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Apr 21, 2013, 12:12 PM
Stealth Plane Works
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Idea

Easily building light and strong vacuum bagged wings


The two most common home building techniques for vacuum bagged wings are stressed skin and independently built spars.

Stressed skin uses a uni-carbon skin to give strength to the wing. It is easy to do, but is heavy. This is because the bigger the distance between the top and bottom skins, the stronger the wing, but most of the uni-carbon will be in places where the wing thickness is less than the maximum.

Independently built spar wings are built by removing a span-wise section of the core equal to the width of the spar, building a spar, and then bonding the three pieces together. This technique gives you a strong, light, wing, but it is a lot of work and it is hard to get the wing back to a clean airfoil shape. If the spar is thicker than the airfoil you have a serious problem to solve.

The technique I describe here combines the advantages of the two techniques. I have built several wings using this technique and it really delivers.

Each step will be described in separate responses to this post.

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Apr 21, 2013, 01:05 PM
Stealth Plane Works
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Step 1, cutting the cores


The cores are cut in two pieces. One is the airfoil with a cutout for the spar, the other a spar core cutout.

The spar cutout in the airfoil is cut from the bottom and does not extend through the top of the airfoil. This keeps the top intact and will ultimately ensure that the top of the finished wing has the correct airfoil shape.



The spar core has the same width as the cutout in the wing core, but isn't as deep, the difference in depth is a tad more than the thicknesses of the spar caps.



In a separate response I'll post a spreadsheet that allows you to calculate how thick to make the caps. The caps have tapered thickness and are optimized to give the optimal strength at each span position. A linear taper becomes unnecessarily strong and heavy the further you go from the center of the wing. This gives you excessively heavy tips and impacts performance adversely.

As you can see from the picture, The spar core has a cutout for a tube. The tube idea comes from Phil Barnes and it makes joining simple. It also serves as a solid web in the spar core. It has both great shear and compression strength.



The last picture shows the tube in the spar core, which is then inserted into the spar cutout.
Apr 21, 2013, 01:21 PM
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Preparing the cores and beds


The trailing edges of the cores are then trimmed. I trim them to 1/8" less than the final chord so the trailing edge will have skin bonded directly to skin with no foam.

If the panel consists of multiple sub-panels they need to be bonded together. I use epoxy making sure there's no bleed outside the joint.

Next add end caps and the center section center ribs. The ribs need to fit the foam cores exactly. If you are building a center section with dihedral you need to sand the dihedral angle into the center ribs.

I bag the wings in the top beds. To make sure the panels come out 100% correctly with no twist I don't use the beds from the cores, but cut top beds that are over length and have any desired dihedral built into them. I then bond the beds to particle board that is cut to the shape of the beds. Particle board is very stable and doesn't warp and twist. The end result should be a bed that the panels fit snugly into with no gaps or overhangs.

These beds can be used over and over again and I make them durable by adding a skin of glass to them. I use 2 oz glass cloth and paint thinned epoxy onto the skins. After I have painted the skins I wipe them down with paper towel to ensure that there are no air bubbles or wrinkles.
Apr 21, 2013, 01:38 PM
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Step 3, preparing the spar cap layup


I build the spar cap from 1" wide strips of uni-carbon tape from Soller Composites. You can use 4.7 oz tape or 9 oz tape. The former is easier to wet out and the latter requires fewer layers.

The number of layers and the taper is determined from the attached spreadsheet embedded in a zip file, SparCalculator.zip.

The data you need to enter into the spreadsheet is the thickness of each layer of the layup, the span of the wing, the number of layers in the center and an exponent.

The number of layers in the center determines the strength of the wing center and is a matter of judgment. I use Mark Drela's Supra plans as a guide because I know that his design is pretty indestructible. I ensure that the cross section of the spar is proportionally similar to Mark Drela's. For an electric plane that doesn't need to be winch launched you can back off considerably. I would go with about half the cross sections. You can also make the bottom cap thinner than the top.

The exponent determines how quickly the bending moments decrease along the span. An exponent of 2 would correspond to a constant chord wing. The spreadsheet that I embedded uses an exponent of 1.8, which is more conservative. You could go more aggressive by using exponents bigger than 2.
Apr 21, 2013, 01:56 PM
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Step 4, bagging the spar into the cores


This is the exciting step.

