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How to build a balsa sheeted foam core wing.

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How to build a balsa sheeted foam core wing.

Old 08-30-2006, 03:13 PM
  #26  
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Default RE: How to build a balsa sheeted foam core wing.

majortom,

That is a great weight reduction for mearly 20 bucks. I hope I could achive the same weight reduction eventhough it will be on a smaller plane than yours. However, as you know, I spent even more money [:'(] to get the conest grade (4-6 lbs sq.ft) instead. I really feel motivated to find a digital scale now... The balsa was shipped today and I expect my order to arr in a week or so. Lets hope one can trust online purchase for balsa and that they are actually inspecting the goods, as they claim, before they ship it away. I guess I will find out soon.
Old 09-02-2006, 11:36 AM
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Default RE: How to build a balsa sheeted foam core wing.

Having weighed & sorted every sheet, I'm happy to report that the lighter half of the 100 sheets averages 8.7 pounds/cu ft. The distribution of weight is definitely humped to the light side, with a relatively small number of individually heavy sheets tailing out on the top end of the distribution. So I have a small number of sheets that I can use where I need wood strength, and I can get good value out of pretty much the whole pack. So yes, I would do that again.
Old 09-03-2006, 10:08 AM
  #28  
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Default RE: How to build a balsa sheeted foam core wing.

When sheeting a foam wing, you need to sand the mating edges of all the balsa sheets. This will assure that all the edges are perfectly straight. Then, when you glue the edges together--you are gluing a straight edge to another straight edge. Trying to glue balsa skins together without sanding them will lead to wrinkles and then bumps and warps in the finished product.

I got a piece of 3" aluminum angle at least 1 meter long. Secure it to the workbench and use sandpaper with adhesvie back along the vertical edge. Now, you lay the balsa sheet flat and slowly and carefully sand it back and forth a few strokes untill the entire edge is perfectly straight. Do this on BOTH side of EVERY sheet.

I like to pre-fab all my skins. It usually takes 4 or 5 pieces of balsa for each side of each wing panel. So, get 5 sheets and lay them out flat on the bench. Place them on top of wax paper. Push each piece up to the next piece and assure that the mating surfaces are going to line up perfectly without any wrinkles or warps.

Now use masking tape to secure all the sheets together. Just use a piece of masking tape along the glue joint between all the sheets of balsa. If your using 5 sheets of balsa, then you'll have 4 joints to tape. Run a strip of tape down each joint and tape all the sheets together to make 1 large skin.

Now pick it up and flip it over. Tape on the bottom.

Now you just slide the sheets over to the edge of the table and let the joint flex open a little bit. Run a bead of glue into each joint. You can do 2 joints and then spin the sheet around and do the other 2 joints.

I like to use regular carpenters wood glue for glueing the edges together. After you put glue on all the joints, slide the skin back on the bench and allow any excess glue to squeeze out. Wipe it off with a wet paper towel. Allow the glue joints to dry for several hours.

Paint images attached.
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Old 09-03-2006, 10:23 AM
  #29  
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Default RE: How to build a balsa sheeted foam core wing.

Majortom,
You got a good batch.

Rcpilet,
I think I will use true edge and an exacto blade for the sake of simplicity and because I dont have 1 m long angle to use. What grade paper are you using for this purpos and for the sake of smoothing out the skin?
Old 09-03-2006, 10:59 AM
  #30  
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Default RE: How to build a balsa sheeted foam core wing.

I use 80 grit paper on the angle.

Okay--you have skins now. Leave the masking tape on one side. This is going to be the OUTSIDE of the skins. This is going to be the surface that you will see when your done.

Sand the glue off the joints with 100 grit. Be careful. Just a little bit. There should not be that much sanding if you wipes off the excess glue witha wet towel.

Now you need to trim your skins to shape. I trim mine to overlap the leading edge by about 6mm or 8mm. You will not wrap the skin over the leading edge. Just let the excess hang out over the front of the wing. I trim the trailing edge to overhang about 6mm. Trim the length of the skins to overhang the root and tip by about 8mm.

Now you need to prep your foam:
Use a cloth to wipe off any dust. A tach cloth works well.

Lay the foam wing panel down in the bottom core. Line it up perfect and tape it to the bottom core with masking tape. The top core is left aside for now, so that you can apply the balsa skin to the top of the wing panel.

If your using glue, such as epoxy you'll need to apply glue to the foam AND the balsa skin.

Apply your glue to the balsa skins and wipe off as much of it as you can get. I use an old credit card to squeeze off as much glue as possible. The balsa skin should just look wet. That is all. You should not have pooled up glue or be able to see any puddles of glue. Wet the entire skin well and spread the glue around liberally, then squeeze off as much as possible with a credit card.

Now use the same technique on the foam.

If your going to use contact cement, you'll need to use this same method to apply the cement to both the skins and the foam.

If your using a polyurethane glue--you only need to apply the glue to the foam. But, I use a spray bottle and mist my balsa skins lightly with water.

