Measurement of CofG

On Jan 11, 4:29*pm, "Dan Marotta" wrote:
Why don't the manufactures publish the height to raise the tail as a
difference between the axle center lines above ground? *Then you don't need
to cut or calculate.

Oh, gee... *That'd be too simple.

One possible reason is that the height is not a constant for any
glider that has a compressible pneumatic tyre or a sprung landing
gear. Both have deflection that depends on the glider mass. That
same deflection would need to be applied to the tail height. It may
not be neglible at max gross wt.

Speaking of sprung landing gear - in some cases the deflection changes
not only the height but also the distance between the tyre/ground
contact point and the datum. That applies to modern Schleicher single
seaters and probably other gliders. The best plan is to do the weight
and ballance as defined by the manufacturer.

I also calculated my own pilot arm as I considered the generalities in
the manual to be unacceptable. That can be done with a reasonably
accurate bathroom scale under the tail as the calculation is
independent of weight on the main gear.

Andy (GY)

...."sprung landing gear." That's why I specified axle centers rather than
simply raising the tail. I didn't consider sprung gear and your information
is good on that account.

As to following the manufacturer's recommendations, I'm all for that. My
question should have been "why do they specify such a complicated method for
gliders with unsprung gear" though I didn't mention the suspension part.
BTW, my LAK-17a specifies an angle of 100:2.9.


"Andy" wrote in message
...
On Jan 11, 4:29 pm, "Dan Marotta" wrote:
Why don't the manufactures publish the height to raise the tail as a
difference between the axle center lines above ground? Then you don't need
to cut or calculate.

Oh, gee... That'd be too simple.

One possible reason is that the height is not a constant for any
glider that has a compressible pneumatic tyre or a sprung landing
gear. Both have deflection that depends on the glider mass. That
same deflection would need to be applied to the tail height. It may
not be neglible at max gross wt.

Speaking of sprung landing gear - in some cases the deflection changes
not only the height but also the distance between the tyre/ground
contact point and the datum. That applies to modern Schleicher single
seaters and probably other gliders. The best plan is to do the weight
and ballance as defined by the manufacturer.

I also calculated my own pilot arm as I considered the generalities in
the manual to be unacceptable. That can be done with a reasonably
accurate bathroom scale under the tail as the calculation is
independent of weight on the main gear.

Andy (GY)

On Jan 11, 3:29*pm, "Dan Marotta" wrote:

Why don't the manufactures publish the height to raise the tail as a
difference between the axle center lines above ground? *Then you don't need
to cut or calculate.

Oh, gee... *That'd be too simple.

I specify W&B leveling with a level and wedge on the aft fuselage
because it is the simplest and easiest way of getting the glider
level. You make the wedge and keep it in the glider's toolbox, or you
use a digital level as somebody else suggest. I don't see what's so
hard or complicated about that.

I have actually designed in a couple of internal surfaces that are
parallel with the glider's x axis, but they are in under the wing spar
and you can't see them while actually doing the leveling. With the
level on the aft fuselage, you can actually see it while you are
raising and lowering the tail to find the level.

To specify the level in terms of height of the axles, you have to know
the distances of the axles from the x axis, which is not simple
because the gear might have an oleo strut (as does mine), and you
don't know if the tailwheel location has been changed or modified.

Also, when leveling to the axles, you have to know what you're
leveling to. If you have a hangar or shop floor known to be level,
you're golden. But if you're doing a W&B in the field or on grass or
another uneven surface, then you need to construct a water level or
other surveying tool. By then, the bubble level on the aft fuselage
starts to look pretty good.

Thanks, Bob K.

Thanks for a good technical reply.

Now I just have to check and see if any of my measuring devices have decimal
inch/cm graduations. Seems most are graduated in 1/16th, etc... Let/s
see... That would be 200 inches long and 2 and 14.4/16 inches high. Oh,
crap! There's that pesky decimal again. I know - I'll make my triangle
2,000 inches long and 29 inches high! Now, if I could just find a surface
on the glider where I can make that fit.

Really, how accurately can you measure 2.9 inches, mark it, and cut it? I
don't have a machine shop. This all reminds me of the old Air Force adage:
"Measure with a micrometer, mark with a grease pencil, cut with an axe".

I've been out of school for a long time, so my calculation is probably
wrong, but it looks like 100:2.9 is an angle of 1.6618 degrees. Will a
digital level get that accuracy? Is that accuracy really necessary?

What's the good of a parallel surface if it's not accessible? Why not make,
say, the arm rest parallel to the longitudinal axis? Then you could simply
place a carpenter's level on the arm rest and, voila!

And, BTW, you shouldn't really be doing a weight and balance in a grassy
field, no matter how level it is. The slightest breeze will generate some
measureable amount of lift and throw your weight measurement off. And if
that weight change is not enough to be concerned with, then I challenge the
need to measure the angle of the fuselage to the thousandth of a degree
(measure with a micrometer).

