iPhone ships with 3-axis accelerometers and a touch
screen. iPhly uses these as primary inputs. Tilting the phone
controls pitch and roll--as if moving the stick in a full-size
airplane or the right stick on a Mode 2 transmitter. To increase
throttle, you swipe up on the screen. You swipe down to
decrease. Swiping left or right controls the rudder. This is
similar to the left stick. Similarly to the left stick on a
transmitter, throttle stays where you put it, but rudder returns
to center if you lift your finger off it. Rudder trim buttons
control where exactly it returns to.
I notice that I don't look at the screen much when I'm
flying--I get most of my feedback from observing the
model. Nonetheless, iPhly sports a spiffy UI that emulates an
attitude indicator in case you want to see how it interprets your
gestures. I (ab)use glideslope and localizer bars to indicate
throttle and rudder position.
I notice some pilots use both hands with iPhly--you can see an
example of this technique towards the end of the YouTube video. I
find it natural to use one hand. I move my wrist to control the
stick, and slide my thumb to control rudder and throttle. Having
all controls in one hand makes it embarrassingly easy to catch your
model in flight.
The Technology
iPhly connects an iPhone and an off-the-shelf RF module
together to control an R/C model. Most hobby transmitters
understand the PPM protocol. Pulse Position Modulation has its
origins in the late 1960s and enjoys wide implementation. It is
also a relatively simple pulse train--simple enough that an iPhone
can generate this waveform as an audio stream. It's almost, but
not entirely unlike software radio. You feed the signal from the
headphone jack into the module's input, and voila, you can fly
airplanes with your phone.
This approach requires minimal hardware "glue" between the
phone and the module. It bypasses the need to access hardware in
the phone that is not available to mortals. This approach also has
some limitations--most importantly, audio resolution limits servo
position resolution. iPhone ships with 44.1kHz audio and that
limits the maximum resolution to 44 steps per channel. As a
licensee in the Made For
iPod/Works with iPhone program, I'm working with Apple to
release a more elaborate device that communicates with the phone
using a serial protocol. My intention is to offer both types of
devices so that customers may choose the price/performance point
they prefer.
Compatibility
PPM is the common denominator in R/C. All RF modules accept it
as input, and most transmitters accept it as "buddy" or "student"
input. In this case iPhly replaces the student
transmitter. Depending on the module you use, you may need to use
simple components to match electrical input and output levels. The
Spektrum DSM module requires a single resistor. A buddy cord for
Hitec Eclipse requires two resistors and a transistor. In some
cases, the only thing you need is a stereo cable. JR and Spektrum
radios use 3.5mm audio plugs for buddy cords: you plug one end
into the phone, the other into the TX and that's it.
Here is what I have observed working so far or have reliable
reports of user success:
I welcome reports of users' successes or failures with other
equipment.
iPhly can also act as a joystick for Windows PCs through the SmartPropoPlus
driver. This setup requires only a stereo cord from the phone's
audio jack to the PC's microphone or line input.
Status
iPhly is in active beta testing. Blade mCX helicopters and
Vapor airplanes are flying regularly with this system. If you wish
to become a beta tester, you can see the requirements and
conditions on the beta-testing
page.
Legal
I am not a lawyer. I share the following as my best
understanding and encourage everyone to stay within their
understanding of their local laws and regulations.
In the United States, FCC certifies 2.4GHz modules as complete
transmitters. The encoder you connect to a module is irrelevant as
far as the radio operation goes--this is how Spektrum can market
their modules for use in Futaba transmitters. This means that you
can legally use any 2.4 module with iPhly, including Spektrum DSM,
Futaba FASST, Hitec Spectra 2.4 and a host of Chinese
modules--Assan, Corona, FrSky, FlySky, FlyDream, etc.
FCC certifies 72MHz and 27MHz modules together with their
antennas, which are part of the transmitter enclosure. This means
that even though you can physically plug a 72MHz module into
iPhly, connect any 110cm antenna to it and have it fly your 72MHz
receiver, FCC regulations prohibit such operation. However, you
can feed the iPhly signal into a buddy-box connector of your 72MHz
transmitter and fly that way. If you are a Ham radio operator, you
can use 50MHz modules with iPhly.
Important! Beyond legal issues, operating an RF module without
an antenna is likely to damage it or at least heat it up
severely. If you use a MHz module with iPhly, it is your
responsibility to connect an appropriate antenna to the relevant
output.
