By Steve du Pont
Soaring June 1964
Three View Drawings Performance
It appears timely to report on the Helisoar Aircraft, Inc., HP-10 project.
In 1961 I considered several sailplanes for this project and chose Dick Schreder's HP-10. Partly because of its simplicity, particularly of the wing, and partly because of the outstanding performance demonstrated by Dick in the HP-10 in the U.S. Nationals in 1961, where HP-10 held first place for four meet days and won the Stroukoff award for best goal-and return flight. Dick's article in the February, 1961, Soaring had pointed out that little additional performance was to be gained in aspect ratios above 20 to 1. A look at a tapered wing indicates the complexity of building it. The Possibilities of applying modern manufacturing methods in the honeycomb concept used in HP-10 were attractive.
Since early in 1961 when the Type Certificate application was filed, there have been over 653 drawings made of parts and assemblies, the sailplane had been tooled and a complete manual for the homebuilder, including 241 special drawings, has been prepared. Our consultants, DeVore Engineering Service of New York, have run a stress analysis of virtually every part. These data have nearly all been submitted to FAA under the T.C. application and a substantial portion of the work approved. In most areas the safety margins are well above the required strength. We do not state if or when a type certificate will be obtained, but the work is progressing in an orderly fashion.
HP-10 customers are advised of the findings of the analysis and if changes are indicated, the customers are notified and the necessary materials made available. Meanwhile, kits are sold for Amateur Built Experimental Certification under CAM-1.
Fabrication of kit parts is to aircraft standards of quality by aircraft people under subcontract. All material is bought under affidavit and inspection has been run by a FAA-approved aircraft inspection department as carefully as if a type certificate had been granted.
All parts are manufactured to tools, and areas such as welding, critical bend radii, press forming of bulkheads and ribs, stretching and rolling of fuselage skins, heat molding Royalite leading edge, rolling canopy bows and the like are done, with the kit-builder left to trim to size and burr, set the parts up on simple wooden fixtures to our instructions, drill and rivet together. All material is included down to the last washer and rivet from which to build the sailplane, except instrumentation. Alclad duraluminum alloy sheet is used. It is also possible to purchase fuselage and wing in partly completed form, still in compliance with the Amateur Build Certificate as a home-built kit.
The simplified wing is indicative of the refinement for straightforward assembly, with its eight preformed honeycomb skins, its six ribs, and simple assembly procedure. It is based on the 65 (sub 3) 618 laminar airfoil. An airline captain without previous sheet metal experience built his basic HP-10 wing in 167 hours including the wooden holding fixture.
In January, performance tests were run against George Moffat's world record-holding Schreder designed and built HP-8 and at 60 mph the un-smoothed Helisoar HP-10 prototype in rough prime finish, performed at an honest 35-to-1 at 60 mph based on known performance of HP-8, which in the light of the many 40-to-1 claims being made on every side for the retractable gear "super" sailplanes, shows up pretty wel for 48-foot span and 20-to-1 aspect ratio.
As to ease of flight, it stalls at below 40 mph indicated with 45 degree flaps, and 46 indicated with no flap, and is, like the other Schreder designs, thermalled at 50 mph and up with 6 to 12 degree flap. Its -8 degree flap is used for high speed running, and with 75 degree down, the 32-foot span flaps will bring it down as steep an approach as could be asked for.
Quick-removable instrument panel and windshield are featured for fast maintenance of equipment at contests, and we are pleased with the results of our program to simplify rigging. With no wild claims, the ship can be rigged and de-rigged by two or three people in a perfectly reasonable time, and has been completely rigged and de-rigged for trailer and back by one person with the help of trestles.
While we make no promises, the usual program of improvements is planned with the policy of making them fit existing kits at a later date. The list includes such items as a new quick wing joint, droop ailerons, faired wing tips housing the aileron counter weights, clamshell wheel fairings, a possible retractable wheel, and of course that dreamed of self-launcher.
It is gratifying to be able to put into production such a development as Dick
Schreder's unique and excellent HP-10.
Wing Area................................................. 114 sq. ft.
Constant Chord ............................................ 28.5 in.
