medium. “It’s that transition between [air and water] during
which you have to not destroy everything,” says Edwards.
To keep the Flimmer intact, the team experimented with
various landing styles using a test vehicle called the Test Sub
2. Many configurations were tested, including plunge landings
(like a duck diving straight down into the water) versus pelican
landings, in which the bird spreads its wings to slow down, puts
its feet down, and skids across the water to slowly set down.
“The only way to land is flat,” says Edwards. Accordingly, the
Flimmer lands more like the pelican – slowing down until the
moment it touches the water. It also features a traditional boat
hull, so it can touch down and slowly scrub off extra energy.
Parachutes, or spins into the water were options as well, but
the team wanted one solid vehicle with no frangible parts.
“Structural analysis was really important,” explains Edwards.
“You don’t want to over-build because it carries more weight
in the flying mode, but you can’t under-build because you still
have to splash down and puncture the hole in the water.”
The process included much trial and error, but the team also
used computational fluid dynamics (CFD), and a non-linear
solver developed by NRL. “[The solver] breaks up fluid into
really small solids and computes, for each very small point, what
the forces and moments are on that particle and then sums up
the whole mesh to get a number,” explains Edwards.
The team also used finite element analysis (FEA) for structural
modeling and completed linear aerodynamics for the general
airplane shape. “A little bit of intuition” was also key, says
Edwards, who explains that sitting around the table with napkins
sketches and kicking around ideas was surprisingly important.
“[Brainstorming] really sets the stage for determining the
general shape,” he adds, but stresses the importance of the
experimental process. “We broke some stuff in the water, which
gave the team insight into what to do different the next time, but
that’s kind of the fun part.”
Future of Full-Finned Flight
The program is currently wrapping up as funding comes to
an end, but the technologies developed as part of the Flimmer
program will continue to be refined.
“I think we’ve ended at a good point,” says Edwards. “We’ve
shown that the vehicle can fly in its full-weight, full-finned
Specifically, the team at NRL will continue working on
increasing the efficiency of the vehicle’s flapping mechanisms.
“It is hard to beat a spinning propeller,” says Edwards.
Although unlike a spinning propeller, the flapping propeller
makes very high thrust levels almost instantaneously. “In one
stroke we can make full thrust,” he adds. However, flapping
propulsion still poses certain issues, and the team plans on
exploring buoyancy driven mobility modes as alternative means of
propulsing under water.
“Regardless of what you do in water, flying is a really interesting
way to get you right where you need to be,” says Edwards, who
uses the example of the oil spill in the gulf.
“No one knew where the oil was coming from or going to,”
says Edwards. “Flimmer could have been deployed from the
shoreline to fly around and survey for oil slicks.” Once identified,
it could have splashed down to swim underwater and try to
identify from where oil was spilling.
This is one potential use of the technology, although Flimmer,
in its current form, may not be representative of how the
research will be used in a final product. However, the underlying
technology will enable similar missions that weren’t possible
without multiple mode vehicles.
“It seems like there is probably something still hiding …
some really good trade between the two,” says Edwards. While
Flimmer is the first example of a multimodal drone for use in
the sky and at sea, Edwards admits that the design can still
be improved upon. “One day we’ll look at the design as all of
these ideas come together and people think about the problem
in a different way and go ‘ah, ha – now we can do flying and
swimming in the same vehicle,’” he adds. “It will be interesting
to see what that is in the future.”
Several images stitched together showcase the Flimmer’s unique landing style.
The Flimmer is launched from the ground via a pneumatic
launcher. (All image credit: U.S. Naval Research Laboratory)