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Old 05-22-2014, 08:47 AM   #1
mclarkson
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Default Cool tips for cutting dihedral?

Let's say I have a wing with no dihedral, such as this one:



And I decide that it really needs some dihedral. (Maybe 4°)

I am not much of a craftsman. Before I started building foamies a few years ago, I'd never built anything in my life. So I have no table saw, no band saw ... no fancy tools of any kind.

I'm unlikely to buy a table saw ($money$) or try to build a precision hot wire cutter (*skill*).

Is there a trick to measuring / scribing a wing to cut at the proper angle? I cheap tool I might not know about?

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Old 05-22-2014, 08:54 AM   #2
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A standard woodworker's Try-Square should be all that's needed to get a perfectly square cut line: http://www.technologystudent.com/equip1/try1.htm
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Old 05-22-2014, 09:09 AM   #3
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Ah ... but I don't want a perfectly straight cut - or, at least, not one that's perpendicular to the wing. I want a cut that will give me an angle when I glue the sides back together. The attached photo shows what that cut would look like (though really exaggerated to make it clear.)


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Old 05-22-2014, 10:51 AM   #4
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ok.. If I were doing it I'd make a straight cut with the Try-Square then prop the wing up at the desired dihedral angle with the edge requiring bevelling aligned with the edge of a thick table or similar work surface with a good deep edge. Hold the wing firmly in place with pins or whatever works.

Taking a large sanding block, use the thick edge of the table as a guide to sand the bevel into the wing. Consentrate on keeping the block flat against the edge.

Best not use your new mahogany dining table for this
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Old 05-22-2014, 11:01 AM   #5
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JPF is right cut the wing, prop it up on one side, then sand it with a block sander, use a smooth board with a line drawn on it with a magic marker for alignment, it wont take very much sanding, then epoxy the two half’s back together.

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Old 05-22-2014, 12:09 PM   #6
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Me ?

I'd only cut part way through leaving top still joined.

I'd then place one wing flat on a flat board raised of the bench upside down.... place weights on it.
I'd then ease the other wing panel down till tip touches bench creating a V slot in the wing centre where I cut.

Two ways now ...

a) Fill with Gorilla Glue / PU so it expands and fills all gaps .. creating a foam infill.
b) Cut a wedge of foam to insert with glue keeping the dihedral created on the bench.

Let set ... then shape / sand to desired shape.

It's actually only the opposite of taking dihedral out !

Nigel

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Old 05-22-2014, 12:23 PM   #7
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All of the above will work, I've only one thing to add. If you know (or can trace) the airfoil, cut a couple blocks of balsa or foam with the lower profile of the airfoil and place these under the wing while you cut to keep the wing from rocking while you sand or cut.

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Old 05-22-2014, 03:27 PM   #8
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Id just cut a wedge and glue it in place

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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Old 05-22-2014, 03:39 PM   #9
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MIGHT need to brace the dihedral joint by inserting something such as popsicle sticks. The joint will never be quite as strong as the original molded wing.

Leaving the top still connected tends to not work because the top surface is always curved. Leaving appx 1/16 uncut on the bottom works if the wing has a flat enough bottom.
I would just go ahead and cut all the way through then sand in the dihedral as described by others. Its easier than trying the other methods.
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Old 05-22-2014, 04:45 PM   #10
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I'm a little confused,is this a top wing or lower wing mount. which ever it is i agree with Nigel. score out enough material in a straight line and don't cut all the way through. then weight it down and glue the seam closed.

if this is a bottom wing,cut out the material on the top side as this gets gets hidden by the fuse when installed.

opposite is true for a top Mount wing,cut notch in the bottom and set the angle desired,fill the notch with a wedge of foam thats going to be very thin and the glue line remains hidden. one last question....why add dihedral. just curious.
i build balsa kits and modify some to add ailerons,when doing this i remove most dihedral so the ailerons are more effective. i will admit,i built 2 of these park flyer's and this one is a bit squirrelly to fly. the red one was my third or forth build and i left more dihedral...it flys better or should i say easier.




edit:fuhber is right about the curved wing,i didn't think of that. but i leave the question as posted above.


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narrow is the place to land...wide is the space to crash....choose the narrow way!
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Old 05-22-2014, 05:03 PM   #11
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Why add dihedral?
Maybe its a home brew airplane and he had a core cut but forgot to ask them to cut in dihedral.
Or flight test indicated it just doesn't fly right without it.

Depending on the purpose of the airplane and how the wing is mounted you might want dihedral for maintaining neutral or positive roll stability when applying rudder. A flat wing mounted low tends to give adverse roll on rudder.

I often adjust dihedral angles....
The FourStar series I tend to reduce dihedral 50%. Flat is just wrong, half original dihedral is just about perfect for knife edge.
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Old 05-22-2014, 07:38 PM   #12
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Originally Posted by fhhuber View Post
MIGHT need to brace the dihedral joint by inserting something such as popsicle sticks. The joint will never be quite as strong as the original molded wing.
Depends ... but yes I would generally agree ... the sticks would also help to maintain the dihedral as well as strengthen the joint.

