Technical Corner
October 1979
We will plan to make the Technical Corner a regular feature of the Newsletter.
VariEze materials tested to support Space Shuttle NASA’s Dryden Test Center recently flew an F-15 fighter with a wing addition constructed of three pound urethane foam skinned with four plies BID/Epoxy. These wing additions were subsequently covered with space shuttle tiles, to test their ability to withstand loads expected during reentry. The bare BID/foam additions were flown to 1.5 times the maximum expected dynamic pressure of the shuttle - 1100 lb per square foot, at 1.4 mach, 660 mph indicated speed. This is a dynamic pressure of nine times that of a VariEze at red-line speed.
The equivalent flat-plate drag area of an entire VariEze is less than 1.5 square feet, or roughly speaking, an EZ has the same drag as a square flat board 14 inches on a side. With this small amount of total drag, adding drag-producing additions will have a large effect on the aircraft’s performance. For example, a cross-over exhaust system will slow the cruise speed by 10-15 mph, due to the drag produced by the blunt bumps required on the cowling. This increment has been verified by everyone who has made this exhaust change. The bumps also cause turbulence that increases prop noise.
Moisture change - moisture on a wing from rain will effect its lift. This effect is small on a conventional aircraft, ie, the Grumman Tiger descends 500 fpm if untrimmed entering a rain shower, but is easily trimmed out. A Canard aircraft generally has a much larger trim change in rain because its high lifting wings are located far apart. We do not fully understand the reasons for this, but the following characteristics exist for most VariEze’s: if a trimmed EZ enters light moisture or light rain it will climb, requiring about 1/2 lb to 1 lb push to maintain level flight. In heavy rain, most EZ’s trim nose down, requiring a mild aft stick pressure to fly level. The trim change varies with speed, being barely perceptible at 70 knots and higher as speed is increased. One EZ flyer reported a heavy aft force required (15 to 20 lb) when making a 150-knot (172 mph) descent through a heavy rain shower.
We often hear the following comment from EZ pilots who have just flown their airplanes, my speed is low, I set the power for 2650 rpm (75% power) and only get 166 mph true’. You are not at 75% power, just because you have set the rpm for that power on the Cessnas you are used to flying. Remember, the EZ has very wide speed range for a fixed-pitch prop. To get adequate take off performance, the EZ has to turn high rpm at 75% cruise. Refer to the following chart for a clean 0-200 EZ with Ted’s prop and wheel pants. At 8000 ft and 2650 rpm you are generating only 49% power (a good economy cruise). To get 75% power at 8000 ft you have to use full throttle and turn 2990 rpm at a TAS of 193 mph. We generally do not cruise at 75% - we prefer 60% or less to conserve fuel and reduce the noise level. 60% gives a TAS of 178 mph and fuel flow of only about five gal/hour at peak EGT. Static rpm - if you have a static rpm of less than 2450, with the 0-200 or 0-235 engine, you will have poor takeoff performance, due either to too much prop or a sick engine. Be sure your tach is calibrated. The hand held Heath kit thumb tach works well. Can even be used in flight, just point it back from inside the cockpit.