Chapter 12

Keep thy airspeed up, lest the earth come from below and smite thee.
- William Kershner

12.1 Trimming for Flight
When modelers discuss their new creation they like to be able to say,"It flew right off the drawing board."

Given the variables that are introduced by dimensional tolerances, minor warps and weight distribution problems the chances of a perfect first flight are slim.

The following ten steps for flight trimming a new rubber model are taken from the Internet:

  1. Locate the CG
  2. Balance model without prop and motor to located CG
  3. Glide model
  4. Adjust for smooth glide
  5. Check stability by launching into slight dive and slight climb
  6. Readjust CG for stability if necessary
  7. Mark location of new CG (if changed in step 6)
  8. Install prop/motor and rebalance to established CG
  9. Test fly under power
  10. Adjust flight pattern with thrust line

Although the above steps were written for a rubber-powered model the idea of separating the center of gravity adjustment and the thrust line adjustment (for powered flight) applies to most model aircraft. It is always best to make one trimming adjustment at a time. If two or more adjustments are made at the same time, you won't know which caused an improvement or poorer performance.

You will find each of the ten steps explained in much more detail at: and click modeling tips and click 10 step.

12.2 Aerodynamics for Pilots
There is a valuable reference on the Internet entitled, See How it Flies. The author makes the point that there is a difference between aerodynamics books for engineers and aerodynamics books for pilots. See How it Flies is written for pilots. Chapter 3 of that book deals with airfoils. The book is on a Web site. See:

On page 14 of 29 we see that it is a misconception to believe that wings must be curved on the top and flat on the bottom. Modelers have often followed the practice of early full-scale designers in making the bottom surface of the wing concave. It's called undercamber. We can still have a cambered wing airfoil with both upper and lower surfaces convex. Let's stop here for some definitions.

When viewing a wing cross section we see an airfoil. If we draw a straight line from the leading edge of the airfoil to the trailing edge, we have a chord line. If we draw a curved line from the leading edge to the trailing edge, staying always halfway between the upper surface and the lower surface, this is called the mean camber line. The maximum difference between this and the chord line is the amount of camber.

If the airfoil is symmetrical, where the top surface is a mirror image of the bottom surface, it has zero camber. On page 15 of 29 we see that under ordinary conditions the amount of lift produced by a wing depends on the angle of attack, but hardly depends at all on the amount of camber. On page 16 of 29 we see that even a thin flat "airfoil' shape will produce lift, if the wind strikes it at an appropriate angle of attack. But, we learned that when flying the flat wing AMA Cub. (See 6.1) In a related Web site NASA provides simplified definitions of a wing's geometry at:

12.3 History of Airfoils
It is interesting to note the variations in historical conventional wisdom regarding airfoils.

In 1889 Otto Lilienthal published his findings on bird flight as the basis of aviation. [Lilienthal, Otto. Bird Flight as the Basis of Aviation. Hummelstown, PA: Markowski International Publishers, 2001. (Unabridged facsimile of the original work)]

On page 45 we see diagrams of airflow over a flat plate at various angles of attack. On page 53 we find that experimental results showed that a "very slightly curved surface (analogous to the bird's wing) possessed pre-eminently those qualities which we considered essential for economy in flight." And on page 54: "Since it is probable that the whole secret of flight is to be found in the properties of such slightly curved surfaces, we must investigate them more minutely later on."

If we fast-forward to a book about the Lockheed "Skunk Works" we find the designers of the Stealth Fighter being required to design wing shapes consisting of flat panels, needed to deflect radar. [Rich, Ben R. and Leo Janos. Skunk Works, A Personal Memoir of My Years at Lockheed. Boston: Little, Brown, 1994]. On page 30 Dick Cantrell, Head of the Aerodynamics Group, was required to toss aside basics of fundamental aerodynamics, "in deference to new technology understood only by witches and mathematical gnomes." The airplane "would be so deficient in lift-drag ratio that it would probably need a computer the size of Delaware to get it stable and keep it flying." It did fly, computer aided, one not the size of Delaware.

It is worthwhile to gain some understanding of how airfoils work because they are used by wings, tails and propellers to move air. In fact, they are used in compressors, turbines and other components inside modern turbojet engines.

12.4 Related Web Sites

Return to Chapter 11

Proceed to Chapter 13

Copyright 2002, Robert S. Munson. All Rights Reserved