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bpcw001

Lear 35A fuel burn question

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Hi,

 

I'm trying to get my fuel planning for the Learjet 35A sorted out.

I know about the rule posted by CW46:

 

1500lbs 1st hour fuel burn,1200lbs 2nd hour, 1100lbs 3rd hour and 1000lbs for the remaining hour for a M0.78 (430-460kts) cruise (although I found that to be a little too optimistic for the Flysimware Lear).

 

What would be the approximate figures for econ-speed, max. range cruise (what speed/mach would that be anyway?)?

 

Unfortunately, I wasn't able to dig up any figures in the original pilot manual that is linked here https://drive.google.com/file/d/0B44Egs5qiC_bbmZad3RuR1dQVjA/view?invite=CMOOkI0C&ts=564bff1c&pli=1.

There are a lot of figures about v-speeds, climb gradients and stuff, but I somehow missed the cruise fuel burn part.

Maybe someone can point me to this information in the manual with a brief explanation on how to use it for fuel planning.

 

Thanks

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Well, after some more testing and adjusting my planning to take cruise wind and additional mileage due to departure/approach procedures etc. into account, I found the 1500/1200/1100/1000 rule to be almost spot on with the Flysimware Lear for the typical >FL350 cruise at M0.72.

 

Fine thing!

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Dave,

 

A normal cruise would be .77-.78 mach about 440kt TAS, the fuel burn in the FSW version is too high.With a full fuel load you should be able to fly about 4+45 and land with 1000 lbs.Consulting actual cruise performance charts would not be of much help unless the model actually was built around them.The rule of thumb hourly burn was based on a normal cruise speed and for endurance ending up FL410 - FL450.

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Intersting. The Flysimware Lear fuel burn more or less works out when using a M0.71-M0.72 cruise, which I've always considered to be quite slow.

So, if the normal econ cruise is supposed to be M0.77-M0.78, the fuel burn modeled is definitely too high.

Hopefully Flysimware will tweak this accordingly.

 

Or we could try to tweak the fuel_flow_scalar in the .cfg file outselves if we knew by how many percent the Flysim fuel burn is off the real thing.

 

Any suggestions?

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I don't know.

 

I use FltPlan.com for my flight planning, and use the Learjet 35 profile on their website.

 

Once I enter the route, FltPlan.com calculates best altitude, time enroute, fuel burn, true airspeed, groundspeed, etc. according to the forecast enroute weather.  Note that FltPlan.com is primarily for real world flight planning.

 

When flying the Flysimware Learjet according to the flight plan, I find that the fuel burn is usually quite accurate.  If it is off, it is not off by much.  The calculated flight time is usually quite accurate, no more than two minutes off.  (By the way, I use ASN for weather.)

 

I generally cruise at .72 to ,73 mach.  Predicted TAS and groundspeed are pretty close to what I actually see in flight.

 

Just my experience.

 

--Randall Griffin

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At M0.71-0.72 I found the figures to be quite accurate as well. However, according to Gary who apparently has some real world experience flying the Lear 35, the jet should cruise at M0.77-0.78 at that fuel burn rate.

 

Maybe Flysimware can comment on this?

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In the real world you would be hearing a lot of  ATC "can you keep your speed up, I have over taking traffic following" The airplane was meant to fly high and fast, .77 is conservative as a lot of operators flew it right up to Mmo.Bottom line is this, the rule of thumb numbers worked for us and I was in the airplane for all of nineteen years.

 

Regarding the FSW version I tried a trip a about three months ago with full fuel and .77 cruise at 390 and was out fuel about 3+45 into it.I actually prefer to sim with old piston engine stuff but try an endurance flight with full fuel and note the hourly fuel burn off until you hit dry tanks.If you can't get 4+45 or better something is not right. Heck, the longest individual flight I ever made was 5+08 and we got jerked around with an early descent and still showed 800 lbs on the ramp.That was KSFO - KTEB and no, I would not want to do that again as it is like sitting on a horse for too long.

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Gary,

 

thanks for the info. Your real-world input is highly appreciated.

 

Will try and adjust the fuel_flow_scalar accordingly for a M0.78-M0.80 cruise.

