aircraft weighing; reliable or not?

[Steve Pierce]

2.25" looking at my original weight and balance on my Super Cub. 2" on the TCDS, I guess that shows how unimportant it was to Piper.

 

[dgapilot]

2.25" looking at my original weight and balance on my Super Cub. 2" on the TCDS, I guess that shows how unimportant it was to Piper.

Steve, does it change when you put extended gear on? Never worked on a Cub with extended gear, just curious.


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[hotrod180]

Pretty tough to beat using the firewall as a datum,
like most (but not all) of the post-war Cessnas.
Unlike extended gear legs, leading edge cuff or slats, etc--
I don't think I've ever seen a mod that moves the firewall fore or aft.

 

[buckchop]

Soooooo. Did you watch them weight the plane. How was the fuel removed? Did you use the same scales? Had they been calibrated recently? Was the plane weighted in flight level attitude both times? Did you weight the parts you took off and the parts you put on or just go by paperwork? Did they leave something in the plane on last weight in. (Under seat storage/baggageares/pod/seats/map pockets. Unless you have calibrated scales and the same people doing the weighting without you watching who knows. How does it fly? Can you tell the difference between 6 and 12 gal of fuel when flying? Most of the cubs winning Vadez are in the 1200 lb class.
DENNY

so if most cubs are at 1200lbs that compete at valdez, then my modified 160hp, pa12 weighing in at 1217lbs is good???

 

[skywagon8a]

2.25" looking at my original weight and balance on my Super Cub. 2" on the TCDS, I guess that shows how unimportant it was to Piper.

Steve, does it change when you put extended gear on? Never worked on a Cub with extended gear, just curious.

So Dave, Which of the two is correct? The original Piper W&B or the TC? If the original sheet isn't available, I would go with the TC.

 

[dgapilot]

When I do a weighing I’ll usually draw chalk lines on the floor using a plumb Bob squares and even a laser. Then use a tape measure. Once I have the actual I’ll compare to either the TCDS or the W&B report in the flight manual. +- .5 inches isn’t going to make much difference, the biggest issue is not getting the airplane level. If the numbers are way off from the published numbers search out why. Sometimes the tail post was used, sometimes the tailwheel resting on the scale. If the tailwheel, what’s the arch of the spring. Spring arch variations can be on the magnitude of 3” or more. Repairs to the fuselage can cause variations in tube locations and if the firewall tubes are perpendicular to the C/L of the fuselage. I know of oneSC where the lower longerons were replaced about 4 times, the airplane crashed at least 3 times. Last rebuild the flat back (it is a PA-18A was replaced with a conventional PA-18 upper deck. I don’t think there is anything “to print” on that fuselage any more. We didn’t do a weighing, just did calculated W&B on it. It flies fine except it could use a little more up trim. When towing a slow glider even with full up trim you still need to hold a little back pressure.


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[supercrow]

Buckchop: I think your 12 is right in the ballpark. 1100 pound 18s and 12s are nice but there aren’t as many out there as you might think in my opinion anyway. With flaps larger engines and metal props equipped to be used, most are in the 1200 pound range or more some of the nicest flying cubs I know are in the 1200 pound range if they are balanced correctly. I have Weighed some 18s with full greenhouse and large tires that are 1250 or more. Just my two cents.

 

[skywagon8a]

When I do a weighing

I was thinking of using the 2" as a means of leveling.

 

[DENNY]

so if most cubs are at 1200lbs that compete at valdez, then my modified 160hp, pa12 weighing in at 1217lbs is good???

That is a great weight for a Modified 12 especially if you have big wings.
DENNY

 

[The Kid]

I don't understand skywagons questions of 2"? I am using the 2.25" of the wheels aft of the wle per Piper's drawing. But, I am NOT using the 186" aft of the wheels as an arm which is at the rudder vertical post, but rather the axle of the bushwheel, which is an additional 16" aft of the rudder tail post, for the arm as that is where the scales were located making a total of 204.25" aft of the wle. But that is for the cg, of course. I am still concerned of the weight but there is nothing more I can do for that except to get the correct figure as I have a ton of lightweight mods now. Weight increases stall speed which then increases takeoff and landing distances. The whole idea of a super cub is very short take off and landing distance which drew me to the plane initially. But, of course, it's still fun to fly but I'm always looking for funer.