Leaving the top still connected tends to not work because the top surface is always curved. Leaving appx 1/16 uncut on the bottom works if the wing has a flat enough bottom.
I would just go ahead and cut all the way through then sand in the dihedral as described by others. Its easier than trying the other methods.
Sorry - But I disagree .... my method of cutting most of way through but leaving top joined actually helps keep wing aligned ... and saves all that angled sanding jazz ..
The top taking up a curve is only in the centre and can in fact a) be better on the eye than the sharp V, b) sits better in wing seat top or bottom wing mounts.

As to whether it's a top or bottom wing makes no odds - the cut is still in the bottom and V'd to infill.

I've done this plenty of times and it's worked a treat for actually very little work....

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Old 05-22-2014, 09:34 PM   #13
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Solentlife, I must be misreading you. That sounds like it'd give me anhedral. The 'top' of the wing needs the most material removed. What am I misunderstanding?

Originally Posted by hayofstacks View Post
Id just cut a wedge and glue it in place
That's exactly what I'd do but ... I can figure out how to calculate the exact wedge to be cut so that the airfoils match exactly and I get the amount of dihedral I want.

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Old 05-22-2014, 10:06 PM   #14
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I agree with Nigel; one vertical cut part way up from the bottom, invert the wing, prop it up in the center to the degree of your choice with the tips on the table, and fill the gaps with your preferred method. Simple, and it will keep the wing aligned. I'd use Gorilla glue, but realize not everyone else would be used to working with it as I am.

However, to use the method you illustrated in your second post, I think the sanding block would be the way to go. I'd use a piece of 2 x 6 with both ends cut square (you'll have to do right/left wings don't forget) and align the leading or trailing edge of the wing with the edge of the board so that both sides will be square when you join them. If you don't have the means to square up the board ends, you could buy one at Loew's or Home Depot and they will cut it for you.

Good luck; it'll be easier than you probably thought.

Tom
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Old 05-22-2014, 11:01 PM   #15
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So you're talking about adding material to the leading and trailing edges, rather than removing material from the center.

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Old 05-23-2014, 01:46 AM   #16
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Originally Posted by mclarkson View Post
Solentlife, I must be misreading you. That sounds like it'd give me anhedral. The 'top' of the wing needs the most material removed. What am I misunderstanding?


That's exactly what I'd do but ... I can figure out how to calculate the exact wedge to be cut so that the airfoils match exactly and I get the amount of dihedral I want.
Easy. find the main spar, align the top of wing with some tape. Place bottom wing flat and prop other wing up until your happy. mesure gap, sand to fit, glue in place. if you want/need to, beef up main spar. i wouldn't worry about it much personally. well cured and ppropperly preped epoxy will be.stronger.then the surface that its glued to.

exact degrees are just a number. i strongly prefer the that's about rightmethod, and it has served me well.

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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Old 05-23-2014, 02:10 AM   #17
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Chellie & jetplaneflyer nailed it. It's a model plane, not rocket science space shuttle. Determine centerline, hack the wing apart, line up cut edge with a table edge, propped up to whatever dihedral you want, and grind. I prefer a board with #40 grit sticky glued on it for quick grinding - #40 grit, time to quit . . .

No fancy dihedral splices for me. I butt glue the wing halves with epoxy, then wrap joint with light weight fiberglass, slop resin on it, lay a piece peel ply on the goo and squeegee. Strip off peel ply when cured, no sanding required, nice smooth fix.
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Old 05-23-2014, 03:53 AM   #18
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Originally Posted by tr4252 View Post
However, to use the method you illustrated in your second post, I think the sanding block would be the way to go. I'd use a piece of 2 x 6 with both ends cut square (you'll have to do right/left wings don't forget) and align the leading or trailing edge of the wing with the edge of the board so that both sides will be square when you join them. If you don't have the means to square up the board ends, you could buy one at Loew's or Home Depot and they will cut it for you.

Good luck; it'll be easier than you probably thought.

Tom
Yeah
A sanding block, with a center mounted piece of hardwood about 6 inches long to make certain that the sanding block stays perpendicular during the sanding process.

As for cheap sanding blocks, I made one out of a piece of oak hardwood, a power sanding belt, a couple of 10-24 screws, a short piece of aluminum flat ware and a wood dowel. Had it for over 10 years.

If interested, I can dig up photos on how it was made.

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Old 05-23-2014, 11:16 AM   #19
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Originally Posted by mclarkson View Post
Solentlife, I must be misreading you. That sounds like it'd give me anhedral. The 'top' of the wing needs the most material removed. What am I misunderstanding?
You FILL the opened up cut ... the cut in the bottom of the wing is OPENED up to a V .. filled ... this creates the dihedral.

Why remove material and actually REDUCE wing area as well ?

It also keeps the work under the wing where least seen ... top stays nice.

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Old 05-23-2014, 11:17 AM   #20
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Originally Posted by mclarkson View Post
So you're talking about adding material to the leading and trailing edges, rather than removing material from the center.
??

Fill the V ...

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Old 05-24-2014, 09:02 PM   #21
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I decided Solentlife's method seemed the least muss and fuss, although it created some problems of its own because of the way the wing mounts to the plane. But, anyways ...






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Old 05-25-2014, 09:12 PM   #22
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The results:

YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.

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