Engineering A Better Prop

The groundbreaking MT 5- and 7-blade propellers
Trey Henderson

Have you ever found yourself rolling out onto the tarmac thinking “man, I hope I have enough runway?” What about climbing out thinking “are we at altitude yet?” Or maybe throttling up and thinking “I hope my low damping factor 4 blade aluminum propeller doesn’t dwell in the resonance range of reactionless mode fatigue cycling too long?” If so, you’re not alone.

I had the opportunity to sit down and talk with Chris Finnoff, a storied aircraft salesman and founder of Finnoff Aviation, who sells pre-owned Pilatus PC-12s. He also founded Finnoff Aviation Products, which develops new after-market products including the STC on the Pratt & Whitney PT67A-67P engine and the 5- and 7-blade propellers from renowned German company MTPropeller.

Chris Finnoff, while running Pilatus Business Aircraft, had advocated developing a five-bladed propeller. Quite independent of Chris, the engineers at MT-Propeller had been having these same thoughts. Then, after Chris left Pilatus and formed Finnoff Aviation Products, he discussed the project with another engineering friend, Mike Dennis, who told him that he had heard that MT propeller’s Gerd Muehlbauer, was looking into the same development. Chris approached Muelbaurer, which was the beginning of their relationship and subsequent business plan.

Finnoff’s involvement in aviation stems back to his time in college at the University of Colorado Boulder. Initially an engineering physics student studying the effects of subatomic bombardment of materials in cyclotrons, Finnoff eventually discovered his true calling: aviation. In the second year of his engineering physics degree, Chris decided to change course. Pairing his love for aircraft with a nudge from one of his professors, he changed majors to aeronautical engineering. The University of Colorado Boulder had, and still to this day has, one of the best
aeronautical aerospace engineering programs in the country.

Not only did Finnoff excel in technical fields such as his engineering studies during college, but he also exhibited natural entrepreneurialism. Chris ultimately started multiple companies while in school perhaps the most influential of them was The Spoke, a small shop where he learned to repair and sell bicycles. Chris’s father owned a machine shop at the time and had connections to the European manufacturers who sold the tooling he needed for his shop. These manufacturers also sold bicycles and were interested in breaking into the American market. This was a golden opportunity for Chris to put his interest in mechanical systems and business leadership to the test. Renting out a small garage in Boulder, Colorado, Chris catered to the rising number of passionate cyclists in the area What started as a small endeavor in a rented garage turned into multiple shops, and Chris eventually became one of the largest bicycle retailers in Colorado.

With the newfound income from his flourishing bicycle company, The Spoke, Chris was able to take one step closer to his aeronautical future by starting flight training and building hours toward his dream to become an engineering test pilot or an astronaut. It was his
entrepreneurialism and salesmanship, though, that would ultimately be the driver of his direction in life.

Finnoff graduated from CU Boulder with his Bachelor of Science in Aeronautical Engineering. At the same time, he graduated from selling bikes to selling aircraft His first full time job was with Denver Beechcraft, a distributor for Beechcraft across the Rocky Mountain states. According to
Finnoff, “Beech would give me a set of keys to a new airplane and a credit card and say, ‘go sell this plane.’” Just as he was a talented bicycle salesman, Chris turned out to be an equally talented aircraft salesman.

Chris eventually found himself rubbing shoulders with aviation hall-of-famers such as Harry Combs of Combs Aircraft, Bill Lear of LearJet, and Olive Ann Beech of Beechcraft.

During his time in sales, he also identified some niches where he could flex his engineering skills on specialized products while at Learjet, Chris was the manager of advanced aircraft sales and marketing where he worked on some truly unique projects. One of which was the development of a highly-modified Learjet 35 that was outfitted with 4 engines and a high aspect ratio wing specifically designed for highaltitude fight. At this time, the U-2 program was coming to an end, but commercial and government entities still had strong need for high-resolution aerial imagery. Additionally, the National Science Foundation took interest in the specialized LearJet as its 7, ft fight ceiling was capable of taking atmospheric samples not previously accessible for research organizations such as the National Center for Atmospheric Research (NCAR) Boulder, CO.

The development of this research and reconnaissance aircraft led to the creation of a spinout company from LearJet known as LearTek, which Chris found himself at the helm of as President and CEO. Chris and his team made major strides toward the development of this aerial science platform, but unfortunately, the future would have different plans. The Air Force decided to reinstate the U-2 program with the intent to build a feet of newer, more advanced U-2s that provided much of the same science and military utility as LearTek’s aircraft.

