Simviation Forums

Posted: **Sat Jan 10, 2009 8:37 pm**

no1 know what aircraft u using if u jus post tha link at least give tha aircraft type and their model #

Posted: **Sat Jan 10, 2009 9:33 pm**

no1 know what aircraft u using if u jus post tha link at least give tha aircraft type and their model #

The aircraft isn't the point. I don't think you need a formula. Just fly at an alitude for a designated period of time. At the start take the fuel, at the end take the fuel remaining. Subtract them, there's how much fuel you used at that altitude for that amount of time, graph it. Repeat at different altitudes. You should have your answer. This should work with any aircraft.

8-)

Posted: **Sat Jan 10, 2009 10:06 pm**

no1 know what aircraft u using if u jus post tha link at least give tha aircraft type and their model #

The aircraft isn't the point. I don't think you need a formula. Just fly at an alitude for a designated period of time. At the start take the fuel, at the end take the fuel remaining. Subtract them, there's how much fuel you used at that altitude for that amount of time, graph it. Repeat at different altitudes. You should have your answer. This should work with any aircraft.

8-)

That will give you fuel flow, as in gallons per hour.. you can get that with a fuel-flow gauge..

I think what he's after, is finding a fuel-flow vs airspeed comparison. And that can get complicated.. because it's not only altitude dependent, it also depends on the type of engine. AND if it's turbo-charged (or turbo-normalized)(Like a Legacy is).. it depends if the aircraft.cfg is accurate.

Another wrench in the works is that MSFS doesn't model fuel flow accurately.. but it is close enough when at theoretical optimum, that it's worth figuring out.

Entering data into a spread sheet would be nice for reference, but it will become pretty apparent that you're hitting a sweet spot as you fly.. manipulating manifold pressure and prop rpm. Turbo-normalized aircraft kinda add the twist, in that they can maintain high manifold pressure, well above where you'd want to fly a Legacy, without supplimental oxygen 8-)

Posted: **Sat Jan 10, 2009 10:12 pm**

its lancair legacy 2000.

i understand i can fly the same path over and over and over at each 100 or 200 fool elevation, then do it again at a different power setting, this would only be a few hundred runs...

or i could find out the formula used and in 15 seconds could get a color coded graph that shows where is most efficiant to fly and for each altitude, something like this....

Well I assumed you wouldn't be making that many measurements. In that case, no that wouldn't work.

8-)

Posted: **Sat Jan 10, 2009 10:28 pm**

Your making this more complicated than it needs to be.. You have fuel-flow gauge in that plane, and an airspeed indicator. The only tricky math for you will be calculating TAS (unless you cheat and have realism settings to show TAS instead of IAS).

Performance will be relatively predictable and linear. And the BIGGEST factor for economy is winds aloft ;)

Posted: **Sun Jan 11, 2009 12:00 am**

You could also install a gauge into the panel that shows you your fuel burn rate

Posted: **Sun Jan 11, 2009 11:24 am**

Like I said.. the MSFS fuel consumption model (formula), is not completely accurate, but it IS pretty close.. especially if you fly realistically.

Obviously it's a function of:

'total engine displacement', 'fuel system type', 'engine rpm', 'manifold pressure', 'mixture setting', 'density altitude'.

You can't access or "see" this algorithm, any more than you could for a real airplane. You have to take known parameters and make educated calculations and assumptions.

Now, you can go into an aircraft.cfg file, and change some of the constants (even tweak some of the scalars).. or even get into air-file editing and access variables that you cannot get to via a cfg file. However, assuming we're dealing with a reasonably accurate model, there's no point in doing that.

I'm still not understanding what it is that you want to know

Posted: **Tue Jan 13, 2009 11:44 am**

Assuming the model for the aircraft is reasonably accurate, and ignoring any errors in the FSX model,

Posted: **Tue Jan 13, 2009 5:35 pm**

coefficient of drag... as speed increases drag increases at an increasing rate

altitude... as altitude increases drag decreaess at a decreasing rate

altitude... as altitude increases engine performance decreases at a decreasing rate

altitude... as altitude increases prop effectivenes decreases at a increasing rate

You're stating the obvious.. like:

Manifold preassure.. as you open the throttle, it goes up.

Vertical speed @ Vy.. as weight increases, it decreases.

Those are all true statements...

Just like i suspect there is a formula to calculate speed based on engine rpm and prop pitch and altitude and angle of attack and temperature.

No there's not.. There are too many variables. Even if you narrowed it to one, specific airplane; things like wing efficiency and overall drag change in non-linear ways by AoA.

Now.. granted, in theory, you can take ONE certain airplane with a fixed prop, and come up with a performance chart with the variables of;

Posted: **Tue Jan 13, 2009 5:40 pm**

He wants to know, how to figure out the best speed, and altitude, for this particular plane to fly at, while attaining the best fuel economy.

Posted: **Tue Jan 13, 2009 6:05 pm**

Here.. this should keep you busy..

http://static.scribd.com/docs/65j45ua8q ... VIEW=width

Posted: **Tue Jan 13, 2009 6:06 pm**

Isn't Google wonderful.

Posted: **Tue Jan 13, 2009 6:22 pm**

Yes.. it is

Posted: **Tue Jan 13, 2009 6:36 pm**

what are you guys talking about?

Posted: **Tue Jan 13, 2009 7:53 pm**

im flying this plane

http://www.simviation.com/simviation/do ... hp?ID=2493

and as i go higher i notice the fuel usage gets lower.
as i fly slower i notice the fuel usage gets lower.

how do they calculate this, and how can i look at the formulas to enter it in excel and look for a optimal speed and altitude to fly at for greatest economy?

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