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joejohn
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airplane gliding

Thu Mar 24, 2022 3:47 am

they say if an airplane experiences double engine failure, it won't just fall out of the sky (I find this hard to believe)
they say because the airplane is gliding through the air
if the airplane won't crash and will just glide through the air, then why do we hear on the news airplanes crash?
 
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SAAFNAV
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Re: airplane gliding

Thu Mar 24, 2022 1:46 pm

Considering these glider flypasts, and no engine to power them:
https://www.youtube.com/watch?v=W4bt4ac8Aps

and this plane with a double engine 'failure' landing without crashing
https://www.youtube.com/watch?v=brsY7CZrxLA, I`d say there's some pretty sinister forces at work. We must launch a probe to determine
 
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AirKevin
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Re: airplane gliding

Thu Mar 24, 2022 3:06 pm

joejohn wrote:
they say if an airplane experiences double engine failure, it won't just fall out of the sky (I find this hard to believe)
they say because the airplane is gliding through the air
if the airplane won't crash and will just glide through the air, then why do we hear on the news airplanes crash?

What are you using as a basis for comparison. Air Canada 143, Air Transat 236, Taca 110, and US Air 1549 are four incidents where they lost both engines and managed to glide and land with no loss of life. As for crashes, Asiana 214, American 965, Turkish 981, and Gol 1907 are four accidents I can think of that crashed despite having perfectly running engines. So I'm not sure what your point is here.
 
kalvado
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Re: airplane gliding

Thu Mar 24, 2022 3:09 pm

There were only few events with dual engine shutdowns, and airplane made it to the ground - some were even flying after that adventure. If you will, this is similar to car running out of gas or suffering engine failure - it is possible to pull over and call for help, but there are many accidents - both car and airplane - caused by other reasons.
There is much more in maintaining controlled flight (or drive) than running engine. Some flavor of control failure - mechanical or human - is the biggest issue in both cases.
 
N1120A
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Re: airplane gliding

Thu Mar 24, 2022 6:19 pm

The two primary reasons aircraft crash are CFIT and loss of control. And by crash, I don't mean make forced landings, but where there is loss of life and/or the the aircraft is written off. The vast majority of SINGLE engine engine failure incidents lead to no loss of life, no injury and repairable damage. In lighter aircraft, twins actually have a higher incidence of accidents because of loss of control on the loss of a single engine. Think about that.

Airplanes are made to fly, not to drop.
 
bluecrew
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Re: airplane gliding

Thu Mar 24, 2022 6:33 pm

N1120A wrote:
The two primary reasons aircraft crash are CFIT and loss of control. And by crash, I don't mean make forced landings, but where there is loss of life and/or the the aircraft is written off. The vast majority of SINGLE engine engine failure incidents lead to no loss of life, no injury and repairable damage. In lighter aircraft, twins actually have a higher incidence of accidents because of loss of control on the loss of a single engine. Think about that.

Airplanes are made to fly, not to drop.

Another thing I would state too... most accidents and incidents we study and learn from are situations that in hindsight are very avoidable. Complacency, lack of SA, basic airmanship failures.

Engine failures and dual engine failures especially create that "oh ****" moment that tends to crystalize your SA quite quickly. You're less likely to react poorly to a difficult situation when you're already in the difficult situation, as opposed to a low stress, low pressure regime where it's all a normal day at work until the GPWS tells you that you're about to hit a mountain.
 
ACDC8
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Re: airplane gliding

Thu Mar 24, 2022 8:12 pm

Aircraft have a glide ratio, meaning a specific distance it can travel (with no power) for every specific amount of altitude it looses. Its been a while, but IIRC in a C172 its something like 9:1, so for every 1000 feet of altitude you lose, you can travel 9000 feet of distance - of course, other factors always play into the situation. Airliners are no different.

Why do we hear about airplanes crashing, well, thats because there are a hundred different reasons why an aircraft crashes, not just loss of power. As for loosing power, that is something that is trained for by the pilots and as pointed out above, is perfectly possible to safely land an aircraft that has lost all power.

And no, aircraft don't just "fall out of the sky", in just about every accident, there is a chain of events that lead to the accident.
 
Thrusty69
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Re: airplane gliding

Thu Mar 24, 2022 10:50 pm

Paper airplanes fly. So do gliders.
 
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Starlionblue
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Re: airplane gliding

Sat Mar 26, 2022 3:00 am

ACDC8 wrote:
Aircraft have a glide ratio, meaning a specific distance it can travel (with no power) for every specific amount of altitude it looses. Its been a while, but IIRC in a C172 its something like 9:1, so for every 1000 feet of altitude you lose, you can travel 9000 feet of distance - of course, other factors always play into the situation. Airliners are no different.

