Measurement of CofG

On Thursday, January 12, 2012 12:48:09 PM UTC-5, Cliff Hilty wrote:
I did this with a stop watch and 5 gallon buckets under
the dump valves and had the "Weigher" tell me when I got to gross, stop and
record both time and gallons dumped, then continued to 9 lbs per sq ft,
stop and record, then dumped the rest. This gave me a very accurate amounts
in both gallons and time (seconds to dump for any wing loading I may want
to fly with.

Um, no, dump speed is limited by vent (in flight and on ground).
Lower pressure on vent in flight slower dump time in flight...
Also, for that glider, enlarging vent holes makes it safer
if you fall off a ridge and need to dump fast...

Originally Posted by Dan Marotta

Hey, Rob!

ZAP was my second glider. I flew her between April '88 and Nov '91.

Blast from the past!

Dan,

Great to hear a little history on her!

I'm trying to treat her well and keep her flying down here in the Texas Hill Country outside of Austin.

She went through a rough patch from '94 to '04...didn't fly and trailer leaks caused water damage on the fuse under the dolly which Gehrlein repaired...now only ~1040 TT. Would love to put 50-60 hours on it this season...put about 25 hours on in '11.

Pass along any history to me at aggies78 at gmail dot com.

Rob

...."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)

When we did my ASW-15 we took the weights just balancing the wings
level, no stand. Fore and aft level was done as you describe (my
manual has about the same information as yours) with the exception I
noted about using a digital protractor instead of making the triangle.
As my ship was exported from Germany in 1998 we had copies of the
W&B's they did there for my maintenance engineer to use as a guideline
for making the new W&B and equipment list. The records that
accompanied my ship when it was exported show that the W&B was done
every second year when it was in Germany! I could get you scans of the
forms though being of German origin they would be useful as a
guideline at best.

Sometimes you can get a surprise when doing a W&B. My club has a Grob
102 Standard III. When the A.D. requiring the installation of lead
mass balance in the control system was done we naturally had to
reweigh the glider. The last time it had been weighed was by a
previous owner many years before we bought it. The new weight was
about 80 pounds LESS than the last one. Aircraft generally don't LOSE
weight over the years, especially when you've just bonded a bunch of
lead to them. The maintenance shop double and triple checked their
procedures, recalibrated their scales and reweighed the glider several
times. The new weight was accurate. We're still trying to figure out
what the hell was going on with the previous weighing. Water ballast
left in the tanks maybe?

On Jan 11, 8:51*pm, RAS56 wrote:
Hagbard Celine;807559 Wrote:

If the shop has a digital protractor you could also convert the rise/
run to degrees, put the protractor on the specified part of the tail
boom and raise the tail until it reads the correct angle.

Arctan(rise/run) = angle in degrees

About 2.1 degrees, sloping down towards the tail in this case.

Same question, hopefully same answer...could use a little more info.

I'm trying to put together a W&B event at our club, we're bringing in
outside help from another club with a set of scales and the experience
of knowing what to do.

WRT positioning the glider at the correct angle, my ship is an ASW-19b.
Although my owners manual has a page dedicated to CG info, there's
nothing in there about what procedure to use to actually weigh the
thing. It's just a page with a CG envelope on it.

Now I did find on page 30 of the manual (a page that has general
dimensions as well as specs for control deflections) a side view drawing
with info presented as the thread starter discussed, mine says 1000 by
45.

SO, to do the procedure correctly, construct a small triangle with those
dimensions, inflate the main wheel to proper pressure, put a level on
the triangle, then raise the tail till I get a level bubble? It would
seem to make sense that this is done with wings level laterally as well,
correct? Lightly (fingertips) or is a wingstand under a tip ok? THEN,
take the weights?

Finally, this will be the 4th W&B for this glider. All the ones done
previously have had a "form" with a glider drawing on it and appropriate
spots to fill in main wheel/tail wheel weights, etc. Where can we obtain
"blanks" to fill in our ships for our event? We will have an A&P IA
overseeing the process and signing off logbooks. As I said, I'm
coordinating, so I'm trying to get all the info/materials each owner
will need to have on hand at weigh-in so the event goes smoothly.

Thanks much,

Rob
ASW-19b
ZAP

Ps-sorry if this double posts...

--
RAS56

At 22:00 12 January 2012, Dave Nadler wrote:
On Thursday, January 12, 2012 12:48:09 PM UTC-5, Cliff Hilty wrote:
I did this with a stop watch and 5 gallon buckets under
the dump valves and had the "Weigher" tell me when I got to gross,
stop
and
record both time and gallons dumped, then continued to 9 lbs per sq
ft,
stop and record, then dumped the rest. This gave me a very accurate
amounts
in both gallons and time (seconds to dump for any wing loading I may
want
to fly with.

Um, no, dump speed is limited by vent (in flight and on ground).
Lower pressure on vent in flight slower dump time in flight...
Also, for that glider, enlarging vent holes makes it safer
if you fall off a ridge and need to dump fast...


Dave you missed my point any glider that you fly it will work for on the

ground to get to your desired take off weight. No water meters ect for
calcs also I use a 3 gallon bucket and time the fill rate to decide how
much to put in each wing to get to what I want. 9 gallons per wing works
for me and its easy and quick to do once you have the initial weight snd
balance.

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.

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.

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.

I like the idea of a "beeping" level which would allow a single person to
complete the operation. And I *really* like the design of Wayne's "wedge".

Bob, thanks for stating what I've always felt, i.e., the TLAR method is good
enough (2-7/8"). I was (wrongly) getting the impression that people were
stuck on precision which I couldn't attain. It would seem pointless to
measure the angle to a gnat's ass and then fly with boots and a heavy jacket
one day and shorts and sneakers the next.

One more time - Wayne, I LIKE the design of your wedge. I think I'll build
one. And ask the manufacturer why they don't include at least a drawing for
a device to level the fuselage.


"Bob Kuykendall" wrote in message
...
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.