At 07:24 AM 11/9/2005, you wrote: > >From the discussion then, I get the impression that the quarter cord >figure is empirical - but pretty close to the calcutated figures for RAF48 >and AS504x wings.
The quarter chord location is not empirically derived. It is located at 25 percent of the chord. It is useful in aerodynamic calculations for a variety of reasons. The aerodynamic center is also not empirically derived. It is a real point in the airfoil and it is defined as the point at which the pitch moment is constant. This is the twisting force that an airfoil generates as it generates lift. It is typically a nose-down force. A symmetrical airfoil will have a pitch moment of zero. Airfoils with a trailing edge that are reflexed upwards can have a positive pitch moment and may be used in flying wings. >I guess that the important thing in designing a wing is that the spar is >as close as possible to the centre of lift, so as to minimise twisting >forces inside the wing. Minimum twisting forces in a wing will generally occur with a spar that is located somewhat behind the aerodynamic center. With a typical airfoil, this is generally about 30-35 percent. >I guess also that the most important consideration regarding CG is that it >must never move aft of the center of lift, you want to ensure that if the >fan stops pulling, then the plane will tend to dive rather than nose up >into a stall. > >If there a rule of thumb for calculating safe CG? The only real way to calculate this before flight testing is to determine the neutral stability point. If the center of gravity is at the neutral stability point, a change in the pitch control force will result in a constant change in the angle of attack. An airplane is generally neutral in roll stability; if you give it some roll force on the stick, it will roll at a nearly constant rate until you remove the roll force from the stick. Neutral in pitch does exactly the same thing. The farther forward the center of gravity is from the neutral stability point the more stable the aircraft in pitch. For a conventional aircraft configuration, the neutral stability point will be somewhere between 30 and 40 percent of the Mean Aerodynamic Chord. Your aft-most CG limit is then fixed at least 5 percent in front of that point. My own KR based design has a calculated neutral stability point at 34.5 percent. Until flight testing is completed, my aft-most CG limit is 29 percent of the MAC. If you want a rule-of-thumb and you have a conventional looking airplane then start with a CG of 20 percent and move the allowable range through flight testing in small increments. Don Reid - donreid "at" peoplepc.com Bumpass, Va Visit my web sites at: AeroFoil, a 2-D Airfoil Design And Analysis Computer Program: http://aerofoilengineering.com KR2XL construction: http://aerofoilengineering.com/KR/KR2XL.htm Aviation Surplus: http://aerofoilengineering.com/PartsListing/Airparts.htm EAA Chapter 231: http://eaa231.org Ultralights: http://usua250.org VA EAA Regional Fly-in: http://vaeaa.org