Why do we hear about airplanes crashing, well, thats because there are a hundred different reasons why an aircraft crashes, not just loss of power. As for loosing power, that is something that is trained for by the pilots and as pointed out above, is perfectly possible to safely land an aircraft that has lost all power.

And no, aircraft don't just "fall out of the sky", in just about every accident, there is a chain of events that lead to the accident.


I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.

A common problem with jets is not being able to get down fast enough on approach. In a propeller plane, you can use the props to get you down, or even slip if you're in a light aircraft. In an airliner, descent requires advance planning. A typical descent starts 120 nautical miles from the runway, and much of it will be at idle power. If you get a significant shortcut when already on approach, it's time to go full speedbrake.

The saying is that "you can go down or slow down, but not both".
 
Woodreau
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Re: airplane gliding

Sat Mar 26, 2022 1:31 pm

not an airliner, but here is a recent video of a plane last month that lost all engine power and made a forced landing. The pilot managed his energy really well and even taxied off the runway using residual energy.

https://youtu.be/CEMlny_ExuU

Commentary
https://youtu.be/C30b2OQDj_8

At least he didn’t bail out of his plane like the faked engine failure that crashed
 
ACDC8
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Re: airplane gliding

Sat Mar 26, 2022 9:51 pm

Starlionblue wrote:
I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.

Oh for sure, gliding characteristics of modern airliners is incredible, and with the the altitude they fly, gives them a big advantage over a single engine prop.
 
BoeingGuy
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Re: airplane gliding

Sun Mar 27, 2022 12:53 am

ACDC8 wrote:
Starlionblue wrote:
I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.

Oh for sure, gliding characteristics of modern airliners is incredible, and with the the altitude they fly, gives them a big advantage over a single engine prop.


Modern jetliners like the 777 have at least a 20:1 glide ratio. The 787 glide ratio is supposedly so high it’s proprietary information. I don’t know the exact number. I assume Airbus models are equally efficient.

The rule of thumb that I’ve heard is 2 miles gliding for every 1000 of altitude loss, but the Air Transat A330 well exceeded that.
 
Redbellyguppy
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Re: airplane gliding

Sun Mar 27, 2022 1:31 am

If I’m descending into, say, Sacramento, on a flight from Seattle, from FL400 I’ll pull the thrust to idle over Redding, or thereabouts. If I do it properly and ATC has no traffic issues, I wouldn’t need to apply thrust until I’m at 1000 ft… and only that because of the drag from configuring. If I had no engines obviously I’m going to throw out stable approach criteria and do whatever it takes to make a piece of concrete.
 
N1120A
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Re: airplane gliding

Sun Mar 27, 2022 5:44 am

Starlionblue wrote:

I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.


The only light single that probably glides worse than a 172 is a Cirrus. A Mooney will do 11:1 (maybe more). A DA40 will do 13:1. As noted, most modern airliners do 20:1 or more. And they're doing it from up high.

Bob Pearson had to slip C-GAUN into Gimli, and did it successfully.
 
LH707330
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Re: airplane gliding

Sun Mar 27, 2022 11:42 pm

BoeingGuy wrote:
The 787 glide ratio is supposedly so high it’s proprietary information. I don’t know the exact number. I assume Airbus models are equally efficient.


Couldn't one infer the 787's L/D fairly accurately (+/- a few %) based on known cruise engine SFC, thrust, and burn in a variety of gross weight conditions? Or is that only useful for cruise L/D, not best glide speed L/D?

N1120A wrote:
Starlionblue wrote:

I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.


The only light single that probably glides worse than a 172 is a Cirrus. A Mooney will do 11:1 (maybe more). A DA40 will do 13:1. As noted, most modern airliners do 20:1 or more. And they're doing it from up high.

Bob Pearson had to slip C-GAUN into Gimli, and did it successfully.


172s are indeed bricks when it comes to glide range. The Bye E-flyer 2 is reportedly 20:1. They needed to compensate for the battery density by sharpening their pencils on the aero front, and appear to have succeeded. I can't wait for those to reach flight schools, it'll force students to learn proper energy management skills in descents and landings.
 
phugoid1982
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Re: airplane gliding

Mon Mar 28, 2022 2:00 pm

If anyone cares, a simple formula to calculate max gliding range is Range=(L/D)*height. L/D max is the same as CL/CD and this occurs when the zero lift drag coefficient is equal to the induced drag coefficient. The speed for this flat glad varies with the square root of (1/density) so as you descend you will slow down incrementally.
 
