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How do you determine best glide?

Posted: Tue Dec 27, 2016 8:08 am
by flyingturtle
While binge-watching the "Mayday" series on accident investigations and in the context of certain South American pilots who revolutionized the concept of MZFW, a question cropped up:

In cases like the Gimli Glider or Air Transat 236, pilots need to find the speed with the best lift/drag ratio. How do you arrive at that number? Do manufacturers shut down all engines in test flights (except the APU) and observe the flight characteristics?

And what do pilots actually optimize when they have to glide - the maximum time of flight, or the maximum distance covered?


Thanks in advance for your insights!

David

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 9:31 am
by Branzino
The best gliding speed is really the best Lift to Drag ratio, so it can be mathematically calculated without the need to shut down engines.

https://www.mathworks.com/help/examples ... ide_01.png

That being said, I'd also be interested in learning how L/D ratios are calculated in flight testing.

flyingturtle wrote:
And what do pilots actually optimize when they have to glide - the maximum time of flight, or the maximum distance covered?
David


From my very limited personal experience, most powered pilots only have trained for gliding at maximum distance. It's glider pilots that would trouble themselves with debating distance vs. duration.

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 9:35 am
by thepinkmachine
Best glide speed is best lift/drag speed. It can be pretty accurately calculated by engineers and wind tunnel tests. No need to shut down engines to find it...

In modern jets it is continuously displayed to the pilots - on the Airbus it is "green dot" speed, same on the Embraer. On Boeings it is the "up" speed, i.e. minimum maneuvering speed with flaps up. Or so I believe, as I have never flown a Boeing

In all engine-out scenario you either go for windmill start speed (generally 280-300kts) if you decide to try restarting the engines. If this is not an option (e.g. out of fuel), you go for "best glide" speed for maximum distance. "Best endurance" speed would be somewhat less than "best lift/drag", but it is not displayed to pilots and not used in procedures.

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 10:46 am
by WIederling
thepinkmachine wrote:
"Best endurance" speed would be somewhat less than "best lift/drag", but it is not displayed to pilots and not used in procedures.


If you want to "feel for it" you'd go for least sinkrate, right?

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 11:24 am
by thepinkmachine
WIederling wrote:
thepinkmachine wrote:
"Best endurance" speed would be somewhat less than "best lift/drag", but it is not displayed to pilots and not used in procedures.


If you want to "feel for it" you'd go for least sinkrate, right?


"Minimum sink rate speed" will be a better term than "best endurance".

I suspect flying at this speed would yield a small gain in terms of sink rate, at a cost of range and, more importantly, stall margin - as this speed will put the airplane close to the stall...

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 3:23 pm
by benbeny
How do you determine sacrificing altitude vs cabin pressure in terms of total engine loss?

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 3:39 pm
by WIederling
benbeny wrote:
How do you determine sacrificing altitude vs cabin pressure in terms of total engine loss?


Reaching the required altitude ( 8000') in the required time (??) is hard prioritized.
No way around it. From there you can go for most economic glide.

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 3:43 pm
by thepinkmachine
benbeny wrote:
How do you determine sacrificing altitude vs cabin pressure in terms of total engine loss?


You don't, at least not on the Bus. Reaching a suitable airport/landing site is of the highest priority.

In absence of bleed pressure cabin altitude will rise slowly, but it should not be a rapid decompression. The procedure calls for oxygen masks on for pilots.

As for the passengers, if the cabin altitude rises high enough, oxygen masks will drop. 15 minutes or so of pax oxy should be enough for the airplane to descend to an altitude where breathing is possible...

Re: How do you determine best glide?

Posted: Tue Dec 27, 2016 4:18 pm
by benbeny
thepinkmachine wrote:
benbeny wrote:
How do you determine sacrificing altitude vs cabin pressure in terms of total engine loss?


You don't, at least not on the Bus. Reaching a suitable airport/landing site is of the highest priority.

In absence of bleed pressure cabin altitude will rise slowly, but it should not be a rapid decompression. The procedure calls for oxygen masks on for pilots.

As for the passengers, if the cabin altitude rises high enough, oxygen masks will drop. 15 minutes or so of pax oxy should be enough for the airplane to descend to an altitude where breathing is possible...

I see. Nice to know how the philosophy behind any procedure. Thank you :)

Re: How do you determine best glide?

Posted: Wed Dec 28, 2016 1:55 pm
by OldAeroGuy
And since there is only about 20 minutes or less of flight time available wit all engines inoperative, O2 available time is hardly an issue.

Re: How do you determine best glide?

Posted: Wed Dec 28, 2016 6:32 pm
by flyingturtle
Ah yes, there's a mathematical function that describes the lift for a certain speed, and another one describing drag for a certain speed, and then you have the L/D ratio for any specific speed. That's one part...

thepinkmachine wrote:
Best glide speed is best lift/drag speed. It can be pretty accurately calculated by engineers and wind tunnel tests. No need to shut down engines to find it...


...thanks for the second and less obvious part of the answer! Could you also gather data from a test flight (e.g. find best "glide" at, say, 20% N1 and 10% N1 and then interpolate from there?


David

Re: How do you determine best glide?

Posted: Wed Dec 28, 2016 9:46 pm
by Revelation
flyingturtle wrote:
Ah yes, there's a mathematical function that describes the lift for a certain speed, and another one describing drag for a certain speed, and then you have the L/D ratio for any specific speed. That's one part...

thepinkmachine wrote:
Best glide speed is best lift/drag speed. It can be pretty accurately calculated by engineers and wind tunnel tests. No need to shut down engines to find it...


...thanks for the second and less obvious part of the answer! Could you also gather data from a test flight (e.g. find best "glide" at, say, 20% N1 and 10% N1 and then interpolate from there?


David


Glider pilots are very familiar with 'min sink' and 'best glide' speeds. When you're in a thermal you fly min sink speed so you maximize your time in lift. When you are in between thermals you fly best glide speed so you maximize performance to the next source of lift.

http://www.5c1.net/Glider%20Performance%20Airspeeds.htm shows how you can find them using a 'polar' and/or by in-flight experiments.