How fast am I flying?
This question prompted David Walden to start working on his version of an airspeed indicator for BASE…
Like everyone else I get my cues from visuals, noise, feeling and body position. But how good am I really at knowing my speed? The hero in me says I’m fine; reality says otherwise.
The more I jump, the less I seem to know, and last year I gave myself a scare by flying too slowly. In certain situations I’m quite good at convincing myself that I have enough speed… just to get over that ridgeline or pass the notch or whatever. A quick look at The List and you’ll see I’m not the only one who does this. The problem is that even though we train this stuff, we are not infallible.
So after a lot of procrastinating followed by a lot of work, here is my version of an airspeed indicator for BASE.
OK, so what is it?
It’s not a new idea, but something I’ve thought about for a looooong time…
An LED mounted on my helmet that changes color depending on my speed. Speed is measured by GPS, and normalized for sea level. Because I usually only jump in zero or light wind, the speed given is a rough approximation of indicated airspeed that could be got from a pitot tube.
The LED is angled towards my face and is bright enough to see in direct sunlight. I’ve mounted it just at the periphery of my vision.
The unit also has a removable SD card which holds a configuration file and stores log files.
So… there are only really three speeds I’m interested in:
- RED: Too slow! Any speed below my best glide speed is too slow
- BLUE: Best glide speed (max L/D) give or take a few km/h
- GREEN: Fast. Flying faster than best glide and with more energy.
How it looks with my (very rough) polar curve…
Speed Monkey in action
Below is footage from one of my test flights in Jan this year – pretty much my favourite jump in NZ, Shark’s Tooth. The jump begins with a transport to the entry point for the line. There’s a ridge-line you can cross to enter the line higher, which I’ve done in the past. But now I got a red light from the Monkey and so bailed to enter the line lower and safer…
Video: David Walden flying the Speed Monkey
Speed thresholds / colors
Thresholds for each color can be changed. The full range of colors are…
- DIM GREEN – Standby mode when not moving (so not distracting at the exit)
- FLASHING RED – Very low speed
- RED – Low speed
- BLUE – Best glide speed, give or take
- GREEN – Faster than best glide
- OFF – Very fast
However not all colors need be used, so for example I’ve got it set like this for myself…
- Dim green (standby) = less than 80 km/h
- Flashing red = 80 to 130
- Blue = 130 to 140
- Green = more than 140
Maybe I’m stupid, but I’ve found having minimal colors is more useful as I can process the info more automatically while everything is going on. Flashing red means danger, that’s simple enough for me.
The speed thresholds are set in a text file on the SD card, which can be easily edited.
Jumps are logged to the SD card automatically. 10 data points per second by default. Logging only starts once I’m moving and stops at the landing. What happens while I’m standing at the exit is that it continually buffers 2 seconds of data, so that once it detects I’m in freefall it can go back and write that buffer to the log. So the jump is logged completely from push to landing, and nothing more. The log file is compatible with the Flysight viewer.
True airspeed vs indicated airspeed.
True airspeed is your actual speed through the air. In zero wind this is the same as the total 3D speed given by a GPS.
Indicated airspeed is what you feel, and what is measured in aircraft with a pitot tube. At sea level (and standard air density) it’s the same as true airspeed, but at higher altitude its less. For example at 3,000m if your indicated airspeed is 150 km/h then true airspeed will be around 175.
We stall at a particular indicated airspeed regardless of altitude, so this is the measurement we want to know while flying. Speed Monkey takes GPS total speed, makes the assumption there is little or no wind, and using the altitude and a simple algorithm, converts it to indicated airspeed. The result is a little rough because atmospheric pressure and density are variable. More info here.
It won’t work well if there’s much wind, because GPS total speed will deviate from true airspeed.
The 3D speed of the GPS is logged as well as the indicated speed and the LED color. So I can use the log file to fine tune the speed thresholds where I think my best glide is.
It looks like a Flysight
This is not a Flysight. Flysight is a genius tool for training flying performance. This is for safety, it helps me avoid making some stupid mistakes.
I don’t use Flysight audio for BASE because I find it distracting. When everything is happening fast I prefer instant access to information using a visual indicator rather than audio.
- Malfunctions. Could be bad to get an incorrect green light. Need to use some common sense.
- Reliance. Could make me lazy about using my other senses for judging speed?
- Distracting? Depends on where it’s mounted and familiarity. I only check it occasionally… when I’m transporting, entering a line, going for glide and such.
- Skydiving. I don’t think it’s much use, because usually there’s some wind on a skydive.
- Attachment to helmet. Semi-permanent attachment using tape and double-sided sticky. A full face would need a different (smaller) LED ‘finger’.
- SD card. Log is written to SD which has to be removed to copy to computer.
This is my fourth prototype. It’s been way more work than I expected, but I’m stoked with how it’s turning out. Version one was the size of a small brick on my helmet. Version 4 is 5x5cm and 2cm thick. It needs more field testing, and by more people.
It would be nice to incorporate the antenna inside the box. I found it was affecting the accuracy, and I don’t know much about RF so for now it’s outside.
The next step would be to design a printed circuit board. It’s a big step (for me at any rate), and I need to see what sort of interest there is for this first.
- Atmega32u4 8MHz microcontroller
- Ublox 8 GPS: Zoe-M8Q 72 channel (GPS, GLONASS, BeiDou, Galileo constellations), 18Hz max update rate
- 180mAh rechargeable lipoly battery
- Programmed in C++, to be open source on Github.
François Gouy gave me the idea years ago by hooking up LED’s to a pitot tube (mounted on a wind vane on his helmet).
Michael Cooper set the bar with Flysight and opened my mind to what can be done with a GPS.
Where to from here
At this point it’s nothing more than an idea, a personal project, and a prototype. If anyone wants to build one I can itemize the hardware components and make my code available. If there is strong interest I can work on getting a PCB done, but there’s always going to be some amount of manual assembly in building the LED ‘finger’. I welcome any comments/ suggestions/ criticism. Cheers!
FFI : Contact Dave or read about Speed Monkey on his website
- Speed Monkey - 9th February 2021
- Speed Monkey - 9th February 2021