Curtin is the Vice President of AeroVironment, Inc.
and Director of its Aircraft Design and Development
Center, which he joined in 1980 at age 15. He worked
part time through high school and college and began
full time work in 1988.
1996, he received the Aviation Week Laurel Award, and
was inducted into the Aviation Week Hall of Fame in
1997. His solar aircraft teams also won the NASA Group
Superior Achievement Award in 1999, and his Micro-UAV
teams won awards from the Defense Advanced Research
Projects Agency (DARPA) and the Association for Unmanned
Vehicle Systems International, also in 1999. He holds
multiple patents for aircraft in his area of expertise,
as well as a B.S. and M.S. in Physics from California
State University Northridge.
and his wife, Bonnie, live in California with a Yorkshire
Terrier named "Hamlet" and two cats. In his free time,
Bob enjoys working on automobiles, mountain biking,
building & flying model airplanes, and fiddling with
his personal computer.
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Curtin responds :
Bill Pawley asked:
your solar powered wing, it appeared that take-offs
and landings can create some of the greatest windows
of stress and difficulty. Did you consider launching
in mid-air at high altitude (as an example: disconnecting
from a series of attached weather ballons)to reduce
launch stress and/or difficulties? Did you consider
using a lighter-than-air-gas to fill in void spaces.
I know it took countless hours of sweat, tears and dollars
to get where you are; but you made it look like fun.
are correct that if we compare stresses in the structure
when the aircraft is flying (supported by aerodynamic
lift) to when it is resting stationary on the ground
(supported by its landing gear), they are very different.
Of course the landing gear structure is designed for
landing loads. Even though the loads in the wing are
significant while the aircraft sits on the ground, it
turns out that loads induced when the aircraft flies
through turbulence drive the design of most of the wing
and landing are always the most difficult portions of
an aircraft's flight. Of course we hope to limit our
exposure to these events by designing an aircraft that
stays aloft for up to six months. By landing infrequently,
we will be able to carefully select our take-off/landing
time and location to avoid bad weather. We have looked
at launching small airplanes from weather balloons.
However, it would be difficult to gently lift something
as large as a 250-foot span solar powered airplane with
weather balloons. Also, the transition from balloon
to aerodynamic lift would be tricky to manage.
at sea level weighs about 0.1 pounds per cubic foot.
At the cruising altitude of Helios, about 60,000 feet,
air weighs about 0.01 pounds per cubic foot. The volume
of the Helios wing is about 1000 cubic feet. Evacuating
all the air from the wing, would provide about 100 pounds
of lift at sea level and 10 pounds of lift at 60K feet.
Of course it would be very difficult to evacuate the
wing. If, as you suggest, we replaced the internal air
with helium, we would get 90 pounds of lift at sea level
and 9 pounds of lift at 60K feet. A sample of air is
about 10 times heavier than Helium if both samples are
the same volume and at the same temperature & pressure.
Unfortunately the weight of the seals and systems required
to contain and manage the Helium would weigh many times
the 9 pounds of lift achievable at 60K feet.
Chad Fritz asked:
the 'Eternal Wing' portion of 'Flying Free', there was
a brief mention of utilizing fuel cells as a secondary
power source. When do you think they'll have a fuel
cell system suitable for your application? What component
of the fuel cell poses the largest hurdle? I thoroughly
enjoyed the program and would like to salute you on
your many accomplishments.
Chad Fritz (real-life Okie from Muskogee)
are developing a energy storage system (ESS) that will
be charged with excess solar power available during
the day and discharged during the night to provide power
to keep the aircraft aloft. Under our NASA program,
we are currently scheduled to perform multi-day flights
in the summer of 2003. The ESS is often described as
a fuel cell system or even as a "fuel cell". However,
this name is misleading because the fuel cell is only
one of many subcomponents that make the ESS possible.
It will not be possible to attribute the ultimate success
of the ESS to any one subcomponent; there are several
challenging subcomponents and good engineering of the
overall system is extremely important. The good news
is that no new basic physics or chemistry needs to be
discovered or developed. Fuel cells have been in use
for decades. The largest hurdle is properly balancing
development efforts between the subcomponents such that
the resulting ESS is optimized for the solar aircraft.
Dave Johnson asked:
am an engineer and private pilot. My question is about
the Helios aircraft: What certificate does the operator/remote
pilot have and how do you stay in contact with ATC?
