Making Sense of Best Glide and Glide Ratio

[Max Clark]

I'm updating procedures and my checklist after new avionics.

Every checklist (and Foreflight) I've bought has a different Vg and Glide Ratio listed. Section III of the POH lists 85 KIAS (flaps up) for engine failure after takeoff. Then there's the actual POH/AFM entries that list different Vg based on weight, and a super helpful glide distance chart.

Foreflight has 89 KIAS and 11.5:1. I didn't dig into this or question it before - lesson learned. This makes sense assuming max gross takeoff and ~2.5 hours of fuel burn, which is likely not where I'll be most of the time. Worse my prop AFMS has a note that "it decreases glide distance approximately 7%

What number have you burned into your brain for Vg? What do you use for Glide Ratio?

[Ibra]

Best glide ratio does not change with weight, it's pure geometry: the B747, PC12, P51 have some nice glide ratios yet they are "pretty heavy" 

The propeller has lot of effect, I would say between 1:10-1:13 in M20J, however, I am damn sure it will not work if an engine fails unless I forgot to put Avgas 

Best glide speed Vbg increase with weight (like stall speed increment but one need to whole polar curve) and headwinds (decrease with tailwind), in thoery, one should calculate vertical speed (fpm) to ground speed (kts), then find indicated airspeed that maximise the ratio. I have done hundreds of hours optimising glides in Astir and Discus, none of that was relevant when engine quit in C172, I just glanced at iPad for my options, put it at 80kts and started looking outside.

It's mostly about having a speed I  mind and doing some planning, what matters is where your aiming point or selected field moves with respect to the horizon and the aircraft, if it goes up you won't make it, if it goes down you will make it, this applies irrespective of calculations 

In M20J, I plan to fly 90kias and plan 1:10 to arrive at 2000ft overhead my landing spot

On a side note, I wonder what numbers go into "smart glide" by  GTN? 

Edited Sunday at 10:55 PM by Ibra

[PT20J]

15 minutes ago, Ibra said:

On a side note, I wonder what numbers go into "smart glide" by  GTN? 

Glide ratio and one glide speed are entered in config mode. There is no adjustment for weight. But after Smart Glide activates the GFC 500 in IAS mode, you can adjust the airspeed up or down with the thumbwheel.

[PT20J]

2 hours ago, Max Clark said:

I'm updating procedures and my checklist after new avionics.

Every checklist (and Foreflight) I've bought has a different Vg and Glide Ratio listed. Section III of the POH lists 85 KIAS (flaps up) for engine failure after takeoff. Then there's the actual POH/AFM entries that list different Vg based on weight, and a super helpful glide distance chart.

Foreflight has 89 KIAS and 11.5:1. I didn't dig into this or question it before - lesson learned. This makes sense assuming max gross takeoff and ~2.5 hours of fuel burn, which is likely not where I'll be most of the time. Worse my prop AFMS has a note that "it decreases glide distance approximately 7%

What number have you burned into your brain for Vg? What do you use for Glide Ratio?

You should use the speeds in Section III of the POH/AFM for your airplane. Best glide occurs at a specific angle of attack, so the airspeed will vary with weight. This is because lift = weight and if the angle of attack is constant then CL is constant so the airspeed has to be variable. The heavier you are, the faster you need to fly and the faster will be your descent, but the glide angle will not chsnge.

[Max Clark]

Foreflight has the glide ratio of 11.5:1. I've seen numbers of 11.4/11.46:1 so quasi consistent.

The Hartzell 3 blade reduces distance by 7%, which takes me to 10.6:1.

Should I just shorten this to 10:1 and have confidence I'd reach anything inside of the ring ignoring everything outside of it?

[MikeOH]

49 minutes ago, Ibra said:

In M20J, I plan to fly 90kias and plan 1:10 to arrive at 2000ft overhead my landing spot\

^^^. THIS. ^^^

I figure both back pockets are going to be touching each other if I have an engine failure...No way I'm risking any more of a helmet fire by trying to optimize glide speed based on exact weight and trying to figure out wind speed and direction at several altitudes below me.  I've got Foreflight set to 10:1.  My biggest fear is going for an airport close to the edge of the glide ring vs. accepting a for-certain makeable off-field site.

[midlifeflyer]

12 hours ago, MikeOH said:

 

I figure both back pockets are going to be touching each other if I have an engine failure...No way I'm risking any more of a helmet fire by trying to optimize glide speed based on exact weight and trying to figure out wind speed and direction at several altitudes below me.  I've got Foreflight set to 10:1.  My biggest fear is going for an airport close to the edge of the glide ring vs. accepting a for-certain makeable off-field site.

THIS for me as well.

Many years ago, I did a C172 checkout for a 20,000 hour retired airline pilot. When we got to the simulated engine failure, he was at best glide and heading toward a landing spot within 6 seconds. Once I realized what he was doing, I started to test it out with other airplanes. 

All he did was look out the window and pitch for the most familiar attitude - level cruise. Turned out that a level cruise attitude resulted in a speed close enough to best glide that the difference was inconsequential. So inconsequential that trying to be perfect has a negative impact. Even seems to account for weight. I’ve been using and teaching it ever since. I used that technique recently in an “impossible turn” demo. It wasn’t in a Mooney, but I’m linking it below anyway.  

Tweak if you need to with a touch of trim. If your autopilot has an IAS mode, use it.  But  only once underway to your landing area.