Airfield Models Mail and Frequently Asked Questions

Construction Techniques

May 05, 2015



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Airfield Models (http://www.airfieldmodels.com/)Mail and Frequently Asked Questions about Construction Techniques

These are questions I have received from visitors regarding construction and building techniques and related issues.

Other Mail and FAQ Pages

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On shear webs, I think you may want to look at ANC 18.  Here is the part that discusses shear webs and spars:  Wood Beams in Aircraft.

In my current designs I use 1/64 ply for shear webs  The samples I have weighed are about the same as contest 1/16 balsa so there is little or no weight penalty.  I use box spars with the shear web on front and back faces and vertical balsa siffeners at rib locations.

After looking at this document and discussing it with other model builders, some with engineering backgrounds, I think the best grain orientation for shear webbing is at 45 degrees.  Since the 1/64 ply has three laminations the front and rear webs probably should have their 2 ply directions at 90 degrees to each other.

I understand the the very strongest webs would be at 45 degrees to the spars in both directions ó in other words plywood but at 45 degrees instead of vertical.  In my case, I would laminate balsa to make them lighter just because Iím OCD like that.

But, Iím not that motivated either so unless itís very critical (thin wing when absolute highest strength is necessary) Iím sticking with the vertical webs.  I added the link you provided to the shear webs page.  Thanks!

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Which do you think would be better, the Adjusto wing jig that you mention on your site or the magnetic building system?

For straight wings (no taper) I prefer to build on the magnetic board.  For tapered wings I prefer the wing jig.

Thatís how it goes overall.  No tool does everything so having a variety gives you a better chance to have the ďjust rightĒ tool in any given circumstance.

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Please can you help me out with these solder clevises?  I am at a loss.  I have tried but just end up with burnt fingers and a black and blobby mess.  And the funniest thing about it all is the clevis fell off the pushrod when I was done, landed on my foot and burnt a hole in my new Nike's.  I am thinking about throwing the solder clevises away and using Z bends instead.

I feel your pain ó or at least I used to.  Until somebody showed me how to solder I had the same problems ó cooked joints, solder that didnít flow, burns on various areas of my body, and people around me learned all kinds of colorful new words theyíd never heard before.

The first thing you need to do is make sure what you want to solder is clean.  Put some alcohol in a small container and soak the clevis or use a pipe cleaner with alcohol on it to clean the inside of the clevis.

Clean the wire with steel wool or very fine sandpaper.

When everything is clean and dry, apply non-corroding paste flux (available from any hardware store) to the wire where the clevis will go.  You can also apply some inside the clevis with a toothpick.

Slide the clevis on the rod where you want it.  There is a seam in the clevis shank.  Arrange it so that the seam is up. Thatís where youíre going to apply the solder.

Make sure the tip of your soldering iron is clean and tinned.  If itís black and cruddy youíll never get it to work.  While the iron is cool, clean the tip with steel wool. After itís heated you can dip it in the flux and then melt a little solder on it.  Most people wipe the iron on a wet sponge, but that only works if the tip was tinned properly to begin with.  Being very careful shake excess solder off the tip.  The tip should be bright and shiny.

Apply the tip to the underside of the clevis shank.  Apply solder to the seam on the opposite side (top). When the clevis is heated enough youíll see solder melt and flow into the clevis.  Apply a little more solder to the end of the clevis to make a small fillet between it and the rod.

Remove the soldering iron and let the solder cool.  Do not move the clevis or rod while the solder is molten or youíll get whatís called a ďcoldĒ solder joint.  A cold joint is dull looking and extremely weak.

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I was told that I could wet a wing skin with a combination of ammonia and water and it would make the skin flexible enough to form it around leading edges and not crack or break.  Have you ever heard of or done anything like this?  If so could direct me on how to do this and all other details.

I have done that, but itís a last resort.  Ammonia does very bad things to wood.  For one thing it turns it to pulp while itís wet and can create undulations where the tape held it while it dried.  These waves in the wood will be present after the wood dries and nothing will make them go away.  It doesn't always happen, but you have to be careful how much pressure you use and how well it's distributed.

When water and especially water and ammonia is used to bend wood it makes the wood somehow different.  I canít say exactly how, but it does not behave the same.  The grain also opens up more than if you hadnít wet it first.

If you must wet the wood, try warm water first.  Donít use ammonia unless you absolutely have to.  Normally leading edge sheeting will bend into place without any wetting necessary.  If the wood is so hard that it needs to be wetted then the wood should probably be replaced with more appropriate sheet.

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I am looking for an easy way to make a full-chord wing fairing at the fuselage joint on an airplane having a removable wing.  This is for scale effect.  A removable belly pan is also involved so there needs to be a break at the leading edge.

Do you have a system for adding this feature on my small 1/12th scale P-47? I can envision starting with the waxed paper over the wings and using epoxy/micro balloons and shaping it while it's in a putty stage of cure.

There are several ways to go about it. The lightest way is the most difficult.  Cut triangle pieces of sheet and glue them to the fuselage.  Sheet over the triangles with thin balsa.  Usually the triangles are concave on the top.

The heaviest way is to use a putty ó most commonly epoxy and micro-balloons.  It is done just as you have said but I would use plastic food wrap or the clear backing from iron-on covering rather than waxed paper.

One way Iíve done this is to put plastic on both the wing and the fuselage.  That allows me to remove the fillet after itís cured and shape it separately.  I add chopped fiberglass to the epoxy/micro-balloon mix to strengthen it somewhat.