I'd like to go on an around-the-world tour with the Lear and would definitely want to get the fuel burn about right for this.

Let's see what I can come up with.

 

As for the flight level profile during this long-range cruise, what would you suggest?

Start at which flight level? Go up to which level after what time? Use a full pax/cargo/baggage load?

 

Thanks

Dave

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So, how about that:

 

Takeoff 11:30 UTC, MTOW, full fuel

Climb to FL420. After one hour FL430

Cruise at M0.79-M0.81

 

16:00 UTC: 900 lbs left after 4h30min

16:47: engine flameout due to fuel starvation, after a total flight time of 5h17min.

 

According to your rule of thumb (1500/1200/1100/1000), the fuel capacity of 6238lbs translates into a flight time of roughly 5h27min, so we're pretty close already.

 

With lighter loads and higher levels it might be possible to stretch this even further.

That's what you get with "fuel_flow_scalar        = 1.50" in the aircraft.cfg file.

 

Fuel consumption still too high? You mentioned that the rule of thumb is quite conservative. But then, I did the tests with max payload and gunning forward at around M0.8.

 

What do you think?

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If you are near max weight with ISA aloft a direct to 370-390 should be doable as initial altitude.There are three sets of charts for cruise being high speed, normal and long range.In each case to operate efficiently above 410 means your are around 14000 -14500 lbs weight range with ISA temp.The weight and temperature dictate how high you fly.These numbers below are from the performance charts of a Aeronca TR / FC200 autopilot equipped 35A but close enough and show the maximum weight and limiting temperature to achieve this performance at 450. 

 

Hi speed - Mach.743, TAS - 419,  Fuel flow 957,  ISA, 14000 lb

Long Range - Mach.77,  TAS 424, Fuel flow 936  ISA -10,  14000 lb

Normal cruise - Mach .77,  TAS 424 Fuel flow 1000 ISA -10, 15000 lb

 

Not a dramatic difference but the point is you will burn off fuel before step climbing up to final altitude.The charts were there to consult but we used 14000 lb and ISA as a general rule as to when cruise at 450. 


They may have modded the fuel flow from when I looked at it back then.I'll have to check it again but spring has sprung and dusting off the warm weather activity stuff right now is getting the priority !

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OK. So I need to set up a flight to check whether I can achieve about M0.75 with a fuel flow just shy of 1000 lbs per hour at a weight of approx. 14000lbs in ISA conditions at FL450.

 

The Flysimware Lear lets you take her up to FL420 and higher even when fully loaded. Strange. The aircraft probably should refuse to climb any higher than the FL370-390 you mentioned when payload and fuel are maxed. Maybe a problem with the flight model.

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Not knowing how they code these things for sim world I would assume they would have to incorporate all the actual engine and flight performance data to have a realistic flight model.I think it is a decent representation of a Lear 35 but would I say it is completely accurate to the real aircraft? No, but I would say that to the other $40 vendor products out there also.

 

I just don't think at this price point you can expect much more than a generic flight model.

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I fully understand this, and I don't expect or demand anything. I, too, think that what we already have is a fine and thoroughly enjoyable representation of the Lear 35A, otherwise I wouldn't bother spending that amount of time with it.

 

I am just trying to tweak as much into a realistic direction as it is possible for me, within the limits given.

Hence, I will limit myself to getting the cruise fuel burn *about* right so that the real-world flights can be done without prematurely running out of fuel and that fuel planning can be done in a somewhat reliable way.

 

Anyway, again thanks for your most valuable input as a real-world Lear 35A pilot

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So,

 

a small correction from my former value of 1.50 to fuel_flow_scalar = 1.49 yields the following:

 

at ISA conditions, FL450, 14000lbs aircraft weight, fuel flow 950lbs/h, M0.74-M0.75.

 

This should be close enough and should go well with the "rule of thumb" observed for the 1.50 value which was still a bit too high.

 

Going to fly with these settings now.

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Dave,

 

Great, keep us posted, would also like to get the fuel flow a little more accurate and it looks like you might be there :wink:

 

Cheers

Martin

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