effect of changed thrust line.

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)

"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.
--
Jim in NC

On Thu, 13 Nov 2008 23:28:09 -0500, "Morgans"
wrote:


wrote

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,

Good to see you around, again. :-) Howz the project going?

Did you have a web page or blog with your project on it? I lost everything
with a total computer melt-down a while back.

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

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.

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.

"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?

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.
--
Jim in NC

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