Interpolation

[jdwatson]

We're all familar with cruise performace charts. I've always wanted to generate some custom charts based upon the POH's figures.

The Cessna's that I fly have performance tables that are at even altitudes from SL to 20,000' (even higher for the T206H) with 3 temperatures. For a given altitude and temperature, pick a "MP & RPM" pair and you can see the %BHP, KTAS, GPH. What about all the temperatures between those on the table ?

I was wondering if anyone had any ideas on how to get all the values in between ? If wanted to interpolate between tables and temperatures what would be the procedure ? I know it's a job for spreadsheet software, but I'm not a spreadsheet guru by any stretch of the imagination.

 

[poadeleted20]

If you interpolate the altitudes, interpolate the temps with them. Thus, if you have, say, 120 KTAS/2500 RPM at 6000 feet/3C and 122 KTAS/2550 RPM at 8000 feet/-1C, you can figure 121 KTAS/2525 RPM at 7000 feet/1C.

 

[Areeda]

JD,

Take a look at the settings in the table. Usually the difference between the 4 values you will be interpolating is ~ 1" of MP and 50 RPM. How accurately can you set those values?

For all practical purposes take the closest one.

Joe

 

[KennyFlys]

JD,

Take a look at the settings in the table. Usually the difference between the 4 values you will be interpolating is ~ 1" of MP and 50 RPM. How accurately can you set those values?

For all practical purposes take the closest one.

Joe

That's what I tried to explain to a guy a while back. He kept asking what the purpose was of having so many different settings. He would look at the same fuel flow, BHP and speed for two separate settings. I simply put it like this...

You can set your MP and RPMs to whatever makes your heart content. But, if you're serious about endurance, saving money on fuel and most important... knowing exactly how you're burning that fuel for management purposes, you'll want to stick with those numbers in the book. You have variation in settings with the same results so you know exactly what those settings will give you.

There's method to the madness. Although, I think the madness is ignoring those tables like so many do.

 

[midlifeflyer]

What about all the temperatures between those on the table ?

Following up on Joe's post, do a manual interpolation where the difference in temperature values gives the largest difference in MP/RPM/fuel flow to get the change in MP/RPM/FF for each 1° or 5° or 10° change in termperature. Combine that mentally with the accuracy at which you can determine the numbers that fit the variables and set power.

Looking at that, decide for yourself whether the intermediate values are helpful to put on a chart or checklist. If you think it would be helpful to you, go for it.

 

[jdwatson]

In the G1000 birds, the MP & RPM are single digit precision, so setting those can be as precise as I want to be. I know how to do the interpolation, I don't like the time it takes me to do it.

So I was thinking, get a computer to do the work for me.

 

[KennyFlys]

In the G1000 birds, the MP & RPM are single digit precision, so setting those can be as precise as I want to be. I know how to do the interpolation, I don't like the time it takes me to do it.

So I was thinking, get a computer to do the work for me.

The only thing really worth expanding on is temperature.

 

[mikea]

The only thing really worth expanding on is temperature.

I have always thought I wanted to interpolate for VFR cruise altitudes, turning the number for n000 feet into those for n500 feet.

 

[jdwatson]

The only thing really worth expanding on is temperature.

Even that would be challenge. Those darn dashes at certain temps and settings. At what temperature would the setting be allowable ? I'll just go back to guessing (aka interpolation). The Lean Assist on the Cessna G1000s is really good.

Too bad one cannot interpolate between two tables in a spreadsheet. That would make it a snap.

 

[KennyFlys]

Even that would be challenge. Those darn dashes at certain temps and settings. At what temperature would the setting be allowable ? I'll just go back to guessing (aka interpolation). The Lean Assist on the Cessna G1000s is really good.

Too bad one cannot interpolate between two tables in a spreadsheet. That would make it a snap.

Lean Assist is handy. The fuel flow meter and totalizator are tools for verification but the tables are what saves you.

I'm sure a spreadsheet could be written for use between tables but may require the Lookup function. The other option would be drawing the tables as you desire for eight and ten thousand in two separate groups of cells in the same rows. Then, create a group by interpolation between them for nine thousand.