Allegro-Lite Tail Notes
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Home > Articles & Tips Index > Mark Drela's Allegro-Lite 2m > Allegro-Lite Tail Notes

[Courtesy of Mark Drela, April 2000]

The 3/16" plank makes the elevator, and the 1/4" plank makes the fin+rudder.

To make the elevator:

1) Cut out and sand the elevator to the right planform.

2) Carefully make the spanwise cut for the spine, noting the gentle radius in the middle. The radius can be more gentle than on the plan, but I wouldn't make it sharper.

3) Slice off the radiused "point" of the back elevator piece and glue on the hard balsa insert of exactly the same size. Titebond is OK here, but avoid excess oozing onto the soft wood (tough to sand).

4) Glue the two elevator pieces together with slow epoxy, sandwiching the carbon spine between them. Chordwise stretched masking tape strips are good for applying pressure all along the seam.

5) When cured, sand the elevator to the right spanwise thickness taper, so it has a 5% thick rectangular "airfoil" all along its span.

6) Sand in the true airfoil.

To make the fin+rudder:

1) Cut out all the 45 degree-grain rudder pieces and glue together. Ambroid is best for this since is sands well on the soft balsa (Duco Cement and Sig-Ment are perfectly good substitutes for Ambroid). Cut out the fin in one piece. The latest tail pdf drawing shows how the pieces fit on the 1/4"x3"x18" plank. Make sure the hinge edges are straight, without any gap between fin and rudder.

2) Make the edge slices for the hinge strip with a new #11 blade as shown on the drawing. A 1/4" deep slice is OK -- a bit wider than half the hinge strip.

3) Sand in roughly a 20 deg bevel on each side of each edge (4 bevels total). This will give a 40 degree rudder deflection clearance. The bevels don't quite meet in the middle of the 1/4" face at the slice -- leave roughly a 1/16" flat spot.

4) Squeeze Ambroid into the slices, and make another blade pass to coat the inside of the slices. Also pre-coat the hinge strip with Ambroid. When dry, test-fit the hinge strip into the slices. Widen and/or deepen the slice if necessary. Then squeeze more Ambroid into one slice and install the strip. When set, glue the strip into the other piece the same way, thus joining the fin and rudder. Leave a 0.02"-0.03" gap to allow the strip to flex.

5) Apply the basswood or hard balsa laminations to the bottom of the fin+rudder. Wet the strips and glue on with Titebond. Apply tension when pulling them around to prevent compression cracks. Hold down with narrow tape strips.

6) Sand the spanwise taper to a rectangular airfoil. Make sure the taper is equal on both sides to the hinge strip stays in the middle!

7) Sand in the true airfoil.

Note: The hinge strip is extremely tough, so you can treat the whole vertical tail like one piece of wood during sanding.

- Mark

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