26point2
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Re: airplane gliding

Mon Mar 28, 2022 7:03 pm

Engines or no engines a plane will crash when it hits something. If your engineless plane can glide to a runway then....no crash.
 
vikkyvik
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Re: airplane gliding

Mon Mar 28, 2022 8:12 pm

Woodreau wrote:
not an airliner, but here is a recent video of a plane last month that lost all engine power and made a forced landing. The pilot managed his energy really well and even taxied off the runway using residual energy.


Just had to say....wow! What a perfect landing! Great energy management.
 
Spock540
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Re: airplane gliding

Tue Mar 29, 2022 1:27 pm

N1120A wrote:
Starlionblue wrote:

I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.


The only light single that probably glides worse than a 172 is a Cirrus. A Mooney will do 11:1 (maybe more). A DA40 will do 13:1. As noted, most modern airliners do 20:1 or more. And they're doing it from up high.

Bob Pearson had to slip C-GAUN into Gimli, and did it successfully.


Try a PA28-201: the plane falls from the sky like a stone as soon as you go idle.
 
N1120A
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Re: airplane gliding

Wed Mar 30, 2022 12:03 am

Spock540 wrote:
N1120A wrote:
Starlionblue wrote:

I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.


The only light single that probably glides worse than a 172 is a Cirrus. A Mooney will do 11:1 (maybe more). A DA40 will do 13:1. As noted, most modern airliners do 20:1 or more. And they're doing it from up high.

Bob Pearson had to slip C-GAUN into Gimli, and did it successfully.


Try a PA28-201: the plane falls from the sky like a stone as soon as you go idle.


I'm assuming you mean a PA28R-201. Just keep the gear up and they glide a lot better. I have over 100 hours in Arrows.
 
Yikes!
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Re: airplane gliding

Fri Apr 08, 2022 2:44 am

joejohn wrote:
they say if an airplane experiences double engine failure, it won't just fall out of the sky (I find this hard to believe)
they say because the airplane is gliding through the air
if the airplane won't crash and will just glide through the air, then why do we hear on the news airplanes crash?


First, I would suggest you forget about what "they say" and ask the experts upon which there are a great many on this forum, about questions such as yours.

Every, and I mean EVERY airplane is inherently a glider once it loses its principle source(s) of propulsion.

Two terrific examples are the Air Canada "Gimli Glider" incident and the 2001 Air Transat A330 into the Azores incident. Total engine losses at cruise altitudes resultant in successful recoveries of the aircraft with no significant injuries or loss of life and minimal damage to the aircraft.

I hope that gives you a bit of insight into your question!
 
flipdewaf
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Re: airplane gliding

Sun Apr 10, 2022 9:14 am

phugoid1982 wrote:
If anyone cares, a simple formula to calculate max gliding range is Range=(L/D)*height. L/D max is the same as CL/CD and this occurs when the zero lift drag coefficient is equal to the induced drag coefficient. The speed for this flat glad varies with the square root of (1/density) so as you descend you will slow down incrementally.

I’m having an argument with myself whether this is correct or not. In flight the point where zero lift drag and lift induced drag are equal is where you get max endurance (minimum drag) but this isn’t maximum specific range, this is when UL/D is at its maximum or at the speed when the tangent from the drag slope intersects the origin.

Now the bit that I’m wrestling with is does the energy source change (from fuel to gravity) change the aerodynamic equation? Will gliding at max L/D give you max time aloft and gliding at max UL/D still give you the greatest glide distance?

I feel I can convince myself either way and therefore neither way.

Fred


Sent from my iPad using Tapatalk
 
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Starlionblue
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Re: airplane gliding

Sun Apr 10, 2022 1:57 pm

flipdewaf wrote:
phugoid1982 wrote:
If anyone cares, a simple formula to calculate max gliding range is Range=(L/D)*height. L/D max is the same as CL/CD and this occurs when the zero lift drag coefficient is equal to the induced drag coefficient. The speed for this flat glad varies with the square root of (1/density) so as you descend you will slow down incrementally.

I’m having an argument with myself whether this is correct or not. In flight the point where zero lift drag and lift induced drag are equal is where you get max endurance (minimum drag) but this isn’t maximum specific range, this is when UL/D is at its maximum or at the speed when the tangent from the drag slope intersects the origin.

