Head-scratcher New W&B

ARFlyer

ARFlyer

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Just got all my paperwork from our engine swap and new JPI. Upon loading in the new W&B to ForeFlight I discovered it’s impossible for me to fly with a passenger without 125lbs of ballast in the rear baggage. I have never heard of a 172 that could only carry basically one person.

Our new weight is 1616lbs with a CG of 36.93” vs our old weight of 1561lbs @ 40.68”. The new CG seems very far forward. The MAX weight of two people up front is 190lbs a piece to be able to fly without ballast.

Attached is the FB screenshots. The red is the new and the green is the old data.

Something just seems off unless we really do have a extremely nose heavy 172 now.
 

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I have a hard time believing that a JPI install added that much weight. Seems like a computation error was made somewhere?

Did the airplane get weighed?
 
Do it on paper.

Foreflight's computations are only as good as the background mumbo-jumbo it uses.
 
ARFlyer said:
Just got all my paperwork from our engine swap and new JPI.
Click to expand...

What engine did you swap from and to? A 55 pound gain seems like a lot for a (more or less) straight remove/replace swap, even if you added an engine monitor.
 
If the aircraft had not been weighed since new 55lbs is not unusual. I would however get the aircraft weighed on a good set of certified scales.
 
Moving the cg 4.5” is a mistake. Either someone did the math wrong, or someone made a horrible mistake in deciding to install too heavy an engine.
 
I dont remember my mistake exactly, but when I re-weighed my 150 last year for the first time in 30 years, I had a similar issue. It turned out to be a math error. Mine gained about 14 pounds of mystery weight over the years, but luckily we were removing 20+ pounds of extra junk and only adding a 10 pound child seat.

If the new weighing was done from scratch with scales, go over all the numbers and re-do the math if they are available. If not, looks like you should ask the AP to do another weigh and include all the data.
 
According to the numbers, you added 55 pounds at an arm of 18.6 inches. Does that make sense?

What were the actual weights at each wheel (or the numbers from the W&B calculations)? Easy enough to check the math.
 
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In our '57 172 we carry about 100 pounds of ballast unless we have someone in the back seat. Otherwise like you we are out of the envelope on the front side. Seems crazy toting 100 pounds of rocks wherever you go. Wish I could put less weight farther back. If I was a typical FAA 170 pounder it wouldn't be as big an issue. We also don't typically sit a little farther back for leg room increasing the arm which also helps a little.
 
What model 172? Is your front seat CG variable? What measurement are you using? In average position anything you add to the airplane should move CG aft. Something’s wrong with your load settings. It’s been a long time since I owned a Hawk XP but even with that motor and CS prop I don’t recall and CG issues.

Did you watch the weighing? Was the airplane leveled properly?
 
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DFH65 said:
In our '57 172 we carry about 100 pounds of ballast unless we have someone in the back seat. Otherwise like you we are out of the envelope on the front side. Seems crazy toting 100 pounds of rocks wherever you go. Wish I could put less weight farther back. If I was a typical FAA 170 pounder it wouldn't be as big an issue. We also don't typically sit a little farther back for leg room increasing the arm which also helps a little.
Click to expand...
Is there an STC to put the battery back there? That would help a lot.
 
kyleb said:
What engine did you swap from and to? A 55 pound gain seems like a lot for a (more or less) straight remove/replace swap, even if you added an engine monitor.
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mondtster said:
I have a hard time believing that a JPI install added that much weight. Seems like a computation error was made somewhere?

Did the airplane get weighed?
Click to expand...
Capt. Geoffrey Thorpe said:
According to the numbers, you added 55 pounds at an arm of 18.6 inches. Does that make sense?

What were the actual weights at each wheel (or the numbers from the W&B calculations)? Easy enough to check the math.
Click to expand...


It was a 360 swap with a 0 time 360. The only stuff removed was the engine gauges and a JPI installed. I attached the W&B sheet.
 

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Stewartb said:
What model 172? Is your front seat CG variable? What measurement are you using? In average position anything you add to the airplane should move CG aft. Something’s wrong with your load settings. It’s been a long time since I owned a Hawk XP but even with that motor and CS prop I don’t recall and CG issues.