Bottom line - that's what the manufacturer says to do and I'll try my best
to do it that way, but I'm not going to lose any sleep over it.

"Bob Kuykendall" wrote in message
...
On Jan 11, 3:29 pm, "Dan Marotta" wrote:

Why don't the manufactures publish the height to raise the tail as a
difference between the axle center lines above ground? Then you don't need
to cut or calculate.

Oh, gee... That'd be too simple.

I specify W&B leveling with a level and wedge on the aft fuselage
because it is the simplest and easiest way of getting the glider
level. You make the wedge and keep it in the glider's toolbox, or you
use a digital level as somebody else suggest. I don't see what's so
hard or complicated about that.

I have actually designed in a couple of internal surfaces that are
parallel with the glider's x axis, but they are in under the wing spar
and you can't see them while actually doing the leveling. With the
level on the aft fuselage, you can actually see it while you are
raising and lowering the tail to find the level.

To specify the level in terms of height of the axles, you have to know
the distances of the axles from the x axis, which is not simple
because the gear might have an oleo strut (as does mine), and you
don't know if the tailwheel location has been changed or modified.

Also, when leveling to the axles, you have to know what you're
leveling to. If you have a hangar or shop floor known to be level,
you're golden. But if you're doing a W&B in the field or on grass or
another uneven surface, then you need to construct a water level or
other surveying tool. By then, the bubble level on the aft fuselage
starts to look pretty good.

Thanks, Bob K.

On Jan 15, 8:57*am, "Dan Marotta" wrote:

Really, how accurately can you measure 2.9 inches, mark it, and cut it?

I think that just on the far side of 2-7/8" would do just fine.

...it looks like 100:2.9 is an angle of 1.6618 degrees. *Will a
digital level get that accuracy? Is that accuracy really necessary?

Most digital levels will offer repeatable measurements to 0.1 degrees,
and I think that that is close enough. In this case I'd feel fine
about a reading of 1.7 degrees. For my fuselage, the exact tailboom
slope is 1.213 degrees, but 1.2 or even 1-1/4 degrees would be fine.

What's the good of a parallel surface if it's not accessible? *Why not make,
say, the arm rest parallel to the longitudinal axis? *Then you could simply
place a carpenter's level on the arm rest and, voila!

Thanks, the armrest trick is a good idea, I might adopt that; it would
be useful for people who have digital levels that beep when they're
actually level.

Thanks again, Bob K.

At 16:57 15 January 2012, Dan Marotta wrote:
Thanks for a good technical reply.

Now I just have to check and see if any of my measuring devices have
decimal
inch/cm graduations. Seems most are graduated in 1/16th, etc... Let/s
see... That would be 200 inches long and 2 and 14.4/16 inches high. Oh,

crap! There's that pesky decimal again. I know - I'll make my triangle
2,000 inches long and 29 inches high! Now, if I could just find a surface

on the glider where I can make that fit.

Really, how accurately can you measure 2.9 inches, mark it, and cut it? I

don't have a machine shop. This all reminds me of the old Air Force
adage:

Why would you want to use inches specifically? the ratio 2000:29 could be
inches, millimeters, bananas or ay other unit you might wish to use, that
is why it is expressed that way, works whatever system of measurement you
care to use. A distance of 10000mm with a drop of 145mm would seem to me
pretty easy to set up.

As Bob mentioned, building a wedge for your gilder is a good solution the
will provide accurate results time after time. Here is the wedge that I use
for my HP-14. I'm sure the dimensions required to build a similar device
are available for production aircraft.
http://www.soaridaho.com/Schreder/Co...uselage_Level/

Wayne
HP-14 "6F"
http://www.soaridaho.com/Schreder



"Bob Kuykendall" wrote in message
...

On Jan 11, 3:29 pm, "Dan Marotta" wrote:

Why don't the manufactures publish the height to raise the tail as a
difference between the axle center lines above ground? Then you don't
need
to cut or calculate.

Oh, gee... That'd be too simple.

I specify W&B leveling with a level and wedge on the aft fuselage
because it is the simplest and easiest way of getting the glider
level. You make the wedge and keep it in the glider's toolbox, or you
use a digital level as somebody else suggest. I don't see what's so
hard or complicated about that.

I have actually designed in a couple of internal surfaces that are
parallel with the glider's x axis, but they are in under the wing spar
and you can't see them while actually doing the leveling. With the
level on the aft fuselage, you can actually see it while you are
raising and lowering the tail to find the level.

To specify the level in terms of height of the axles, you have to know
the distances of the axles from the x axis, which is not simple
because the gear might have an oleo strut (as does mine), and you
don't know if the tailwheel location has been changed or modified.

Also, when leveling to the axles, you have to know what you're
leveling to. If you have a hangar or shop floor known to be level,
you're golden. But if you're doing a W&B in the field or on grass or
another uneven surface, then you need to construct a water level or
other surveying tool. By then, the bubble level on the aft fuselage
starts to look pretty good.

Thanks, Bob K.