I am training in a SR20 G6. My instructor says that the max power percent at takeoff is impacted by the density altitude. Is there a chart or easy way to calculate the anticipated percent power is I have the density altitude?
You should find everything you need in Section 5 of the POH.
Thank you for the reply. I re-reviewed that section. I see how pressure altitude impacts take-off / landing distance. What I am looking for is when I am taking off, full throttle, what is the % power I should see based on the density altitude. It would help me know that I am truly getting full power. thanks, Keith
On an SR20 G6 you should see about 29 inches of MP and close to 2700 RPMs and that is an indication of the proper power.
He’s asking about non-sea level density altitudes, though, and how increased DA (decreased MP) will affect percent power. Specific percent power calculations are not elucidated in the POH.
And I doubt they ever will be. A POH is designed to create performance tables based on RPM and MP as the percent power gauge is a computer calculated number.
Got it. Appreciate everyone’s quick response. Will focus on RPM and MP.
Also remember that the mixture is important too. If you are at a high density altitude in a non turbocharged plane, your best power will not be at the full rich mixture you need at sea level.
So the real answer to your question is complicated. What you need to remember is that performance is inversely proportional to density altitude, and that you need to consult the POH for the appropriate performance numbers for the conditions.
How’s this for theory…
At lower altitudes, a climb of 1000’ results in approximately a 1" drop in MP. 1" is about 1/30th of SL pressure so there are a bit over 3.3% fewer air (oxygen) molecules in the cylinder which means you should, in theory, produce about 3.3% less power for each 1000’ you climb.
I believe the POH also gives fuel flows with MP/RPM. So the natural extension to the OP’s question is when and how to adjust mixture. I’ve said this before. The RSA 10 fuel servo acts a lot more like a altitude compensating than Cirrus likes to admit. My experience is that while there is some mixture adjustment for high DA it’s much less than one would expect if they’re used to a 22 NA.
That jibes nicely with the rule of thumb that full throttle gets you 75% power at 7500 MSL.
3.3% * 7.5 = 24.75% power loss
Never knew that rule of thumb, but that does mirror what I would get taking off out of L35 (Big Bear City, CA, elevation 6,700’). About 75% power in the winter and 70% power on a summer morning. On a hot summer day, power was 0% because I couldn’t take off no matter how light
Another way to remember it has to do with the approximate standard pressures.
You get 100% power at sea level where standard pressure is about 1000 mb
At 5000 feet maximum power is about 85% where standard pressure is 850 mb and at 10,000 feet the best you can do is 70% and the pressure is 700 mb.
By 18,000 feet (500 mb) all you can get is 50% of the sea level rated power.
All that applies to normally aspirated aircraft only.
Density altitude causes several crashes every summer. I don’t know of any POH that adequately addresses density altitude. Of course, FAA pamphlet P-8740-2 tries to make sense of it. Lets use an airport like Big Bear, Truckee, Sedona, Grand Canyon about 6000’ elevation and 30 C. Some pilots use the takeoff distance chart of 6000’ pressure alt and 30 C showing a takeoff roll of 2078’. (SR22/gen1). But, the atis said density altitude 9000’. Should you use 9000’ PA and 30 C? That shows a takeoff roll of 2853. Using the KOCH FAA density altitude takeoff and climb chart, the ‘normal’ takeoff distance should be increased 210% bringing the takeoff roll to over 3600’. The climb factor on the KOCH shows the climb rate would decrease to about 240 fpm. Be careful out there!
Short answer is no.
Pressure altitude is the altitude your altimeter shows when set to 29.92. Density altitude is pressure altitude corrected for temperature.
If you go to your trusty E6-B computer and check, you will see that a pressure altitude of 6000 feet and a temperature of 30C equals a density altitude of almost exactly 9,000 feet (actual DA os 8992 but 9000 feet is close enough). And that’s what was given on the ATIS in your example.
So the performance chart in the POH which inputs Pressure altitude and Temperature is what should be used.
Using your 6000 foot PA and 30C my SR22 would require a ground roll of 2095 feet for takeoff, 3159 feet to clear a 50 foot obstacle and should have an initial ROC of about 915 fpm at a takeoff weight of 3400 pounds.
Looks to me you are using a 6000’ PA for climb,not the density altitude. . Should you not be using a 9000 PA/DA for takeoff distance and climb? (that’s where the airplane thinks it is…) My guess is more like 700 fpm climb, and 2850’ ground roll.
Yes I am. But the chart corrects for temperature just like the takeoff distance.
Go to 6000 feet pressure altitude, then move right to the 20 and 40C temps. Take the average of them to see the ROC at 30C.
So, without calling 6000’PA at 30C, a ‘density altitude’ the numbers in the POH are actually density altitude?.
Yes. Again, pressure altitude corrected for temperature is density altitude.
So when you use the POH chart to look up performance at 6000 feet pressure altitude and 30C, that is the same as looking up performance at a density altitude of 9000.
Got it… thanks. Looking at a long, hot summer here in the foothills of California.