The HP-15

By Richard Schreder

With the new Open Class sailplanes going to 21 meters (British Sigma) and 22 meters (German Nimbus and SB-9), it appears that flying in this class may soon become awkward and expensive as participating in the America's Cup Race. The pilot of limited means, muscle, and acquaintances is going to be forced to fly and compete in machines of reasonable size, weight, and cost.

The recent liberalization of the Standard Class rules to permit retractable landing gears and fixed-hinged flaps makes it possible now for designers to build simple, low cost sailplanes with outstanding performance. The HP-15 is probably the first sailplane to be designed to the new Standard Class rules. Its superior performance is provided by a very high aspect ratio (33 to 1) all-metal-wing. The high wing loading (8 lbs. per sq. ft.) and high aspect ration yield high-speed performance heretofore unattainable in 15-meter designs. The high aspect ratio and low span loading, combined with a new high max. lift airfoil, fixed-hinged flaps and drooping ailerons, allow the HP-15 to circle at normal speeds but lower rates of sink than most current Standard Class ships.

Construction of the fuselage and V-tail follows conventional HP-11, 12 and 14 configurations, except that the 1-inch square-steel-tubing cockpit framing has been replaced by aluminum tubing. The landing gear has hydraulic shock struts. A spring-loaded elevator trim and anti-balancing tabs have been incorporated to provide adjustable control-stick loads. And the tail wheel is steerable for easy ground handling.

The most outstanding feature of the HP-15 is its wing construction. All bending loads are carried in very heavy, rolled to contour, wing skins. Each panel contains only three ribs, a plastic leading edge, two skins, and two bend channels spars. The normal interior structure was originally to be replaced by high-quality urethane foam blocks completely filling the cavity enclosed by the skins and spars. But we are now shifting from urethane to honeycomb. In any event, assembly labor is drastically reduced to about that normally required to produce a main spar.

The flaps are simply 4-fee-long, prisms, with no framing other than a rib at each end. The flaps are attached with piano hinges to the bottom of the rear spar, and are driven torsionally from the fuselage end.

The ailerons are of a similar construction and mounting, and are driven by push-pull tubes supported by guides attached to the aft side of the rear spars. These tubes are readily accessible by lowering the flaps.

A very ingenious method of securing the two sings together makes use of two curved 1/4-inch diameter pins, which pass through holes in interlocking fingers riveted to both the the and bottom skins across the root of each wing panel. The flap drive and the attachment of the wings to the fuselage are automatic when the wings are brought together. The wing and tail fairings are permanently attached to the fuselage. The tail surfaces are folded by lower spring-loaded, tapered pins into detents.

Assembly and disassembly from or into the trailer can be accomplished in less than five minutes. The wings are supported on carts, and the fuselage on a dolly. All of them are on wheels and engaged automatically when pushed into the trailer. The trailer is all aluminum, designed for as-simple-as-possible construction. It weighs 600 lbs. and is completely equipped with hitch, lights, and color-coded, easily-connected, quick-disconnecting wiring harness. The wheels and tires are matched to the owner's car.

 Both the HP-15 and trailer are designed for easy assembly by the home builder with only hand tools, drill, rivet gun, and air compressor. Kits will be made available for both the sailplane and trailer. They are priced at $2995 and $695 respectively, while construction time is estimated at 600 and 100 man hours each.

Specifications and performance are as follows:

Calculated performance


Design speed


Max. speed, smooth air


Max. speed, rough air


Max. speed, aero-tow


Max. speed, winch/auto-tow


Stall speed, no flap


Stall speed, full flap


Max. L/D ratio

45 at 55

Min. sink

1.6 fps at 45




49.2 ft.


23.2 ft.

Height at tail

45 in.

Cockpit width

24 in.


2 degrees


3 degrees


0 degrees

Taper ratio


Empty weight

330 lbs.

Gross weight

600 lbs.

Wing panels

90 lbs.

Fuselage and tail

150 lbs.

Wing area

75.0 sq. ft.

Flap area

12.4 sq. ft.

Ailerons area

4.6 sq. ft.

Stabilizer area

7.0 sq. ft.

Elevator area

5.0 sq. ft

Airfoil: Schreder

69 - 180


The decision of CIVV (formerly CVSM) last February 17th in Paris, France, to permit the use of fixed-hinged flaps on Standard Class sailplanes renews interest in this often controversial subject. Because flaps have been forbidden up until this time in Standard Class ships, most soaring pilots, including myself, came to feel that there must be some great performance improvement to be realized from flaps with complicated gaps, slots, moving hinges, and large area-increasing features.

