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From time to time 1-26 pilots have asked me about the slide-rule-type glide calculator that Jo and I have
been using for the past 15-plus years which is mounted to the instrument panel in #196. I will try to
explain what it is, how it works and a little about how it was made. First off, some people don't realize
that in addition to the slide rule, there is a table that goes with it which is mounted on the left side
of the cockpit. So the glide calculator is in two parts: the table and the slide rule itself.
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Background and Terms
There are several pieces of information that are factored into the kinds of calculations which are, or
were (before electronic computers such as the Cambridge "x-Nav" and such) done with a hand-held glide
calculator. The only difference between the various calculators (circular, chart, slide rule) is the
way they are arranged and therefore the way the information is entered for solving the different soaring
problems. The differing arrangements offer more or less convenient solutions for the various problems.
Height - In feet for 1-26 purposes. - The number of feet the glider is (or needs to be) above the
point we want to fly to. That point is often set by the elevation of an airport, but it can also be any
goal such as the height of an obstructing ridge or a minimum height over a town or a Control Zone.
Distance - In statute miles, since a 1-26 airspeed indicator is in miles per hour.
Glide Ratio - A numerical ratio which results primarily from comparing Height with Distance--BUT,
for different parts of our problems, it is also influenced by other factors such as the wind. Here it
is read on the "Miles" scale opposite the index mark (which I wrongly labeled L/D). This is because
that index is at 5280 feet (the number of Feet in a Mile).
Those three factors--Height, Distance, and Glide Ratio--are manipulated with the slide rule part of the
calculator I am describing. If the pilot knows any two of the three, the slide rule will quickly show the third.
The Table of Glide Ratios also deals with three factors directly. Built into the figures of the table
is the performance of a 1-26 when it is flown at various speeds. We don't have to deal directly with
that nor do we have to apply alterations for ballast or bugs.
MacCready Speed Ring Setting - (Across the top of my table.) - This has several other names such
as thermal setting, speed ring setting, etc. I have grown accustomed to expressing this as a single digit
number which is in hundreds of feet per minute. (Or knots if you prefer, since a knot is essentially 100
feet per minute.)
Winds - Also in miles per hour to conform to the airspeed indicator numbers. I use the minus sign
(-) to mean headwinds since they reduce our performance. It is important to realize that these Winds numbers
are actually "Wind Components". We must estimate the Component based upon what we know about the actual
wind strength and its direction relative to our course. Wind changes with time and with altitude, and our
course changes often. We will probably never know what the actual "Wind Component" is at any given time,
but we can learn to make estimates which are sufficiently good to solve our problems. The Winds factor is
the one which dictates that our glide calculation problems are solved only "Approximately", never "Precisely",
even though we may be dealing with some numbers expressed to two decimal places! Some of our GPS based
navigators give wind indications, but none that I know of show "Wind Components".
Glide Ratio - Like above, these ratios are numbers representing "Miles of Distance per mile of Height".
Because these ratios are plotted for different MacCready numbers, they DO take into consideration the performance
of a 1-26 flown at the appropriate "MacCready Speed" as would be indicted by a speed ring or speed-to-fly
indicator (either electric or pneumatic). Because different horizontal lines are given for the various Wind
Components, the wind's influence on the glide IS also included. I have purposely left the ratios expressed as
two decimal place numbers in order to remind myself of the change in size of adjacent ratios. In the problems
to be solved, the Glide Ratio settings on the two pieces of equipment are adjusted to match each other, or else
one piece of equipment is used to find the Glide Ratio setting for the other piece.
Operation
- The most common glide calculation problem to be solved is: Can I get to some goal (like the home field) from here?
We know where we are and from a map or GPS we can find the Distance to be flown. We know our altitude from
the altimeter, and by subtracting the elevation of the goal from it we find the Height available. I leave it
to each individual to select his own fudge factors and desired arrival height. Personally, at Hobbs (3,700 MSL),
I just subtract 4,000 feet from the altimeter reading and figure I can get down on some part of this very large
airfield. But I also use other fudges that I will mention later.
On the slide rule, set the Height opposite the Distance and read the needed Glide Ratio opposite the index mark.