Another way is to cut a thicker sheet of balsa to match the airfoil and glue it to the fuselage.  Carve to shape.

Choose your poison.

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I have read your article How To Make Tapered Wing Ribs.  I have read other articles dealing with stacking balsa between templates and Drilling and bolting them together.  What no one has said was how the stack was drilled successfully without destroying the balsa.  Can you explain to me how you drill the holes in the ribs you make?

I use several methods for drilling balsa depending on what type of balsa Iím dealing with.

For very thin and soft balsa I used sharpened brass tubing.

For harder balsa of any thickness I use brad point drill bits.

For thicker soft balsa I cut teeth in a brass tube such that the teeth angle away from the direction that the tube rotates.

The ribs in the article are soft contest balsa except for the tip and root ribs.  I stacked up about 5 blanks and used my drill press with a brad point bit. It did chew up the balsa somewhat.  It didnít matter in that I only clamp the ribs together one time so as long as the bolt could pass through and be tightened to prevent the rib blanks from shifting, it didnít matter if the holes werenít perfect.

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What procedure do you use in sanding the wing parts in preparation for the covering without breaking any of the ribs and spars?

I use a long T-Bar sander and sand front to back.  I move frequently between groups of ribs (the ones covered by the T-bar) so that I donít sand too long in one spot.  I also overlap by moving over a set number of ribs each time ó three, for example.

The wing needs to be braced so you can hold the sander with both hands. Take your time and work slowly. If you start scrubbing youíll screw something up that is hard to fix.

It is possible to sand a spar down to match the ribs without hitting the ribs, but again, it means bracing the wing, using both hands and going slowly. Sight from the end so that you can see if the sander is rocking and hitting ribs. When the spar is way oversize, put masking tape on the ribs to protect them. When you start hitting the tape, remove the tape and from then on go slow, be careful.

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I want to scrap-build a 1975 Sun Ray from RCM plans.  The bottom wing is a gull wing.  The instructions tell you to construct the gull wing spar from a single continuous piece of birch plywood (38" span).

This seems heavy (and silly) to me.  I was thinking of constructing a balsa spar and laminating it with fiberglass.  Then, I was toying with the idea of fully sheeting the wing and fiberglassing that too.  I thought it would be best to glass it in sections, feathering and overlapping layers of cloth where the gull wing bends.  Will that be strong enough?

Before going into the answer here's a warning.  RC Modeler Plans (RCM) are still being sold but they are not delivering.  Why the State Attorney General hasn't stepped in I don't know.  But DO NOT attempt to purchase any RCM plans.  You'll just lose your money. Hopefully when you figure it out your credit card company will get your money back.  But whoever is behind it is scamming people.  I hope it's not anyone who was actually associated with the magazine because it was a great magazine for the many, many years it was published.

Having never a built a model such as this, I canít say from experience how you could lighten the structure while maintaining necessary integrity.  I would follow the plan for the most part, but I would use light woods as usual and remove excess ďbeefingĒ which is almost certainly present given the year the model design was published.

That being said, I donít think youíll save any weight by building a fiberglass spar.  The effort would be considerably more than cutting out a plywood spar and in the end it may end up heavier and might also be weaker.

I would probably extend the plywood spar slightly into the out panel, but not the full span.  The landing gear is located at the outer gull so the spar must be strong enough to prevent the wing from breaking during normal landings.

If money is no object and you have a way to safely fabricate it, a carbon fiber plate spar should save weight and add considerable strength.  I have not cut CF other than to trim little bits here and there.  Iíve heard that scroll saw blades pretty much bounce off the stuff.  You could probably cut it with a scroll saw but it may take hours.  A band saw may work better.

If you choose to go with the plywood then you may be able to make some cut-outs in it as long as you keep them away from the gull angles.

As far as glassing goes, I agree with your approach.  Trying to glass the wing in one piece will probably be frustrating and may leave areas where the glass didnít lay down properly.  But lightweight fiberglass is not going to strengthen the gulls enough to do away with a spar strong enough to handle the landing gear loads.

If you can conceive of building more than one of this model then you may want to try some things and see how they work out.  If your modifications hold up then you may want to go even further on the next one.

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I was looking through your website and I thought I found it but I can't seem to find it again.  If I was mistaken I would really like your method of building tapered wings or please help me to find the article.

The method I use to cut the ribs is about halfway down on this page:

Model-Building Tips - General Construction

Itís an old concept (not mine) that I have used with excellent success.

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I have (once again) gotten myself interested in building model airplanes.  I used to stick to White wings paper airplanes (and design my own) because they are cheaper to design, build, and buy.  However, I've finally decided to move on up in the world and build balsa models.

I've got an electric engine that used to belong to a plane my dad bought for me one Christmas not too long ago, but I dismantled the airplane (to see how it was built) not long after I got it, and most of the pieces were lost in the shuffle.

I've decided to design one and build it (I'm actually going to college for Aerospace Engineering, so this should be a lot of fun).

How do I mount an unsheeted wing with a small dihedral?  I'm thinking about making the dihedral 5 degrees and the wing will be mounted to the top of the fuselage.  What kinds of coverings are available that don't iron on yet are light and strong?  Thanks for your time (and for your website).

A well built model of any type is a good model and one to be proud of.  I have some White Wings and have enjoyed them.

Does the wing have to be unsheeted?  If you can at least sheet the center youíll have an easier time of it.

If you donít use any sheeting at all youíll need a block glued between the ribs and against the trailing edge of the wing.  Drill through the block for a pair of bolts that will thread into hardwood blocks or a plywood plate in the fuselage.

