ARM from MOMENT?

J

JasonM

Pattern Altitude
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JM
I have an Ipad app I am trying to setup for weight and balance and it asks for the ARM for everything, but the POH for the 172N I am flying shows only specs for MOMENT.

Is there an easy way to calculate/get the ARM if I have the moment? Feel like I should know how to do this :mad2:
 
Moment = Arm * weight
Arm = Moment / Weight.
Weight = Moment/Arm (Just to be complete)
 
Weight X Arm = Moment

Moment / Weight = Arm

Moment / Arm = Weight

M
----
W A

The good old WAM formula
 
JasonM said:
Is there an easy way to calculate/get the ARM if I have the moment?
Click to expand...

Nope, gotta have 2 out of 3, can't solve an equation with only 1 known value.
 
Thanks for that info. I took that info and looked in the POH again, then noticed I only have the moment when using the weight of the fuel and other factors to get the moment.

for example, How would I get the arm of the fuel tanks when empty?

So if ARM = MOMENT / WEIGHT, so if there is no calculated moment, but a weight of 2300, that wouldn't get me what I am looking for.

I must be approaching this the wrong way or missing something.
 
Hi JM.

I think you are missing something. I don't understand your question :(.
 
AuntPeggy said:
Look at the illustration 6-3 on page 6-6 of your POH.
http://www.scribd.com/doc/3801614/C172N-POH
Click to expand...

Well that helps for most of what I am after.:) Cant believe I missed that.:mad2:

It still doesn't show me the fuel ARM. How can I get that value? I didn't want to just search the internet because of how important weight and balance is and I figured I should go strictly by the POH.
 
somorris said:
Hi JM.

I think you are missing something. I don't understand your question :(.
Click to expand...


The weight and balance app setup asks for this.

Fuel ARM = ??

Pilot ARM = ??

Rear Passenger ARM = ??

That is what brought this up. AuntPeggy helped me with the others by pointing out that I missed that ARM Loading Arrangements chart, but I am still stuck with the Fuel ARM.
 
JasonM said:
Well that helps for most of what I am after.:) Cant believe I missed that.:mad2:

It still doesn't show me the fuel ARM. How can I get that value? I didn't want to just search the internet because of how important weight and balance is and I figured I should go strictly by the POH.
Click to expand...

Figure 6-6 and compute it from what you see there.
 
Edit: ^^^^ Or that works too.

Well, from that link, you have the following (figure 6-5):

W = 240lbs
M/1000 = 11.5

Using WAM - you can figure that one out:
A = 11500 / 240
A = 47.92
 
Last edited:
The only reason you need the ARM is to calculate the Moment. If you already have the fuel moment, you know the fuel weight, why is the ARM important. You are looking for maximum weight and center of gravity, which is calculated after you have all the moments calculated.
 
somorris said:
The only reason you need the ARM is to calculate the Moment. If you already have the fuel moment, you know the fuel weight, why is the ARM important. You are looking for maximum weight and center of gravity, which is calculated after you have all the moments calculated.
Click to expand...

Except when you need the arm. A good example of when you need the arm is detailed above, in the numerous posts you must have skipped to come tell the OP he doesn't need the arm.

:rolleyes:

A hint: If you know the weight, but not the arm or moment, but you do know the the moment for a different weight, you can figure out the arm, and then figure out the moment for the weight you need. AKA:

2x + 3y = 4z

Solve for x.
 
I did miss it. Thank you for clarifying.
 
SkyHog said:
Edit: ^^^^ Or that works too.

Well, from that link, you have the following (figure 6-5):

W = 240lbs
M/1000 = 11.5

Using WAM - you can figure that one out:
A = 11500 / 240
A = 47.92
Click to expand...

That makes sense. :yes: Thank you. Thanks everyone else as well :D
 
JasonM said:
Would this be trusted as much as the POH? That was my main concern in looking outside of the POH. I just didnt feel comfortable getting data outside of the actual POH that came with the airplane.
Click to expand...

The TCDS, while not commonly taught as a reference point for such questions, is an excellent resource for this. Its what the FAA uses to certify Airworthiness of a type.
 
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