Led by this delusion, I drew up a super ship for 1969 and 1970 that would have 8-lb. Wing loading for cruise and 5-lb. wing loading for thermaling. The change in area was to be accomplished by sliding the flaps and ailerons aft an additional 67% of the wing chord.

Upon drawing up performance curves of the two configurations, I was shocked to find that the minimum sink of the 8-lb. per. sq. ft. retracted wing was lower than the minimum sink of the 5-lb. per square ft. extended version. If the following favorable assumptions were not realized, the comparison would be even more dramatic:

    1. Such a wing could be constructed.
    2. Span lift distribution is uniform.
    3. Extended airfoil is as efficient as basic.
    4. Wing weight could be held to 120 lbs. per panel.
    5. No losses would be encountered from tracks, supporting arms, etc.
    6. Torsional strength could be maintained.


With the aspect ratio dropping from 33 to 1 down to 20 to 1, the max. glide ratio drops from 45 to 1 to 36 to 1, and min. sink from 1.6 to 1.75 feet per second. What has fooled everybody for years is that we have disregarded the reduction of aspect ratio caused by the increase in chord. It now is apparent that the only help that can be expected from flaps is to slow the ship down so that it can circle in small-diameter thermals when higher wing loading are used for better cruising performance (as with the Sigma.) The fixed-hinge flap will do this just as well as the Fowler type. The simple flap gives a bonus of many additional advantages, as listed at the end of this article. With this information available, I was able to put together a much simplified, easier to build, lower-priced ship having superior performance. Approval of flaps and retractable gear made my "super ship" HP-15 a future Standard Class sailplane. There are two common objections to flaps used as dive brakes:

  1. The operating loads are high at high speeds. This problem has been partially solved by permitting 5-seconds for extension instead of the previous 2-second time limit. Narrowing the chord and using air-paddled or hydraulic boosters will eliminate the difficulty entirely.
  2. Raising the flaps on final approach at low airspeed to stretch a glide would be dangerous. This last objection is not a valid problem. The technique for making approaches to landings should be the same for either 90-degree flaps or dive-break equipped sailplanes:
        1. A rectangular pattern should be set up parallel to the runway at 600 feet when downwind and abreast the center of the landing field.
        2. The airspeed should be held constant at about 20 mph above stalling speed (25 on gusty days) until the landing flare-out.
        3. Brakes or flaps are extended on base leg and/or final to steepen the approach as necessary, with the airspeed maintained as above.
        4. The brakes or flaps are retracted as soon as an undershoot is apparent in order to flatten the glide.
        5. If the pilot tries to stretch his glide and can't make the selected field, dive brakes will not help him. Flaps, however, will keep the ship airborne a few seconds longer while it is losing another 10 mph. The 90-degree flapped sailplane will land 10 mph slower than a no-flap ship of the same wing loading, or 15 to 20 mph slower than the same ship with extended dive brakes.

In conclusion, the legalizing of fixed-hinge flaps for Standard Class sailplanes will permit designers and builders to provide safer, simpler, and cheaper machines with improved cross-country performance. The breakdown of advantages of 90-degree flap over conventional DFS dive brakes is as follows:

  1. Lower landing speeds.
  2. Steeper decent at slower speeds.
  3. Much lower terminal velocity.
  4. Permit save landings in shorter fields.
  5. Reduce landing damage in rough fields.
  6. Strengthen wing by shifting spar loading inboard.
  7. Increase longitudinal stability.
  8. Improve pilot visibility by lowering nose during approach and landing.
  9. Lighten wing structure by eliminating structural cutouts.
  10. Improve performance by eliminating air leakage and contour discontinuities at critical locations on wing.
  11. Simplify construction and maintenance by eliminating dive-break bell-cranks and actuating systems from inside of wing.
  12. Improve stall characteristics.
  13. Flap dive break is non-icing and would not jam above icing levels in cloud.
  14. Lower take-off speeds.
  15. Lower tow speeds.
  16. Improved flight characteristics by shifting minimum drag bucket at low and high speeds.
  17. Improve flight characteristics by aligning fuselage with airflow at low and high speeds.
  18. Improve thermaling performance by permitting turns of smaller diameter.
  19. Cheaper to build.


Soaring June 1969