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Old Jan 11, 2014, 04:23 PM
The Prez....... again
kenh3497's Avatar
United States, IA, Rockwell
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Strength of #2 hardware

I'm debating weather or not to use an internal linkage to the elevator of my 1/4 scale Kraft Super Fli. If I decide to do this, there is only enough room for a #2 clevis (steel of course) between two of the uprights in the fuse. I don't think there is enough room for #4 stuff in there??? In YOUR opinion, would a #2 steel clevis and it's hardware be strong enough for this application.

I'm hoping for an all up flying weight of 10 to 13 pounds or so.

One way or the other is not a deal breaker but I thought it nice to clean up the rear of the airframe with an internal elevator link.

Ken

EDIT I'm also tossing the idea around of attaching the horizontal stab with four #2 screws to allow a bit of adjustibility of the incidence. The whole tail assembly gets flying wires for added strength as did the full scale version.
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Old Jan 11, 2014, 05:35 PM
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richard hanson's Avatar
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Apparently you a looking at a setup with no access to the clevis after assembly.
Personally I would not do that. A model that size needs both elevators solidly joined - as done on old control line planes then the horn welded to the joining wire ( steel 1/8" dia )
The clevis, 4/40 type Sulllivan which has an end clip.
I would also slot thru the rear, so that the entire stab and elevator assy could slide out the rear of the model for final assy/or adjustment.
a tailpost and rudder assy would be removable.
a slot in the fuselage would index the post.
This would permit adjusting the stab -which would mount on two ply plates using 4 machine screws threaded into hardwood spots in the stab.
Done right very light -using a steel and CF pushrod to forward mounted servo.
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Old Jan 11, 2014, 05:49 PM
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BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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Ditto on what Richard suggested.

As for using four 2-56 screws that would be fine. But the heads are so small that you'll need to ensure suitable hardening of the surface where the heads seat. I'd also suggest that if the screws will be the locating keys that the stabilizer side of the shear plane be hardened with something like 1/32 ply or more so that bumps against the stab do not cause the narrow screws to cut into the structure and let it pivot. Or use some sort of keying in addition to the screws.
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Old Jan 11, 2014, 07:37 PM
The Prez....... again
kenh3497's Avatar
United States, IA, Rockwell
Joined Jul 2011
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Quote:
Originally Posted by richard hanson View Post
Apparently you a looking at a setup with no access to the clevis after assembly.
Personally I would not do that. A model that size needs both elevators solidly joined - as done on old control line planes then the horn welded to the joining wire ( steel 1/8" dia )
The clevis, 4/40 type Sulllivan which has an end clip.
I would also slot thru the rear, so that the entire stab and elevator assy could slide out the rear of the model for final assy/or adjustment.
a tailpost and rudder assy would be removable.
a slot in the fuselage would index the post.
This would permit adjusting the stab -which would mount on two ply plates using 4 machine screws threaded into hardwood spots in the stab.
Done right very light -using a steel and CF pushrod to forward mounted servo.
Quote:
Originally Posted by BMatthews View Post
Ditto on what Richard suggested.

As for using four 2-56 screws that would be fine. But the heads are so small that you'll need to ensure suitable hardening of the surface where the heads seat. I'd also suggest that if the screws will be the locating keys that the stabilizer side of the shear plane be hardened with something like 1/32 ply or more so that bumps against the stab do not cause the narrow screws to cut into the structure and let it pivot. Or use some sort of keying in addition to the screws.
The 1/8 joiner will be used and was/is the plan if I go with the enclosed pushrod. Already looking at carbon pushrods weather it's internal or external. If I go external, I'll split the elevators and each will have a separate pushrod each. I was planning on some ply in the appropriate places with blind nuts and washers for the stab mount.

So... would you approve of internal linkage with the wire joiner & welded, brazed or silver brazed steel horn? 2-56 or 4-40?

Ken
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Old Jan 11, 2014, 08:48 PM
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BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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For that size I'd suggest carving a little clearance to allow for a 4-40 link. Or since this would be buried perhaps make up something custom.

Silver solder would work fine. But I would not rely just on a simple plate with hole that the wire passes through and a solder fillet. Instead I'd suggest that you fabricate not only the flat brass or steel plate but also have a close fitting THICK washer of brass or unplated steel on either side. That way when the whole shebang is silver soldered you'll have the extra surface area to the wire courtesy of the well bonded side washers. Or instead of washers even 1/8 to 3/16 inch lengths of 5/16 or 3/8 round rod with a hole to fit the wire to act as double thick washers. The surface area that these side doublers add for the silver solder to the wire and back to the horn would be seriously bullet proof.
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Old Jan 12, 2014, 10:22 AM
The Prez....... again
kenh3497's Avatar
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I agree just a simple plate would not suffice. I have considered a thick "washer" or wheel collar something or other to reinforce the connection between the wire and the horn. I could even manufacture a proper reinforcement with my miniature lathe.

