Differences between automotive & airplane engines

Lou wrote:

I'll agree with the automotive engine with PSRU being heavier, but are
you sure about your other statement "the lighter the better"?
I'm currently looking at an engine that is 100lbs lighter than the one
recommended for my plane. Although cutting 100lbs from the total
weight is a dream come true, it brings up the question of weight and
balance. I can move the engine forward to make up the difference in
balance, but I don't know how far or how to find out.
Lou


Googling this group for weight and balance yields 25 pages...
So I picked up one of mine here (Dec 10, 2002) and included
Brian's note at the bottom...
(Unfortunately, there was no link attached, so here is the text).



There is a lot of smoke and mirror magic around weight and balance
because so many people understand it so poorly.

At the heart of all of it, though, is a rotational force about a
reference point. the rotational force is called a MOMENT, and
the reference point is called the DATUM.

Sometimes the datum is located at the tip of the spinner.
Sometimes it's located at the main gear axles.
Sometimes its located at the leading edge of the wing.
It doesn't particularly matter where it is located, as long
as you use the same location to work the problem.

You'll often see the term STATION. This is the distance from the
datum to a particular place on the aircraft. Say, for instance,
the instrument panel?

The station numbers change according to where the datum is placed.
But the instrument panel stays in the same physical location.
It's all an offset from a zero point.

One reason to place the datum at the tip of the spinner is because
all the station numbers are positive. No negative distances to
confuse things.

One reason to place the datum at the axles is because the datum
is station zero.zero. Multiply the weight on the wheel times
zero (the ARM is zero at the datum) and the moments for that
wheel come out to zero. Makes the arithmetic a little easier?

And, the reason to place the datum at the leading edge of the wing
it because that's where we are going to wind up anyway. The results
of our CG calculations will finally boil down to a point some given
distance aft of the leading edge.

CG range is often refereed to in terms of a percentage of the wing
chord. Say 25% would be the forward CG limit, maybe 33% would be the
aft limit. So our end number actually refers to a distance aft of
the leading edge. The actual numbers will be different, depending
on where the datum is located, but they all (hopefully) come out
at the same place on the airplane.

First rule:

weight x distance = moments pounds x inches = pound inches (!)
So,
moments / inches = pounds
and
moments / pounds = inches

Practical example:

A bowling ball, held at the chest, has a certain weight.
Held at arms length, it has exactly the same weight!

But due to the longer distance (called ARM) it has a much higher moment.
\
THAT's what feels so heavy.
That rotational force.

So, to solve your little weight and balance question.

The only distance from anything. that matters, is the
distance from the CG of the instrument to the DATUM
specified for that aircraft.

If you have a "before" weight and balance already done,
multiply the weight of the instrument times the distance
from the datum given in the "before" problem.

Then add that moment to the airplane's moment,
and the instrument weight to the airplane's weight.

Divide the new moments by the new weight and you get the
new CG location.

Does that help?

Or do you maybe feel like I sometimes do after some
of your answers???


From: Brian Anderson - view profile
Date: Tues, Dec 10 2002 12:22 am

Jim,

EE's are the brightest of the lot - - - they can measure and calculate
things you can't even see.

The revised CG calculation is straightforward, but you need to calculate the
original moment first, i.e. the total weight [W] x the arm from the datum
[D]. Add to this the additional moment for the instrument, i.e. 8 lbs x the
distance of the instrument CG from the datum [d]. The resulting moment is
[W*D] + [8*d]. Divide this by the new total weight [W+8], and the result is
the new CG location from the datum.

Hence, new CG location = [[W x D] + [8 x d]]/[W+8]

I know even an elderly EE can follow that. After all, I is one too.

Brian