IEEE Electrification Magazine - March 2016 - 52

A comparison of the available specifications of various
flying cars is given in Table 1.

Flift

Propulsion System architectures

Fdrag

Fthrust

W
Figure 11. The forces sustained by the flying car during cruising.

This value of climbing time fulfills the requirement of the
design goal. In this case, the lift force (the propulsion force
from the engine) needed is
FL = mg + ma = 16.40 kN.
Induced power is the power required to produce lift.
Assuming the efficiency (h) is 0.80,
F
PInduced = hL VC = 123 kW.
When the flying car is in cruising mode, the forces sustained by the flying car are shown in Figure 11, where
Flift = W;
Fthrust = Fdrag .
From Flift = W, the lift coefficient C L can be obtained. The
lift is always larger than drag in a typical flight condition,
thus making the thrust required to overcome the drag less
than the lift produced. For this class of vehicles, the ratio of
lift coefficient to drag coefficient is in the range of 10-15.
Assuming C L = 12C D, we get
Fdrag = 0.5 * C D * t air * Vcruise2 * S,
where S is the reference area that is the wing area and t air
is the density of air.
Parasite power is the power required to move the airframe through the air. It is the power required to overcome parasite drag. The power of parasite is
Pparasite = Fthrust * Vcruise /h = Fdrag * Vcruise /h.
For the selected design example, we get Pparasite = 134 kW.
The rotor profile power is the power required just to
turn the rotors. It is the power required to overcome the
rotor aerodynamic drag force. Assuming the rotor profile power is the same as the induced power, the total
power needed is
Ptotal = PInduced + PParasite + PRotor = 380 kW.

I E E E E l e c t r i f i cati o n M agaz ine / March 2016

For a large vehicle such as the U.S. Defense Advanced
Research Projects Agency (DARPA) Transformer vehicle
(http://mashable.com/2014/02/11/darpa-transformerdrones/), a separate propulsion system could be used for
lift, flight propulsion, and ground propulsion. However,
for smaller vehicles, using a separate electric/hybrid system will add additional weight, volume, and cost. In addition, packaging the individual systems into a car could be
a major challenge. Also, if the thrust required for vertical
lift is provided by the same engine (or motor) that is used
for cruising, this engine will be too large and heavy to
have higher efficiency during cruising. The thrust mismatch will result in a higher energy requirement and,
thus, more fuel consumption, leading to a more limited
flying range. Hence, depending on the performance and
range requirements, separate engines may be required
for cruising and VTOL operation.
A simplified propulsion architecture for a flying car is
shown in Figure 12. The engine is used for providing the
ground propulsion and for the flight propulsion. It will not
provide the VTOL capability. For the ground propulsion, the
propeller is disconnected using the clutch, in which case it
works similarly to an automobile. During takeoff and flight,
only the propeller is driven from the engine.
The automotive hybrid propulsion can be extended to
flying cars by including the capability for flight and for
VTOL, as shown in Figure 13. For ground propulsion, an
auxiliary power unit (APU) consisting of an engine-driven
generator and a power converter is used for charging a battery that provides electric power to the motor driving the
wheels of the vehicle. This is similar to the architecture of a
series hybrid automobile. A turbine engine/internal combustion engine is used for VTOL and for flight propulsion.
During VTOL, the engine, together with the gearbox, drives
the vertical takeoff fan. Once the cruise altitude is reached,
the engine drives the propeller for the horizontal flight. The
smooth transition from vertical takeoff to horizontal flight
is achieved by separately controlling the tilt of the vertical
takeoff fan.
The APU function in Figure 13 can be combined with the
starter/generator operation of the engine, and the propeller
can be driven electrically as shown in Figure 14. Initially, the
engine is started using the battery power, and the active rectifier unit functions as an inverter to convert the dc voltage to
variable voltage and variable frequency to provide the electric power to start the engine. Once the engine accelerates,
the electric machine acts as a generator and provides power
to charge the batteries. The inverter unit converts the battery
voltage to ac to provide the power to the propulsion motor
for ground propulsion. For the flight operation, the same dc
power from the active rectifier output can be used to drive
the propeller using an electric motor and an inverter. For

http://www.mashable.com/2014/02/11/darpa-transformer

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2016