A lift reserve indicator (LRI) is a differential pressure measurement device that provides an indication of how much lift is available to the pilot. Whilst I'm not schooled in how it might be different from an angle of attack (AOA) indicator, to me they seem the same. Here is a great tutorial on how to build one, which is half of what I plan to construct.
The different zones are calibrated to mean the following.
| Red | Lift available is not sufficient for level flight. Upper boundary represents stall speed. |
| Yellow | Bottom boundary is just enough to support level flight. Remainder represents slow flight/final approach region. |
| Green | Lift available exceeds what's necessary for level flight. |
A 0-2 in H2O differential pressure gauge is used as the LRI. The Dwyer Minihelic II 2-5002 Differential Pressure Gauge, Range 0-2.0"WC is one such gauge. Its specs can be found here. Should you prefer a horizontal or vertical implementation, the Manostar FR51HV/VV would fit your bill (my apologies, however I do not know where to procure either version). The input to either is a standard AOA probe, fashioned by one's self or purchased. I will be using my Dynon heated AOA pitot probe.
I also want to have an audible indication of my LRI status. For that I will need a transducer which will convert pressure differential into a voltage. Then I will need to take that voltage and convert it into sound for the pilot. My understanding is that the F-4 Phantom had a similar setup to help pilots fly at the edge of the flight envelope (I assume other military aircraft have the same instrument). Here is a description from the F-4 flight manual and an image of the indicator (click here and search for "AOA" to learn more about that instrument).
The transducer I will use is the Honeywell ASDXRRX001PDAA5, which is available from Digi-Key, Mouser and Newark, among others. It converts +/- 5 in H2O to 5-95% of 0-5 V. It has 0.091" male ports. Tubing that would work might be this nylon 1/8" OD from Grainger. One can also use the remaining "PT 1/8 CLR PLASTIC" tubing from from the empennage kit or directly from Van's.
The output from this differential pressure transducer can then feed a simple voltage-to-frequency converter when the voltage reaches a lower threshold. Here is one such circuit example, based on the LM331 and another here. Another example would be a simple VCO (voltage controlled oscillator). When I complete the design, I'll post more. However with GRT's recent addition of AOA, I may not need the circuit and will just keep the "steam" based LRI.
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