Monday, July 23, 2012

Stabilizer Incidence Angle

The stabilizer front spar lays directly on top of the fuselage upper longerons.  It is clamped there by a block of wood which sets under the longerons and is bolted to the front spar.  The brace wires are adjusted to keep it square to the fuselage center line. There is no elevator trim.  The angle of incidence of the stabilizer is not adjustable because the front spar of the fin bolts to brackets on top of the front spar of the stabilizer.  Probably not the cleverest arrangement, but it is simple.  Also the upper wing fore and aft position is adjustable with the rigging to balance the plane.

The factory fuselage drawing shows the upper longerons flat the full length of the fuselage.  On my fuselage the upper longerons are bent down 5/8" from the last fuselage bay (forward of the stabilizer front spar) to the tail post, as shown here.  The older drawing of the tail (7012) shows the same angle.  This bend lifts the stabilize angle about 3 degrees.  When Frank Pavliga test flew his NINE with the longerons straight he found a very tail heavy plane.  There are no other flying NINEs to compare handling. It looks like 5/8" is about what is needed to correct the tail heaviness.  If I ever find the lift, drag and pitching moment curves for the Aeromarine 2A airfoil I can do the math to figure this out more accurately.  The airfoil was tested at MIT in Dec. 1922 and the curves were apparently published in the 1925 edition of Handbook of Instructions for Airplane Designers.  Google claims to have the 1925 edition but its not.  I'm sure MIT has the original data and the Air Force museum should have the handbook since it was done at McCook Field.

Since I have a fuselage, this angle would have continued to exist on mine without a problem.  Unfortunately I don't have the fin so all this has been needed to figure out the correct layout of the fin to fit with the fuselage, rudder and stabilizer.

At this point I'm ready to get back to making the jig and fin.  I'm looking forward to fitting all this to the fuselage.