Now the bit that I’m wrestling with is does the energy source change (from fuel to gravity) change the aerodynamic equation? Will gliding at max L/D give you max time aloft and gliding at max UL/D still give you the greatest glide distance?

I feel I can convince myself either way and therefore neither way.

Fred


Sent from my iPad using Tapatalk


I don't think the energy source changes anything, but that is an interesting question.

Image
 
phugoid1982
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Re: airplane gliding

Sun Apr 10, 2022 6:42 pm

flipdewaf wrote:
phugoid1982 wrote:
If anyone cares, a simple formula to calculate max gliding range is Range=(L/D)*height. L/D max is the same as CL/CD and this occurs when the zero lift drag coefficient is equal to the induced drag coefficient. The speed for this flat glad varies with the square root of (1/density) so as you descend you will slow down incrementally.

I’m having an argument with myself whether this is correct or not. In flight the point where zero lift drag and lift induced drag are equal is where you get max endurance (minimum drag) but this isn’t maximum specific range, this is when UL/D is at its maximum or at the speed when the tangent from the drag slope intersects the origin.

Now the bit that I’m wrestling with is does the energy source change (from fuel to gravity) change the aerodynamic equation? Will gliding at max L/D give you max time aloft and gliding at max UL/D still give you the greatest glide distance?

I feel I can convince myself either way and therefore neither way.

Fred


Sent from my iPad using Tapatalk


It's easier for me to explain mathematically. I'm not sure where you got that max endurance occurs at L/D max which is Cd0=Cdi which is incorrect. As for U*(L/D) I think you got that from the breguet range equation for horizontal flight which when you use the small angle approximation, Wcos(theta)=L so W=L so that approximation applies for gliding flight. Remember U (assuming you meant velocity) is a function of Weight also so the force balance gives U=sqrt(2W/rho*S*Cl). Hence U (L/D) when you multiple through is proportional to sqrt(L)/D not L/D and this is for steady powered flight. As for gliding flight, the easiest way to derive this is to draw a force balance diagram. (I apologize in advance because i'm sure you know this but i'm doing for the benefit of the non engineers on this forum so please don't feel like i'm being a condescending jerk.) The force balance setting thrust to 0 yields Wsin (theta)=D and Wcos(theta)=L. So dividing through D/L=tan (theta). If you draw a triangle of the descent phase you'll also see that tan (theta)=height/horizontal distance covered (Range). Therefore (D/L)=H/R or rather (L/D)H=R which is the same as (CL/CD)*H=R. To find the relationship between CD0 and CDi for L/Dmax take the derivative of (CL/CD) with respect to CL and set to zero keeping in mind that CD=CD0+CDi and you will get that exactly CD0=CDi.

Now wrt to endurance, remember range is about minimizing loss of height per unit distance. Endurance is about minimizing sink rate. Therefore the sink rate is given by Vsin(theta) which is V(Theta) for small angles and equal to DV/W from the force balance. Substituting V=sqrt(2W/rho*SCL)) you'll get that sink rate is minimized when CD/CL^(3/2) is smallest. Again, taking the derivative of this quantity wrt to CL you you will get that this occurs when 3Cd0=Cdi. At the end of the day the Velocity for max endurance is slightly smaller than that for max range. That's why when sailplane pilots encounter thermals they fly slower to maximize time in the updraft and then once it dies accelerate to Vmax range to cover the most ground and find another thermal.

Hope I make sense,
Vik
 
flipdewaf
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Re: airplane gliding

Mon Apr 11, 2022 3:57 pm

Duplicate
Last edited by flipdewaf on Mon Apr 11, 2022 3:58 pm, edited 1 time in total.
 
flipdewaf
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Re: airplane gliding

Mon Apr 11, 2022 3:57 pm

phugoid1982 wrote:
flipdewaf wrote:
phugoid1982 wrote:
If anyone cares, a simple formula to calculate max gliding range is Range=(L/D)*height. L/D max is the same as CL/CD and this occurs when the zero lift drag coefficient is equal to the induced drag coefficient. The speed for this flat glad varies with the square root of (1/density) so as you descend you will slow down incrementally.

I’m having an argument with myself whether this is correct or not. In flight the point where zero lift drag and lift induced drag are equal is where you get max endurance (minimum drag) but this isn’t maximum specific range, this is when UL/D is at its maximum or at the speed when the tangent from the drag slope intersects the origin.

Now the bit that I’m wrestling with is does the energy source change (from fuel to gravity) change the aerodynamic equation? Will gliding at max L/D give you max time aloft and gliding at max UL/D still give you the greatest glide distance?

