Calculating True Airspeed; Best Glide Speed

I’m a pilot. I think it’s in our nature to obsess over numbers. (And I’d really, really like - psychologically, if for no other reason - to see whether or not the plane I’m flying is a “150 knot” airplane...)

True Airspeed

There are several ways to calculate this:

• The simplest way to get a “pretty close” number is to take 2% of your calibrated airspeed, multiply it by how many thousands of feet you are MSL, and add it to the calibrated airspeed indicated. So, if I’m flying along at 145 mph indicated (calibrated, 143 mph) at 5,000', that works out to: ((143 * .02) * 5) + 143 = 157 mph true (÷ 1.15 = 137 ktas)

• A bit more accurate is the true airspeed calculator built into a Garmin GPS unit (GNS430W, GTN650, etc), which factors in barometer, OAT, etc. (and will calculate your head/tail wind component too).

• There’s also the “4-way GPS ground speed” method, used, e.g., in this M20F evaluation. (That plane, a ’67 which was constructed differently and is generally faster than the later models, was also lacking a bunch of the LASAR speed mods mine is wearing ...) If the winds aloft aren’t strong, this might be “close enough,” and it’s certainly straightforward.

• The most accurate GPS ground speed test seems to be this one: Horseshoe Heading Technique. Though I’ll admit, my algebra is rusty AF. Fortunately, Craig Cox wrote a True Airspeed Calculator Java applet (with source code provided) that performs the necessary calculations.

Best Glide Speed

So, the 50-year-old owner’s manual for a plane I occasionally fly doesn’t have any information regarding best glide speed! I’ve been using the value listed for a similar (slightly smaller) model made a couple of years earlier, but in the process of putting together this spreadsheet, decided to actually, you know, research it a bit. Someone on MooneySpace reports it as being 104 mph / 100 mph (prop windmilling / stopped) for a ’67 M20F, and this Transition Guide for a 1967 M20F lists similar numbers (105 / 100 respectively). I’m using the 105 mph number.

Anyway. Best glide speed is calculated at gross weight. There’s a heady formula for calculating it properly, but the “close enough” rule of thumb is to lower the speed 5% for every 10% the aircraft is below gross weight. (And then, if there’s a headwind, add 50% of the headwind component (which you'll have from flight planning or from spinning in the numbers on your 430 during an earlier moment of pre-panic boredom.)

I threw together this awful spreadsheet that creates a look-up table for my usual configurations and different fuel quantities, while hoping I never need it ...