Gerd R. Muehlbauer Says 'Competitors' Claims Are Misleading

In an open letter to the industry, MT-Propellers president Gerd R. Muehlbauer says that claims made by rival propeller manufacturer Hartzell about MT-Propellers are misleading to the aviation industry.

Muehlbauer says that Hartzell's claims focus on four areas, and in the letter, he offers data he says refutes those claims.

According to the letter, Hartzell has released a video showing an MT-Propeller failing in a simulated bird strike. Muehlbauer says that every certified propeller has to show compliance of bird impact resistance per CS-P 360 (EASA Requirement) or FAR 35.36 (FAA Requirement) at the most critical location (about 4 inches from the tip) and the flight condition which will cause the highest blade loads (take off power and blade angle of about 15°).

"Since all our propellers are certified according to these standards, we have demonstrated that our blades will withstand such impacts of a 4 lbs bird without major propeller effects," he writes. "Hartzell shows a video of such a test with one of our blades, but with a far too high blade angle ( about 40° ) with the result of a failure at about mid-blade section. There is no blade, whether aluminum, carbon fibre, aramid fibre, glass fibre or wood composite which will survive such a test and this is why it was performed not within the rules, with the sole intent to destroy it."

Muehlbauer says there is no real case ever reported of such a bird strike condition in more than 110 million flight hours with more than 50,000 blades flying.

Muehlbauer also says that Hartzell's claim that their props are more prone to throw blades is false. "Every certified propeller has to show compliance of centrifugal load tests per CS-P 350 (EASA Requirement) or FAR 35.35 (FAA Requirement) to demonstrate a load of twice the maximum centrifugal load at maximum rotational speed."

Muehlbauer says the company's large size blade root with 11 lag screws will hold a centrifugal load of 72 tons vs a maximum load of 9 tons at a usual take off rotational speed. "In other words, our blade retention is 8 times better than required. It further has been fatigue tested to a simulated flight of 20.000 hours without failure," he writes.

The company president goes on to question on what criteria Hartzell based a comparison to say that it's propeller blades are "10 times stronger" than the MT-Propeller blades. He also defends the airfoil design. "These airfoils we use are a special development with the help of the German DLR (German National Aerodynamic Research Institute ) taking into account the relative thicker airfoils, required for the properties of spruce.

"[The] fact is, that or natural composite blades have nothing in common with the known wood blades, used in the early days (late 1940 and early 1950) of General Aviation in the U.S.A. Our blades are high tech products which outperform any other blade type during take-off and climb and in most cases also in cruise. Because of our aerodynamic calculation methods in selecting the pitch distribution, our blades are also much quieter, so noise reduction (inside the cabin and fly-over) is a today’s requirement. Since we do all calculations and tests in house with our well educated engineers and test pilots, we have direct comparison to OEM propellers,
with either metal or carbon blades on more than 200 STC's we did, from powered sailplanes to regional airliners, certified by EASA, FAA or many other CAAs."

In summary, Muehlbauer writes that the GA market is tough and unforgivable and competition is always good as long as it is fair, resulting in most cases in better products.

"The success of our propellers in the biggest General Aviation marketplace on earth, the U.S.A. tells us, we are on the right track," he said.

In a response to the letter emailed to ANN, Hartzell Propeller Executive Vice President JJ Frigge said "Whether it be at trade shows, one on one discussions with airframers and modifiers or working on propeller/engine collaboration projects, our customers have indicated a strong interest in understanding their product choices.

"In this instance, we believe that blade material selection, structural composite at Hartzell and wood laminate for MT, leads to important differences in airfoil design, blade shank retention and, ultimately, overall performance.

"We welcome a continued dialogue on these topics so that we can work with customers to help them make the best choice for their aircraft."

FMI: www.mt-propellerusa.com, www.hartzellprop.com