I feel I can convince myself either way and therefore neither way.

Fred


Sent from my iPad using Tapatalk


It's easier for me to explain mathematically. I'm not sure where you got that max endurance occurs at L/D max which is Cd0=Cdi which is incorrect.

Assuming constant SFC then we are looking to minimise D to minimise fuel flow rate and therefore endurance.
phugoid1982 wrote:
As for U*(L/D) I think you got that from the breguet range equation for horizontal flight which when you use the small angle approximation, Wcos(theta)=L so W=L so that approximation applies for gliding flight.

At any given weight and constant SFC the maximum specific range is when the function f(U) UL/D is highest no?
phugoid1982 wrote:
Remember U (assuming you meant velocity) is a function of Weight also so the force balance gives U=sqrt(2W/rho*S*Cl). Hence U (L/D) when you multiple through is proportional to sqrt(L)/D not L/D and this is for steady powered flight.

Wouldn't it go proportional to (L^(3/2))/D
phugoid1982 wrote:
As for gliding flight, the easiest way to derive this is to draw a force balance diagram. (I apologize in advance because i'm sure you know this but i'm doing for the benefit of the non engineers on this forum so please don't feel like i'm being a condescending jerk.)

No problem, youarent being condescending at all, this is exactly what I love about this place and you highlight the piece I hadn't thought about correctly.
phugoid1982 wrote:
The force balance setting thrust to 0 yields Wsin (theta)=D and Wcos(theta)=L. So dividing through D/L=tan (theta). If you draw a triangle of the descent phase you'll also see that tan (theta)=height/horizontal distance covered (Range). Therefore (D/L)=H/R or rather (L/D)H=R which is the same as (CL/CD)*H=R. To find the relationship between CD0 and CDi for L/Dmax take the derivative of (CL/CD) with respect to CL and set to zero keeping in mind that CD=CD0+CDi and you will get that exactly CD0=CDi.

Id taken the assumptions of steady level flight for my baseline but if I had used climbing flight (negatively)it would have made more sense.
phugoid1982 wrote:

Now wrt to endurance, remember range is about minimizing loss of height per unit distance. Endurance is about minimizing sink rate. Therefore the sink rate is given by Vsin(theta) which is V(Theta) for small angles and equal to DV/W from the force balance. Substituting V=sqrt(2W/rho*SCL)) you'll get that sink rate is minimized when CD/CL^(3/2) is smallest. Again, taking the derivative of this quantity wrt to CL you you will get that this occurs when 3Cd0=Cdi. At the end of the day the Velocity for max endurance is slightly smaller than that for max range. That's why when sailplane pilots encounter thermals they fly slower to maximize time in the updraft and then once it dies accelerate to Vmax range to cover the most ground and find another thermal.

Hope I make sense,
Vik

It does all make sense but I am going to do my own FBD to make sure I have grasped it and can understand it.

Fred
 
flipdewaf
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Re: airplane gliding

Mon Apr 11, 2022 4:47 pm

Of course it isn't L^(3/2)/D, the U is Cl^(-1/2)
 
phugoid1982
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Re: airplane gliding

Mon Apr 11, 2022 6:24 pm

flipdewaf wrote:
phugoid1982 wrote:
flipdewaf wrote:
No problem, youarent being condescending at all, this is exactly what I love about this place and you highlight the piece I hadn't thought about correctly.


I appreciate that. I too enjoy these mathematical digressions. I didn't want to cheat and go to my aircraft performance books and pull out a formula. It was quite fun working it out on my own. One thing my late father who was a brilliant engineer always taught me to heart is always derive everything from first principles and don't just memorize equations which is good advice. Considering i've been in industry so long I wanted to see if my brain cells are still functional. It's also tough to explain without visuals so I found this pdf from Princeton which might be helpful.

http://www.stengel.mycpanel.princeton.e ... cture9.pdf
 
MPadhi
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Re: airplane gliding

Thu Apr 14, 2022 10:06 am

Starlionblue wrote:
A common problem with jets is not being able to get down fast enough on approach. In a propeller plane, you can use the props to get you down, or even slip if you're in a light aircraft. In an airliner, descent requires advance planning. A typical descent starts 120 nautical miles from the runway, and much of it will be at idle power. If you get a significant shortcut when already on approach, it's time to go full speedbrake.

The saying is that "you can go down or slow down, but not both".


