effect of changed thrust line.

In article , "Morgans"
wrote:

"Alan Baker" wrote

Read this. Read it again and again until you get it:

Don't take moments about anything other than the centre of mass.

Sorry if I don't take your word for it, so if you have some online sources,
lay 'em on me.

Sorry, man, but it's just basic physics.

You can take them about any fixed point, but the essence of it is that a
rigid body rotates about its center of mass.

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

"Alan Baker" wrote

Sorry, man, but it's just basic physics.

You can take them about any fixed point, but the essence of it is that a
rigid body rotates about its center of mass.

Yes, but I believe the problem is that you are not seeing the whole picture.

For example, what forces are involved that cause an airplane to not be
rotating around the center of mass while at cruise?

The answer is that thrust is cancel by drag. Where those two forces are
located is behind what amount of thrust differential is needed.
--
Jim in NC

In article ,
"Morgans" wrote:

"Alan Baker" wrote

Sorry, man, but it's just basic physics.

You can take them about any fixed point, but the essence of it is that a
rigid body rotates about its center of mass.

Yes, but I believe the problem is that you are not seeing the whole picture.

For example, what forces are involved that cause an airplane to not be
rotating around the center of mass while at cruise?

The answer is that thrust is cancel by drag. Where those two forces are
located is behind what amount of thrust differential is needed.

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

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

"Alan Baker" wrote

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

Now, you are starting to get close. You are splitting hairs, though.

Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line, and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

A change of 1/2 degree would be close, but the best answer will be to try it
and see.

Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is made
successfully, there won't be any rotating going on. ;-)
--
Jim in NC

In article ,
"Morgans" wrote:

"Alan Baker" wrote

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

Now, you are starting to get close. You are splitting hairs, though.

No. I'm understanding the situation and so far, you've not shown that
you do.


Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

If your claim that thrust and drag line always canceled each other out,
then that last statement would not be true, would it?


Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line, and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

They can be kept close by keeping the change in torque about the centre
of mass the same as they were in the original design...


A change of 1/2 degree would be close, but the best answer will be to try it
and see.

Where did you get that figure? Show your work if you're going to try and
be quantitative...


Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is made
successfully, there won't be any rotating going on. ;-)

Yup. And in order for that to happen, you need to keep the same
relationship with the centre of *mass*.

:-)

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

On Fri, 14 Nov 2008 16:56:51 -0800, Alan Baker
wrote:

In article ,
"Morgans" wrote:

"Alan Baker" wrote

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

Now, you are starting to get close. You are splitting hairs, though.

No. I'm understanding the situation and so far, you've not shown that
you do.


Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

If your claim that thrust and drag line always canceled each other out,
then that last statement would not be true, would it?


Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line, and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

They can be kept close by keeping the change in torque about the centre
of mass the same as they were in the original design...


A change of 1/2 degree would be close, but the best answer will be to try it
and see.

Where did you get that figure? Show your work if you're going to try and
be quantitative...


Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is made
successfully, there won't be any rotating going on. ;-)

Yup. And in order for that to happen, you need to keep the same
relationship with the centre of *mass*.

:-)
Using an engine offset calculator, and assuming the CL is over the CM
and it is 78" behind the prop center,1.5 degrees would have the prop
center 2.045" below the CM. If I lower the engine 2 inches, the angle
needs to change to 2.969

If theCM is farther back, the angle change will be less.
If I only drop the engine 1 inch instead of 2, the difference is half.
If I drop it 4 degrees, the angle WOULD be over 3.5 degrees - 4.432,
to be exact.

ASSUMING the CM is somewhere very close to the CL (which it MAY not
be)

In article ,
wrote:

On Fri, 14 Nov 2008 16:56:51 -0800, Alan Baker
wrote:

In article ,
"Morgans" wrote:

"Alan Baker" wrote

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

Now, you are starting to get close. You are splitting hairs, though.

No. I'm understanding the situation and so far, you've not shown that
you do.


Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects
the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

If your claim that thrust and drag line always canceled each other out,
then that last statement would not be true, would it?


Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line,
and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

They can be kept close by keeping the change in torque about the centre
of mass the same as they were in the original design...


A change of 1/2 degree would be close, but the best answer will be to try
it
and see.

Where did you get that figure? Show your work if you're going to try and
be quantitative...


Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is
made
successfully, there won't be any rotating going on. ;-)

Yup. And in order for that to happen, you need to keep the same
relationship with the centre of *mass*.

:-)
Using an engine offset calculator, and assuming the CL is over the CM
and it is 78" behind the prop center,1.5 degrees would have the prop
center 2.045" below the CM. If I lower the engine 2 inches, the angle
needs to change to 2.969

If theCM is farther back, the angle change will be less.
If I only drop the engine 1 inch instead of 2, the difference is half.
If I drop it 4 degrees, the angle WOULD be over 3.5 degrees - 4.432,
to be exact.

ASSUMING the CM is somewhere very close to the CL (which it MAY not
be)

Define: "CL".

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

On Fri, 14 Nov 2008 19:48:00 -0500, "Morgans"
wrote:


"Alan Baker" wrote

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

Now, you are starting to get close. You are splitting hairs, though.

Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line, and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

A change of 1/2 degree would be close, but the best answer will be to try it
and see.

Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is made
successfully, there won't be any rotating going on. ;-)


U guess the way to intelligently assess this is to determine the
longitudinal center of balance of the finished plane, and project the
thrust line of the engine (from the plans) back to where the 2
intesect. THAT point is where I want the new thrust lines to intersect
as well. If I drop the engine 2 inches I change the angle down enough
for the thrust line to hit the same point and the flight
characteristics SHOULD, theoretically be the same.
No?
Any reason this should not work?

In article ,
wrote:

On Fri, 14 Nov 2008 19:48:00 -0500, "Morgans"
wrote:


"Alan Baker" wrote

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

Now, you are starting to get close. You are splitting hairs, though.

Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line, and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

A change of 1/2 degree would be close, but the best answer will be to try it
and see.

Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is made
successfully, there won't be any rotating going on. ;-)


U guess the way to intelligently assess this is to determine the
longitudinal center of balance of the finished plane, and project the
thrust line of the engine (from the plans) back to where the 2
intesect. THAT point is where I want the new thrust lines to intersect
as well. If I drop the engine 2 inches I change the angle down enough
for the thrust line to hit the same point and the flight
characteristics SHOULD, theoretically be the same.
No?
Any reason this should not work?

If I understand your language correctly, not that I can see.

The "longitudinal center of balance" is just the longitudinal position
of the centre of mass, and as such the "line" with which the thrust line
would intersect would be the vertical line through the CoM, which would
be very close to the moment line which would be the line perpendicular
to the TL that passes through the CoM.

What you want is for the aircraft to react to change in thrust in the
way its designers intended, and to do that, you need to keep the moment
arm the same, but keeping the vertical line segment the same should be
pretty close in any rational aircraft layout.

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