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Dispatch: LRC workaround?


Rodeo
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Edit: SimBrief now has added MACH 0.74 and MACH 0.72 cruise profiles to their CL60 profile, so the discussion below is mostly irrelevant now.

 

I've requested that SimBrief add an LRC cruise schedule to their CL60 profile if possible, to make it easier to plan range-topping missions such as London to New York westbound:

https://forum.navigraph.com/t/cl60-cruise-schedules/7076

In the meantime, the most economical profile available is MACH 0.77. For those who have access to the actual fuel tables for the CL60, is there a way the LRC cruise could be expressed as a fuel factor relative to MACH 0.77 cruise, let's say over a ~3,500-3,600nm air distance?

SimBrief's fuel factor will also apply to climb and descent, but over longer ranges this matters somewhat less. I guess, for planning purposes, we would have:

  • a fuel factor that applies to a specific air distance
  • we would scale said fuel factor based on actual air distance (either linearly or using another recommended method)

Regards,

Tim

Edited by Rodeo
SimBrief came through
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With some help from a kind user with ForeFlight Performance Plus, it seems the difference between LRC and MACH 0.77 over ~3,000+ nautical miles perhapsvaries between 2 and 3 percent or so, depending on payload.

So a very rough guideline could be a fuel factor of M01 for each ~,1000nm (or ~,1500nm to be more conservative) of air distance or so.

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It also seems ForeFlight appears to climb the aircraft a bit more aggressively than SimBrief does (given same route and payload, 2000 feet higher more or less around the same waypoints); if you  were to use e.g. a 250/250/0.72 climb, I suspect you could use a P2000 altitude offset on SimBrief and still climb quite comfortably (SimBrief plans for 250/300/0.78 in climb).

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Note that to fly LRC, you will need to decrease speed as you get lighter in weight, which is counter a constant Mach required on NATs, for example. 

A Q400 SAR chart I found online. Note the red line showing decreasing KTAS with decreasing weight (from bottom to top). 

sar_q400.jpg

Edited by mraviator
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I thought I heard/read the opposite? MACH 0.74 near MTOW and eventually 0.72 when you're light (my assumption being that you slow down because, as you get lighter, you can fly slower -- less thrust, less fuel used -- while retaining a sufficient safety margin vis-à-vis the aircraft's equivalent of "green dot" speed, since it would go down along with the weight).

Either way, the FMS can maintain LRC speed for you, so you'd use that, except in oceanic airspace (whichever constant MACH number agreed upon with ATC). It would affect fuel burn though, so maybe indeed the more conservative figure above is more appropriate for oceanic flights :)

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I think we are saying the same thing - faster when heavy, slower when lighter.

I took a look at the CL650 SAR Chart a while ago and the LRC line did not quite make it to M0.77 at heaviest weight and was significantly lower than that at lightest weight. 
 

I just bought the aircraft yesterday and only have four flights under my belt so far, but great to hear LRC can be maintained via FMS! More to discover.

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4 hours ago, mraviator said:

I think we are saying the same thing - faster when heavy, slower when lighter.

Now that you've edited your post, I can't remember for sure why I misunderstood you, but I seem to recall you used the term accelerate (rather than decelerate) originally ;)

Edited by Rodeo
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6 hours ago, mraviator said:

I just bought the aircraft yesterday and only have four flights under my belt so far, but great to hear LRC can be maintained via FMS! More to discover.

Took me forever to find out, but there it is:

2111932212_Screenshot2022-01-10at05_15_50.thumb.png.156c662fe0fd1604a1c83938d387be1e.png

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On 1/11/2022 at 6:25 AM, esperez0802 said:

Is there any way to change or add different climb and descent profiles, like 250/280/.72? That would be useful too.

Turns out SimBrief was able to do that, it's available now :)

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