The Need For Speed: Making A Flying Legend Fit For The 21st Century

The moment you lay your eyes on the Polen Special, you instantly perceive a piece of aviation history. Its sleek fuselage and its get-out-ofmy- way red color portray an unrelenting need for speed. After all of these years, the Polen Special airplane, conceived and hand-built by Dennis Polen in 1972, is still one of the most revered homebuilt aircraft in the world.

At the beginning of the 1970s, commercial sports airplanes either cost too much money or lacked in performance. Polen's decision to build his own arose from those two facts. Essentially a scaleddown WWII fighter minus the armament, the Polen Special has a ground speed in excess of 300mph (480km/h), and is still the fastest four-cylinder airplane in the world.

This airplane is literally one-of-a-kind. Efforts were made over the years to convince Polen to mass-produce his airplane, or at least make its blueprints available. But Polen remained true to his commitment to keep the plane entirely to himself. After all, the Polen Special is not an airplane for amateur pilots. Intended to fly high and fly fast, the Special spends most of its air time in Class A airspace, above 18,000ft altitude under instrument flight rules and subject to air traffic control clearances and instructions, with the pilot breathing oxygen. Dennis Polen took enormous pride in having built such an amazing flying machine. Who could blame him for a little bit of pride of ownership? Over the years, the Polen Special has achieved a truly "Classic" status among the aviation aficionados living in the American Pacific northwest, many of whom have their own Polen "sighting" story that inevitably involves seeing the plane's tail. As the years passed, the plane's eminence only grew stronger.

In 1997, Dennis Polen made the decision to sell the plane after no longer being able to fly it himself due to a chronic illness, but only under the condition that the plane be put into the capable hands of a pilot who was qualified enough to fly it safely and care for it lovingly. Enter Dick and Debbie Keyt from Granbury, TX.

A Perfect Match

Flying is in their blood. Debbie and Dick Keyt have been the proud owners of the Polen Special starting in 1997. Since then, the plane has been shown around the US at various air shows, always attracting tons of attention.

Dick Keyt is an American Airlines pilot and an Air Force veteran with an aerospace engineering background. His wife Debbie is a former American Airlines flight attendant and a licensed pilot herself. If foster care for airplanes imposed a set of requirements, Dick and Debbie would pass by birthright. Upon assuming "guardianship" of the Polen Special and flying it at various air shows for several years, Keyt wanted to embark upon making modifications to the plane's aerodynamics and powertrain, but without chipping away at its soul. First, he wanted to redo some of the inefficient "plumbing" underneath the cowling (the sheeting covering the engine in the plane's nose) and thus increase the power output of the engine. Second, Keyt wanted to redesign the cowling based on the new engine arrangement and improve aerodynamics. For the

Flying is in their blood. Debbie and Dick Keyt have been the proud owners of the Polen Special starting in 1997. Since then, the plane has been shown around the US at various air shows, always attracting tons of attention. latter, he would team up with the University of Texas at Arlington (UTA) to do an aerodynamics analysis of the entire aircraft. For both of these tasks, Keyt needed to digitize large segments of his plane.

Portable Leica Hand Scanner Fits the Bill

Digitizing difficult-to-reach parts is a breeze with the Leica T-Scan.

Leica laser tracking systems are used extensively for portable measurement and inspection by the aerospace industry. A laser tracker is comprised of a high-speed tracking 3D laser interferometer and precision angular encoders that deliver a measuring rate of up to 3,000 points per second, with a measurement distance up to 40m (131ft) when used with a corner cube reflector. The precision measurement capabilities of the laser tracker enables aerospace manufacturers to validate designs, build-and-inspect, confirm close tolerance work, perform alignments and part mating, and capture 3D coordinate data on-the-fly.

Leica has introduced two technological enhancements to their laser tracker product line. The first product is the walk-around T-Probe, an ultralight handheld probing device that operates without cords or cables. The second product is a new high-speed hand scanner known as the Leica T-Scan. This product enables users to digitize complex surfaces and large objects easily and quickly with very high precision. The laser tracking systems, combined with wireless probing and rapid non-contact scanning, uniquely deliver portable data acquisition technologies that capture, visualize, and process spatial data right on the shop floor.

So when Keyt wanted to modify his plane's aerodynamics and powertrain, the Leica T-Scan paired with a LTD700 Leica Laser Tracker was ideally matched to the specific job requirements. The Leica T-Scan offers three crucial advantages when compared to similar scanning solutions. First, it is virtually impervious to environmental light and the type of surface being scanned, thus requiring no surface preparation in most cases. Second, Leica T-Scan operates in a measurement volume of up to 98ft., which allows for the entire airplane to be scanned with just rudimentary repositioning of the equipment involved. On top of that capability, the point-cloud data collected by the Leica T-Scan requires no post-processing to generate a 3D rendition of the scanned object. All data is processed in real-time right on the spot, and is immediately usable.

Denny Deegan, a Leica Geosystems Application Engineer, performing a scan of the plane's underbody. Deegan can follow the scanning progress in real-time on an application PC, making the necessary adjustments without changing his position. Thus, major time savings are accomplished.

Denny Deegan, a Leica Geosystems Application Engineer, arrived in Texas ready to perform the job. Leica TScan's advantages over similar scanning solutions enabled Deegan to scan the entire plane in merely 16 hours, gathering over 32 million data points. This is a savings of several days when compared to other scanning technologies that require extensive surface preparation and massive post-processing of data. Keyt followed the progress of the scanning job on a computer running the T-Collect software from Steinbichler, which showed the scanned engine segments in real-time as they were being digitized. Various improvement ideas kept popping up in his head almost as quickly as Deegan was able to scan different surfaces.

Sleeker, Faster, Higher

"One of the uses for this data will be to design a plenum chamber on top of the engine that would seal all of the air coming into the front inlets and would be independent of the cowling itself," explains Keyt. "The new plenum chamber will be completely embedded inside the cowling. This will optimize the air flow management in the engine, letting us squeeze more power out of it. The more power I have at my disposal, the harder I can push the plane. This is what reverse engineering is all about."

Another major improvement that will be possible after having a fully digitized rendering of the engine bay is to reposition the ductwork, bringing the outside air into the turbo charger.

The hand scanner easily maintains a line of sight to the Leica Laser Tracker. Varying luminous intensity of the parts being scanned is not an issue.

Keyt continues: "Based on the 3D renderings of the engine, we will figure out how much depth we have and what the clearances are off the exhaust pipes. In the current design, the turbo charger has to suck the air in. If we can ram the air into the turbo charger instead, we could lower the discharge temperature, giving us higher performance. Right now, the intercooler cools the air coming out of the charger down from 300°F (150°C) to 200°F (93°C). This is very thin air, with comparably little oxygen to burn. The cooler we get the air, the denser it gets, increasing the oxygen content significantly. The more oxygen there is to burn, the more fuel you can pump into the combustion chamber, increasing the horsepower output. We hope to be able to drop the air temperature further to about 150°F (65°C)."

The second important design change will be to design a new cowling based on the new engine component arrangement.

"I will team up with UTA, with benefits for both parties. The university gets excellent raw data for teaching purposes. As far as I know, there have been just isolated cases of entire planes being digitized. In return, the university will allow me to pinpoint where airflow interferences are and how we can make the cowling more efficient," concludes Keyt.

All these improvements are intended to increase the airplane's already impressive aerodynamics and power output, letting Keyt fly his "Special" at higher altitudes and at greater speeds. These improvements will make sure the plane remains competitive in the sport aviation field in the years to come, living up to Dennis Polen's legacy and exposing this important piece of aviation history to even more flying enthusiasts.