When you are finished applying the glue, set the skin on top of the foam and line it up as best you can. Now put the top foam core on top of the skin and prepare to compress it. Use a LOT of weight and distribute it evenly. A couple hundred pounds of weight is a minimum. Minimum 100 kilos. Better off with 150--200 kilos.

Allow the glue to cure for 24hrs--36hrs.
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Old 09-03-2006, 11:10 AM
  #31  
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Default RE: How to build a balsa sheeted foam core wing.

Okay, you got 1 skin on. Some guys will do both skins at once--but I've screwed it up before, so I just do 1 at a time now. I'm not the best builder either.

Just pull the wing panel out of the cores and do the same thing on the other side.

When you have both sides sheeted, now you can sand off the excess on the root, tip and leading edge. Stand the wing up on the leading edge and sand it back and forth across a long piece of sand paper. Sand SLOW. Sand EVENLY. Don't get in a hurry.

If you do not want to stand the wing up on the leading edge to sand it--it will be fine to use a sanding block. But just remember to use a very long sanding block. At least 12" (30cm). If you get a sanding block much lionger than 18" (45cm) it will be cumbersome and hard to handle well.

SMALL SANDING BLOCKS ARE USELESS. ALL THEY DO IS SAND RIPPLES AND WAVES INTO BALSA. NO SMALL SANDING BLOCKS

Use a large sanding block to sand the excess off the root and tip.

Then glue on the leading edge balsa block. Shape the leading edge balsa with a razor plane or sanding block with 60 grit paper. When you get close--drop down to 80 grit and then 120 grit.

How you shape the leading edge is up to you. Some people try and shape them uniformly across the leading edge and gradually taper the curve to match the thickness of the wing as it tapers down and gets thinner near the tip. I prefer a very blunt and fat root section and an almost razor sharp tip. This helps snaps and stall.
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Old 09-03-2006, 11:22 AM
  #32  
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Default RE: How to build a balsa sheeted foam core wing.

Rcpilet,
Thank you for this, great how to contibution to this thread.
Old 09-03-2006, 01:21 PM
  #33  
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Default RE: How to build a balsa sheeted foam core wing.

Here is an example of how I like to shape my leading edges on aerobatic models.
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Old 09-03-2006, 01:40 PM
  #34  
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Default RE: How to build a balsa sheeted foam core wing.

I assume to simulate the wash-out the full scale planes has? However, this not so for the aerobatics as you do want an aerobat to easy go into a spinn (I am a full scale pilot since 10 yrs.). Another reason it to simulate a laminated wing profile to achive less drag (read fuel efficency).

One of the skydivers at the skydiving club, where I am one of the pilots, is a aeroplane mechanic teacher by trade and has access to all the gadgets in the school where he teaches. He promised me today that he is going to try to help me to laminate/balsa sheet my foam core wings using the new vaccum technique.
Old 09-03-2006, 02:26 PM
  #35  
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Default RE: How to build a balsa sheeted foam core wing.


ORIGINAL: Rate1

I assume to simulate the wash-out the full scale planes has? However, this not so for the aerobatics as you do want an aerobat to easy go into a spinn (I am a full scale pilot since 10 yrs.). Another reason it to simulate a laminated wing profile to achive less drag (read fuel efficency).
No. There is no washout in an aerobatic model like an Extra. Thats not why I do it.

I shape the tips sharper to make them STALL faster than the root. The sharper angle of the leading edge at the tip will make the wingtip stall sooner.

You WANT an aerobatic plane to stall sometimes. You need the wing to stop flying in order to perform certain maneuvers--like snaps.

With a leading edge that is uniformly shaped and just gradually tapered--sometimes the wing will not want to stall for a VERY LONG time. When it finally DOES stall--it is very violent. It tends to make the plane twist up in a ball and go very deep into a snap or tumble.

With the sharply tapered tip--the wings will stall sooner--but less violently. You don't always WANT the most violent snaps that a plane will produce. This is especially true in IMAC or Pattern type flying. You want SMOOTH. Not erratic and violent.

It depends on the plane too. I have built enough kits and flown enough different types of airframes, that I kind of know how much to taper the leading edges. If this is your first big kit--you may want to just sand the leading edges uniformly. Have fun flying the plane and build a few more kits. Experiment slowly with the leading edge shapes and see how it effects different wings of different planes.
Old 09-03-2006, 02:45 PM
  #36  
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Default RE: How to build a balsa sheeted foam core wing.

"I shape the tips sharper to make them STALL faster than the root"

This is the same purpose as the wing washout on full scale planes (not on aerobatics because of stated reason)

"If this is your first big kit--you may want to just sand the leading edges uniformly."

I will most likley follow your advise = stick to original drawing = uniform LE's. Obviously I don't want to risk my first 1/4 scale kit, and will instead concentrate on a light weight and best build I can come up with, and not manipulating too much with the aerodynamics of the construction. All the wing profiles (of a well designed rc model) uses well established wing profiles that has been developed by many engineers (in the 50's) that took into account Raynolds number together with other importaint aspects. The result ended up as several finished wing profile (that a model mfg choises when design their planes) that has a lot of work into em wich I will not take the risk of destroying. Obviously, with experience and knowing what to achive altering a profile there is no prop. However, with my limited building experience I rather not go that route.
Old 09-03-2006, 06:16 PM
  #37  
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Default RE: How to build a balsa sheeted foam core wing.