I was leaning to the 4-40 hardware. Thank you for helping me over the fence

Ken
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Old Jan 12, 2014, 12:30 PM
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I have used the system I noted on 30% TOC aerobatic models - 42% IMAC models and over one hundred pattern models -which are smaller -being 7-11 lbs
We learned a little---
The aft section of the fuselage needs to be torsionally very stiff- Box the entire section under the stab-and forward the stab at least one stab chord. this does not need to be heavy
thin, hard , cross grained orientation, balsa works very well.
The plates used to mount the stab - again firmly anchored along sub bulkheads. - 3/16" thick 7 ply plywood gives good glueing area and screws can be countersunk
the screws do not need to be large I used 4/40 on most models.
We used 5/8" dia spruce dowel sections full depth of the stab. these can easily be drilled and hardened with CA The Dalotel weighs 11 lbs electric powered 935 squares 78" span.
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Old Jan 12, 2014, 02:51 PM
The Prez....... again
kenh3497's Avatar
United States, IA, Rockwell
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The fuse is all trussed up on all four sides. It seems to be plenty stiff. While you can feel it give a bit when you twist it, it takes far more to twist than the balsa tail parts will withstand before breaking.

Build thread here with ALL my trials and tribulations, well at least most of them

http://www.rcgroups.com/forums/showthread.php?t=1956662

This is my first "scratch build". I've never drawn plans, nor built anything this big so I'm in need of a little guidance.

Ken
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Old Jan 12, 2014, 03:22 PM
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Looks very nice - If it were mine --I would fill with vertical grain 1/16 12 lb balsa ,between the upper fuselage stringer and the top longeron.
The intersection of the turtledeck and the top longeron at LE of the horizontal stabilizer , is a weak area and can twist.
when you add your stab mounting plates you will also need a flat outer edge of the fuselage at these points.for covering
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Old Jan 14, 2014, 10:29 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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I agree with Richard's suggestion. The sides are pinched pretty close together at the tail and by having the box become more of a tall and narrow rectangle you lose a lot of the torsional stiffness from the diagonals. I'd even close in the box by inletting cross grain 1/16 sheet into the bottom up to that third upright from the tail. Again because it'll aid by more stiffly "closing the box".

Or if it helps consider the sheeting for this narrow tail portion as a wing leading edge "D" tube. In fact even putting flat cross grain fill between the top longerons under the decking for the last portion up to that same upright is not a bad idea.

Once the fuselage flares out as wide as it is at the third upright the diagonals in the open bays ahead of that will do the job you're expecting.

This will also provide a firmer support for the bracing wires you said you're using on the tail. When you're talking angled lines it's surprising how the force vectors can multiply when loads are applied. So boxing in the tail as Richard is suggesting is a primo idea for a model that will be tossed around willy nilly.
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Old Jan 15, 2014, 07:20 PM
The Prez....... again
kenh3497's Avatar
United States, IA, Rockwell
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I did some twisting last night by grabbing the motor box and the tail post. There was much less twisting than my traditionally built box solid side CAP (in my avatar) I will take your suggestions under advisement.

As for "willy nilly".... Why would anybody do that to an aerobatic airframe

Ken
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Old Jan 15, 2014, 07:55 PM
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It's not the twisting that is the only issue. If you use bracing wires around the tail you also need to take into account the point load and distortion of the longeron where the bracing wires attach. If they can distort to the side or inwards you lose a lot of the stiffening from the wire.

I do see that one of the sets of uprights is spaced pretty close to the rear post. If that's where the bracing wires will anchor that's all well and good. But consider how thin it is from side to side.

If the goal is to use the model for serious judged flying duties I'd say it's not worth the risk and I'd add the sheet fill to stiffen the rear area. If it's for your own giggles and amusement then you're right and you'll likely find that it's just fine. The infill is only going to give you that last few percent in any event.
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Old Jan 15, 2014, 09:14 PM
The Prez....... again
kenh3497's Avatar
United States, IA, Rockwell
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OK, now I see what you are after. The tail wheel mount will be extended forward take care of the brace wire mount. Adding a little extra wood in the back will be easy to really stiffen things up. I am concerned about the tail becoming to heavy though. I'll just have to be careful what, where and how much I add.

Ken
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Old Jan 16, 2014, 07:34 PM
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It took me a while to consider the boxing in that Richard suggested as well. But it made more sense to me when the functional bracing wires were included.

As Richard suggested it would not take a lot of wood. 1/16 sheet fill for all four sides would easily be enough. Even with the glue included you're only looking at maybe 1/3 to 1/2 oz if light but firm wood is used.
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Old Jan 16, 2014, 07:55 PM
The Prez....... again
kenh3497's Avatar
United States, IA, Rockwell
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I marvel at how this thread went from asking about "how strong is #2 hardware" to a discussion on fuselage design. THAT'S OK though, as it all has to work together to make the whole thing NOT fall apart in the air.

Don't you just love RCG

Ken
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