What would you do? (Checkride)

[EdFred]

I've posed this problem to my friend's 8th grade daughter just to see if she can lay out a plan as to the steps she would take to try and solve. She has/is taking algebra but no physics as of yet. Just want to see if she can come up with a concept.

Will report back.

 

[MauleSkinner]

Can also use the Koch chart.

Problem with a Koch chart is that it doesn’t have a way to account for reduced (or increased) weight, so you’ve actually got to do a takeoff at the weight in question and determine your takeoff distance under some known pressure altitude and temperature combination that you can use as a baseline.

The problem with THAT is you don’t end up with the distance that the manufacturer would come up with, and so applying some percentage as a safety factor would have a basis in reality, and would actually be a “safety” factor, not just a baseless number that makes you FEEL safe.

 

[David Megginson]

A dog doesn't know what an analogy is. Clearly, spiders don't either.

Nor do most first-year university English students (I'm a former prof).

 

[MauleSkinner]

A dog doesn't know what an analogy is. Clearly, spiders don't either.

Nor do most first-year university English students (I'm a former prof).

I’m afraid that finding out what an anal ogy is would be like realizing the doc has one hand on each shoulder.

 

[David Megginson]

I’m afraid that finding out what an anal ogy is would be like realizing the doc has one hand on each shoulder.

The missing "r" makes all the difference.

 

[MauleSkinner]

The missing "r" makes all the difference.

Not sure whether to “like” that post or not…

 

[idahoflier]

I have my commercial checkride next week.

It's been a week. How did the discussion go?

 

[PeterNSteinmetz]

Problem with a Koch chart is that it doesn’t have a way to account for reduced (or increased) weight, so you’ve actually got to do a takeoff at the weight in question and determine your takeoff distance under some known pressure altitude and temperature combination that you can use as a baseline.

The problem with THAT is you don’t end up with the distance that the manufacturer would come up with, and so applying some percentage as a safety factor would have a basis in reality, and would actually be a “safety” factor, not just a baseless number that makes you FEEL safe.

Agreed the Koch chart will not handle weight variation. One solution is to interpolate between values if given for different weights in the tables.

If not available, it turns out that takeoff distance is treated in chapter 6 of Anderson’s “Introduction to Flight”. Equation 6.94 in edition 3 gives the takeoff run required as proportional to W^2.

So I think the OP would be on a rather good foundation adjusting by that factor. As noted in some of the discussion above, there are a number of issues which factor into this calculation from physical principles and simplifications to be made. Anderson deals with these.

It is a nice book for this sort of thing which allows one to either look up and qualitatively understand these results or dig into it at a college sophomore engineering level.

 

[MauleSkinner]

Agreed the Koch chart will not handle weight variation. One solution is to interpolate between values if given for different weights in the tables.

If not available, it turns out that takeoff distance is treated in chapter 6 of Anderson’s “Introduction to Flight”. Equation 6.94 in edition 3 gives the takeoff run required as proportional to W^2.

So I think the OP would be on a rather good foundation adjusting by that factor. As noted in some of the discussion above, there are a number of issues which factor into this calculation from physical principles and simplifications to be made. Anderson deals with these.

It is a nice book for this sort of thing which allows one to either look up and qualitatively understand these results or dig into it at a college sophomore engineering level.

But you still end up with the same placebo safety factor.

 

[PeterNSteinmetz]

But you still end up with the same placebo safety factor.

All of these things are fairly rough calculations, right? Even the measured values in the tables are fairly rough for 50 year old planes.

Personally, I would be comfortable with this correction factor from the POH tables.

But if I have less than 150% of the predicted takeoff distance available in runway length, I always figure the abort point on the runway and abort if I am not off the ground by then. I would not just blithely go beyond that point assuming I will be ok because this calculation said it should work. Plenty of time to taxi back and think some more about it.

 

[MauleSkinner]

All of these things are fairly rough calculations, right? Even the measured values in the tables are fairly rough for 50 year old planes.

Personally, I would be comfortable with this correction factor from the POH tables.

But if I have less than 150% of the predicted takeoff distance available in runway length, I always figure the abort point on the runway and abort if I am not off the ground by then. I would not just blithely go beyond that point assuming I will be ok because this calculation said it should work. Plenty of time to taxi back and think some more about it.

If you use your actual takeoff data, it comes out pretty close.

 

[PeterNSteinmetz]

If you use your actual takeoff data, it comes out pretty close.

My 69 Cardinal has been surprisingly on the book for performance, though I have heard of others that are not. I recently had the leading edges of wing repainted because there was a lot of rough paint from a bad repair and a top overall and am now about 6 mph better in cruise! Glad I got something concrete to show for the 6 AMU top overhaul.