[simpits-tech] Brainstorming a static & pitot system

[Wayne Ledzian]

In theory, what you propose does not sound technically difficult.  In practice, though, it may be quite a bit more challenging.  I'll address the issues one instrument at a time.

Airspeed indicator:
     This is essentially a delta-P gauge calibrated to read in knots instead of pressure.  You've got your static port taking ambient pressure and your pitot port taking total pressure.  For this gauge, you can either apply vacuum to the static port, or you can apply pressure to the pitot port.  Either will work, but simply moving a syringe to a set position will not be sufficient unless the system is completely leak-free.
     
Altimiter:
     This is a simple pressure gauge calibrated in distance instead of pressure.  For any positive altitude, a vacuum connected to the static port  will provide an altitude reading.
     In addition, while the motion may be 'smoother' than a servo-driven mechanism, it will still have a step size limited by the resolution of whatever device is driving it.  To be accurate to +/- 10 feet over 40,000 feet, you'll need at least 12 bits of precision and very little slop in your hardware.

VSI:
     This is a very sensitive rate-of-change meter.  Fortunately, once the control of the vacuum plenum for the altimeter is worked out, simply connecting the VSI to the same plenum should suffice.


My suggestion:
Three plenums as follows:
 -- One large plenum as a pressure sink, kept pumped down as low as your pump will keep it.
 -- One smaller plenum for the airspeed indicator 'static' port.
    - connect this plenum to the sink tank via a needle valve.
    - Use a second needle valve (servo controlled) to adjust leakage into the airspeed indicator plenum.  Pressure within this plenum is to be monitored and controlled via a PID controller to set desired delta-P.
 -- One smaller plenum for the altimeter and VSI.  Connected and controlled similar to the airspeed indicator plenum, pressure to be actively controlled to match the desired altitude.

With this setup, the high level of precision is required only of your electronic bits, specifically the pressure sensors and the ADC used to read them.