Can an HP-11 compete with, say, a Standard Cirrus?
Yes, says a top Canadian pilot-owner, if you know how to
Get the "H" Back into Your HP.
By John Firth
Soaring May 1973
From the ingenuity and incredible industry of Dick Schreder have come the HP series of sailplanes. What is H.P.? High performance? Maybe. High potential anyway.
The best and most popular of the HP series sailplanes are the HP-11 and HP-14. Dick produced the '11 around 1960 and flew it to third place in the 1963 Argentina World Championships. That it is still a potent performer shows what a marvel it must have been in those days. The HP-14 appeared at the last moment (as usual) before the 1966 U.S. Championships at Reno, which Dick proceeded to win.
After owning an HP-11 for seven years, I find it still a good competition ship; it is rugged, light, rigs easily, and handles well. The performance is on a par with the latest Standard Class ships, and it will land in a smaller field than any of them. Unfortunately, the average HP does not come anywhere near the original performance claims. Some of this is due to the inevitable optimism of all glider designers; a good deal is the failing of the builder. Though Dick Schreder gives you a mechanically rugged ship, the aerodynamic niceties are not spelled out. Here I hope to show some important items which make the HP-11 or HP-14 perform like a Standard Cirrus. The principles here can be applied on all sailplanes, and home builders will find ideas to use.
The flaps are the biggest assets of the HP's: Unfortunately, for structural simplicity, the undersurface camber was removed from the basic airfoil. (See Paul Bikle's "T-6 Performance," Soaring, Oct. '70) Putting it back makes quite a difference. Figure 1 should make the method clear; the extra 3% wing area is also beneficial, but with flaps down drag increases as well, and landing approaches are even more spectacular. The operating loads are quite high. To assist in balancing them, I recommend the addition of a spring as in Figure 2. If the friction in your flap system will not hold the flaps in position for normal flight operations, add some friction; a clamp on the operating rod works well. The spring adds a safety factor: if the operating rod or crank should fail, the flaps will at least go down far enough for low-speed approaches.
The most elementary failing on homebuilt ships (and others) is the lack of gap sealing, not only on fairings but also on control surfaces. This not only adds drag, but the controls do not work well either. Seal all hinge lines with adhesive tape; plastic-coated paper tape is available in several colors, and seems to last at least a year. Lubricate the hinges, and then clean the entire surface where gape is to stick with a degreasing solvent. Deflect the surface almost full before applying the tape.
Top-surface gap covers make the flaps work better. You need some 0.10 gauge 7075 cut into 2 1/2 inch wide strips (Figure 3). This is hard to come by here in Canada, but maybe Dick Schreder will sell you some. As the gap cover makes the push rods inaccessible, inspect them carefully. Mine were badly worn at the bushings. I recommend replacing the bushings with teflon. Bend the covers to a smooth curve, clamping them between 2 x 4s and pressing against the floor (Figure 4). Round off the back edge and lubricate the flap surface with silicone spray.
The wing root is a critical region on all gliders. To have the best chance of avoiding separation at low speeds, the intersection with the fuselage must be clean and free from excrescences and leaks. I suggest building fiberglass fairings attached permanently either to the wing or turtledeck. You need to have the wings in place to do this. Always tape this fairing to stop leaks, which can trigger flow separation.
Surface finish of the wing is important. Read Paul Bikle's excellent article if you want to know more. ("Polars of Eight," Soaring, June '70) In any case, the bare metal will not allow really good performance. Filling and smoothing is time-consuming and tedious, but the result on the HP-11 is a marked improvement at high and low speeds. Luckily for me, Dave Webb did an excellent job on my wings before I acquired the ship. Before applying filler, the whole surface must be cleaned and etched to allow a good bond. You can use card body filler, or micro-balloons in epoxy or polyester resin. Epoxy takes a day, at least, to set to workable hardness; be sure to mix the resin hardener will, before adding the micro-balloons. More than the normal percentage of hardener may be needed for best results. Ideally, the whole wing should be smooth and wave free; you can detect waves down to about 0.005 inch by rolling a straight edge chordwise over the wing. It should roll smoothly with no hesitation. Pay a lot of attention to the region over the spar. Mine has 1/8 inch of fill here, thinning to non at the leading edge. This seemingly excessive thickness stabilizes the skin under compressive loads. It also restores the wing thickness to 18% (reduced by the flap modification), and increases the overall camber slightly, thus increasing the usable CI . Every little helps. Apply the filler in several stages, waiting until it is really hard before cutting it down. A disc or belt sander, used with great care, is a tremendous help; the final contouring is done with #240 silicon carbide paper on a backing pad, with lots of water.