Youíll also have to glue a couple dowels through the leading edge back into a piece of wood glued between the spars.  Drill holes in the former in the fuselage at the front of the wing to match.

Iím assuming this wonít be a very large plane and youíll be able to get away with that.  If the plane is subjected to much stress then there is the possibility the system wonít hold and you can imagine the result of a wing parting company with the fuselage in flight.

As far as coverings go, the ones that donít iron on are applied with dope.  I suggest you stick to plastic and concentrate on learning about design and construction.  Later when you build better models start looking at better finishes.  I only paint my best models because itís expensive and tons of work.

Painted finishes include doped on silk, silkspan, tissue, Koverall, Stits fabric and some others.  Another finish is fiberglass that is then painted with whatever you like.  But fiberglass can be applied only to solid wood.  You canít apply it over open construction.

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I have an ABS plastic cowling for a 1/4 scale J3 cub and I would like to modify the cowling fit by cutting it in half top and bottom.

My question involves cutting and rigging the cowling so it will not be destroyed by vibration.  This arrangement will make it easier to get the cowling on and will allow the opening form the side mounted engine to be a minimum size.

I would fiberglass the inside of the cowling before cutting it to help prevent vibration from cracking it.  I would make a 1/32Ē plywood lip to glue to one half of the cowling and use #0 or #1 wood screws to attach the second half to the first half.

When you mount the cowl to the fuselage you must support it as perfectly as possible with no distortion.  If the fuselage wedges into the cowl then sand some of the fuselage away so that the cowl fits as smoothly as possible without the fuselage trying to expand the cowl.  If the cowl is bigger than the fuselage then be sure the cowl mounting blocks are right against the inside of the cowl so that when you tighten the mounting screws the cowl doesnít bend in.

Use plenty of screws and donít over tighten them.  Just snug them.  Itís best if you use hardwood blocks and wood screws.  Harden the threads in the blocks with thin CA.

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I have a construction question.  I'm trying to figure out a way to make an access panel.  It's to get to the motor compartment.  The plan calls for dowels and rubber bands but I think that looks ugly.  I would use magnets but I don't want the field to interfere with the motor.  Any ideas?

I've always wanted to build one of those.  It's definitely an attention getter!  I agree that the rubber band method isn't too attractive.

I would simply glue a couple small triangles of 1/16" plywood into the corner in front of the former at the leading edge of the wing and above the dowel.

Use a couple #0 or #1 wood screws to hold the hatch down.  You can either inset a small piece of plywood into the balsa hatch or just harden the holes in the balsa with thin CA and not crank the screws down too tight.

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I am inquiring about what is the correct way to install CA hinges.  The plane I am building has large control surfaces and I am concerned about failure.

I donít use CA hinges because I think they are the worst excuse ever for hinges.  I suggest you use pinned flat hinges, such as Dubro or Robart Hinge Points.  I canít tell you the proper way to install CA hinges having never used them.

I have written a tutorial regarding Robart Hinge Points if youíre interested.

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I am having difficulty building open structure wings that do not warp (more like bows) when the leading edge and trailing edge sheeting is applied.

I use magnets to build with and heavy weights to keep things where they should be.  Before the wing sheeting is applied, the wing is nice and straight.

Once the Sheeting has been applied the wing becomes extremely rigid and the warp is permanent.  The leading edge on the wing I am building does not have a false leading edge but is a 1/2" square stick inserted in the ribs so it looks like a diamond shape.  If you have any suggestions I sure would appreciate them.

Itís difficult for me to tell you exactly what the problem is when I canít see the actual pieces used in the wings you are building.

More often than not balsa sticks are warped.  I cut my own sticks and they almost always bow or warp when they are cut from a sheet that had a straight edge before I stripped the sticks.

Narrow sheets can do the same thing.  For example, a 1Ē wide balsa trailing edge sheet can curve along itís length.  Even with these warped parts I donít have problems building a straight wing.  It has more to do with technique than anything else.

Here are some things you can do:

  • Be sure to acclimatize your wood. Before you start to build, the kit should be in your shop for at least a week. Itís best to leave the box open during this time.

  • Use a straight edge to align spars, trailing edge sheet, etc. on the building board.  Donít trust the lines drawn on the plans.

  • Use slow-drying glue.  It allows the parts to be put into the correct position (straight) before the glue dries.

  • Install warped parts so they oppose each other.

    For example, if there is an upper and lower main spar and they are both bowed at one end, set up the spars so that they bow "in" with the bow at the same end of the wing.  In other words, the concave side of the spar part should be outward.

    If the part is very hard and requires a bit of effort to make it straighten out then it should be replaced.  Use a bowed piece only when it takes very little effort to make it straight.

  • Install upper and lower sheeting the same way.

  • Single parts that donít have a match, such as a leading edge, should be installed so that the bow is forward or backward.  I like to install it so the concave side is forward.

    The spars and trailing edge will prevent the leading edge from curving the wing front to back.  If you install the leading edge with the bow up or down then it can cause the wing to bow in the same direction which I think is what happened to your wing.

  • You say you use lots of weight and magnets to hold things in place.

    Do not place magnets or weight over unsupported sheeting.  For example, if you put magnets on the upper trailing edge sheeting, place them in a location where there is a rib under the sheeting.  Otherwise the magnets will bow the sheeting down while it dries.  When the magnet is removed, the stress will be relieved and can cause problems.

