Hi Chris, First of all congratulations for your project. It is nice to see that young people are interested in understanding the principles of flight. Aspect ratio optimization is a key factor in glider design.
Here is a simplified analysis which I hope will awnser your question(s): The lift/drag (L/D) ratio of your wing is the main driver of the performance of your glider. The better it is, the further the glider will go for a same initial height. Considering that the weight of your glider remains constant with the different wings (this should be true since you keep the same wing area and, I guess, the same balsa thickness and density), the lift required to balance the weight of the glider is the same for all configurations. The lift coefficient of the wing may change from one wing to another but the glider speed will also change to produce the same amount of lift. Then, what mostly influences the L/D ratio is the wing drag. The total drag is the result of two different drag terms: the profile drag and the induced drag. The profile drag is depend of the airfoil shape and will increase with a decrease of the Reynolds number. Without going into much details, the Reynolds number is a measure of the ratio between the viscous forces over the inertia forces. In the case of an airfoil, it is proportional to the wing chord. So a high aspect ratio (AR) leads to smaller chords and thus lower Reynolds numbers. Just for demonstration, we could assume that the profile drag is proportional to the inverse of the Reynolds number squared. Since the Reynolds number is proportional to the wing chord and the wing chord is proportional to the inverse of the AR, then the profile drag would be proportional to the AR squared. This leads to the profile drag curve (red) on the graph below. The induced drag is caused by air flow around the wing tip from the high pressure zone below the wing to the low pressure zone over the wing. This creates cross flows on the bottom (outward flow) and top (inward flow) of the wing which in turn create vortices when the top and bottom flows meet at the trailing edge of the wing. These vortices are lost energy which is a source of drag. The induced drag is proportional to the inverse of the AR (blue curve on the graph). The total wing drag is the sum of the profile and induced drag. It is represented by the green curve on the graph. As you can see, there is a point where the total drag is minimum (at AR=6 in your case). This is the optimal AR for this given glider. See graph at http://pages.videotron.com/jdlbt/tmp/DragComponents.html. (This graph is only for the demonstration as the drag scale shown has no physical meaning) You may then wonder why full scale gliders have much higher aspect ratios than model gliders ? This is due to the fact that they operate at much higher Reynolds numbers (higher speeds and larger wing chords) so the induced drag is much more dominant compared to the profile drag (if you move the blue curve up on the graph and the red curve down, you will see that the point of lowest drag moves towards higher ARs). I hope this helps you a little. If you have any other question, please don't hesitate to ask me. Best regards, Julien de la Bruère-T. Concepteur Mécanique - Ingénierie Turbines Hydrauliques / Mechanical Designer - Hydraulic Turbines Engineering --------------------------------------------------------- Alstom Canada inc., Power Environment Tél.: (450) 746-6500, ext. 3184 Fax: (450) 746-7021 1350, chemin Saint-Roch Sorel-Tracy, Québec CANADA, J3R 5P9 [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
