Cirrus Design again increased and widened the scope of its brand when the company received a supplemental type certificate (STC) for adding a bolt-on performance-enhancing mod to the company's 310-horsepower SR22. The new turbonormalizer (TN) installation provides a substantial increase in performance that boosts economy cruise numbers to 218 knots true airspeed (TAS) at 25,000 feet while burning 17.5 gallons per hour.
The turbonormalizer option expands the performance envelope of the already impressive four-place SR22. The surprising part is that managing the TN engine turns out to be easier than managing the normally aspirated engine. Want proof of this claim? In early October, Cirrus corporate pilot Paul Sallach pushed the throttle and mixture controls — Cirrus airplanes feature a SMART single-movement power lever, which incorporates the propeller governor control into the engine throttle control lever — to the maximum-go position and took off from an airport at 836 feet msl. N162SR (serial number 2088) maintained a 1,000-foot-per-minute climb — at a cruise-climb indicated airspeed of 105 knots — through 12,000 feet, then through the midteens, and on into the flight levels before reaching the airplane's certified ceiling of 25,000 feet. Time from brake release to level off was 26 minutes. Never once did Sallach touch the engine controls.
"We think of it as a 1,000-foot-per-minute elevator," said Sallach as he pointed out the very cool cylinder head temperatures (CHTs) displayed on the engine and fuel-monitoring (EMax) page of the Avidyne FlightMax EX5000 multifunction display. During this extended climb the hottest CHT never got above 354 degrees Fahrenheit.
There are distinct advantages to operating in the airspace defined in mid- levels of altitude — there's a lot less traffic, the ride is typically a lot smoother since the airplane is above thermal activity and low-altitude weather, and air traffic controllers are a lot more likely to grant direct-to routing requests. These advantages, the ease of engine operation, and the 200-plus-knot true airspeeds will undoubtedly make the turbonormalizer a very popular option for Cirrus buyers. The complete TN package — a $62,800 option — includes the turbonormalizer system; a matched set of GAMIjector fuel-injection nozzles; a radical-looking, three-blade wide-chord, second-generation advanced structural composite (ASC II) Hartzell propeller; and a built-in 77-cubic-foot four-place oxygen system from Precise Flight. This high-altitude increased performance package adds 55 pounds to the empty weight, resulting in a useful load that Cirrus advertises at 982 pounds. At this point there seems to be only one drawback — buyers must choose between cool comfort and speed since weight-and-balance restrictions prevent the installation of both the TN option and air conditioning.
Once the airplane was leveled off at 25,000 feet, the power lever was pulled back to the position Sallach termed "the bump." Moving the throttle lever aft slightly slows the propeller rpm down to 2,500 while the throttle remains wide open. The mixture is then leaned to 17.5 gallons per hour. This results in a lean-of-peak (LOP) mixture at 86-percent power. A few minutes later the airplane had accelerated to a TAS of 217 knots.
True airspeed runs also resulted in true airspeeds of 203 knots at 16,500 feet and 196 knots at 13,500 feet. Again, the engine had been "parked" at 86-percent power, burning 17.5 to 17.6 gallons per hour at all three altitudes.
For the sake of illustration let's pencil in numbers for a cross-country flight following the climb to 25,000 feet. Usable fuel in an SR22 is 86 gallons. About 15.7 gallons were consumed in the climb, leaving just over 70 gallons. At 17.5 gph, this will totally exhaust the fuel in a little more than four hours. Even after setting aside a one-hour cruise consumption fuel reserve, the airplane will still cover over 650 nm in three hours.
The turbonormalizer system is automatically controlled by a fixed absolute pressure controller. This unit continually compares the air pressure exiting the turbocharger compressor (which is termed the "upper deck pressure") with a reference pressure and automatically adjusts the exhaust wastegate to maintain the proper upper deck pressure. It's a simple system that requires very little pilot input.
Tornado Alley Turbo (TAT), of Ada, Oklahoma, worked with Cirrus' engineering department to develop the turbonormalizing system for Cirrus. TAT began as General Aviation Modifications Inc., or GAMI, in 1996. AOPA installed one of its TN systems on AOPA's 2001 Bonanza Sweepstakes airplane. For nearly 10 years, TAT has been at the forefront of creating bolt-on changes to make general aviation engines run better.
The Cirrus TN system features two high-efficiency turbochargers — one on each bank of three cylinders — with high-temperature Inconel turbine rotors that push compressed air through large intercoolers to the engine induction inlet. Unlike a turbocharger system, this turbonormalizer, like all TAT systems, doesn't boost the induction air (manifold) pressure flowing into the 310-horsepower six-cylinder Teledyne Continental Motors IO-550 above what Mother Nature provides at sea level; it merely enables the pilot to take advantage of, and enjoy the many benefits of, being able to maintain full-rated sea-level power (100 percent) up to 25,000 feet.
Climbs can be at full-rich or lean-of-peak mixtures. Full-rich climbs, the method pilots are most familiar with, consist of pushing the throttle and mixture controls forward to the stops and leaving them there until reaching the intended cruising altitude. This is the method used to get to 25,000 feet in 26 minutes. Full-rich 100-percent-power operation consumes 36.2 gallons per hour. This method consumed 15.7 gallons of 100LL during the 26-minute climb to 25,000 feet msl. With a fuel load of 45 gallons, two 200-pounders, and 40 pounds of gear aboard, the estimated weight was 280 pounds below the maximum takeoff weight of 3,400 pounds.
Lean-of-peak climbs are also an option. Here's how it's done. Takeoff and initial climb are flown at full-rich mixture. After the airplane is out of the airport traffic area, pull the power lever back to "the bump," or 2,500 rpm. Then quickly pull the mixture knob back to 17.5 gallons per hour. This results in a LOP climb at 86-percent power. Lean-of-peak climbs result in a slightly slower climb rate than the full-rich-mixture method but have the advantage of saving a little fuel. A lean-of-peak climb to 25,000 feet would require approximately seven or eight more minutes than the rich-of-peak climb, but the fuel burn would be reduced by four or five gallons.
Initially all TN installation will be completed at the Cirrus Design facility in Duluth. Plans to develop other installation centers are being considered. Retrofit packages for normally aspirated SR22s are also being considered. The fact that the TN option is approved by STC instead of being included in the airplane type certificate makes the possibility of a retrofit program a viable option if there's enough interest.
The long-awaited turbonormalizing option greatly expands the Cirrus SR22 operating envelope. And that's a good thing for Cirrus and for twenty-first-century pilots who can afford to fly really high and go really fast.
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