Best Glide

[chrixxer]

At a WINGS event, learned best glide isn't a static number, but changes with weight. (Duh. Except, no one had ever 'sprained that before, and it's not in the owner's manual.) There's a formula I'm not going to try to wrap my brain around, or the "close enough" formula of subtracting 5% from the Vg speed for every 10% you are under gross weight. I used the latter and built a quick reference table for common scenarios (me alone, me + my dogs, me + a passenger, me + a passenger and my dogs), at varying fuel states (full (64 gallons), tabs, and then in 10 gallon increments down to 0). The differences are pretty significant; if I'm alone in the plane and lose the engine with 20 gallons on board, my best glide is about 93 mph IAS, vs. 107 mph IAS at gross.

The spreadsheet I used is pretty ugly, but here's the link in case it's useful or interesting to anyone: https://docs.google.com/spreadsheets/d/17MhAKYxRxcNJbLIBu_S_J_vdKS6wBVXro2KpVGV3VPI/edit?usp=sharing. (My "Excel"-fu is rusty AF.)

'Cause, from experience, if I'm in an actual engine out situation, I'm not pulling out (or tapping over to) a calculator ...

(Stuck at home with a sick Labrador, and lost a friend over the weekend to what very well might have been an engine out over mountainous terrain.)

 

[Ragsf15e]

Yep, they publish gross, but it definitely changes.  Mine is 100mph for my f though... surprised yours is 107mph?  Thought our airplane would be same-2740 gross weight?

[midlifeflyer]

I didn't look at the spreadsheet but you might as well key the formula into it. 

But here's a shortcut when flying - normal cruise pitch will produce best glide within about 5 KTS. So, in the clouds, just put the airplane on the AI horizon line. True in all 30+ single makes/models I have flown.

[Yetti]

If it is close to the ground, you don't have time to do math.   Push down immediately, then get it to 100 and start doing the other things your emergency flow.  you don't have time to dig out emergency procedures. 

[201er]

Best glide isn't an airspeed. It's an angle of attack. Much easier to establish and maintain it consistently at any weight or bank angle using an AOA indicator.

[Hank]

24 minutes ago, 201er said:

Best glide isn't an airspeed. It's an angle of attack. Much easier to establish and maintain it consistently at any weight or bank angle using an AOA indicator.

Which so many of our planes left the factory with. About one has been installed for every dozen times you throw AoA comments into our speed discussions.

Meanwhile, speed should vary the same as for landing--subtract 5 mph for every 300 lb below gross at that time. Round numbers are easier to do in your head. Or if near the ground, use the book number and concentrate on a good landing somewhere you can see. 

FWIW, Best Glide in my C is 105mph with the prop windmilling and 100mph with the prop stopped.

[PTK]

Every speed is an AOA! Every time we use a certain airspeed to glean maximum performance it’s always at one correct AOA. That AOA never changes, but the airspeed that achieved it does. The airspeeds we are accustomed to are almost always incorrect. They are only correct at gross weight and 1G! How many times are we satisfying those conditions? Sitting on the ramp maybe!

[DXB]

1 hour ago, midlifeflyer said:

I didn't look at the spreadsheet but you might as well key the formula into it. 

But here's a shortcut when flying - normal cruise pitch will produce best glide within about 5 KTS. So, in the clouds, just put the airplane on the AI horizon line. True in all 30+ single makes/models I have flown.

I never knew this, although it makes some sense and seems like a very useful piece of knowledge.  Thanks.

[MikeOH]

While I've never had to put it to the test, I'm going with pragmatic engineering vs. theoretical best speed.  For my F I'm going to trim for 100 mph and call it a day; I'm going to be a LOT more worried about picking a spot and which direction the wind is coming from (which is going to have a FAR more significant effect on glide distance).

I'm too lazy to do the math, but I'm wondering how much actual distance difference there is when gliding from 5000 AGL at 93 vs. 107 mph...my SWAG is: not too damn much.

Yeah, yeah, yeah someone will say if I come up 10 feet short and hit a tree I should have used the proper speed for my weight at the time...oh, well.

[chrixxer]

5 hours ago, Ragsf15e said:

Yep, they publish gross, but it definitely changes.  Mine is 100mph for my f though... surprised yours is 107mph?  Thought our airplane would be same-2740 gross weight?

