Effect of heavy rain on airfoils

Quite an old paper - 1992.

On a quick read (it's pretty heavy) I wonder if this is applicable to GA aerofoils. The lower speed is perhaps not a factor because these tests were done largely in high lift (landing) configurations.

I'm sure there are some much more experienced glider pilots than me, and I have not flown a glider for years, but I remember flying my ASW19 through heavy rain and it degraded the performance to an amazing extent.

Composite canard aircraft often undergo a nose-down pitch trip change when entering rain, to a greater or lesser extent depending on the canard loading. This was the topic of a lot of discussion 30 years ago when these aircraft were first being built and flown in large numbers. The effect is typically initiated in very light moisture, so much so that pilots may feel the trim change before seeing rain drops on the canopy.

The Cirrus (probably just like most other aircraft with very "fine" aerodynamics) significantly loses cruise performance in rain. If you fly it from clear air into light rain, you immediately lose about 5 knots of airspeed. If it is heavier rain, it's more like 10 knots. This is without any ice, just normal rain.

I recall reading a claim that the Piaggio Avanti changes its pitch significantly upon entering IMC.

It sounded very hard to believe...

I had a glider which seemed to increase the sink rate from less than 2 knots to about 8 in rain, at a cruising speed of say 50 knots. Part of the sink rate may have been that the airmass itself was sinking as the rain cooled it – impossible to tell. But some of it was undoubtedly due to loss of CL at its previous angle of attack.

Such an increased sink rate at substantially the same attitude means increased angle of attack to maintain lift. That is likely to cause a trim change as the centre of lift would move.

Modern glider airfoils are less sensitive to raindrops (or bugs) on the wings. Even so, many high performance gliders are fitted with bug scrapers to prevent significant loss of efficiency. We don’t fly much in rain, anyway.

Chris N.

The flight manual for Scheibe Falke (SF-25) explicitly mentions the effect of rain:

The aircraft utilises a glider airfoil, which is sensitive to rain. Drops of rain on the wing disturb the airflow and reduce the lift. While the dry wing minimum airspeed is about 67 km/h, with wet wing it increases to 80-85 km/h. Water on wings also changes the stall characteristics. Being positively docile in a stall with dry wings, the aircraft will drop a wing when wet. When flying in rain, keep the airspeed above 85 km/h, do not lift off at airspeeds under 85 km/h, and maintain approx. 105 km/h in climb and on approach. Avoid steep turns and other manoeuvres involving high G-forces!

I found this slight more digestible description of the effect:

AERODYNAMIC EFFECT In torrential rain a film of water will form on the surface of the wing. This film will become distorted and "wavy" under frictional stress in the airflow, thus increasing the "roughness" of the wing surface. The roughness will be further increased by the continuing impact of the large raindrops because these will crater the water film. One effect is to move forward the point of transition increasing drag and reducing lift, especially at high angles of attack (compare figure 1 and figure 2). At rainfall rates of 100 mm/hr the increase in drag has been estimated at 5-10%, increasing to 30-50% in 2000 mm/hr rainfall. The other penalty is a reduction of up to 30% in the maximum lift generated. The knock-on effect of increased drag and reduced lift is to increase the stalling speed of the wing. All these factors become more pronounced in a high-lift configuration.