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High Altitude Soaring Performance
Most high altitude flights involve climbing in mountain wave conditions. There have been several notable cross-country flights that involved climbing in wave to high altitude and running downwind or crosswind for great distances. The most recent of these flights is the Hebaud brother's 1450 km flight from France to North Africa. Another great long distance wave flight was Ray Lynskey's 2,000 km flight in New Zealand.
Many questions come to mind when planning a flight of this sort. Some time back, I had a letter published in Soaring Magazine (June, 1992) asking for help with these kinds of questions. I received a number of letters on the subject from some very knowledgeable people. Sharing this good information with other soaring pilots seems in order!
Here's the situation I put forth. Disregard pitot static errors, and consider calibrated and indicated airspeed to be identical. Release our sailplane at 31,000 feet in a standard atmosphere with no wind or vertical motion of the air mass. We maintain 60 knots IAS (indicated airspeed), which is almost exactly 100 knots TAS (true airspeed). Our sailplane's sea level polar shows a sink rate of 140 feet per minute at that IAS. This speed also happens to be at the maximum lift over drag ratio point, and is 43.4 to 1. The minimum sink rate from the polar is 120 feet per minute at 50 knot IAS. The questions follow:
- If the descent rate is timed, what will the actual descent rate be?
- What can I expect to see on my variometer?
- What will my actual glide ratio through the airmass be?
Before getting into the answers to my questions, there are some basics that should be understood. First, indicated airspeed is not a very good measure of an airplane's performance, particularly if the aircraft is flown at varying weights. Angle of attack of the wing chord relative to the airstream is the best measurement of performance for all fixed wing aircraft. The angle of attack at stall, minimum sink rate, and best glide ratio is always the same for each of these benchmarks, no matter what the weight or altitude. This is true as long as the airfoil remains uncontaminated with bugs, water, etc. The Navy and Marines have known this for years, and use angle of attack indicators for approach and to optimize cruise. The Air Force and the commercial airlines do not use angle of attack indication on their aircraft.
Using indicated airspeed as a measurement of performance is simpler and cheaper than mounting and calibrating a sensitive electrically driven angle of attack transducer vane. As soaring pilots, we know that when we add weight in the form of passengers or ballast, the indicated airspeed for stall, minimum sink rate, and best glide ratio increase. Most of us memorize these numbers from a table in our aircraft manual. The angles of attack for stall, minimum sink, and best glide do not change as we add weight. A navy pilot uses the same angle of attack on approach, no matter what his aircraft weight. This assumes no changes in wing shape by using different flap settings.
A good practical definition of high altitude soaring is "soaring in heights above 18,000 feet". At that altitude, atmospheric pressure is about half that of sea level. The time of useful consciousness without oxygen is about 30 minutes, and the relationship of true airspeed to indicated airspeed becomes significant. Flight level 180 is also the floor of the positive control area in the continental United States.
As we climb to higher altitudes, the air becomes thinner, or less dense. However, the indicated airspeed for stall, minimum sink, and best glide remains almost exactly the same. Some errors do creep in at the higher altitudes, but they are not larger than a knot or two. At 31,000 feet in our example, the minimum sink speed will still be near 50 knots IAS, and the best glide speed will still be near 60 knots IAS. The angle of attack of the wing at these indicated airspeeds will be the same as it was at sea level. The thing that has changed is the speed through the air mass (true airspeed). It is 100 knots in our example situation, and since there is no wind, the ground speed will also be 100 knots.
If we time the descent while gliding at 60 knots IAS, it will not be 140 feet per minute. It will be 233 feet per minute. If we slow our aircraft to the minimum sink speed of 50 knots IAS, the sink rate will not be 120 feet per minute. It will be 199 feet per minute. The glide ratio at 60 knots IAS will still be 43.4, but since we are moving faster through the airmass in a horizontal direction, our vertical descent component will increase also. We haven't lost anything - we're just going faster! It's the same effect as ballasting, but there are no changes in indicated airspeed for the benchmarks as there is when adding ballast. An interesting side effect of all this becomes apparent when you think about it. The higher you go, the stronger the lift has to be to maintain altitude or climb!
What you see on your variometer depends on the type. A mechanical vario, such as the Winter will read the actual rate of descent (233 fpm). Electronic pressure transducer varios may read either the true rate of descent (233 fpm), or may read a compensated rate of 140 feet per minute. An altitude compensated computer that accurately computes speed to fly is a necessity if you are going to optimize performance during wave flights at high altitude.
Adding wind, which is always present and is always strong in high altitude wave conditions further complicates the situation. It's not unusual to have wind velocities equal to 75% or more of the true airspeed of the glider. The optimum speed to fly may exceed the structural limits of the glider when attempting to penetrate upwind. Even when maintaining a track parallel to the wave (90 degree crosswind), you must set substantial headwind components into the computer because of your large crab angle. The newest generation of computers that have Global Positioning Satellite (GPS) inputs will make life in the wave easier. They will automatically compute and set the proper wind component for the track you are maintaining over the ground.
A very good source of detailed information on the subject of high altitude wave performance is contained in the January, 1992 issue of Technical Soaring. Another source of sailplane performance data is the Polar Explorer computer program sold by CuSoft Research. It is a professionally written program of very high quality, and contains the answers to just about any sailplane performance question imaginable. My thanks to Dr. Scott Jenkins from the University of California, and Branko Stojkovic, President of CuSoft Research. The information they provided contributed immensely to my understanding of high altitude soaring performance.