  • Be patient!  Allow glue to dry thoroughly before removing the weight or magnets. It is always said to wait 24 hours. That is absolutely correct. 24 hours isnít overnight Ė itís 24 hours.  Itís not just the glue drying, but the wood taking a ďset.Ē  The longer you can leave the wing on the board, the better.  If it's not in the way and you have other things to work on, just leave it.

  • A thick part that is warped should be replaced.  For example, tapered trailing edge stock probably wonít stay straight even if it is bent so that it glues on straight.  Better is to use a piece of straight stock to begin with or carve and sand your own tapered stock.  I usually glue a sheet in place and sand it to shape after it is dry.

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How do you transfer your pattern from the plan to your wood, especially if you have many identical pieces to cut?

There are a lot of ways to transfer the pattern:

  • Put the drawing over the wood and use a pin to punch holes through the outline to create an outline on the wood.

  • Same as above, but then rub colored chalk through the holes onto the wood.

  • Turn the plan upside down over the wood and use a hot iron to transfer the drawing onto the wood.

  • Trace the pattern on to clear plastic such as overhead projector transparency material.  Cut out the patterns and then trace around them on to the wood.

  • Make a copy of the plan and then use spray glue to put a light coat on the pattern only.  Allow it to dry to the point where it is not really sticky and then rub the pattern down on the wood.  The pattern can be peeled off after the part is cut out.  This is the method I always use.

If I have to cut a lot of identical pieces and itís a short stack then I stack up the wood and use my scroll saw.

If itís a tall stack, such as ribs, I make a hard pattern and cut each piece individually using a hobby knife around the pattern.

If the part is a rectangle, such as shear webs in a constant chord wing then I use my table saw.  If I didnít have a table saw, then I would cut around a pattern to make them.

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I have decided that my next project will be a 40-sized sport-scale PT-17 Stearman.  Problem is, I have not been able to find any acceptable plans for this.  Nick Ziroli Jr. has some lovely scale plans for giant size, but I don't want to build something that big (yet).

On the other hand, I found a "stand-off scale" plan for the PT-17 drawn by Nick Ziroli Sr. back in 1973 from Carstens.  There are, however, a few things I need to tweak and I would appreciate your thoughts.

Ziroli's fuselage deviates further from true scale than I am prepared to accept.  He put the engine out front and exposed, which is fine in a trainer or a fun-flyer but I want a plane that looks like a Stearman while in the air.  So, I need to redo the fuselage to match the true outline and put the engine/muffler inside.  This will entail shortening the moment forward of the CG by between 1/2 and 3/4 inch (on a total length of 40"), but not really shortening the length aft.  Obviously, this then requires that the aft section be built much lighter than Ziroli's original to avoid having to load up on lead weights in the nose.

Question 1)  Ziroli called for a solid slab stab and fin/rudder, using 1/4" balsa.  Seems to me that this is too thick and is likely to warp as well.  In your Hammerhead design you used 1/8" sheet with 1/8" ribs sanded to shape, for a total thickness of 3/8 or so.

Since your plane was bigger and more powerful than my planned 51" span and 0.46 engined Stearman, I was wondering if you thought that 1/16" or 3/32" sheet with 3/32" ribs (and with holes drilled in the sheet) would be strong enough - or should I avoid a sheet core entirely and build a completely open empennage framework?

Question 2)  Ziroli's fuselage design calls for a solid 1/8" balsa box with doublers at the nose and formers throughout.  I am wondering if I could go for 1/8" lite ply from the wing trailing edge forward, cut out in the middle for lightening and doubled at the wing root with either ply or balsa.  The aft section would be open frame with Warren trussing (similar to your Hammerhead) - I was thinking 1/4" x 1/4" balsa for this.

The outer shape would retain Ziroli's 3/8" x 1/8" stringers sanded to match the cross section.  Would this be lighter than the original all-sheet box?  Would this be strong enough - or alternatively is 1/4"square framing excessive on this size model?

The wings in the original are fine as they are, although I will probably want to cut holes in the ribs to lighten them a bit.  Any thoughts you have on these issues will be very helpful. Cheers!

Another place to check for plans is Cleveland Plans.

Back in the day almost all models were over-built.  The philosophy was to build models to withstand a crash.  It took a long time for us to realize that isn't the way to do it.

A better philosophy is to build to fly and expect the plane to be totaled in a crash.  At first that may seem foolish, but there are a lot of advantages to building this way.

First, our radios are much more reliable than they used to be.  Radio failure is not the cause of most crashes.

Second, a plane that flies well is less likely to crash.  Why build a plane that always flies like a dog just so that it might survive an unplanned impact with the ground?  Better to have one that flies well for its lifetime however long that may be.

Lastly, a thing a lot of guys don't think about is that a lighter aircraft not only flies better, but the lower amount of mass amounts to less inertia in the event the plane does crash.  What that means is that it has a better chance of surviving impacts.

Great Gonzo is an excellent example of this.  I can't count the number of times I attempted zero ground speed landings with her only to have her drop out of the sky from 15 feet straight on her nose.  Never once was there any damage incurred other than a few broken props.  It was so light that it just bounced off the ground.

FYI, the tail on Hammerhead is about 3/4" thick.  But I would not build the tail for your plane as I did for Hammerhead.  I think your second idea is better.  Use ribs only, but skip the balsa core.

What I would do is cut rectangular rib blanks and use one basswood spar in the middle of the ribs so that it won't show under the covering.  A single, full-span 1/8" x 1/4" spar should be fine.