Flap Span...................................................34 ft.
Flap deflection ............................ 8 deg. up, 75 deg. Down.
Max Speed ................................................ 150 mph
Glide Ratio ........................................ 35 to 1 @ 60 mph
Construction .........................................Metal Monocoque
Wing: 1/2 inch thick aluminum honeycomb molded surfaces. Skins graduated from 0.050 at the root to 0.016 at tip. Leading edge is molded Royalite.
By George B. Moffat, Jr.
Soaring July, 1964
I'll start by sticking my neck out and saying that this all-metal ship seems to be the best buy in performance sailplanes today. Kits sell for about $3,500 from Helisoar Aircraft, Danbury, Connecticut. There are no current plans to manufacture completed HP-10s although an ATC is in the works.
The preformed sandwich construction of the wing accounts for a speedy construction, and the molded shape makes performance less subject to filling than on most metal ships. The rectangular wing takes a bit of getting used to at first but the 37 to 1 L/D helps.
The HP-10 had a poor record in the Nationals, highest standing achieved was 8th place by designer Dick Schreder in 1961. However, Dick was in the lead most of the contest, losing only by failing to complete the task on the last day. The HP-10s entered in subsequent Nationals have not been well enough flown to prove much. In straight flight, I have found that Joe Emmons' HP-10 is as good as the HP-8 at maximum L/D and stays very close up to 100 mph, after which the Eight's extra pound per square foot of wing loading begins to help. Recent tests of the HP-8 against the first completed kit showed very promising performance, considering the unfinished state of the wing and fuselage. I recorded a best L/D of 34 to 1 at 60 mph despite a gap between the wing and the canopy where the tape had blown off.
The handling of the original HP-10 left a good deal to be desired, particularly in yaw stability and coordination; consequently I was interested to spend three or four hours making several flights on the production model early this year. This sailplane has a foot added to the rear fuselage section as do all of the kits. It proves very pleasant to fly, well coordinated and light on the controls with no vices. Stability is positive but not excessive, the pitch stability being so light that a trim tab is not required, an attractive feature on all Schreder ships. Yaw stability is surprisingly good, about like that of the Sisu. Rate of roll is a brisk 4.6 seconds, well under the Sisu. Stall was straightforward and crisp, without wing drop, at an indicated 47 mph with zero flap, 43 mph with 12 degree and about 38 with full flap, 67 degrees.
The ship circles happily with 12 degree flap at between 50 and 54 mph. Judging from my experience with the Eight one would do better at the higher speed and a steep bank angel. The Ten is very easy to hold at constant speed in any bank up to about 55 degrees in fact, it feels a lot like a late model 1-23. On leaving a thermal, the acceleration is brisk, getting to 90 mph and up with no trouble. Since thermals were weak, I didn't get over 110 mph in order to conserve altitude. At the latter speed the HP-10 is dead quiet and feels very solid. The wing is remarkably stiff.
Rigging and de-rigging are not as good as the Sisu or HP-11 although far better than on the two original models. The wing is held together with 20 vertical pins (this will be simplified soon, I am told) which go in easily enough if one has the right horses. The parts are reasonably light and an average crew could probably rig the ship in 10 to 15 minutes.
Since the Ten does not have spoilers, all landing control is through the large flap. This proves very effective and easy to use thanks to an ingenious actuation which reduces loads on the crank. You can adjust the glide angle constantly during the approach by cranking on and of, or just come in high, roll in full flap, and dive for the spot you want to hit. At about 30 degrees nose down I was unable to get the speed above 60 mph. I would not hesitate to put the ten in any field a Ka-6 could get in. The brake is very effective.
The cockpit is large and comfortable, the visibility excellent. One can not adjust the rudder pedals, smaller pilots will require cushions. This of course helps keep the CG where it belongs. One of the many well thought out features in the productions model, indicative of the hundreds of hours of engineering which have gone into it, is the easy canopy removal so one can get at the instruments, tow hook, etc.
To sum up, the HP-10 seems to me a tremendous amount of value for the money in a kit that can be built without special tools or knowledge and in a relatively short time. The performance and handling are of contest winning caliber.