It's a bit of a tangent, but this got me thinking - are there any ways to slow down and go down in an emergency. Let's say there's an in flight fire - would the best strategy be to go as fast and close to your destination. Let's say 550kts, out to 100 miles out. Then slow down enough to extend flaps, gear, and speedbrakes, then put the aircraft in a 90 degree roll so you're dumping all lift whilst having all drag devices out.

Is this feasible in an emergency, or would it create further issues?
 
Woodreau
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Re: airplane gliding

Thu Apr 14, 2022 12:39 pm

You cannot fly 550kts. The most you can do is Mmo/Vmo.

The emergency descent profile if there is no structural damage is flown at Mmo/Vmo at flight idle and speed brake deployed you can get really high descent rates.

You do not want to roll 90 degrees, airliners do not have enough roll authority like fighters do to do a knife edge.

But then again AF447 descended from FL370 to impacting the ground in about 2.5 minutes by stalling the aircraft. So I guess that is the fastest way down if you could figure out how to recover the aircraft.
 
Woodreau
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Re: airplane gliding

Thu Apr 14, 2022 12:51 pm

phugoid1982 wrote:
flipdewaf wrote:
phugoid1982 wrote:


I appreciate that. I too enjoy these mathematical digressions. I didn't want to cheat and go to my aircraft performance books and pull out a formula. It was quite fun working it out on my own. One thing my late father who was a brilliant engineer always taught me to heart is always derive everything from first principles and don't just memorize equations which is good advice. Considering i've been in industry so long I wanted to see if my brain cells are still functional. It's also tough to explain without visuals so I found this pdf from Princeton which might be helpful.

http://www.stengel.mycpanel.princeton.e ... cture9.pdf


Reading your discussions between you two brought to mind the sonar equations and Eckland equations you had to run to try to locate submarines…. You had slide rulers to help with the mental math and a set of speed rulers that you could use to play what if with the assumed speed on the tactical plot to try to establish datum on the submarine.

Then I find out that now (2 decades later) there is an app for that…. Pfft… kids these days.

https://en.wikipedia.org/wiki/Target_Motion_Analysis


But yeah when I was flying gliders you’d slow to min sink speed while circling in thermals to maximize the lift I’d get from thermals. Once outside a thermal and searching for the next thermal and found yourself in sink you fly a faster speed to get out of the sink area faster….. my favorite was flying a ridge, where you get lift from the air being physically lifted by terrain as long as you didn’t get blown over the crest into sink.
 
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zeke
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Re: airplane gliding

Thu Apr 14, 2022 2:41 pm

BoeingGuy wrote:

Modern jetliners like the 777 have at least a 20:1 glide ratio. The 787 glide ratio is supposedly so high it’s proprietary information. I don’t know the exact number. I assume Airbus models are equally efficient.

The rule of thumb that I’ve heard is 2 miles gliding for every 1000 of altitude loss, but the Air Transat A330 well exceeded that.


I would be very surprised if Boeing does not published numbers for the 777/787 in the checklist for all engine flameout, the A330 they say 100nm from FL400 (15:1), and with gear and flap down 9:1.
 
BoeingGuy
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Re: airplane gliding

Thu Apr 14, 2022 5:27 pm

zeke wrote:
BoeingGuy wrote:

Modern jetliners like the 777 have at least a 20:1 glide ratio. The 787 glide ratio is supposedly so high it’s proprietary information. I don’t know the exact number. I assume Airbus models are equally efficient.

The rule of thumb that I’ve heard is 2 miles gliding for every 1000 of altitude loss, but the Air Transat A330 well exceeded that.


I would be very surprised if Boeing does not published numbers for the 777/787 in the checklist for all engine flameout, the A330 they say 100nm from FL400 (15:1), and with gear and flap down 9:1.


Be surprised. The Boeing Dual Eng Fail/Stall checklists do not give any kind of gliding guidance to the crew. The checklist assumes you get at least one engine started and have a nice day. There is no branch of the checklist for what to do in case you don't get an engine started.
 
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zeke
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Re: airplane gliding

Fri Apr 15, 2022 1:10 am

BoeingGuy wrote:
Be surprised. The Boeing Dual Eng Fail/Stall checklists do not give any kind of gliding guidance to the crew. The checklist assumes you get at least one engine started and have a nice day. There is no branch of the checklist for what to do in case you don't get an engine started.


Interesting, not something that is impossible to happen, aware of 3 dual engine failure/dual thrust loss events on the 787, Jetstar, ANA, Royal Brunei.
 
