Fork Tail Devil High speed P-38 - RC Groups
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Mar 21, 2008, 10:48 PM
Engineer for Christ
IBCrazy's Avatar
Build Log

Fork Tail Devil High speed P-38

In this log I will be constructing a high performance P-38J Lightning. Unlike my other performance planes, this one will be near scale to the real thing. I plan on making this one with the heart of a pylon racer.

I estimate the plane will weigh 28-32 oz AUW. I make this estimate based on dual booms, dual motors, dual speed controls, and a fiberglassed wing.

Since I'd like to keep my wingloading below 18 oz/sqft, I decided on a 39 - 1/2 wingspan (8.25" root chord and 4.5" at the tips). That give me a wing area of about 1.7 sqft. The wing is only 1/2" at the center and 3/8" at the tips for a very thin and fast wing. The airfoil is a hybrid (ok, actually I took a USA 27 airfoil then proceeded to sand it down on my belt sander until it was 7/16" thick). So basically, I have no idea what this airfoil would be close to
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Mar 21, 2008, 11:52 PM
Engineer for Christ
IBCrazy's Avatar

Glassing the wing

Now on to glassing the wing. I want this stiff, so I went with 5 oz fiber glass and West System epoxy. At only 7/16" thick I have barely any room for a carbon spar, but this shouldn't need one if I glass it right, so I opted to leave out the spar. To adhere the glass to the wing, I sprayed the wing and fiberglass cloth down with Super 77 spray adhesive. I then wrapped the wing in the fiberglass cloth.

I started with 35ccs of epoxy and spread it over the top side of the wing and let it set for 5 minutes. After 5 minutes passed the epoxy had sunk into the foam and glass leaving a few bare spots, so I mixed 15ccs more. Again I spread this on and let set for another 5 minutes.

Seeing no large dry spots, I then wrapped the wing in wax paper and smoothed out the wing surface with a piece of scrap balsa wood. It took out some of the epoxy, but it should have left a nice even coat. At least I hope it did. Tomorrow I will do the bottom half of the wing.
Mar 22, 2008, 11:40 PM
Engineer for Christ
IBCrazy's Avatar

Motors built

For this plane I decided to build my motors especially for this project. I decided on GoBrushless GBX double stator motors. They tend to run really well at 15 to 16 Amps and can be wound for incredible speed. I knew I wanted a kv of about 2500 and a motor draw of about 15-18 Amps. I already selected the props - counter rotating 5.5X4.5 APCs. I put some numbers into my calculator and came up with three motors:

1. A 6 turn Wye 12 magnet = 72% efficient
2. An 8 turn Wye 8 magnet = 83% efficient
3. An 11 turn Wye 6 magnet = 84% efficient

I opted for the 8 turn 8 magnet since the additional magnet coverage would give me a little more torque and allow for a little more power should I need it. I also am very entertained by the bizzare winding (AaABbBCcC). I consider it worht it for a 1% reduction in efficiency.
Mar 22, 2008, 11:49 PM
Engineer for Christ
IBCrazy's Avatar

Booms almost done

For the booms I needed something slim yet stiff. I went with foam sheeted with 3/16 balsa. The booms are 1 - 1/4 high in the front and taper to 5/8 in the rear. The width also tapers from 1-1/8" to only 1/2" The booms were a little flimsy for my tastes so I glued a 1/8" piece of balsa to the bottom and a 3/32" piece to the top. I will sand them down tomorrow hopefully.

The booms wil be mounted to the wing via 1/4" nylon bolts through blind nuts in the booms. Although I dont plan on ever removing them from the plane, the nylon bolts serve as a failure point for the booms in a wreck or hard landing so it hopefully won't tear the plane up too bad. They also feel more secure than just gluing.

I also coated the bottom of the wing with epoxy this morning. I removed the wax paper a few hours ago and it left a nice finish. The top however, still had dry spots, so I put another coating of epoxy on it. The wing now weighs about 8 oz, but is really stiff.
Mar 23, 2008, 10:07 PM
Engineer for Christ
IBCrazy's Avatar

A little progress on Easter

I only got a little done today on the ForkTail. I sanded the booms round and drilled the wing for the boom screws. I then decided to stress test the wing to see how strong it was.

I placed the 39-1/2" wing between my workbench and the ping pong table and placed a 5 lb weight on the center. Despite the wing being only 1/2" thick it only flexed 3/4"

I'm designing the nose piece now. This thing is going to rip
Mar 24, 2008, 10:46 PM
Engineer for Christ
IBCrazy's Avatar

Nose piece designed!

I finally get around to designing the nose piece of this plane. It is 1-1/2" thick until the canopy. So it should fit a 3S in there comfortably. I also cut out the elevator. I might go with a 4" stabilizer rather than the 3" in the picture for added stability. We'll see. Right now the scale shows 17 oz with the frame and both motors.

I also decided to test my wing a little more. This time I suspended it with 10 lbs on the wing. It still didn't break (although it flexed 1-1/4"). I could still press on it so it can take even more, but I'm not going to try more weight. Remember this wing is only 1/2" thick!!!

I still need to add foam to the nose piece to give it a full contour.
Mar 25, 2008, 02:32 PM
Registered User
critterhunter's Avatar
I think you are doing that weight/wing trick with invisible wires like they do on magic shows. Impressive wing strength. Now if some mad weight lifter starts stacking his weights on your plane you've got no worries whatsoever.
Mar 26, 2008, 11:37 PM
Engineer for Christ
IBCrazy's Avatar

Nose peice almost done

Ver little progress since I made a motor testing stand tonight, but I did get the nose peice almost completed. Let me tell you 3/16 balsa doesn't bend well. I sanded a foam block to glue my nose templates onto, then sprayed it with Super 77.

In order to get the nose to come together I needed to use Gorilla glue and clamp it in a vise. I wet down the nose so hopefully it will retain that shape without high stresses.

Next up, cutting the ailerons, vertical stabilizers and elevator.

May 10, 2008, 11:13 PM
Engineer for Christ
IBCrazy's Avatar

More progress.....Finally

Ok I got back to this project at last. I got the stabilizers made and set up the tail. I still need to hollow out the battery chamber and inatall electronics. I have been a bit hesitant on this one because it is looking like a really nice plane, but I know I'm going to end up wrecking it I'm not worried about a hard wreck but more the belly flop due to the long runway this is going to need.

Due to the wreck potential, I have decided that everything should be bolted together with nylon bolts. This way in the event of a wreck, the plane should break apart at the nylon bolts and leave the plane more or less with only minor damage.

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