Unusual State Alarm and Recovery Director
inexpensive stall warning device based upon known aerodynamic performance of airplane, balancing G-forces against force proportional to dynamic pressure. The basis for this idea evolved early (US 1,885,578 and 504,062).
Such an instrument may be an airtight case having upper and lower halves, with a thin circular diaphragm separating them and suspending a mass element Wm. The pitot tube supplies static plus dynamic pressure (Ps+Pd) below the diaphragm while static pressure Ps is on top. Net dynamic pressure Pd acts upward on the effective diaphragm area Am creating the dynamic force = Pd*Am to support the mass.
It can be calibrated in level flight (1 G) to produce a warning if airspeed drops below Vsw, where Vsw is selected somewhat above minimum stall speed Vs. (enough for warning). At airspeed Vsw, area Am is made to be just sufficient to support weight Wm of the mass. If airspeed decreases, the mass will see less than 1G. If AoA increases, the airplane accelerates up, forcing the mass up as well. In either case, the balance of acceleration and dynamic pressure forces urges the mass down relative to its case, to be measured, e.g., by a strain gage.
It turns out that calibration for level flight also prepares for any accelerated flight
Now, a better way to implement the basic principle: the functionality can be separated into two parts: measure dynamic pressure with a differential pressure transducer and the acceleration force with a G-meter. A single MEMS unit embodies 6-axes of accelerometers plus rate gyros, and costing less than $10, is used in smartphones. A $90 pressure transducer is available. Instead of physically measuring the difference between the two forces, the two output voltages representing dynamic pressure and normal acceleration each can be input to the microprocessor and subtracted there.
It is then possible to measure and display the coefficient of lift function in flight, using a desired warning level Clmax to measure against in flight thereafter. Changes in flight configuration such as weight, flaps (representing variations in the Cl function) can be taken into account.
These sensors address previous deficiencies. As Cl(AoA) descends into a stall, this reversal of acceleration can cause the unaided warning signal to be turned off when the aircraft is actually in a deep stall.
The presence of the sensors and the microprocessor adds capability of tracking the aerodynamic state of the aircraft. More than just alarms, transition from NORMAL state into STALL state would be indicated by nose drop (sensed by pitch rate and loss of lift and/or wing drop sensed by roll rate). In addition, yaw rate may indicate entry into the SPIN state, causing computer-generated vocal exclamations such as “push stick!” or “left rudder!”, aiding a surprised pilot at a critical time.
Possibly, a bracket-mounted actual smartphone could be used with an external pressure transducer plugged into it. (It’s a stretch, but make its camera read (and redisplay) the ASI and negate the need for the pressure transducer?)