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Opal (Full custom performance/glider)

A project log for RC Planes

A project blog about scratch building radio control planes

QuinnQuinn 04/26/2021 at 20:410 Comments

Opal was my first plane that I fullly designed myself, without basing her on any plans.  I had been having a lot of fun with Pearl with the straight wing and wanted to make a higher performance and faster version.

She was made to support swappable wings, with my initial intent to make a moderate wingspan symmetric airfoil one, and a much longer glider wing.  The fuselage is based on an 11mm carbon fiber tube., with horizontal and vertical stabilizers attached with plastic brackets.  Brackets were made with old gift cards, softened and shaped around the tube using a heat gun.

The front end a plywood structure was attached to the tube and held the mountings for the Rudder and Elevator servos, both connected with a pull-pull system instead of rigid control rods.  The tube was mounted in a pair of bulkheads, on the plywood structure.

Balance was intentionally made such that the battery sits exactly middle of the CG, allowing a range of battery sizes without issue.  With a smooth bottom and folding prop, she would hand launch, and belly land, but also have plenty of power with the larger prop and motor than what Pearl has.

In the above picture, you can see the motor in it's aluminum bracket on the right.  This is followed by the ESC, and R/E servos mounted below it.  Those servos are connected to pull-pull wires that go through the CF tube.  Continuing left is a 1300mAh battery, though I usually use a 2200.  Last is the battery voltage telemetry sensor and receiver.  

Opal was laminated from the beginning, having learned how helpful it is for protection, but also added strength.

I had a couple good flights on her until one day there was a crunch, the fuselage cover fell off and it was clear something had gone wrong.  I landed her without power, risking damage dragging the inners on the grass without the protection of the cover.  The aluminum motor bracket had warmed enough, pulling heat from the motor, to soften the glue to the point it detached.  This swung around allowing the prop to impact the wing.  As the cover was partly attached on this bracket, it fell off.  As it was simple foam, it wasn't damaged, though needed to go out to get it.

She wasn't too bad to fix.  I reattached the bracket but this time with epoxy, and doubled with reinforced tape. A piece of tape over the wing dent, and she was restored.

I made 2 wings for her.  The first is 40" full symmetrical airfoil with 30" carbon fiber 6mm tube as a spar.  Full span ailerons give her fantastic roll rate.  This wing on Opal is great for high performance flight.  I'm happy to perform high-g manoeuvres, and fight at high or low speed.  The symmetrical flat wing provides equal inverted performance and a wide envelope.  Her stall characteristics are pretty good, she'll just slightly tip, but is immediately controllable again.  Glide slope, while not true glider performance, is quite good and can catch some strong thermals.  A sample of this airfoil:  (CF tube goes down the middle, between the foam spacers)

The second was intended for soaring with a more traditional airfoil and 90" wingspan.  I added flaps to aid in landing as well.  As a spar, I had a pair of CF tubes, joined in the middle with a metal rod to give some dihedral.  First flight out was not very controllable, and that rod bent, folding the wings and resulting in a crash.  The fuselage plywood structure  needed to be replaced, but was otherwise fine.

I gave up on dihedral and reinforced by directly connecting the two 30" rods, as well as sistering them with a third spanning the middle.  I was able to fly and land this configuration, but it was barely controllable, and the significant bowing to the wing extremely worrying.  I've not flown it again like this as I need to change something.  I'll probably shorten this wing to 60" and give her another try, but really I need to rebuild the wing stronger.

As originally designed, her rudder surface is woefully small.  Eventually I got around to extending it simply by gluing on a piece of balsa.

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