LRI Lets You Get Down Slow

Private Pilot - February 1987


The Huntington Lift Reserve Indicator offers takeoff and approach airspeed information.


Most pilots are very familiar with both angle of attack and airspeed indicators. Normally, input from these two instruments is what we use during the critical liftoff and approach-to-landing phase of every flight.

In some situations, this data isn't enough. Then, what the pilot really wants is an indication of the combination of angle and speed at which the airplane will continue to fly, just above the point of the "mush," the stall and sometimes the spin.

One instrument combines this data and presents it clearly to the pilot. Called the Lift Reserve Indicator (LRI), it has been developed over the past several years by Morgan Gurdon Huntington. The Huntington LRI unit consists of a probe sensor mounted on the underside of the wing, about 25% aft of the chord, and a simple swinging-needle instrument in the cockpit [LRI I]. The wing probe has two holes that serve as pressure sensors...

The new instrument sensor is mounted with its probes at least four to five inches below the wing skin, where stable stream lines can be found. A recent NASA report concludes that these LRI streamlines bear a constant relationship to the pitch line of flight, throughout an 18 degree change in angle of attack.

When Aviation Consumer , a publication not noted for its enthusiasm for many aviation products, borrowed one of Huntington's units for a brief flight test program, it decided to keep it in the company Mooney.

...In the instructions [LRI I], users are advised, " The entire calibration procedure consists of adjusting the angle of the probe so the indicator needle reads one division low (to the left of) zero at the instant of flared touchdown with application of full-up elevator control. Variations of flap setting, center of gravity, wing loading and drag have no bearing on the reproducibility of the LRI calibration.

"Fly the aircraft around the pattern and land as slowly as is prudent by reference to the conventional instruments. At the moment of flared touchdown, with full up-elevator control applied, observe the LRI reading. At this juncture, dynamic lift is at the threshold of supporting the aircraft (one G of lift), elevator authority is lost and the airplane is at its minimum non-mushing airspeed - mushing sink is imminent."

Installation instructions advise that several calibration flights may be needed. Between flights, the bolt that attaches the probe is rotated slightly rearward to steepen the angle of the probe if the indication is too low. Rotate the probe forward to correct too-fast indications.

Owners are advised to check this touchdown calibration at altitude to confirm that the red/white junction (just above zero) also identifies the point of least power for zero sink from level flight. The manual notes that this is the old Mexican bush pilot's way of determining the minimum non-mushing airspeed before beginning final approach to a rough, short field landing.

Owners of the LRI should then scribe a durable mark along the side of the probe where it meets the mounting plate. This reference then should be noted on preflight walkaround inspections.

What's it really like to fly the...LRI? We contacted Raul J. Mercado who keeps his 1976 Cessna 182 at the Riverside, California Airport. Mercado has been flying since 1946 and owns an auto parts business. He is more than enthusiastic about his LRI, stating, " I really trust it, particularly when going into short fields. It records everything where you are and makes it easy to stay ahead of the power curve. I never had to abort a landing with this instrument."

Mercado has had his LRI for 2.5 years and has flown with it more than 70 hours. He says it took four hours to install it. He flies frequently into near by Big Bear Airport, elevation 6750 feet, and says, "I was making an approach to Big Bear and was at 70 knots CAIS. Gusts of wind came quickly and the LRI gave me the extra confidence to stay ahead of the power curve and make a safe landing." He also uses the instrument in the liftoff phase of flight.

Mercado has followed the designer's recommendation and located the instrument on top of the glareshield. Huntington recommends that the instrument be mounted at a distance that is in the "dark focus" area of the eye, about 40 inches away, and should remain in the pilot's peripheral vision.

I requested that Mercado do the flying while I observed the LRI. When the tower gave us a go, he applied the power and, as the LRI needle picked up past the zero calibration, he eased back on the controls to lift the nosewheel and we were off. The pilot used the LRI to monitor his rate of climb and was able to hold a much steeper angle of climb than I would normally use in a 182.

