CF wing spar, LED lights, CF struts. Part 3

[Britpilot]

This is Part 3. Rigid Carbon Fibre wing struts.

Part 1 can be found here: supercubclub.proboards.com/index.cgi?board=tt&action=display&thread=2796

Part 2 can be found here: supercubclub.proboards.com/index.cgi?board=tt&action=display&thread=2793

It took me quite a while to figure out how I was going to fit rigid Carbon Fibre wing struts on my SCLP. In the end it turned out to be quite a fiddly job involving a lot of intricate metalwork. Not for the faint hearted this mod if you want to copy it!
I decided to use 3.2mm diameter carbon fibre rod for the struts. That was the easy part, working out how to fasten the rods to the wing and fuselage was the hard part! After a few experiments this is how I finally did it.

To mount the struts to the wing I made up some mounting plates with threaded inserts. I used # 6 “T nuts” from my local hardware store for the threaded inserts (The “brad” type T-nuts with holes and not the hammer home type with spikes). I epoxied these T-nuts into 35mm diameter plates made out of white gutter alluminium. After removing the stock strut mounts from the wing I epoxied the new threaded inserts into the wing.






As part of my previous battery box strengthening mod (See post : supercubclub.proboards.com/index.cgi?board=tt&action=display&thread=2699) I had installed #10 “rivnuts” on the fuselage for the struts to fasten to and I made a bracket out of 2mm alluminium sheet to fit on to these. I also made the lower strut mounts out the same alluminium and cut elongated holes for # 8 bolts in them for strut length adjustment.





Next I cut out of 2mm alluminium four plates to mount the struts to the wing. I made these plates 20mm diameter with a “tab” to fix the strut to. After I had finished these plates and bent the “tab” to the strut angle I epoxied a carbon fibre rod onto each one.





After drilling a hole for a #6 machine screw in the centre of each plate I screwed them to the wing, marked and cut the CF strut rods to length and epoxied them to the fuselage mounting plate. I did this with the wing installed on the aeroplane to get all the angles correct. (Another fiddly job this getting the dihedral angles and strut angles equal side to side) To tidy up the finished struts I placed heat shrink tubing over the epoxied strut to plate bonds.






I chose to use nylon machine screws and nuts to with shake proof washers to fasten the struts to the aircraft and wing as I wanted the bolts and screws to be the weakest link. My thinking here was that hopefully in the event of a major prang the nylon screws will break alleviating to much damage to the wing and fuselage. At least that’s the idea! For the wing mounts I used #6 nylon machine screws and #8 nylon machine screws and nuts for the fuselage mount. Here is the finished struts on the aeroplane. The wing is now incredibly stiff with the CF rigid struts and CF spar! Good for pulling 12 Gs!









So after all this work was it worth it? It’s like a whole new aeroplane! The stock SC wing must flex a lot in flight because now with my really rigid wing the aircraft is much more responsive and handles gusty conditions much better. Aerobatics are a joy as I have no worry that I’m going to pull the wing off! I have even managed inverted flight for almost 20 seconds. The CF struts are also a lot quieter than the flimsy SC stock plastic struts. I had thought of painting the new struts white to match the aeroplane but now I've grown to like the black CF look.

Flying at dusk with all my LED navigation lights on is really cool! I have not been bold/brave/daft enough to actually fly at night yet but the two bright landing lights and tail light really help while performing aerobatics in low light conditions to work out whether the airplane is flying towards or away from you. The blinking red LED on the vertical stabilizer is not really bright enough though and the narrow 20 degree viewing angle does not help. If anybody knows where I can get a brighter blinking red LED (3000 MCD or more) please let me know. I was testing all the LED lights on the kitchen table the other night when my wife walked in and looked at my SC all lit up and said “Boys with their toys!” I guess that kind of says it all.

[john66]

Very nice work, I may have to try something similar as my struts wont stay on at all. I have some electrical connectors that are pretty similar to the part you made yourself that goes on to the end of the strut that connects to the wing, they're crimp on but im sure would be best to add some epoxy like yourself.

Inspirational stuff, keep up the good work.

PS love the RAF roundels, I have a set myself though am yet to put em on.

[Dillzio]

IMO, the carbon fiber struts seem to be a bit of overkill. They seem to be over engineered to the point they'd probably the the strongest part of the plane. A couple more of upgrades to that cub and you'll be able to break 200mph with that airframe! Also, you say you have a carbon spare through the wing? I thought that with the carbon spar it doesn't need the wing struts?

I think it's better to have flexible, breakable struts so that in the event of a crash the wing is basically held on just by the stretchy rubber bands to adsorb the impact, and you also risk ripping out the wing strut mounts or the battery box. I find that just by attaching the two struts to each other instead of the battery box it makes it much more durable, as it is free to move forwards and backwards, and it eliminates the strain on the battery box all-together. I use two pieces of fishing line and some swivels connected via a wire clasp made from a large paper clip.

Your craftsmanship overall is excellent, I certainly can't fault any of it as you've far exceeded the standard I set for myself of using toothpics, popsicle sticks and epoxy. As long as you've got enjoyment out of the time and money you've put into it then that's what this hobby is really all about. The plane looks fantastic.

Oh, and I know where you can get a bright flashing red LED. This is what I have planned for when I get around to my night flying setup: You go to a sports or bicycle shop and look at their safety lights. They go on the back of bikes for night riding, or you can get ones that clip onto your belt for jogging at night. I have one for my bike that I know you could see from a mile away. You have to strip off all the casing and just use the electronics to save on the weight, but it's easily done.

I actually just picked up a little keychain LED torch for $2.50, the shaft section with the batteries just unscrews, leaving an ultra light, ultra bright headlight just the perfect size for the cub!

So, what are you going to make for it next? I'd like to suggest a perfectly molded cowl made from 1.5mm polished low-weight aluminum alloy. :-)

[Britpilot]

The question is do you engineer an airplane to fly or to crash? I know this is a trainer but it also my first RC aeroplane of consequence and I am using it not just as a flight trainer (for which it has been superb) but also as a engineering project trainer. The skills and techniques I have learnt modifying my cub I will use on a more advanced project further down the RC aeroplane road.

Regarding the wing and do we need struts when a CF spar has been installed? I thought about that and the rubber bands flex and give a lot and without struts. This allows the wing to rock on the fuselage in gusts and high G maneuvers. For that reason I kept the struts. Believe me, flying the Cub with a fully rigid triangulated wing is a delight and a major improvement on the stock setup.

As for the cowl. A nice polished alluminium cowl now that would be a challenge! Where do I get a mini set of rollers to roll the alluminium into shape! By the way I'm using the PZ 10x 8 P51 prop at present. Very good performance from such a flimsy piece of plastic. A lot better than all the Master and APC props I've tried so far. It pulls almost 1Lb 7 ozs on my test scale at WOT with 12.5volts.

BP

[Dillzio]

Ah, I see. Now I think about it those rubber bands don't really seam capable of supporting the whole weight of the airplane do they, particularly if ur pulling up from a dive! Do you have any plans to upgrade the motor? Now you have an extensively reinforced airframe, it would be a shame not to add a ridiculously overpowered motor :-)

Oh, BTW, you do know that the 10x8 prop is a bit too much for the stock setup, don't you? It's acceptable, but WOT should be avoided.

[Britpilot]

I'm giving thoughts about going to a brushless setup. Looking at alternatives at present. Got any suggestions?