Rain effect on lift

October 1982

We recently received a letter from Owen Biliman, reporting on his Quickie accident in which moisture on the airfoil decreased his ability to climb. The result was a destroyed aircraft after striking tree tops. Our answer to him contains some information of interest to EZ flyers. While our research on rain effects is not complete, the information is published here in order to clear up some misinformation floating around.

"Dear Owen,

Thank you for sending along your account of your Quickie experience. The subject of rain-induced boundary layer transition and its effect on trim and performance is one that we have been investigating for several years now. Tests have included fixed and free transition measurements of six different airfoil shapes on the VariEze, Long-EZ, Amsoil racer and Defiant. Full-scale moisture tests have been conducted in the NASA Langley 30 x 60 wind tunnel. I have not published an account of these tests because they still contain some contradictory results.

For example, theoretical predictions call for the largest trim change to exist on the well-contoured aircraft that normally have the most extensive laminar flow. J ust the opposite is true - the best-contoured ones have the least trim change in rain! The trim change of the Long-EZ and VariEze in rain is generally mild. Most trim down in rain, about 25% of the VariEzes trim nose up. There have been several report a strong nose down trim change, outside of the pitch trim capability. In general, these have been fixed with a correction of canard incidence or elevator shape. I know of no rain-induced accidents with the VariEze or Long-EZ, however several have reported extensive increases in takeoff rotation speed and take off distances.

Again, there are variances from one airplane to another. We have done low-level aerobatic maneuvers in driving rain with our Long-EZ’s without noticing any major difference in maneuverability. We have no operational limitations for flying in rain except to throttle back to save the propeller leading edges from erosion. Fixed transition test conducted by applying grit on the leading edges (at 4% chord top and bottom) has shown that maximum lift is reduced significantly, increasing the minimum speed by about 8 knots.

The NASA wind tunnel tests (see the adjacent plot (image is below) of CL with fixed, free and wet surfaces) seem to predict that the EZ has about half the degradation in rain as for fixed transition. This approximately four to five knot increase in minimum speed while wet generally is not a problem since we all seem to fly a bit more conservatively in the weather. Our tests with new airfoil designed to reduce the rain-induced trim have not led to changes on the Long-EZ since they have all shown degraded low speed performance (less lift). The low Reynolds-number of 0.5 million is a particularly difficult section design area.

The Defiant’s canard operates at twice the RN. It has a very mild nose change in rain and no measurable effect on take off speeds. The Defiant doesn’t have a trim change with airframe ice. Long-EZ N26MS has a moderate nose down trim change with rhime ice. Concerning your Quickie and others that have near equal-area tandem wings: we have not conducted fixed transition or moisture test on these, but based on your and others experience, it appears that the transition effect on maximum lift is more severe. This is apparently due to the double effect of loss of CL and the inability to trim to an adequate angle-of-attack. I have referred your letter to Quickie Aircraft Corporation. They no doubt will be conducting tests and/or making recommendations or improvements to prevent recurrence of your accident.

Best Regards,
Burt Rutan’

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