The aerodynamic effect of heavy rain on airplane performance

[Centaurus]

The following extracts are from US Flying feature writer Thomas H. Block published in 1981 and are well worth studying. He states "Water, indeed can cause problems beyond hydro-planing and poor visibiility.

With regard to the aerodynamic effect of heavy rain on airplane performance. The National Aeronautics and Space Administration has been conducting a series of tests to determine the effect of heavy rain on airfoil aerodynamics. The results of these tests have shown that heavy rain can significantly increase drag as well as decrease lift and stall angle of attack. This paper describes a recent effort to use the heavy rain airfoil data to determine the aerodynamic effect on a conventional twin-jet transport. The paper reports on the method used to model the heavy rain aerodynamic effect and the resulting performance degradation.

The heavy rain performance effect is presented in terms of the diminished climb performance associated with increasing rain rates. The effect of heavy rain on the airplane's ability to escape a performance-limiting wind shear is illustrated through a numerical simulation of a wet microburst encounter. The results of this paper accentuate the need for further testing to determine scaling relationships and flow mechanics, and the full configuration three-dimensional effects of heavy rain.

Short extracts from the article follow: Intense rain beating continously against the wing seems to have the same effect on the airfoil as a layer of frost or ice. At higher angles of attack - the sort of profiles we fly during the slow airspeed maneuvers of takeoff, landing or missed approach, it is estimated that all that water can distort the airfoil shape and reduce lift by as much as 30 percent. That's enough to affect the performance of any aircraft.from light aircraft through to heavy jets. After reviewing the official reports on several airline accidents that occurred in heavy rain, the NASA-funded study by the University of Dayton Research Institute points out that "the effect of rain was not taken into account."

We believe that this heavy rain factor - totally neglected in accident investigation - produced significant aerodynamic penalties and resulted in a serious overestimate of wind shear as a cause of thunderstorm - related accidents. Penalties associated with heavy rain can be of the same order as those associated with windshear."

If further testing proves the theory correct, we need to get the word out that heavy rain on takeoff or landing requires additional precautions. By keeping bank angles shallower, pitch attitudes flatter and airspeed margins wider, we could easily avoid the risks. If we stay closer to the centre of the aircraft's performance box and don't depend on book-value performance for runway, climb gradient and airspeed requirments, we will finish our day's flying smiling.

Some pilots still don't believe that a thin layer of ice or frost can roughen the airfoil shape enough to be disastrous, and those are the people who litter our wintertime accident statistics. Now we are told by this new line of NASA-inspired research, too much water at the wrong time can be nearly as bad. Perhaps the time has come for us to add one more thing - -heavy rain - to that list of items we prefer to keep off the wings.

I have seen no literature on the effect of heavy rain on wing aerofoils and certainly never in the numerous flight simulator sessions I have conducted or observed. I think it is vital knowledge for pilots to be aware of.