The Lift Reserve Indicator
by Harner Selvidge, ICS 2015
In my article in the April, 1988, Commanche Flyer on instrumentation in my PA-39, N8981Y, I mentioned that I was soon installing a Lift Reserve Indicator (LRI) as an eventual replacement for my Safe Flight Instrument Company angle-of-attack (AOA) instrument, and promised a report on its operation. After an uneventful installation and calibration, and a number of operational flights, I am very pleased with it, and have the following details to report.
Why have an angle-of-attack, or a lift indicator? Isn't it just one more expensive gadget on an already over crowded panel? Not so! There are many circumstances where it can more surely save your neck than any other instrument. The reason for this is this is that it shows the amount of lift available at any instant, and lift is what keeps you up. The lift is a function of the angle at which the airflow impinges on the wing.
Haven't most pilots gotten along pretty well all these years without and AOA indicator? Well, yes - except for those who stalled out and spun in after take-off and climb-out because of not being able to maximize their lift when a problem developed, and they desperately needed to stay up as long as possible. Or those who stalled out and spun in during the approach, or who overshot the runway because of too high an approach speed. Pilots try to avoid these unpleasant situations by closely monitoring the airspeed indicator to stay above the stalling speed. But what is the stalling speed? And how much above it need you fly? The "book" stalling speed is given in the flight manual. Stay above it and "no problem" - right? Not necessarily. The stall speeds depends on many variables, some of the important ones being 1) Weight (lots of bodies, bags, fuel, ice?) 2) If you are turning in the pattern for approach or departure, what is your angle of bank, or 3) What is the position of your flaps? The angle of attack (AOA) or Lift Reserve Indicator (LRI) will show your margin above independent of any of these variables.
The Lift Reserve Indicator, like an airspeed meter, operates by ram air pressure, requiring no electrical power...It measures the difference in air pressure from two orifices located at right angles to each other on a streamlined probe in the undisturbed air below the wing or fuselage. When the angle of the probe changes with respect to the direction of the air flow, one of the orifices will show more pressure and the other, less.
The sensing probe for the LRI is mounted under the wing, or in the case of the twin, it can be mounted on the nose. The sensing probe comes already mounted on a plate which can be substituted for an inspection plate if there is one in the right place. There was not one in the right place on N8981Y, so we made a hole where we wanted it and used a doubler which came with the kit.
A calibration of the scale on the indicator must be performed for each installation. This will involve two or three flights...Basically what you do is to determine only one point, the point of maximum lift to drag ratio. To do this right, requires some brief periods of precise flying, which any pilot should be able to do. However, make it easy on yourself, and don't make any test flights if it is windy or turbulent. Make all calibration flights in the early morning when the air is stable and quiet.