If vacuum bagging, the taping of the individual sheets together and cutting to shape are the only things that are necessary before the actual core to sheeting bagging. The epoxy resin (laminating) or Gorilla Glue will hold the individual pieces of sheeting down to the foam core (and the edgeas will be percfectly held down - without the need to edge glue the sheets together and this therefore eliminates one sanding step.

The above is shown in Jim Young's video referenced earlier as well as the video from ACP Composites that comes when you get one of their vacuum bagging set-ups.
Old 09-11-2006, 03:36 PM
  #38  
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Default RE: How to build a balsa sheeted foam core wing.

Ok, quick update.

My kit is finaly here and so is the contest balsa I ordered to replace the stock sheeting in order to save weight.

Needless to say I was very curious to see if this stunt would save around 10 oz as expected or not, so I found my sweet little digital scale (acurate to +/- 1 gram=0,035oz) and went to action...

The weight of all stock balsa sheets: 598gram=21,09oz
Weight of the contest balsa to replace the stock: 311gram=10,97oz
------------------------------------------------------------------
Weight reduced by replacing the stock sheeting for contest grade is 10,12oz or 0,63 lbs if you like. Same as slashing the weight of the sheeting by 50%.
Now some scrap and cutting some of the material off is expected while building the kit, so a total of approx 0,5 lbs will be saved on this model by this stunt alone.


I have hooked myself up with one of the Skydivers at my Skydiving club to help me out vaccum sheet the wings. This guy is a aeroplane technician instructor with the keys to the school where he works at (access 24/7). He said they have a 2000 liter airtank at the school used for the students laminating classes. We are planing to laminate my wings on a weekend. Hopefully this will work out without any snags and hopefully I will get myself a set of super lite wings with a minimum amount of resin used. *fingers crossed*


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Old 09-14-2006, 12:43 PM
  #39  
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Default RE: How to build a balsa sheeted foam core wing.

Though I've not built a foam wing in many years, I've cut many cores and sheeted them with both balsa and 1/64 ply. Prepared the skins in large sheets as has been outlined but used 3M spray adhesive. Spray very lightly both the core and sheeting and let dry to touch. Using venition blind strips, locate the sheeting on the wing and remove in order.

The bond is permanent, and light as long as discipline is used to spray lightly... it doesn't take much.

Old 09-14-2006, 12:53 PM
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Default RE: How to build a balsa sheeted foam core wing.

Hello AA5BY

"Using venition blind strips, locate the sheeting on the wing and remove in order." ??? What is venition blind strips??? Can you please explain this in other words please. Being a forigner I didn't understand that last sentence.

So if understand you corr, one can do as normal, core back into its sleefs after sheeting is in place and weight it down to dry, but use 3M spray instead?

Have you tried the epoxy resin way of doing it as well? Why did you prefere your method, what is the name of the 3M spray? Loads of questions I know, but I am trying to find the best, esiest, and lightest way of doing this as I am about to sheet my first set of wings.

Regards,
Old 09-15-2006, 10:54 AM
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Default RE: How to build a balsa sheeted foam core wing.

3M spray adhesive shold be easily found at a home center. It is a contact adhesive, meaning that both surfaces receive the adhesive and that when they touch, they are bonded and no drying time or compression is needed.

A word of caution... I used 3M many times on foam core wings many years ago. It would be wise to ensure that any spray adhesive chosen doesn't attack foam.

To keep the sheet and core seperated while locating the sheet... some strips are needed. I may have mis spelled "venition blinds", I refer to window shades that open and close. Years ago they were metal and now are almost allways plastic. The strips from a discarded window shade work very well as standoff strips keeping the core and sheeting seperated until contact is desired.

A really nice result of using spray adhesive is the speed factor. Once the sheets are prepared, the four skins can be glued onto the cores in a matter of a few minutes and immediately be ready to trim and apply leading and trailing edges and tips. I've often done that with spray adhesive as well.

Another important note: The balsa strips must be prepared as a sheet as several describe above. I don't think spray adhesive would work for individual sheet application as there is no repositioning once contact is made.

For this reason, sanding should be done prior to the glue up of the strips. Fit them together with masking tape on the back side, place on flat surface and sand. Then flex open the joints and using a very small brush, lay 15% thinned titebond on the edges, press flat and wipe excess glue away with slightly dampened rag. Weight down to dry. As others have said, making the sheets up, sanding should be done prior to gluing joints as the hard joints will not sand even to the surrounding balsa.
Old 09-15-2006, 11:15 AM
  #42  
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Default RE: How to build a balsa sheeted foam core wing.

AA5BY

Thank you for that. I will see if I can find this 3M spray glue and check it out. This as it could be an easy alternative to the treditional epoxy resin or even the vaccum techniqe. I am intresseted in finding the method that offer the lightest weight together with not being too hard to complete as I have never sheeted foam cores before.

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