I don't know if I can save her - RC Groups
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Nov 06, 2017, 01:04 PM
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gobigdave's Avatar
Discussion

I don't know if I can save her


Long ago, I picked up a Curare from CCK. Beautiful kit! I got myself to the point where all that was left was installing the hardware, sanding, and covering. Unfortunately, life got in the way, and I had to put building on hold for a while, I was able to get started again this past weekend. That's when there were a few choice words screamed, and now I don't know if I can save it.

My problem is with the 2-piece wing of this Curare. I knew aligning this was critical, so I must have measured 20 times when I added the wing tube to the fuselage framing (before adding sides, etc.) It was perfectly straight. I measured fanatically again when I added the sides. That wing tube was level and the distances from the ends to the tip of the tail were equal. That's when life hit, and the 90% built plane went on the shelf.

My plan this weekend was to get the landing gear and motor installed. First, I put the wings on, and then I noticed it didn't look right. I took the wings off and measured the wing tube again. It was still perfectly level, but now the measurements from the ends of the tube to the tail were off by a touch over 3/16". If this were a fun sport plane, I wouldn't like it, but I could live with it. A pattern plane is another story. Something must have shifted slightly while it was drying.

Here's the thing. I can probably cut the tube out of there, but I'm not sure I will be able to line everything up again. I figure cutting the tube out will result in a larger hole in the fuselage. I could make two plates to go over the wing tube inside the fuselage that glue on the inside of the existing plywood wing root on the fuselage. Lining those plates up to ensure a level and straight wing, plus get the incidence right will be a trick.

Any ideas to help this?

Should I just bite the bullet and build another fuselage? This time I will keep an eye on the wing tube more.
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Nov 06, 2017, 01:09 PM
Registered User
jpurcha's Avatar
Have you contacted the kit cutter (sorry I can't remember his name) for some assistance on this problem. I bet he would be willing to help.

Jim
Nov 06, 2017, 01:29 PM
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doxilia's Avatar
Quote:
Originally Posted by gobigdave
Here's the thing. I can probably cut the tube out of there, but I'm not sure I will be able to line everything up again. I figure cutting the tube out will result in a larger hole in the fuselage. I could make two plates to go over the wing tube inside the fuselage that glue on the inside of the existing plywood wing root on the fuselage.
Dave,

From your comments it sounds like the fuse saddle area as well as the wing panels are straight and square. I think your problem is in the tail boom rather than the entire fuse being out of wack. If that's the case, cutting the wing tube free will compound the problem as you will now be aligning the wing to be square with a crooked fuse.

That said, if you're sure your fuse is dead straight, then triple check the alignment of the wings to the fuse AT the saddle. Insure they are equidistant LE and TE from the fuse when mounted (they may be identical panels when off the tube but you need to check ON the tube) AND that they are both level with respect to each other (i.e., uniform dihedral) and equidistant from the table top. Then, check if the tip LE's are equidistant from a point on the nose of the fuse that is centered. If that's all good, chances are good that your tail boom has shifted.

Quote:
Lining those plates up to ensure a level and straight wing, plus get the incidence right will be a trick.
I don't recall how Mark's (CCK) fuse is setup for anti-rotation of the panels but the incidence is typically setup with those elements. Of course, if the incidence of each panel is different, then the distance to the tail can vary easily by something like 3/16 so before you tear our the tube and consider building new fuses, setup the wing to the fuse sides at 100% confidence level. Only then, can you be sure if alignment with the tail is off or not.

I doubt your tube has "shifted" as it is held in place by the framed up fuse on either side. Then braced with ply rings on the inside in all likelihood.

David
Nov 06, 2017, 02:20 PM
Registered User
gobigdave's Avatar
Yes, I contacted Mark at CCK via email, but I haven't heard back from him yet. I didn't expect to today, since I know he's been pretty busy given his most recent posts around this forum.

David, I will check the fuse again tonight, but I'm pretty sure it's straight. I checked the wing tube lengths multiple times, and then I compared wing tube to firewall centerline, wing tube to tail, and wing tube to stab mount. All were off in the same direction.

Anti-rotation is handled by a combination of a tab and a rod. The tab goes through a slot to be screwed down inside the fuselage. The rod slides into a hole in the wing root.

Wing Tube --- Tab Slot --- Rod

All are lined up on center.

Here's a photo of a fuselage before the sides go on.
Nov 06, 2017, 08:05 PM
AMA 73386
jzanutto's Avatar
I am not able to visualize exactly where you are shooting the measurements to/from. Can you use the paint program and draw a cheesy top view of the fuselage and show what you are measuring to and from?
Nov 06, 2017, 08:51 PM
Registered User
gobigdave's Avatar
A = A
B = B

Does this help?
Nov 06, 2017, 09:59 PM
AMA 46133
Quote:
Originally Posted by gobigdave
A = A
B = B

Does this help?
I would put the fuselage on a jig and measure everything. I don't know how long you put the project away for. Over time the fuselage may have warped. Shifting the wing tubes may not do anything except to line up the wing and the stab. If it is a right warp you may have created a natural right thrust. If it is a left warp then you may have an issue.

Balsa wood will dry up over time and shrink.
Nov 06, 2017, 10:32 PM
Registered User
doxilia's Avatar
Dave,

while your measurements are in principle relevant, they are actually somewhat unimportant. Here's why:

If you have two identical wing panels that are mated to a square fuselage, then that aspect of the model can be considered "square". How long the wing tube actually protrudes into each wing is irrelevant. You could have a perfectly sound and square pattern model with a wing tube that is 1/4" shorter on one side than on the other. Now, that's not how we build them (but conceptually we could) and the wings should snug up properly on each side with the same length of tube on each side of the fuse.

