Today's Message Index:
----------------------
1. 03:36 AM - Piet Project on eBay (Jack)
2. 06:08 PM - W/B QUESTION (Oscar Zuniga)
3. 08:09 PM - Re: W/B QUESTION (Ryan Mueller)
Message 1
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Subject: | Piet Project on eBay |
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=390348679103&s
sPageName=ADME:B:SS:MOTORS:1123
Jack
DSM
Message 2
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Chris; I'll pass along a few comments from when I have done the W&B on
my airplane (once after the rebuild, the second time after the engine
change). I go with what others have said... include oil and unusable
fuel in the basic empty weight. However, those are easy enough to
add and deduct. And use a W&B spreadsheet... there are several of
them available from the archives or just ask on this list and you'll
get one or more. Makes doing "what-ifs" much quicker and easier.
So I used the wing leading edge as the datum since the usual CG limits
are stated as a range that is a percentage of the wing chord (60" chord in
the case of our wings). I started out by setting the fuselage level
and stable, wheels blocked or chocked, tailwheel elevated on a steady
stand. I placed a level on the cockpit longeron (actually on the underside
of the longeron), clamped in place, and leveled the fuselage. That's very
important.
I took a line and plumb bob and dropped them over the leading edge of
the wing right where the centersection and wing panel join, but really
anyplace convenient where the plumb bob can drop down to just at the
floor. Let it stop moving and mark that as the reference against
which everything else is measured.
I laid a line of tape down from the center of the contact patch of one
tire across to the other tire as the line that represents the mains.
I dropped a plumb bob from the center of the tailwheel down to the floor
and marked that spot, and also dropped a plumb bob from the center of
the firewall down to the floor and marked that spot, then connected those
with a line of tape representing the long axis of the airplane. you
could also do it from the center of the prop, if you don't have any offset
in your engine thrust line. Now you have everything you need to measure
things from, and you should write down all the essential dimensions
because you'll need them when you go to weigh the airplane. Tailwheel
from datum, mains from datum, fuel tank from datum, passenger and pilot
from datum (although you can determine the latter three by weighing
later).
While the airplane is in the level position you can also measure the
angle of your cabanes and other stuff. You can measure the approximate
wheelbase, but that will change after you install the wings and the
springs or bungees are compressed. You don't need that value for
W&B though.
Knowing where your main gear axle centerline falls relative to the datum
is very useful in light of the recent articles that William Wynne wrote
in the newsletter (thanks, Ryan and others, too). This is a good time
to make adjustments if they are needed, but you'll know more as you
begin to add the wings and accessories and you determine where your
final CG is.
Oscar Zuniga
Air Camper NX41CC "Scout"
Flying Squirrel N2069Z "Rocket"
Medford, OR
website at http://www.flysquirrel.net
Message 3
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Subject: | Re: W/B QUESTION |
Agreed with Oscar on using the leading edge of the wing as the datum, due to
the fact that the Piet is rather unique in that you can shift the wing fore
and aft to adjust the CG (and which number crunching shows is more effective
than hanging the engine farther out front). After you do your W&B (or even
preliminary ones) you find that your CG is too far aft, you can easily
calculate how much farther aft the wing needs to move to bring it within the
chord CG limits in the most adverse scenario. As shown in the article (#2)
the CG and weight of a covered wing is roughly known....you could always
weigh your wing if that gives you more peace of mind, the CG should not
really be that different either way. Just subtract it from the W&B of the
entire airplane, adjust the CG of the wingless airplane in inches based on
how much you want to move the wing, then add the wing back in. You will
arrive at the new CG with your wing notionally relocated.
Might be clear as mud....if you don't have the issues, I would send Dee and
Doc a few bucks for the W&B back issues...I think William did a pretty good
job of making them rather informative. Thanks Oscar,
Ryan
On Wed, Sep 21, 2011 at 8:05 PM, Oscar Zuniga <taildrags@hotmail.com> wrote:
>
>
> Chris; I'll pass along a few comments from when I have done the W&B on
> my airplane (once after the rebuild, the second time after the engine
> change). I go with what others have said... include oil and unusable
> fuel in the basic empty weight. However, those are easy enough to
> add and deduct. And use a W&B spreadsheet... there are several of
> them available from the archives or just ask on this list and you'll
> get one or more. Makes doing "what-ifs" much quicker and easier.
>
> So I used the wing leading edge as the datum since the usual CG limits
> are stated as a range that is a percentage of the wing chord (60" chord in
> the case of our wings). I started out by setting the fuselage level
> and stable, wheels blocked or chocked, tailwheel elevated on a steady
> stand. I placed a level on the cockpit longeron (actually on the underside
> of the longeron), clamped in place, and leveled the fuselage. That's very
> important.
>
> I took a line and plumb bob and dropped them over the leading edge of
> the wing right where the centersection and wing panel join, but really
> anyplace convenient where the plumb bob can drop down to just at the
> floor. Let it stop moving and mark that as the reference against
> which everything else is measured.
>
> I laid a line of tape down from the center of the contact patch of one
> tire across to the other tire as the line that represents the mains.
> I dropped a plumb bob from the center of the tailwheel down to the floor
> and marked that spot, and also dropped a plumb bob from the center of
> the firewall down to the floor and marked that spot, then connected those
> with a line of tape representing the long axis of the airplane. you
> could also do it from the center of the prop, if you don't have any offset
> in your engine thrust line. Now you have everything you need to measure
> things from, and you should write down all the essential dimensions
> because you'll need them when you go to weigh the airplane. Tailwheel
> from datum, mains from datum, fuel tank from datum, passenger and pilot
> from datum (although you can determine the latter three by weighing
> later).
>
> While the airplane is in the level position you can also measure the
> angle of your cabanes and other stuff. You can measure the approximate
> wheelbase, but that will change after you install the wings and the
> springs or bungees are compressed. You don't need that value for
> W&B though.
>
> Knowing where your main gear axle centerline falls relative to the datum
> is very useful in light of the recent articles that William Wynne wrote
> in the newsletter (thanks, Ryan and others, too). This is a good time
> to make adjustments if they are needed, but you'll know more as you
> begin to add the wings and accessories and you determine where your
> final CG is.
>
>
> Oscar Zuniga
> Air Camper NX41CC "Scout"
> Flying Squirrel N2069Z "Rocket"
> Medford, OR
> website at http://www.flysquirrel.net
>
>
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