Differences between automotive & airplane engines

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

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

You weigh the airplane without the engine installed and
calculate a balance point for it. Knowing the weight of the engine, you
then figure the arm at which it needs to be located to bring the
airplane's empty CG to the point the designer calls for it. Not a big
deal at all. Pages 134 and 135 of William Kerschner's Advanced Pilot's
Flight Manual shows how.

Dan

wrote in message
oups.com...

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

You weigh the airplane without the engine installed and
calculate a balance point for it. Knowing the weight of the engine, you
then figure the arm at which it needs to be located to bring the
airplane's empty CG to the point the designer calls for it. Not a big
deal at all. Pages 134 and 135 of William Kerschner's Advanced Pilot's
Flight Manual shows how.

Dan

However, you will also be changing the area and arm relationships of the
side view of the aircraft (there is a name for this which I cannot recall)
and the size of the verticall fin will need to be increased if you are to
retain the same yaw stability. Then, because of the increased area of the
vertical stabilizer, a larger rudder would be needed to retain the original
crosswind landing capability. In addition, due to the increased planform
area forward of the CG, a larger horizontal stabilizer may well be required
to prevent any sort of deep stall or flat spin tendency. Finally, just as a
larger vertival stabilizer requires a larger rudder, a larger horizontal
stabilizer will very likely require a larger elevator.

To put it another way: Engineering is the science of compromise, and an
airplane is a series of compromises flying in close formation.

Peter

"Peter Dohm" wrote

However, you will also be changing the area and arm relationships of the
side view of the aircraft (there is a name for this which I cannot recall)
and the size of the verticall fin will need to be increased if you are to
retain the same yaw stability. Then, because of the increased area of the
vertical stabilizer, a larger rudder would be needed to retain the
original
crosswind landing capability. In addition, due to the increased planform
area forward of the CG, a larger horizontal stabilizer may well be
required
to prevent any sort of deep stall or flat spin tendency. Finally, just as
a
larger vertival stabilizer requires a larger rudder, a larger horizontal
stabilizer will very likely require a larger elevator.

To put it another way: Engineering is the science of compromise, and an
airplane is a series of compromises flying in close formation.

It should not change all that much, I'll bet. If you look at that heavy
engine moving a few inches, and the increased cowl area in front of
aerodynamic center pressure, then look at that long, long arm back to the
fin and rudder, it should only take about a third of the area the engine
added to make it all work out. Increase the fin/rudder height a couple
inches, or add a small dorsal fin, and all will be well in the world. :-

Reminder: all usenet advice is worth what you pay for the advise.

To the OP; what are you using that is 100 lbs lighter, and what was the
original? That is a nice weight savings!
--
Jim in NC

It should not change all that much, I'll bet. If you look at that heavy
engine moving a few inches, and the increased cowl area in front of
aerodynamic center pressure, then look at that long, long arm back to the
fin and rudder, it should only take about a third of the area the engine
added to make it all work out. Increase the fin/rudder height a couple
inches, or add a small dorsal fin, and all will be well in the world. :-

It will change it some. My Jodel came out considerably heavier
than designed, mostly to the use of birch instead of the mahogany
specified in the original French drawings, fabric over all ply surfaces
to meet Canadian requirements, a tailwheel insterad of a skid, and so
on. Since most of the added weight is behind the CG, it was tailheavy
and the engine had to go 11" further forward.
The longer nose side area results in a little less directional
stability with the same tail, and I won't spin it because I don't know
just what the effect of the extra weight in the tail (and the longer
nose arm to balance it) might do to the spin; it might flatten into an
unrecoverable situation. Sure does an awesome slip, though.

Dan

On Fri, 10 Feb 2006 12:46:28 -0500, "Peter Dohm"
wrote:

wrote in message
roups.com...

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

You weigh the airplane without the engine installed and
calculate a balance point for it. Knowing the weight of the engine, you
then figure the arm at which it needs to be located to bring the
airplane's empty CG to the point the designer calls for it. Not a big
deal at all. Pages 134 and 135 of William Kerschner's Advanced Pilot's
Flight Manual shows how.

