ULTRAcooling and the Navajo

Reprint from Papa Alpha 31 Magazine

In my experience, there are at least two things pilots can’t get enough of. One is more power and the other more fuel. Certainly, if you asked pilots for a list of improvements they wanted, it would include more speed, more useful load, greater range, shorter takeoff distance and safer single engine operation. In most cases, these improvements relate back to having more power and more fuel. For example, with more power, you could safely carry more weight and also improve the takeoff and single engine performance. If you had more fuel, you would have more range at the same speed or more speed at the same range, but you may also need more power to carry the extra weight of fuel safely.

I could certainly be described as an advocate of both more power and more fuel, but like you, I am also concerned with efficiency and cost. Before I became involved with the Navajo series of airplanes, I had the pleasure of flying some aircraft that were upgraded with higher output engines. Most notably, an S model Bonanza that was upgraded from a naturally aspirated 285 HP engine to a turbocharged intercooled 350 HP Lycoming as used in the Chieftain. Another was the Aerostar, which jumped from 290 HP to 350 HP with Chieftain engines and was turbo-intercooled in the process. Both the Bonanza and the Aerostar demonstrated impressive climb and cruise improvements and both were much safer and easier aircraft to fly. In addition, they were both more fuel efficient. The Bonanza and the Aerostar engine change programs were accomplished by my company, Machen, Inc., a sister company to American Aviation, Inc.

When we upgraded the Bonanza and the Aerostar we learned a great deal about the importance of proper intercooling. Without it, we lost 90 horsepower at altitude. By optimizing the intercooling systems on those aircraft, we restored the lost power and picked up 10 to 15 knots more speed while reducing fuel consumption and cylinder head temperatures. The engines seemed to run more smoothly and they were noticeably more powerful and fuel efficient.

After completion of these two engine change projects, we had a healthy respect for the development costs involved in putting a completely different engine in an airframe and the price you have to charge to recover these costs. We also appreciated the remendous benefits of proper intercooling such as increased climb and cruising speeds and the opportunity to run efficiently, saving fuel and extending range. Of course this increased performance was also attained at a fraction of the cost of installing a bigger engine. We felt from a business standpoint, American Aviation should concentrate its efforts on developing optimum intercooling systems for the general aviation piston twins. We have completed several installations on the Chieftain, a Standard 310 Navajo, a 310 with the Panther conversion and later the 325 CR and the Pressurized Navajo.

Our philosophy with intercooling has always been to maintain sea level horsepower settings and fuel flows at higher altitudes. That results in “more power” available for the pilot, because, with the standard engine, power is progressively lost as the aircraft climbs to altitude. In addition, more speed on the same fuel meant greater range. We also had the option of leaning to peak EGT without fear of reduced engine life.

The actual intercooling process starts with instrumenting the aircraft and engine to measure the temperature of the air entering the turbocharger inlet and the temperature of the air exiting the trubocharger and entering the engine. We had an advantage with the Chieftain as we had used the identical engine in both the Bonanza and the Aerostar. For peak efficiency, a good intercooling system requires a high velocity inlet and a low pressure exit location. We located both inlet and exit locations by “surveying” the pressure in and around the cowling. This was accomplished by arranging an array of small copper tubes attached to tiny tubing that connected to an airspeed indicator in the cockpit. Some of the copper tubes were designed to act like miniature pitot tubes to measure “total pressure” while others were designed like static ports to measure “static pressure.”

AFter flight testing the Navajo and measuring the pressures around the cowlings we were very pleased to find the lower cowling offered both the high and low pressure areas we were looking for. In addition, this location would also allow the system to be easily installed in the field. Ideally, we wanted the installer to be able to “bolt the system on” with no fiberglass work, as this reatly reduces installation time. By choosing the lower cowling location and mounting the intercooler core to the engine mount, the P-51 style fairing could be easily fitted to the lower reomovable cowling with the same hardware used to attach the wing root fairings and wing tips. Additional work to remove and install the cowlings would be prevented and of course no fiberglass work required.

Performance increases were predictable with increased speeds and shorter times to climb heading the list.

There was also a dramatic improvement in the observed cylinder head temperatures as the aircraft climbed to altitude. We experimented with some of the low noise, economy cruise power settings that we developed in the Navajo engined Aerostar. Using 29″ and 2100 RPM at approximately 15 gph per engine, the Chieftain is extremely quiet and attains speeds where the fuel consumption used to average 19 gph per engine.

After fine tuning the system for performance, and load testing all of the structural components, we embarked on an extensive flight test certification program to show compliance with the Federal Aviation Regulations. These tests included a dive test at 10% more than never exceed speed (or max red line speed), a takeoff comparison test, a series of stalls both power on and off, a VMC test and both twin engine and single engine cooling climb tests.

We refer to the Navajo system as “Ultracooling” because it offers the optimum characteristics of an intercooling installation. By that I mean the temperature of air entering the engine remains relatively cool and basically the same temperature at all altitudes! This phenomenon may tend to amaze even the most experienced aircraft powerplant engineer, but it is a characteristic of our Ultracooling system where the ultracooler core, inlet and exit location, and aerodynamic fairing are carefully optimized.

We are now bringing this technology to the entire fleet of general aviation piston twins and have already completed installation on the Navajo, Pressurized Navajo, Cessna 337, Cessna 340/340A, Cessna 414/414A, Beech Duke, and Aerostar.

As some of you probably know the 340 and 414 Cessna were already intercooled from the factory, but the performance advances they have achieved with Ultracooling simply points out houw technologically advanced and powerful Ultracooling systems are.

We now have more than 200 Navajos enjoying the benefits of Ultracooling and more than 50 experienced installation centers nationwide. Recently, feedback from installers has allowed us to make even further design improvements.

In a recent article about intercooling, the technical write for Flying Magazine, Mr. Peter Garrison said, “In general, an intercooler raises critical altitude, increases cruising speeds at high altitudes, lowers engine temperatures, and magnifies detonation margins.” He went on to day, “with efficient intercooling, you can use mixture settings closer to peak or even at peak, and get much lower fuel flows, so long as turbocharger inlet temperatures stay below limits (normally 1,650°F).”

Mr. Garrison’s remarks seem to echo those of Mr. John W. Wells, Supervisor of Propulsion for Cessna Aircraft Company, who said, “An inercooler is one of those rare devices in aviation that seem to have few compromising traits. It is a passive piece of equipment; there are no moving parts, so there is little to go wrong. Its work is highly beneficial to the engine’s operating temperatures, power output, and longevity.”

After years of experience, let me just say that we agree wholeheartedly with Mr. Wells when he said, “There is no modification as inexpensive or simple as intercooling that will produce such dramatic improvements in (aircraft) range and performance.”

Jim Christy