Jim Smith's Lift Reserve Indicator

Photo: Kathryn Wainfan

Jim Smith's lime green RV-6 sports many performance-enhancing modifications including extended, tapered wing tips, and Ellippse propeller, and extensive modifications to the cooling air outflow path.

Jim's latest addition to his airplane is a home-made lift reserve indicator. A lift reserve indicator is an instrument that gives the pilot a visual indication of the amount of margin from stall the airplane has at its current airspeed, altitude, and angle of attack.

The instrument works by measuring the pressure differential between an angled probe under the wing and the aircraft static pressure. Several commercial LRI systems are available, but Jim made his own, primarily to reduce cost.

The angled probe was added below the pitot tube on the same faired mount under the wing that held the pitot. In the cockpit, Jim re-purposed a commercial water pressure gauge to serve as an indicator.

Photo: Kathryn Wainfan
Photo: Kathryn Wainfan

The proper angle for the probe, and the markings on the face of the gauge were determined by flight test. The gauge is divided in to 3 regions: Red, White, and Green. The top of the red arc corresponds with the initial stall break. As the needle moves into the white, and then green areas, the airplane has progressively more lift margin and can pull more G before stalling.

Jim now uses his LRI as his primary instrument for setting take off and landing airspeed and angle of attack. He reports that the entire system, which he believes considerably enhances the safety of his airplane cost him a total of $54 for materials and the gauge.

7 Replies to “Jim Smith's Lift Reserve Indicator”

  1. You need to publish a detailed article on this lift reserve indicator in an upcoming issue of Kitplanes.

  2. I built something like this for a project in an elect. engineering class, but it read the angle directly, using a vane and synchro/resolver. Mine was pretty complicated, and yours looks like a very elegant solution.

    I have one question: does it work at various airspeeds? For example, if you are at max gross, you will stall at the same angle, of course, but the airspeed will be higher. Or in an accelerated stall? A vane-type device doesn't care what the airspeed it because it always streamlines.

    That is not a criticism; it looks like you have a great invention, but I was just wondering about that.

  3. I vote strongly for a quick article on how to assemble this affordable solution to AOA indicators. It will make most airplanes safer and save lives. This is BIG

  4. Jim did a great work and showed that it is no magic behind the angle of attack measurements. In fluiddynamics laboratories pressure based angle measuremet probes are used since long long time... Detailed information and range of the pressure gauge would be helpful for other DIY projects!

  5. I purchased a Lift Reserve Indicator several years ago for $400. I use it almost exclusively for take off and landing to tell me what the wing is doing. Automatically compensates for density altitude and loading.
    Now the identical instrument copied by a competitor sells for over $2.000. The original inventor is no longer in business.

    A do-it-yourself article would be greatly appreciated.

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