effect of changed thrust line.

wrote

Project is at www.pegazair.on-the-net.ca/ClareSquared

Tanks! I'll go have a gander! Or a goose! g
--
Jim in NC

On Thu, 13 Nov 2008 22:20:25 -0500, 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 (150 sq ft wing,33 ft wingspan,
)

suck it and see.
your elevator should have enough authority to control the resulting
couple.
you should experience some upthrust but tweaking back the throttle
should control it.

your question in the first line....
take the centre of mass as the pivot point.
you have 4 force couples.

lift vs moment arm.
elevator down thrust vs moment arm
wing drag vs moment arm
thrust vs moment arm.

they will summate to zero in equilibrium but
you probably wont know any of the values o those forces.
alan baker will probably give a guestimate.
suck it and see.

I think you'll need the same side thrust but a little more down thrust
as the cessna 150..

Stealth Pilot

In article ,
Stealth Pilot wrote:

On Thu, 13 Nov 2008 22:20:25 -0500, 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 (150 sq ft wing,33 ft wingspan,
)

suck it and see.
your elevator should have enough authority to control the resulting
couple.
you should experience some upthrust but tweaking back the throttle
should control it.

your question in the first line....
take the centre of mass as the pivot point.
you have 4 force couples.

lift vs moment arm.
elevator down thrust vs moment arm
wing drag vs moment arm
thrust vs moment arm.

they will summate to zero in equilibrium but
you probably wont know any of the values o those forces.
alan baker will probably give a guestimate.
suck it and see.

I think you'll need the same side thrust but a little more down thrust
as the cessna 150..

Stealth Pilot

The math isn't all that hard.

Assuming the designed thrust line goes relatively close to the centre of
mass, then if you want to keep the torque created by thrust close to the
same, you need to change the angle of the engine by arctan(h/l), where h
is the amount you're moving the engine up or down and l is the distance
between the propellor and the centre of mass.

I suck at ASCII art, but:


-----T' (new thrust line)
^
|
h |
|
-----T--(old thrust line)--------------------------------C
l (CoM)

h/l is equal to the tangent of the angle TCT'

If the thrust line is not aligned with the CoM to begin with, then
situation isn't quite as simple, but for small misalignments the effect
is small and for larger misalignments the overall change in torques is
smaller in comparison.

Draw few diagrams of the situation and you'll see what I mean. I'm not
even going to try to draw that situation here. Basically, if the thrust
line was already above the CoM and you move it up, then the change is
smaller than the arctangent of h/l and if it was below the CoM the
change is a little greater than the arctangent.

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

In article
,
Alan Baker wrote:

In article ,
Stealth Pilot wrote:

On Thu, 13 Nov 2008 22:20:25 -0500, 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 (150 sq ft wing,33 ft wingspan,
)

suck it and see.
your elevator should have enough authority to control the resulting
couple.
you should experience some upthrust but tweaking back the throttle
should control it.

your question in the first line....
take the centre of mass as the pivot point.
you have 4 force couples.

lift vs moment arm.
elevator down thrust vs moment arm
wing drag vs moment arm
thrust vs moment arm.

they will summate to zero in equilibrium but
you probably wont know any of the values o those forces.
alan baker will probably give a guestimate.
suck it and see.

I think you'll need the same side thrust but a little more down thrust
as the cessna 150..

Stealth Pilot

The math isn't all that hard.

Assuming the designed thrust line goes relatively close to the centre of
mass, then if you want to keep the torque created by thrust close to the
same, you need to change the angle of the engine by arctan(h/l), where h
is the amount you're moving the engine up or down and l is the distance
between the propellor and the centre of mass.

I suck at ASCII art, but:


-----T' (new thrust line)
^
|
h |
|
-----T--(old thrust line)--------------------------------C
l (CoM)

h/l is equal to the tangent of the angle TCT'

If the thrust line is not aligned with the CoM to begin with, then
situation isn't quite as simple, but for small misalignments the effect
is small and for larger misalignments the overall change in torques is
smaller in comparison.

Draw few diagrams of the situation and you'll see what I mean. I'm not
even going to try to draw that situation here. Basically, if the thrust
line was already above the CoM and you move it up, then the change is
smaller than the arctangent of h/l and if it was below the CoM the
change is a little greater than the arctangent.

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).

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

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 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
,
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

On Fri, 14 Nov 2008 07:25:03 -0800, Alan Baker
wrote:

In article
,
Alan Baker wrote:

In article ,
Stealth Pilot wrote:

On Thu, 13 Nov 2008 22:20:25 -0500, 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?


Draw few diagrams of the situation and you'll see what I mean. I'm not
even going to try to draw that situation here. Basically, if the thrust
line was already above the CoM and you move it up, then the change is
smaller than the arctangent of h/l and if it was below the CoM the
change is a little greater than the arctangent.

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).

alan his corvair engine has the thrustline lower than the original
O-200.
so that would be down an extra 3 degrees.

Stealth Pilot