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Dec 08, 2011, 11:12 PM
AndyKunz's Avatar
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Micro Bridi Trainer Build Thread

When I was a kid several decades ago, my father built a Bridi T-20 (aka RCM Trainer 20) from a kit and powered by a Fox .19 RC. It was beautiful, but it didn't last very long.

In 2007 ago my friend Ellis Grumer and I built him one for his birthday, and covered it in a blue and yellow motif similar to his original. Powered by an E-Flite Power 25, it has plenty of authority. I fly it now and then and it has always been fun to fly. It's has a lot of sentimental value, so it doesn't fly as much as some of my other planes.

Tonight I was just itching to build something new (after all, I only have 4 other projects in the works), so I started working on a 1/2 size model of the Bridi T-20. It will be powered by an E-Flite Power 180 (2500kV) on 2S, using an AR6400LBL brick for control.

Hopefully this will be as quick as the Micro Kaos I built last year. Slab-sided fuselages are good in that regard!

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Dec 08, 2011, 11:15 PM
AndyKunz's Avatar
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The fuselage sides are easy. So far I've used 18.5" of a 1/32" x 3 x 36" sheet. I think I can build the whole plane with just 1 sheet.

Next I framed the perimeter (except at the tail) of the sides with 1/8" square balsa sticks. The T-20 has a high wing, so these sides will be extended upward another 3/4" in a few steps.

Time for bed. More on Saturday ...

Last edited by AndyKunz; Dec 08, 2011 at 11:20 PM.
Dec 10, 2011, 11:37 PM
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The cabin area of the fuselage was added. You can see the wing saddle doublers here. The two formers are two cross-grain laminations of 1/32" balsa. At the top of the forward one you can see that it is doubled up - this will be to take the 1/8" dowel that will serve to pin the LE of the wing down.

These were all set square using a small triangle.

After that glue dried (I build with carpenters glue now due to developing an allergy to CA), I joined the sides together, then later the tail and inserted the cross pieces of 1/8" square at the same station as the verticals.
Dec 10, 2011, 11:50 PM
AndyKunz's Avatar
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Fuselage glue drying leaves time to make wing ribs! Here you can see some rough-cut ribs along with the stack of finished ribs. It always pays to make a few extra if you can.

They are roughed out using the ply template and cutting oversize with a knife. The holes are cut using a piece of 3/32" tubing (drills tear thin balsa). They provide alignment for stacking up with a 3/4" 4-40 bolt. Very carefully I belt-sanded the stack, then cut the notches with my jig saw. Be very careful to keep things square!

The stack is kept together as the spars are test-fit. Trim and sand the notches until the spars (1/8" sq) are snug.

The ribs were stacked with identical weights adjacent to one another (wood often varies chord-wise, so take advantage and cut the root ribs from the harder/heavier stuff and tip ribs from the lighter/softer stuff). I pulled one rib from the stack and placed it on the right wing, then the next one (similar weight) went to the corresponding place on the left wing, and so on until all the ribs were placed. Note that there is about 1/2" space in the center between the two panels that will be removed later, when the sides are joined with dihedral.

The LE is 3/16" square (fairly hard), followed by 1/8" x 1/16" which will improve the covering appearance and airfoil, followed by the main spars 1/8" square. All spars are top and bottom on this symmetrical airfoil.

More in a few days.

Dec 11, 2011, 07:16 AM
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Andy, I also had a Bridi RCM Trainer and it was actually the airplane I learned to fly with. I still have the wing from this airplane, but mated with a second fuselage. It hasn't flown since the early 1980s. It's a great airplane although not really a basic trainer. My first airplane with a radio in it was a Sterling J-3 Cub. I immeadiately started a .60 sized VK Cherokee after the cub was done, got the wing built and got my draft notice and had to go into the US ARMY, so I never flew the cub.
Last edited by Mode One; Dec 11, 2011 at 07:23 AM.
Dec 11, 2011, 09:15 AM
AndyKunz's Avatar
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You're right, it was called an "aileron trainer" back in those days. The one in the top photograph is pretty nimble, but having a Power 25 surely contributes to that.

Dec 12, 2011, 09:55 PM
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The wing panels have been sanded to round the LE and taper the TE a little bit, so it makes the airfoil smooth. They are also separated, and the root rib/spar face is sanded flat.

