Williams County Airport

Bryan, Ohio 43506

August 26, 1972

Dear builder,

It was planned to fly the prototype HP-16 in the 1972 Standard Class Nationals held in Marfa Texas. Problems were encountered with skin separation from the structure after two days of flying with water ballast in the wings.

Removal of the skin from the wings revealed that one panel could be peeled off without damaging any of the ribs while another could not be removed without pulling the ribs apart. This indicates that the problem was improper preparation of the aluminum skins prior to gluing.

It appears that aluminum ribs, skins and spars must be more carefully treated before gluing. We are therefore recommending that all metal surfaces be acid-etched and cleaned before gluing. Builders who have not yet covered their wings, please contact us immediately and we will provide information on etching for best gluing results.

Carrying water ballast may be introducing excessive hydraulic pressures so we strongly urge that no water be put into any wings until more information can be gathered on the subject.

Proper venting of all wing compartments is absolutely essential if interior pressures are to be avoided. Using of 1 inch lengths of thin wall, Large diameter plastic soda straws placed in the rib drain notches will help to keep glue from closing these vents when the lower surface skins are glued. Before cementing the top skins in place, all vent holes in the bottom edges of the ribs should be checked to verify that they are open.

The prototype RS-15 was delayed about 6 weeks by my participation in both Nationals. In the meantime, Steve Owen completed and flew his RS-15. He reported that no problems were encountered and that the ship flew normally. V-tails were installed on this ship and the prototype. The all-flying T-tail had had many more problems than the V-tail and the following disadvantages:

1. Heavier, more complicated and difficult to build

2. Extremely difficult of counterbalance and more susceptible to flutter.

3. All flying stabilizers are difficult to solve stability, feel and controllability characteristics.

4. Limited travel elevator and stabilizer type T-tails have small allowable C.G. travel allowances and run out of power if the C.G. is out of limits.

For these reasons, all RS-15 and HP-16 kits will be supplied with V-tails.

Do not expose wings or tail surfaces to high temperatures in direct sunlight unless they are painted white. Dark colored primers or finishes could generate very high interior temperatures of destructive buckling through excessive temperature differentials.

Our number one project is to get the RS-15 prototype flying so that we can get the final parts out to builders at the earliest possible moment.

HP-16 parts are being shipped as fast as they can be assembled.

Sincerely yours,

R. E. Schreder

Post Office Box 488

Bryan, Ohio 43506

November 30, 1972

Dear Builder,

As time goes on we find that proper cleaning and preparation of aluminum sheet is extremely important in getting maximum strength and adhesion from epoxy resins used in bonding skins to wing structure. Lacquer thinner is not being recommended any longer for cleaning aluminum because some brands contain solvents that can leave oily residues.

Methyl Ethyl Ketone may be used to remove traces of oil or ink. The best treatment we have found so far for cleaning aluminum surfaces for best adhesion of glue or paint is to scrub such surfaces with Ajax or Comet powdered cleansers sprinkled on a wet rag. Rinse with clean water and dry with clean towel. If the rinse water will lie flat on a completely wetted surface for 60 seconds, you know that the metal is oil free. So check every square inch of skin surface and other metal glue surfaces by this method. If water rolls off, scrub that area again.

If you don't have a good flat table 24 feet long, one can be made from 1/2" or 3/4" plywood cut into 24 x 96 inch panels and nailed to the top and bottom of two 2 x 8s to make a rigid box-like top. 2 x 4s can be spiked to the sides for legs or the top can be supported by 4 or 5 saw horses. Sight the top for straightness.

The HP-16 wing construction is the simplest and fastest method know for amateur builder construction. Since the method of assembly is entirely different from other designs, it is extremely important that instructions be followed to the letter. Deviations can result in twisted wings, gaps between ribs and skin, very poor adhesion, flat spots, humps, buckled ribs, crooked trailing edges, peeling paint, corrosion and other defects.

Below is a list of dos and don'ts that must be observed if you want a set of perfect wings.

1. Skins must be pre-bent exactly as specified or thew will not maintain contact with the ribs.

2. Rib and spar structure must be straight, free from twist, curvature, sags, humps, etc. as the skin cannot correct poor workmanship underneath.

3. When the skin is strapped in place, there should be no gaps or other defects because once the rivet holes are drilled it is too late to make any more corrections. Do not try to curve a poorly fitting skin by inserting boards under straps to provide local pressure.

4. All metal surfaces to be glued or painted must be thoroughly cleaned and roughened for proper adhesion.

5. All surfaces of the main spar that do not have to be glued must be protected with zinc chromate paint because bare 7075-T651 aluminum will be damaged by corrosion if it is not properly protected DO NOT paint any area to be glued.

6. Ribs must be notched to allow free venting of all interior areas so that no water or air can be trapped in closed areas. Destructive pressures could develop if venting is not provided; so make sure all areas are cleared before gluing any skins on. Trapped water in removable surfaces could induce flutter.

7. The surface used to support ailerons and flap sections must be perfectly flat, straight and free from twist. If this surface or the bars used to clamp trailing edges are not strait, a horrible looking wing trailing edge will result.

8. Removing the straps from the wing after clecos are in place during the gluing operation will result in tighter skin fit on the structure.

9. Also as the glued wing skin is clecoed in place, the wing assembly should be turned over so that the glued surface is against the table surface. This will allow the glue to form fillets at rib skin joints and stop excess glue from running down the sides of the ribs.

10. A strip of foam rubber of polyurethane 1/2 to 1 inch thick and 2 inches wide should be attached to the table surface so that the wing is supported directly under the spar. This will insure that the skin remains in contact with the main spar while the glue cures.

11. Check and double-check your work for accuracy, straightness and freedom from twist. You just can't be too careful if you want to eliminate errors.

12. Be sure that there are no glue lumps or thick tips on the flaps and aileron ribs before gluing the bottom skins on. Such defects will cause unsightly lumps to appear in the skin when the bars are clamped on the trailing edges and thy will remain forever after on the finished surface.

13. Don't use automotive body fillers or finishes other than zinc chromate and lacquer primer surfacer to finish the wings. You may wind up with a cracked, peeling or lumpy surface that defies all efforts to remove or correct it. White lacquer can be applied over the zinc chromate, white lacquer-primer surfacer preparation.

14. Don't paint wings any color other than white as exposure to the sun in warm climates could generate destructive temperatures that would exceed limits of glue and foam or cause buckling of skin that might break it loose from the ribs.

Sincerely yours,

BRYAN AIRCRAFT, INC.

R. E. Schreder