Hobbistar 60 - Bolt-on wing conversion
 

My Hobbistar trainer's stock method of securing the wing is rubber bands over dowels.

http://farm4.staticflickr.com/3732/1...e8a45398_z.jpg

Not much else is stock about this plane, though... the wing has been rebuilt, a lot of the covering has been changed, it's been converted to a tail dragger and at the time of this writing I've just finished installing a DLE 20 2-stroke gaser in place of the original glow engine. So it's high time I converted the wing to bolt on.

I've spent a lot of time thinking about this mod and I've asked a lot of questions. I've settled on a 4-bolt approach using 1/4-20 nylon bolts. The front bolts will go straight down through the thickest part of the wing while the rear bolts will go through a stock reinforcement near the trailing edge of the wing. Here's a rough diagram - not to scale.

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The bolts will thread into tapped holes in 1/2" plywood mounts. The mounts will be removable for access to the plethora of stuff packed into the fuselage.

Someone gave me the idea of lining the wing holes with a sleeve to prevent the wing from being crushed when the bolts are tightened. I went to Home Depot and challenged a rep in the Tool section to help me find something suitable. Our search led us to another rep in the plumbing section and the perfect solution.

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Yep, that's right. A toilet supply tube. A 1/4-20 nylon bolt fits perfectly.

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I plan to mount the sleeves flush with the upper and lower wing surfaces. The top of the mounts are flush with the wing saddle so the sleeves should come into direct contact with the mounts.

I was skeptical of the strength that half an inch of tapped plywood and a nylon bolt could provide. Never having tapped wood before, I tried it on a scrap piece, hardening the threads with thin CA before threading in the bolt.

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I made it tight. REAL tight, to the point I thought the threads would strip or the head of the bolt would break. Neither happened, and I was convinced.

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There was a little housekeeping to do on the wing. First step was to cut off the control rods that were the attachment point for the stock single servo.

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Next was the replacement of the wing strap screws. I hate, hate, HATE cheap Phillips screws so I cut down some #4 button head hex screws. Pictured are a POS stock screw, a 3/4" #4 button head hex screw, and the same type of screw cut, tapered and paired with a 4-40 washer.

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Much better.

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Here are the wing mounts, test-fitted inside the fuselage.

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I started by aligning the top of the rear mount flush with the top of the wing saddle and clamping it in place. I used a pin vise to drill through the fuselage into the mount (see the red arrows).

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I'll be attaching them through the fuse with two 3/4" #6 screws on each side.

I am onboard again...subscribing.

When I converted the wing on my Avistar to a bolt on, I cut it in half and made a new center rib out of ply with a tongue on the front. Added some hardwood in the front of the saddle for the tongue to fit into and more at the back for the bolts. I also removed all the dihedral too.

I wish I hadn't removed the aileron torque rods though as it would be easy to cut the stock ailerons and mod them for flaps. i had already put two servos in the wings for the ailerons anyway.

Still need to cover the wing and install engine/radio , etc. Trying to decide if I want to make a cowl for the engine like the RCM article for the Tiger. http://www.rcmplans.com/issues/reque...-092000-1.html

Bill S.

Good to have you back again!

Good ideas, Bill... I want to keep the dihedral for stability and I don't want to put any more work into this than I have to.

Both mounts installed.


http://www.rccanada.ca/rccforum/atta...ine=1387509057




Ground the control rods down further and marked the working area on the bottom surface near the trailing edge.


http://www.rccanada.ca/rccforum/atta...ine=1387509057




Rear bolt holes are drilled into the wing.


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I'm going to hold off on the front ones while I reconsider the position of the bolts and the mount.

Count me in! I've been reading your DLE 20 thread and it looks great. I agree your front wing mount does look far back. I've been wanting to do this mod for awhile on mine, and can't wait to see what you come up with and how it performs. Keep up the good work!

I converted mine 10 years ago. Hate the rubber bands. Mine is still flying and working great. Did mine similar but a little different. Let me know, I can post pictures if your interested.

Terry

Thanks for the props!

I can't wait to see what I come up with too! :p

Yes please, Terry - pictures would be appreciated.

