Abstract :
It has long
been the dream of aviation and automobile enthusiasts to have a vehicle that
will bring them the best of both worlds. Many drivers stuck in rush hour
traffic have fantasies about being able to push a button and watch their car’s
wings unfurl as they lift above the stalled cars in front of them. Just as many
pilots who have been grounded at an airport far from home by inclement weather
have wished for some way to wheel their airplane out onto the highway and drive
home. This yearning has resulted in many designs for roadable aircraft since as
early as 1906.
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| Flying Cars |
‘Flying
car’, ‘roadable aircraft’, ‘dual-mode vehicle’ and other terms are used to
describe the all-purpose vehicle that can fly like an airplane and drive on the
highway like an automobile. Make it amphibious and we have the perfect all-purpose
vehicle! Nevertheless, this might be taking our ideas a bit too far.
A designer
of a flying car will encounter many obstacles, including conflicting
regulations for aircraft and automobiles. As an automobile, such a vehicle must
be able to fit within the width of a lane of traffic and pass under highway
overpasses. It must be able to keep up with normal highway traffic and meet all
safety regulations. It must also satisfy vehicle exhaust emission standards for
automobiles. Therefore, the wings must be able to fold (or retract) and the
tail or canard surfaces may have to be stowable. The emission standards and
crashworthiness requirements will add weight to the design. The need for an
engine/transmission system that can operate in the stop and go, accelerate and
decelerate environment of the automobile will also add system complications and
weight.
For flight,
the roadable aircraft must be lightweight and easy to fly. It must have a speed
range at least comparable to existing general aviation airplanes. Conversion
from aircraft to car or vice versa must be doable by a single person and the
engine must be able to operate using either aviation fuel or auto fuel. Ground
propulsion must be through the wheels and not via propeller or jet which would
present a danger to nearby people, animals or other vehicles.
some of the
specifications to be determined by the design approach are:
Ø Range
Ø Endurance
Ø Rate of climb
Ø Cruise speed in air
Ø Cruise speed in land
Ø Airworthiness standards
Ø Automobile safety and emissions.
Additional
challenges to be noted are
· The need to
have acceptable in-flight wing aerodynamics while being able to retract, fold, or detach and stow the wing
for road travel,
·
The need to ‘rotate’ on take-off,
· The need to
find an engine/transmission combination which could meet the conflicting demands of ground and air travel,
·
The need
for dual-mode control systems, and the need to meet rigorous stability and performance requirements in both modes
of travel.
The design
of a satisfactory wing is a dominant part of any roadable aircraft layout. As a
‘car’ the vehicle must fit into standard roadway widths. The resulting vehicle
footprint (aspect ratio) is less than unity. This is regarded as inefficient
for an aircraft wing planform. A wing of reasonable aspect ratio must then be
capable of being extended from the body (fuselage) for flight and somehow
stowed for highway use. There are many ways to do this including folding wings,
rotating wings, telescoping wings, and detachable wings. These could be stored
in, under, or over the car configuration. Alternatively, they could be towed
behind the car. All such designs impose structural compromise and weight
penalties. The use of the wing for a fuel tank location would also be ruled out.
The
take-off problem reflects the differing stability requirements of automobiles
and airplanes. Most modern aircraft are designed with a tricycle landing gear
arrangement with the rear or main wheels placed only slightly behind the center
of mass (center of gravity). This allows easy rotation in pitch to a reasonable
take-off angle of attack after ground acceleration. Placement of the rearwheels
in the optimum location for the main gear of an aircraft would result in a very
unstable car. It would have a tendency for its front wheels to lift off the
road at highway cruise speeds near the desired take-off speeds for the
aircraft. Cars are designed to minimize the likelihood of the wheels lifting
off the road at highway speeds! Some roadable aircraft designs have attempted
to solve this problem by having a conventional aircraft tail section that is
removed for road travel. This effectively moves the center of mass further
forward between the front and rear wheels. Others have employed a car type
suspension with wheels or axles that can be extended or retracted to give the
needed angle of attack for take-off.
Further
complications arise due to the need for the wing on the airplane to develop some
lift during the take-off run while the automobile must produce as little lift
as possible at highway cruise speed. Removing or retracting the wings for the
car layout will obviously solve most of the highway lift problem.
Aircraft
piston engines are designed to be run at constant rpm for long periods of time.
Automobile engines are designed to operate over a wide range of rpm and are coupled
to a transmission to make possible combinations of torque and power suitable for
a variety of operational needs. Aircraft engines must also be capable of
efficient operation over a wider range of altitude than car engines.
Air-cooling is normally used with aircraft engines while water-cooling is
usually used for automobile engines. Both a water-cooling system and a
transmission system will add extra weight not common in most aircraft designs.
Some flying car designs have proposed using separate engines tailored to each
mode of travel. This is on the assumption that two optimized engines may not
weigh much more than a single dual-mode engine and drive train, and that the
improved efficiencies may allow lower fuel consumption. Other designers have suggested
the use of an engine and transaxle from a small 4WD automobile with the drive
for one set of car wheels attached to the wheels and the other to the
propeller.
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