Did you watch the weighing? Was the airplane leveled properly?
Click to expand...

Ours is a N model with the 180 STC since ‘01
 
Might have been wrong previously. When I go through old W&B sheets I saw all kinds of crazy changes that shouldn't have been.
 
get the plane weighed. last time i put in some new stuff and took out a crap load of old ones, she gained for 25 lbs ... those calculations over the years are not accurate
 
Maybe an error by the AP in calculating the empty CG. Arithmetic might not be their strong suit.

- Subtract wheel location from datum plane location, repeat for three wheels.
- Multiply three pairs of numbers.
- Add those three products.
- Divide that sum by the sum of three other numbers.

Ask a random person to do all that. Most would panic. An AP isn’t much different.
 
ARFlyer said:
Ours is a N model with the 180 STC since ‘01
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You have the heaviest O-360 C172N I have heard of. My empty weight is 1478 @ 38.6. The other O-360 C172N's I know of are 1504 @ 37.46 and 1512 @ 37.95 respectively. Something sure seems off. Were the fuel tanks empty when it was weighed?
 
-8 on the nose wheel seems kinda far up there. Every plane is different, so it might be right. They have to weigh the plane at a certain level, and the arms of the wheels are documented for your plane as well. I’d make sure it was done correctly with the corr3ct numbers for your make and model.
 
So it gained 50 pounds. That weight would have to be added at a location 76" (six feet) in front of the datum, which I believe is the firewall, in order to shift the CG as much as it did. That seems unlikely.
 
The moment column is labeled moment/100 - except that they values printed for the wheels is the actual moment (not divided by 100) - which would be OK except that the fuel correction is divided by 100 to get 11.04 instead of the 1104 which would be consistent with the other moments calculated. That gives you an inaccurate total moment (should be 60,775 if not divided by 100 ) and thus the CG is wrong (37.6 using the numbers given).

I assume the tanks were totally empty when weighed? That's why the addition of 4 gallons.
 
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NoHeat said:
Maybe an error by the AP in calculating the empty CG. Arithmetic might not be their strong suit.

- Subtract wheel location from datum plane location, repeat for three wheels.
- Multiply three pairs of numbers.
- Add those three products.
- Divide that sum by the sum of three other numbers.

Ask a random person to do all that. Most would panic. An AP isn’t much different.
Click to expand...
I’ve also seen a couple of occasions where they rolled the airplane on scales but used the arm for the jack points or the gear attach point instead of the wheel location.
 
Capt. Geoffrey Thorpe said:
The moment column is labeled moment/100 - except that they values printed for the wheels is the actual moment (not divided by 100) - which would be OK except that the fuel correction is divided by 100 to get 11.04 instead of the 1104 which would be consistent with the other moments calculated. That gives you an inaccurate total moment (should be 60,775 if not divided by 100 ) and thus the CG is wrong (37.6 using the numbers given).

I assume the tanks were totally empty when weighed? That's why the addition of 4 gallons.
Click to expand...
I also find it interesting that the weight on both mains was identical…I smell a shortcut/lie.
 
I have a report from a local 172P with a recent Airplains 180 conversion, weighed recently at 1583 at 39.05"
 
bnt83 said:
View attachment 97612
Click to expand...

Yes! Bnt83 found a big error.

For the moments, it appears that the AP goofed by dividing by 100 for the airframe, but not similarly dividing by 100 for the unusable fuel.

Then they added those two numbers, which did not have the same units. Bad stuff, mixing the units.

That goof results in an error of about 0.6 inch in the empty CG, by my mental calculation. Correcting that error puts the empty CG pretty close to the previous value.

The AP might have made other errors as well, but that mixup in units is a whopper that mostly explains the ridiculously big shift in empty CG.
 
idahoflier said:
You have the heaviest O-360 C172N I have heard of. My empty weight is 1478 @ 38.6. The other O-360 C172N's I know of are 1504 @ 37.46 and 1512 @ 37.95 respectively. Something sure seems off. Were the fuel tanks empty when it was weighed?
Click to expand...
It does seem like the OP empty weight was more in the 182 category
 
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