Qld Flyer

I have a Robart incidence meter and it states that you need to level the fuselage so that the datum line is parallel to the ground. How do you determine the datum line on ARF's when that information is not generally provided? Or can you just level the fuselage say with the tail plane parallel to the ground and then do all measurements from there? I am only using it to make sure the wings etc are close before a maiden flight.

cheers

airbusdrvr

First of all, an ARF probably has the wing saddle pre-cut and it is what it is. You could use the incidence meter to see if the wings are straight. Just mount the wing and check that the angle shown on the meter is the same on both sides of the plane. Mount the incidence meter at the wing root on both side and compare the number, it should be the same. Then mount the meter at the wing tips and the two reading should be the same. If angle numbers are given for your arf between the wing and tail, you can measure the angle at the wing and then the tail and see if the difference equals the number you are looling for.

Rodney

If you want the plane to go through the air in a level (as observed as if you were sitting in the cockpit) you just prop or jig up the fuselage to that attitude and then measure then angle of the wing(s), stabiliser, engine thrust line etc. Normally (if a monoplane) you will want a bit of positive angle on the wing, the stabiliser somewhat negative with respect to the wing and some down thrust and right thrust on the engine/motor. How much angle? Usually the plans indicate a starting point but if just guessing with a symmetrical airfoil on the wing you will want about a 2 to 4 degree positive on the wing with the stab about 3 degrees less with about 3 degrees down thrust and 2 to 3 degrees right thrust. Now most kits or ARF's will have the wing angle pretty well locking in by the way the fuse is built so, in that case, just use that as a reference and set the stab to a negative 3 degrees with respect to the wing.

Lnewqban

Most of the drawings for airplanes and model airplanes have a datum line, to which all angles are referred.
That datum line is just a reference line and it may be defined by the designer in an arbitrary manner.
Most of the times, the central line of the fuselage is selected and it is represented as a perfectly horizontal line.
That central line of the fuselage is normally the central line of the aerodynamic shape (side of fuselage) that will produce the minimum drag during normal horizontal flights.

Read this how-to:

http://www.robart.com/how_to/incidence_meter.aspx

Regards!

Qld Flyer

Thanks for that. I shall give it a go.

Gray Beard

My Feedback: (-1)

Most ARFs have a zero incidence setting on the stab, that's most!! Most of the time you can put the incidence meter on the stab to locate level or just use a small bubble level.
Line, datum and arbitrary in the same breath??? The datum is a line or point that measurements are taken, there is nothing arbitrary about a datum.

Rodney

A datum line is an arbitrary line. It can be anywhere the person using it wants it to be and in any direction. It is simply defined as a point or line to which all other measurements are referred to. and for convenience, is usually aligned with the desired attitude of the fuselage when the plane is in flight. Back before aircraft, this was also commonly referred to a "water line" as use in nautical drafting and generally meant the line at which the depth in the water would be consistent when the ship was afloat. This terminology was carried over into the aircraft drafting effort but has slowly been called other things including , in some case, the datum line. Bottom line, you can specify any reference you want as long as all other points are in relation to that arbitrary definition.