effect of changed thrust line.

On Sat, 15 Nov 2008 06:44:21 -0800 (PST), stol
wrote:

On Nov 14, 2:59Â*pm, wrote:
On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself



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


This is all good till you consider that cowling you spent days
trimming to get it to fit perfectly will now be junk.


Not a chance. The cowling has not even been designed yet, much less
built or trimmed.

This plane has not been completed - still a work in progress.
One of the other local builders is building with an O200 and has his
mount that I can compare to.

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

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.



I don't know who you are, nor what your qualification really are.

But most of what you have espoused here I strongly disagree with.


You dance pretty well, though.

Richard

In article ,
cavelamb himself wrote:


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.



I don't know who you are, nor what your qualification really are.

But most of what you have espoused here I strongly disagree with.


You dance pretty well, though.

Richard

Nope. I just don't like bull****.

Angling an engine "up" implies only the physical orientation of the
engine.

Now if you want to talk about what makes for a stable configuration with
respect to the *aircraft*, that's an entirely different story.

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

On Fri, 14 Nov 2008 11:08:57 -0800 (PST),
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

Plane is designed for 72 inch prop. I will be running a 68" prop, so I
have 2 inches more to play with. Also running bigger wheels and tires,
which gives me another 1/2 inch at worst case,perhaps 1 1/2 with a
full load of air. Not sure if I will need to go down 4 inches - 2 will
likely do it but I needed a number to ask the question.

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 ,
"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

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.

In article ,
Jim Logajan wrote:

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.

But by taking moments about the CoM, you can isolate the contribution of
the engine and work to keep it the same despite the change in its
location.

That's precisely why you don't do it with respect to loci that change.

Let's say the engine is mounted such that it is acting through the CoM,
OK? In that case, changes in thrust cannot *possibly* cause any net
torque, right?

OK, move the engine up or down, and if you reangle it to set the thrust
line through the CoM, then the same situation holds true.


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.

Not dangerous for the engine, though, right?

--
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