Chapter 11

Everything should be as simple as it is, but not simpler.
- Albert Einstein

It would take several books to begin to cover the scope of this subject. In fact, Don Ross has done that. I cited his book, Rubber Powered Model Airplanes in Chapter 9 at 9.1. The companion book, [Flying Models, Rubber, CO2, Electric and Micro Radio Control, Hummelstown, PA: Aviation Publishers, 1998] is also useful. Books such as these, and monthly magazines, provide modelers with the basic information needed to get started in the hobby.

I have been working with novice modelers for seven years. During that time I have noted certain problems that novices have to work through. We'll look at some examples in the following discussion.

11.1 The Plan
Check the drawing carefully for errors. Since you are likely to build over it, any problems will be transferred to your model. I have seen drawings with one wing half (unintentionally) shorter than the other. Compare precut parts or parts printed on sheet wood with the plan. Sometimes they don't agree. Wing ribs printed too short for the plan, for example.

When enlarging or reducing a plan and printing it out in 8.5 x 11 inch panels, distortion may be introduced. Check to see that straight lines are straight.

Some copy shops have digital printers. They can enlarge a plan significantly and not increase line width. They cost more but are worth it. You will be working with the plan for some time. Some fuselage formers (the pieces added to a square structure to give it a rounded shape) are notched for stringers (the strips of wood running foreward and aft over the formers). Despite the kit manufacturers best efforts, the stringers that run through the notches often take a wavy path. If you are working with printed sheet wood cut the notches after laying the stringers in place and marking their correct location.

11.2 The Fuselage
The fuselage serves as a foundation for the flying surfaces. If it is not square and true it will misalign other parts making flight trimming difficult. Longerons are structural members that usually extend from nose to tail in the fuselage. Choosing matched pieces of material for longerons is the key. If those on one side are stiffer than the other side the fuselage will bend to one side, assuming a banana shape. Hold a few inches of one end of the longeron pieces flat on a table with the rest extending horizontally over the edge. Press lightly on the extended portion. You will feel differences in stiffness if they exist. See Appendix E at E.3.1.4 for more stiffness testing methods.

On rubber-powered models longerons must withstand compression and twisting caused by a fully wound motor. If balsa is used, choose hard, straight grained, matched pieces. Some modelers substitute basswood on larger models.

If the cross section of the fuselage is rectangular, ensure that the sides are set at 90 degrees to the plan when gluing cross pieces in place. Test with a small drafting triangle, a try square or the cardboard corner of a tablet back.

Bill Warner uses thick balsa triangles pinned to the plan to fixture fuselage construction in his excellent book about constructing the FAC Moth. [Warner,Bill. Building the Flying Aces Moth. Blue Ridge Summit, PA: TAB Books, 1992. P14-21]

Plans depict parts that are mounted at an angle to one another. Measure your model to ensure that it conforms to the plan. One can make an inexpensive tool for this purpose from a plastic protractor. Drill a small hole in the center of the protractor and fasten a thread through the hole. Tie a washer or other weight about 6 or 8 inches from the hole. You may extend the base of the protractor by gluing a piece of wood along the straight edge. Proceed as follows to measure the angular difference between the horizontal stabilizer and the wing. (this is referred to as the decalage angle)

Set the fuselage firmly in place. Hold the protractor against the underside of the stabilizer. Record the angle defined by the weighted thread. Without moving the fuselage do the same thing with the wing and record the reading. Subtract one reading from the other and you have the decalage angle you built into the fuselage. Compare with the angle measured on the drawing. Often the wing or stab do not have a nice flat bottom to perform this measurement. In Chapter 13 we will look at airfoil shapes and how chord lines can substitute for flat bottoms when making these measurements.

Most designs call for some downthrust. This is the angle (front of shaft down) that the prop shaft makes with the fuselage datum line. What is the datum line? This is an elusive item. On a rubber model some consider it to extend from the prop hook to the rear rubber attach point. Others use the upper longeron, or make a line 90 degrees to the fuselage side uprights. Pick one you will be able to measure from on the model and draw it on the plan.

As before, set the fuselage firmly in place. Position the propeller vertically and position the base of the protractor against the front of the propeller and record the angle defined by the weighted thread. Position the protractor on your chosen datum line and record the angle with the prop. Compare the two measurements to ensure the model agrees with the plan. If it does not, or you need to adjust the angle after flight trimming, get out your sanding block and change the angle.

Turn the fuselage on its side and do the same thing to check any required side thrust angle. This is usually right thrust to overcome propeller torque while running.

11.3 The Wing
Wings may be designed with no twist, with twist in one wing, or with twist in both wings.

Free flight models are usually trimmed to fly in circles. This causes the inside wing to drop. By twisting that wing to a higher angle of attack (trailing edge down) it acts like a control surface (aileron) and maintains level flight. So much for why, now how to measure? Set model firmly in place and measure the angle of the wing close to the fuselage with our modified protractor, record the angle. Measure the angle near the wing tip and record. Subtract one from the other and you have the angle of twist you built into the wing. Compare to design instructions.