Just brainstorming here...and this may be crazy. So...I went and looked at the photos on the DLE 20 thread. Build a strong bulkhead just above the fuel tank where the wing front meets the fuse. (This will take some good measuring. Anyway, on to my idea. Instead of putting the dowels in the wing, mount them facing to the rear in the new bulkhead, but lower than the wing surface as it cross the fuse. OK so now you have 2 dowels facing the rear. Build a cross bulkhead on the wing.... the bulkhead extending above the wing surface. Then drill 2 holes through the new bulkhead that the dowels will slide into. Can be drilled ahead of time....
So to install the wing, you place it rearward of the dowels, line up the two holes and slide it forward until the holes on the rear of the wing match the holes in the rear attachment block. Put in the rear bolts and you are good to go.
Hope that is not too confusing or making it too complicated.;)

Found a picture that I can refer to as an example. In the picture, the balance blocks of the Cessna would be turned 90 degrees, and under the wing inside the fuse. They extend beyond the lower part of the wing so you can slide them in. If in the right place, the bulkhead on the wing with the holes pushes up right against the new bulkhead above the fuel tank and the rear bolts go right where they are now.

http://www.rcuniverse.com/forum/atta...mentid=1949353

I think I understand... My main concern is space under the wing - there's a gas tank there now. And it's certainly more involved than what I have planned... but something to consider!

True about the gas tank, but if it doesn't come to the top of the bulkhead and sits below it like the picture on the DLE thread it might. I will leave it to you since you can sit and stare at it and I can't. :cool:

My version
 

Here are a few pics that show how I did mine.

You're right about the fuel tank - there's plenty of space above the tank. I won't rule out the dowel idea but it's still more complicated than I want at this point.

Excellent - I see you mounted the front bolts near where I was planning to... tell me, did those bolts hold up well in that position? Did either one ever break because of the wing pulling against it in flight?

Wing bolts don't have to be nylon. Use aluminum or steel bolts if your gut tells you the nylon might fail. In a mishap the nylon bolt sheering isn't going to save you from damage anyway. I redid my Hobbistar 60 several years ago and never could come up with a clean easy bolt on conversion and ended up staying with rubber bands (YUK). I didn't think there was enough meat (strong wood) in the upper windshield area to use dowels and I wasn't willing to reconfigure the fuselage to do it. It's not an easy plane to do that to. Maybe the two bolts back near the c/g like you were thinking and tlj125 showed in his photos PLUS a horizontal tab sticking out forward under the L.E. and into a slot made under the windshield area would provide an attachment at the front and rear of the wing that you would have faith in. Make the tab out of 1" aluminum strap material or hardwood. (and dump the nylon bolts too)

I have flown this plane year after year for over a decade and have never broken a wing screw or mount. Flown off grass, tarmac, water, and snow with no issues. Recently recovered the fuse with monocote. Still have to do the wing. One of my longest lived planes. Its the energizer bunny of the fleet. Good luck with yours.

Terry

If you have the room, I like tlj's idea better, and it is simpler. Only thing left is to figure out the way to strengthen the mount points on the wing.

I have to disagree with you on that score - last year I cartwheeled my Pulse 125 on landing and besides the prop the only damage was to the wing bolts, which sheared exactly as designed.

Great news - and I see you used Nylon bolts. Thank you!

I do too... especially since my plan is so close to his!

At the back the wood is extra thick, so with the anti-crush sleeves it'll be fine. At the front I will also use the anti-crush sleeves, plus a thin sheet of plywood - I'm thinking 1/16", covered in white - between the holes and the bolts.

While I was thinking about where to put my forward wing bolts, I checked the length of the bolt against the thickness of the wing at my first chosen position.


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Nope, that's not going to work! I didn't find any 1/4"-20 nylon bolts longer than 2-1/2". Ergo, the location of the forward bolts has to change... so I put the dowels back in the fuselage, banded the wing on and marked holes in my new location.


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Holes drilled.


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Anti crush sleeves epoxied in. They will be cut flush to the surface of the wing.


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While I was at it I put the rear ones in.


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I wrapped up the evening's work by blackening the wing mount screws and washers with a Sharpie.


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That's better.


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grosbeak,
What do you use to print the labels on the plane (under the switch above) ?

Bill S.

I have a Dymo handheld electronic label maker - it uses the D1 cartridges. I bought a black-on-clear cartridge for these labels.

I have used inkjet water transfer decals in the past with success.

