Airfield Models - How To

Make Perfect Wing Saddles on Model Airplanes

May 02, 2015



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Airfield Models (http://www.airfieldmodels.com/)How to Make Perfect Fitting Wing Saddles for a Model Aircraft

This tutorial will show you a way to achieve perfect fitting wing saddles.  It will not show you how to set up the wing properly.

Prior to performing these steps, the wing should already be mounted true and square to the fuselage using wing bolts or rubber bands and dowels.

For years I attempted to fit and sand wing saddles to achieve a perfect fit and was never successful.  I could get a perfect fit in both saddles but by that time the wing wouldn't be level.

My solution is to sand the saddle as close to a perfect fit as possible and use small shims (when necessary) to level the wing.  The shims can be 1/4" square pieces of 1/64" plywood or material of suitable thickness.

The shims will not be bearing the load of the wing.  All you need to do is strategically place them where they'll keep the wing where it is supposed to be.  After the wing is squared, leveled, etc., the wing saddle can be cleaned up into something presentable.

Also see

Tools and Materials

  • Shim material if needed
  • Epoxy and micro-balloons or Silicone sealant
  • Waxed paper, plastic food wrap or plastic iron-on covering (Monokote)
  • Masking tape
  • Spatula (balsa scrap works)
  • Single edge razor blade
 
 

Tip: If you use epoxy and micro-balloons, you should make the wing saddles before covering the airframe.

If you use silicone, you should do so after covering the airframe because covering will not stick to silicone.

Time required to do this is twenty to thirty minutes.  You will need slow-drying epoxy and micro-balloons or silicone sealant.

I usually do not use silicone because it is difficult to trim neatly, it can not be sanded and it does not take paint or iron-on coverings.

If you choose the micro-balloon method, then I suggest you do not begin unless you know you will be around when the epoxy begins to set.  That is the best time to trim off the excess.  The epoxy is much harder to trim after it is fully cured.

Unsighly gaps in wing saddle are easy to fix. Note the unsightly gaps around the wing saddle.  I think these gaps seriously detract from the model's appearance.  If it is your model I am looking at I will keep my opinion to myself, but that is what I am thinking. I'm also wondering why you have four different kinds of bolts holding your engine on.
Another big gap between wing leading edge and fuselage.

There is an unacceptably large gap at the leading edge as well.  The muffler exhausts underneath the fuselage directly down the centerline.  This is a great place for all that exhaust oil to enter the airframe.

I will fill the gap with wood first and then use epoxy and micro-balloons to finish it off.

Fixing this gap will wait until after the wing saddles are done.  Doing too much at once is a sure-fire recipe for mistakes.

Wrap the wing with a non-stick material.

You can wrap the wing using anything thin that epoxy (or silicone) will not stick to.  I use masking tape on the opposite side of the wing to hold it in place.

For straight wings I usually use waxed paper because it is a little easier to work with.  I use plastic food wrap for tapered wings because it can be stretched tightly in all directions.

You can also use an iron on covering, such as SolarFilm.  Just iron it down normally and peel it back off when you're finished.

Regardless of what you use as a release film, wrinkles will be faithfully reproduced in the finished wing saddle so be sure to smooth it out.

Mix micro-balloons with epoxy so that it is just thick enough that it won't run.

Normally when I mix micro-balloons with epoxy I mix in as many balloons as the epoxy can absorb.  For wing saddles, I mix in only enough that the mix won't run which is far less.

The mixture will not compress properly if it is too thick which can distort the wing - thus creating a distorted wing saddle.

The amount shown here (less than 1/2 ounce) made the entire mess in the next several images.  The wing saddle area is 3/8" thick and about 8-1/2" long.

Work fast you do not want the mix to get too thick before you put the wing in place.  I have noticed that micro-balloon/epoxy mixes tend to cure faster than straight epoxy.  I think it is because the friction created when mixing in the micro-balloons tends to heat up the epoxy and accelerate the curing process.