If a model is flown at too high an angle of attack (nose high), it will lose speed, and lift, causing the wing to stall. It will then usually fall off on one wing and lose altitude abruptly. (The dreaded "tip stall")

Since wings usually stall near the tips first, designers may incorporate "washout" (leading edge down) into the outer portion of the wings. This causes them to fly at a lower angle of attack avoiding the tip stall. This makes the aircraft less likely to enter into an uncontrollable attitude.

Measure the angles as with the single wing twist above. Wood workers use "winding sticks" to check a board for twist. They set a stick at right angles to the board, on top of the board, at each end. By sighting along the board, over the tops of the two sticks they can easily see any twist in the board. You can do the same thing with your wings. Rubber band a long balsa stick under each wing, near the wing tips, at right angles to the wing span. Sight along the span and you can easily see the twist and check whether you built the same washout into each wing, for example. During assembly of major components such as wings to fuselages it is useful to employ some type of fixture to position the parts correctly before gluing. This is well illustrated in the following Web site: and click Attaching wings to biplanes.

Due to the variations in density (and therefore weight) in any material, especially wood, it is a good idea to check the balance of a wing. One wing should weigh as close as practical to the other. Some modelers compare the weight of wing ribs and spars to be installed on one wing half to the other half before construction to assure equal weight distribution. If a one-piece wing doesn't balance on the center rib, add a bit of weight to the light wing tip.

11.4 Warps
Flying surfaces (wings, tail components) should be free of unwanted twist. Warps make consistent flight trimming almost impossible. When covering with paper tissue, tape the tissue to a picture frame (empty), or a frame built up of sticks. Preshrink the tissue by spraying it with water or alcohol. Remove the tissue when dry. Determine the tissue grain direction by tearing a corner. It will tear easiest with the grain.

Modelers use many different kinds of adhesive to attach the covering. Alternatives include glue sticks, thinned white glue, thinned model cement, lacquer and clear dope. The following discussion will be based on the use of clear dope. Brush two thin coats of dope on the framework and let dry.

When covering a wing position the paper with the grain running parallel with the leading edge. This minimizes sag between ribs. Adhere the covering to the framework along the center rib by brushing dope thinner on the tissue, this goes through the tissue reactivating the dope causing the tissue to adhere. Then smooth the tissue out toward the wing tip for 3 or 4 times the width of the wing. Using thinner, adhere the covering at one point only in the center of a rib at that location (point X). You now have the paper smoothed in a triangular pattern. The base of the triangle is along the center rib and the tip of the triangle well out on the wing at point X. Let the adhesive dry. Next adhere the tissue to the leading and trailing edges alternating every couple of inches and smoothing as you go until you reach the rib that contained point X. Repeat the process for the remaining span to be covered. This procedure reduces the likelihood of pulling the covering tighter in one diagonal direction than the other, thus reducing a built-in tendency to twist (warp) when shrunk again with water or alcohol, or when a finish is applied. As a further precaution, pin the wing or other component down to a flat surface while allowing the covering to dry thoroughly.

When applying plastic covering, the same procedure can be followed if the adhesive is activated with heat. Shrinking is also accomplished by applying heat.

When transporting models to a flying site in a hot car, or subjecting tissue-covered models to humid conditions, you may still encounter warping. Check for warps before you fly.

11.5 Glue Joints
The cellular structure in wood can be compared to a bundle of soda straws running up a tree trunk. That's how moisture is transferred up to the leaves. This cellular structure is parallel to the length of a balsa stick. When we look at the end of a board we see "end grain". We are looking into the "pores" or ends of the "soda straws." When we look at the side of a stick (at 90 degrees to the end grain) we see edge grain.

Woodworkers avoid gluing end grain to end grain, or even end grain to edge grain, because the glue seeps into the pores of the end grain and "starves" the glue joint making it weak. They overcome this by using special joint shapes that get some edge grain involved in the joint. Examples include the mortise and tenon (tongue on one piece into a slot on the other) or the dovetail joint often seen in drawer corners.

When modelers install very porous balsa sticks as cross pieces in fuselages or tail structures, one of the pieces has end grain. Experienced modelers will apply a bit of glue to this end grain and let it dry briefly, then reglue it to mate with the edge grained component. This is called "double gluing". Don't overdo the glue so that a fillet of glue is formed, that results in an unwanted weight increase.

11.6 Wood Grain Types
It is important to consider the direction of wood grain when selecting the wood to be used for a particular part. When we view the end of a log, we see the circular growth rings in the wood. Wood sheets may be sliced from the log so the rings pass through it tangent to the sheet surface (A-grain); at right angles to the surface (C-grain); or randomly between A-grain and C-grain (B-grain).

Each of the grain types above has particular characteristics that should be considered for a given application. For specific recommendations see Another consideration regarding wood grain is related to grain direction in individual parts. If grain runs the length of a stick it will be stiffer than if the grain ran across the stick, in which case it would obviously break easily. When laminating sheets of balsa to make a wheel, for example, glue the sheets with the grain at right angles to one another for greater stiffness and resistance to buckling.

11.7 Propellers
Carving propellers is almost a lost art, but I'm including a few references for those who would like to try it.

To carve a prop see: and click modeling tips. To make an inexpensive plastic prop see:

Return to Chapter 10

Proceed to Chapter 12

Copyright 2002, Robert S. Munson. All Rights Reserved