The International Cessna^® 170 Association

Gentlemen

Would any of you have a good advise about how to check the horizontal stabilizer (HS) incidence?
The 170 Book has some information about this but the numbers given by the book are for 170 and 170B models. What would be the number for the A model?

I understand that HS incidence is related to the wing’s incidence. For stability, HS incidence shall be smaller than wing’s incidence (and off course CG within the specified envelope)

So, also important is how we measure this incidence. I wonder the following procedure:

1) Level the airplane:
Some say that the reference for leveling the airplane is the upper door sill. Other say that the reference is that long fuselage side skin that goes from the door down to the tail. What is your understanding of this? Also, when we level the airplane (lets say, using the appropriate reference) does this mean that wing incidence (referred to a horizontal line) is 0 (zero)?

2) Measure the HS incidence:
Having leveled the airplane, measure the HS incidence using a “good” level. But here the difficulty is what reference to use for the level. Front and rear HS spars?
Anyhow, lets suppose that we used the correct reference and we got the HS incidence. This incidence will also be referred to a horizontal line (as fart as I am using a level). Is this the number specified at the 170 book?

Maybe it is not easy to explain all this without a piece of paper . . .
Any advice will be much appreciated.
Thanks!

Thomas Shaw

According to cessna,the bottom surface of the upper door sill should be used for levelling the airplane,for weight & balance purposes. For checking wing & horiz stab angles of incidence,who knows? You might want to talk to someone who rebuilds Cessna's (like after a wreck) for a living.
The article in The 170 Book sez the wing incidence is the difference between a line thru the bolt holes at the wing root ( I assume the spar attach bolts) and the line of the fuselage butt plate. I assume that the horiz angle of incidence is measured against the same datum. I also assume that the angle listed for the B model horiz stab is the same for the ragwing & A model,but I have no basis for that assumption!
The article also sez that the horiz angle is the difference between a line across the top of the front & rear spars & the top line of the fuselage butt plate (again).
Why all the checking? Are you rebuilding the airframe,ir do you suspect that the flying surfaces are out of rig?

Eric

Eric,

Some years ago the tailcone was fully rebuilt after a hard ground loop. Since then, the airplane was not flown and I am just wondering how good (or bad) was the "workmanship" because I know that during the rebuild no jig was used!

Just on thing: when you say "the fuselage butt plate" I guess that you mean that long skin sheet (one each side of the fuselage) that goes all the way from the doors to the tail right?

Thanks for your answer.


Thomas

I was just quoting The 170 Book atricle when I referenced the "fuselage butt plate". Your guess is as good as mine.

Eric

The Horizontal Stabilizer angle of incidence is measured across the top of the horizontal stab. spars (ront and back) without regard to the airfoil shape of the skin. (Press the skin down so it won't interfere with the straight-edge.) It's angle is compared to the fuselage splice plate, which is the long piece of aluminum that runs the length of the fuselage on each side. That angle should be -2 degrees, 48 minutes.
An easy way to do this is to use a long, metal carpenter's level to extend the stabilizer line across the spars, forward to be compared against the upper edge of the splice plate.

I know this is an old thread, but I got a definitive answer from Cessna regarding this measurement on a 48 Ragwing.

Horizontal Stabilizer Angle of Incedence = -4 degrees measured against the leveling reference, upper door sill.