Horizontal stabilizer

[Dferr34]

Does anyone know if there is an stc or mod to add weights to the elevator of a 170A like the 170B?

[n2582d]

Don,
There's a reason you guys with the '48 170's and 170A's have such huge biceps. It's the great exercise you get keeping the elevator off the stop as you taxi! I think Service News Letter (SNL) 76 from 1948 may answer your question:

SNL 76, Click to Enlarge

So, in my opinion, the short answer is no; it would not be permissible to add balance weights to the 170A elevators. I understand why one would not want to install an unbalanced elevator on a C-190, but don’t understand why a balanced elevator wouldn’t work on a C-170A, but apparently it doesn’t work. There are no STC's or Cessna Service Letters or Accessory Kits that I can find which permit this. But take a look at the chart at the end of SNL 86-44.

Click to Enlarge

It gives a range of between 0.0 to +67.3 in.-lbs. underbalance. On the C-170B the elevator weight is around 14" ahead of the hinge line. If the elevator is similar on the 170A that would mean that if it takes over 4.8 lbs. to balance the right elevator you need to add some weight to the elevator horn. But the figure also seem to imply that if you had 4.8 lbs. of lead in horn and it balanced, you would be within specs. It's interesting to note that the numbers are so different for the 190/195 which apparently has the same elevator, but with a balance weight. It's also curious to me that the Vne for the 170A and the 170B is the same at 160 mph. One would think that with an unbalanced elevator the 170A would have a lower Vne.

[n2582d]

Does anyone know if there is an stc or mod to add weights to the elevator of a 170A like the 170B?

I've run across a 337 form for a 1948 C-170 where they added 6 pounds to each elevator "according to manufacturer's instructions". This form, because it predates Oct. 1, 1955, is considered "approved data" if someone wanted to replicate the alteration on a '48. I'd want to see those "manufacturer's instructions" before I signed off such an alteration. Wonder if Cessna would still have record of that? You'll find the 337 form here.

[GAHorn]

I’m apprehensive about adding weights to the elevator of a 170A simply because someone’s 337 was dated prior to 1955. Cessna’s SLN-76 (posted previously) is pretty specific about that not being acceptable, and just because someone prior to 1955 did it to their airplane doesn’t meet my definition of “airworthy” in light of Cessna’s specific warning. (That 337 is no proof that modification was successful or if it remained on the airplane.)

[n2582d]

Absolutely agree George. That's why I wrote, "I'd want to see those "manufacturer's instructions" before I signed off such an alteration." For anyone contemplating such an alteration AC23-629-1b will probably convince you that an unbalanced elevator isn't so bad; there’s a lot to consider when adding weight to control surfaces.

[Bruce Fenstermacher]

I've flown many hours of balanced and unbalance elevator equipped 170A and B. Never did I wish my A with an unbalanced elevator have a balanced elevator like my first A model. To be totally forthright for those who wouldn't know, my first A model was modified in nearly every way but the data plate to B a B model. So the first didn't have an A model elevator, it had a B model elevator. They are different is shape and trim system besides the balance weights. Bet the trim system difference might be why Cessna say's not to balance an A model elevator.

My point really here is, I wanted lots of things for my true A model, but a balanced elevator was not one of them. I think you would be disappointed wasting the effort to balance an A elevator.