THE CARTERCOPTER
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Significance of μ-1
and the Technical Issues Involved
At 7:40 AM on June 17, 2005, while flight-testing
for a U.S. Army contract out of Olney Airport in Texas, the CarterCopter
reached μ-1 (Mu-1). This is the first time in history that any rotorcraft
has reached μ-1.
The
condition was achieved during normal flight-testing while collecting data on a
newly developed speed controller for the rotor. The milestone attempt was not
planned but evolved as flight-testing proved the rotor to be very stable as the
rpm was decreased.
Test pilot, Larry Neal, was decreasing rotor rpm in
small increments when he neared μ-1. With all systems stable the decision
was made to proceed to μ-1. Data from the flight shows that the airspeed
was 170 mph and the rotor was slowed to 107 rpm giving a μ value of 1.
Previously, the lowest rotor speed achieved was 115
rpm. The μ-1 flight time was just 1.5 seconds before Neal reduced the
throttle to slow the aircraft, but the aircraft was operated continuously above
μ 0.9 for over 20 seconds, and the high μ flight was accomplished
without incident. The pilots commented that the aircraft was so smooth that
there was no vibration or noise to indicate that they were in a rotary wing
aircraft, let alone one flying at 170 mph with the rotor slowed to 107 mph.
The CarterCopter is the prototype aircraft of
Carter Aviation Technologies (Carter). It is the technology demonstrator of
Carter's Slowed Rotor/Compound (SR/C) Aircraft Technology and has been in
flight-testing since 1998.
This historic flight was the culmination of more
than 12 years of research and development. According to Jay Carter, "This
[reaching μ-1] has been our goal since we first began flight-testing in
1998. To prove our technology we needed to do something that no one else had
ever done. We have had several setbacks, but no one on the team ever lost
faith."
Significance
of μ-1
As the rotor rpm/tip speed is reduced (mu
increases), the drag on the rotor decreases dramatically. As an example to put
this in perspective, if the rotor speed during helicopter takeoff mode is 300
rpm, and this is reduced to 100 rpm in high speed cruise where a wing can
provide the lift, the drag on the rotor is reduced approximately by a factor of
27. Once the rotor rpm has been slowed, the rotor drag is basically a function
of the rotor blade area, which is very small relative to the area of the rest
of the aircraft.
Because the rotor provides the lift for VTOL and
slow speed flight, the wing can be sized for high speed, reducing the wing area
by a factor of 2 to 4 under what would be needed if it had to support the full
weight of the aircraft at those slow speeds. This combination of a small, low
drag wing with a slowly turning rotor is known as a Slowed Rotor Compound
(SR/C) aircraft, and combines the efficiency and speed of conventional fixed
wing aircraft with the VTOL and slow speed flight of helicopters.
Aerodynamicists have known since the 1930's about
the benefits of SR/C. In the mid 50's and early 60's, both the U.S. and British
governments funded SR/C research (with the McDonnell XV-1 and Fairey Rotodyne,
respectively). Both did remarkably well for first prototypes as they both
achieved forward speeds of around 200 mph (the Rotodyne even set the official
world speed record for rotorcraft), however there were a number of problems
that many thought were unsolvable, or at best impractical to resolve. It was
this thinking that lead to the focus on tilt rotor development (V-22 Osprey).
In hindsight, it can be said that those early SR/C projects were abandoned too
soon.
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