effect of changed thrust line.

On Nov 13, 7:20*pm, wrote:
How does a person determine what the proper height of an engine should
be when building an airplane? If a particular engine design mandates
the prop is 4 inches, say, lower than where it would be with the
engine originally installed, what effect will it have on handling, and
what changes in downthrust might be advised?

We are building a Pegazair, and my Corvair engine would need to have
the cowl higher than ideal to keep the crank centerline at the same
hight as say, an O200. Weight wize, the engines are just about
identical as equipped Have not determined the center of gravity of the
engine yet, to determine the overall length of the mount.

For those unfamiliar with the plane it is a highwing STOL 2 placer
roughly the same size as a Cessna 150 *(150 sq ft wing,33 ft wingspan,
)

I've posted a spreadsheet to calculate a new thrust angle based on
changing the waterline location of an engine. The data needed is
horizontal distance from center of propeller to CG, original vertical
distance from center of propeller to CG, original thrust angle, and
new vertical distance from propeller center to CG. The formula is not
sensitive to vertical CG location, an estimate will do. What matters
is the change in the engine location.

http://www.spiretech.com/~guynoir/sl...downthrust.xls

On Sat, 15 Nov 2008 20:32:00 -0800 (PST), guynoir
wrote:

On Nov 13, 7:20Â*pm, wrote:
How does a person determine what the proper height of an engine should
be when building an airplane? If a particular engine design mandates
the prop is 4 inches, say, lower than where it would be with the
engine originally installed, what effect will it have on handling, and
what changes in downthrust might be advised?

We are building a Pegazair, and my Corvair engine would need to have
the cowl higher than ideal to keep the crank centerline at the same
hight as say, an O200. Weight wize, the engines are just about
identical as equipped Have not determined the center of gravity of the
engine yet, to determine the overall length of the mount.

For those unfamiliar with the plane it is a highwing STOL 2 placer
roughly the same size as a Cessna 150 Â*(150 sq ft wing,33 ft wingspan,
)

I've posted a spreadsheet to calculate a new thrust angle based on
changing the waterline location of an engine. The data needed is
horizontal distance from center of propeller to CG, original vertical
distance from center of propeller to CG, original thrust angle, and
new vertical distance from propeller center to CG. The formula is not
sensitive to vertical CG location, an estimate will do. What matters
is the change in the engine location.

http://www.spiretech.com/~guynoir/sl...downthrust.xls


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.

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

--
John Kimmel


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

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.