box Detail

These photos show the completed joiner boxes. I have wrapped the joiner boxes with Kevlar tow for a little additional bursting strength. Note that the panels are sanded to near finished before completing the joiner blade fitting and installation. The fit of the joiner blade is important. You do not want any slop up or down. If you find that you have sanded too far, you can build up and edge or side of a blade with a thin skim of epoxy and slowly sand back down to a better fit.

Note: It is a good idea to add a locating pin, made from a 1/8 dowel, toward the trailing edge of the wing joint. This will help with any twist themight occur during heavy loads.

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Fuselage Construction

This photo shows the assembled fuselage sides with their doublers. I have three words for you, aliphatic, aliphatic, aliphatic. Use aliphatic resin when and where ever it is possible. Its dry weight is only 20% of wet weight, it sands nicely and the set up time is only about twenty minutes. Full cure over night. The key to making a really nice Mirage is low weight, so take the little extra time needed for aliphatic resin.

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Antenna Tube

This photo shows the addition of an antenna tube. The tube allows for a cleaner radio installation and the extra weight is negligible. I have run the tube under the stiffeners that run vertical from the doublers to the tail.

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Bulkheads

This photo shows the bulkheads in their appropriate positions. I have moved the F2 bulkhead forward to allow for the additional spoiler servo. I moved mine up 1/4". I could have gone a little more and still had plenty of room for the battery and receiver.

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Everything in it's Place

All of the bulkheads and cross members have been installed. Time to start closing it up. I still need to cut the stringers and square off the tail. That will be done after the tail feathers have been tested for fit.

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Modified Bulkhead

I created a new F3 bulkhead for my forward wing mount.

  • I traced the bulkhead off the plans so that the top is a half circle that is just 1/8" from the inside of the canopy outline. I cut it from 1/4" birch plywood.

  • The forward wing mount, is a 1/4" hole drilled just above the center of the wing's lead edge, per the plans.

  • The three holes running across the bulkhead are for the pushrods on either side while the spoiler cord access is down the center. These three holes are 5/16". The pushrod holes are roughly in the same positions as the original holes with the spoiler cord access running down the center.

  • Also, I installed a 1/4" birch plywood plate between the sides of the fuselage and under the trailing edge of the wing for a wing bolt mount.

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Canopy Tray

The canopy tray needs to be fit just a bit. You will need to sand it down to fit. The sides should be about 1/32" short of the fuselage sides to allow for the thickness of the canopy.

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Wing Mounted

These photos show how my wing mount fit together.

  • First, make sure that the wing's center section sets correctly on the fuse.Sand the sides to achieve a good fit. It is important to keep the appropriate angle of attack.

  • Temporarily mount the wing in its correct position and hold in place with rubber bands and tape. Use a long, sharpened 1/4" OD brass tube to drill a hole through the lead edge of the wing using the 1/4" hole in the bulkhead as a guide. The hole will open up into the1/4" wide void that was created in the center section of the wing by the two half ribs.

  • Temporarily install the 1/4" wing mount dowel in the lead edge of the center section and put back on the fuselage.

  • Install your tip panels and check the wing alignment. Make sure that the trailing edges of your wing tips are equal distance to the rear of the fuselage.When you are all lined up, take a piece masking tape and place it across the back of the center section down to the top of the fuselage to mark and hold its position.

  • Drill a hole to accept a 1/4-20 bolt, through the wing and the mounting plate,in front of the trailing edge stock and down the center line.

  • Remove the wing from the fuselage and tap the plywood mounting plate for a 1/4-20wing bolt.

  • Carefully enlarge the hole in the wing so that you can push you wing bolt through the wing without having to work too hard. It is better to have ita little tight than a little loose.

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Wing Mount Dowel

This is what your dowel should look like installed in the wing. Now that you have the mounting plate tapped, its time to glue in the dowel.

  • Fill the 1/4" void in the center section with epoxy and micro balloons.

  • Wrap plastic-wraparound your wing. This is to keep the wing from getting glued tothe fuselage.

  • Push the dowel into the epoxy filled void. Some epoxy may ooze out through cracks.This is why you need to wrap your wing.

  • Put the wingback on to the fuse and bolt it into position until the epoxy has set up.

  • After the epoxy has set up, remove the center section and clean up any excess epoxy.

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Tail Grouping

The construction of the tail is pretty straight forward. Use aliphatic resin to keep it light. It wouldn't be a bad idea to use some hardwood across the center spar of the stabilizer. Or, find some really hard balsa. Do not be afraid of the off-center tail and elevator. IT WORKS GREAT AS IT IS! I know it is hard to believe for some, but it flies beautifully the way it was designed. Keep it simple, keep it light, and you will have a great flying model that will last you for years to come.

Good luck and good lift!