While LearTek’s venture may not have been successful in the long run, the leadership and experience Finnoff gained during his time at the organization provided the platform he needed to dive deeper into business leadership and administration. Chris soon after founded Turbo West Aviation, which became one of the most successful sales and service centers for the Piper Cheyenne and Beech Aircraft in the Rocky Mountain area. Turbo West employed over 100 mechanics, had specialized divisions for avionics and propulsion, and even hosted a helicopter sales and maintenance team. Chris spent the better part of 20 years operating Turbo West until he sold the company in 1994.

The same year he sold Turbo West, Chris was approached by Pilatus to create a business plan to drive the future of the PC-12 program and refine the dealer network Chris assembled an all-star team of technical and business-minded people including Ignaz Gretener who is today the Head of General Aviation at Pilatus and Oliver Maysefield who served as the Chief Engineer on the PC-12, and Oskar Bruendler who was, at the time, the Chief Financial Officer and Senior Vice President at Pilatus. The plan was fully supported by visionary Oscar Schwenk, then President and CEO of Pilatus Aircraft in Stans, Switzerland. In addition, he hired two people from the U.S.: Dave Wodraska and Greg Klingler, and from Switzerland Thomas Bosshard to head up the Completion Center. Thomas is the current President
of Pilatus Business Aircraft in Broomfield, CO.

In forming the PC-12 business plan, Chris and his team identified that 60-70% of the PC-12 market existed in North America. It was clear then that a new company would need to be formed to tap that sector of the market. This led directly to the formation of Pilatus Business Aircraft, Ltd., (but according to Chris, everyone close to the project just called it “PILBAL.”)

Chris was asked by Schwenk to be the founding President of PILBAL and to put the business plan into reality. The Swiss decided to locate the new company in Broomfield, Colorado. When the program started, there were 17 Pilatus PC -12s that had been sold. Chris executed the business plan, set up the Completion Center, and hired several of the PILBAL employees to help. Then he proceeded to set up a Dealer Program throughout the United States to handle the sales. He developed a marketing plan, secured a Demonstration PC-12 and hired a Flight Demonstration team to show the new airplane. Chris was CEO and President of PILBAL December 1994 to 2001. When Chris left Pilatus, there were over 200 aircraft in the fleet.

Finnoff spent some time working with a few startups before stepping back into business ownership with his own company in 2005. It was then that he formed Finnoff Aviation with the idea of purchasing older PC-12s and upgrading them for resale. Since his time as President at Pilatus, Chris had been interested in developing a 5-bladed propeller upgrade for the PC-12. Chris identified a myriad of reasons why 5 bladed designs are better than 4 bladed designs: less takeoff length, better climb capability, and a quieter ride are a few of the big performance reasons, but there are also numerous engineering reasons why a 5-bladed propeller makes a better solution than a 4-bladed prop.

One day, Chris and a good friend of his, Mike Dennis, who owns and operates Oregon Aero, were chatting about the advantages of a 5-bladed prop when Mike happened to mention a rumor he had heard: MT-Propeller was looking into developing a 5-bladed propeller for the PC-12. After talking with Mike, Chris picked up the phone and made a call to the German propeller company. He reached out to MT- Propeller’s Owner and President, Gerd Muehlbauer, to gauge his interest in working together on the 5-bladed propeller project.

Finnoff saw MT-Propeller as the ideal company to collaborate with on the development of this new product. MT works on a wide scope of innovative and fascinating projects such as supplying propellers for General Atomics’ Predator drones, developing the lift and propulsion fans for large hovercraft, and tackling the demanding environments of diesel- powered propeller aircraft. In Chris’s own words, “MT is an unbelievable company.” MT does all of their own engineering, they build all of their own hubs and governors, and the company leadership is full of passionate pilots and engineers.

It wasn’t long after their phone call that Finnoff found himself in Phoenix, Arizona meeting with Muehlbauer to discuss the formation of a joint program to develop and qualify a 5-bladed propeller. In exchange for becoming a distributor of the new propeller, Chris offered Gerd access to his PC-12 flight test aircraft. The deal was set, and Gerd made one more trip to the U.S., this time to inspect Finnoff’s aircraft. In a bit of humor, Chris himself admits “he was probably checking to see if [Finnoff Aviation] was actually viable.”

MT-Propeller and Finnoff Aviation worked closely on the development of the propeller, with MT doing numerous test flights and working through countless design iterations to optimize nearly every characteristic of the new propeller. At this time, Finnoff Aviation had already begun upgrading PC-12s with Pratt & Whitney PT6A-67P engines, which ended up being one of the greatest advantages of testing on Finnoff aircraft. The new, more powerful engine allowed testing at multiple power levels. With this flexibility, MT was able to certify the propeller on both the 67P and 67B engines much more easily.