Use a firm balsa leading edge.  Once the whole thing is glued together, sand the surface to an airfoil shape.  It's not as hard as it seems at first.  Just take care to keep it symmetrical from one side to the other.

I would use 1/16" ribs for the tail surface.  They will look closer to scale and save you a bit of weight.  Take a look at the photo page for Rustik to see how the fin/rudder assembly was built.  That's more or less what you want to do, but add the spar and skip the sheeting.  As long as you don't do high-speed dives, it will be fine.

Forget about lite ply in the fuselage and everywhere else for that matter.  Lite ply doesn't carry its own weight and is generally a poor choice for model aircraft construction.

Use contest balsa for everything but spars and leading edges.  You can probably make the fuselage sides from 3/32" balsa with a 1/64" doubler if there is a structure built around the basic box.  Otherwise I would stick to 1/8" sides and still use a 1/64" doubler.

1/4" square longerons are the way to go.  3/16" square probably won't be stiff enough and the weight savings will be negligible.  I built Great Gonzo using 3/16" longerons but I wouldn't trust them to build a fuselage that is stiff enough for a .40 size model.

What you can do is make the stringers from thinner stock ó maybe 1/16" x 3/8".  They'll be a little harder to work with because they'll try to flip on their side when you bend them to match the curve of the fuselage.

Start by gluing them at the forward most position and use thin CA to glue them in place working your way back.  Draw lines on the fuselage sides to keep the stringers straight.  Wavy stringers look bad under covering.

I also suggest using pull-pull controls to the tail surfaces.  That will save you at least a few ounces of weight behind the CG.

Another idea is to finish the tail surfaces with tissue and dope.  It will work well, although you'll have to handle the model carefully so that you don't punch holes through the tissue covering.  Tissue and dope will be lighter than Monokote or Coverite.

One way to get some weight up front is to build one of the William's Brothers scale engine kits.  They're fairly heavy which is good and bad.  Good because it helps with the balance, but bad because all weight is bad.

A way to save a lot of weight on formers and increase the strength is to make balsa plywood.

You can cut the formers to have about a 1/4" outline and they'll still be strong enough and stiff enough to work well.  Use your judgment on how much to cut out of a former.  If in doubt, make the cut-out small at first.  You can always cut out more if you think it will be ok.

If you choose to paint the model, spray light coats and sand between every coat.  That will keep the paint thin and light.  Thin coats of paint are much less likely to peel or chip and weighs less than standard weight iron-on films (standard weight being Monokote, Oracover, etc.).

If the model is truly a .40 size, then I recommend using the O.S. .52 4-Stroke.  It's an awesome engine and a perfect match for a Golden Era biplane.

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The aileron in the kit that I am building is bowed.  There does not seem to be any twist to it.  My question is how to get the bow out.

The only method I know of to *attempt* to remove a bow is to bend the wood past what would be straight and hope for the best.

The reason you go past straight is that the wood will spring back slightly.  Before you bend the wood you can steam it, moisten it with water using a paper towel (don't soak the wood) or use heat.

If the aileron stock wasn't that great to begin with, then a bow might just be the "last straw" and prompt me to replace the aileron instead of mess around with it.

Because I build with magnets, I can set up a few fixtures to hold the piece so that it is sitting on the leading edge and adjust the fixtures so that the piece is bent as indicated above.

Another way is to place the aileron on a dowel that is padded with a rag or paper towels and then place weights on the ends.

No matter what you do, you're guessing as to how much to bend it.  Normally about 100% is a good place to start.  In other words, if the ends are 1/4" higher than the center, then bend the ends 1/4" past center in the other direction.

Once you get the piece jigged how you like it, let it stay there about 24 hours so that it will take a set.  You might need to repeat this more than once to get the bow completely removed.  Whether it's worth the trouble rather than replacing the part is your decision.

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I really liked reading your site on wing building.  I'm going to re-build one I crashed the other WINDY day.  Building is 1/2 the fun to me.  What type of wood is used in a trainer wing spar?  The broken one seems to have a reddish color to it.  Also where's a good place to order balsa wood from?

As far as your wing spar goes, there is no set material to make them from. Normally they are made from balsa, but sometimes they are spruce or basswood.  It all depends on what the designer was thinking when he was at his drafting table.

Repairing spars is a tricky proposition.  If you haven't done it already, then I strongly suggest that you seek out a better builder in your area to give you first hand guidance.  Spars are critical structural members and if they aren't repaired properly you can expect catastrophic failure of the wing.

What you'll need to do is put the broken pieces back in place as well as possible.  Then splice in pieces to fill the gaps so the spar is one piece again with no missing areas.

That will not be strong enough on its own.  You should then take some 1/32" or 1/16" plywood and splice a piece on the front and the back that extends about 5" on both sides of the break (10" in all).

Toward the end you should taper the plywood to a point. Otherwise what you will be creating is called a "stress point" where the wing will naturally want to break again.  That point will be where the plywood ends.  The taper spreads the stress out over a larger area so the wing can bend instead of break.

It is easier to draw than to explain.  Again, seek a good builder to help you out.

Sources of balsa are listed at the end of the Balsa Wood page.

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I will be starting to build with magnets mostly due to your site and all the positives that your site provides on building this way.  The one question I have is when building tail feathers.  How do you hold parts down so they don't bow up?

The technique I use depends on how the surfaces are built.