 

[hotrod180]

A buddy of mine was having trouble with leveling his PA22-20.
Prick punch mark was there, but the hole above in the door frame was missing.
His info said the firewall & tailpost should both be plumb,
but they didn't match up.
He finally found a piper drawing that showed a horizontal reference line (HRL),
that's what he used.
Another friend with a heavily modified PA-12 had the same experience,
after talking to my pacer buddy he managed to find a similar piper dwg for the PA12 showing the HRL.
Once you locate the HRL points on the airframe, it's easy to level using a laser (hi-tech), or a water level (old school).

 

[skywagon8a]

I don't understand skywagons questions of 2"? I am using the 2.25" of the wheels aft of the wle per Piper's drawing. But, I am NOT using the 186" aft of the wheels as an arm which is at the rudder vertical post, but rather the axle of the bushwheel, which is an additional 16" aft of the rudder tail post, for the arm as that is where the scales were located making a total of 204.25" aft of the wle. But that is for the cg, of course. I am still concerned of the weight but there is nothing more I can do for that except to get the correct figure as I have a ton of lightweight mods now. Weight increases stall speed which then increases takeoff and landing distances. The whole idea of a super cub is very short take off and landing distance which drew me to the plane initially. But, of course, it's still fun to fly but I'm always looking for funer.

Let me try to explain. There seems to be a disagreement of whether the dimension should be 2" or 2.25", which appears to depend upon which "official" document is used as a reference. My use of 2" is based upon the FAA's TC data sheet where it gives the arm of the main tires as being 2", thus the axle center. This number is used solely as a means of leveling the airplane by dropping a plumb bob from the wing leading edge and measuring back to the axle center line 2". All of the equipment items on the TC 1A2 have an applicable arm published. This arm is applicable when the main tire center line is 2" aft of the WLE datum. If 2.25" is used for the distance, should all of those TC arms be increased by 0.25"?

Not that 1/4" will amount to a hill of beans in the grand scheme of things, it's just a matter of detail to accuracy. Similar to Bob Turner's issue with some using the wing leading edge for the datum and some using 200" ahead of the WLE. When you mix apples and oranges, you eventually get accumulated errors.

When actually weighing the airplane there should be prolific use of plumb bobs and tape measures for all data points. When calculating additions and subtractions, the numbers in the TC ought to be used. Or if all the numbers are listed on Piper original sheets, use them. Don't mix the two.

When all else is done, it is the loaded CG which will dictate the performance characteristics. The CG more so than the weight, though the weight is also important.

 

[KevinJ]

The whole idea of a super cub is very short take off and landing distance which drew me to the plane initially. But, of course, it's still fun to fly but I'm always looking for funer.

Does it feel shorter after taking weight off of it? Take offs are where I feel the weight off of one the most. They feel snappier. Landings seem to feel best with a bunch of lead in the tail.;-)

 

[hotrod180]

.....My use of 2" is based upon the FAA's TC data sheet where it gives the arm of the main tires as being 2", thus the axle center. This number is used solely as a means of leveling the airplane by dropping a plumb bob from the wing leading edge and measuring back to the axle center line 2"....

I disagree.
Fron TCDS 1A2 (for Piper PA18):
"Datum: wing leading edge
Leveling means: plumb from PK screw in upper door frame channel to center punch mark on enclosure door rear hinge"

..... All of the equipment items on the TC 1A2 have an applicable arm published. This arm is applicable when the main tire center line is 2" aft of the WLE datum. If 2.25" is used for the distance, should all of those TC arms be increased by 0.25"?....

I say no, they will not change. MLG wheels can be at 2", minus 2", or wherever-- doesn't matter.
The TCDS calls out the arm of everything from the datum, which is the WLE.

.....When actually weighing the airplane there should be prolific use of plumb bobs and tape measures for all data points. When calculating additions and subtractions, the numbers in the TC ought to be used. Or if all the numbers are listed on Piper original sheets, use them. Don't mix the two.

I definitely agree with this though.

 

[buckchop]

That is a great weight for a Modified 12 especially if you have big wings.
DENNY

Yea 18” extended, alerons moved to the tip, 8’6” flaps, 4 position flap knotch, Charley Centers leading edge cuff, CC wing tips with fence, CC mid wing fence, atlee tie downs, all new wing except spars, but built with Dakota cub parts, my big saving is fabric interior and the Oratex fabric. Still havent installed the vortex generators but least they r powdercoated. Hahahahaaaa

 

[skywagon8a]

I disagree.
Fron TCDS 1A2 (for Piper PA18):
"Datum: wing leading edge
Leveling means: plumb from PK screw in upper door frame channel to center punch mark on enclosure door rear hinge"

When the plane is leveled with the PK screw and the punch mark (if you can find them), the main tire arm is +2" per the TC. Therefore, by dropping a plumb bob from the leading edge and measuring 2" to the main axle.....the plane will be level. PK screw and punch mark not needed.

