# Calculating the moment for a new piece of equipment

Discussion in 'Maintenance Bay' started by Lisa J., Jan 5, 2022.

1. ### Lisa J.Filing Flight Plan

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I am in the process of upgrading the propeller on my Cessna. The new prop will obviously be in the same spot as the old one but it is 22.5 pounds heavier. I have the "arm" length and the weight of the new prop assembly but how do I calculate the moment of that new piece of equipment? Then how to I change the weight and balance of the airplane official W&B sheet. Yes I will have an IA involved in all this but I need to do the calculations first to even see if this modification is worthwhile.
Thanks so much for your help.

Lisa

2. ### Capt. Geoffrey ThorpeTouchdown! Greaser!PoA Supporter

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Moment is arm times weight. Add (edit: Subtract) 22.5 time the arm to the aircraft empty moment. Note: You may need to divide by 1000 to get numbers that match the book...

This may help, but it's my video, so don't watch if that sort of thing offends you.

Last edited: Jan 5, 2022
3. ### Bell206Final Approach

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You multiple the arm by the weight to get the moment. But whether the number is positive or negative depends on where your aircraft datum is. And there is also the weight of the existing prop to include into the new empty weight calculation. So perhaps post the model aircraft and the figures of the current prop and we can direct you in the right direction.

4. ### Greg BockelmanTouchdown! Greaser!

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Weight times arm equals moment. The prop is 38.8 inches forward of the datum on a 172 . Assuming this is a 172, take the weight of the old prop times 38.8 and that gives you the moment to subtract. Weight of new prop times 38.8 is the moment to add. Simple as that. The difference should be about 854

5. ### murpheyTouchdown! Greaser!PoA Supporter

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6. ### MagmanLine Up and Wait

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The Arm is the distance from the Datum to the Center of Gravity of an item.

Assuming both props have identical hub thickness then they should have the same

Arm.

Moment =Weight x Arm

You can do this several ways.

Aircraft. Aircraft. Aircraft
Empty Weight. X. C.G. Total Moment

- Orig.Prop Wt x + Arm = - Moment. A
Or. C
+ New Prop wt. x + Arm. = + Moment. B

___________________________________________

New Empty New New Moment
Weight. C.G.

You can also use the weight change x Arm to simplify calculations as in C

7. ### Cap'n JackFinal Approach

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You have the answers you need- now welcome to PoA!
What Cessna? A propeller 22.5 pounds heavier would be quite significant for the ones I fly.

8. ### Lisa J.Filing Flight Plan

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I have a Cessna R172K Hawk XPII. This is the 210hp IO-360KB Continental engine. This airplane is nose heavy from the factory with that big engine in it and constant speed prop. I am "considering" the Hartzell three blade top prop. But the true issue is what will this do to my CG and the loading of this airplane. I've owned this airplane for 40 years and am well used to it and it's CG issues and putting 22.5 pounds extra way out on the nose may be an issue and I want to see the numbers. So I'm working the math as we speak.

Thank you all for taking your time with your responses here. I so appreciate it. I'll let you know what I come up with. But now you know what I'm contemplating.

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9. ### sourdough44En-Route

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If less weight would help, any consideration if an MT composite prop is an option? Not sure it’s available for the aircraft, likely cost more, then availability issues. If already nose heavy, I’d at least look into it.

10. ### midlifeflyerTouchdown! Greaser!

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If there is an STC applicable to the XP, perhaps it has some of the information you are looking for. Might even think in terms of rear ballast to offset the extra weight up front. I knew people who did that. The non-derated 210 HP engine, even with the time limit for max HP, can certainly handle the extra weight (I loved flying the XP in the Rockies).

11. ### Lisa J.Filing Flight Plan

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This is the answer: Moment =Weight x Arm

So with that information I was able to figure the moment for that prop and it's a negative number as it's forward of the datum.
Doing the calculations with two people in the front seats and full fuel the airplane would only have enough capacity left for 153 pounds in the rear seat or baggage compartment and this would have to be used for rear ballast to offset the prop. So for me this makes this project not worth the expense. There is nothing wrong with the stock McCauley that is on it now and it works fine. If nothing else this got me to learn the math instead of just using the factory loading graphs. Interesting idea but truly isn't feasible in all practical matters.

Thank you all so much for taking your time to weigh in on this subject and help out. PoA is an amazing forum!

Regards,

Lisa

P.S. Yes the Hawk XP R172K is a fun airplane to fly as it truly has the power for flying high. When I needed a rating to fly a 182 for CAP it was an easy transition as the XP is just a little 182.

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12. ### PugsCleared for TakeoffPoA Supporter

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Have about 30 hours of Hawk XP time and really liked it! It did fly a bit better with a sandbag in the baggage compartment though!

13. ### Lisa J.Filing Flight Plan

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The plane truly loves a back seat passenger or some baggage/ballast. Nose up trim works good too. it's definitely not a regular 172 flying.

14. ### PugsCleared for TakeoffPoA Supporter

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Yep. As you said, made my 182 transition easy.

15. ### pfarberPre-takeoff checklist

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Forward of the datum means a negative arm. Get your signs right or the numbers will be off.

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16. ### MagmanLine Up and Wait

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With any W&B it’s not a bad idea to sort of step back and see if your result

is what the change should be.

Moving a light item a couple inches should not result in the EWCG moving 4 inches.

A misplaced decimal point can wreak havoc will calculations.

It’s not a bad idea to check your aircraft W&B from year 1 or last weighing.

There are a few I have found that were really scary.

The dirty secret is NOBODY checks the person that computed the Report.

17. ### Jim CarpenterPre-takeoff checklist

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The Hawk XP POH I have for a 1977 with a 195hp engine shows the prop at an Arm of -41.0. Look in the equipment list table of Ch. 6 Weight and Balance for the arm for your model. The new prop would presumably mount at the same arm station as the original prop, but if not, that info should be in the install documents of the new prop.

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18. ### SkyDog58Touchdown! Greaser!

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Just some questions regarding the portion that I bolded.

You say that the new prop would only leave you with 153 pounds for the rear seat with full fuel and two up front. So currently, you only have 175.5 pounds available under the same loading? That seems to be a little on the low side for the XP, though I do not have a POH in front of me.

Next, the way I read what you wrote, you say that the 153 pounds would be at least partially used up as ballast. But I would think that if you did normally have ballast back there but then wanted to take another person or gear for a trip that you would remove the ballast for that trip as your extra pax or gear would take the place of your ballast. No need for both.

Am I missing something?

19. ### MagmanLine Up and Wait

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Maybe? If not?

Might be!

The Arm is not at the point a unit attaches.

The Arm is from the Datum to the unit Center of Gravity.

A different configuration to a Prop ( or Transponder , ELT OR ?) may cause the Arm

to vary an inch or two.

A good example of this is when upgrading Starters.

The Starter folks do include this in their instructions.

My recall is Cessna generally uses the Lower Firewall as the Datum.

The distance from that point TO THE CENTER OF GRAVITY OF THE PROP is the Arm

and in this case would be negative .

This in turn gives you a Negative Moment and provides a Nose Heavy Tendency.

20. ### SnowmassLine Up and Wait

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It is very unlikely that a 22 pound heavier prop will be a problem unless you already have an inability to raise nose enough for a power off stall. The effect of moving the CG forward yields a bit greater pitch stability and a minuscule increase in drag. carrying 20# in the baggage compartment would probably offset the heavier prop if needed. Or maybe a 5# weight in the tail. The math in the posts tells you how to calculate (even for an American).