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Author Topic: Bristol 72 Racer 30" span (1/10 scale)  (Read 1064 times)
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Bill G
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« Reply #25 on: March 11, 2017, 05:48:34 PM »

"wishbone" gear legs

That is so descriptive - love it!
Yeah I saw someone use it on another site, so I stole it.  Grin  Fits perfectly.  I tend to think more automotive, calling it a control arm assembly.
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Bill G
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« Reply #26 on: March 14, 2017, 10:11:41 PM »

Dry fitted all the hinges in the control surfaces, which is one of those less exciting, gotta get out of the way tasks. One nice thing about the relatively thin SD8040 airfoil, is that even with deep chord ailerons, the relatively small hinge gap set by the Dubro flat pin hinges provides reasonable aileron travel. Aluminum tubing used for the wing bracing cable guides can also be seen in the wing photos. Came across a few small servos used for the ailerons. I bought them at a local swap from a vendor I knew, although I thought they were a bit pricey at around $8. Turns out they're digital servos, and reportedly decent quality also. Note the harness routing in the wing panels, where running the harnessing toward the front of the wing helps with providing forward weight. Every bit counts. The motor is now installed, where the motor mount shims will be some of the few items that will be easily serviceable on this model, once completed.

Performed a weight check with everything conceivable on the scale, weighing 18.5oz. Items still to be added are the dummy engine ducting farings, wing farings/wheel housing farings, covering, and light over painting the Microlite red covering for an opaque appearance. 22oz should still be a reasonable AUW, comparing with models such as a 30" Guillows P47 with a geared 370 bl inrunner, at over 22oz. This subject probably has a bit more wing area, although less desirable aspect ratio. Looking at prop data, I should be able to get over 100W from the Rimfire 28-26-1000 motor, with the planned prop. The motor actually came from the same previously mentioned swap meet, from a nice guy that almost sold it to me for free. The model probably shouldn't go much over 22oz however, for the motor to still be adequate. I did fly a 36oz Rake M1C with the next larger Rimfire motor on 3s with a 12"x6 prop, but that's not exactly recommended either.

Now working on the engine details, with the exhaust holes marked out on the fuselage.  I have a scheme worked out for the engine cooling duct farings inside the nose opening, using sections of thin sheet plastic. The "divider" farings between the cylinders will probably be made from sculpted balsa.
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Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
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Bill G
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« Reply #27 on: March 17, 2017, 02:01:33 AM »

Slowly finishing the engine details now, as they will be more difficult to complete, once the wings are soon permanently installed. A balance test (pic 1) showed the CG to be at the planned location. Can't complain with a subject like this, although the aft covering/paint weight and the majority of the wing weight will move the balance rearward. To keep the balance point in the ballpark, 10 grams of lead was added in the forward lower cowling, before installing the engine duct farings. (pics 3 and 4) The engine ducting will also help contribute to forward weight. Before installing the engine duct farings, the pushrods were installed, with 1 per cylinder as a simplification.

The engine duct farings are made from clear sheet sections (1 per clyinder seen in pic 2) with balsa formers located between the cylinders, forming the shape of the ducting. This method is working well and turned out to be easier than I expected to install, with 4sections now in place. The individual farings are glued to the balsa shims, with a slight bend placed at the "tabs" that fit between the cylinders. (pic3) The seams between the farings will not be seen, as there will be a ducting "divider" between each cylinder, which will be centered over the seams. The dividers will probably be sculpted from 1/4" balsa.
Attached files Thumbnail(s):
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
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Bill G
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« Reply #28 on: March 23, 2017, 11:45:40 PM »

Finished the engine details, installing nine individually sculpted and then custom fit ducting fins between the cylinders. They require slow trimming/sanding to size, which isn't the most exciting work. The fins are tapered from their attachment points on the crankcase to the inner cowling, as well sculpting the leading edge. To minimize the wood grain appearance, the 1/4" balsa sheet sculpted fins were primed, sanded, and then painted silver. The silver was first painted on the inner cowl faring, as it would be difficult to get in between the cylinders with a brush, after installing the fins between the cylinders. There are also also two scale vent tubes at the top of the fuselage, where the access door slide latch makes up one of them, with a piece of aluminum tubing glued over it. The second vent is glued into a hole drilled into the access door.

One more major detail area to go, with the wing/wheel housing farings. One more hurdle is now completed, with the wheel bays cut into the wing panels. The plug in wings were priceless for this task. First the inner wing ribs were cut for the wheel clearance. That part was easy, as you could visually line up and mark off the required opening area, with the wheels loosely placed over the axles in the retracted position. Next the bottom portions of the second wing ribs outward were trimmed, for wheel clearance. Those cuts were gradually made, as the wheels were cycled up and down, while slowly moving the wings inward. As planned, the ribs did not need to be fully cut though, although there was a bit less of the ribs remaining than expected. The tops of those ribs were laminated with 1/16" balsa, shaped to match the airfoil.
Attached files Thumbnail(s):
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
Re: Bristol 72 Racer 30" span (1/10 scale)
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Konrad
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« Reply #29 on: March 24, 2017, 09:46:37 AM »

Wow, didn't see that coming! That is the wheels retract into the upper fairing!
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Cut it twice and it's still too short!
Bill G
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« Reply #30 on: March 27, 2017, 09:28:52 PM »

Wow, didn't see that coming! That is the wheels retract into the upper fairing!
I'll have to get more pics up soon.  I have the bottom wing panels faired in (easy part) and am working on the top farings.  Keeping scale proportions for everything forces you to keep the tire clearances between the upper farings at a minimum, like the full scale.  One good thing is that the symmetry between both sides is as good as could possibly be expected.  It's nice to make the same mirrored parts for each side, with mimimal tweaking during installation. I made cardstock templates for the upper faring plates, which require slowly trimming down to size, as in a few thousandths at a time, when you get close.  Everything will be fabricated with balsa.
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