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Designed for Success

By Staff Writer | August 1, 2015
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By Frank Lombardi

It’s been said that once you develop the stick-and-rudder skills necessary to fly the machine, being a good helicopter pilot is all about good decision-making and risk management.

I agree wholeheartedly and try to practice this at all times. The greatest pilots have the best situational awareness. What they learn is added to a mental database that, over time, becomes wisdom. Here’s a story that happened to me. One day, I was in the produce aisle of a supermarket shopping for fruits and vegetables. Next to me was an attractive lady doing the same. At some point, she took note of the embroidery on my polo shirt. “Excuse me, but what’s that on your shirt?” she asked.

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Since pilots are taught always to think about the next two things to do, I responded quickly, “Oh, that’s called a ‘Fenestron.’ Let me explain what that is.”

I proceeded to explain that all single-rotor helicopters need something to counter the twisting force created by the main rotor acting on the fuselage and allow the pilot to control where the nose points. The Fenestron does this, I said. It has the same purpose as a conventional tail rotor, yet creates its side force through use of a ducted fan. I added, “It’s a very successful design.”

Luckily, I was next to the tomatoes, so I grabbed one, pulled out my pocket knife, and cut it in half horizontally (I paid for the tomato, of course) and pointed to the thick outer skin.

“See, the shroud protects the fan blades from striking anyone or anything, increasing safety,” I said. “It also improves the efficiency of the fan blades by virtually eliminating drag-inducing tip vortices, and shields the blades from the wind effects of forward flight and main rotor downwash.”

I explained that the shroud also aids in lowering the static pressure at the lip of the duct, which helps draw in air. “Since the duct aids the flow in becoming fully developed almost immediately,” I said, “the diameter of the fan can be about 30 percent less than that required of an equivalently powered conventional tail rotor.”

At that point, she looked at her watch, probably to mark the time that she learned such an interesting fact.

I may have cut her off as she went to speak. But I couldn’t miss the opportunity to explain how a Fenestron also is quieter than a conventional tail rotor, due to its many (usually 8-12) shortened blades and absence of tip vortices.

I pointed to the inner part of the tomato and explained that the most-evolved Fenestrons wisely have their blades spaced unevenly around the hub, and use stationary stator vanes to help quiet acoustics and smooth the airflow even more.

At this point, I felt it was important to tell her that all this wonderful engineering comes at a price. Fenestrons are mechanically complex. Building a sizable one with power comparable to or greater than a conventional tail rotor results in a significant weight gain, since the shroud must be structurally incorporated into the vertical tail. In addition, its thickness typically needs to be about 20 percent of the fan’s diameter for proper aerodynamics.

So most Fenestron fan diameters end up being even smaller than their conventional tail rotor equivalent. This increases power requirements, and puts about a 12,000-pound limit to the weight of the helicopter they are designed for. “Unfortunately, engineers must always make compromises,” I said. “Does that answer your question?”

“Oh. Actually, I just thought it was a bear riding a unicycle. But thanks for that explanation,” she replied. “I’ve got to be going… my groceries are spoiling.” She quickly hit “bingo fuel” and sped away.

Now, I am not going to get into the minutia of conditional probability. But as a pilot and math guy, I can tell you that odds—no matter how remote—can sometimes catch up with you. This brings us back to the decision-making, risk management and situational awareness I referred to earlier.

I’ve worn that polo shirt many times since that incident, with nary a word from anyone. Then, on just another regular “clear and a million” day, it happened. This time I was at a gas station.

“Excuse me, but what is that on your shirt?”

I looked to my left. The question had come from a pretty lady fueling right next to me.

“What, this?” I replied. “Oh… it’s… a bear riding a unicycle.”

She laughed out loud. “OH MY GOSH, THAT IS SO ADORABLE! I’ve never ever seen anything like that! Forgive me, but what did you say your name was?”

See? Lesson learned.

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