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  #1  
Old 09-28-2020, 04:20 PM
schmoboy schmoboy is offline
 
Join Date: Jan 2005
Location: Crystal Lake, IL
Posts: 5
Default Converting a DA Performance Number Down to Sea Level?

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!
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  #2  
Old 09-28-2020, 05:05 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 5,518
Default

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.
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RV-7
Lord Kelvin:
I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.
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  #3  
Old 09-28-2020, 05:51 PM
koupster's Avatar
koupster koupster is online now
 
Join Date: Dec 2013
Location: SLC, UT (KBTF)
Posts: 242
Default 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
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  #4  
Old 09-28-2020, 06:07 PM
schmoboy schmoboy is offline
 
Join Date: Jan 2005
Location: Crystal Lake, IL
Posts: 5
Default

Thanks for that link. That is a very useful tool.

Quote:
Originally Posted by koupster View Post
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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  #5  
Old 09-28-2020, 06:38 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 5,518
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Email on the way- - -the Koch chart is a good one !!
__________________
Bill

RV-7
Lord Kelvin:
I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.
Reply With Quote
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