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vo - lan - tor (vo-lan'ter) n. A vertical takeoff and
landing aircraft that is capable of flying in a quick,
nimble, and agile manner. --intr. & tr.v. -tored,
-toring, tors. To go or carry by volantor.
[Lat. volare, to fly. Fr. volant,
to move in a nimble and agile manner ]
The Skycar volantor developed by Moller International is capable of vertical
take-off and landing (VTOL) much as a helicopter and flies from point of
departure to destination much like an airplane. However, the Skycar volantor
is uniquely qualified to travel short distances on the ground as an
automobile as well. All this and incredibly, its easy to fly! Actually
a computer does the flying. The pilot need only move the controls in the
direction he wants to go so that little skill is required. (Still for the
time being, the operator will need to have a private pilot's license until
the ease of operation and safety are thoroughly demonstrated.) The Moller
Skycar is a volantor capable of these remarkable achievements through the use of an
arrangement (array - collection - grouping) of proprietary technologies.
Favorable power to weight ratio is the basic qualification for
VTOL. However, in order to create a safe, environmentally responsible
and economically feasible method of transportation Moller International
had to take into consideration a number of components including airframe
and engines.
The Airframe
A VTOL aircraft with its larger installed power must be aerodynamically
efficient at high cruise speeds if it is to use that installed power
efficiently. Also, if the airframe of the volantor is not appropriately
aerodynamic, fuel consumption increases and its maximum travel distance
(range) becomes unacceptable. The ideal airframe must also be lightweight
so the craft can obtain a favorable power to weight ratio. Lastly,
it must be strong for stabilization and safety.
The determination of aerodynamic efficiency comes down to the following:
- When the aircraft is moving at high speed, does
the propulsive air move efficiently through the propulsion or
thrust system?
- Does the aircraft have a small frontal area?
- Does the aircraft have a small wetted area (surface area in
contact with the airstream)?
- Is the vehicle sufficiently streamlined to ensure that its
aerodynamic surfaces are free from airflow separation and therefore
present a clean aerodynamic design?
- Does the configuration achieve a good lift/drag ratio at high
cruise speeds?
Some light planes built today,
particularly those in the experimental or homebuilt category do
an excellent job satisfying the above conditions. A measure of
aerodynamic performance is the passenger transport efficiency
(PTE) as measured by:
PTE = (Passenger Miles / Gallon)
A few four-seat aircraft have a PTE near 70 at 250 MPH although they
will generally have a fairly high landing speed without STOL
(Short Takeoff and Landing) provisions (flaps, slats, etc.).
The key to a successful high-speed design with a high PTE is finding a way
to simultaneously satisfy the five stated aerodynamic requirements. For
example, a long, very narrow aircraft could certainly be streamlined
and have a small frontal area, but might have an excessive wetted area.
- An efficient VTOL aircraft requires the propulsive airflow to
move almost horizontally through the system during cruise because
even a modest bending of the flow can introduce a substantial
drag due to momentum losses.
- A frontal area under 25 ft2 is
realistic for a VTOL aircraft carrying up to four passengers.
- A wetted area to frontal area ratio under 15 is achievable with
an efficient airframe design.
- The drag coefficient based on
wetted area (CDwet) is a good measure of the aircraft's freedom
from airflow separation. A well-designed aircraft should achieve
CDwet of .005 at cruise while a state-of-the-art design arrived
at after extensive wind tunnel testing could have a CDwet <.004.
- Lift/Drag Ratio (L/D) will be high with a large wing area and
a high aspect ratio. However, this high L/D will only occur at
speeds significantly lower than desired cruising velocity. A high
aspect ratio is also hard to achieve in a light vehicle where
the fuselage becomes a lifting body, due to its size relative
to the small wing area required for efficient high-speed flight
with a modest payload. For this reason, the maximum lift/drag
ratio of a powered lift aircraft is likely to be less than that
for a conventional airplane, but could match or exceed its PTE
at higher cruise speeds (>250 MPH).
The Skycar volantor's composite airframe is constructed mostly of FRP (fiber
reinforced plastic) which enables it to be both lightweight and strong. We
have our own 250 mph wind tunnel in which we have performed over 1000
hours of detailed flight testing using both powered and un-powered models
to ensure that we have chosen an optimum design.
The Engine
As seen from the example above, VTOL aircraft requires a great deal
of power to obtain lift-off and perform a safe landing. Since the
engine must lift its own weight along with the weight of the craft
it must be lightweight. Economy is another area that requires much
attention. Purchase price, operating costs, and maintenance costs are all
factors which, if too high, can make the engine impractical. Lastly, to
be truly efficient, an engine must be environmentally friendly. Ideally,
clean burning engines capable of using the most readily available fuels
would provide the best option.
The need for a moderately high disc loading results in a relatively
high installed power in order to hover. For the installed powerplant
weight not to become excessive, the engines must be light for their
power output. The key element in determining the VTOL aircraft cost
may then become the powerplant. If, for example, one uses 200 lb/ft<
disc loading, it can be shown that:
(Installed Horsepower / Gross Vehicle Weight) ~ 0.5
In this case, a VTOL aircraft with a modest payload requires installed
power in the 1000 HP range and an engine HP to weight ratio
near 2. A turbine can meet this weight requirement; however,
a small 100 HP turbo-shaft can cost $100,000, while a single
1000 HP turbo-shaft can cost $300,000. The smaller turbo-shaft
gives a poor specific fuel consumption, while the single engine
provides no back up. Any design using turbo-fan engines will
expect even higher engine costs. A turbo-fan using disc loading
like the Harrier generates only one half pound of lift for every
horsepower. Hence, the Harrier requires approximately 40,000 HP
with little payload capability in the VTOL mode.
To meet the 2 HP/lb requirement in a small fuel-efficient form, only
two engine alternatives appear to be possible at an economical cost:
- A turbo-charged or super-charged fuel injected 2-cycle engine.
This engine would need to be developed.
- A rotary engine that employs aluminum housings, peripheral porting
and an air-cooled rotor. Engines of this design are in existence.
Moller rotary engines were developed from technology obtained from
Outboard Marine Corporation (OMC) and are of the Wankel-Type. During each
rotation of the rotor a four-stroke spark ignition combustion process
occurs in each of the three pockets of a triangular rotor. After one full
rotation of the rotor the engine has completed the four-stroke process
three times. They therefore provide a high power-to-weight ratio at
a reasonable cost and are very small for their power output. The 150
HP model used in the M400 can be easily carried by one person. Eight
Rotapower engines are used in the production model volantor.
Wankel-type rotary engines in general are very reliable as a result of
their simplicity. The number of moving parts in a Moller rotary engine
(dual-rotor) is approximately seven percent of those in a four-cylinder
piston engine. The rotapower engine is also multi-fuel capable. Moller
rotary engines in the volantor are typically configured to run on unleaded
gasoline however, we have recently demonstrated the engine's ability to
run on diesel to the Army and on Natural Gas to another organization.
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