AN IDEA FOR A NEW McCREADY POINTER

August 2 2004 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

There are two McCready speed-to-fly pointers that appear on the analog vario display portion of the Brauniger IQ Comp GPS variometer. One of these pointers represents the glider's average climb rate over the last 10 minutes of climbing. This can be thought of as a predictor of the expected average climb rate for the near future. The other pointer is called the "active McCready pointer" and essentially represents the optimum airspeed-to-fly for a given set of conditions including wind, updrafts or downdrafts, and expected future thermal climb rates. More precisely, the "active McCready pointer" points to the expected future thermal climb rate for which the glider's current airspeed would be exactly optimal for the fastest cross-country racing performance, considering the polar curve, and considering the current conditions, including wind, and including any updrafts or downdrafts that are present. When the "active McCready pointer" points to "zero", the glider is flying at the airspeed that will yield the flattest possible glide path, given the existing wind conditions, and given any updrafts or downdrafts that are present. It's not the purpose of this article to get into a comprehensive discussion of McCready theory, but in very general terms, a pilot should try to continually adjust his airspeed on glide so that the "active McCready pointer" always is aligned with the average-past-climb rate pointer.

Many other vario manufacturers are using a similar system.

Note that neither of these McCready-related pointers considers the glider's position in relation to a chosen goal or target waypoint.

I propose that any sophisticated, yet elegant, vario design should have a third pointer, which could be called a "glide-to-target" pointer. This pointer would give the pilot information about his overall situation with respect to the target, considering the glider's physical position in space with respect to the target, and also considering the apparent headwind or tailwind. This pointer would point to the highest "active McCready pointer" value at which the glider would be expected to reach the chosen target, given the current wind conditions and given the glider's current position with respect to the target, in air with no vertical motion.

For example, imagine that the "glide-to-target" pointer points to "1200 fpm". This indicates that the pilot will be able to successfully race to the target, in air with no vertical motion, if he flies at an airspeed that keeps the "active McCready pointer" also pointing at "1200 fpm". If the pilot flies at a slower airspeed, so that the "active McCready pointer" points to less than 1200 fpm, then his situation in relation to the target will improve, and the "glide-to-target" pointer will creep upward. The "glide-ratio-to-target" pointer will also creep upward if the pilot encounters an increase in tailwind, a decrease in headwind, or a strong, widespread updraft. If the pilot flies at a faster airspeed, so that the "active McCready pointer" points to greater than 1200 fpm, then his situation in relation to the target will deteriorate, and the "glide-to-target" pointer will creep downward. The "glide-to-target" pointer will also creep downward if the pilot encounters an increase in headwind, a decrease in tailwind, or a strong, widespread downdraft. When the "glide-to-target" pointer is on zero, then if the glider encounters no lift or sink, it will just barely be able to make the target by flying with the "active McCready pointer" on "zero". If the "glide-to-target" pointer points below "zero", then the glider cannot make the target at any airspeed unless it encounters an updraft or a favorable change in wind conditions.

In essence the "glide-to-target" pointer would serve as a "gas gauge", telling the pilot how much reserve altitude he has, in relation to the target and the prevailing conditions. More precisely, the "glide-to-target" pointer tells the pilot how much margin he has in his angular relationship to the target. This is better than an approach altimeter display. For example, if the "glide-to-target" pointer points to "1200 fpm" then this means the same thing--that the pilot can make the target, in air with no vertical motion, by flying at the airspeed that puts the active McCready pointer at 1200 fpm--regardless of whether the glider is half a mile from the target or 10 miles from the target. In contrast, an approach altimeter reading (other than zero) has to be interpreted in the context of the distance remaining to target. For example, a 500' margin of altitude means something very different if the glider is half a mile from the target, than if the glider is 10 miles from the target.

This "glide-to-target" pointer has many advantages over existing approach altimeters. For example, a vario designer has to decide whether an approach altimeter will be based on the glide path that will result if the pilot flies at the airspeed that will yield the flattest glide angle, given the existing winds, or based on the glide path that will result at the glider's current measured airspeed, or based on the glide path that will result if the pilot flies at the optimal airspeed for cross-country racing in thermal conditions according to McCready theory. If the latter approach is adopted, then the pilot is somewhat at the mercy of the vario's opinion about how strong the thermals are. (With the Brauniger IQ Comp GPS variometer, if both an airspeed probe and a GPS are connected to the vario, then the approach altimeter is based on the last of these three systems when the glider is climbing, and on the second of these three systems when the glider is descending.)

The "glide-to-target" pointer would provide the pilot with a unified, easy-to-understand display that would work exactly the same way during a climb as during a glide, without making any assumptions about the strength of current or future thermals or about the airspeed that the pilot will choose to fly. This simple pointer would convey a great deal of information about the glider's position with respect to the goal, with a minimum of additional clutter. This would be a valuable supplement to the approach altimeters that currently exist on most competition-level varios for hang gliding and paragliding. By watching the "glide-to-target" pointer, the pilot would be free to customize his last thermal climb to match his own estimate of the current climb rate. Then when the pilot went out on glide toward a chosen target, the "glide-to-target" pointer would keep him continually updated about how his angular situation relative to the target. Again, the "glide-to-target" pointer can be thought of as a kind of "gas gauge". If at any time the pilot encountered a thermal that was stronger than the current position of the "glide-to-target" pointer, then it would be to his advantage to stop and circle until the "glide-to-target" pointer had risen to a value equal to his current climb rate. So the "glide-to-target" pointer would provide valuable information as to when a pilot should pause to "fill up his tank" with extra altitude so that he can race to the target at a faster airspeed, even if he already has the target made at a lower airspeed.

Also, as a glider passes through patches of lift and sink, the "glide-to-target" pointer will let the pilot know exactly what "active McCready pointer" position he should be using for optimal racing performance, given his current angular relationship with the target. Existing approach altimeters do help a pilot find a glide speed that will successfully carry the glider to the target, but generally this speed is only optimized for air with no vertical motion, and is not the optimum speed to fly while the glider is passing through a burst of lift or sink. The "glide-to-target" pointer would much more adaptive because it would be easily used in combination with the "active McCready pointer", which responds to updrafts and downdrafts.

The "glide-to-target" pointer would put all the decision-making power in the hands of the pilot rather than in the hands of the mysterious inner workings of an approach altimeter. Once the "glide-to-target" pointer becomes widely adopted, the current approach altimeter system will seem as archaic as the old style speed ring that had to be manually adjusted to match the pilot's estimate of the current average thermal strength. The "average thermal strength" pointer, in combination with the "active McCready pointer", currently allows the pilot to take a dynamic, adaptive, integrated approach when he is climbing in thermals in the middle of a task, and also as he glides between thermals in the middle of a task. The "glide-to-target" pointer, in combination with the "active McCready pointer", would allow the pilot to take the same dynamic, adaptive, integrated approach when he is climbing with a defined target within reach, and also when he is gliding toward a defined target.