The smaller this ratio is, the easier it will be to fly quickly to the goal. Use that needed Glide Ratio to enter
the Table along the appropriate estimated Wind Component (horizontal row) to the nearest larger Glide Ratio number.
[Fudge number two.] Set the speed ring or thermal knob to the MacCready Ring Setting read from the top of that
column and fly to the goal.
If the Glide Ratio number doesn’t appear on that Wind Component line of the Table--You Can’t Get There From Here,
and must climb again. Here is a small piece of advice: On a day with strong convection, avoid using marginally
low speed ring settings unless you have good reason to believe you will really fly through good lift along the way.
Strong convection means you will encounter strong sink as well as strong thermals. With little reserve, you might
encounter only the sink and not be able to make it. For this reason, in the western U.S. I try to use a MacCready
Setting of at least 3 during all but the very last part of the day when convection has smoothed out. This allows
me to slow down to conserve altitude if necessary.
- Once the slide rule has been set, we come to the second type of glide problem: Am I really making good the
Glide Ratio I started out with?
It would be foolish to set up a glide and blindly follow it without checking on actual progress. This is where
Jo and I really like this Glide-Slide system the best. Once the slide rule has been set, it remains set and
visible in front of the pilot and doesn’t require any handling. Checking on progress only means looking at the
remaining height and the distance to go (altimeter and GPS) and seeing how these trend compared to what the
slide rule indicates. If resetting the slide rule to a smaller Glide Ratio number is appropriate, then the
speed (MacCready Setting) is too low for the Wind we have been encountering, or perhaps we have been flying
through an above normal amount of lift. If I think the wind is actually different, I reset the slide rule
AND the MacCready number on my speed ring. If I believe that lift is mostly responsible, I keep going without
change, figuring I may get the other side of the coin soon! If that doesn’t happen, I can reset them and fly
faster later. If the Glide Ratio tends toward larger numbers on these periodic checks, then an immediate
resetting of the slide rule which will result in a lower MacCready Setting (slower speeds) is indicated.
By checking frequently and making small adjustments, it is possible to stay pretty close to optimum and finish
the glide as planned.
- Next is the classic racing final glide problem: How high should I work this thermal in order to make the
best time to the finish line?
For any given climb rate, there is only one height to climb to which will give the shortest time/fastest overall
speed from there to the goal. This height is determined only by the instantaneous rate of climb and the Distance
to go. We must be careful to use an accurate judgment for this rate because the common tendency to overestimate
our climb rates can cause serious trouble here. If the climb rate changes, a different height is needed. We
climb to the proper altitude and then fly with the MacCready Setting appropriate for the climb rate at the time
we reached that altitude. If a pilot climbs too high and has to fly faster to get down to a reasonable finish
altitude, he has wasted time climbing. If he doesn’t climb as high as he should and therefore must fly slower
in order to get to the finish, he has wasted time gliding. This problem has only one correct solution.
So how does the Glide-Slide help?
Assume you are in a thermal which will take you high enough to run for the finish line. You watch your
instantaneous climb rate (but averaged over perhaps 20 to 30 seconds) which gives your MacCready Setting.
Enter the Glide Table with that number and read down to the appropriate Wind Component line. That shows
you the Glide Ratio to be used. On the slide rule, set this Ratio opposite the index mark. Determine the
Distance to go by map or GPS, and opposite that Distance is the Height at which you should leave the thermal.
The primary thought during the climb should always be “instantaneous rate of climb“. If that changes, the
problem must be reworked and the speed ring/thermal knob reset for the new situation. As the climb rate changes,
a different Height to leave is needed. Better climb rate--higher climb. Poorer climb rate--less needed Height.
If the wind is moderately strong, the distance can be changing at the same time. This can be a busy time in your
flight, but you must keep a good watch outside the cockpit for your safety and mine!
When at the proper height, be sure the MacCready Setting of the Speed-To-Fly device is properly set to the latest
instantaneous rate of climb and the slide rule matches it. Leave the thermal, fly by the STF device, and check
your progress with the slide rule as frequently as possible at first. If the wind is not as you expected, you
must know as soon as you can. From there on, watch your progress and adjust as you learned in #2 above.
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