MAR 2015

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: https://experimenter.epubxp.com/i/471466

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Page 28 of 36

EAA Webinars INTERACTIVE | EDUCATIONAL | MOBILE What would you like to learn? EAA offers a series of free live webinars moderated and presented by aviation experts on a variety of topics. Register today! EAA.org/webinars EAA Webinars are supported by Sheet Metal for Kit Aircraf EAA SportAir Workshop instructor Mark Forss walks you through the terms, tools, and skills required to build the aircraf of your dreams. Visit EAA.org/Shop or call 800-564-6322 to order or for more information. EAA.org/Shop.com Copyright © 2015 EAA $ 29 .95 $29.95 is the EAA Member price. Non-member price is $36.95. EAA Experimenter 29 As I tucked my head into the cockpit, I asked the owner to blast away, and there was no horn. But out of the corner of my eye I saw the airspeed indicator spin around. Yes, you guessed it; the owner misunderstood my vague request and hit the pitot tube instead of the stall warning slot with the air blast. They are within a couple of feet of each other. When I looked back at the airspeed indicator, it looked like the attached photo, with the needle stuck at about 75 knots. We fruitlessly tried sucking on the pitot tube and tapping on the airspeed gauge, to no avail. I removed the instrument from the panel, and the next day we packaged it up and shipped of to Century Instrument in Wichita, Kansas. Century is probably the largest instrument repair company in the United States. I am told it does hundreds of instruments per week. If anyone could fi x the airspeed indicator, it was Century. It didn't take long after Century got the instrument that we re- ceived a call saying the internal diaphragm was ruptured, and the unit was not economically repairable. The amount of $425 got us a replacement instrument. It was an expensive mistake. Airspeed indicators are extremely sensitive. To give you a feel for it, a rough calculation of dynamic pressure can be done with the equation P d =1/2 v2 where P d is the dynamic pressure, is the density of air, which is 0.0765 pounds m /feet3 at normal sea level day temperatures and pressures, and v is the velocity in feet per second. For example, 75 knots gives 0.15 psi, and 200 knots results in just over 1 psi. It is no wonder that the airspeed indicator went berserk when we hit it with 100 psi! This is why you never want to even blow on a pitot tube with your mouth. Also be careful when washing the plane. A blast from a water hose, or even worse, from a pressure washer can result in a damaged airspeed indica- tor. Ditto for altimeters and vertical speed indicators, but they are not as sensitive as the airspeed indicator. By the way, the stall warning system did work when we sucked on it, but I still have the fl avor of dead bugs on my tongue. All homebuilders should think about a stall warning or angle of attack (AOA) for their birds. The National Transportation Safety Board estimates that a stall is involved in more than half of our fatal accidents. Noncertifi cated systems are available for less than $200. I hope this little discussion helps you with building and main- taining your aircraft. Richard "Dick" Koehler, EAA 161427, is an active pilot, A&P mechanic with inspection authorization (IA), an instructor for the EAA SportAir Workshops, and EAA technical counselor for EAA Chapter 186.

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