When it was released in 2010, the new MT 5-bladed propeller was head and shoulders above any competing propeller available for the PC-12. At the time there were no other 5-bladed propellers available for the PC-12, which put them at a unique advantage. It wasn’t until 2016 that competition entered the market when Hartzell, Pilatus’s propeller company of choice since the ‘60s, released their answer to the 5-bladed propeller question. However, despite having 6 extra years to engineer their product, the new propeller from Hartzell still came up short. According to Chris, “the Hartzell propellers are good propellers, but the MT’s have a lot more virtues and technology development in them.”

To truly appreciate the additional technology MT developed for their propeller, one has to understand the basics of rotor dynamics and material properties. As a pilot, you’re likely familiar with RPM restrictions. Your PC-12 operating handbook instructs you to steer clear of RPMs below 950. Maybe you’ve wondered why, or maybe you’ve never questioned it, but there’s a critical reason why PC-12 pilots should typically avoid this operating range. When passing through – or worse – loitering in this RPM range, the vibrational environment the propeller is exposed to is very near its own natural vibrating frequency. When an object is stimulated with
a frequency close to its own natural frequency, the magnitude of the vibration tends to amplify; this is known as resonance. Magnitude amplification through resonance can lead to increased mechanical stresses and even catastrophic failure of just about any type of structure: bridges, spacecraft, cars, and, yes, even aircraft. Luckily, this failure doesn’t happen immediately. All materials have something known as a fatigue lifetime, which simply means “how many stress cycles can this object (for example, a propeller, a bridge strut, or a car axle) undergo before failure,” which is why a pilot can safely throttle through the resonance range so long as they do so reasonably quickly.

The resonant frequencies or “modes” that a propeller will experience is influenced by many factors of its material makeup, construction, and design choices. This is where the advantages of the MT 5-bladed propeller become obvious. First and foremost, all 4-bladed propellers, regardless of material or other design parameters, exhibit something known as a reactionless mode. If you could imagine your propeller in slow motion while it is operating in a reactionless mode regime, you would see that 2 of your 4 blades bend forward, and simultaneously the other 2 bend back toward the fuselage of the aircraft. This asymmetric force on the hub of the propeller creates enormous stress on the structure that inches your propeller closer to the end of its fatigue life, and consequently, closer to catastrophic failure.

Well designed odd number bladed propellers, on the other hand, have the ability to avoid this operating mode. Notice I say “well designed.” In addition to simply adding another blade, great care has to be taken to properly damp the vibrations, which is another area where MT have excelled in their design. MT have engineered their blades with what they call “MT Natural Composite Technology.” This natural composite technology consists of a beech and spruce wood mixture that undergoes extremely high pressure compression, essentially fluidizing the cellular structure of the wood resulting in near-isotropic material properties, all the while carrying the benefits of a wood composite, most notably a higher damping factor than synthetic compsites such as carbon fiber. The higher damping factor of the natural composite contributes to lower tip deflection and the 5-bladed propeller’s unlimited fatigue life which results in no RPM restrictions.

In comparison, Hartzell’s 4 and 5- bladed propeller design is available in two more traditional materials: aluminum and carbon fiber. While the Hartzell 5-bladed propeller is a major step up from its 4-bladed counterpart for many reasons, the materials chosen for its design means that it is subject to classical reactionless modes and, therefore, a limited fatigue life.

Not content with their already sizable lead in the propeller market, MT continues to push the boundaries of propeller engineering to this day with their recent announcement of the “Silent 7,” the world’s first 7-bladed propeller available to the general aviation market. In developing this new breakthrough product, MT has made use of advanced new aerodynamics simulation paired with their natural composite technology. The Silent 7 carries some unique and impressive advantages, namely: significantly quieter operation than any 4- or 5-bladed propeller on the market, up to a 7 knot increase in high-speed cruise beyond the MT 5-blade, and last, but definitely not least, “the coolest-looking propeller on the market,” says Finnoff himself. (And I think I would have to agree.)

In closing our conversation, Chris expressed his fondness for the PC-12, saying that “It’s our favorite airplane in the world. We believe the PC-12 is the most versatile, most capable, and safest of any turbine-powered airplane. [Finnoff Aviation Products] spends 90% of our time and our money on developing products that enhance the great ability of the Pilatus products.”
As of March 2021, The Silent 7 is now certified in both the United States and Europe and is available from Finnoff Aviation Products.

Trey Henderson is a graduate of Embry-Riddle Aeronautical University. After receiving his Bachelor of Science in Aerospace Engineering in 2017, Trey worked as a contract design engineer for many of the large space exploration companies including SpaceX, Blue Origin, and United Launch Alliance. Today, Trey is a design engineer for a private fusion energy startup which is developing a high-field, high-temperature-superconductor-based tokamak reactor.

Courtesy of POPA. Originally Published in POPA Magazine’s Summer 2021 edition.