Dan

However, you will also be changing the area and arm relationships of the
side view of the aircraft (there is a name for this which I cannot recall)
and the size of the verticall fin will need to be increased if you are to
retain the same yaw stability. Then, because of the increased area of the
vertical stabilizer, a larger rudder would be needed to retain the original
crosswind landing capability. In addition, due to the increased planform
area forward of the CG, a larger horizontal stabilizer may well be required
to prevent any sort of deep stall or flat spin tendency. Finally, just as a
larger vertival stabilizer requires a larger rudder, a larger horizontal
stabilizer will very likely require a larger elevator.

To put it another way: Engineering is the science of compromise, and an
airplane is a series of compromises flying in close formation.

Peter

You are correct - but 4-6 inches on a 20 foot plane does not make a
significant difference in the yaw and required rudder size. And 4 to 6
inches can correct for a fair amount of weight.

clare at snyder.on.ca wrote in message
...
On Fri, 10 Feb 2006 12:46:28 -0500, "Peter Dohm"
wrote:

wrote in message
roups.com...

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

You weigh the airplane without the engine installed and
calculate a balance point for it. Knowing the weight of the engine, you
then figure the arm at which it needs to be located to bring the
airplane's empty CG to the point the designer calls for it. Not a big
deal at all. Pages 134 and 135 of William Kerschner's Advanced Pilot's
Flight Manual shows how.

Dan

However, you will also be changing the area and arm relationships of the
side view of the aircraft (there is a name for this which I cannot
recall)
and the size of the verticall fin will need to be increased if you are to
retain the same yaw stability. Then, because of the increased area of
the
vertical stabilizer, a larger rudder would be needed to retain the
original
crosswind landing capability. In addition, due to the increased planform
area forward of the CG, a larger horizontal stabilizer may well be
required
to prevent any sort of deep stall or flat spin tendency. Finally, just
as a
larger vertival stabilizer requires a larger rudder, a larger horizontal
stabilizer will very likely require a larger elevator.

To put it another way: Engineering is the science of compromise, and an
airplane is a series of compromises flying in close formation.

Peter

You are correct - but 4-6 inches on a 20 foot plane does not make a
significant difference in the yaw and required rudder size. And 4 to 6
inches can correct for a fair amount of weight.

You are right. I should have at least asked the weight of the original
engine.

clare at snyder.on.ca wrote:

To put it another way: Engineering is the science of compromise, and an
airplane is a series of compromises flying in close formation.

Peter


You are correct - but 4-6 inches on a 20 foot plane does not make a
significant difference in the yaw and required rudder size. And 4 to 6
inches can correct for a fair amount of weight.

There is also the issue of the CG of the engine. They don't all balance
at the same point. One could be 100lbs lighter but have most of the
weight shifted to the front. Mounting in the same geometrical position
would not move the planes CG. (Just a theoretical. I don't know of a
specific case.)

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

On 10 Feb 2006 05:55:48 -0800, "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

calculate it out yourself.
weight times distance from the datum for the original engine will need
to equal weight times distance for the new engine.

ie moment arm of the old engine needs to equal the moment arm of the
new engine.

for your calculation purpose you can select an arbitrary point on the
fuselage as your datum point.
the manufacturer's data for the engine should show the cg position of
the engine. measure the distance from that cg point to the datum.
multiply the engine weight by the distance you just measured.
the result is the moment arm.

now divide the answer above by the new engine weight and you will get
a number. that number is the distance from the datum to the cg
position of the new engine that maintains the original aircraft
balance.

if you dont have the cg position of the engine you can work it out
easily.
(think about the consequences of dropping the engine as you consider
this next bit. it needs to be done carefully!)
just hang the engine up on a rope from a part somewhere on the engine
and draw a vertical line from the rope down across the engine.
hang it up by a different position on the engine and draw another
vertical line. the vertical lines will intersect at the cg.

(btw hang it so that you can see the side elevation of the engine)

just to work out a hypothetical answer.
suppose you make the pilot's door knob the datum point.
measuring from there to the engine cg gives say 52 inches.
the engine weighs 180 lbs.
multiplying that gives a moment of 9,360 inch lbs.

the new engine is 100 lb lighter. say 80lbs.
9,360 divided by 80 = 117.

the CG of the new engine needs to be positioned 117 inches from our
datum of the the pilots doorknob to keep the existing aircraft cg.

there you go lou. that should make it easier.
btw take into account all the stuff that changes as well.
cowlings and engine mount if they have any significant weight.
and include the prop and spinner in the engine weight.
Stealth Pilot
Australia

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