For built-up stick tail surfaces I put a magnet (without plates) on each side of the sticks.  The magnets are set up such that they are attracted to each other (with the stick in between) which puts pressure on the sticks.  Then I just push the sticks down on the board and they tend to stay there.  Weight can be added if necessary, but if the part is bowed to begin with then I normally don't use it.

For solid surfaces that have edge-joined parts or cross-grain tips, I put a straight edge with a magnetic strip attached to the bottom on the board.  Then I push a straight edge of the largest piece of the surface against the magnetic straightedge.  That locks in one side.

I use magnets to push the edge-joined parts against the larger part.  I normally put more magnets or other weight on top of the structure to hold it down while it dries.  I normally use Ambroid glue to edge-join because it dries fast and sands well.

Tail surfaces tend to bow after they are built.  Long, flat surfaces just donít like to stay flat.  For that reason, I almost always build them thicker than what is called for.  For example, if the tail is to be 3/16Ē thick, then I build it from 1/4Ē wood.  Then I can sand it flat after it is complete.  A bow isnít a big deal, but it bothers me enough to do the extra work to make sure my models donít end up with them.

Warps are a different story and are more than an annoyance.  My structures donít warp, but they do bow sometimes and I take care of them as above.  Warps are normally due to using warped wood to begin with or from putting too much pressure on the structure when gluing it together.  Ensuring that parts mate well before gluing them allows a modest amount of clamping pressure to be used without building in unwanted stresses.

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While reading thru your site I found mention of shaping the leading edge and was wondering if you could elaborate more on this.  I am putting together my first kit and need to shape the leading edge.  I have planed down the edge and now need to provide the round shape.  The issue I have is this wing tapers down from one end to the other so the leading edge will differ in size.  How do you sand down the edge to the rounded leading edge?

What youíre asking me to describe is really a technique that has become second nature to me through sanding a heck of a lot of leading edges.  Iíll try to explain it, but what you need to know is that for it to become automatic for you, youíre going to have to build a lot of planes.

That doesnít mean you canít get every leading edge you ever shape right, but youíll have to stop more often, check that youíre on track and make a lot of corrections.

I work on the top and bottom of the leading edge separately until itís ready for finish-sanding.

First, I plane the top so that it is tangent to the ribs for the full span, but leave about 1/32Ē or so left to sand.  When I say, Ďtangentí what I mean is parallel to tangent.  Tangent would actually be flush with the front of the rib, but I donít plane down that far.  It is too easy to remove too much wood.

If the leading edge tapers as you describe, then I start towards the tip and with each stroke of the plane I move more towards the root and plane all the way to the tip.  This technique removes more wood from the tip than from the root.  I adjust as necessary.  If Iím removing too much wood at the tip, then I might stop short of the tip for a few strokes until it evens out.

Once the top is tangent, I use the plane to start rounding over from the back edge of the leading edge to the front.  Again, I work slowly and check my work frequently.  When Iíve gotten as close as I feel comfortable getting with the plane, I flip the wing over and repeat the same steps for the bottom of the leading edge.

Now itís time to switch to sandpaper.  I use a long T-Bar sander with 150 grit paper to rough in the shape.  Again, I work on the top and bottom separately.  I sand spanwise while rounding the block over the front of the leading edge.  I use this block until the shape is exactly what I want and the back of the leading edge is flush with the ribs.

Now I switch to finer sandpapers to finish sand.  By the way, I shape the everything before I even think about finish sanding.  If you finish sand one part and then shape another, you still have to make the two parts mate.  What is likely to happen is that there wonít be enough material in the part you finish sanded. Better to not do any more sanding than necessary until everything can be sanded at the same time.

In some cases, itís harder to sand a part after other parts are glued around it.  In this case, it is often best to sand the part while you can. Use your judgment and think ahead.

One thing youíll need to learn how to do is vary the amount of pressure you put on a sanding block to deliberately sand in or remove a taper.  That is a ďfeelĒ thing and thereís no real way to tell you. You just have to do it a few times and see what happens.  For example, you can shape a tapered leading edge from start to finish with nothing but a sanding block.  Put more pressure on the end that has the most taper until the angle is right. Then sand evenly from then on.

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I just read your tutorial on rib-making, and read that you cut out the "central" part of all your ribs for weight reasons.  Do you think I can do the same on a Giant Scale (77") Ziroli Stearman PT17?  Or does the cutout reduce the strength of the rib by too much?

Yes, I believe the ribs will be strong enough even with the interior portions removed.  You will have to use your judgment though, as I havenít built this aircraft.

I would not modify ribs that are load-carrying.  For example, ribs near the center section, interplane struts or cabane struts should probably be left intact except for possibly holes to pass servo leads.

Other than that, I wouldnít worry too much.  Ribs are more for providing shape than for strength.

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I read with interest your article on plywood landing gear laminated with f-glass cloth.  I need to build a set for a scratch built plane and wondered if you have any further suggestions.  I was going to use 1/32 ply but it may too difficult to bend.

Seems like your gear originally might have been too flexible.  How can we stiffen it up without getting into carbon fiber?  Any further suggestions will be appreciated and keep up the good work of passing on your projects.

Eli Whitney tried 61 times before his cotton picker worked!!

I used 1/64Ē ply simply because it was much easier to bend without fear of it cracking in the bends.  I ended up adding another lamination of 6 oz glass cloth and then a 6th lamination of ply to the original gear.  It is very stiff now and I think itís going to work well.  Iíve played around with it a bit Ė bending, twisting, etc. and it seems good.

The plane is almost finished, so Iíll be able to see what happens in a real-world environment.  Until then, all I can say is that I trust it or I wouldnít use it, but the proof will be seeing it hold up after a few hard landings.

