Computing best decalage for desired speed and trim is actually fairly involved. I know of no one good reliable simple formula that you can use to calculate decalage (angular difference between wing and horizontal stab) for all sizes and types of soaring gliders. If you pressed many designers they would probably have to admit to you that a fair amount of empiricism (read flight trimming) is typically involved in setting up decalage on a new design. Decalage is effected by many other glider design parameters including designed range of speeds, CG location, Airfoil pitch moment, wing's drag moment, wing downwash angle of air at tailplane, etc. A variety of approaches to computing decalage are possible. These methods vary in their reliability and difficulty. Method 1 - Best Approach - Plane Geometry Software If you are lazy and want to get the right answers I would suggest you investigate some very fine glider design software called "Plane Geometry". You can do all the necessary calculations on paper to get a good estimate of decalage (but it is an arduous and error prone process). For routine work it is much easier (and less error prone) to use a computer program to do the grunt work for you. Blaine Rawdon (a rather gifted professional aerodynamicist and active aeromodeler) has written a truly excellent Excel spreadsheet that greatly simplifies the process of accurate soaring glider design. Plane geometry is very helpful for answering questions like optimum decalage, best fuselage deck angle (angle the wing mounts on the fuselage for minimum drag), tail length and tail surface sizing for best performance and handling, control surface sizing to achieve known good effective response from control surfaces, etc.. I know of no other single tool that provides as much excellent information as Plane Geometry (initially, you may be a little overwhelmed with the amount of information Plane Geometry provides. You can begin by only using the parameters you need and over time learn more about more exotic parameters and use them when you need them). Michael Shellim has a fine review of Plane Geometry on his web page that should give you a good idea of how the interative process of using the design tool works. You can view Mike's informative page at http://www.rc-soar.com/hardsoft/planegeo.htm Plane Geometry's results for decalage, fuselage deck angle, and stabilizer geometric incidence are the most reliable of any of the calculational methods I have tried. Plane Geometry is easily worth the approximately US$20 which it costs if you design model gliders. One thing that Plane Geometry does not do is predict the performance of a proposed glider design (Plane Geometry does not have a database of airfoil polars/performance). You must use other tools to identify an airfoil/wing design that should have good Lift/Drag, sink rate, etc. Plane Geometry will help you implement that design by providing vital design information like decalage, CG range, tail sizing for stability/performance, etc. Plane Geometry focuses on the design issues of conventional gliders with tailplanes (cruciform and V-tail). Design Problems relating to tail less gliders (flying wings) and canards are not directly covered by Plane Geometry. Decalage Method 2 - Designer�s rule of thumb (Good starting point for an estimate of decalage but less accurate/reliable than Plane Geometry Software) Almost every thing else affects decalage at least a little (CG location, airfoil pitch moment, wing's drag moment, downwash angle of air at tailplane) so the final setting is often a matter of trimming rather than computation. For thermal soarers trimmed for good performance at speeds near maximum lift to drag 5 to 6 degrees from the zero lift angle of attack of the wing is usually a good starting point for an initial guess of decalage. For example, the zero lift angle for RG15 is approximately -2.52 degrees so a beginning suggestion for decalage for an RG15 thermal soarer would be somewhere around 2.48 degrees. Decalage Angle (RG15 thermal soarer) = AoA~ -2.52+ 5 degrees = 2.48 degrees Note: Decalage for slope racing gliders is typically a little different than for thermal soarers. I would suggest adding 3 degrees to the airfoil zero lift angle as a designer�s guess for the decalage (thermal gliders are tuned for optimum performance at speeds near those for optimum Lift/Drag while racers are trimmed for performance at high speed and typically require a little less decalage than soarers when trimmed that way). Example (Slope Racer) Decalage Angle (S2062) = AoA~ -2.03 + 3 degrees = 1 degree The decalage for a racer needs to be small to preserve the speed range of the glider. Typically very small angles (<= 1 degree) are involved which are difficult to measure and to build accurately. This desgner's rule of thumb is a surprisingly good initial guess of a value of decalage but you will have to typically resort to flight trimming to get final settings. ________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
