The International Cessna^® 170 Association

Proud new owner of a '48 170. I was working up W&B scenarios, and it appears this plane has a very far forward CG.

Current documentation shows Empty Weight is 1379.86 with CG at 37.96.
The last time it was weighed (before P. Ponk gear and some avionics changes) it was left main 645, right main 645, tail 100, for an empty weight of 1383 and CG 37.91.

Using Bruce's spreadsheet, I put in myself (186) and my instructor (170), and with 33.5 gallons of fuel, we are out of forward CG limit (especially when accounting for my short legs). We put a case of oil in the baggage compartment, and with half fuel, we were just inside the CG envelope. The airplane felt very nose heavy. Indeed, when my instructor flew it solo, he had trouble getting the tailwheel to stick down on pavement, said it felt bouncy.

I've read quite a few threads here, and it seems like on the last weighing, the tail was actually a bit lighter than most.

First question is it common to need ballast to correct forward CG? I would need 100lbs in the baggae compartment to get the CG towards the middle of the envelope where I'd like it. We plan to reweigh the plane, to make sure the numbers are all correct, but the way it handles seems to be consistent with far forward CG. So I am wondering if there's an actual issue that needs to be corrected.

Second, related question. The plane has a 12.5 gallon aux tank installed in the left wing. I think I have all the records, and the 337 and log book state that a flight test would be performed to determine unusable fuel, but I can't find a record of that. I assume the arm for this is +45, but how can I figure out the additional unusable fuel?

TIA.

Forward CG is common in 170s. With full fuel and about 440lb in the front seats, I can get my B-model back to the forward limit with about 30 lb in the baggage area. 100lb in baggage with full fuel, puts my CG at 41.5 (limits are 40.3 to 46.4 at that weight). Fuel burn moves the CG forward, but not as fast as the CG forward limit moves forward at lower weight, so the zero fuel CG is actually further inside the envelope,than at gross.

Unfortunately if there is no documentation for the unusable fuel for your additional tank. You would need to experiment to find it. I'd personally just use the unusable fuel for the right side.

To weigh the aircraft it is actually pretty hard to drain all the fuel from the system or even know when your at unusable fuel. But it is pretty easy to fill the tanks the same to the top, weight the plane, then subtract the fuel for the weight and moment you want for other calculations.

Weighing aircraft is not an exact science. Meaning two well meaning, very careful people, following the instructions to the T, can have slightly different results.

The first discrepancy is how accurate the means of measuring the top door sill to level is. Just a hair one way or the other will result in different arm. A second common mistake weighing a 170 is that the arm for the tail wheel must be for the Scott 3200 you likely have installed, not the incorrect, but standard smaller Scott 3-24B. If your aircraft records are typical of all aircraft whose records I've reviewed, you will find a mistake someplace. I can probably count on one hand those I've found correct.

Miles all ready stated 170s tend of be more nose heavy with fuel than not and ballast in the rear is common.

Most mechanics make the mistake of weighing the aircraft with oil. Oil is to be drained when weighing then re-added according to the loading chart. It is so small most guys miss it!

Thanks for all the feedback. There are actually "placards" next to the fuel caps on each wing giving different useable fuel measures. I hadn't put fuel in it when I posted so hadn't noticed that. But I can still not locate anything in the documentation or logs as to how it was determined, so I've no confidence it is accurate. But I've got very little interest in sitting in the plane for the full range, so I guess my main concern is knowing how much fuel is on board for W&B, and less about usable vs. unusable. To that end, I picked up one of the universal fuel dipsticks and will calibrate it for each side.

I plan to help with the weighing, so I will be sure to remember to put in full fuel, no oil, and to account for the tailwheel.

I've already learned a ton here reading old threads, and appreciate all the collected wisdom.