The tail should have gap covers similar to the flaps; bend the strips enough to remain in contact for 15 degrees of surface deflection. This not only reduces drag, but improves redder efficiency. If you still feel short of yaw control, try adding 6 inches to the tail height. The work is not excessive, and the higher aspect ratio tail gives you more improvement than the increase in area suggests. Also, fill in the recess in the root ribs to reduce slot leakage. Do not use foam here; it can get wet and freeze.
The fuselage has many areas which need attention for minimum drag; you should pay almost the same attention to smoothing the contours of the front fuselage as to the wing. Use a straight edge (as on the wings) to detect abrupt changes of curvature. The air intake should be recessed into the nose and have rounded edges. The front canopy should fit perfectly with no steps or leaks. Attach the plexiglass with silicone rubber, instead of the normal metal strips. Use the same stuff to mold a gap seal in place, all around. The biggest problem is the center canopy, where the pilot gets in. It should have smooth contours and no leaks. The problem is not simply to make the seal, but to allow for expansion and contraction also. Something on the lines of Figure 5 seems to work.
Needless to say, the wheel doors should fit properly with gaps sealed. Foam strip and RTV rubber again. Add a cover to the wheel well too. If you have trouble with the wheel pushing the doors open every time you hit a bump, add a gear lock to hold it in the "up" position.
The usual tail wheel can add about a pound of drat at 100 mph. Any thinning and fairing you can do here will help. At the rear end, leave a fair-sized hole to allow the cockpit air to escape. Fill all other holes with something; they only create noise. Which brings me to my last point.
Noise is environmental pollution, especially in the cockpit. It makes you inefficient. You may think your sailplane is quiet, but after those Ford adds, and with the FAA on the warpath with measuring devices, do you perhaps begin to wonder if your ship is as quiet as it could be? After all the other good work you have now done, the ship probably sounds pretty good from the outside, but for serious competitive flying, or just plane peace and quiet, do some acoustic treatment. Try adding a foam-coated bulkhead to seal off the tail-cone behind the wing (not forgetting a hole to pass the cockpit air.) Cut some pieces of soft foam to seal around the flap torque tube. Add a cockpit lining of thin carpet, and you will have a really quiet ship. Watch that airspeed!
Maybe a few personal comments on how to operate your "new improved" HP-11 will be of help, if Dick Schreder will forgive me.
Starting logically from takeoff, select "up" flap like the new fiberglass drivers before you start to roll; gives you better aileron control. Concentrate on direction and when this is fully under control, raise the tail with forward stick. When a glance at the ASI shows 35-40 knots, pull on 20 degrees of flap, and the ship will leave the ground cleanly, under full control, without change of attitude. Bleed off the flap as the speed rises. I find it also more comfortable to follow the towplane using flap rather than using elevator.
Despite Paul Bikle's comments on the T-6, 7-8 degrees of flap at 45 knots seems right for thermaling at 30 degrees of bank. In smooth thermals I can get down to 42-43 knots. Experiments in wave, against a Skylark 3, showed that 10 degrees of flap at 40 knots does not noticeably increase the sink rate. However, lateral control gets a little sloppy. At the high-speed end, minus 5 degrees and 90 knots gives L/D around 22 as determined by comparison flying.
Those of you who have let down from great altitude in a hurry know how uncomfortable full flap decent can be, if only because you slide forward. A comfortable 2000-fpm decent can be made by stalling the ship gently, with the wings level, and holding the stick back. The ship then settles like an elevator, to quote an eyewitness. Be prepared for a rapid spin entry if a wing drops, and have a good look beneath you before you do it. Recovery is quickly effected by releasing the back pressure. The ship gives a little shake and starts again around 45 knots.
