smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

MRC And LRC

Thu Jun 14, 2012 9:18 pm

I was reading up recently on Maximum Range Cruise (MRC) and Long Range Cruise (LRC). From what I can gather, LRC was developed from MRC because a slight speed increase gave a good extra advantage in speed/time in exchange for not such a detrimental effect on fuel burn.

I also read that for MRC, the aircraft needed to be constantly operated at their optimum altitude. For older generation aircraft this was apparently quite difficult with all the associated power changes.

I am just curious in regards to, what is the reason for so many power changes that would be required on an older generation aircraft for an MRC compared to an LRC?

When they say older generation aircraft, are they referring to aircraft without A/T?
 
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glen
Posts: 259
Joined: Thu Jun 30, 2005 4:43 pm

RE: MRC And LRC

Fri Jun 15, 2012 7:29 am

Quoting smartt1982 (Thread starter):
I am just curious in regards to, what is the reason for so many power changes that would be required on an older generation aircraft for an MRC compared to an LRC?

With MRC you are flying with the speed of lowest drag:



So every time you are a only a little bit left (slow) of your optimum speed your drag increases. Thus, if not adding thrust to overcome this situation, this additional drag will slow you further down, requiring even more thrust to come back to MRC. At the moment you are again exactly at MRC you have to reduce again thrust to the thrust required for MRC. Otherwise the aircraft will increase its speed further requiring again a thrust correction.
So with MRC you are at a very unstable point of the speed-drag curve, requiring constant corrective action to stay there. Especially in a real life environment with changing winds and not knowing the very exact power setting for your actual condition (altitude, temperature, weight) to stay there.
Flying LRC (usually 99% range compared to MRC) on the other hand you are slightly right of your minimum drag point. Thus every increase of speed brings automatically an increase of drag, forcing the aircraft back to its original speed without thrust adjustment. Same is true for speed decrease with drag decrease, allowing the aircraft to accelerate to LRC without adding thrust. So you are in a stable situation where the speed can fluctuate around the speed corresponding to your actual power setting, without needing continuous corrective action.
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
 
smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

RE: MRC And LRC

Fri Jun 15, 2012 8:59 am

Cheers Glen that is really helpful, thanks. Should have been able to see that myself, better start doing more study!

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