Thread: Why does dive CG test work? View Single Post
 Apr 02, 2003, 11:05 AM Lex Mierop Lex Mierop Guest n/a Posts RE: [RCSE] Why does dive CG test work? Richard, It's all about the trim. When flying a nose heavy plane at regular thermal cruise speed, you have to add up trim to get it to fly straight and level. The "up" trim setting is causing an aerodynamic force on the tail is pushing down to compensate for the extra nose weight. The amount of aerodynamic down force required to stay level is constant for a given amount of nose weight. The amount of aerodynamic force created by a flying surface varies with the speed of the airplane. As it goes faster it is more effective (better Reynolds numbers). So what you start with is a nose heavy plane that is trimmed for level flight at cruise speed. The tail forces balance the nose heaviness of the model. Now increase the speed, and what happens? The nose heaviness remains constant, but the downforce on the tail increases, causing the model to pitch up. The inverse is true on a tail heavy model. To trim for straight and level at cruise speed, the tail requires down trim (an upward force on the tail to compensate for the lack of weight in the nose). Increase the speed, and the upforce on the tail increases, causing the model to pitch down. That's a tuck. -l -----Original Message----- From: invicta421 [mailto:richardh@examen.com] Sent: Wednesday, April 02, 2003 12:51 AM To: soaring@airage.com Subject: [RCSE] Why does dive CG test work? I have been pondering this question for the last few weeks, but haven't satified myself with an explaination yet. Most aerodynamics seem to make intutive sense to me, but this doesn't. The test I am speaking of is when you put the sailplane into a 45 degree dive and see if it pulls up, flys neutral or tucks under. A page on how to do this can be found here: http://www.polecataero.com/articles/cg_art.shtml But this page doesn't explain why... to me it seems that a nose heavy glider should tuck not rise when the CG is in front of the average lift on the wing. Any help understanding this? Thanks -Richard RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to soaring-request@airage.com. Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to soaring-request@airage.com. Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.