Circuits and approaches can be flown slower than on most high-performance sailplanes. In light winds, 45 knots gives a good margin over the stall, if you use more the 20 degrees of flap. The full 90 degree flap gives a very steep approach at this speed. If you are dropping short, 40 to 50 degrees can be taken off with no danger of stalling (see Bikle's curve for CI max, Soaring October 1970.) Raising the flap once you are firmly on the ground will allow better use of the wheel break and better aileron control.
Well, that should keep you all busy for the next couple of winters. Good luck.
David Webb's letter to editor printed in Soaring July 1973.
As one of the builders (with Ben Price) of John Firth's HP-11, and originator of some of the mods mentioned in John's article (Soaring, May '73), I would like to add a word of caution about modifying the aircraft without careful thought.
The flap extension mentioned was actually pieces of 0.040" aluminum bonded and riveted between the upper and lower trailing edge flap skins during initial construction, and was not just bonded onto the top skin as shown in the sketch. The flap extension was minimal and was, from memory, approximately 1 1/2" at the root tapering to 3/4" at the tip. This extension was, of course, carried through to the ailerons as well. However, with this mod, it is almost impossible to wind the flaps down at high speed. This must be done progressively as the speed bleeds off. We felt it was preferable to leave the operating loads high to discourage putting too great a strain on the flaps. (See Paul Bikle's "Flap Failure" comments on the HP-14, Soaring, October '70, page 30.)
The original idea was to put some camber back in the flap and slightly increase the wing and flap area and, hopefully, the low speed performance of the aircraft to make it suitable for Canadian thermals. Additionally, and for the same reasons, the wingtip shape was modified to the now familiar form -- a fixed linkage aileron droop mechanism interconnected with the flap was added, and the aileron differential movement was increased to compensate at soaring speeds.
I agree with John's assessment of the HP-11. I think it was one of Dick Schreder's best designs and, suitably cleaned up, it performs well. Sometimes I feel that I acted hastily when I sold the aircraft to John in early 1968, since it still appears to be competitive with aircraft of comparable span.
In conclusion I should point out that the above mods were carried out without involving Dick Schreder and were "unofficial".
Fort Erie, Ontario, Canada
JOHN FIRTH's letter to editor printed in Soaring July 1973.
In response to Dave Webb's letter on the HP-11 (Soaring, July '73), which was triggered by my article (Soaring, May '73), I acknowledge that the major modifications were made by the builders. The intent of the article was to awaken homebuilders and HP owners to the real potential of the HP's, not to claim all the improvements as my own brain waves. However, having done 400 to 500 hours of work on the ship in the last five years, I feel justified in writing the article.
The figure of 1 1/2 inches at the root and 3/4 inches at the tip for the flap extension are correct. However, the original construction has not been entirely satisfactory; the riveting of the 0.040 strip between the upper and lower flap skins resulted in some profile distortion and the camber change has not stayed uniform. Also, the filler cracked and bulged. As most builders will have completed aircraft, I devised another scheme. I believe that a properly executed epoxy or RTV 152 bond for the extension strip will be satisfactory in combination with the lower surface glass reinforcement. This should provide better stiffness and stability than the method used by Dave. However, perhaps 0.032, with countersunk rivets on the top surface would be advisable.
Dave Webb expresses concern that the flap extension, combined with the balance spring, will result in the pilot being able to overstress the flap. The vertical dive case may be worse, but it is highly unlikely that the deployment speed will rise higher. For normal humans, winding down the HP-11 flap (two urns of the handle) above 60 to 65 kts. Is physically impossible. With the extension and the springs, this speed remains the same. Paul Bikle's over-vertical dive case was extreme, and the failure occurred 22 knots above the 80 knots maximum speed for deployment. Thus I see no danger in adding the counterbalance spring; indeed, it would seem to add safety.
Dave's comment regarding "approved mods" is well taken, but surely this point is well understood by the builder of an experimental ship. I hope those eager to climb into fiberglass will note Moffat's comments (Soaring, July '74) on the virtues of a stiff metal structure. George turns out to be right most of the time.