The Aviation Consumer

June 1984

(Excerpt)


...an instrument designed to eliminate the stall/mush accident and go one- up on the angle of attack indicator.

...The LRI responds to three things...: angle of attack, indicated airspeed (in engineering terms, dynamic pressure, or q) and the character of the airflow around the wing. The LRI probe looks like an overgrown rectangular airspeed pitot. It is mounted under the wing at about the quarter-chord point (that is, about a quarter of the way back from the leading edge). The probe projects about six inches down and forward at about a 60-degree angle. The top and bottom surfaces of the resulting "arrow head" shape contain static ports, which are plumbed back to the cockpit instrument.

In a nutshell, the LRI measures the difference in pressures at the top and bottom surfaces of the probe. Like the airspeed indicator, it is simply a differential pressure gauge...At normal cruise speeds, the top surface of the probe strikes the air at about a 60-degree angle, the bottom surface at about 30. The top surface, facing more directly into the wind, registers a much higher pressure than the bottom surface. A large pressure differential results, and under such conditions, the LRI needle is pegged to the max. But at slower speeds as the angle of attack increases, the top surface rotates away from the wind, while the bottom turns into it. The pressure differential is thus smaller, and the needle responds accordingly falling to the left on the dial . As angle of attack increases and airspeed decreases, the pressure differential continues to fall, until the probe points directly into the wind and both faces strike the air at a 45-degree angle. At this point the pressure differential is zero.

The instrument dial has three areas: a red band to the left (the low pressure regime), a white band in the middle and a green band to the right. The red band indicates that the aircraft is mushing or stalled, (that is, it has no lift reserve). The white band is the proper range for landing; a "normal" approach should be flown with the needle at the top of the white. The green band indicates a very large lift reserve and that airspeed is too high for proper landing.

Every LRI installation must be individually calibrated. The reference point is the top of the red arc; the probe angle is adjusted so that the needle reads top-of-the-red when the aircraft settles down onto the ground in a full-stall touchdown. Calibration normally takes three or four landings, each followed by minute probe adjustments.

Installation, in most cases is no big deal. In the case of the Aviation Consumer Mooney, the probe was installed on one of the inspection plates on the bottom of the wing. The two air tubes were strung along the wing to the cockpit instrument. The instrument itself was mounted on top of the glareshield on the left side, just above the airspeed indicator for easy cross checking...LRI's are installed as minor modifications according to FAA Form 337, which requires the signature of an A1 rated mechanic. Since the LRI does not tie into any other aircraft system, this seems reasonable.

Calibration of our aircraft took about four landings...to adjust the mounting angle of the probe until the needle hovered just into the red as we plunked down on the runway in a full-stall landing. We then perfumed a series of airborne calibration tests for airspeed and LRI readings.

Later we re-flew the calibration tests at a much lighter weight, and confirmed one claim: that the LRI automatically responds to weight changes. At the lighter weights, the LRI consistently showed more lift reserve (that is, less angle of attack) at the same airspeed. For example, at gross weight, the LRI read "top of the white" - the ideal approach speed - as 73 mph. With only the pilot aboard, the optimum LRI reading came at 69 mph.

Thus, the pilot is able to make all approaches, regardless of weight, at one LRI reading, in effect ignoring the airspeed indicator. This saves the guesswork of trying to adjust target approach speed for estimated aircraft weight.

We've not yet put in enough hours behind the LRI to evaluate it under all conditions. But so far, we like it. Comments one Aviation Consumer pilot: "If you put the LRI needle on the hash mark you're virtually guaranteed a good landing. There's just the right amount of flare energy left, enough for a decisive flare, plus a couple of seconds of float before it settles. It's real easy to float a mile in the Mooney, but proper use of the LRI prevents that."...

The more we fly it, the more we find ourselves, naturally and unconsciously, watching the LRI instead of the airspeed during a tight approach in gusty conditions...Perhaps the best way to sum up our judgment of the LRI is to recount this fact: our original intention was to install the LRI only temporarily, strictly as a test. But after flying the LRI for a while, we don't think we'll give [this] instrument back.