thrust line for engine and not mounting engine on this thrust line

Hello can someone please clear something up for me about the thrust line
I am interested in knowing what changes will happen if?
using a pusher type configuration for reference
I notice that many longez type aircraft vary a bit where the engine is
mounted in reference to the thrust line.
few align engine below line of main wing, but many align engine above this
line
my question is
is there some reference data I can read that would show
if you lower engine thrust line below main wing (this will happen) and to
compensate for the lowering of the engine you must (?) to correct this
problem created
and
if you raise engine above the wing line then you must do (?) to correct the
problem

what problems are you creating buy not aligning engine properly
and what is needed to be done to correct whatever problem you are creating

thanks been following this group a while but haven't seen any reference to
this
off line answers or explanations or help is welcome
thanks for any help tom laudato tom at youbuysell.com

tommyann wrote:

what problems are you creating buy not aligning engine properly
and what is needed to be done to correct whatever problem you are creating


Ever tried to push a large piece of furniture from the side rather than
the middle. Remember how it gets all squirrly and doesn't really want
to go where you want it to. That's the sort of problem your looking at,
except that you can't just stop and go to the other side.

Airplanes have all sorts of centers. Everyone's well versed in CG,
cause that's the one we get taught about in flight training. We don't
hear much about the rest, because as pilots we can't change them much.
Story's different when we put on the builder's hat.

Not only do we have a CG, but there is a center of lift. Those
published numbers from the kit manufacturer or plans publisher don't
mean much if you change the position of the wing, thus moving the center
of lift. The CG must be just forward of CL or you risk not being able
to pull the nose up on takeoff (if it is to far forward), or
inadvertently entering a deep stall (if it is even a little behind).

Then there is the center of drag, CD. This one is fun because it is in
two dimensions. Make your wings assymetrical and the plane will pull to
one side or the other. This is because the drag is offset to one side.
This is the laterall element. Then there's the vertical element.
Suppose you put those big draggy wings on top, like on a Cessna. Short
final, deer on the runway, you firewall the throttle. The nose wants to
pull up. On a Cherokee, you have to be careful, or you might find
yourself nosing over into the runway. The engine is providing the
center of thrust, CT. You can create all sorts of havoc, or very docile
behaviour, depending on how you arrange CD and CT.

That propeller spinning moves the air around the plane in a circle, till
it hits the tail fin and tries to turn you to the side. Especially
notable on takeoff where you need to add a lot of rudder to keep going
straight. One solution is to angle the tail fin to the side a degree or
so. Another is to cock the engine to the side a bit. How much angle is
optimized for a certain flight condition by the designer.

So the CG is forward of CL. This is by design. The plane stalls, then
falls. You want it to fall nose first, so that it will stop falling
very soon thereafter. The downside is that the tail has to provide down
force to keep the nose up. Lots of extra drag there. The drag can be
reduced if the engine is pointed up just a bit, though. Drag the
airplane through the air by pulling it's nose up.

On a pusher, all of these things will be reversed, or maybe not. If you
decide to play with these factors, find out what you're doing first.
The designer should have done the math and flight test to optimize the
parameters for reasonable flight regimes. If you start fiddling with
them willy-nilly, the best that you can hope for will be a very draggy
bird that is constantly fighting with itself to get through the air,
just like a car with a bad alignment job. The likely outcome is that
you will have a bird with some strange and possibly unpredictable ways.

The likely outcome is that
you will have a bird with some strange and possibly unpredictable ways.



Center of lift also moves around a good bit depending on AOA, airspeed,
load, type of airfoil, etc. And its movement is non-linear. Otto
Lilienthal suspected this and I think there was a discussion of it in
his book. Octave Chanute and Wilbur Wright debated it at great length
in 1901, and its in thier letters. And finally, Wilbur and Orville
discovered just how much it can move, and in what odd unexpected ways,
when they conducted the wind-tunnel experiments in 1901.