A mylar sheet is used to ensure that the cores don't end up getting bonded to the beds. The joints on the top of the cores are taped to prevent bleed and the core is placed in the prepared beds on top of the mylar. Place plastic duct tape on each side of the spar cutout to prevent any overflow from sticking to the beds.

The top cap is laid up directly in the cores. Wet them out thoroughly and then lay them into the spar channel. Use a 1" roller to get rid of excess epoxy and air bubbles and finally get rid of excess epoxy with paper towel.

Make sure that you have continuous layers of uni-carbon across the center panel and spread any joints across the spar.

Wet out the tube cutout in the spar core and then insert the tube. Don't worry about the spar core breaking along the continuous portion. Put a bead of thickened epoxy along the slot. Wet the sides of the spar cutout in the cores with epoxy and then place the spar cores into the cutout with the open slot against the top cap. Finally lay up the bottom cap lay upon layer on top of the spar core.

The end result should be that the wet spar sits in the cutout and nowhere sticks over the top.

Put a mylar sheet on top of the cores and then add a breather.

Now put the whole thing, including the beds inside a vacuum bag and keep the layup under vacuum until the epoxy has cured.

Including the beds is a really important step because it ensures that the cores are pressed into the correct airfoil shape.
Apr 21, 2013, 02:01 PM
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Step 5, bagging the skins


After cleaning up the cores you need to spackle the bottom of the cores so the bottom airfoil is true and there are no dips into the spar cutout.

The wing skins are almost exclusively Kevlar laid up on the bias. Just make sure that there is a sheet of uni-carbon in the center to ensure that dorks don't cause wrinkles in the skins.

From this point on the skin layup and bagging is conventional.

Simple, eh?

Questions?
Apr 21, 2013, 06:02 PM
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jp_fritz's Avatar
Thanks for posting this. I like the idea.
Two questions.
What type of foam are you using?
Is the spar core the same as the wing core?

James
Apr 21, 2013, 06:48 PM
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Hl60


I currently use HL60 for both the wing and spar cores. I plan to switch to lighter foam for the wing cores and use a slab of Spider foam I have for the spar cores.

Anker

Quote:
Originally Posted by jp_fritz
Thanks for posting this. I like the idea.
Two questions.
What type of foam are you using?
Is the spar core the same as the wing core?

James
Apr 21, 2013, 08:51 PM
Jim C Patrick
jcpatrick's Avatar
My-oh-my! What a difference 5 years makes.

Nevertheless, this is a very slick implementation and it should make a nice, strong wing. I assume (!) you are using a CNC hot-wire? Those are some very clean foam cuts.
Apr 21, 2013, 09:33 PM
Dean
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Pics for post #4 & #5?
Apr 22, 2013, 08:14 AM
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Yes, they are CNC cut.

Anker

Quote:
Originally Posted by jcpatrick
My-oh-my! What a difference 5 years makes.

Nevertheless, this is a very slick implementation and it should make a nice, strong wing. I assume (!) you are using a CNC hot-wire? Those are some very clean foam cuts.
Apr 22, 2013, 08:18 AM
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Coming. I didn't have a wing at those stages of build to photograph.

I started this thread because I'm helping someone build an e-Supra and I thought I mights as well share the technique with a broader audience.

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Quote:
Originally Posted by A10FLYR
Pics for post #4 & #5?
Apr 22, 2013, 08:29 AM
Stealth Plane Works
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Spar calculator exponent


I omitted to explain that using an exponent smaller than 2 is prudent because the wing gets thinner and thinner as you approach the tips. The 2 exponent for a constant chord plank still holds.

Notice that the distances in the spar calculator spreadsheet are given in inches from the center, so you need to cut your uni-carbon strips at twice the lengths. You definitely want unbroken strips across the center.

I also add another layer or two in the center, just to be safe.

If you build the wing with center dihedral another advantage is that positive G loads will try to press the top cap through the center ribs, which is unlikely to succeed. But for negative G loads the forces will try to lift the cap out of the wing, so there a good bond is essential.

Will Lipscomb has found a foaming agent that causes epoxy to expand. I plan to use that when I get my hands on some to ensure that there are no open voids in the spar.

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Apr 22, 2013, 12:02 PM
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Nov 21, 2013, 09:06 PM
Launching!
Dan.Cummins's Avatar
Anker,

I love the simplicity of your spar design.

Are their any issues with the top spar cap not being tied to the top skin?

Do you winch launch this wing design.....

Thanks for a great thread.

Cheers!

Dan


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