By the way, this container of micro-balloons was purchased in Germany in 2001.  It is much larger than anything I have seen in the states yet cost less than the small bottles I have purchased in the U.S.  Why is it that American micro-balloons cost so much???

I am guessing it is because American sand is better than European sand and therefore makes higher quality glass which equates to more expensive micro-balloons (which are made from glass in case you did not know).  My other theory is that we're simply being gouged by the micro-balloon tycoons.

Put the micro-balloon/epoxy mixture on the wing saddle. Get over your indignation of American micro-balloon prices quickly because the mixture is beginning to set up.  Trowel it on the saddle area nice and thick.  Use plenty so that there will not be any voids.
Mount the wing to the fuselage.

Carefully put the wing in place but do not push it down.  Bolt it down slowly and evenly.  The mixture should squeeze out and look something like this.  If you did not use waxed paper or cling-wrap you have now glued the wing permanently to the fuselage.

This technique works with rubber-banded on wings as well.  Use just two or four rubber bands that are not too tight and put them on evenly.  It is helpful to have someone hold the wing to the fuselage so it is not shifting around while you are putting the bands on.

Tip: If you use micro-balloons and epoxy then you should not attempt to remove excess mixture while it is still wet.  The mixture will be pulled from the wing saddle which creates voids instead of filling them.  Just let the mix set up for a while.

If you use Silicone then you should remove the excess while it is still wet.  It is a real pain to remove silicone after it is dried - especially if it is stuck to the covering on the fuselage side.  It's not a bad idea to mask the side and trim the tape to match the wing saddle.

Excess micro-balloon/epoxy mixture squeezes out of wing saddle when the wing is tightened down. The excess is easy to remove if you remove it at the right time.
Check the wing incidence before the mixture sets. Check the wing incidence again while the mixture is wet and can be removed if necessary.  Make sure the wing is level (looking from the front or the rear of the model) to ensure you have tightened the bolts fully and evenly.  In this image the wing is about 1/4 positive which is exactly what I wanted (somewhere between 0 and 1 - closer to 0 being better).

If for any reason you have to remove the wing at this point then remove as much of the micro-balloon mix as possible and start over.  There will not be time to reuse what you've already mixed.

Check the stabilizer incidence too. Just to make sure everything was as it should be I checked the stab again to ensure that it is level.  I have checked my incidence meter against this level to make sure they jive.
After the mixture sets, but before it is fully hardened, remove wing from fuselage.

After the mixture is set, but before it is fully cured remove the wing with the waxed paper still attached or remove the wing from the waxed paper and then peel the paper from the fuselage.  I chose the latter.

Be careful when removing the wing not to distort the mixture.

Remember - the epoxy is not fully cured at this point.  It is best to leave the wing off for now.  If you can not help yourself, at least dust the wing in the saddle area with a little baby powder or dry micro-balloons to prevent the wing from becoming permanently adhered to the fuselage.

Note that the epoxy/micro-balloon mixture migrated around the wing bolts.  I have never had a problem popping the bolts loose and the mixture can be easily picked from the bolts after it has fully cured.

 Trim off the excess micro-balloon/epoxy mixture. Place a razor blade flush against the fuselage side and trim off the excess mixture.  The razor blade refused to stay where I wanted it while I was taking the photo, but the top of the blade should be angled so the excess is trimmed toward the saddle (that would be down in the above photo).

If you hold the blade so it cuts away from the saddle, it is possible for the micro-balloons to stick the blade and be pulled from the wing saddle.

I have had no problem getting iron-on coverings to stick to the epoxy/micro-balloon mix.  Heat from the iron makes the epoxy tacky.  Once it is cooled the covering is held firmly in place.

 
 

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How to Build a Fuselage for a Model Aircraft
Make Pushrod Exit Fairings for Flying Model Aircraft

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Copyright 2002 Paul K. Johnson