Cruise Altitude Question

[Jester]

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

[Saratoga]

Well if you are in the Baron twin, you have no business being up high anyways. Not turbocharged. Amazing you hit 18,000. Provided you work the mixtures correctly, the best cruising altitude is always up high.

Let me restate: Heading west in a piston, the best crusiing altitude is low, where you hid the lightest headwinds and can just get there asap burning some fuel. Heading east, as high as you can get to a certain extent, find the right balance.

The manufactures find the best cruise altitude where speed meets power, pretty much the best altitude to fly at their long range cruising speed.

Statistically speaking, the best cruising altitude period for jets is 36,000 feet. That is the altitude where a jet enginen is working at it's absolute optimum (for lower power anyways), least gas for the most power. For a standard piston, 5000-6000 is a good long range cruising altitude. For turbocharged, dash up to 15,000 or so to take advantage of winds as necessary. Turboprops, get up as high as you can really, 22-25000 feet. Jets like I said, usually start fairly low then climb to 36 as weight allows.

In reality, picking altitudes is a hit and miss game, things (the weather) change and you can end up really getting the short end of the stick. But at least you have somewhere to start.

[Jester]

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).

[Saratoga]

Ya those engines just struggle up there. If you want to fly higher than that though, there is an addon here at SimV in the 2004 props section that turbocharges the Baron. You could probably hit 22,000 or so with it turbocharged.

Thanks for the comment on the screename. It has multiple meanings, first for the ship, and secondly as a tribute to the plane that probably saved my life.

[OTTOL]

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?

[glb]'Ceiling: There are two ceilings.
Absolute ceiling. The altitude at which the rate-of-climb is zero. ...........Performance and Service ceilings.
The performance ceiling (civil)is the altitude at which rate-of-climb is
typically 150 ft/min.  '

T[/glb]   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  ;)

[Saratoga]

Dunno if you read Flying magazine, but if you do and just got the latest version, it's got some great turboprop cruising altitude stuff (centered around the B200) in the article about the 6,000th King Air.