However, my point is that it doesn't matter whether the tube's are square wrt each other. You have to measure the actual airframe. If your model were built to 100% perfection then using the two tubes would be a reasonably proxy for squareness but as we can't build to 100% perfection, you should take airframe metrics. The tubes are just there to support the assembled model.

I agree with Hansen, you should put your fuse in a jig and assess it first. The wings and stabs are irrelevant if the fuse is not straight. Like I mentioned above, if the fuse has banana'd, moving the sleeve in the fuse to match the banana is just going to result in a banana split...

David
Nov 07, 2017, 03:58 AM
PhoenixFlyer
PhoenixFlyer's Avatar

I agree with Hansen, you should put your fuse in a jig and assess it first. The wings and stabs are irrelevant if the fuse is not straight. Like I mentioned above, if the fuse has banana'd, moving the sleeve in the fuse to match the banana is just going to result in a banana split..


David. Best answer.

V
Nov 07, 2017, 09:47 AM
Registered User
gobigdave's Avatar
Banana Split. That was great!

Ok, I will get everything together in a jig and measure it up. I need to clean up one wing panel slightly (forgot to angle the root rib on one panel). I want to add bit of wood to reduce the gap on the bottom while ensuring both panels are exactly the same length. They are already when measured on top. I also plan on adding filets so no gap will ever be visible.

Mark, from CCK, suggested I build it up and see how it flies if it's not too far off. If it is, he can cut me up the parts for a new fuselage.

Thanks for talking me off the ledge again.
Nov 07, 2017, 11:58 AM
AMA 46133
>>Ok, I will get everything together in a jig and measure it up.<<

Out of the blue I "collected" a tool that a friend suggested. It turned out to be a very worthwhile piece of equipment. I believe I paid $80 for it.

http://smile.amazon.com/Bosch-GLL2-4...+level+GLL2-40

I don't know if it is still available. Maybe an upgraded version is available now.

https://www.rcgroups.com/forums/show...light=Kwik+Fli

Scroll down to post 52 and you will see some pictures on how I used the laser level to line up my wings/stab/vertical fin. If your firewall is not offset for down and right thrust you can stand the fuselage up on the nose and laser the straightness of the fuselage vertically.

I looked at your diagram a few posts above and realized you are only measuring the distances between the wing tube and stab tube. They may be perfectly parallel even if the fuselage is bent. You need to triangulate the tip of the wing tube to the extreme end of the fuselage to square the fuselage and the wing tube first. The reason is that the stab tube may be offset a tad.

Just remember while you are building a model, anything is possible up to and including building a new fuselage/wing or what ever. So start with what you can one thing at a time and you will get back into the groove. To build a model is a test to see how well we can cover up our screw ups.

>>Mark, from CCK, suggested I build it up and see how it flies if it's not too far off. If it is, he can cut me up the parts for a new fuselage. <<

Yes, sometimes a slightly crooked fuselage may turn out to fly better than a perfectly straight one. The reason, I have seen a lot of fuselages actually were designed with right and down thrust on the firewall and/or the nose ring. The fuselage sides on both sides (for a wood fuselage) are not the same and the engine mount is offset to the left so the prop washer can hit the thrust line. Is that fuselage straight? Or intentionally bent?

The good thing is that you have a kit cutter that is willing and able to come to the rescue and cut you a new fuselage. So you can be sure of a perfect plane eventually.

>>Thanks for talking me off the ledge again.<<

Oh yes, don't jump yet.
Nov 07, 2017, 12:27 PM
Registered User
cannondale1974's Avatar
Quote:
Originally Posted by gobigdave
A = A
B = B

Does this help?
That's the actual fuse right? It looks like the tail of the fuse is crooked to the right, (to my eye). If this is the case, just cut the tail of the fuse down the centerline, reglue and sand/trim as necessary. My 1/2 cent worth.
Nov 07, 2017, 01:28 PM
Registered User
doxilia's Avatar
Quote:
Originally Posted by cannondale1974
That's the actual fuse right? It looks like the tail of the fuse is crooked to the right, (to my eye). If this is the case, just cut the tail of the fuse down the centerline, reglue and sand/trim as necessary. My 1/2 cent worth.
The problem is that photos can be very deceiving. A perfectly straight fuse can look quite bent if shot slightly to the left/right from centerline.

However, if you are right and the fuse is bent to the right, it appears that the tail boom is actually straight with the vertical fin assembly slightly off. Fixing this may be viable but given the little I know of Mark's re-design, it is a rather complex interlocking fuse which might make the repair somewhat tricky.

In any event, going back to my previous comments, measuring wing tubes to stab spars is rather misguided. Assuming the tube extends from the fuse sides identically on either side as do the stab spars, any difference in metrics is in my mind just a proxy for an issue. To really be sure you are not square, you should measure up the assembled airframe. If it is not square, then the problem most likely can't be fixed by moving the wing tube. You probably have a banana fuse at some point. Hansen's laser is indeed an excellent gadget!

Good news is whipped cream hasn't been added yet so there is no split to speak of!

David
Nov 07, 2017, 07:54 PM
Registered User
gobigdave's Avatar
I already had that laser level in my Amazon wish list. I was going to order it as part of the "jig". I was originally planning on using that to help get my stab, etc. all aligned, but now I can use it for this as well. I also saw another use in the Novi Arrow build thread. It was used to make a straight line around a curved surface -- very useful when covering starts.

Regarding the photo making things look out of whack. I'm pretty sure it's just the photo. When I sight it, it sure looks straight.
Nov 07, 2017, 08:17 PM
AMA 46133
>>Regarding the photo making things look out of whack. I'm pretty sure it's just the photo. When I sight it, it sure looks straight.<<

Yes that is the inherent issue with a wide angle lens. The lens distortion creates a curve line when you are off center.


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