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| Figure 1. Consumer-grade coaxial rotorcraft UAS. |
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| Figure 2. Gyrodyne QH-40 DASH. |
The Gyrodyne QH-50 DASH (Drone Anti-Submarine Helicopter)
was the first vertical takeoff and landing (VTOL) UAS, which is presently one
of the most ubiquitous designs among consumer and professional users alike. The
original QH-50 design was developed to carry a single reconnaissance Marine via
a coaxial rotorcraft vehicle. In 1960 the design was changed to an unmanned
vehicle and the QH-50 began service. President John F. Kennedy personally
approved the aircraft program after observing a demonstration in 1962. The
aircraft consisted of two counter-rotating lift surfaces rotating on a single
axis. A reciprocating Porsche engine was originally used to power the craft,
but a turboshaft engine was eventually used in it’s place. The aircraft was
designed to carry two anti-submarine torpedoes and could also be fitted with
transmitting cameras for remote reconnaissance. The QH-50 was used extensively during
the Vietnam War for reconnaissance and spotting of naval gunfire (observing the
accuracy of impact to assist accurate corrections) (Evans, 2011).
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| Figure 3. Aerialtronics Altura Zenith ATX8 |
Modern rotorcraft UAVs come in many shapes and sizes. One
design that has gained recognition in professional and heavy-lift applications
is a coaxial multi-rotor. In this application, an even number of electric
motors are placed in counter-rotating pairs at equal axis around a central
airframe. An example is the Aerialtronics Altura Zenith, an octo-rotor in the “X8”
configuration (four main arms with a coaxial pair at each end). A flat-axis quad-rotor
aircraft is considered by many to be the most efficient design for simplicity,
low power consumption, and low weight (Bouabdallah, Becker, and Siegwart, 2007).
However, for professional videography and other applications, safety is an
important consideration in the engineering design process, specifically in redundancy
of controls.
The main similarity between the two systems is the use of
coaxial rotor technology. Coaxial rotors counteract the main problem of torque
in traditional single-rotor helicopters. They also prevent a condition called “retreating
blade stall” that occurs at high forward speeds. A 1997 NASA Technical Paper by
Colin Coleman found that a coaxial rotorcraft system “requires 5% less power
than an equivalent single rotor.”
The main difference between the QH-50 and Altura Zenith
(besides the obvious size difference) is the flight control, specifically in
the yaw direction. Yaw of a rotorcraft is accomplished by increasing or
decreasing relative airframe torque in opposition to the direction of the
rotor, which turns the aircraft. However, the QH-50 was a turbine-driven
aircraft and could not easily speed up and slow down the rotors to create the
torque effect needed to turn. The solution was to install flaps on the blade
tips that could be deployed to quickly slow the rotation of a rotor and
decrease the torque in that direction. Modern electric multi-rotor aircraft
such as the Altura Zenith are much more responsive. Torque effect is induced by
quickly speeding up and slowing down rotor pairs via computerized control of
motor speed.
Electric VTOL aircraft are still relatively inefficient and
do not have the endurance of fixed-wing aircraft. One technology that could be
applied to future unmanned rotorcraft is called “slowed rotor/compound,” where
a main lift rotor is slowed during forward flight while the aircraft is driven
forward by a propeller. It is different from an auto gyro aircraft in that the
rotor is powered. This would allow true VTOL access to small landing areas
while maintaining the speed and efficiency of fixed-wing flight (Carter
Aviation Technologies, 2016).
References:
Bouabdallah,
S., Becker, M., & Siegwart, R. (2007, September). Autonomous miniature flying robots: coming soon! - Research,
development, and results. Robotics &
Automation Magazine, IEEE, 14(3), 88-98. doi: 10.1109/M-RA.2007.901323
Carter Aviation
Technologies. (2016, April). Carter
Aviation seeking partners for UAS business development. [Press release].
Retrieved from http://www.cartercopters.com/pdfs/ CarterCopterBrochure-June2015.pdf
Coleman, C.P.
(1997). A survey of theoretical and
experimental coaxial rotor aerodynamic research (NASA Technical Paper
3675). Moffett Field, CA: Ames Research Center.
Evans, S.S.
(2011). The incredible story of the QH-50 DASH – The first unmanned helicopter
turns 50. Vertiflight, 57(1), pp.
36-39. Retrieved from http://www.aero.psu.edu/Facilities/ images/36_DASH_QH-50.pdf
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