If you look at how I did mine, the wood has been counterbored so the screws can work at the standard length. The blocks are glued with Hysol to the main spar in the wing top and bottom. The wood I used was hard maple. The maple goes all they way through the wing. I cut through the covering and sheeting with a #11 exacto and found the spar then opened up the hole to have a tight fit with the maple blocks. I cut the blocks to size before gluing, tracing the wing profile into the block and shaping with a dremel after crosscutting. Then drill and counterbore holes(9/32 and 7/16 drill bits). Then locate the mating blocks in the fuse, glue (again with Hysol), and "line" drill and tap from the holes in the wing. I later added the screws to the blocks in the fuse, just in case.

Looking as hpw you are doing yours, I don't see the load applied to the spar. Maybe I am just missing it.

Terry

I was hoping to avoid putting any internal structure in the wing. That hope was short-lived. More in the next few minutes.

My next step was to cut all of the sleeves flush. I protected the surrounding covering with some masking tape and grabbed the flush-cut saw.

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I needed to gauge the angle of the sleeve to determine the new position of the forward mount - a transfer punch through the sleeve made a handy visual reference.

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The angle, transferred to the fuse.

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New holes drilled.

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Forward mount in new position.

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Wing bound to fuse.

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Correct size for 1/4"-20 tab - 13/64" bit.

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Drilling through the rear sleeves.

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Rear bolt holes drilled and tapped with bolts in for a test fit.

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Rear bolts in!

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Front bolts in!

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You may have noticed that I have done nothing to reinforce the leading edge where the bolts pass through - that's because I really didn't want to open up the wing to add reinforcement. The sleeve prevents the bolt from crushing the wing, although it is only attached by epoxy to the upper and lower skin in the wing. The little voice in my head, which I do try to listen to, told me this was a likely failure point. I decided to try it anyway. When the front bolts were in, I picked up the plane by the wing (near the roots). It held. I tossed the plane a few inches up and caught it by the same place on the wing. It held. Success!

Alas, when I attempted to bore out the sleeve just a little bit for the bolt to slide more easily, the bit torqued the sleeve a little too much, and the little voice was proven right.

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Nothing for it now except to open things up.

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Luckily, I had a big balsa block from when I had to rebuild one of the wingtips on this very wing last winter. Now, even though balsa is technically hardwood (look it up!), it's not especially hard or crush-resistant. However, even the little cautionary voice in my head agrees that this is not a problem - the sleeve will prevent the balsa from being crushed, there is plenty of surface for glue adhesion between sleeve and balsa block, and the balsa block will be glued to the wing on four surfaces.

Here's the shaped block.

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Glue in, skin on.

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I decided that the other wing would have to be done too. Note the makeshift clamp to hold the balsa block against the internal leading edge.

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Next step will be to reattach the wing and drill the new holes.

Lookin good... Doesn't it follow...every plan when starting never is where it ends up? Another thought, although not a critical one. You might want to put in grommets on the front holes.... just 1/2 on each hole. Since the leading edge is curved and the screw head bottom is flat, the grommet will allow a more spread surface to hold the front.

Hooked, great idea. Thanks for the tip!

Okay, there were a few interim steps. :P


Skinning the port wing patch with a little plywood. Clamped:


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Cured:


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Sanded:


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I also fabricated a new forward mount because the original broke while I was working on it. Not a good sign! I had had to remove a considerable amount of the centre section to provide clearance for the forward wing strap and I guess it was just too much.


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I removed a layer of ply from the centre section on this mount too. You'll see that a little further down.




Drilling the wings (again)... Trusty Forstner bit.


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Holes drilled.


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Sleeve material in for an alignment check.


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I roughed up the outside of the sleeve to provide better epoxy adhesion.


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And although there would be a lot more epoxy contact between sleeve and wing this time, I bored out the sleeve before installing it in the wing.


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Here's a view of the starboard lower wing surface with epoxy applied.


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Here are the sleeves, glue cured and cut flush with my trusty flush-cut saw.


Starboard upper:


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Starboard lower:


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Port upper:


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Port lower:


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With the wings joined, I bolted them on at the rear and bound them with elastics, hopefully for the last time! You may recall that to tap for 1/4"-20 you need to start with a 13/64" hole. However, the wing tube inner diameter is closer to 1/4". I started by chucking a 15/64" brad point bit in my drill, inserting in the sleeve and running it in reverse to mark a centre point, seen in this photo.


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With a centred mark, I switched to the 13/64" bit and bored through the new forward mount to create the wing bolt holes. Note the reduced centre section.


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Next step was to chuck the 1/4"-20 tap in the drill and slowly tap the holes...


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... followed by some CA for the threads.


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Finally!


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