How to measure the speed of your Skywagon.

In the July newsletter, David Paule wrote about testing the speed of his 180. He experienced a fairly wide variation of airspeeds, perhaps partially due to atmospheric conditions, but also due to his methods. Since we all seem to be messing with our airplanes these days, I thought I would write to suggest some methods for testing airspeed.

David notes that crosswinds could affect his results, and tried to choose a day when winds were light. To understand this better, we have to go way, way back to our private pilot ground school. Remember the whiz wheel we used to do flight plans? When we had a cross wind, we would compute the crab angle needed to track our desired course. Then we would compute our resulting ground speed. It would always be less than our true airspeed, because we were using some of our forward energy fighting the crosswind. This is the source of some of David’s variations between runs, and even a 10 knot difference in crosswind component can make a couple of knots difference in forward ground speed.

The real reason the crosswind affected David’s results, was because he was using a ground reference (I-25) and flying a ground track. Cessna, Beech and others for years have avoided this problem by flying a heading, rather than a ground track. This way, all the forward energy is being used in the direction being measured. Using this technique, the pilot flies a heading of true north, and counts 10 section lines. Then he reverses course, and flies a heading of true south, counting 10 section lines. The key is that he allows the airplane to drift sideways as it will, and does not use any energy fighting the drift. You can do this in any direction, using any landmarks, as long as you fly exactly 180 degrees different heading in each direction, and average the results.

This is often done between VORs. Fly the magnetic heading between the two VORs, start the clock when the OBS needle indicates station passage over the first VOR, then stop it when you pass the second. Then fly the reverse course. Average the two times, and measure the distance between the VORs on your sectional chart to use in your calculations. The course selected on the OBS must be 90 degrees from the course between the VORs, so you can detect when you go by the station at the far end. You probably won’t pass right over the station, because you are holding the heading, and you will drift sideways with any crosswind you might have.

When you want consistent results from one day to the next, it is important to eliminate all the variables. You must fly at the same weight, the same power setting (a digital tach helps here), the same density altitude, and even the same barometric pressure. Also, the air should be smooth, since turbulence will slow the aircraft. David has an extra problem on Colorado’s front range with the mountain wave that is almost always there to some extent. Of course, if the air is rising or falling, and the aircraft climbing or descending, speed can vary quite a bit. Remember that in a low pressure center, air is generally rising, and in a high, the air is sinking. I have found 1 to 2 kts variation due to barometric pressure. Also, it takes 4 or 5 minutes for your aircraft to settle down to speed, so start your run 5 minutes from your landmark.

If you use GPS or Loran to measure speed, you must make your two runs exactly with and against the wind aloft. This means trying different headings to find the max ground speed, then reversing course.

Another consideration is safety. Probably, with all the staring at instruments involved, you should have someone along to watch for traffic, or an assistant to help watch instruments and take data.

When you modify your aircraft with beacon removal, speed kit, big gnarly engine, or whatever, test it before and after. Find out if you got your money’s worth, and tell the rest of us. That is one of the things this club is all about.

Fly safe, and have fun.