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Final Glide Calculator

"Whiskey Tango one mile out". And here he comes at Warp-Factor 12 trailing a long misty contrail as he whooshes across the line and pulls up in a long chandelle for a landing on the cross runway. Have you ever wondered how competition pilots figure out how fast to fly so that they have some altitude left when they cross the finish line at zero feet? I did too. You can cut grooves in the turf several miles out if you don't figure the final glide right. And then as if that weren't bad enough, the farmer will charge you for helping him plow his field. Everyone I talked to said Glide Calculators. A couple of pieces of plastic with a rivet in the middle, numbers around the edges and two pages of instructions. They come in two flavors: 21:1 and 35:1. Neither one fits my plane, but 35:1 is close. They're not expensive. I bought one.

It worked really well on the kitchen counter. I could figure how fast to fly, how high to start and how far to go. There were even some curved windows that let me factor in headwinds and tailwinds. But it never worked as well in the cockpit. It took a while, but I finally figured out why. The glide calculator fit my pocket. The instructions fit better on the kitchen counter.

So how do world champions do it? For that matter, how do guys who fly regionals just for fun do it? Contest pilots have a lot more to do in their cockpit than I do in mine, and even less room. I couldn't see them reading instructions on final glide. I started looking in cockpits. Know what I found? The hot shoes use computers! Skinny little things the size of an instrument with displays not much bigger than my watch. Some of them even have more than one. They program those things on their kitchen counters where they can read the instructions. Then they punch in height and distance on final glide and the speed-to-fly shows up on the display. And there I was trying to get one of those plastic things to work. It wasn't even for my plane.

What was I going to do? I wasn't about to spring for one of those computers. Not when some of them were worth more than a good sailplane. On the other hand, I already had a computer. It was on my desk at work. It was just a little big for the plane. But I didn't need the computer in the plane. I just needed knowledge that was in the computer. Why couldn't I do my calculating on the kitchen counter just like the big boys do, and take it to the plane in a form that would fit and wouldn't need any instructions?

The first thing I did was pull out the polar chart in the owner's manual. Leaving aside speculation about the accuracy of those numbers, it's a good place to start. No two gliders fly exactly alike even if they're the same make and model. Gap sealing and surface condition can make an obvious difference in performance between two planes. It's not surprising that the polar isn't exact for a given plane. But that's not important here. What is important is that we start with a representative set of numbers that we can work with to tailor a final glide chart for our own plane. Competition pilots get the numbers they put into their computers from the same place. We're just going to use a different display window. Theirs is a computer display. Ours is a computer printout.

There's a lot of information on the polar chart. Frankly, I hadn't taken a good look at the chart since I bought the plane. One look to find the max l/d and min sink speeds and I put the book away. This time I looked a little more closely. The curve I always looked at is the top one showing airspeed against sink. That's useful, but it's the bottom curve showing l/d and airspeed that lets us build a final glide calculator.

First list the airspeeds you want to use as your reference points. My airspeed reads in knots. I used five knot intervals. Use whatever's on your airspeed indicator. Then go across on the top curve and pick off the sink rates for each speed. Next, go to the bottom curve and find the l/d for each speed. The latter is key to the rest of the chart so read it as accurately as you can. Now divide 5280 by the l/d for each speed for the sink rate in feet per mile at each speed. One nice surprise is that, while the sink rate rises alarmingly with airspeed, the sink per mile doesn't go up as fast. You're coming down faster, but you're also covering a lot more ground. No wonder the hot shoes are running around at the high end of the speed range. Those fast runs don't cost them as much as you would think from looking at the sink rate alone. They have to climb more, but boy do they get there fast. If they make up more time on the run than it costs them on the climb they're ahead of the game. No dummies they.

I`m using knots for airspeed and miles for distance. I think distance in miles. You can do it in kilometers or nautical miles if you like. That's the nice thing about this glide calculator. You can use any scales you want to. It's your plane isn't it? Now comes the hard part. Multiply the sink rate per mile by the number of miles for each distance reference. I use 5 mile rings so I used five mile intervals. If you're using a calculator and paper, this gets tedious. Get a calculator with a constant capability and it's not too bad. Or you can use a spreadsheet on your personal computer and extend the formula for one speed across the rest of the speeds. At the same time. It's fun watching the calculations ripple across the whole chart.

What's nice about using a spreadsheet is that this chart is a starting point. I made mine without allowance for pattern altitude so that I wouldn't be confused about which altitude I was using. I know these numbers will take me to the ground. I plan to use the chart to make altitude runs on the field at different speeds and refine the chart to more accurately reflect the performance of my plane. When I'm comfortable with it I'll add pattern altitude to the whole chart. The spreadsheet will let me update it without having to make all the calculations again.

The chart as it stands is for still air. But think about the effect of wind on performance and you can use it in wind too. Add the headwind to the speed you plan to fly to find the real performance line for that speed. A 10 knot headwind will make 70 knots fly like the 80 knot line. The same goes for a tailwind except that you subtract the wind from your speed. You can fly the 60 knot line at 70.

It may not have blinking lights. It may not have a keyboard. It`s only a piece of paper. But as a final glide calculator, it's not bad.

OPEN CIRRUS FINAL GLIDE TABLE
FINAL GLIDE HEIGHT TO ZERO ALTITUDE
Min Mile

KnotsL/DSinkSink5 M10 M15 M20 M25 M30 M35 M40 MKts
404096132660132019802640330039604620528040
4544108120600120018002400300036004200480045
5042126126630126018902520315037804410504050
5539144138690138020702760345041404830552055
6036168146730146021902920365043805110584060
6533192160800160024003200400048005600640065
7030228176880176026403520440052806160704070
7528282189945189028353780472556706615756075
80253182101050210031504200525063007350840080
85233602301150230034504600575069008050920085
902142025012502500375050006250750087501000090
951948027813902780417055606950834097301112095

Posted: 2/25/1996 By: Peter King


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