effect of changed thrust line.

wrote

Don't bother with center of mass. It's not really relevant.

Indeed

Angling the engine up 3.8 degrees would lead to trouble. That's a lot
of angle.

Absolutely

Thrust works against the center of DRAG, which is much harder
to locate than CG.

Thanks for a verification of my thinking.
--
Jim in NC

In article ,
cavelamb himself wrote:

wrote:
On Nov 14, 8:25 am, Alan Baker wrote:


Did a quick little check:

As an example, a Cessna 150 is about 25 feet long and from looking at
wikipedia's little jpeg, the centre of mass should be about 5 feet
behind the propellor disc.

So if you raise the thrust line 4 inches, you need to angle the engine
up an additional 3.8 degrees; arctan(4/60).



Don't bother with center of mass. It's not really relevant.
Angling the engine up 3.8 degrees would lead to trouble. That's a lot
of angle. Most engines are aligned with the longitudinal axis or
parallel to it (the waterline) or angled *down* a bit (Ercoupe has
lots; Cherokee and its brethren have some, 172 has none at all) and
some are angled to the side a bit as well to control P-factor.

Thrust works against the center of DRAG, which is much harder
to locate than CG. Lowering the thrust line would tend to raise the
nose more on powering up, which would require more nose-down trim to
control, which would lead to a bigger drop in attitude when the power
is removed.
But I don't think four inches lower is going to be a big
deal. The loss of ground clearance, OTOH, is significant for a STOL
airplane.

Dan


And angling an engine UP is a real BAD (tm) idea.

Really?

So when you're flying with a large AOA, it's BAD(tm)?

Please.

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

In article ,
"Morgans" wrote:

wrote

Don't bother with center of mass. It's not really relevant.

Indeed

Angling the engine up 3.8 degrees would lead to trouble. That's a lot
of angle.

Absolutely

Thrust works against the center of DRAG, which is much harder
to locate than CG.

Thanks for a verification of my thinking.

Sorry.

But all forces on a body act around it's centre of *mass* when
considering its rotation. You can take the moments about anywhere fixed,
but the CoM is what actually matters.

As for angling the engine 3.8 degrees up being a bad idea, how can that
possibly be? When you pull up to a high AOA, does the engine suddenly
explode? No.

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

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

In article
,
wrote:

On Nov 14, 8:25 am, Alan Baker wrote:

Did a quick little check:

As an example, a Cessna 150 is about 25 feet long and from looking at
wikipedia's little jpeg, the centre of mass should be about 5 feet
behind the propellor disc.

So if you raise the thrust line 4 inches, you need to angle the engine
up an additional 3.8 degrees; arctan(4/60).


Don't bother with center of mass. It's not really relevant.
Angling the engine up 3.8 degrees would lead to trouble. That's a lot
of angle. Most engines are aligned with the longitudinal axis or
parallel to it (the waterline) or angled *down* a bit (Ercoupe has
lots; Cherokee and its brethren have some, 172 has none at all) and
some are angled to the side a bit as well to control P-factor.

The longitudinal axis is purely notional. What is it defined by?

Go ahead: write a definition in words that will let anyone deduce what
any arbitrary aircraft's "longitudinal axis" is...


Thrust works against the center of DRAG, which is much harder
to locate than CG. Lowering the thrust line would tend to raise the
nose more on powering up, which would require more nose-down trim to
control, which would lead to a bigger drop in attitude when the power
is removed.

Which is why you change the angle...

If the thrust line is changed to keep the torques the same, then no need
for increased trim, right?

But I don't think four inches lower is going to be a big
deal. The loss of ground clearance, OTOH, is significant for a STOL
airplane.

Dan

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

In article ,
wrote:

On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself
wrote:

wrote:
On Nov 14, 8:25 am, Alan Baker wrote:


Did a quick little check:

As an example, a Cessna 150 is about 25 feet long and from looking at
wikipedia's little jpeg, the centre of mass should be about 5 feet
behind the propellor disc.

So if you raise the thrust line 4 inches, you need to angle the engine
up an additional 3.8 degrees; arctan(4/60).



Don't bother with center of mass. It's not really relevant.
Angling the engine up 3.8 degrees would lead to trouble. That's a lot
of angle. Most engines are aligned with the longitudinal axis or
parallel to it (the waterline) or angled *down* a bit (Ercoupe has
lots; Cherokee and its brethren have some, 172 has none at all) and
some are angled to the side a bit as well to control P-factor.

