Math Quiz - Glide Ratio conversion to Foot Per Minute Descent Rate
Purpose: many software apps paint a glide ring by asking for the glide ratio (examples: ForeFlight & Garmin), however at least one software app (Avare) wants the value in feet per minute. Sure, getting this number is possible by simply trying it in flight, but where is the mental challenge fun in that ;)
Looking for either affirmation or debunking (along with corrections) of logic and formulas from a better set of experts.
For a decent rate at best glide, I had to put some think time into it and am focusing on the following formula:
} Decent Rate in ft / min = Available Altitude (in feet) / Time to Remain Airborne (in minutes)
Supporting assumptions and calculations are:
- Use 1,000 feet Available Altitude.
- Time gets reduced to minutes.
- Best Glide Speed is easy: straight out of the POH.
- Glide Range in NM for 1,000 feet altitude is just as easy: Use POH Section 5's Glide Range for 1,000 feet altitude.
++ If the Glide Ratio is already known, another path to Glide Range in NM is: Glide Ratio * 1,000 / 6,076.12 (constant for converting feet to nautical mile). This should equal the value per the Glide Range graph for NM traveled based on 1,000 foot altitude.
- Time to remain airborne (in minutes): Glide Range in NM for 1,000 feet altitude * 60 (minutes) / Best Glide Speed (per POH).
Any errors here?
And if correct, sure it might seem simple. But I could not find any examples on the Internet and it took some time to wrap my head around how to get the relationships to work.
Comments
I think you're right, but I'm not smart enough to do these problems without making a sketch like above. Between best glide speed (KTS), glide ratio(x:1), and descent rate (FT/MIN), if you have any two you can find the other one. Keep in mind this analysis is only meaningful when IAS=TAS and you're in still air, so that airspeed and ground speed are identical when predicting glide range. I've also used the small angle equality to simplify the math. Bob