|Aug 28, 2005, 05:41 PM|
Joined Aug 2005
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!
|Aug 28, 2005, 09:36 PM|
Joined Feb 2003
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, 10:45 PM|
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.....
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.
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.
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