For a real-world demonstration of what happens when you screw up your
CG calculations, check out the Stormy Petrel 3:
hometown.aol.com/wright1902glider/page3.html Interesting how it can
balance itself on the downtubes without any tie-downs, and yet it can't
lift off and blow away in a 30mph wind? When I static tested it 3 weeks
after this photo, I found that the actual CG was 23" behind the
hang-point. Opps.


Harry

Challenger utilizes a higher reduction drive when people want to swing a
longer prop. I have no direct experience doing it on mine, but I've heard
no negatives either. One would expect to have to trim a bit.
"wright1902glider" wrote in message
ups.com...
The likely outcome is that
you will have a bird with some strange and possibly unpredictable ways.



Center of lift also moves around a good bit depending on AOA, airspeed,
load, type of airfoil, etc. And its movement is non-linear. Otto
Lilienthal suspected this and I think there was a discussion of it in
his book. Octave Chanute and Wilbur Wright debated it at great length
in 1901, and its in thier letters. And finally, Wilbur and Orville
discovered just how much it can move, and in what odd unexpected ways,
when they conducted the wind-tunnel experiments in 1901.

For a real-world demonstration of what happens when you screw up your
CG calculations, check out the Stormy Petrel 3:
hometown.aol.com/wright1902glider/page3.html Interesting how it can
balance itself on the downtubes without any tie-downs, and yet it can't
lift off and blow away in a 30mph wind? When I static tested it 3 weeks
after this photo, I found that the actual CG was 23" behind the
hang-point. Opps.


Harry

Forces and moments are balanced in straight flight. The net center of
lift (slightly ahead of the main wing CL) is the sum total of tail and
main wing lift and is opposite to CG force and at same point. The tail
lift is negative. When rotating up the net lift is forward of CG, when
rotating down the net lift is behind of CG.

For canards both wings lift - that gives a slight efficiency edge to
the design.


Ernest Christley wrote:
The CG must be just forward of CL or you risk not being able to pull the nose
up on takeoff (if it is to far forward).........

I am not clear on your thrust line use.
Some amphibian aircraft have engines much higher than the main wings.
The offset of the prop thrust line to the main wings will affect fuel
efficiency - the prop introduces more moment force pitching the
aircraft down and more elevator force is required to balance this -
thus more drag on the aircraft. Generally you will see canards engine
thrust line pretty close to the main wing
level - but usually higher to keep the prop clear of ground on
takeoff/landing.

I am not clear on your thrust line use. Some amphibian aircraft have
engines much higher than the main wings. The offset of the prop thrust
line to the main wings will affect fuel efficiency and performance -
the prop introduces more moment force pitching the aircraft down and
more elevator force is required to balance this - thus more drag on the
aircraft. Generally you will see canards engine thrust line pretty
close to the main wing level - but higher if anything, to keep the prop
clear of ground on takeoff/landing.

Correcting your problem? Reposition your engine mount so that at least
the engine thrust line is horizontally centered. I suppose you can tear
up the design more and do the vertical centering.

Thanks for the input

aircraft. Generally you will see canards engine thrust line pretty
close to the main wing level - but higher if anything, to keep the prop
clear of ground on takeoff/landing.

Correcting your problem? Reposition your engine mount so that at least
the engine thrust line is horizontally centered. I suppose you can tear
up the design more and do the vertical centering.

how high up can you go with the engine keeping the engine thrust line
vertically centered before you start getting bigger problems
i see many longez with engines up from horizontal center of both wings of 12
inches or so
thanks for the input to all that have taken the time to answer tom

I read somewhere about Super Cub guys in ALaska putting shims under the
lower mounts at the firewall to up the thrust line a few degrees.

Kent Felkins

"tommyann" wrote in message
...
Thanks for the input

aircraft. Generally you will see canards engine thrust line pretty
close to the main wing level - but higher if anything, to keep the prop
clear of ground on takeoff/landing.

Correcting your problem? Reposition your engine mount so that at least
the engine thrust line is horizontally centered. I suppose you can tear
up the design more and do the vertical centering.

how high up can you go with the engine keeping the engine thrust line
vertically centered before you start getting bigger problems
i see many longez with engines up from horizontal center of both wings of
12
inches or so
thanks for the input to all that have taken the time to answer tom