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There is currently a big shift from internal combustion engines to electric motors
for powering model vehicles of all sorts - airplanes, helicopters, boats, and cars
- and of all control modes - autonomous (free flight), radio control, and control-line.
The state of motor and battery technology has passed the point where the weight
and thrust available with electric power meets or exceeds that of engines for most
applications. Costs are pretty much at parity as well when you compare engine vs.
motor and fuel vs. battery acquisition and cost of ownership over the life of the
power system.
All sorts of useful electronic peripheral equipment has been developed for use
with electric motor power: programmable electronic speed controllers, motor cutoffs
based on altitude and/or elapsed time for free flight, motor timer/speed controls
for control line, and even engine noise generators to give life-like sound to otherwise
eerily quiet war birds and commercial transports, to name a few. These devices had
made the switch to electric power nearly seamless for most flyers. There is probably
little demand for a spray bottle of burnt fuel residue for coating the model after
a session, but I personally would like an of Eau du Running Fox 35 air freshener
just for the sake of nostalgia.
As a life-long control line model airplane flyer with only sometimes moderate
aerobatic success, I have followed the writings and videos of modelers who have
mastered the arts of building, building, flying, adjusting, and even repairing contest-grade
stunt machines. I remember being in awe the first time I heard a perfectly-adjusted
engine break into a high-speed 2-cycle mode when the model pulled upward from horizontal,
settled back into a richer 4-cycle mode on a downward path and back to horizontal,
and then repeated the sequence maneuver after maneuver. The practice is commonplace
today, but I have to wonder who was the guy who first decided to exploit the phenomenon
to make a more perfect flight?
The 2- vs. 4-cycle alternation is due, per my understanding, to the gravitational
effect on the fuel flow rate to the carburetor when the engine and fuel tank are
in various positions. In a nose-high attitude the fuel in the tank and in the fuel
line are lower with respect to the carburetor than when in a horizontal position,
so more suction (vacuum) is needed in the line to provide the same rate of fuel
flow. For a given engine speed (RPM) the suction is constant, so the greater weight
of fuel in the line does not flow as quickly, resulting in a leaner (faster) engine
run. As the engine increases in RPM the suction will increase accordingly, providing
more suction to counter the weight of the fuel. Depending on where the fuel mixture
is set, that could cause the engine to oscillate between 2-cycle and 4-cycle operation
in that transition area. However, because everything is happening so fast, there
is no time for that to occur. Besides, all maneuvers are actually flown in an increasingly
upward arc - even the triangle when viewed in the 3-dimensional path it is flying
on the surface of a hemisphere - so the fuel weight is continually increasing as
the angle of flight (not necessarily angle of attack) is continually increasing.
As the model transitions from vertical back to horizontal at the top of the maneuver,
the weight of the fuel is returned to normal so the engine kicks back into the 2-
and 4-cycle threshold realm. Then, as the nose of the model goes down, the fuel
mixture is richened additionally and a slow-running engine is the result. The whole
process helps keep the airspeed of the airplane more constant throughout the maneuvers.
So, with the advent of electric propulsion that easily provides sufficient thrust
for the entire stunt aerobatic pattern, the control line community is now living
with a constant thrust output at all flight attitudes from the power plant. I have
not read in, for instance, Bob Hunt's column in Model Aviation whether this is a
problem. Masters like Bob can no doubt adapt to any situation, but then again electric-powered
models have not been running away with all the top prize positions. Maybe the absence
of automatic variable thrust is one of the reasons. I should write to Mr. Hunt and
ask him, but maybe someone will read this article and ask him for me. My question
is why hasn't someone designed such a motor speed control yet that detects the models
attitude and adjust the motor speed accordingly?
I'm throwing this idea out to companies like
Winged Shadow Systems, who make some ingenuous peripheral products like the
How High altimeter and the
Thermal Scout thermal detector, and the
Sky Limit altitude/time limit motor cutoffs. Surely those guys can design and
affordably market a dynamic, attitude-aware motor control for electric-powered control
line airplanes. I provide here a basic outline of the concept, what I title "A
Programmable Dynamic Attitude-Aware Motor Speed Control for Electric-Powered Aircraft©." While its indented initial application
is for control line aircraft, it is possible to extend the usage to free flight
and other modes of flight.
The first thing that would be needed is a device capable of sensing the airplane's
attitude; i.e., whether it is climbing, diving, or in level flight. That information
would suffice to command two or three three unique motor speeds, which could easily
be programmed by the user in terms of percentage of full speed or as an absolute
number. The controller would convert the speed data into the pulse train format
required by readily available electronic speed controls (ESCs). A simple
mercury-filled
or electrolytic tilt sensor can do the job for providing two or three discrete motor
speeds. If you want a continuously variable speed controller that can respond to
all attitudes throughout a 360° rotation (rather than just two or three), a
more sophisticated sensing device is required (an
Tilt Sensor IC, often based
on MEMS). Any attitude sensor will need to have a response time fast enough to keep
up with the model's flying speed and be able to operate properly even under conditions
other than an acceleration of 1 G toward Earth's center (the direction we normally
experience).
A good place to start looking for motions and position sensing gear is the robotics
websites. Robotics hobbyists have found all sorts of nifty parts for sensing and
controlling their platforms. The radio controlled multicopters (quad, penta, hexa,
etc.) that are extremely popular now are chock full of low-cost, high accuracy sensors
for detecting tilt and acceleration on all three axis. ROHM Semiconductor is well-known
for its sensors and happens to manufacture the
RPI-1040, a 4-direction detector that measures 3.1 x 3.1 x 0.8 mm,
weighs next to nothing, and has a 10 µs response time. Because the flight
path is always on the surface of a 3-dimensional hemisphere, two orthogonally-mounted
tilt sensors would likely be required with a microprocessor algorithm that receives
an orientation fix at power-up with the model sitting on the ground with wings level.
The conceptual block diagram below outline my idea.
Please contact me if you are interested in pursuing the design and testing; I
do not have the time or equipment necessary to tackle it on my own.
Here is the text of the above conceptual drawing:
Programmable Dynamic Attitude-Aware Motor Speed Control for Electric-Powered
Control Line Aircraft©
Kirt Blattenberger -- Copyright December 30, 2012
This concept for a programmable dynamic attitude-aware motor speed control for
electric-powered aircraft uses one or more sensors to detect and report the aircraft's
3-dimensional attitude to a microcontroller, which subsequently calculates an appropriate
rotational speed for the motor (rotations per minute, e.g.) and sends an appropriately
formatted pulse train to the external electronic speed control (ESC), although an
on-board ESC may be incorporated. Motor speed settings may take the form of an absolute
number or a percentage between 0 and 100%. The user programming interface may consist
of a set of onboard switches and/or an off-board serial electrical interface and/or
parallel electrical interface. An ability to store and recall one or more programs
should be provided without requiring an external power source to be connected. Although
any response to the tilt sensors may be programmed, the primary intention is to
detect whether the aircraft is in horizontal flight or whether it is climbing or
diving, and then command the motor's speed accordingly in order to effect a flight
speed throughout a maneuver that is more consistent than would be with the motor
running at a constant speed. The process would attempt to emulate an internal combustion
engine's ability to switch between 2-cycle and 4-cycle modes (thereby changing engine
RPMs) when assuming various attitudes, although other uses are possible.
Posted September 10, 2022 (updated from original post
on 12/30/2012)
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