At Oshkosh’s AirVenture, the legendary designer Burt Rutan introduced his “flying car” design called BiPod, which caught a lot of media attention. However, the German company Carplane (Carplane.com) developed a similar – and as they claim – much more sophisticated design years earlier.
Carplane issued a specific technical and design response to counter Rutan’s claim:
1). We welcome all positive contribution to roadable aviation.
2). We particularly welcome Burt Rutan to roadable aviation.
3). We thank Burt for validating our twin-hull roadable concept.
4). Main Similarities: Twin hulls, hybrid/electric drive, steering wheel in road-mode/stick in air-mode.
Main differences between BiPod and CarPlane
a). The Carplane(R)’s mode-conversion is push-button in 15 seconds. The BiPod’s mode-conversion involves detaching 4 stabilizers & 2 wings (+ 2 aileron linkages), then re-securing each stabilizer at one end and each wing & a canard at both ends. Burt Rutan states, this takes “1 person about 10 minutes” because “the wings have skateboard wheels on them”.
b). Carplane(R) entry is via large side-doors. BiPod entry involves climbing over the cabin wall.
c). The Carplane(R) is flown & driven from the same cockpit. BiPod drivers/pilots must change cabins to continue onward after mode-change.
d). In Road-Mode, the Carplane(R) generates road-hugging “downforce” (negative-lift) via its canard and via the wedge-like slant of its wings. Its tail plane veers downward to act like a spoiler. Burt Rutan states the BiPod’s mid-wing flap creates downforce in road-mode. However, the wings stored underneath would appear to prevent any Bernoulli effect on the underside. And on the upper side, the flap appears to merely deflect – like an air brake. [Its canard and fins over the rear wheels would appear to create downforce.]
e). The Carplane(R)’s wings are stored tips low/forward and shielded by the canard, thereby reducing drag. The BiPod’s wing-roots face perpendicular (flat) to the oncoming airflow,
f). The Carplane(R) conforms to road regulations in regard to wheelbase & overhang. The Bipod’s rear overhang exceeds 50% of its wheelbase. A 50% limit prevents “weathercocking”.
g). Large flaps migrate the Carplane(R)’s Center of Lift (CL) aftward, thereby facilitating rotation around the rear axle. Its empennage extends rearward, thereby shifting weight aft and lengthening the moment-arm for improved elevator-leverage. Its elevator is large and its effectiveness is enhanced by its location directly in the prop-flow. Its canard augments nose raising and is enhanced by ground effect.
The BiPod has battery & occupants fore of the Center of Lift. Its rotation is impeded by an unusually oblique angle between its Center of Lift & the rear axle (65Åã). Elevator area is reduced (to make room for motor-mounts), thereby reducing pitch-force. Elevator effectiveness is reduced further due to limited prop-flow under the horizontal stabilizer (because its prop-arcs must remain clear of the wing storage area below). Burt Rutan states that “a burst of wheel-power causes rotation”.
He does not explain how sufficient traction is attained near maximum lift. (Note: The BiPod’s wheel- powered runway-hops (“wheelies”) were performed with only one occupant on board at low speed. Taxi tests are normal. We wouldn’t mention it if Burt hadn’t stated, “I never show an aircraft until it’s flown”.)
h). The Carplane(R)’s conventional 26Åã CL/rear-axle angle enables normal flare and touchdown. The BiPod’s unusually oblique CL/rear-axle angle (65Åã) predisposes it to slap-down landings (accentuated further by elevator-size (small), elevator-moment (short) and nose-end weight).
i). The Carplane(R) has 15” block-section auto tires (block-section is required for road certification of a 4-wheeled vehicle). The BiPod has 10” (front) & 12” (rear) rounded-section motorcycle tires, thus reducing brake-effectiveness and hampering cornering (due to lack of tilt).
j). Architects design garages & parking spots to accommodate “compact-cars” [defined as
2.2mW/5.5mL/2mH (7’2”W/18’L/6’5”)]. The Carplane(R) conforms to compact-car size. The BiPod exceeds it in both width & length.
k). The Carplane(R)’s seating dimensions are identical to a 3-series BMW. Elbow grooves enable normal armrest width. The BiPod has prone seating, no side elbow-space and occupants share the cabin with a wheel-well and a lithium battery.
l). The Carplane(R) has a front bumper-bar, front wheels which deflect the cabin upward upon frontal impact over an oncoming obstacle, a rear shape which deflects the vehicle upward upon rear impact and a tubular-steel cocoon around each cabin. The BiPod cabin has a battery at chest height in front of each occupant and a motorcycle engine directly behind.
m). The Carplane(R) has medium aspect ratio and a high-wing configuration enabling wing-crab for a relatively high crosswind component. The BiPod has only slightly longer wings at mid-height, limiting crosswind capability.
n). The Carplane(R) is designed for roadable LSA certification (max. 650kg/1430lbs., 120kts./136mph).
It has a non-laminar-flow airfoil, low stall speed and high stability (laterally: due to twin-hulls,
dihedral & high wing/pendulum-effect; longitudinally: due to 3-surface configuration & centralized
weight). It has high control-responsiveness throughout its envelope due to the long rudder & elevator
moments enabled by empennage extension. It’s designed for low-experience owner-pilots. Scaled
Composites has registered the BiPod with the FAA as a glider. Burt Rutan cites its high aspect ratio
and claims it has LSA weight. Burt also cites 197mph cruise (LSA max. is 136mph).
o). The Carplane(R) has a single 1.65m-diameter, 2-blade prop covering an area of 5.18m2. Although
the split air-inflow increases noise, its location high & substantially between the fuselages yields a net
reduction of the noise-footprint. The BiPod has two 81.5cm (32”)-diameter, 4-blade props covering a
total area of 2.56m2 – less than half Carplane(R)’s. [Prop efficiency decreases with radius as
p). The Carplane(R) has inter-hull joins at varying heights and cross-braces to counter divergent torsional forces which act separately on each hull due to road vibrations, uneven surfaces and especially bumps. The BiPod’s hulls’ joins are all located at mid-fuselage although each hull is longer and farther apart, accentuating leverage.
q). The Carplane(R) employs a secondary spar to counter wing-torsion. The BiPod doesn’t.
r). Carplane(R) development in its current form started Nov. 2007 and was disclosed via Int. Patent Application on July 28, 2008. BiPod development started Nov. 30, 2010 and was disclosed July 15, 2011.
6). We encourage all roadable aircraft development as long as it doesn’t infringe on anyone’s IP or patents. We also welcome anyone proving a claim to have advanced the field of roadable aircraft and expanded the knowledge of the aviation community.
This statement spurs an intensive discussion about roadable aircraft as well as competing design models.
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