Mounting Clark-Y with 4-bolts.. any suggestions? - RC Groups
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Feb 17, 2013, 06:13 AM
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sroge's Avatar

Mounting Clark-Y with 4-bolts.. any suggestions?


I just finished my scratch built fuselage and I am now going to build the wing:

fuselage length: 75"
fuselage width: 4"

wingspan: 87"
chord: 16"
dihedral: zero.

As this plane is supposed to be an easy and uncomplicated one I decided to use an unmodified Clark-Y on it and use the whole wingspan for ailerons and flaps.

From the beginning my idea was to build the wing as having two halves being joined by an aluminum or carbon tube, each half having two holes close to the wing root. So the 4 bolts keep the wing halves together and mount the whole wing to the fuselage as well.

So I in the end there will be 4 t-nuts on top of the fuselage.. now I have to decide how to build the wing—especially the area covering the fuselage for it will contain the holes for the nylon bolts.

Does anyone have a good suggestion on how to build this area?

I would like to build the wing with a d-box design with the d-box from the LE to the wing joiner, embedding the wing joiner inside the d-box back wall. But how can I integrate the holes for the nylon bolts in a way that offers a good transfer of the forces between fuselage and wing? Let the screw cross the whole wing and rest the head on the upper side of the wing? Or rather let the head be located deep inside the wing and make a larger hole on top for a screwdriver..? What might be the best way?

Last edited by sroge; Feb 17, 2013 at 06:20 AM.
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Feb 17, 2013, 12:15 PM
B for Bruce
BMatthews's Avatar
You need something to take the crush loads around the screws and to transfer the loads from around the screws to the main spar. I'm a fan of having the two main hold down bolts sitting right by the main spar so the loads are most directly passed to the spar. This is the normal method on full size airplanes as well. Then two more smaller bolts part way back to the trailing edge ensures consistent alignment.

It doesn't appear that your fuselage has the mounting bolt plates or blocks yet so this may still be an option for you to consider.

But many modelers seem to be caught up with the idea of the wing bolts being located at near the leading and trailing edges. If this is what you're planning then for a model of this large a size I'd suggest that you makes one rib for each panel from 3/4 inch spruce lumber from the local lumberyard. This special rib will line up with the T-nuts of course. It'll be light enough for a model of this size and weight but it'll allow you to drill through and then countersink for the heads of the screws to sit on a flat area. The perfect tool for drilling the countersink is a Forstner drill bit. Don't go too deep though. You only just need to set the head in far enough that it sits on flat wood. I'd use a 1/2 inch Forstner drill and then a washer so the loads don't tend to want to crush the wood. The added strength of these two special ribs will connect the flight loads from the bolts to the spars decently well.

You won't tighten the bolts a lot. You only need to pinch them down with a firm but light torque. It's the G loads during flight that need to be spread out. The screw head by itself would be too likely to pull into the wood and cause the wing to become slightly loose in flight.
Feb 17, 2013, 01:35 PM
Registered User
I'm with Bruce on this, but didn't think of those heavy ribs (good idea hower!). I was thinking you'd need a ply box for the wing joiner, which needs to be near the front spar, (On or in would be ideal). Right behind the box and glued to it, full depth bocks for the bolts to fuselage connenction, drilled for the bolts, very much like earlier mentioned ribs. Rear bolt blocks may not require a joiner, but could be added to re-enforced ribs or even a rear spar.

Having the bolts serve double duty is practical and efficient.
Feb 17, 2013, 01:50 PM
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sroge's Avatar
Thank you for your reply! Because this is a scratch build I did not know much about my future weight distribution—so I was not even sure where I will have to put the wing within a certain range. So indeed I am free to put the bolts whereever I want. But to me it also seems quite reasonable to put them close to the main spar—with an aerodynamic center of 25% chord and a max thickness at 30.5% chord placing the main spar and the joiner in this area the TE bolts can be of smaller diameter.. yes I think this is what I want.

As my fuselage width is only 4" it may also be useful to have a space of 2" between root rib and the next rib and place the support for the bolt between both ribs. This is not a long distance and if at least the first two ribs are made of plywood there should be enough strength. Or am I wrong?

Feb 17, 2013, 01:58 PM
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sroge's Avatar

yes, I am going make the first two ribs of plywood. So as for the rear bolts it will also be easy to get a strong connection.
Feb 18, 2013, 01:28 AM
B for Bruce
BMatthews's Avatar
The main bolts at the main spar is the wise and proper method. Ideally the bolts should pass down through blocks of vertical grain lumberyard spruce "2x4" wood that is glued well to the main spars for the full depth of the wing. And be sure that your T-nuts are in wood that is well gusseted or even skip the T-nuts in favour of deeper blocks of plywood which is threaded with a die and the threads hardened by soaking with thin CA then re-threaded to clean up after the CA kicks off. Such threaded blocks are every bit as tough as a T-nut for this sort of use.
Feb 18, 2013, 04:04 AM
You know nothing....
Stuart A's Avatar
Originally Posted by BMatthews
skip the T-nuts in favour of deeper blocks of plywood which is threaded with a die and the threads hardened by soaking with thin CA then re-threaded to clean up after the CA kicks off. Such threaded blocks are every bit as tough as a T-nut for this sort of use.
I came across a guy,slope forum I think,who used the exact same method as above,But using wood screws.His reasoning being that the coarser thread of the screw was better suited to wood than a machine screw.
works well over the long term apparently.
Feb 18, 2013, 12:10 PM
B for Bruce
BMatthews's Avatar
The big wood working outfit Lee Valley did a study on machine threads in wood and found that they hold really well even compared to wood screws. In fact in larger sizes the machine screw threads won out for pull out strength. The key seems to be getting a good thread fit.

The idea of hardening the threads with the CA glue is mostly for long term stabilty. Screwing the bolts in and out of raw wood is going to cause the short grain side of the hole to splinter away over time unless they are hardened up. The long grain side will still hold things just fine regardless. The other advantage I found with hardening the wood with CA is that once it kicks off I can run the threading tap down the hole again so it's easy to turn the screws in by hand as the CA hardens all the little whiskers and the tap cuts them away to leave a clean thread.
Feb 18, 2013, 12:29 PM
Hope to get out of life alive
kenh3497's Avatar
Many but not all wood screw threads are tapered. IMO, a machine thread is a better choice.

Feb 18, 2013, 01:59 PM
You know nothing....
Stuart A's Avatar
I tend to agree with you guys.Not advocating,just reporting
Feb 19, 2013, 01:00 AM
B for Bruce
BMatthews's Avatar
The key is that the guy used the CA soaked wood blocks as a subsitute for T nuts. And that idea is a keeper regardless of the type of screws used.

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