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 other light aircraft.
Issue link: http://experimenter.epubxp.com/i/81495
Wait a minute. You already know those answers. All you have to do is read the company's advertisements or check out its promotional literature. Look through any airplane magazine and you'll probably see an ad with speeds listed like this—Cruise: 110 knots, Stall: 32 knots, Rate of Climb: 1,500 fpm. Well, there you go. Then you notice a fl ight review of this very airplane in the issue you're reading. You're just having a kismet day! You read the article and notice the author's performance numbers don't match the company's advertised numbers. So you track down another review of that plane in a different magazine, and now you have a third set of performance numbers. Looks like kismet ain't all it's cracked up to be. You're confused, maybe skeptical, or at least curious. The picture in the ad shows the same N-number airplane as the ones cited in the fl ight reviews. It's the company's airplane, so how can the cruise, stall, and climb performance numbers be different? It would be helpful if the folks who wrote fl ight reviews presented their fi ndings for the same conditions. Sea level/standard day is the most commonly used reference, but any properly specifi ed reference would do for the sake of comparison. How about 8,000 feet density altitude using 75-percent power with the plane loaded to maximum weight at takeoff? Still not perfect—different planes will burn different amounts of fuel during their climbs from different density altitudes of airports—but it's certainly better for comparison. The reader would then have more confi dence in published numbers and an independent validation of the manufacturer's claim. And there'd be no math required. But that's not the real world. So, it looks like it's up to you to perform your own fl ight tests and calibrations to come up with your airplane's real numbers. Over the next several months we'll talk about how to collect the data you'll need to determine those important airspeeds and the rates and angles that go with them. Then we'll talk about fl ight control system characteristics, stability and control, and handling qualities. And every now and then, we're going to discuss how you interface with your airplane. Some call this human factors. This historically underrated aspect of aviation has a direct effect on safety and goes a long way toward pilot workload and ultimately the pilot's fl ying enjoyment. Knowing what to test is like knowing the strings on a guitar. It's a start, but knowing where the notes are doesn't make you a musician. Learning how to tickle them the right way is the real challenge. Same with airplane testing. Is your airplane ready? Are you, or your test pilot if it's not you? Your airport? Will you have a chase plane? Test conductor? Kneeboard cards or fancy video recorder? Lots of questions that should be answered before that fi rst fl ight. We'll cover that as well. We're not aiming for rock stardom here, but defi nitely somewhere beyond garage band. Since the only FAA-mandated performance numbers that must be determined during your initial fl ight testing are those three airspeeds mentioned earlier, let's start there. The thing about speed is it comes in several fl avors. Comparing indicated airspeed from one source to true airspeed from another doesn't make a lot of sense. The problem is published airspeeds are not always identifi ed as indicated or true, and you can't tell just by looking at them. Okay, quick review: – The speed you read from the airspeed indicator. I know that many people call this indicated airspeed, but I don't. VO VI – Indicated airspeed is VO after it's been corrected for instrument error. If you connect the airspeed indicator to a benchtop test set, the discrepancy between the accurate test set and the indicator reading is instrument error. VC – Calibrated airspeed is VI after it's been corrected for errors arising from the plumbing of the pitot and static lines. Notice if you replace your airspeed indicator with one that has a different instrument error, your calibration will be off. That's why I prefer EAA EXPERIMENTER 41