So, digging through my owner's manual, turns out no Vg is specified. The 107 must have been a carry-over from my E (I duplicated its checklist and then revised per the F's owner's manual). Other sources say 104 or 105 windmilling, 100 if the prop is stopped, for at least a '67 F.

[chrixxer]

4 hours ago, Yetti said:

If it is close to the ground, you don't have time to do math.   Push down immediately, then get it to 100 and start doing the other things your emergency flow.  you don't have time to dig out emergency procedures. 

My emergency procedures are two taps away in ForeFlight, and live on my yoke. I've had to use them twice, both times < 2000' AGL...

[chrixxer]

20 minutes ago, MikeOH said:

I'm too lazy to do the math, but I'm wondering how much actual distance difference there is when gliding from 5000 AGL at 93 vs. 107 mph...my SWAG is: not too damn much.

IDK. I hope my days of being in a position to find out are over. But it was an area they emphasized in the FAAST training, FWIW...

[carusoam]

This data sort of applies to most Mooneys...

Each PIC would have to verify the numbers that work for their plane...

But the strategy is the same.

Note the glide speeds vary over the entire range of weight...

From memory, my best glide speed changes 10kias from heaviest to lightest... 90 kias at max weight, 80 kias at the lightest...

knowing your weight at the time would be nice... having an AOAi would be pretty cool...

Keep in mind, finding this data in the electronic POH probably took longer than a glide from cruise altitude...  

 

PP thoughts only, not a CFI...

Best regards,

-a-

 

A close-up that shows better detail of the three lines...

For the O drivers that are error checking... the note reminds us to have closed cowl flaps... 

For the mathematicians... the three lines should converge at the origin... (why are there three lines?)

For anyone familiar with converting miles to kms.... does the kms on the bottom of the chart match the miles on the bottom of the chart? (60 mi should be about 97 km) the scale seems a bit wonky?

[Jim Peace]

5 hours ago, Yetti said:

If it is close to the ground, you don't have time to do math. 

https://getyarn.io/yarn-clip/8d00bd73-7981-4108-b16f-b36098477741#HyHoSIGjJS.copy

[Yetti]

I wonder if the Porsche is the same one used in 48 hours

 

[carusoam]

Upon further review...

Of the above glide distance chart for the O....

The close-up that shows better detail of the three lines...

For the O drivers that are error checking... the note reminds us to have closed cowl flaps...   Os don’t have cowl flaps...

Anyone take an O above 24k’?

A heavier plane doesn’t glide as far as a lighter version of the same plane... increased drag of the increased AOA, required to maintain the proper glide speed...

For the mathematicians... the three lines should converge at the origin... (why are there three lines?) there are four weights given, four glide speeds given, and only three lines on the chart... I don’t think the three lines mean anything in particular.

For anyone familiar with converting miles to kms.... does the kms on the bottom of the chart match the miles on the bottom of the chart? (60 mi should be about 97 km) the scale seems a bit wonky? Choosing to glide in kms will yield a better glide distance...?

This is for Rev. E of the chart...  need to look for a better Revision...

Best regards,

-a-

[chrixxer]

14 hours ago, carusoam said:

knowing your weight at the time would be nice... having an AOAi would be pretty cool...

 

Yeah, the weight thing is why I put together the quick "cheat sheet" for common configurations. It's never going to be exact, but it should be close enough in the proverbial pickle.

I'm seriously pondering installing the AV-20-S. I have a spare 2" instrument hole in my panel, and as I'm still rocking the vacuum AI/DG, having an electronic backup attitude source would be nice to have. Not 100% sure about the "probeless" thing, but the FAA NORSEE'd it, so ...

Edited June 22, 2019 by chrixxer

[gsxrpilot]

1 hour ago, chrixxer said:

I'm seriously pondering installing the AV-20-S. I have a spare 2" instrument hole in my panel, and as I'm still rocking the vacuum AI/DG, having an electronic backup attitude source would be nice to have. Not 100% sure about the "probeless" thing, but the FAA NORSEE'd it, so ...

I've not had any luck getting the AOA in my AV-20S calibrated. It seems to be pretty useless. The clock is nice though ;-)

[RogueOne]

I am doing 100MPH and focusing on a landing area.  Very sorry to hear of the loss of your friend and hoping your buddy feels well soon...

Happy thoughts.  Life can be a B..I itch sometimes.