As of now, Rustik has several flights and the gear has performed flawlessly.  I store the model standing on the spinner to prevent the gear from warping due to the weight of the model.

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Is it possible to build a Gee Bee Z into a biplane?  There is one on my flight simulator and it looks awesome.

I donít see any reason it could not be made into a biplane.  Gee Bees are known to be squirrelly aircraft and making it a bipe might make it even more so.  I would narrow the wing chord a little if I were to do this.  Most Gee Bee models have longer moments than what is scale to make them easier to fly.

This would be a fairly advanced project, however, so I suggest you get plenty of building in before you attempt it.  I think it could be a very unique and fun project and should be a very attractive model.  Designed properly it may be an excellent flyer, but again, building experience will be essential before you attempt it.  Otherwise it will be an exercise in misery and frustration Ė take it from someone who jumped in way over his head too early.

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I ordered a set of Palmer plans for the 11-foot C-130 and if you are not familiar with Palmer there is minimal information provided with the plans and no advice/information offered after the purchase of the plans.  I have a couple of questions about the construction of this aircraft and must search for answers.  For instance, the main strength of the  fuselage lies in a section about 4 foot long, by 9 x 11 inch rectangle located directly under the wings and cg.  This is made of 1/4" x 1/2" medium hard balsa sticks running lengthwise and crosswise making this rectangular box.  Cross bracing is provided by 1/4" sq. sticks at various places.

Then the roundness of the fuselage is created with balsa formers at various fuselage stations, longerons, and planking.  Well, my question would be: Do you think this rectangular box would provide better strength to the fuselage if those 1/4" x 1/2" sticks were of Basswood instead of Balsa or would the change be so minute compared to the increase weight of the basswood?

Palmer is only suggesting 49 MVVS engines on this airframe so he suggests building it as light as possible. Then he turns around and suggests the use of 1/8 inch foam board instead of light (contest grade) balsa.  I can only obtain 3/16 inch form board here, and yes it is cheaper then balsa, but question if it is better and the practicality of foam board vs. balsa.  The foam board I have weighed is heavier then light balsa!  Any thoughts on your part would be appreciated.

Youíre going to have to take my comments for what theyíre worth here.  Iíve never built anything like the project youíre working on, so Iíll go from general experience and you can adjust fire.

I would use medium density balsa for the longerons and not use plywood anywhere that it isnít absolutely necessary.  For example, if the design calls for lite-ply formers, I would replace them with balsa except in strength-critical areas as Iíve already mentioned.

Many designers draw nice outlines and an adequate structure, but also over-build their designs.  I donít have an engineering degree, but from years of experience, Iíve learned that a lot of wood can be removed from a model making it much lighter and still be airworthy.  On this plane, I suggest you skip the FoamCor and use balsa instead.  Itís stronger and lighter as you have said.  It is more expensive, but my way of thinking is that if I canít afford to build it right, then I canít afford to build it.

Think about where the forces are going to be in this plane Ė the engine mounts, wing attachment, wing center section and landing gear.  Most of that load is carried by the skin or the spars.  The lighter you build it, the less load on all areas so building light has a snowball effect in a good way.

The box construction is there for a couple reasons:  One, it makes it much easier to install equipment in the fuselage than if the inside were round.  Second, and more importantly, it allows you to build a straight box around which the rest of the fuselage is built.  A round, planked fuselage is amazingly strong and light.  If it werenít for building ease, the box would not be necessary.  If you were to build just the outer formers with planking and reinforcement where necessary (wing mount, empennage, etc) your fuselage would be adequately strong.

However, it is very difficult to build a round fuselage that is straight Ė hence the box.  This plane has 2.0 cubic inches of engine total, so keep that in mind.  Try not to think of it as a four engine airplane, but one having a single 2.0.  The fuselage as designed is probably over-built already.  1/4 x 1/2 balsa box is fairly substantial.  Now it is a big box, so it has to be built straight and the cross-braces should be as close to a perfect fit as possible to prevent it from flexing.  Once you get this monster planked, you will not be able to twist this fuselage no matter how hard you try.  If you glass it, which is what I would do, it will be that much stronger.

For the most part, there is very little force on ribs.  When designs start getting larger, designers make ribs thicker to make the plane easier to build.  Thicker ribs are stiffer and easier to work with on the board.  But in the air, they do very little so that ease of building has a price.

If you want to use 1/8Ē ribs, then cut a big chunk out of the middle.  You can remove at least half the weight of every rib.  I wouldnít mess around with ribs in the nacelle area too much though.  But it is only a .49.  Donít make the mistake of thinking that each nacelle has to carry that 2.0.  A 3/16Ē firewall is more than adequate for each engine.  If the firewall is small enough, you might even get away with 1/8Ē, but Iíd have to see the design before recommending that.

Another area where I often see planes being over built is the thickness of the skin and sheeting.  Often on larger planes the skins are too thick.  I believe they do this to give you lots of material to sand to get it flat.  However, if you do a good job sanding the ribs before sheeting and sand the skins before gluing them on, then you can get away with thinner skins to begin with.

I would think this plane would be strong enough with 1/16Ē skins on the wing that are subsequently glassed.  Iíve seen planes of this size with 1/8Ē skins which is ridiculously thick. At most they should be 3/32Ē but I believe you could use thinner skins with no compromise to the strength.  Whatever you use for skins, make sure they are the lightest wood you can find.

