Say you wanted to build a glider to do the usual glider things. How big would you make it?
Given that fuel load is not an issue, and the mass of the APM is insignificant, would you make the wingspan 1.5 meters or 3.5 meters? Or bigger than full size?
Does a larger wing have a higher L to D, better overall glide ratio and more ability to catch thermals? Or is it just that we tend to size things in relation to ourselves and "big" means in relation to a human carrying aircraft.
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Hi, I'm new, but I'm a mechanical engineer with an aerospace background. I believe I may be able to help you out.
The L/D max is well approximated by the equation to the left. It's proportional to the square root of the wetted aspect ratio. The wetted aspect ratio is the span squared divided by the total surface area of the UA. For a flying wing, this is approximately the span divided by the mean aerodynamic chord (MAC).
Figure 3.8 attached shows trends in completed commercial systems for different kinds of UA for their (L/D)max vs AR_wet. Note that Cf is the skin friction coefficient (dependent on material selection and how tight the seams/screws are, etc.) and that e is the span efficiency (typically 0.8-0.9, but use as low at 0.65 for initial sizing).
Long story short, L/D goes up relatively quickly with long, thin wings (for a constant fuselage size, AR increases with span, increasing L/D even further). Of course, as mentioned, other costs go up too. You'll need better structural efficiency to see the benefit, meaning more expensive materials and/or manufacturing, TOL becomes more difficult, and so does transportation and so on.
Source: Designing Unmanned Aircraft Systems: A Comprehensive Approach
Fig 3.8.png
l.d max eq.PNG
Big in my eyes is anything above 5m wingspan, the reynolds numbers work better at a larger size, of course the airframe can lift your standard payload for longer or more for the same. The difficulty comes with launch, recovery and transportation. I would not go over 1.5m in size until you have flown a platform of that size many times successfully. Trent from my Geek show is a fantastic example of experimenting building confidence and changing stuff methodically and then getting it right. His latest flight video shows very well that it works.