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Converting a DA Performance Number Down to Sea Level?

schmoboy

I'm New Here
What is the proper way to convert a performance metric down to Sea Level if it has been determined at a given density altitude?

For example, if I determine takeoff performance in feet at a given density altitude, how do I get the length it would be at sea level?

And can I do the same with climb performance numbers if I determined Vx and Vy at a given density altitude?

Thanks!
 
I asked this a while back and did a lot of research. You are welcome to the Navy and other flight test manuals on hand.

Meanwhile, I am getting my popcorn.
 
Koch Chart

Just use a Koch chart in reverse. Rather than starting with density altitude, one inputs pressure altitude and temperature. The chart gives you takeoff and rate of climb adjustments. If the chart tells you to add 100% to your sea level - ISA takeoff roll, then you know that the sea level - ISA number would be half of the roll you measured at the higher DA. Likewise with the climb rate.

Here's an online Koch chart https://www.takeofflanding.com/
If you input 3000 feet PA and 15*C, it tells you that a runway of 1000
feet would be equivalent to an ISA sea-level runway length of 680 feet.

Cheers, David
RV-6A KBTF
 
Thanks for that link. That is a very useful tool.

Just use a Koch chart in reverse. Rather than starting with density altitude, one inputs pressure altitude and temperature. The chart gives you takeoff and rate of climb adjustments. If the chart tells you to add 100% to your sea level - ISA takeoff roll, then you know that the sea level - ISA number would be half of the roll you measured at the higher DA. Likewise with the climb rate.

Here's an online Koch chart https://www.takeofflanding.com/
If you input 3000 feet PA and 15*C, it tells you that a runway of 1000
feet would be equivalent to an ISA sea-level runway length of 680 feet.

Cheers, David
RV-6A KBTF
 
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