I say no, they will not change. MLG wheels can be at 2", minus 2", or wherever-- doesn't matter.
The TCDS calls out the arm of everything from the datum, which is the WLE.

All of the arms called out by the TC assume the airplane is level. The main wheels will be at +2" if the plane is level. If another dimension is used, the plane will not be level in accordance with the TC. It would take some complicated (to most) mathematics to figure the difference from the published numbers, which would also necessitate the consideration of the vertical location of the item. Granted, the differences will be slight.

 

[BC12D-4-85]

https://www.faa.gov/sites/faa.gov/files/2023-09/Weight_Balance_Handbook.pdf

Have a look if interested. I did during last winter's flight review and am going to do it again. Chapter 3 describes the actual tech of weighing.

Gary

 

[hotrod180]

When the plane is leveled with the PK screw and the punch mark (if you can find them), the main tire arm is +2" per the TC. Therefore, by dropping a plumb bob from the leading edge and measuring 2" to the main axle.....the plane will be level. PK screw and punch mark not needed......

That's assuming stock leading edge & stock gear.
Both are often modified, as in a cuffed leading edge & extended and/or raked gear.

The nice thing about using the firewall for a datum is that I'm not aware of any mod that moves it fore or aft--
unlike the landing gear or leading edge.
Unlike PK screws & prick punch marks, I've never failed to find the firewall.
And it's super-easy to measure from-- esp inside the fuselage.

 

[skywagon8a]

That's assuming stock leading edge & stock gear.
Both are often modified, as in a cuffed leading edge & extended and/or raked gear.

I always assume an unmodified airplane when using the data on the TC.

The nice thing about using the firewall for a datum is that I'm not aware of any mod that moves it fore or aft--
unlike the landing gear or leading edge.
Unlike PK screws & prick punch marks, I've never failed to find the firewall.
And it's super-easy to measure from-- esp inside the fuselage.

When actually measuring, the firewall is an excellent reference. Take into consideration the thickness of the firewall blanket. Make certain that information is documented in the paperwork you generate and the difference between the firewall and the "approved" datum is noted. Someone later down the road may use the numbers in the TC causing the W&B information to go down hill.

What will you use for the seat, baggage measurements and the cg of the fuel? Those are dependent upon a average center of gravity of a body or baggage. When you change a datum, there are many other changes which must be made and documented.

Yes the Cessna 180 does use the firewall, other planes don't. Keep it simple, rather than confusing the next guy.

 

[dgapilot]

I always assume an unmodified airplane when using the data on the TC.


When actually measuring, the firewall is an excellent reference. Take into consideration the thickness of the firewall blanket. Make certain that information is documented in the paperwork you generate and the difference between the firewall and the "approved" datum is noted. Someone later down the road may use the numbers in the TC causing the W&B information to go down hill.

What will you use for the seat, baggage measurements and the cg of the fuel? Those are dependent upon a average center of gravity of a body or baggage. When you change a datum, there are many other changes which must be made and documented.

Yes the Cessna 180 does use the firewall, other planes don't. Keep it simple, rather than confusing the next guy.

Using the firewall can be handled just like using 60" forward of the LE, convert everything to the station you are using as the datum, or convert everything back to the original wing LE position (or 60" ahead of the LE). It really doesn't matter what you use as a datum as long as ALL other points (including the envelope) are based on THAT datum. As others have stated, make sure your W&B sheet references what datum you used so the next guy can actually figure out what you were doing. Lots of things can trip you up, wing LE cuff or slats, change in AOA (Shouldn't change it much but it will change it), extended gear or raked gear, tailwheel or rudder post, if tailwheel the spring arch will change the location, when going from original brakes to Cleaveland the CG location of the wheel changes due to the brake caliper hanging off the back but the center of the wheel doesn't. In some 50 years of working on airplanes very few have had a "correct" weight and balance. Every airplane I've owned I start with the oldest W&B document and build a new one with an excel spread sheet identifying EVERY change and building a new Equipment List (most people seem to neglect having a current Equipment List which is actually a regulatory requirement).

 

[bob turner]

Plus one. Re-read that one. The only caveat - I would remove the comment about the cleveland caliper. It is true that it slightly changes the aircraft CG, but it might be confusing when it is in between discussing the location of weighing points.

I too rarely see a good weight and balance, and I always have to generate an equipment list.