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I've been fiddling around with trying to make a blended wing body aircraft. While modelling, I heard from someone that the design reminded them of the Swift Killer Bee (now Grumman Bat). One distinct feature that I noticed from the design of the Bat is their use of an inverted winglet. Does anyone know why the design features an inverted winglet rather than a conventional one? The more recent designs of the UAV also features a conventional winglet in addition to the inverted one. Are there any benefits for this design?

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I would also like to know if anyone has an idea on how the UAV handles landing? Is the inverted winglet capable of supporting the stresses experienced during landing? I can only find that the Bat comes with an optional landing gear while normally it is supposed to be caught by a net.

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    $\begingroup$ FWIW those aren't winglets. They're vertical stabilizers also called "fins" on flying wings. They provide a different function from winglets though sometimes they CAN be designed to also function as winglets (see the Prandtl flying wing) $\endgroup$
    – slebetman
    Commented Jan 12, 2017 at 4:44

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Without knowing what was going on in the minds of the designers it's difficult to tell exactly, however there are a few reasons as to why the vertical stabilizers are featured both above and below the wing.

In the sUAS market there are a huge amount of products which offer very similar capabilities. It is quite possible that the stabilizer design of above and below rather than in a single direction is to differentiate the product in a crowded market. This is similar to business jet design wingtip design - while it is known drag may be reduced the devices offer aesthetic appeal and different designs stand out to buyers.

The design also allows for the same surface area with a smaller packing footprint. Due to the bulbous shape of the fuselage the potential package size of the bat is smaller with the stabilizers as designed rather than simply above or below.

The stabilizer design is symmetric around the horizontal plane, meaning that in a side gust condition the UAS will feature less tendencety to roll and will instead simply be 'shifted'.

Again in a side gust condition the balancing of forces above and below the attachment point cancels out the moments and therefore may allow for a less strong joint (weight savings).

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Winglets in general increase aircraft performance by decreasing induced drag caused by air spilling over the wing tips from the bottom to the top of the wing (sorry, I forgot the technical term). The winglets reduce this spillover.

Since the aircraft is a pusher, there is the distinct possibility of striking the prop on the ground on takeoff or landing. The inverted winglets will act as a guard to prevent this (as the inverted V-tail design on the predator drone protects its pusher prop from the same such prop-strike).

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