Around the Patch

The trouble with data.

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In each December issue, this magazine publishes a comprehensive listing of aircraft kits for sale at the time we go to press (late September). And while much of the information is available elsewhere-from promotional material and web sites-there are a few bits and pieces that you wont find anywhere else. One example is the number of completed aircraft, which, with a rare few examples, is not something companies like to publish in sales literature or on their web sites.

We get that data point the old fashioned way: We call the company and ask. Same for the rest of the figures, which are provided to us by the kit manufacturers. This interaction is important. In the course of reviewing the raw materials for the guide, we often find that the kitmakers web site is either quite old or in error. Sometimes both.

Every year, refining and error-checking the buyers guide is like climbing Everest. There’s just so much data. Nearly 300 aircraft or aircraft sub models, times 32 individual fields. In fact, there are more fields, because we ask for data thats not published-the maximum speed of the airplane at sea level, for example-to help us create the what-is-LSA code, as well as others that help us know if the airplane is currently available as a kit, perhaps now just available as plans, or off the market entirely. All told, there are more than 10,000 pieces to the puzzle that we print and place into our online guide, not including the photos.

We have added a new dimension to our data reduction this year. With the help of our own Barnaby Wainfan, I included a simple stall-speed estimator into a preliminary spreadsheet of all the aircraft data. It assumed a CLmax (maximum trimmed lift coefficient of the wing) of 2.0. According to Barnabys numbers, this is at the low end of whats achievable with slotted or Fowler flaps, and at the high end of plain flaps. Comparing the estimated stall speed with the claimed speed helped us find the outliers.

For those that fell outside the predicted range, we contacted the company for clarification. A few offered more realistic numbers, while others stood by the submitted figures. In part, its the industrys penchant for publishing indicated airspeed at the stall thats to blame. Pitot-static systems are notoriously inaccurate at high angles of attack. The right way to measure airspeed at stall is with an articulated pitot head on a boom, which very few manufacturers care to get into. Bottom line: Many, perhaps most, of the published stall speeds aren’t accurate.

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This year, I also inserted a basic range calculator that helped us find numerous errors. By estimating fuel consumption from 75% of the given horsepower (using a conservative 0.45 pounds/hour/horsepower for four-strokes, 0.55 for two-strokes and 0.65 for turbines) then calculating endurance from the stated fuel capacity, its easy to multiply by the claimed cruise speed to get range. We had a few with dramatically overstated range, which, in a lot of cases, we traced to incorrect cruise figures or fuel capacities. We also caught a few companies gaming the system, that is offering range figures at very conservative power settings and cruise at max allowable power. You can have one or the other. For that matter, Ive always disliked stating range, particularly as the companies do-to fuel exhaustion (like we fly that way!)-because its never realistic.

We also found mixing and matching with engines, using, say, the 100-horsepower Rotax 912S when referring to cruise speed and climb performance, but then citing the 80-hp 912 also used in the airframe for the range, empty weight and price.

When you reduce the issue, performance claims are only as good as the claimant; what really matters is how your airplane truly performs. It mystifies me that builders, even today, seem to make decisions based on the spec sheet. A 10- or 20-knot cruise-speed difference is immaterial in the face of the quality of the kit, the integrity of the company and the designs flying qualities.  

Marc Cook has been in aviation journalism for 20 years and in magazine work for more than 25. He is a 4000-hour instrument-rated, multi-engine pilot with experience in nearly 150 types. Hes completed two kit aircraft, an Aero Designs Pulsar XP and a Glastar Sportsman 2+2.

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