I wasn't really sure exactly where to post this, so I figure here would be the closest I could come. If not, please accept my apologies.

I've been doing some higher altitude flying in a Beechcraft Baron Twin and was wondering exactly how they (the manufactor) comes up with the "cruise altitude" specs. Anyone know how they come up with it?

I've been doing some test flights with the aircraft, comparing air speed based on a fixed head pressure and fuel flow at different altitutes. The purpose was to find  the "sweet spot" for the aircraft for max performance for a given head pressure and fuel flow per hour.

For example (all head pressures were 17 and fuel flow was 15 gallons per hour):

@18,000ft
165 kts

@13,500ft
176 kts

@10,500ft
182 kts

@8,500ft
183 kts

Based on those numbers, I'm inclined to say that the best flight altiitude would be between 8,500 ft and 10,500 ft.  Is this a valid method for finding the sweet spot, or am I way off?

John

Thanks Saratoga.
At least it turns out that I wasn't crazy, keep the Baron under 10,000 ft AGL and it is a happy camper.

Thanks again,

John

P.S. Nice handle, the Saratoga was my sister ship while I was stationed on-board the USS America (CV-66).

Quote:


   single engine service ceiling is determined by using a rate of 50fpm.

I haven't done a whole lot of experimenting with the Baron but I have done a lot of real world flying and experimenting in the Chieftain, which I would consider to be in a similar "ballpark" to the Baron.
Typically, when flying a turbine aircraft, you want to fly as high as possible, without exceeding L/Dmax. Essentially the higher an engine goes, the less fuel it burns. The limiting factor is, now, wing efficiency.
Long story short: A turbine pilot flying long distance, wants to fly as high as the book will allow to minimize fuel burn.
After years of flying jets, I went back to flying high performance piston twins for a while and approached these aircraft with the turbine pilot's mindset.
What I found in the Chieftain, is that once you reach an optimum altitude( between ten and fourteen thousand feet for the Chieftain),exceeding that target is a wasted effort.
Just through knowledge of basic aerodynamic principles, I know that true airspeed(TAS) increases with altitude. What happened though, was that the wing rapidly became less efficient(it approached L/Dmax), so that the calibrated airspeed(CAS) decreased with altitude increase at a greater rate than the increase in TAS(typically about 5ktas/per 1000').

example(using the PA-31):

@10,000' IAS(uncorrected)is typically 150kts. That equates to approximately 175kts(true).

@17,000' IAS is 130kts, which equates to 170ktas.

The only factor, at this point, which may make the higher altitude worthwhile, might be long range fuel burn. What I found, though, was that the mixture could be leaned with the higher altitude but the tradeoff came with the additional power needed to maintain that cruise speed.
In layman's terms; the aircraft was now behind the power curve.

Try this site to make your calculations a bit easier. ....

http://www.flightplan.za.net/page3.php

just remember, if you want to try these numbers on your own, to factor in standard temperature lapse rate