Uncategorized

Finding the Lesson

By By Frank Lombardi | October 10, 2014
Send Feedback

Legend has it that I showed up for my interview at the police aviation unit wearing a pocket protector. I categorically deny this. At that time, there existed a crusty, weathered Vietnam-Era instructor pilot who was heard to say, “I don’t care where you put that dweeb, just don’t put him on my team.” Well, you know how that request was answered. He had to live with me for years. I recently saw that since-retired aviator and he wanted to know when I was going to write “something good.” That’s not all. Lately my phone has been plagued with texts from coworkers and pictures of them being lulled to sleep by my writings. I think they may be sending me the message that my columns are getting a bit too technical for their tastes.

Sorry guys, I guess it’s true that I’m a bit of a nerd about all this stuff. I can’t help it. For me, every aspect of flying brings new challenges to my understanding, new lessons to be learned. But sometimes it can be its own challenge to make some of the deeper topics sound exciting in this limited space.

Well this month, at your request, there will be no boring lecture, engineering mumbo jumbo, or formulas. Just light, easy reading. So before you continue reading, take the magazine outside and find a spot in the shade of your aircraft. In fact, I’ve cleared my mind of all equations and I’m writing this poolside, with a glass of sweet tea, while watching a fly buzz around the glass.

Advertisement

Funny thing about flies…did you ever notice how a fly can seem to hang onto your windshield at darn-near highway speeds? It’s pretty amazing, but not really magic, once you realize that it can do that because it’s standing inside the boundary layer. See, the boundary layer is a very thin layer of air slowed by the friction of the surface it flows over.

Skin friction causes the air very close to the surface to slow down from the freestream velocity you’d see on your airspeed indicator, essentially down to zero at the surface itself. While this is good for the insect as it tries to hold on, it is actually a contributor of drag. As the flow travels along the surface, it slows, loses energy, and the boundary layer thickens. It will eventually separate from the surface and become turbulent, causing a big drag rise.

Keeping the flow attached longer by smoothing a surface can reduce drag. Sometimes being able to control when and where this separation happens can actually help decrease it too. It’s why a golf ball has dimples and why some airfoils have gurney flaps at their trailing edge (see The Leading Edge, April 2012).

There have been a few experiments on rotor blades involving boundary layer control with suction, to delay retreating blade stall and reduce rotor power requirements, but none proved to be very practical. Anyway, sorry I digressed. I know I was trying to keep it light.

Oh, another interesting thing about bugs: They can be used as a sort of poor-man’s flow visualization device. If you are sitting in the shade of your aircraft like I suggested, get up and look at its nose, and take notice of all the bug splats. You’ll be able to see the direction of the airflow change by way of the splat direction. Above the airflow’s stagnation point – the point where the flow velocity is zero – the bug splats will streak upwards.

As the streamlines of air approach the stagnation point, the splats will become more circular until they look like a bullseye. That’s the point where the airflow separates from going over the nose to under the belly. After that, the splats will start to streak downwards. Seemingly useless information, I’m sure, yet I was involved in at least one aircraft program where the bug splats helped determine that the helicopter was flying at an excessively nose-low attitude. The bug splats were actually streaking downwards above the pointiest spot on the nose, showing a very large area of flat-plate pressure drag! How exciting! And no wind tunnel necessary.

Sorry, I keep getting off-topic. But I’m sure you guys will agree with me when I say that- Hello? Hello?

Oh well. I guess you’re right about the engineer in me. Some things will never change. Enjoy your nap. Zzzz

Receive the latest rotorcraft news right to your inbox