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Matt6461
Posts: 2835
Joined: Wed Oct 16, 2013 9:36 pm

Re: Updated: Sir Tim Clark: Emirates next batches of A380s will be powered by RR

Fri Dec 07, 2018 2:11 am

Eyad89 wrote:
Matt6461 wrote:

We can reasonably predict fuel burn performance from 3 metrics: weight, SFC, and L/D




Isn’t the weight factor already considered in the L/D calculations (induced drag), why count it again separately?


There's a long answer and short answer:

The short is that cruise L/D does not vary with weight. (except to a very small extent related to the variation of Reynolds numbers with altitude, and the small delta to gravitational acceleration with altitude, among other even smaller factors).

The long answer involves explaining the short answer.
Are you aware of the effect of air density on induced drag?
 
JoeCanuck
Posts: 4580
Joined: Mon Dec 19, 2005 3:30 am

Re: Updated: Sir Tim Clark: Emirates next batches of A380s will be powered by RR

Fri Dec 07, 2018 2:52 am

Taxi645 wrote:
JoeCanuck wrote:
Regardless, none of it works if the 380 doesn't get the Ultrafan in a timely manner...and really I don't think RR can pull it off.


As said, why would Airbus and EK go through all this trouble building and buying a plane past it's competitive cycle at a trickle rate to bridge the gap to ultrafan and not built in contingency for one or two years delay? Of course they wouldn't. As if they would say, well we managed to pull through to 2025 but now there's a delay so we'll pack our bags, throw away hundreds of millions and call it a day.

They'll have margin built in in case there are delays with ultrafan, but yeah it's not easy sailing to get there.


If everybody decides to commit to the 380neo, then sure, they'll eat a delay. But so far, nobody has committed to it. The Ultrafan is being developed regardless, and will go on the 350neo. The XWB engines are more efficient than the T900's and the 350 line is busy. There isn't the same urgency for that neo so the problems that will be caused by the inevitable delays, aren't as big of a deal.

That one or two year delay, (I'm a pessimist so I think closer to 3 or 4), still puts the 380neo a decade away from EIS. That's a lot of resources tied up for a long time slowly doling out an aircraft nobody else wants.

It's still a good plane. It's not the most versatile, but it is the best people mover ever. As it is, it's a useful tool for EK, but they pretty much have as many as they need. EK isn't in a hurry to take the ones they have on order. They knew all about the T900, when they made the switch, and I'm guessing the only reason they did switch, is that they sucked huge guarantees out of RR.

If RR lives up to their side, EK wins. If RR doesn't, EK has an easy out and they still win.

EK will be flying at least some of the 380's it has for another 15 or 20 years and they will need some fleet renewal aircraft.

Airbus would love to sell more ceo's and the neo, and I'm sure EK would love to buy some...for the right deals. I'm sure they are negotiating about this very thing, right now. It's all going to depend on how quickly RR can develop the ultrafan, how long Airbus can justify keeping the line open and how long EK is willing to pay and wait.
What the...?
 
Eyad89
Posts: 439
Joined: Fri Sep 02, 2016 10:47 pm

Re: Updated: Sir Tim Clark: Emirates next batches of A380s will be powered by RR

Fri Dec 07, 2018 1:09 pm

Matt6461 wrote:

The short is that cruise L/D does not vary with weight.




No, Matt. It is only that the MAXIMUM possible L/D for a given plane is independent of weight. Maximum L/D is an intrinsic property of a design that is not affected by weight, speed, angle of attack, or altitude. But L/D isn't the same as maximum L/D. Commercial airplanes cruise at a speeds and altitudes that wouldn't achieve maximum L/D.

For a given flight, a plane would be achieving a certain L/D that is determined by simply dividing lift by drag (or Cl by Cd). For a given speed and altitude, weight has a direct impact on induced drag and eventually total drag and L/D at that moment. Having said that, the best L/D ratio this plane can achieve remains unchanged even if weight, attitude, or speed change.





Matt6461 wrote:
Are you aware of the effect of air density on induced drag?


Sure, induced drag increases as air density decreases ( higher altitudes)
 
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Matt6461
Posts: 2835
Joined: Wed Oct 16, 2013 9:36 pm

Re: Updated: Sir Tim Clark: Emirates next batches of A380s will be powered by RR

Fri Dec 07, 2018 1:38 pm

Eyad89 wrote:
Matt6461 wrote:

The short is that cruise L/D does not vary with weight.




No, Matt. It is only that the MAXIMUM possible L/D for a given plane is independent of weight. Maximum L/D is an intrinsic property of a design that is not affected by weight, speed, angle of attack, or altitude. But L/D isn't the same as maximum L/D. Commercial airplanes cruise at a speeds and altitudes that wouldn't achieve maximum L/D.

For a given flight, a plane would be achieving a certain L/D that is determined by simply dividing lift by drag (or Cl by Cd). For a given speed and altitude, weight has a direct impact on induced drag and eventually total drag and L/D at that moment. Having said that, the best L/D ratio this plane can achieve remains unchanged even if weight, attitude, or speed change.





Matt6461 wrote:
Are you aware of the effect of air density on induced drag?


Sure, induced drag increases as air density decreases ( higher altitudes)


There is more than one non-dimensional L/D value. Optimal Cruise L/D is like max L/D in being generally independent of weight.

I'd encourage you to take this discussion to my fundamentals thread in TechOps.
 
Eyad89
Posts: 439
Joined: Fri Sep 02, 2016 10:47 pm

Re: Updated: Sir Tim Clark: Emirates next batches of A380s will be powered by RR

Fri Dec 07, 2018 2:20 pm

Matt6461 wrote:
Eyad89 wrote:
Matt6461 wrote:

The short is that cruise L/D does not vary with weight.




No, Matt. It is only that the MAXIMUM possible L/D for a given plane is independent of weight. Maximum L/D is an intrinsic property of a design that is not affected by weight, speed, angle of attack, or altitude. But L/D isn't the same as maximum L/D. Commercial airplanes cruise at a speeds and altitudes that wouldn't achieve maximum L/D.

For a given flight, a plane would be achieving a certain L/D that is determined by simply dividing lift by drag (or Cl by Cd). For a given speed and altitude, weight has a direct impact on induced drag and eventually total drag and L/D at that moment. Having said that, the best L/D ratio this plane can achieve remains unchanged even if weight, attitude, or speed change.





Matt6461 wrote:
Are you aware of the effect of air density on induced drag?


Sure, induced drag increases as air density decreases ( higher altitudes)


There is more than one non-dimensional L/D value. Optimal Cruise L/D is like max L/D in being generally independent of weight.

I'd encourage you to take this discussion to my fundamentals thread in TechOps.



Sure, would provide the link for the thread? I couldn't find it ..
 

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