effect of changed thrust line.

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

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.


--

Richard

(remove the X to email)

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.

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

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?

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.

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:

Not if you use wedge washers...

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



The smallest of those are 1" in dimeter.

Do you think that's big enough???



--

Richard

(remove the X to email)

In article ,
cavelamb himself wrote:

Alan Baker wrote:

Not if you use wedge washers...

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



The smallest of those are 1" in dimeter.

Do you think that's big enough???

Why would it matter if the SMALLEST of something is BIG ENOUGH?

Surely even you are bright enough to realize that the there must
logically be larger ones than the SMALLEST of something...

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

On Fri, 14 Nov 2008 16:16:21 -0600, 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?

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?

Not on this plane. I'll get pics of the mount design on line soon.
I've put mounting tabs on the top and bottom rear so I'm mounting it
like a Conti O200, but using 1" diameter Licoming type homebuilder
mounts.The typical bed mount would interfere with my 180 degree header
system.

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.

wrote:

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


Not on this plane. I'll get pics of the mount design on line soon.
I've put mounting tabs on the top and bottom rear so I'm mounting it
like a Conti O200, but using 1" diameter Licoming type homebuilder
mounts.The typical bed mount would interfere with my 180 degree header
system.



How will the mount attach to the engine case?
I don't recall how the aft end of the engine is arranged.






--

Richard

(remove the X to email)

On Fri, 14 Nov 2008 22:53:23 -0600, cavelamb himself
wrote:

wrote:

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


Not on this plane. I'll get pics of the mount design on line soon.
I've put mounting tabs on the top and bottom rear so I'm mounting it
like a Conti O200, but using 1" diameter Licoming type homebuilder
mounts.The typical bed mount would interfere with my 180 degree header
system.



How will the mount attach to the engine case?
I don't recall how the aft end of the engine is arranged.


I'll get pictures, but I used a chunk of auminum channel, cut away to
make a "U" shaped bracket that bolts to the top surface of the engine
case, with "ears" to which mounting blocks are fastened, immitating
the top ears of an O200 case. The bottom has an angle boted down each
side, like the typical bed mount but without rubber isolation, with
mount blocks fastened to them as well,.

Very similar to the way it is mounted on my engine test stand,
pictured on my website.