The bottom line is that (again, I havenít seen this design) I think you would do better to ďunbeefĒ this design than to beef it up.  No matter what you do, if this thing crashes, itís going to explode, so may as well make it light so it flies well.

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What technique did you use when making grooves in the wing spars for the shear webs?  Was there some tool or procedure required to make straight, uniformly deep grooves?

I used to use my router table, but now I use my table saw.

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When gluing the ribs and webs to the bottom spar, how do you go about keeping them perpendicular to the spar?  I did not see any clamp or brace on each rib to ensure it dries straight.

I cut the webs so that they hold the ribs in the proper position.  For most ribs that means perpendicular, but for the center ribs on a wing having dihedral, I cut the proper angle on the shear web.  Before the glue sets up, I add the upper spar to ensure the shear webs are standing perpendicular.

If I wait until they are dry before adding the upper spar, then if the webs do not align properly, I will have to cut them loose.  It is better just to complete the job and get it right the first time.

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You said that you keep the ribs straight with the use of the webs which are cut square.  Two questions arise:

1. What if the plans do not call for webs?

I generally add webs to most planes I build.  They are light, add tremendous strength and also help prevent the wing from twisting ó especially unsheeted wings.

2. Making the webs square helps align them perpendicular left to right, but what about making them perpendicular when looking at the plane from the side, i.e. leaning forwards or backwards?

I align ribs over the plan or over gridlines to ensure they are straight fore to aft.  As far as leaning forward or backward, any decent kit or plan will tell you how to build the wing with the ribs on the board.  For flat bottom airfoils, the wing is built flat on the board.

For symmetrical or semi-symmetrical wings one of two methods is used most commonly.  Either a stick is placed under the ribs at the trailing edge so that the ribs align properly with the main spar flat on the board or the ribs have tabs on the trailing edge that align them with the main spar flat on the board.  The tabs are removed after the wing is removed from the board.

As far as ensuring the webs are perpendicular, I notch both the top and bottom spar and make sure I can get both spars glued in place before the glue on the webs sets up.  The notches align the webs vertically.

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I built a 4*60 with CA glue mostly, some epoxy and minimal wood glue.  I have discovered that I may have some allergic reaction to CA.  I am a bit cautious now with CA and certainly would like to migrate to using more Titebond.  Here are a few questions which I hope you could throw some light on (hope you do not mind me using the 4*60 manual as a reference, and I assume that you've built this popular model before) :

1) Which is better, the regular Titebond or the waterproof Titebond ? Do they have the same drying time and strength ?

2) If you take a look at step 2d of your 4*60 manual, how would you apply Titebond?  With CA it is pretty easy, just dry assemble, and hit the joints with CA.  Now with Titebond, it seems like you would have to apply beads of glue along the edges of every rib, shear web, diagonal braces, etc before you assemble EACH piece. How do you clamp the assembly ?

3) For applying balsa sheeting onto the stabs (step 42b), do you use Titebond?  Does warpage become an issue here ?

First, I have not built the 4 * 60 kit that you are referring to.  However I am familiar with the aircraft.  It is a typical sport R/C model using typical construction methods as far as I can tell.  A lot of people have reactions to CA and have either switched to "odorless CA" or a different type adhesive.  I do not have a reaction to CA but I do not use it for general construction because it is messy, expensive and does not have a long enough working time.  I have also found that it does not speed up my construction.

I have used Titebond for many years and have never had any type of problem with it.  I recently tried Titebond II but found that it seemed to dry a little more rubbery and tended to clog sandpaper.  When that bottle was gone I switched back to regular Titebond.

This How-To shows how I build a typical wing using slow drying Carpenter's glues.

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In your latest section about creating the ribs, you show how you cut out a portion of the middle of the ribs.  I was wondering if that takes away from the strength of the ribs or if the real pressure is not on those areas.

This question was in reference to the article Create a Set of Constant Chord Ribs.  A section has been added to that page to address this question specifically.

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I am looking into scratch building a Embraer 135 RJ, because I have some friends who fly them and I just want to fly R/C airliners. I have not decided on what power plant I am going to use......(i.e. gas or electric DF).  I was hoping to use gas because of the size and scale like performance.  I am not sure what the materials should be... foam or fiberglass considering strength, since the wings do not have a huge wing area.  What about actually sculpting the airframe?  Is the process a hard thing to do?  What about getting a hot wire system?  Are they worth the money?

In a subsequent e-mail this builder indicated the fuselage diameter to be approximately 8".

Keep in mind that I have never done this, but this would be my approach for ease of building and accurate outlines as well as light weight.  I looked up this airliner on the web so I could see what it looks like.

If the plane was smaller then I would say cut the whole thing from foam and then fiberglass it.  But with a fuselage that large around I would build a box made of balsa.  Around the box I would strategically place formers.  Between the formers on the outside of the box I would glue blocks of foam and then carve it to shape.  You could build the whole fuselage from foam but you would have to glue blocks of foam together first and then cut fuselage sections.

The last option is to build the fuselage from balsa and plank it.  Thatís what I would do if it were a really large airplane, but probably not for the size youíre building.

I think you should definitely make the wing from a foam core and sheet it.  No matter how you build the model, I would definitely fiberglass it.

As far as buying the foam cutting equipment, youíd probably have to for a project like this unless you know someone who has it and is willing to loan it to you or help you out.  If you build more airliners then youíll become an expert with the tools and each model should be better than the last.

If this is your first model, then try to look at it as a learning experience and donít be discouraged if the end product isnít quite what you envisioned.

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