Measurements for calculating CG on 172M

B

blue@172

Filing Flight Plan
Joined
Dec 9, 2020
Messages
15
Display Name
blue@172
Apologies if this question is too obvious..
In the POH section on airplane weighing procedures, the equation to determine the CG uses the empty weight combined with measurements (A) between firewall front face & main wheel centers and (B) between nose wheel center & main wheel centers. Is there a document somewhere that specifies these measurements for a Cessna 172 (M specifically)?



6-4.jpg
 
Don’t you have a W&B in your plane’s documents? It should show the arms you’re looking for.
 
IIRC you have to measure after leveling. The measurements change as the

nose strut telescopes and the main gear flexes. IIRC you have to raise the

main gear some when leveling the aircraft.
 
blue@172 said:
Is there a document somewhere that specifies these measurements for a Cessna 172 (M specifically)?
Click to expand...
The Cessna service manual usually has a station diagram in Section 1 with some reference measurements. Or you can simply measure those points yourself from the Datum, in inches, which is the lower part of the firewall face. Just remember measurements going aft are positive and measurements going forward of the Datum are negative.
 
The page of the POH just before the page with that Figure 6-1 "Sample airplane Weighing," has the leveling and weighing procedure spelled out. It does not list actual measurements, evidently (just a guess) those distances may be somewhat variable.

Edit: Attached are the appropriate 2 pages from the 172M POH. As near as I can tell, that info is not in the service manual.
 

Attachments

Last edited:
You have to do the measurements yourself. There is enough variability that Cessna doesn't give those measurements. Yes, it's a pain, but so is the rest of the W&B if you're going to do it right.

Level the airplane as per service manual. It will look weird, tail really high. Nose strut usually collapsed, even some air out of the nosewheel tire, maybe. Mains might need blocking up. Plumb bob off the lower firewall to just above the floor, a length of lockwire stretched tight across the main legs and centered on the axle centerlines. Measure between the plumb bob string and the nosewheel's axle center, and between the string and that lockwire. A taildragger is more hassle, since the tailwheel will be on a stand on the scale, so you need a plumb line from the fuselage somewhere ahead of the stand and then measure between the tailwheel axle and string, and between string and main gear wire, and add them up.

Make sure all non-essential stuff is out of the airplane. Make sure the doors are shut. Make sure the fuel tanks are either empty or full. Make sure the oil is at full.
 
Oil full? Not for a CAR3 airplane. Oil should be drained. If not, oil weight must be deducted.

The airplane is required to carry a W&B to be airworthy. Look at it to see how the empty CG was calculated. If you think the arms are incorrect? Level the plane (empty, in weighing configuration) and check them. An owner can’t modify the W&B, so if you think it’s wrong? Talk to a mechanic about getting it corrected.
 
Nope. Give stations but not wheel stations that you have to measure.

As weight increase the nose strut compresses and moves aft while the

main gear flexes forward.

It’s not “ roll on- roll off”.

I’ve had to block the mains like Dan said on the M’s and N’s I’ve done.

Also note comment on Unusable Fuel which is in TCDS also IIRC.

Bets are being taken on amount of weight increase?
 
Stewartb said:
Oil full? Not for a CAR3 airplane. Oil should be drained. If not, oil weight must be deducted.
Click to expand...
From the 172 TCDS:

1754944233332.png
So, for serial numbers 17262578, 17261445, and 17274009 and on, full oil is required. The OP's airplane is a 172M, and the 172M serial number range is given as:

1754944858003.png

The full oil also applies to the 172P and Q:
1754944261033.png

Now, for unusable fuel, you need either full tanks or totally empty tanks so that total fuel minus unusable can be subtracted from full tanks, or unusable added to empty tanks. And we also need to remember that unusable fuel is NOT the fuel left in the system after draining at the strainer. It is as given in the TCDS and reflects that fuel which won't flow from the tanks when the airplane is in certain attitudes such as a Vx climb or power-off, full flap glide.

We get the same info in a different form from the 172M POH:

1754945675673.png
 

Attachments

  • 1754944411781.png
    1754944411781.png
    13.9 KB · Views: 5
Last edited:
IIRC Cessna wants you to remove tank drains and then add Unusable Fuel per TCDS.

Unless specified like above I’ve drain at the carb with aircraft level..
 
Magman said:
IIRC Cessna wants you to remove tank drains and then add Unusable Fuel per TCDS.

Unless specified like above I’ve drain at the carb with aircraft level..
Click to expand...
The Service manuals tell you to drain the system by removing the tank quick-drains AND the plug at the bottom of the fuel selector. But most of those plugs have never been removed since the airplanes were built and aren't about to come out easily now. I've had to take the selector out and take it apart and then fight that plug out. Of course, the tanks have to be drained first. It would have been nice if Cessna had installed shutoffs at the tank outlets, lockwired open, for stuff like that.
 
I bought a 9/32 square socket to take those plugs out.

Of course it doesn’t matter if there was a failed attempt with vise grips!
 
You have to measure those distances on the airplane being weighed. They can be different on each airplane. And the distances affect the calculated CG. Which you really want to be accurate.
 
blue@172 said:
Apologies if this question is too obvious..
In the POH section on airplane weighing procedures, the equation to determine the CG uses the empty weight combined with measurements (A) between firewall front face & main wheel centers and (B) between nose wheel center & main wheel centers. Is there a document somewhere that specifies these measurements for a Cessna 172 (M specifically)?



View attachment 143924
Click to expand...
You are comparing data from a 172RG with a 172M? You do realize the CG in a 172RG moves about 2” aft when the gear is retracted?
 
Same thing. Only different.

Maybe the RG POH was convenient for illustration purposes only?
 
You must log in or register to reply here.
Back
Top