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Starlionblue
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Re: airplane gliding

Fri Apr 15, 2022 1:45 am

MPadhi wrote:
Starlionblue wrote:
A common problem with jets is not being able to get down fast enough on approach. In a propeller plane, you can use the props to get you down, or even slip if you're in a light aircraft. In an airliner, descent requires advance planning. A typical descent starts 120 nautical miles from the runway, and much of it will be at idle power. If you get a significant shortcut when already on approach, it's time to go full speedbrake.

The saying is that "you can go down or slow down, but not both".


It's a bit of a tangent, but this got me thinking - are there any ways to slow down and go down in an emergency. Let's say there's an in flight fire - would the best strategy be to go as fast and close to your destination. Let's say 550kts, out to 100 miles out. Then slow down enough to extend flaps, gear, and speedbrakes, then put the aircraft in a 90 degree roll so you're dumping all lift whilst having all drag devices out.

Is this feasible in an emergency, or would it create further issues?


As mentioned you would not be doing 550 knots, at least not with the aircraft in one piece. At most something like 330 indicated. :)

There's no need to slow down in an emergency descent. The goal is to get down to a safe altitude ASAP.

To get down fast you need to fly fast*. Accelerate to Vmo (in practice just under for a bit of margin), speedbrakes out, idle thrust. If you have the gear down you'd descend slower since the gear limit speed is well below Vmo (250kts on the A330).

Descent rates in excess of 6000fpm are typical. In case of suspected structural damage, you'd keep the speed where it was, and consequently descend slower.

90 degree roll is not recommended. Besides, not possible on the 'bus in Normal Law. It would definitely create "further issues".


Another case is uncontrollable fire. Keep your speed as high as possible until you're about 15nm from the runway and 4000 feet-ish. Then idle thrust, full speedbrake. At gear limiting speed, gear out, then flaps out in stages at the limit speeds. This gives you the fastest possible approach. Do give yourself a bit of margin because if you leave it too late you'll be going too fast to land and have to go around, which would be unideal in this situation. Again though, you're not going down and slowing down at the same time. You go down, then slow down.


* This also applies to normal approaches. If you're high on profile keeping your speed up lets you descend faster. If you are instructed to "descend to 10000 and slow to 210," but are currently fast and high, the best strategy is often to descend fast to 10000, then slow. Don't try to do both at once.
 
celestar345
Posts: 106
Joined: Wed May 08, 2013 5:35 pm

Re: airplane gliding

Fri Apr 15, 2022 3:32 am

zeke wrote:
BoeingGuy wrote:
Be surprised. The Boeing Dual Eng Fail/Stall checklists do not give any kind of gliding guidance to the crew. The checklist assumes you get at least one engine started and have a nice day. There is no branch of the checklist for what to do in case you don't get an engine started.


Interesting, not something that is impossible to happen, aware of 3 dual engine failure/dual thrust loss events on the 787, Jetstar, ANA, Royal Brunei.


Still can remember for the Jetstar incident Boeing just completely passed the blame to the fuel biocide treatment. Doing the inspection aftermath really ain't much fun...
 
MPadhi
Posts: 134
Joined: Sun Jan 25, 2015 5:33 pm

Re: airplane gliding

Wed Apr 20, 2022 11:00 am

Starlionblue wrote:
MPadhi wrote:
Starlionblue wrote:
A common problem with jets is not being able to get down fast enough on approach. In a propeller plane, you can use the props to get you down, or even slip if you're in a light aircraft. In an airliner, descent requires advance planning. A typical descent starts 120 nautical miles from the runway, and much of it will be at idle power. If you get a significant shortcut when already on approach, it's time to go full speedbrake.

The saying is that "you can go down or slow down, but not both".


It's a bit of a tangent, but this got me thinking - are there any ways to slow down and go down in an emergency. Let's say there's an in flight fire - would the best strategy be to go as fast and close to your destination. Let's say 550kts, out to 100 miles out. Then slow down enough to extend flaps, gear, and speedbrakes, then put the aircraft in a 90 degree roll so you're dumping all lift whilst having all drag devices out.

Is this feasible in an emergency, or would it create further issues?


As mentioned you would not be doing 550 knots, at least not with the aircraft in one piece. At most something like 330 indicated. :)

There's no need to slow down in an emergency descent. The goal is to get down to a safe altitude ASAP.

To get down fast you need to fly fast*. Accelerate to Vmo (in practice just under for a bit of margin), speedbrakes out, idle thrust. If you have the gear down you'd descend slower since the gear limit speed is well below Vmo (250kts on the A330).

