Alright, so if you watched this, and you thought what I thought then your thinking... (Helium!)
A VTOL using Helium is genius, a little complex by their description.. but the idea is simple enough to mold!
I like the fact that the engines are only running for a short time, to the the plane high enough to glide down, using very little power after the first climb!
Comments
ah in that case you don't want a gravity plane so much as you want to make a hybrid airship because you're not going to get much benefit from only going up 400 feet.
but this is good because even a a model Gravity plane would be huge because of the weight of the airframe, batteries, tanks, pumps and everything needed to make the idea work. but a hybrid airship ca be a semi rigid or even non rigid in design which makes things lighter and cheeper.
there have been a few degins that use this hybrid airship idea.
Lockheed-Martin P791
the skycat
Piasecki PA97 Helistat
the Dynairship
basic idea is these aircrafts use their lifting gass counter act the weight of the craft then use wings or propellers to give the rest of the needed lift to get off the ground.
https://www.youtube.com/watch?v=tPbu5UeW4uk
Heres the video, sorry guys.. DIYdrones.com keeps rejecting my video embeds? its weird..
But since we cant fly over 400ft anyways, this was proposed more as an energy saving idea?
If the helium can lighten your plane, you can fly more gear at slower speeds giving your longer flights!
You might be able to use it the way they describe, to lift to max altitude, then glide down all on little engine power but 400ft may not give you long glide times, if u load your plane up?
If you google around there's been a little bit of work on this subject. The main trouble discussed was the conflicting objectives between high glide ratio and high speed to overcome the wind. A cruise missle was suggested as an example of an airframe optimized for high speed.
Depending on your objective, the ability to fight the wind might not be an issue. The application being discussed was RTB functionality for high altitude balloon microgravity experiment payloads.
Another issue I found was the difficulty of converting the potential energy in to work without experiencing issues with wing/aileron flutter from exceeding maximum speed.
I looked to the lockheed U-2 for inspiration because of it's history of high altitude flight and high glide ratios. The concept of "Coffin Corner" seems to be a factor.
http://en.wikipedia.org/wiki/Coffin_corner_(aviation)
Would be very interesting to see what you come up with.
Are you thinking of the mechanical fish gliders type things, but air vs water????
Sail up, then glide down. The atmosphere is much more turbulent and variable than the ocean. TO & LDG will be a major problem.