How do you use an incidence meter? - RC Groups
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Aug 28, 2005, 06:41 PM
Registered User

How do you use an incidence meter?

I took the Piper Cub on it's maiden voyage Thursday. I bought it used and had to put some time into it to take care of the obvious fixes. Immediately after lift off it veered left over the flight line, pit area and headed towards the club's outhouse located in the parking lot (maybe it knows where it should be). I managed to get the wings level long enough to sort of land it (it ended up on it's back). Funny enough there was no damage and our club expert agreed to take it up after I explained that "it wasn't me, something's wrong with the plane!" As soon as it became airborn again he yelled, "How did you fly this thing as far as you did?" He actually landed it on the runway sort of sideways in every direction and promptly asked me, "did you learn anything about buying used planes? Get rid of it." It looks like the fuselage is a little twisted, not alot, but the wing could be the main problem. He also said something about engine thrust. I have an incidence meter which I picked up used with no instructions. Can anyone explain how to use this in simple terms without using words like "datum line" and other terms a novice does not understand? I'm not one to give up if it can be fixed without rebuilding the whole plane. If it is twisted, can airframes be twisted back into shape by using moisture and clamps? It's all wood construction. This has become a personal challenge so any help from someone who has experience in this would be appreciated, even if the answer is to use it for a ceiling hanging or wall clock! Thanks!
Last edited by rcwilbur; Aug 28, 2005 at 06:45 PM. Reason: missing word, wrong word
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Aug 28, 2005, 10:36 PM
It would help if we knew which incidence meter you have.

The 'datum line' is a line on the airframe used as a zero reference, that is, the datum line is the line on the airframe which is set to level and once level all incidence angles are taken from that line.

If the model really is a Piper Cub and is anywhere near scale, the datum line is the bottom of the window openings.

Level the left window opening front to back, and then level the model side to side using both window bottom openings.

Having leveled the model, now level the incidence meter and set the pointer to zero.

With the incidence meter zeroed, clamp the meter on the wing and read the incidence directly.

There is a full tutorial on incidence meters on the Robart web site.

EDIT: Having looked a bit more closely at the photo, that model may be of a Piper Cub, but it's 'stand-back-about-a-quarter-mile' scale.
Aug 28, 2005, 11:45 PM
B for Bruce
BMatthews's Avatar
That ain't no Piper Cub. Looks more like a free flight old timer adapted to RC with less dihedral and added ailerons.

But in any event....

First off it sounds more like your wing has some serious warps or one side is somehow way different than the other. What you need is not just incidence checking but a total model alignment. A lot of the alignment procedure is just common sense and the ability to set up the model and use some procedures to measure and set the alignment. The incidence guage is just a tool that makes it a bit easier to measure angles of the wing and tail and those angles are just a part of the overall procedure.

Basically you need to do to your model what a surveyer does to a new parcel of land. This requires long straightedges, strings, tape measures, spirit levels and that incidence guage you got. They are all tools of the trade.

A general order of the steps you need to do.....
  • The fuselage needs to be checked for any bowing or other unwanted curves. If any bowing or curving exists as seen from the top view then you can still use the fuselage as is but it may require you to establish a new centerline that allows you enough of a fudge factor to align the engine thrust line to that new centerline and to position the wing on that new centerline both span wise and skew wise in correct alignment.
  • The fin and rudder must be set on the true centerline, or if the fuselage is bowed, on your new centerline to within about 1/2 of a degree. I suspect that fin and rudder misalignment or warps is the cause of your present problem. This is important. If your fin is angled to the (new) centerline then cut it off and re-attach it in alignment. Jigs or braces to hold it while you double and triple check it and finally glue it are encouraged.
  • The wing needs to be checked to ensure it's centered span wise on the fuselage and is sitting square across the (new) fuselage centerline.
  • To set the wing measure from the centerline and set the wing even in span to either side of the (new)centerline (not neccesarily the fuselage sides if it's bowed). Once that is set you need to measure from identical points on the trailing edge that are out from the centerline to a point on the fuselage centerline back near the fin. These two diagonal measurements must be exact within about 1/16 inch. Doing so sets the wing's skew so it's square across the (new)centerline and square to the fin that you already check/remounted.
  • Now check the side thrust angle. Replace the prop with a piece of 1/2 x 12 plywood or wood that is flat and has a hole accuratley in the center of it. With the stick horizontal measure from the (new)centerline back by the fin to the tips of the stick to determine if there is any side thrust. If anything there should be a couple of degrees of right thrust. On a 12 inch stick that would be in the order of about 3/8 inch difference in the two measurements with the left side measurement being the longer of the two.

This completes the top down alignment. Now it's time for the side view alignment of the down thrust and wing and tail incidence angles. To avoid all this datum angle stuff I suggest you block up the landing gear and tail so that the wing is level at 0 degrees incidence by the incidence meter. This will be the reference for the rest. Be sure to use the guage on the wing very close to the fuselage for this part. You'll see why later.
  • Turn the "prop" stick to vertical and use the incidence guage to check the downthrust angle. You may need a small square to extend a right angle of the thrust line out from the "prop" so you can use your incidence guage. Or get a little plastic slope guage from a hardware store. It's sort of a level but has a protractor guage in it and can meausre angles. Use something of this sort to measure the downthrust which should be something like -3 to -6 degrees compared to the wing (set to 0 and being used as a reference). If it's outside that range then shim the mount until it's at -4 to -5 degrees downthrust compared to the level wing.
  • Now measure the stabiliser angle. Use a straight edge to set the elevator trim so the elevator is in perfect alignment with the stabilizer. Now measure the incidence with your guage. It's probably -1 to -4 at that point. If it's outside of that then use the elevator trim to set it to about -2 to -3 as a starting point for your next flight.

That's the thrust and incidence angles done with.

The last thing in this survey job is to check for warps. Since you have the model set so that the wing incidence is 0 use your incidence guage at various spots out along each wing panel to check for any differences. This is why you used the guage close to the fuselage before. Any twist in the wing would have made your wing angle leveling step useless otherwise.

The wing should be flat or have some washout (less angle of incidence at teh tips compared to the center). Be sure to use the same trick with the straightedge to set the ailerons to 0 deflection before doing any angle measurements. Measure out from the fuselage at the 1/4, 1/2, 3/4 and tip just before the curved parts start. The angles should all be 0 or go slightly negative as you move to the tips such that the tip angle is maybe -2 to -3 degrees. Also the corresponding measurements on each side should be the same within one degree or less. If not then use the covering iron to warp the wing until you have these matching angles.

If you go through these steps in this order I'm sure you'll find that at some point one or more of the checks will find a problem. If the model is setup as per the above it'll fly unless the wing's airfoil section has some major shaping difference from one side to the other.
Aug 29, 2005, 06:18 PM
Registered User
You guys are incredible. Thanks for having a look and taking the time to help out!
Happy Landings!

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