Does it have a 'fail-safe' mode should it lose ground
control? Other high flying aircraft such as the U-2
reach a point where the stall speed approaches max speed.
Does Helios approach this condition?
There are currently no laws that define a specific certificate
a remote pilot must have. However, it does seem that
eventually remote pilots may be required to have a certificate
similar to the certificate a pilot needs to fly on instruments
alone. Usually the remote pilot of Helios talks to air
traffic control through a normal phone line. However,
if the remote pilot is within radio range of air traffic
control, then he uses the standard air traffic frequency
used by other commercial or military traffic. Helios
flies so slow that it doesn't approach a point where
stall speed is the same as maximum speed. I think the
U2 has this problem because stall speed starts to approach
the Mach limit of the airframe. Even as high as 100K'
the stall speed of Helios will be about 200mph, which
is well below the speed-of-sound.
Keith Tacia asked:
was particularly interested in the flying wing part
of Scientific American. If these planes are supposed
to fly for six months, what happens if one or two of
the engines go out? Does the plane start to lose altitude,
do you have time to send up a replacement wing to bring
the damaged one down? Also, wouldn't adding communication
equipment for phones and television add too much weight
to the craft?
The electric motors that spin the propellers are mechanically
very simple. From a mechanical perspective, they are
similar to a conventional ceiling fan. Simplicity helps
us make very reliable motors. There are enough motors
on Helios that normal flight can be continued even if
one or several motors fail. The remaining operating
motors will simply work a little harder to make-up for
failed motors. After a motor or two motors fails, another
aircraft will be sent to replace the one needing service.
is designed to carry a 100Kg communications payload.
The payload will require careful design to package a
large communications capability within the 100Kg weight
budget. The payload will be similar to others currently
launched on satellites. However, Helios flies one-thousand
times closer to earth than a geo-stationary satellite.
This means that it will take one-million times less
power to send radio signals to and from Helios (the
power received by a radio-frequency receiver is inversely
proportional to the square of the distance to the transmitter).
The reduced power requirement will enable Helios to
carry small, light-weight radio transmitters.
Robert Sterchak asked:
really enjoyed the "Flying Free" episode of Scientific
American Frontiers. One thing that I was wondering about
was the material that you use, not only to create the
large flying wings, but also for the Small "flapping"
and "gliding" planes. I appreciate any information you
could provide. Thank you.
The type of materials used depends very much on the
airplane design. Small flapping aircraft that flap fast
are usually covered with a Mylar type film that is about
one-thousandth (0.001") of an inch thick. Very lightweight
gliding aircraft can be covered with much thinner materials
that are a fraction of one-thousandth of an inch thick.
Main structural members are often made from balsa wood,
Styrofoam, graphite, and/or fiberglass. I recommend
that you visit your local hobby shop to get information
and materials required to build small aircraft. Picking
the ideal material for each piece of an aircraft's structure
is important because the power required to fly goes
up proportionally to weight to the three-halves power.
The key items that influence the design of an airplane
are its mission, the payload weight it needs to carry,
and the type of propulsion system selected. Typically
these influencing factors are expressed as a list of
requirements in the preliminary design stage of the
design of a new aircraft. Preliminary design is probably
the most important phase of the design process.
Teresa Lynch asked:
enjoyed Scientific American tonight and its feature
on your escapades. My husband is a sculptor and has
been building ash-wood armatures that are oftentimes
suspended. One of his sculptures is an imaginary plane
and is hanging at the Portland, Maine Jetport. His latest
series requires covering the ash armatures (sculptural
shapes) with a transluscent/transparent gossamer-like
material that can be made to be snug and conform to
a variety of shapes. The material on your gossamer condor
looks exactly like what he might want. Can you tell
us what type of material it is and what manufacturer/outlet
might have it available Thank you very much.
And keep flying,
I actually can't tell you specifically the types of
plastic film we use, because we consider the information
proprietary. Also, we have used many different covering
materials over the years. I don't recommend that you
use the same covering that was used on the Gossamer
Condor, because its strength degrades rapidly when exposed
to sunlight. Degradation in ultra-violet light is probably
a factor you want to consider for a sculpture that will
be on permanent display. I recommend that you inquire
at your local hobby shop to get samples of plastic films
available from several manufacturers.