[RogueOne]

Garmin pilot has a “glide ring” that I will use in addition to a 696 with nearest to see if possible to find a sanctuary in a storm.

[midlifeflyer]

22 hours ago, DXB said:

I never knew this, although it makes some sense and seems like a very useful piece of knowledge.  Thanks.

The beauty of it is typified by how I learned it. I was a wet behind the ears CFI doing a 172 checkout for a retired airline pilot with 20 gazillion hours. When I pulled the power he looked out the window, pitched for level flight visually,  found a place and headed for it. Never even looked inside the cockpit - to troubleshoot - until after he was  heading to his chosen landing area, already at best glide. No playing around. 

Heading to the landing area at best glide within about 6 seconds. Compare that to the too typical "hunting" for best glide while valuable seconds tick away.

its become a staple. Both personally (and I have had a power loss in the clouds) and when teaching.

(Don't worry. I managed to teach him something too :D, but the lesson I learned was better)

[MB65E]

Also remember best glide in a head wind isn’t best glide in a tail wind. 

-Matt

[Cyril Gibb]

23 hours ago, carusoam said:

 For anyone familiar with converting miles to kms.... does the kms on the bottom of the chart match the miles on the bottom of the chart? (60 mi should be about 97 km) the scale seems a bit wonky?

It's correct. Nautical miles not statute miles.

[carusoam]

Thanks Cyril!

I got caught up using the wrong conversion factor... 

Best regards,

-a-

[0TreeLemur]

On 6/22/2019 at 10:55 AM, chrixxer said:

Yeah, the weight thing is why I put together the quick "cheat sheet" for common configurations. It's never going to be exact, but it should be close enough in the proverbial pickle.

I'm seriously pondering installing the AV-20-S. I have a spare 2" instrument hole in my panel, and as I'm still rocking the vacuum AI/DG, having an electronic backup attitude source would be nice to have. Not 100% sure about the "probeless" thing, but the FAA NORSEE'd it, so ...

I installed the AV-20-S.  Here are the pluses.  It can display OAT, buss voltage, and TAS on the main screen.   I agree with @gsxrpilot that the timers are nice.   It has two user timers, plus an "engine run time" timer that starts counting when the buss voltage exceeds 13-ish V, which occurs during runup in a/c with generators.   It also has a "flight time timer" that starts when it thinks the pressure differential across the pitot and static lines is enough for 40 kias.   I use that one to time when I switch fuel  tanks.  It also has a mems-based attitude indicator that I find to be well in agreement with my vacuum driven one.   I bought it for that feature because a good timer with a backup AI is cool.

On the other hand, in the less-than-cool category, it has an OAT sensor that uses the DAVTRON temperature probe.   It is calibrated using a menu-based "trim" setting.   When I calibrate the temperature probe vs. whatever temperature source I can obtain at altitude, it seems consistent.  But, when I compare the indicated TAS vs what the Garmin 430W tells me is the TAS when I change the temperature, pressure, and CAS on that menu, the AV-20-S indicates 4-6 kts higher.   The Garmin 430W agrees with three-course groundspeed method of making TAS measurements within about 2 kts.  So, whatever algorithm the AV-20-S is using is not that accurate.   I have found that I can adjust the temperature "trim" setting to get the TAS to match, and then the temperature is off.  Something is not right in the TAS calculation based on the air temperature.

It advertises having a "probeless AoA sensor".  In my experience, the probeless AOA indication on the AV-20-S is useless.   In theory it should work at 1g based on measured V, dP/dt and nose angle relative to the horizon based on the mems AI.   However, as g deviates significantly from 1.0, due to turning or some other acceleration, another variable is introduced to the equation that the probeless AOA sensor cannot resolve related to angular accelerations.  I contacted the company and was told something to the effect that the probeless AOA has not yet been calibrated in a Mooney.  I think that was a polite way of saying "fuhgeddabouddit", so I have.  I turned it off because it was distracting.  

It has the ability to serve as a g-meter, but since I don't do aerobatics and I know to fly in the green arc when it is rough, I don't ever use it.   My 2-cents worth.   I'm glad I have it for the useful timers and backup AI with 30-minute battery life.   The other functionality is of questionable utility in my opinion as a rank private pilot.  YMMV.  I do not recommend it for its purported ability to accurately indicate AOA.