Percent Power, again

[hevansrv7a]

OK, I'm a little confused again.
* MAP is a function of pressure altitude.
* Available oxygen is a function of density altitude.
* Various charts, GRT %PWR calculation use MAP and a correction for "altitude", which kind not stated.

So, does this mean that for a given combo of MAP and RPM at a given altitude that the actual power can vary as much as the difference between pressure altitude and density altitude?

If so, this means I was right that %PWR is a very rough concept and that when near 75%, the magic number for leaning, one must excercise care.

Comments would be welcome!

[Kevin Horton]

The engine charts are based on pressure altitude. Once you have calculated the power for the pressure altitude, there is a temperature correction described on the chart to account for the effect of temperature.

Yes, it is very difficult to determine whether you are at 74% or 76% power. But there is no magic about that 75% power number. The higher the power the greater the risk of causing problems by leaning. The lower the power the lower the risk. There is no cliff edge at 75%.

[Ironflight]

You know how Lycoming recommends "no leaning below 5,000' at full power, then lean for smooth operation in the climb above that altitude...."? I always check the %power on my GRT when I climb through 5K, and with full power and climb RPM, it can vary between 78 - 82 percent! The bottom line is that the Lyclone is not a jeweled watch of a power plant - I wouldn't get to hung up on a couple percent either way. Like Kevin says, 75% is not a cliff - it is a point on a continuum. I keep the "Fuel Flow vs. Percent Power" chart on my knee board, and use that at higher power settings to make sure I am close to what Lyc recommends.

Paul

[L.Adamson]

Originally Posted by Ironflight

You know how Lycoming recommends "no leaning below 5,000' at full power, then lean for smooth operation in the climb above that altitude...."?

And to all the "ocean level flat landers" out there , with normally aspirated engines; remember that at higher altitude airports such as KSLC at 4227' , you must actively think about leaning before takeoff, as density altitude is usually much higher than 5000'. No leaning, will result in a very noticeable loss of available takeoff power. Going full rich for landing will also have a loss of available power should a go-around be required.

As mountain flying guides suggest; actively think about leaning proceedures for takeoffs above 3000' when density altitude is a factor.

L.Adamson

[elippse]

In von Mises' "Theory of Flight", there are some good formulas for power vs density ratio, dr. Testing has shown that the power available at a given density ratio is (dr-0.15)/0.85. But remember, your forward speed will generate dynamic (ram) pressure which will in turn increase the density of the air in the induction system. So try this: dr = (1-dalt*0.688E-6)^4.256. Typically in summer dalt will be 1500' to 2000' above your baro altitude. You can also use the fact that dalt increases 63'/degF, 113.4'/degC above standard temperature for that altitude.Now get your static pressure, sp. sp=sea-level baro*(1-baro alt*6.88E-6)^5.256. Divide MAP by this and then multiply this by dr to get the induction dr. Notice: if MAP is lower than sp, your power is less. Now do the previous (dr-0.15)/0.85 to get your available fraction of sea-level power at WOT. Simple, huh? Also, use the obtained sp to see how your MAP compares to static pressure to see your total induction pressure drop-dynamic pressure recovery. If you are going really fast, you are going to have to take into account the rise of induction temperature due to stagnation, the conversion of speed to pressure. That works out to (TAS ft/sec)^2/11876 degF. That would give 5.9F at 180 mph TAS, which would decrease power about 1%. But be careful; when you do all these exact calculations, you may find that your HP numbers are much lower than the HP numbers your friends are telling you. But take heart, and don't feel bad. The first liar never stands a chance!