Sport VFR Instrument Project

SportVFR is an DIY electronics instrument system for very simple VFR.  I'd like to build a HummelBird some day.  This is a hand-prop, no-electrical, 37-hp stick and rudder human kite.

I am a minimalist, in all regards.  So I want the simplest possible flight system.  Historically this would have meant "steam gauges" or mechanically actuated gauges.

Powered Day VFR, non-electrical flight requires the following gauges (per CFAR part 91);

For flight (nothing can be GPS derived, must be measured)

• Airspeed Indicator 
• Non-Sensitive Altimeter - doesn't require adjustment for local barometric pressure
• Magnetic Compass

Engine Instruments

• Tachometer
• Oil Pressure
• Oil Temperature

To purchase all these instrument in mechanical gauges would cost ~$300.  While that is far from prohibative, it is nowhere near the most cost effective way anymore.  In just the last 5 years small "First Person Vehicles" (aka Drones) have grown out of the R/C community.  These vehicles have created a accurate and cheap AHRS (Attitude and Heading Reference System).  Add to that the open-source Arduino community making small and powerfull [enough] microprocessors and accessories. All of the above is $51, about the average price of one mechanical instrument.

Here's the BOM:

$1.98 Arduino Pro-Mini (or any compatible arduino-based device)

$1.59 16x2 LCD alph display 

$7.54 10-DoF IMU/AHRS, this is 3-axis gyro, accellerometer and compass, with an onboard barometric pressure sensor

$12.56 Differential pressure sensor (airspeed)

$13.98 Oil Pressure Sensor

$9.79 Oil Temperature Sensor

$3.49 Hall-Effect proximity sensor (Tachometer)

Total: $50.93

Display:

 Arduino has great libraries for interfacing with the 16x2 LCD display

Sensors:

The sensors come in three flavors, digital-signal, analog-signal and pulse-count.

The AHRS uses a digital signal to transmit the current setting of the device.  These signals are digital values.  If we wanted to display artificial horizon or turn-bank indicator, we could do so with this device, but it would require additional computations.  Fortunately all we really need is the compass and the barometric pressure, both are first-order values that don't require any fancy math or interpreation.  Just take the values hit a lookup table and display the scaled result.

 

The Oil Temperature, Oil Pressure and Airpseed pressures sensor are analog devices.  Oil Pressure ranges from 10-180ohms electrical resistence between 0 and 80psi.  There are more clevar ways to get high accuracy from this signal, but I just want a rough number and it's much simple to setup a simple voltage splitter using a 500ohm resistor ahead of the sensor and a small capacitor to filter noise.  The Oil temperature is very similar but can use a slightly higher resistance (less current) 1.5kOhm.  The voltage at the split goes into an Analog Input chanel and can be poled. All sensors will need to be calibrated and scaled, but this can be done using common tools (like 0C ice-water and 100C boiling water for the oil temp).

The final sensor class is pulse-count... this is for the tachometer.  It would sound odd that you can count every revolution of a fast moving prop and still do all the stuff above, but actually, it's pretty easy for these little guys.  We'll use a low-level processor trick called an "interupt handler" which will wake up every time the tach line rises and incriment a small counter.  Then once a second we'll take the count * 60sec and voila; Revs Per Minute.

I/O Summary:

- 2 serial interfaces (LCD and AHRS)

- 3 10-bit Analog Signals

- 1 digital pulse signal

Attacking the coding:

- Create the tach signal interupt

- Setup the LCD display "Hello SimpleVFR"

- Wire the oil temp sensor and read values from it

- Create helper functions for scaling/mapping analog inputs

- Setup the AHRS and read the barometric pressure

- Read the compass

- Start laying out the output to the display from the collected values.  

Layout (in roughly PFD order):

[ ASI ] [ HDG ] [ ALT ][v or ^ for vsi]  

[ OT  ] [ RPM ] [ OP  ] 

---- EXAMPLE ---

KTS     HDG    ALT

101   021  3120^

121  2310    63

 OT      RPM          OP

- Add the Oil Pressure and Air pressure sensors and use above calibration/scaling algorithm

- Test in a car to see if airspeed response is linear (acceptable)

- Make values for - ASI hi/lo, OT hi, OP lo and RPM hi - than limits "blink" 

- Calibrate all the instruments, I'd like to avoid having a special calibraiton mode, if possible.  I should be able to make a version that output raw values and use that to adjust.

Summary:
It's simple and achievable.  If anyone wants to follow along and see how it comes out, great.  I'm really making this site to get my own ideas out.