At 11:44 AM 18/12/2008, you wrote:
I recently read some stuff about compressed air engines (on the
internet, where else!). They have been around for some time and
were, for example, used in mines because they were safer than steam
or early internal combustion engines. There has been a recent surge
of interest in using such engines in vehicles because they have no
emissions, can be efficient and that modern materials, eg carbon
fibre, can greatly reduce the weight of air tanks. This excludes
the emissions from power stations generating electricity which
drives the compressors needed to "refuel" the engine.
This got me thinking about compressed air's potential for self
launching or sustaining gliders and I wonder if someone could do the
numbers on the following scenarios to test their viability.
Standard carbon fibre cylinders are available which are about 2 ft
long, 8" diameter, weigh 12 lbs and hold 88 cu ft of air at 4500
psi. I think they are used in SCUBA diving. Four could probably be
fitted into a fuselage; would the following work as a self launcher
or a sustainer?
1 As a fully throttleable jet
2 As a propellor driven by air outlets at the tips, cf the Sud-Ouest
Djinn helicopter of 1955.
It has been suggested that at slow , eg glider, speeds a narrow
column of fast moving air as from a jet is less efficient than a
large area of relatively slow moving air, eg from a propellor.
Thanking you in anticipation.
.
Graham
Forget the jet when using compressed air.
Tip jet propellor or an air driven piston engine or turbine might be
the way to go. Prop efficiency will be only 50% or so. Those scuba
tanks don't give nearly enough air. My very rough calculation showed
about enough energy to acheive liftoff speed. No climb.
Yes a narrow column of fast moving air as from a jet is less
efficient than a large area of relatively slow moving air. Thrust is
mass x velocity change. Energy is mass x velocity squared. So for the
same thrust you can use 1/2 the mass but twice the velocity and use
twice the energy. This is why gliders have large wingspans - push the
most mass of air down slowly so as to use the least energy.
Mike
Borgelt Instruments - manufacturers of quality soaring instruments
phone Int'l + 61 746 355784
fax Int'l + 61 746 358796
cellphone Int'l + 61 428 355784
Int'l + 61 429 355784
email: mborg...@borgeltinstruments.com
website: www.borgeltinstruments.com
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