Airspeed Indicator - Variations

Variations

The Lift Reserve Indicator (LRI) has been proposed as an alternative or backup to the Airspeed Indicator (ASI) during critical stages of flight. This is an elegant device but is rarely found in light aircraft or even transport jets. The conventional Airspeed Indicator is less sensitive and less accurate as airspeed diminishes, thus providing less reliable information to the pilot as the aircraft slows towards the stall. The actual stall speed of an aircraft also varies with flight conditions, particularly changes in gross weight and wing loading during maneuvers. The ASI does not show the pilot directly how the stall is being approached during these maneuvers, whereas the LRI does.

The LRI shows the pilot directly the Potential of Wing Lift (POWL) above the stall at all times and at any airspeed, so it is more descriptive and easier for the pilot to use. The LRI uses dynamic differential pressure and Angle of Attack to operate. It is very fast acting and extremely accurate at low airspeeds, thus providing more reliable information to the pilot as airspeed diminishes and becomes critical.

The LRI uses a three zone, red-white-green display. During flight, the green zone is well above the stall where flight controls are firm, angle of attack is low, and the unused POWL is high. The white zone is near the stall where flight controls soften, angle of attack is high, and the unused POWL is diminished. The top of the red zone defines the beginning of the stall. The severity of stall increases as the needle travels deeper into the red. During the takeoff, the LRI uses dynamic pressure to operate and will not lift the needle above the red zone until enough airspeed energy is available to fly.

The pilot adjusts the instrument to indicate the edge of the red-white zone during minimum airspeed practice at altitude, indicating the aircraft has zero POWL beyond that point. Since the wing will stall at the same angle of attack at any airspeed, once properly adjusted the LRI will indicate the red-white edge anytime the stall is approached. This includes landing stalls, climbing stalls, and accelerated stalls. After adjustment, the black line in the center of the white indicates maximum angle of climb and maximum angle of descent with enough reserve lift for the landing flare. With practice, the pilot can use the LRI to determine the exact moment for liftoff with minimum ground roll and maximum angle of climb combined.

The LRI has been well received by STOL pilots and pilots of experimental or home-built aircraft. The LRI is very useful for short field landings, short field takeoffs, and slow speed maneuvers such as steep turns, steep climbs, and steep descents, and also allows pilots of fast or "slippery" aircraft to land with little or no float very reliably. Since the LRI is so useful at the critical lower end of the flight envelope, most pilots will use the LRI as a complement to the ASI, using the LRI for slow speed work and the ASI for cruising and navigational work.