effect of changed thrust line.

wrote:
OK - got some more info.

The center of mass is something like 34 inches behind the firewall
and roughly 7 inches above the top engine mount point on the firewall.
so roughly speeking 13 inches above the prop centerline.
The prop flange with the O200 is 29.75 inches from the firewall.
This means it is 63.75 inches from the prop flange to the CM.(center
of mass)

This means there is NO WAY the thrust line is aligned anywhere close
to the center of mass.
This would require a downward displacement of almost 15 degrees.

THAT is not going to fly - PERIOD. We are hitting about 5.5 inches
BELOW the center of mass

If we aim for the middle of the rear stabilizer, about 183 inches from
the prop flange, 1.5 degrees down is 5.5 inches above the prop center,
which is about the middle of the rear of the fuselage and roughly 10
inches below the center of the rear horizontal stabilizer .

If I want to hit the same spot with the engine down 1.5 inches, i need
to change the angle to 1.875 degrees.
2 inches goes to 2 degrees.
2.5 inches would be 2.15 degrees, +/-
3 inches would be 2.31 degrees
4 inches would be 2.58 degrees.

Does this make any sense??
It sounds right to me.



I'd fly it.

--

Richard

(remove the X to email)

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.

On Sat, 15 Nov 2008 23:11:33 -0600, cavelamb himself wrote:

wrote:
OK - got some more info.

The center of mass is something like 34 inches behind the firewall
and roughly 7 inches above the top engine mount point on the firewall.
so roughly speeking 13 inches above the prop centerline.
The prop flange with the O200 is 29.75 inches from the firewall.
This means it is 63.75 inches from the prop flange to the CM.(center
of mass)

This means there is NO WAY the thrust line is aligned anywhere close
to the center of mass.
This would require a downward displacement of almost 15 degrees.

THAT is not going to fly - PERIOD. We are hitting about 5.5 inches
BELOW the center of mass

If we aim for the middle of the rear stabilizer, about 183 inches from
the prop flange, 1.5 degrees down is 5.5 inches above the prop center,
which is about the middle of the rear of the fuselage and roughly 10
inches below the center of the rear horizontal stabilizer .

If I want to hit the same spot with the engine down 1.5 inches, i need
to change the angle to 1.875 degrees.
2 inches goes to 2 degrees.
2.5 inches would be 2.15 degrees, +/-
3 inches would be 2.31 degrees
4 inches would be 2.58 degrees.

Does this make any sense??
It sounds right to me.


I'd fly it.

You would. But you have NEVER been known for your intelligence. ))))

Dudley Henriques wrote:
On Sat, 15 Nov 2008 23:11:33 -0600, cavelamb himself wrote:


wrote:

OK - got some more info.

The center of mass is something like 34 inches behind the firewall
and roughly 7 inches above the top engine mount point on the firewall.
so roughly speeking 13 inches above the prop centerline.
The prop flange with the O200 is 29.75 inches from the firewall.
This means it is 63.75 inches from the prop flange to the CM.(center
of mass)

This means there is NO WAY the thrust line is aligned anywhere close
to the center of mass.
This would require a downward displacement of almost 15 degrees.

THAT is not going to fly - PERIOD. We are hitting about 5.5 inches
BELOW the center of mass

If we aim for the middle of the rear stabilizer, about 183 inches from
the prop flange, 1.5 degrees down is 5.5 inches above the prop center,
which is about the middle of the rear of the fuselage and roughly 10
inches below the center of the rear horizontal stabilizer .

If I want to hit the same spot with the engine down 1.5 inches, i need
to change the angle to 1.875 degrees.
2 inches goes to 2 degrees.
2.5 inches would be 2.15 degrees, +/-
3 inches would be 2.31 degrees
4 inches would be 2.58 degrees.

Does this make any sense??
It sounds right to me.


I'd fly it.


You would. But you have NEVER been known for your intelligence. ))))






--

Richard

(remove the X to email)

There is not one right answer here.

What we are looking for is predictible and managable responses.

That's why the Cooper-Harper index exists and is written the way it is.


http://en.wikipedia.org/wiki/Cooper-Harper_rating_scale

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?

Alan Baker wrote:
Jim Logajan wrote:
Running the thrust line through M does _not_ guarantee you wont get
any couple.

It guarantees you won't get a couple from the thrust.

I think I see one of your problems. How many forces are needed for a
couple? Can one of those forces pass through the center of mass?

In article ,
Jim Logajan wrote:

Alan Baker wrote:
Jim Logajan wrote:
Running the thrust line through M does _not_ guarantee you wont get
any couple.

It guarantees you won't get a couple from the thrust.

I think I see one of your problems. How many forces are needed for a
couple? Can one of those forces pass through the center of mass?

One. There only has to be a one force on a body in order for it to
experience to angular acceleration. That force just has to act on a line
that is not through the centre of mass.

Questions for you:

Do you believe that in the absence of any other forces, if the
aerodynamic forces act on a line that is not through the centre of mass,
will the aircraft experience angular acceleration?

If you add thrust acting through the centre of mass to the situation,
will it change the angular acceleration?

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg

In article
,
Alan Baker wrote:

In article ,
Jim Logajan wrote:

Alan Baker wrote:
Jim Logajan wrote:
Running the thrust line through M does _not_ guarantee you wont get
any couple.

It guarantees you won't get a couple from the thrust.

I think I see one of your problems. How many forces are needed for a
couple? Can one of those forces pass through the center of mass?

One. There only has to be a one force on a body in order for it to
experience to angular acceleration. That force just has to act on a line
that is not through the centre of mass.

I apologize, I was using the term "couple" incorrectly.

But that doesn't matter to my argument. So ignore the first bit and
answer my questions below:


Questions for you:

Do you believe that in the absence of any other forces, if the
aerodynamic forces act on a line that is not through the centre of mass,
will the aircraft experience angular acceleration?

If you add thrust acting through the centre of mass to the situation,
will it change the angular acceleration?

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg