effect of changed thrust line.

In article ,
cavelamb himself wrote:

Alan Baker wrote:

Not if you use wedge washers...

http://www.instron.us/wa/acc_catalog...ref=http://www
.google.com/search



The smallest of those are 1" in dimeter.

Do you think that's big enough???

Why would it matter if the SMALLEST of something is BIG ENOUGH?

Surely even you are bright enough to realize that the there must
logically be larger ones than the SMALLEST of something...

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

Alan Baker wrote:
In article ,
cavelamb himself wrote:


Alan Baker wrote:


Not if you use wedge washers...

http://www.instron.us/wa/acc_catalog...ref=http://www
.google.com/search


The smallest of those are 1" in dimeter.

Do you think that's big enough???


Why would it matter if the SMALLEST of something is BIG ENOUGH?

Surely even you are bright enough to realize that the there must
logically be larger ones than the SMALLEST of something...


BECAUSE WE GENERALLY USE 3/8" OR 1/2" DIAMETER BOLTS ON ENGINE MOUNTS.

--

Richard

(remove the X to email)

Alan Baker wrote:
In a glide in a low wing aircraft:

Total aerodynamic force (lift and drag!)
^
|
|
M (Centre of Mass)
|
C (Centre of Aerodynamic Pressure)
|
|
Weight (no down arrow head... ...sorry)

It is late and I'm about to go to bed, but after seeing your force diagrams
like the one above I wrote a private e-mail to the original poster with the
advice that while you get many concepts correct you also appear to apply
them incorrectly on occasion, so your advice should be disregarded as
suspect. Nothing personal - but someone's time, money, and life are
involved. Getting things wrong is pretty common - we all do it - even Nobel
prize winners in physics have had their physical intuition fail them
(Feynman wrote of one such case in one of his books.)

Anyway, all the force diagrams you drew appear to have important errors.

And for what it is worth, I have a BSc in physics and was just reviewing a
textbook on basic mechanics a week ago (J. P. Den Hartog's "Mechanics" -
inexpensive and worth buying by the way) but even so I would have to think
the problem through carefully before attempting even a qualified answer.

In article ,
cavelamb himself wrote:

Alan Baker wrote:
In article ,
cavelamb himself wrote:


Alan Baker wrote:


Not if you use wedge washers...

http://www.instron.us/wa/acc_catalog...ref=http://www
.google.com/search


The smallest of those are 1" in dimeter.

Do you think that's big enough???


Why would it matter if the SMALLEST of something is BIG ENOUGH?

Surely even you are bright enough to realize that the there must
logically be larger ones than the SMALLEST of something...


BECAUSE WE GENERALLY USE 3/8" OR 1/2" DIAMETER BOLTS ON ENGINE MOUNTS.

1. Do you think this is the only supplier of wedge washers in the world?

2. Why did you ssy, "Do you think that's big enough???"?

3. You don't suppose the original poster could just design the engine
mounts to accomodate the different bolt angles do you? I mean, it's not
like he's building a kit or something...

....no, wait.

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

Alan Baker schreef:

...no, wait.

No I won't.
Ploink.

In article ,
Jim Logajan wrote:

Alan Baker wrote:
In a glide in a low wing aircraft:

Total aerodynamic force (lift and drag!)
^
|
|
M (Centre of Mass)
|
C (Centre of Aerodynamic Pressure)
|
|
Weight (no down arrow head... ...sorry)

It is late and I'm about to go to bed, but after seeing your force diagrams
like the one above I wrote a private e-mail to the original poster with the
advice that while you get many concepts correct you also appear to apply
them incorrectly on occasion, so your advice should be disregarded as
suspect. Nothing personal - but someone's time, money, and life are
involved. Getting things wrong is pretty common - we all do it - even Nobel
prize winners in physics have had their physical intuition fail them
(Feynman wrote of one such case in one of his books.)

Anyway, all the force diagrams you drew appear to have important errors.

And for what it is worth, I have a BSc in physics and was just reviewing a
textbook on basic mechanics a week ago (J. P. Den Hartog's "Mechanics" -
inexpensive and worth buying by the way) but even so I would have to think
the problem through carefully before attempting even a qualified answer.

I'm sorry, but the problem is simple.

You want the new engine configuration to behave like the old one. To
achieve that, it must have the same effect on pitch as the old one. To
achieve *that*, it must have the same moment arm with respect to the
center of mass.

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

In article ,
jan olieslagers wrote:

Alan Baker schreef:

...no, wait.

No I won't.
Ploink.

Suit yourself, but running from factual arguments never appears to be a
good idea to me...

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

In article ,
wrote:

On Fri, 14 Nov 2008 16:56:51 -0800, Alan Baker
wrote:

In article ,
"Morgans" wrote:

"Alan Baker" wrote

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

Now, you are starting to get close. You are splitting hairs, though.

No. I'm understanding the situation and so far, you've not shown that
you do.


Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects
the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

If your claim that thrust and drag line always canceled each other out,
then that last statement would not be true, would it?


Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line,
and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

They can be kept close by keeping the change in torque about the centre
of mass the same as they were in the original design...


A change of 1/2 degree would be close, but the best answer will be to try
it
and see.

Where did you get that figure? Show your work if you're going to try and
be quantitative...


Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is
made
successfully, there won't be any rotating going on. ;-)

Yup. And in order for that to happen, you need to keep the same
relationship with the centre of *mass*.

:-)
Using an engine offset calculator, and assuming the CL is over the CM
and it is 78" behind the prop center,1.5 degrees would have the prop
center 2.045" below the CM. If I lower the engine 2 inches, the angle
needs to change to 2.969

If theCM is farther back, the angle change will be less.
If I only drop the engine 1 inch instead of 2, the difference is half.
If I drop it 4 degrees, the angle WOULD be over 3.5 degrees - 4.432,
to be exact.

ASSUMING the CM is somewhere very close to the CL (which it MAY not
be)

Define: "CL".

--
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

On Nov 14, 2:59*pm, wrote:
On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself



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


This is all good till you consider that cowling you spent days
trimming to get it to fit perfectly will now be junk.