effect of changed thrust line.

On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
wrote:

wrote:

Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
inches.
One inch change in prop height according to your spreadsheat makes a
change of 2.14 degrees.
I cannot buy that. Particularly since it would go from 1.5 down to .64
up.
Here is REV A, with a couple math errors fixed:
http://www.spiretech.com/~guynoir/sl...thrustreva.xls


That looks a lot closer.

What is it based on?

wrote:
On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
wrote:


wrote:


Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
inches.
One inch change in prop height according to your spreadsheat makes a
change of 2.14 degrees.
I cannot buy that. Particularly since it would go from 1.5 down to .64
up.

Here is REV A, with a couple math errors fixed:
http://www.spiretech.com/~guynoir/sl...thrustreva.xls



That looks a lot closer.

What is it based on?

"Vector Mechanics For Engineers: Statics and Dynamics", by Beer and Johnston, 5th edition, Chapter 4: "Equilibrium of Rigid Bodies",
page 126, equation 4.1. Equation 4.1 can be easily found on the internet, just type "statics" into Google and go to the first hit,
which should be Wikipedia.

All my spreadsheet does is keep the distance from the thrust line to the CG the same with different engine locations, so that the sum
of moments about the CG remains the same (as in: Zero. See equation 4.1).


--
John Kimmel


I think it will be quiet around here now. So long.

John Kimmel wrote:

wrote:

On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
wrote:


wrote:


Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
inches.
One inch change in prop height according to your spreadsheat makes a
change of 2.14 degrees.
I cannot buy that. Particularly since it would go from 1.5 down to .64
up.


Here is REV A, with a couple math errors fixed:
http://www.spiretech.com/~guynoir/sl...thrustreva.xls




That looks a lot closer.

What is it based on?


"Vector Mechanics For Engineers: Statics and Dynamics", by Beer and
Johnston, 5th edition, Chapter 4: "Equilibrium of Rigid Bodies", page
126, equation 4.1. Equation 4.1 can be easily found on the internet,
just type "statics" into Google and go to the first hit, which should be
Wikipedia.

All my spreadsheet does is keep the distance from the thrust line to the
CG the same with different engine locations, so that the sum
of moments about the CG remains the same (as in: Zero. See equation 4.1).



I tried the first sheet.
Set the offset to 4 inches and got 8 something degrees angle!

That may have been a right answer for the problem you've set up.

But no way it would work on the airplane.



--

Richard

(remove the X to email)

cavelamb himself wrote:


I tried the first sheet.
Set the offset to 4 inches and got 8 something degrees angle!

That may have been a right answer for the problem you've set up.

But no way it would work on the airplane.



I checked the math on my rev a spread sheet more carefully than on the original, and included a check feature within the spreadsheet
itself. I also checked the spreadsheet graphically using a cad program. The spread sheet as currently posted will work for all
aircraft, but if you don't have a good understanding of the vector mechanics underlying it in the references I included in my previous
post, you should not use it.

If you think there is an error in my spread sheet, you are welcome to identify it, correct it, and re-post it, or you can show me the
problem graphically with a free body diagram and I'll make the correction, but your post above doesn't give me enough information.

--
John Kimmel


I think it will be quiet around here now. So long.

On Mon, 17 Nov 2008 20:41:30 -0800, John Kimmel
wrote:

wrote:
On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
wrote:


wrote:


Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
inches.
One inch change in prop height according to your spreadsheat makes a
change of 2.14 degrees.
I cannot buy that. Particularly since it would go from 1.5 down to .64
up.

Here is REV A, with a couple math errors fixed:
http://www.spiretech.com/~guynoir/sl...thrustreva.xls



That looks a lot closer.

What is it based on?

"Vector Mechanics For Engineers: Statics and Dynamics", by Beer and Johnston, 5th edition, Chapter 4: "Equilibrium of Rigid Bodies",
page 126, equation 4.1. Equation 4.1 can be easily found on the internet, just type "statics" into Google and go to the first hit,
which should be Wikipedia.

All my spreadsheet does is keep the distance from the thrust line to the CG the same with different engine locations, so that the sum
of moments about the CG remains the same (as in: Zero. See equation 4.1).


Only problem I see is you still have it set up with the thrust above
the CG. The center line of the prop is significantly lower than the
CG. Do I just use negative numbers? Or should the spreadsheet still be
corrected?

wrote:


Only problem I see is you still have it set up with the thrust above
the CG. The center line of the prop is significantly lower than the
CG. Do I just use negative numbers? Or should the spreadsheet still be
corrected?

If you downloaded the corrected version, the numbers you see when you open the spreadsheet a
60 -13 -1.5 -14 -2.408899631

CG is 60" aft of propeller center. Propeller center is 13" below CG. Thrust angle is 1.5 degrees down.
If you move propeller center down 1" to 14" below CG, the new thrust angle will be 2.4 down.
--
John Kimmel


I think it will be quiet around here now. So long.

On Tue, 18 Nov 2008 21:20:43 -0800, John Kimmel
wrote:

wrote:


Only problem I see is you still have it set up with the thrust above
the CG. The center line of the prop is significantly lower than the
CG. Do I just use negative numbers? Or should the spreadsheet still be
corrected?

If you downloaded the corrected version, the numbers you see when you open the spreadsheet a
60 -13 -1.5 -14 -2.408899631

CG is 60" aft of propeller center. Propeller center is 13" below CG. Thrust angle is 1.5 degrees down.
If you move propeller center down 1" to 14" below CG, the new thrust angle will be 2.4 down.


That's what I suspected - and it seams to work. The resulting
reduction in the angle is counterintuitive however.