Thrust works against the center of DRAG, which is much harder
to locate than CG. Lowering the thrust line would tend to raise the
nose more on powering up, which would require more nose-down trim to
control, which would lead to a bigger drop in attitude when the power
is removed.
But I don't think four inches lower is going to be a big
deal. The loss of ground clearance, OTOH, is significant for a STOL
airplane.

Dan


And angling an engine UP is a real BAD (tm) idea.


Lowering the thrust line to below the center of aerodynamic drag would
cause nose up - OK I get that. Now where is the center of drag on a
peg? and it will DEFINETLY change with flying attitude - ie with the
flaps on, or the slats extended.

Which is why one should take moments about a fixed point...

....say the centre of mass. :-)


I guess what it boils down to is it will not be a HUGE effect.
On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
roughly 1/2 degree. One 1/8" washer at the firewall and one at the
engine rubber on both sides will make 1/2 degree change if I need to
do a bit od "fine" tuning.

Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
offset - guess I'll put in about .875 and see what happens

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

In article ,
cavelamb himself wrote:

wrote:


Lowering the thrust line to below the center of aerodynamic drag would
cause nose up - OK I get that. Now where is the center of drag on a
peg? and it will DEFINETLY change with flying attitude - ie with the
flaps on, or the slats extended.

I guess what it boils down to is it will not be a HUGE effect.
On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
roughly 1/2 degree. One 1/8" washer at the firewall and one at the
engine rubber on both sides will make 1/2 degree change if I need to
do a bit od "fine" tuning.

Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
offset - guess I'll put in about .875 and see what happens

I thought about this a bit last night.
And a couple of thoughts seemed worth relaying.

First, (and most obviously) a new mount will be needed.
So build it as close as you can guess to what you'll need.

Adjusting the mount at the firewall end strikes me as a bit "iffy".

More that a washer or two makes for a noticeable misalignment between top
and bottom bolts. When torqued down, something it GOING to give.

Either the mount gets twisted or the firewall support structure does.
Or both?

Not if you use wedge washers...

http://www.instron.us/wa/acc_catalog...ref=http://www
..google.com/search


The engine end, if rubber cushioned would be a lot more compliant.
Might consider all that when designing the new mount.

The Corvair would use a bearer style mount, wouldn't it?

Rubber pads front and rear would give quite a bit of adjustment room.

I think Stealth got it right.

Same side alignment and a touch more down.

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

Alan Baker wrote:
In article ,
"Morgans" wrote:

wrote

Don't bother with center of mass. It's not really relevant.

Indeed

Angling the engine up 3.8 degrees would lead to trouble. That's a lot
of angle.

Absolutely

Thrust works against the center of DRAG, which is much harder
to locate than CG.

Thanks for a verification of my thinking.

Sorry.

But all forces on a body act around it's centre of *mass* when
considering its rotation. You can take the moments about anywhere fixed,
but the CoM is what actually matters.

I know you are trying to be helpful and you know that objects in free space
rotate about their center of mass, but I'm fairly confident that aspects
such as the center of pressure and affects on control surface authority
need to be taken into account when changing the thrust line.

As for angling the engine 3.8 degrees up being a bad idea, how can that
possibly be? When you pull up to a high AOA, does the engine suddenly
explode? No.

A high angle of attack during cruise would presumably place the wings
closer to their stall angle. I presume that is what makes it dangerous.

"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

First: yes, any *fixed* point will do, which the centre of drag is not.

Second, the math is easiest when you pick the point that is actually the
one about which the body will rotate.

I think I get your point, about the plane rotating about the center of
mass, but I do not believe that a change in the amount of thrust and its
location in relation to the center of mass is what is relevant, in this
issue.

For a plane to not require a change in trim with a change in power, and not
to rotate, the thrust line must be in line with the center of drag in level
flight, no?

No.

Thrust, drag, lift weight are the way we normally coin the forces on an
aircraft, but the reality isn't quite that.

That formalism has both drag and thrust operating in the horizontal
axis, but we know that the engine's thrust will only actually be
horizontal at one particular angle of attack. Hence, only then will the
thrust line be aligned with the drag line.

That's why I keeps saying take all the moments about a fixed point to
really have a clear picture.


The downthrust or upthrust is added to partially negate the rotation caused
due to the distance the thrust is away from the center of drag.

Forget all about drag for a moment: you can boil down all the forces of
lift and drag -- the aerodynamic forces -- to a single force through the
aerodynamic centre of pressure. Now imagine an aircraft with the engine
off and the prop feathered and in a trimmed out glide.

Got that pictured in your mind? Good.

Now: when you start the engine and add its thrust into the equation,
what is the one direction in which you can apply that thrust and not
cause the aircraft to pitch.

Where must the axis of the thrust vector be?

Through the centre of mass. Period.

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