August 14, 1968

Dear Builder:

I flew HP-11 No. 2 for the first five days of the 1968 Nationals at Elmira.

Joe Perrucci, the builder, had reported that he had not been very successful in making a diamond goal flight but felt that the problem was probably his flying. I soon found that I had difficulty keeping the ship in the air. Its minimum sink was 300 feet per minute, about twice what it should be. Close examination of the sailplane disclosed many gaps and discontinuities which were adding up to many losses. Also suspect was the buffed polish finish on the wings and tail.

Between contest flights we added tape, elevator gap seals, cardboard fairings and sanded the wings. Performance improved dramatically and the minimum sink dropped to less than 150 feet per minute. Joe installed the wheelwell doors later and telephoned to report that the HP-11A now flies like a different sailplane. He no longer has trouble keeping the ship in the air.

To the best of our knowledge, no one has made an investigation into the reasons why a glossy wing has low maximum lift and high drag at low speeds but I suspect it is caused by laminar separation. Try sanding yours with 400 grit wet or dry paper.

Once more I would like to point out that elevator and rudder controls should always be checked after the V-tail has been unfolded because it is possible for the drivers to slip out of their pockets if either surface is tilted past vertical center when in the folded position.

Another possible problem can arise if the landing gear is retracted without air pressure in the shock struts. Unequal extension of the struts while in the folded position can cause jamming of the gear or separation of one piston from its cylinder. Always make sure that the pistons hit solid oil ½" before bottoming in the cylinders. This will help prevent damage on hard landings.

Be very careful about letting light weight pilots fly your ship without compensating ballast. The size of elevators and their movement are designed to give satisfactory control responses within a limited center of gravity range.

Getting the c.g. too far aft can result in loss of longitudinal stability, poor spin recovery or loss of control during tows or low speed flight. C.G. ahead of the forward limit can prevent proper flareout for landing.

The HP-14T with only 15° up and 10° down elevator could be especially critical if the c.g. is too far out.

In general, if your ship won’t trim out in the 60 to 80 mile per hour range, you are out of the limits. Every sailplane should have a careful weight and balance made up from actual weights and measurements. The cockpit should be placarded to show the allowable pilot weight variation and a note to add ballast for anyone out of the allowable range. We will be glad to help builders or owners who have any questions on the subject.

We bought enough 16 x 20 Aldott color prints of the HP-14 to supply all builders. This is a beautiful picture, taken over the Davis Mountains near Marfa and is one of Alex’s best. They make excellent executive-type gifts. Our price is $12.50 each, postpaid.

Builders who have not yet installed HP-14 push-pull tubes in their rear fuselage are requested to double check the layout of their guides. The correct alignment will place the centerline of the tubes 3/32" above the horizontal centerline of the No. 12 bulkhead and 1 ½" on either side of the vertical centerline.

We installed a hydraulic flap operating system on the HP-14T prototype. Flaps can be lowered at any speed up to 120 mph by actuating a reciprocating handle on the left side of the cockpit above the rail. Flaps are raised by holding the handle back. This allows hydraulic oil to bleed from the actuating cylinder to the reservoir and permits the flaps to raise smoothly. An indicator mounted on the side rail permits setting the flap at any desired position.

Drawings and parts are being made up now. As soon as they are ready, all builders will be notified. Cost of the installation kit is estimated to be about $100. This system can be retrofitted into HP-10,11,12,13 or 14 sailplanes.

Fiberglas HP-14 wing fuselage fairings are being fabricated. Price is estimated at $50 per set. Current owners who desire either or both of these new items should place their orders as soon as possible to aid us in manufacturing sufficient quantities.

If original builders who have sold their ships will send us the names and addresses of the new owners, we will send future newsletters to them.

Sincerely yours,

(Article courtesy of Alex Upchurch, who is co-owner of HP-14 #12, C-FWHZ)