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rufowler

Does Power % = Throttle %?

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[Warning: newbie question!]  ;-)

 

In the specs & reference materials that come with an aircraft (which I supposed is are light versions of the official pilot's operating handbooks), many of the V-speeds, range approximations, and fuel consumption info will often be listed in association with a "Power Percentage."  i.e.:  The design cruise speed for my Piper PA28 is 125 knots @ 75%, 116 @ 65%, and 102 @ 55%.

 

Does that % correspond to the % of throttle in FSX?  (If you hover the mouse over the throttle in the cockpit, it will display how high it is.)  I assumed it was the same thing, but then I noticed that most of my aircraft perform a lot slower when the throttle is set to those #'s, even at just above sea level with a very light payload.  Perhaps I'm just confused though.  

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In short, no.

 

In a single engine piston aircraft with a fixed-pitch prop, a given power setting comes from a combination of RPM and mixture setting at a given pressure altitude and temperature.

 

If the aircraft has a constant speed prop, the power setting is achieved with a combination of Manifold Pressure, Prop RPM and mixture setting (at a given pressure altitude and temperature)

 

Here's a link to a Cessna 152 POH. Scroll down to page 74 for the power setting chart.

 

http://www.aboveallsba.com/Airplane%20data/POH/C152-POH.pdf

 

Let's say, you were cruising at 4,000 feet ASL (standard altimeter setting) and the outside temperature was right at standard. To achieve 75% Power, you would set the RPM to 2450 and lean the mixture as per the recommended procedure. You would also expect to burn 6.1 Gallons per Hour.

 

For altitudes and temperatures in between, you would just interpolate between the values on the table.

 

 

Hope this helps.

 

DB

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So it seems like if anything, the "power %" is more a function of propeller speed -- and how it is set in various altitudes and temperatures -- rather than of the % of throttle displayed in FSX, no? Basically that throttle % doesn't really factor into the actual "power %."  Is that right?  (If so, I guess there's a bit of calculation to do anytime I want to hit a certain power %.)

 

Related to that (I think):  Is "performance cruise" basically the quickest route to get to a destination while "economy cruise" allows for the least fuel consumption?

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You are correct. Generally, the power setting decision and calculation would be done during the pre-flight planning, with wind, pressure and temperature information readily at hand. The POH would of course be available in flight if you needed to change power settings for any reason.

 

Note as well that the expected cruise speeds on the chart are KTAS (Knots True AirSpeed). The Airspeed gauge in the aircraft reads Indicated Airspeed (KIAS). True Airspeed will be higher than Indicated Airspeed as altitude increases.

 

RE: Performance cruise vs. Economy cruise, Yes, they would basically be your go-to power settings for whenever the situation dictated speed or fuel efficiency.

 

For example, if while en-route, ATC tells you to expect a hold and a waypoint ahead....prudence dictates that you slow to economy cruise immediately.....heck, if you slow down enough you might not even have to hold by the time you get there!...and you save some gas.

 

Conversely, if you need to arrive at your destination by a certain time and have encountered higher than forecast headwinds, go for performance cruise if fuel is not a concern.

 

 

Note though, that for a Piper PA28, the difference between performance and economy cruise might only be a few knots. For turboprop and jet aircraft, the difference could be 50-100 Knots, so it can make a huge difference.

 

Cheers,

DB

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Thanks so much for this explanation!  I think I got it.  I did know about the KTAS, though not quite sure how to convert it other than to apply the rule of thumb by adding 2% of the KIAS for every 1000 ft of altitude.  Is that an ok way to figure it out?

 

Also, just looking at the original POH for the PA28-181 and it seems that the difference bet economy and performance is really small -- as you predicted it's literally only 3 knots.  The max range, however, jumps pretty dramatically from ~500 to ~600 nm at the same altitude, temp, etc.  That somehow doesn't seem possible, but I suppose this correct, or at least feasible?

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Ahh, I see now. Looking at the Piper charts it looks like they reference "Best Power" and "Best Economy" for a given power setting.

 

 

"Best Power" and "Best Economy" are techniques for leaning the mixture.

 

Reference the POH here: http://www.redskyventures.org/doc/other-poh/PA28-II-POH-retyped-NotreDamePilotsInitiative.pdf

 

 

On page 35, in the top-middle of the chart, you will see the difference in fuel flow between Best Power and Best Economy for 55%, 65% and 75% power.

 

Without going into too much detail about it, leaning for best power will net you a bit faster speed, cooler engine temperatures and slightly higher fuel burn.....while leaning for best economy will be a few knots slower, higher engine temps and lower fuel consumption. Much has been written about the merits of either method. I don't think a consensus has yet been reached as to which is best for your engine in the long run.

 

Typically, best power is achieved by leaning to obtain maximum RPM / airspeed. Best economy is leaning mixture to peak EGT. Many aim for something in between.

 

 

Regarding the difference in range between best power and best economy.....looking at the fuel burn difference between the two for 75% power........10.5 GPH vs 8.8. Over a 4-5 hour flight, that works out to an extra 30-45 minutes of fuel at 120+ Knots, so the extra range is accounted for.

 

 

Regarding True Airspeed, for an aircraft like the Piper, the 2% per thousand feet rule should work just fine.

 

 

Cheers,

DB

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Thanks, DaviiB!  That was the same reference document I was using too. :)

 

Well it's good to know I'm not completely reading this wrong.  I suppose it does kind of make sense, even though intuitively it seemed like such a small difference in speed ought not account for such a big difference in range.  Cheers.

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