Descent rates in excess of 6000fpm are typical. In case of suspected structural damage, you'd keep the speed where it was, and consequently descend slower.

90 degree roll is not recommended. Besides, not possible on the 'bus in Normal Law. It would definitely create "further issues".


Another case is uncontrollable fire. Keep your speed as high as possible until you're about 15nm from the runway and 4000 feet-ish. Then idle thrust, full speedbrake. At gear limiting speed, gear out, then flaps out in stages at the limit speeds. This gives you the fastest possible approach. Do give yourself a bit of margin because if you leave it too late you'll be going too fast to land and have to go around, which would be unideal in this situation. Again though, you're not going down and slowing down at the same time. You go down, then slow down.


* This also applies to normal approaches. If you're high on profile keeping your speed up lets you descend faster. If you are instructed to "descend to 10000 and slow to 210," but are currently fast and high, the best strategy is often to descend fast to 10000, then slow. Don't try to do both at once.


Fascinating as always! Thanks for the detailed reply!
 
CosmicCruiser
Posts: 2565
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Re: airplane gliding

Wed Apr 20, 2022 4:32 pm

90deg bank, you're now aerobatic.
 
thepinkmachine
Posts: 486
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Re: airplane gliding

Thu Apr 21, 2022 7:13 am

BoeingGuy wrote:
ACDC8 wrote:
Starlionblue wrote:
I'll add that the glide ratio of an airliner is way better than a Cessna 172. More like 1:15-20. For every 10000 feet, you can expect to glide 30 nautical miles at best glide speed. Plenty of time to try to restart the engines and look for landing options.

Oh for sure, gliding characteristics of modern airliners is incredible, and with the the altitude they fly, gives them a big advantage over a single engine prop.


Modern jetliners like the 777 have at least a 20:1 glide ratio. The 787 glide ratio is supposedly so high it’s proprietary information. I don’t know the exact number. I assume Airbus models are equally efficient.

The rule of thumb that I’ve heard is 2 miles gliding for every 1000 of altitude loss, but the Air Transat A330 well exceeded that.


Haven’t seen the numbers either, but the 787 is capable of flying a 3 degree ILS with both engines out… On autopilot and with autoland! (At least in the simulator)
 
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zeke
Posts: 17343
Joined: Thu Dec 14, 2006 1:42 pm

Re: airplane gliding

Thu Apr 21, 2022 9:04 am

At what speed ? In what configuration?

Sounds like something I should try if I have some spare time in the sim ;)
 
thepinkmachine
Posts: 486
Joined: Tue Apr 28, 2015 4:43 pm

Re: airplane gliding

Thu Apr 21, 2022 11:28 am

zeke wrote:
At what speed ? In what configuration?

Sounds like something I should try if I have some spare time in the sim ;)


If I remember correctly, last time I tried it, we intercepted the G/S in clean configuration and at UP/Gdot speed and the plane happily flew the profile at constant speed. Then at ~5-6 NM we started extending flaps, but kept the gear up until 2-3 Nm out. We crossed the threshold at Vref+5 :) I think it should work nicely on the 350 as well…
 
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zeke
Posts: 17343
Joined: Thu Dec 14, 2006 1:42 pm

Re: airplane gliding

Thu Apr 21, 2022 12:55 pm

I think I need to gather data to validate your opinion ;)
 
thepinkmachine
Posts: 486
Joined: Tue Apr 28, 2015 4:43 pm

Re: airplane gliding

Thu Apr 21, 2022 2:09 pm

I’ll be happy to learn the results! :bigthumbsup:
 
kalvado
Posts: 3726
Joined: Wed Mar 01, 2006 4:29 am

Re: airplane gliding

Thu Apr 21, 2022 2:53 pm

thepinkmachine wrote:
BoeingGuy wrote:
ACDC8 wrote:
Oh for sure, gliding characteristics of modern airliners is incredible, and with the the altitude they fly, gives them a big advantage over a single engine prop.


Modern jetliners like the 777 have at least a 20:1 glide ratio. The 787 glide ratio is supposedly so high it’s proprietary information. I don’t know the exact number. I assume Airbus models are equally efficient.

The rule of thumb that I’ve heard is 2 miles gliding for every 1000 of altitude loss, but the Air Transat A330 well exceeded that.


Haven’t seen the numbers either, but the 787 is capable of flying a 3 degree ILS with both engines out… On autopilot and with autoland! (At least in the simulator)

3 degrees is a glide ratio of 19. I believe optimal configuration glide ratio for 787 was quoted as "over 20". Apparently, gear would make things worse, though...

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