Calculating CofG for J170C

[J170 Owner]

Okay, I have pilot=80kgs, baggage=10kgs and fuel=70kgs. Using the info in the POH I have 80 x 297 = 23760, 10 x 920 = 9200 and 70 x 451 = 31570. Dividing the sum of the moments by the sum of the weights; 64530/160 = 403.3

 

Now 320kg empty weight + 80kg pilot + 10kg baggage + 70kg fuel = 480kg. The intersection of 480kg and the resultant arm of 403.3 is way, way ouside the CofG envelope. What am I doing wrong???

 

 

[djpacro]

Empty aircraft moment

 

 

[J170 Owner]

Empty aircraft moment

Yes, climbed up on top of the cupboards, dug out my Warrior PA28 books and had a read. Silly me.

 

 

[facthunter]

That could be a senior's moment.Nev

 

 

[J170 Owner]

Okay, now I am including the aircraft moment (after my senior's moment) my calculations are somewhat disturbing. Empty weight 320kg + Pilot 80kg + baggage 20kg + 80L fuel puts me behind the rear limit. Do the calculations for YOUR Jab J170 and see what you come up with. I flew my Jab back from Bundaberge with two POB, full tanks and at least 20kg of baggage in the back but the plane was just fine. This stuff never gave me grief before! The index units for my plane are 75500 or 75 on the chart. Prove me stupid, I dare you....

 

 

[GregF]

Your sneaky missus has chucked in an extra 10Kg of extra baggage

 

 

[J170 Owner]

Your sneaky missus has chucked in an extra 10Kg of extra baggage

The 'GF' won't get in 'that tiny thing'. She said much the same when I flew a Tobago around. Everything is 'tiny' to her, everything.

 

 

[djpacro]

Interesting text from Service Bulletin JSB 029-1:

 

Some J170 aircraft operate quite close to the rear of their CG envelope and in some cases the extra weight added by this new ventral fin will be sufficient for the aircraft to exceed its rear CG limit in some load cases. Operators must calculate the effect that changing this part has had on their aircraft and assess if any ballast or operating changes are required. Depending on the category in which the aircraft is operating these calculations may need to be carried out by an approved Weight Control Authority.

I wonder where they recommend the ballast to be fitted if you don't want to move the engine forward?Sample empty weight index in the POH is 49.5 i.e. sample empty aircraft moment arm of 180 mm is a lot further forward than yours at 236 mm. I'm not going to work through that loading system in the POH, it is so complicated, but just looking at the sample charts there would confirm your problem with the empty cg being so far aft.

 

 

[J170 Owner]

Yes, I have started the 'handling upgrade' and that ugly, ugly ventral fin goes on next week. About 1.5kilos up front needed to bring the CofG forward. Still find it amazing that I could fly with 2 POB and full tanks.

 

 

[J170 Owner]

My plane has a basic empty weight of 320kg and a empty arm of 240mm giving a moment of 76800. The load sheets in the POH use the term aircraft index units and these are (on the chart) 40 to 180. I cannot see what the connection is, what am I missing here? what index units would correspond to my aircraft and how is that determined?

 

 

[djpacro]

Only a guess but I'll bet it's right (you will need to check, it must be in the POH somewhere): divide the moment by 1000.

 

 

[J170 Owner]

Only a guess but I'll bet it's right (you will need to check, it must be in the POH somewhere): divide the moment by 1000.

Yes, divide by 1000 is correct. I downloaded the manual because my copy is (where else) in the plane.

 

 

[J170 Owner]

Your sneaky missus has chucked in an extra 10Kg of extra baggage

I swapped her for a GPS unit zonks ago.