Wow, what fun! Following Bob Windt's plans, I constructed a gas powered r/c model. Bob's plans call for an aluminum thrust duct, but as on
my electric models, I used pink foam. (Once you start building with foam,
you become convinced foam can do anything. See my description of the Webers' foam
propeller for an elaboration on this theme. And be sure to visit
again soon, as I expect foam engines will be discussed in the near
future.)
The model weighs 3 pounds, is 3 feet long, is made of pink 2 inch thick
extruded polystrene foam, uses an OS 15 FP .15 engine (that's .15 cubic
inches of displacement) swinging an 8 inch diameter prop with 4 inch pitch,
and has a bag skirt made of plastic trash bags.
Although the engine isn't nearly as powerful as
the typical .40 size airplane engine, I was stunned by the
increase in power over the electic motors I'd been using. Model airplane
fuel can be messy, and model engines can be hard to start and requires all
sorts of starting paraphenalia, but the hassle is worth it! I've been
having a lot of fun with the model. Even on very gusty days, the model
has had enough power to maneuver around, and there is no problem
whatsoever getting over the hump. On snow, the model can often power its
way out of tire tracks and can zip across well-trod snow fields. The
only challenging surface so far has been grass. Right now, the grass is
still frozen when it appears, and the model loses its cushion as it climbs
on top the spiky grass blades. I expect that the model will have similar
problems in the spring, it doesn't weigh enough for its
area to push down the grass and keep the air gap small enough to prevent
the craft's cushion from escaping too fast. But we'll see -- I'll report
back come springtime!
In the picture below, you can see the model at rest just after a trip which involved
going across pavement, into a pond, onto a floating ice shelf in the middle of
the pond, and then back across the water and back onto the pavement. You
can clearly see the craft's integrated design (one engine pushes some of
the air out the back for thrust and some underneath the craft where a
small fraction of the air is shunted into the skirt and the rest is
directed underneath the craft). The plastic trash bag skirt
material can be patched with tape but is surprisingly hardy as long as
you keep sudden landings to a minimum. At the front of the craft, I embedded
a film cannister filled with pennies for ballast. Even with the pennies,
the craft will tip over every once in awhile when you turn it abruptly at
top speed. Full-size hovercrafts have similar behavior -- it is best to
slow down before turning. The cockpit cover protects the receiver and
servos and is held in place with wooden pegs and a rubberband.
