Updated: Mar 27, 2020
Determining wing dimensions
Physics team began the preliminary wing design by researching and selecting an airfoil profile. We chose the Clark-Y because of its popularity in ultralight aircraft and desirable properties such as a tame and “mushy” stall. To get a conceptual starting point for the required reference area, we examined similar ultralight airplanes and their wing designs and noticed the reference area typically ranges from around 120 sq ft to about 180 sq ft. One specification of ultralight aircraft is that they must have a stall speed at or below 24 knots. Based on the maximum lift coefficient of the Clark-Y profile and the 24 knot stall speed limitation, we were able to write the following equation to solve for the minimum reference area:
This resulted in a reference area of approximately 166 sq ft which fits right within the anticipated range. Now that we had a general idea of the reference area we could roughly solve for our wing's Reynolds number. The Reynolds number is a value that describes the flow characteristics of air over the wing. This led us to the conclusion that we actually have a higher maximum lift coefficient and a lower MTOW. We then downsized the wing to 150 sq ft which keeps a wing loading of about 2.7-3 (depending on the pilot and the material choices). We are opting for a higher wing loading because it minimizes drag and improves maneuverability. We then looked through a few textbooks (cited in the engineering notebook, release date TBD) in order to come up with a desired aspect ratio, and decided upon a range of 4-7.5. Higher aspect ratios minimize lift induced drag due to their shorter chord and smaller wing tip vortices. However, this is at the cost of longer, and therefore heavier, wings. This is because longer wings have longer lever arms and exert a greater force along any point in their span requiring a larger and heavier spar to bear the additional load. Higher aspect ratio wings also have a higher zero lift drag due to their long leading edge, but this is relatively minimal in comparison to the large inefficiencies of lift induced drag (at least at our slow cruising speed). Low aspect ratio wings, on the other hand, are lighter and have a lower zero lift drag but a much greater lift induced drag. Since our ultralight will have lithium batteries as a power source, which have a much lower power density than avgas, efficiency is a primary objective. With this in mind, we opted to start with an aspect ratio of 6 which gives us a wingspan of 30ft and a chord of 5ft.
*Editors note: the thought processes and design choices presented in this article don't necessarily represent those implemented into the final design and are subject to change. Flight Club Aerospace is a group of amateur students with no formal education in any fields of engineering. We present this information for educational purposes only, with the understanding that it is not to be re-created without adequate professional oversight and mentorship. For our latest designs and updates, please see our most recent blog posts.