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dstagl
02-06-2002, 04:26 AM
OK, I'm wondering if there's something I'm not doing right or if this is really the way it is in the real world. I have problems getting up to a cruising altitude where I keep losing speed until I stall. I lower the VSI in the autopilot on the way up, but I still have problems getting up say getting the Learjet up to 38000. Is this realistic or is there something I'm missing?

Thanks!
Dave

Simon Evans
02-06-2002, 04:29 AM
Most aircraft won't zoom straight up to that kind of altitude, you'll need to step climb as the fuel load lightens. This replicates the real world where the fuel load for a given flight is calculated and full fuel isn't carried unless it's needed.

If you reduce fuel to less than 50 percent in the menu you'll find the aircraft will bowl along quite happily up there.

Simon Evans

Frederf
02-06-2002, 05:38 AM
I never got the stepped climb deal, isn't it less efficient to level off at a lower alt?

Simon Evans
02-06-2002, 06:25 AM
No, because efficiency is based upon various factors...

A jet engine is more efficient at higher altitudes, therefore it pays to get there sooner rather than later. Even the new generation of feederliners like the ERJ and the CRJ have a steep climb profile and aim for the stratosphere as soon as possible, no matter how short the cruise phase will be.

However, if the climb itself takes the aircraft to inefficient corners of the flight envelope - like very high power settings not leading to increasing IAS or altitude because of fuel and/or passenger load, then it makes more sense to stay lower, use the lower thrust settings, and weight for the weight to reduce.

There are several ways to achieve this but the most common are the `stepped climb` with a climb-level-climb-level-climb-level configuration until cruise altitude is reached, or there is cruise climb, where a lower but constant rate of climb is maintained.

In most ATC-controlled environments, they don't like aircraft wandering from one flight level to the next at their leisure so the stepped climb is most often used. Elsewhere the cruise climbmay be preferred.

Hope this helps.



Simon Evans

Silverblade
02-06-2002, 06:29 AM
Commercial and general aviation jets all use the step method to reach cruising altitude with full or near full loads, just as Mr. Evans said.

Less efficient? Less efficient than what? As you've experienced, a fully loaded aircraft can't reach a higher altitude than a lighter one of the same performance... due to laws of physics.

By the way, I don't know about the Flightsim model of the Lear, but in real life it cruises happily along at 41,000... its published cruise altitude.

Silverblade

http://www.flightsim.com/dcforum/User_files/3c529f6b3fd541d6.jpg

frigatebird
02-06-2002, 07:25 AM
Try setting the fuel really low as an experiment....say a figure of 10 in each tank. You should get straight up to FL410 with no problems then. You'll get some good dead stick practice when you run out of petrol fairly soon though, so be prepared!

TRex
02-06-2002, 07:28 PM
Not familiar with Lear, but jet is jet, so there is also another point. Jet engine efficiency drops with low IAS. Just for example ... On some 767 (loooong time ago, when I learned this) I couldn't maintain airspeed when I reached desired altitude at ~185 KIAS. I descended a bit to get up speed and when reaching same altitude at 220 KIAS I could even accelerate.

dstagl
02-06-2002, 08:06 PM
OK, so what is the solution to step climbing to high altitudes while using the ATC so that you don't lose your IFR?

Dave

peterfitness
02-06-2002, 09:37 PM
I am not sure if this is completely realistic, (and if it's not, I'm quite sure someone will tell me so), but I have had no trouble with jets of all sizes reaching their cruising altitude using the following method. Firstly I select auto throttle then set the speed to 250kts, which is I believe the maximum permissable below 10000 feet, (at least in the U.S.A.), at an initial climb rate of 2500 feet per minute. Once reaching this altitude, I change to mach number, gradually increasing it until I reach 30000', at the same time reducing the vertical speed in 500 fpm steps to about 1500 fpm, or even less. On reaching 30000' I reduce the vertical speed to 300 to 500 fpm and increase the mach number to the appropriate (for the aircraft type) speed, until the final cruising altitude is attained, usually in the 35000' to 41000' range. As others have mentioned, fully loaded aircraft do not usually climb immediately to their desired cruising altitude, as the benefit of more efficient engine operation at high altitude is negated by the effort of dragging a full load to that altitude. This applies to long sectors where aircraft typically carry large fuel loads, whereas aircraft operating short sectors, in most cases, would not carry a full fuel load. I hope this helps, and if anyone can correct me on my procedure, please do so.
Peter.

tjrush
02-06-2002, 09:56 PM
The Lear 45 as modified by Al Whitney suggests the following climb procedure:

Below 10000 ft - 245 kts, +3000 fpm
FL100 to FL200 - 315 kts, +2000 fpm
FL200 to FL300 - M 0.73, +1500 fpm
FL300 to FL400 - M 0.74, +1200 fpm
FL400 to FL500 - M 0.76, + 800 fpm

Max initial cruise FL350 or FL370 with full fuel.
When aircraft weight < 17,000 lb (in his configuration this
is when fuel is less than 4,000 pounds), climb to FL450

dstagl
02-06-2002, 10:36 PM
Great, I'll go try these things.

Thanks!
Dave