Finding Best Cruise Altitude

[Tom Goodrick]

(This is copied from an obscure part of this Forum so people can find it more easily.)

It's too bad this will get stuck at the end of this long article where few people will read it. So anyway, I recently started flying FS2002 on my laptop. Unfortunately none of my XML gauges work on the laptop even though I know I developed many of them when flying FS2002. I needed to do some speed checks to find the best altitude for cruise. First I determined the proper settings for each aircraft to match specified speed, altitude and fuel flow for each aircraft. Then I made flights in standard conditions (clear weather) setting the fuel flow for 75% power at each of these altitudes. Speed at full throttle was used if I could not get the desired fuel flow for 75%.

Altitude_Cessna 182____Cessna 182RG__Mooney Bravo _ Beech Baron 58
________15.0 gph______15.0 gph______17.2 gph______2x16.2 gph
2000_____135_________150___________162________196
4000_____138_________153___________164________200
6000_____140_________156___________167________203
8000_____142_________156___________171________200
10000____138_________151___________174________196

The Bravo is the only one that is turbocharged. It could fly higher and fatser but that is impractical because you and everyone on board would need to wear oxygen masks and because descents must be made at 500 fpm to protect the ears of all on board. Pressurized planes can fly high because they can descend fast without hurting ears and no one has to wear an oxygen mask.

Some specs for the Bravo show cruise at higher speeds using 85% to 89% power. That is not conducive to good engine life. This is not done by people who want good engine life.

These results are very typical for non-turbocharged engines where the power maxes out at about 6000 - 7000 ft. But I found in my experiemnts with these aircraft in FS2002 that using a high value of the power scalar will mess up this altitude/power relationship. There is a way of calculating the engine parameters needed to make a given max power value. This should be done. (I need to review it.) Without that, use the fuel flow for 75% power as a guide to setting power. Adjust drag to match cruise speeds with only a little use of the power and thrust scalars.

Some people want you to adjust the mixture for "best economy." It is best to adjust for best power (lean to maximize fuel flow) and then readjust the throttle to get the fuel flow for 75% power. Using 60% power instead of 75% power gives good economy. This is what is generally mean't by "Economy Cruise Power." Given no other guidance just reduce the 75% fuel flow by 20% and use that as your economy power setting.

If you have a pressurized aircraft, your range of altitudes will be appropriate to the type of aircraft: Tprops 14000 - 28000 ft, jets 30,000 - max op alt.

[Tom Goodrick]

A "couple of years ago" I wrote some articles dealing with power for piston aircraft in FS2004. I wanted to find them yesterday but couldn't. I am now looking through my old notebooks. I need to find the formula that we came up with for calculating power from engine parameters. If anyone has it, please email me.

Here's a quick summation of the problem and of my attempts to deal with it.

FS has always had a problem with piston power. There is one entry in the aircraft.cfg file for rated engine power. But tests have shown this means absolutely nothing. You can put garbage in here and see no difference in performance. So the question is "How can we set up an aircraft model to give the actual rated power?

I first approached this by attempting to estimate the power and to show it in flight in "Power Gauges." At first these were based on fuel flow which is a valid measure of power. Power is proportional to fuel flow. Some of these gauges seemed to work quite well and are still in use. But a better way was to read the actual value of power being produced in the sim at any point in the flight. This was done both in a general "Power Panel" gauge that shows many related parameters for tuning engines. Then I used the same method to make 'Power Gauges". All my power gauges show percent max power at any time during flight. The problem with piston engines is that we must read the actual power being developed and then divide by the max rated power to get a percent power. Percent power is the only value that a pilot can easily use in flight when frequently changing aircraft as many of us do.

So this means that my good power gauges all have a number associated with them that is the max rated power. There is a different gauge for each engine.

Using one of these gauges may be the best way to tune a flight model to get the right power. You set up conditions for 75% cruise and then adjust the engine parameters to see 75%. Having done that, the next steps in development of FD files is to adjust drag to get the proper speed and then adjust the fuel flow to get the proper fuel flow at that condition.

For turboprop engines, I found a way of calculating the actual percent shaft horsepower being developed in the sim from the standard engine performance parameters available in the sim. The nice thing is that there is only one "TPower gauge" that works with all turboprop aircraft. I do still have documentation for that development. It remains to be seen how useful that is in setting the power for turboprops. There is indication that power alone is insufficient for proper operation.