The International Cessna^® 170 Association

Good Afternoon Everyone,

I'm having some W & B issues and perhaps you all can assist. Today I was finishing by bi-annual / re-current training and we were calculating W & B in assorted configurations. The current W & B for my plane (1952 B Model) is as follows:

Empty Weight 1383.87 lbs.
Moment is 58903.45 in. lbs.
Index is 42.57

Now with me at 170 lbs and full fuel, I'm at 1790 lbs. with an index of 59.10. This puts me outside (FWD) of the CG limits.

Myself and one other front passenger at 260 lbs, full fuel and I’m at 2050.87 lbs. With an index of 68.54 – again, outside (FWD of the CG limits.

Some observations – It is always outside in the FWD area and when compared to 3 other (yes I know it’s different but just as a comparison) 172s that seem to be about the same empty weight with indexes running from about 48.5 to 57.8. Is that because they are tricycle gear and the CG is further back on that type of plane?

Right now I’m on the road and can’t look at the W & B history until I get back but I have to be a little suspect of my index at 42.57. If it was in the 48 to 52 range, all the examples / weight combinations I tried would have worked or have been right on the line.

Just can’t believe just me at 170 lbs and full fuel is out of balance in this plane, especially since the trim in that configuration is almost on the neutral mark during hands off level flight.


Any thoughts? What kind of numbers are you guys getting?


Kyle Takakjian

Me thinks your math is wrong. According to my 170 W&B calculator your numbers with you and full fuel would be:

Wght 1820.57
CG 42,16
Moment 76759.5

Well within the CG range.

BTW my airplane is more nose heavy than yours with the following numbers as weighed.
1317.90
38.18
50321.42

I just sent you via email my Excel program. Have fun with it

Kyle,

You might be confusing index with the cg location. Index is simply moment/1000. The cg is the total moment (index times 1000) divided by the total weight. Using your empty airplane numbers for example, the index is actually approximately 58.9. It is the cg that is 42.57 inches (behind the plane of the firewall).

Miles

Kyle - a forward CG when loaded with two adults up front was one of the first things I noticed about my 170. With myself and another 200 pounder in the front seats and full fuel I need to add weight to the baggage compartment to bring the CG back into the envelop.

I agree with Miles. Your index is 58.9 (58,903 / 1000).
Your empty CG is 42.57" behind the firewall.

So using your numbers, adding 170# Pilot (index = 6.1), 222# fuel (index 11.5) and 14# oil (index -0.5) you get a new weight of 1790 and index of 76.0.



76,000 / 1790 = 42.5" C.G. Well within envelope:


Add a 260# passenger with an index of 9.4 and now your weight is 2050 with an index of 85.4. 85,400 / 2050 = 41.7" C.G. Again well within limits.

Now with all that being said, I suspect your empty weight numbers because like Robert said, most 170's with two adults up front and nothing in back will have a C.G. forward of it's limit.
My empty weight numbers are 1398#, index of 52 = 37.2" C.G.

Here is the rest of my W&B chart:

Well I got home and had a chance to dig into the logs and check the last W & B calculations. Turns out you guys were right again. I have a sheet that I keep in the plane if I need to do a W& B other than 5 standard loads I have recorded already. What I did WAS to mix the index with the CG figure .

Of course with Bruce's spread sheet it's easy as pie but when the correct numbers are in the right places, even on the paper charts - everything works out fine.

Bruce - that program is really neat, and it prints to boot. Someone had a short thread on the value of this association. well once again this is another example of how useful and helpful the membership is.

Have a great weekend everyone...........and thanks for your responses

Kyle