Re: [mfnren] BASE specific gear
Ren, this is a flight computer based on iPhone and
this idea of using an accelerometer to measure L/D. By its principle, accelerometer axis needs to be aligned with the relative wind, hence the vane and the long pole that extends it into undistorted airflow.
I'm going to write a bit more detail for those who might be interested, since up to this point it's not been widely known yet.
L/D Magic app, with the first accelerometer-based prototypes tested in 2011 and launched to the App Store in 2013, is an app that goes beyond just measuring g1 and g2 as explained in the link above - that would only allow measurement of lift-to-drag ratio, and it would be very noisy by the virtue of dividing one noisy number by another; it actually
blends measurements from various sensors via their physics interdependencies and magically synthesizes the internal parameters of the system - flight parameters like lift and drag coefficients, absolute lift and drag forces, horizontal and vertical momentary and sustained airspeeds (even when you're not in sustained flight), and, of course, L/D and GR. To do its magic, it uses the mathematical technique called
Kalman Filter, the same as used in inertial navigation systems in missiles, drones, satellites, etc. Its input is noisy measurements from various sensors; "inside" the filter is the physics of the process - differential equations of time evolution of various internal variables describing system state; and its output are these internal variables, smoothed in a statistically optimal way.
L/D Magic can operate in two principally different modes - one is using relative wind and iPhone's accelerometer as described above, the other is using GPS (in which case the vane and the pole are not needed). Each has its pros and cons.
The pro of the former is that it's totally independent of the wind, it only depends on
relative wind. So, it solves the biggest issue with GPS-based glide ratio measurement devices - with them, you never know what your true GR relative to air is, let alone what true L/D is. It also solves another problem, inherent to GR: in hunt for better GR you might think that you've found a better body position while in fact you're simply bleeding your speed and trading it for better glide without realizing that you're ever slowly slowing down and inevitably will end up in a "stall". L/D Magic measures true L/D, which is much better indication of the flight regime than the current GR (which you see with your eyes anyway, if you're not 10,000ft above the ground - GR simply shows where your impact point is - the "accuracy trick" we all know from canopy piloting). The con here is that using only accelerometer it cannot reliably measure the speed, because the speed is essentially integrated by Kalman Filter from initial conditions and acceleration measurements, but drifts due to noise adding up in the long run and causing calculated speeds stabilize at wrong values. (To solve this problem, I'm working on a more advanced Kalman Filter that uses not only accelerometer and gyro, but GPS and compass as well.)
This is an example of recorded flight output data processed with Kalman Filter that uses only accelerometer:
The pros of the latter are that you don't need a vane and a pole, you simply put GPS on your helmet (note that the internal GPS in iPhone is not good for our purposes, as I already talked about it
here). And it measures speeds more accurately. The cons are, a) it depends on wind being nil (which is rarely a case in skydiving, but more frequent in BASE) - this is because GPS cannot distinguish between ground speed and air speed; and b) it doesn't give as precise L/D as the vane-based models, because GPS measurements are less accurate and the max update rate is only 5-10Hz compared to 100Hz of accelerometer and gyro.
Currently, L/D Magic is compatible with these external GPS units:
- Dual
XGPS150A (5Hz) and
XGPS160 (10Hz)
-
UltiMate GPS (5Hz)
Not released yet, but tested, is the support for
Bad Elf GPS Pro (10Hz + logger)
Also (although I haven't tested yet, but have reasons to believe they should work) Bad Elf's GPS's for 30-pin and Lightning ports - like Emprum, they pass data directly to Location Services in iOS and thus should work out the box with pretty much any app using location.
(full disclosure: I'm not affiliated with any of these companies/products other than using their SDK's to enable my app to work with their hardware)
L/D Magic is the culmination of several years of R&D, starting with discovering
Wingsuit Equations in 2006, first realizing the idea using RC model airplane data logging hardware (
Z-Device) in 2007:
analog L/D Meter in 2010:
and now, we have these powerful computers in our hands, with accelerometer, gyro, magnetometer, GPS, barometer, beautiful screen and wireless sound, capable of running a military-grade Kalman Filter in real time at 100Hz update rate... The future is here. It's magic... pure fucking magic!
Yuri
P.S. If anyone (or knows someone) would be interested in designing and running production of vane mounts for various models of iDevices (iPhones 5 and 6 and iPods 5th gen) and later, Android devices, please drop me a line at
ldmagicapp at gmail dot com. (It might be as simple as printing them on a 3D printer.) To get the idea how it looks and works like, see
https://vimeo.com/108326309 and
https://vimeo.com/108428363. And if anyone uses an external GPS with at least 5Hz update rate, compatible with [nonjailbroken] iPhone running iOS8 or later, and wants to see the support for it added to the app, let me know, too.