Dermal Dynamics

(Posted July 1998)

Like much of the country, Seattle has been undergoing a bit of a hot spell lately. While our record temperatures in the high '90s probably don't impress some of you, running the mercury that high does produce a mighty fine environment for open-cockpit aviating.

The Fly Baby is great in this weather. Just preflight, roll the plane out of the hangar, and plop into the cockpit. No coat, no gloves...not even a helmet, if just going up to shoot some landings. Looks of envy from all the Cessnas and Pipers taxiing with the doors propped open, and lots of kids waving from the Park-n-Ride located at the arrival end of runway 3-4.

The (for Seattle, at least) unusual experience of coatless open-cockpit flying has given me the opportunity to study the reaction of human skin to high-speed (well... FLY BABY speed, at least) slipstreams.

I tend to fly with my left elbow propped on the side of the cockpit, just like one does when the car window is rolled down. Either my canvas or my leather flying jackets react the same... the fabric flutters in the ~100 MPH breeze.

When I fly without a coat, the flesh around my elbow reacts in a different fashion. With my elbow only slightly bent (like I'm reaching for the instrument panel), the skin itself flutters like the coat fabric. As I draw my hand back, bending my elbow more acutely, the rippling slows until two standing waves seem to "freeze" in place.

The first wave is just before the bend in the elbow, and the second, larger one is just behind the bend. These extend almost all the way across my arm, with a pretty broad radius of bend. The cross-section of the standing wave is pretty much like a shark's fin, gradually sloping up the windward side and chopping back down at the leeward side.

Generally speaking, the skin seems to take on the appearance of a skin-divers wet suit. About the right apparent thickness, about the same ability to fold. If I lost fifty pounds or so, things might look different...:-)

The standing waves feel OK, but in truth, the rippling is a bit uncomfortable. Not pain as such, just a little bit of "whip-crack" feeling from where the skin flaps.

I just recently discovered another Dermal Dynamics phenomenon, one that is kind of pretty to look at. I've been making most of my flights in the late evening when the sun is well down in the west. To shade my eyes when flying west, I've taken to placing my elbow on the cockpit coaming with my arm going up and hand lightly clutching the windshield edge for support.

With the sun thus behind my forearm, I have noticed a Kirillian-photograph nimbus around the forearm. While my arm is out of the direct slipstream, enough comes around the edge of my windshield to flutter the hair on my forearm. Backlit by the sun, it's quite interesting to look at.

The amount the hair flutters depends on how far around my forearm it's located. For a frame of reference, let's assume the wind is hitting my arm from the 12 o'clock position. The hair at nine and three o'clock is directly back-lit by the sun, and is fluttering so fast it's a blur. But as the wind curls around my arm, the lift produced raises the hair but flutters it less and less. By the eight and four o'clock positions, the individual shafts are pulsing at a quite slow rate.

Sound weird (and a might gross) but it's actually quite pretty. I might try to haul a camera along and get a picture of it.

Over the past several months, I've mentioned the high noise levels produced by the slipstream impacting the cups of my headset. This isn't engine noise itself, it's the effect of prop blast striking the headset cups and inducing a low-frequency warble into my ears that can get quite painful. Flying without a headset, with just earplugs, is very comfortable but of course, one can't use a radio. As I've reported here, a Lightspeed ANR headset couldn't seem to handle this type of noise as well, and the ANR circuitry had fits when the slipstream changed as I moved my head to look for traffic. Pastor Dave sent me some sound-absorbing material to try to insulate the inside of the cups of my current headset and change its fundamental frequency, but between mike booms and other interior hardware, there isn't a way to smoothly install it.

One possible cause of the noise has been ruled out. My flying helmet is a full-head one, without a provision for a radio headset other than small holes (~3/4") on either side. Wearing the headset over the leather, there was the possibility that the resulting poor seal was the cause of the noise. However, I bought another helmet at the Arlington Fly-In. This one has the big openings on either side so that the headset makes direct contact with my head. Alas, it doesn't seem to affect the noise level.

The cups themselves are the problem. Back when I flew the NORDO Fly Baby, I occasionally used a headset-style hearing protector and got the same noise levels. So the solution involves the aerodynamics of the headset.

One possibility arose as I shopped around a local avionics store. Telex makes its headset model 5x5. Instead of the usual cups and speaker, it transmits noise to the ear via a small plastic tube into an earplug. No external cups=no slipstream noise.

Initially, I rejected the 5x5 because of the way the mike boom was supported. The headset package includes a little band like a Walkman headset to keep the sound transducer in place and support the microphone boom. It also includes a clamp for using the bow of the pilot's eyeglasses instead, but it looked like the goggles would interfere with it. I was willing to give it a try, though.

The man at the avionics store (American Avionics at Boeing Field) said that, instead of using the headset's earplug, many pilots were getting custom-molded plugs. He referred me to an auditory specialist at Boeing Field. Not only do they make the custom ear casts, they can make them to incorporate a right-angle fitting so that the acoustic tube goes forward instead of straight out to the side like the stock earplug.

This opened the possibility that I could wear the headset *underneath* my current flying helmet...leaving no anachronistic signs other than the slim mike tube on one side. Even better, the cast earplug can include a bar for holding the sound transducer and holding the mike, thus eliminating the clumsy eyeglass bow clamp.

One drawback: The headset itself was $220, and the ear cast would be another $80...and none of it would be returnable if it didn't help my situation.

On my way back from Arlington several weeks back (there was a gear-up landing at my home field just as I entered the pattern, so I had to set down at a nearby field to wait) I met an EAA buddy using the 5x5 headset. He was quite enthusiastic about it. I mentioned I'd been tempted to try one. Turns out he had an older model as a spare and offered to lend it to me.

One problem: The older model Telex 5x5 did *not* use the little soft earplugs and plastic acoustic tubes. It uses, instead, hard-plastic earplugs connected to solid metal-tube couplers.

I couldn't wear it with my helmet, but the warm weather lent itself well to flying without one. I tried the headset the other night, with the thermometer hovering around 97 degrees.

It worked well. The slipstream noise was almost completely gone. More engine noise was apparent, but the overall noise level was quite a bit below that experienced with the conventional headset. The radio was a lot more understandable as well.

The only problem was the microphone. Every time I keyed the mike, I got a loud POP over the headset. Almost to the level of pain.

Is it a characteristic of just this older model, or is the "pop" something that is going to happen with any earplug-type headset?

So now, I'm practically back to square one. I'll probably bite the bullet and give the newer model 5x5 a try. For now, though, I've got a lower-cost option in the works. I've ordered a headset with sculpted ear cups instead of the usual hemisphere design. We'll see if the slipstream noise decreases with this more-aerodynamic system.

Ron Wanttaja

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