The wing tips on this model are very simple. It's just a 1/8" sheet 1/2" wide with the front rounded a little. To save some weight, I used a 1/4" brass tube to cut some holes. The excess spars are cut off with a Dremel wheel (they're fragile), then the pieces are angled and glued back on. Finally they sanded to fit everything together.
Dec 12, 2011, 10:11 PM
AndyKunz's Avatar
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While the glue is drying on the wings, let's bend up some wire for the landing gear.

Although it's overkill for a model this light, I used .047" piano wire because using smaller would let the wheels wobble too much. These wheels, by the way, are pretty nice - they're tail wheels from the Parkzone SR-10 (PKZ5204).

The nosegear was wrapped around a 1/8" dia rod in a vise. I need a DuBro bender for micro nosegear!

Note the bends on the mains. The area to be soldered is cleaned with sandpaper. The wires are mounted to a block to hold things in preparation for soldering and thin wire is wrapped around the joint. After soldering, a little clean-up with some alcohol and they're all nice and shiny!

The wheel retainers from the PKZ5204 bag are sliced to make a donut about 1/16" thick, which is then epoxied into place on the axle. This will keep the wheel from binding on the wire bend. The remaining piece is drilled with a .041" bit, then jammed onto the axle to retain the wheels. Cutting the wire with pliers will leave a burr that will keep the retainer from popping off.
Dec 12, 2011, 10:19 PM
AndyKunz's Avatar
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The cheeks get 3/16" soft balsa laminated inside. We will be sanding a bunch later on to streamline it a bit. There is a notch in the firewall to allow the motor wires to fit under the battery hatch. Note also that there is a piece at the base of the windshield for gluing the sheeting to later.

The nosegear will be free castering. The bracket is made of hardwood - the small hole for the wire is drilled first, then the big area is cut out for the wheel collar to go in. Note that the small hole is not centered - this allows it there to be a little epoxy fillet inside it without interfering with free rotation.
Dec 12, 2011, 10:30 PM
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While I had the motor installed, I marked the center of the shaft position on the two cheeks. I then used a French curve to draw the desired shape of the cowl cheeks.

Next, I rough-cut the nose to shape. This was followed by very carefully smoothing on the belt sander. This was a little aggressive, so I stopped that quickly and went back to hand-sanding it.

After the top and bottom sheeting is applied, the final sanding of the nose will occur, but that's a job for another day.

Dec 14, 2011, 11:23 PM
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After sheeting the top and bottom with 1/32" cross-grained, the excess balsa is removed and sanded flush with the sides. This gives a very squared-off look that is what we want before putting a radius on all the corners and streamlining the nose a bit.

While things are easy to access, 1/8" x 1/4" medium rails were glued into the front of the cockpit area to allow easy mounting and access of the AR6400LBL brick. The high location allows the battery to slide as far forward and back as necessary beneath it for proper balance and larger battery sizes if desired.
Dec 14, 2011, 11:32 PM
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While on the subject of mounting radio gear, the aileron servos will go into the wing. They are mounted on 1/8" square balsa rails on the two ribs, and a small piece of 1/8" square goes between the ribs to provide a solid edge for the covering hole.

A hole is cut with a sharpened brass tube to provide a conduit for the servo lead. I prefer to use McDonalds straws for this purpose - they are quite light and fit perfectly with the size K&S tubing I use for cutting the hole.

The servos are set at a height that will allow me to cover the wing with UltraCoat Lite with the servo installed. The straw could be removed to save weight if one desires.

Dec 15, 2011, 11:09 PM
Grumpa Tom
Kmot's Avatar
This is a nice looking model and a great build thread!

I too developed allergy to CA and now I use a cartridge respirator and breathe clean sweet air whenever I am working with stinky chemicals.
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Dec 16, 2011, 07:39 PM
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Hey there, Andy. You've got some nice looking planes on the links you provided...Very cool.
Here's a couple pics of what I've been up to : First is a 50" 1913 Ponnier Monoplane. I buy as little pre-built items as possibe, opting to just make my own.
Second pic is a Nieuport XI parkflyer. I'm working towards getting this 'used and weathered' look down & hoping I can pull this plane off by new years. Pretty much everything is built, I just have to cover and detail.
I'll be following along here with your build.
Dec 16, 2011, 07:46 PM
AndyKunz's Avatar
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Beautiful workmanship there!! I had a Comet Nieuport 11 that was Peanut sized as a kid. I have been thinking about making another, but there's only so much time

We have to do something about that transmitter manual in the second photo...


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