dilan12345
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Lift to Drag ratios at Take-off, Cruise and Landing for A320

Tue Mar 14, 2017 6:44 pm

Hey guys, does anyone know what the Lift/Drag ratios for the a320 are in the different phases of flight. I know at the optimum cruise speed, the L/D ratio is around 18, but i'm not sure what it is for take-off and landing. All help is appreciated.

Thanks
Dilan
 
dilan12345
Topic Author
Posts: 9
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Thu Mar 16, 2017 12:53 pm

anyone :)?
 
flipdewaf
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Thu Mar 16, 2017 1:05 pm

As a first order of magnitude guess I would say look at the fact that its probably fairly close to the limits when lifting off with one engine so look at the thrust from a single engine and the total weight.

Fred
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GalaxyFlyer
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sat Mar 18, 2017 3:48 am

What possible use is knowing the L/D while configured?
 
flipdewaf
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sat Mar 18, 2017 1:42 pm

GalaxyFlyer wrote:
What possible use is knowing the L/D while configured?

To know the drag, then you can work out required engine thrust at each stage of flight.

Fred
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longhauler
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sat Mar 18, 2017 3:55 pm

flipdewaf wrote:
To know the drag, then you can work out required engine thrust at each stage of flight.
Fred

You already know it. Maximum thrust (or flex thrust) for take-off, fixed climb thrust when climbing, idle thrust for descent. The only variables are cruise (dependant upon Cost Index) and approach (dependant upon how early one configures for landing which is dependant upon ATC/traffic).

Cruise variables can be determined from Cruise perfomance charts as shown on the other A320 thread, and approach thrust/fuel can only be determined by history plotting by actual airport and time of day data as it is very very rare that an aircraft will be allowed to configure in the most timely and efficient manner.

But the concept of L/D ratio is interesting as it is not something that is normally quantified in airline operations. The engineers that designed the aircraft would quantify it, as it would directly reflect efficienty. However .... best L/D ratio is something that A320 pilots do know, as it is displayed on the airspeed indicator when flaps are retracted ... it is referred to as "green dot".

In normal use, Green Dot would be your ideal minimum clean speed both on departure and approach, as to stay near it would always be most efficient. In abnormal use, Green Dot would be your initial target speed during a high altitude engine failure for terrain clearance. I say "initial" as after drills and check lists are completed, and terrain verified, then normal engine out descent profiles would be followed.

For your information, at 77 tonnes, GD is about 235 knots, at 60 tonnes, it is about 205 knots below FL200. (Above it, you add 1 knot per additional 1000'.)
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!
 
flipdewaf
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sat Mar 18, 2017 8:42 pm

longhauler wrote:
the engineers who program the buttons you press and make the charts you read however find l/d values pretty useful.

Fred
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Starlionblue
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sun Mar 19, 2017 2:19 am

longhauler wrote:
However .... best L/D ratio is something that A320 pilots do know, as it is displayed on the airspeed indicator when flaps are retracted ... it is referred to as "green dot".

In normal use, Green Dot would be your ideal minimum clean speed both on departure and approach, as to stay near it would always be most efficient. In abnormal use, Green Dot would be your initial target speed during a high altitude engine failure for terrain clearance. I say "initial" as after drills and check lists are completed, and terrain verified, then normal engine out descent profiles would be followed.

For your information, at 77 tonnes, GD is about 235 knots, at 60 tonnes, it is about 205 knots below FL200. (Above it, you add 1 knot per additional 1000'.)


On the 330-300. "Green Dot" (best L/D speed) is calculated as follows according to the FCOM.
With all engines running:
‐ Below 20 000 ft equal to 0.6 × weight (tons) +107 kt
‐ Above 20 000 ft, add 1 kt per 1 000 ft
With at least one engine-out, subtract 10 kt, in the above situations.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
thepinkmachine
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sun Mar 19, 2017 5:19 am

It's even easier on the 320. Gdot speed = weight in tons x2 +80

A hint to L/D ratio may be found in "all engine flameout" check list. It suggests that at Gdot you can fly approximately 3NM/1000ft, while in approach config it is 1.5NM/1000 ft (which is L/D=9). It think is not very accurate though, and rather conservative.
 
flipdewaf
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sun Mar 19, 2017 8:17 am

I'd guess that green dot gives you largest distance in an all engines families situation, is that right? I'm not sure that's actually max L/D. I think max L/D would give you maximum time in the air, ML/D or VL/D would be max distance but it was a long time ago since I last visited it.

Fred

Fred
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thepinkmachine
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sun Mar 19, 2017 8:53 am

flipdewaf wrote:
I'd guess that green dot gives you largest distance in an all engines families situation, is that right? I'm not sure that's actually max L/D. I think max L/D would give you maximum time in the air, ML/D or VL/D would be max distance but it was a long time ago since I last visited it.

Fred

Fred


Max L/D = max glide distance

Min sink rate is a bit more complicated calculation, which I don't remember at the moment.


Cheers!

Pink
 
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Starlionblue
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sun Mar 19, 2017 8:58 am

flipdewaf wrote:
I'd guess that green dot gives you largest distance in an all engines families situation, is that right? I'm not sure that's actually max L/D. I think max L/D would give you maximum time in the air, ML/D or VL/D would be max distance but it was a long time ago since I last visited it.

Fred

Fred


Green dot is not max range speed, which would give the longest distance flown. Green dot is max L/D, which is max endurance speed (max time in the air).

On the 330, green dot all engines is 10 knots higher than green dot engine out. This seems a bit arbitrary so I suspect there's a "pad" in there .

On the 350, green dot all engines is not an exact number difference from green dot engine out. My guess is that the system is more advanced and gives you an "exact green dot" in all situations.

In the pic below, you can see that Max L/D (green dot) is the lowest point on the drag curve, while max range speed is where a straight line drawn from the origin intersects the drag curve.

Image
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PRNJ
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sun Mar 19, 2017 9:42 am

Mathematically the maximum Lift to drag ratio is obtained by the following formula -- (L/D)Max = 1/2 times Square root of ( pi multiplied by span efficiency factor multiplied by Aspect Ratio of wings divided by Zero Lift Drag coefficient) . Span efficiency factor can be taken as is more or less one .Aspect ratio is the ratio of the span to mean chord of the wing or square of span divided by wetted area of wing. Then L/D ratio will not change with weight or wing loading for a particular aircraft.
 
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dlednicer
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Sun Mar 26, 2017 5:09 pm

There is a whole chapter on this in Fred Obert's book "Aerodynamic Design of Transport Aircraft"
 
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zeke
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Re: Lift to Drag ratios at Take-off, Cruise and Landing for A320

Mon Mar 27, 2017 3:04 am

PRNJ wrote:
L/D ratio will not change with weight or wing loading for a particular aircraft.


I don't agree with this, take the A330 for example it has active CG control which uses fuel to being transferred from the wing to the tail to reduce the cruise drag at essentially at the same mass. Lift is also reduced/increased as the downforce in the tail changes with the trim fuel.
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