Around the field and the LRI came into use again down final approach. The pilot let the needle creep back toward the white marked area of zero-lift reserve, but stayed in the green [LRI I] until he started to flare. Touchdown was made with the controls full aft, just as it should even with a nosewheel, and there was no bounce.

We continued around the airport for a couple of touch-and-goes. Each had no "sag" on lift rotation; the angle of climb was steep, but according to the LRI, well within limit, and all landings were smooth and bounce-free. The LRI had done its job well.

...Art McCoy...from Gold Hill, Oregon...flies a Cessna 172 with a Horton STOL kit from a 1300-foot dirt strip. He has more than 320 hours total flight time and installed the LRI before he began operating out of his strip. "I wanted the extra safety of what the LRI could give me. On final, I have to clear an obstacle approximately 35 feet high 200 yards north of the strip. On takeoff to the south, I have to clear 500 volt power lines approximately 50-60 feet high and 400 yards from the end of the strip. I feel confident in these conditions with the LRI. Also, I'm sure that the Horton STOL kit helps alot. I can rotate at 48-50 indicated with the LRI two or three lines above the red and have a rate of climb that feels very positive. There is no mushy or edgy feeling as climbout is 800-900 fpm, depending on the wind."

McCoy says it took three tries to get the LRI set properly after installation. He thinks this probably was because of the extreme camber cuff on the leading edge of the Horton STOL kit. He says the unit works well in the rain and has had no failures in 18 months of use, and if he were to change aircraft, he would retain the LRI.

Gordon E. Evans of Camarillo, California, has the instrument on his fully IFR Piper Seneca II, along with full deice equipment and a Robertson Stol installation. He is a former U.S. Marine Corps jet pilot with 2400 hours total time. He reports excellent results with the LRI system...Evans reports that the instrument is particularly useful on short, rough runways and during climbout through the first several hundred feet. He says the unit earned its keep during two round trips to Punta Colorado and Las Palmas near the tip of Baja California. "The aircraft was at or near gross weight," says Evans. "Crosswinds were gusting more than 30 knots at 60 degrees to 80 degrees from the runway on the uneven resort gravel strips. I used almost full control input and differential power was required. The LRI was my primary reference instrument."

Ron Clark of Santa Barbara, California, has had the LRI in both a Cessna 210 and a Mooney 201, logging 200 hours in a four year period...

Clark thinks the instrument is useful on takeoffs from short fields and where maximum angle of climb is required. " While the instrument would be interesting for any test pilot or crop duster", says Clark, "the rest of the people better go buy themselves one." Clark, too, says he would install the LRI in any other aircraft he would to purchase.

Joseph G. Hoffman, a private pilot with more than 2000 hours, has the LRI in his Rare Air America conversion of the Cessna P210 with a 350-hp Lycoming engine up front. He has flown about 80 hours with the unit and reports no problems. He would install a new one if he were to change airplanes. "The instrument is a very good continuous reference to avoid trouble", he stated.

We talked with Capt. John Powers, a 747 check pilot from Ridgefield, Connecticut, who spent a day installing the LRI in his classic Cessna 170. He says the only installation problem was translating engineering language to pilot language. His wife was in the process of getting her private license in the taildragger when the instrument was installed, and she is now working on her instrument rating.

According to Morgan Huntington, "partial panel IFR is a 'breeze' flying with the LRI because this single instrument regulates airspeed, precisely controls aerodynamic pitch - and because its uniform indication ensures constant density altitude for any power setting."

Powers says he is very impressed with the LRI on both takeoff and landing. Flying in wind-shear conditions, the 747 captain said, "I was much more confident with the LRI than with the airspeed. (You get) quantitative information that can be predictably reproduced. One very blustery day going into the Brandenburg Airport in Kentucky, there were rain squalls with a speed variance of as much 10 mph. The LRI had a very quick response, much like a flight director. I'm pretty well sold on the thing."

"You don't have the excessive two to three-second lag found in the airspeeds," he says. "It's very impressive and easy to fly."