In a corner of my hangar lies a graceful white pod, the remains of an American Eaglet motorglider my uncle built in the 1980s. Lovely on paper, it was a beast in the air; after a terrifying test tow, my uncle sawed the wings in half so that it might never pose a threat to anyone again. Later, Derek Piggott, in a PIREP for Gliding Magazine, wrote that “it was the only time when flying a glider [that] I continued with a tow because I was too scared to release.” The Eaglet eventually made its way to my shop as an item of curiosity. The longerons appeared to be constructed of curtain rods and the control fittings made use of fishing tackle (this is not a joke). However, in the ’80s, this was sometimes what Experimental aviation looked like—and sometimes still does.
Things have changed substantially since the Eaglet was designed. An example of just how far homebuilt sailplanes and motorgliders have come is Bob Kuykendall’s HP-24, a high-performance, 15-meter glider aimed at the Experimental market. With the upward spiral of avgas, and words like “eco” and “green” working their way into the aviation lexicon, the U.S. is seeing a renewed interest in soaring. Of course, there has always been a diehard core of soaring pilots in America, but their ships have come almost exclusively from Europe. With the HP-24, that may be about to change.
Brad Hill’s HP-24/Tetra 15 is the first flying example of the new HP-24 kit.
Asymmetrical Wings
The HP-24 builds on the legacy of Richard Schreder, a soaring pilot and homebuilt-glider designer whose HP series of gliders were offered as kits in the ’60s and ’70s. The HP-18 was his last and most popular kit design, but with the rise of European glass ships, the HP-18 was no longer a serious contender at the contest level. Nevertheless, they remain popular with soaring clubs; and, in 1999, Bob Kuykendall and Steve Smith, formerly of NASA and known as “Dr. Smith” to you Mythbusters fans, decided to develop a set of HP-18 “super wings” that would keep the stock spar, but use a more modern airfoil.
Parts for the autoconnect arm and funnel. This autoconnect does just that—automatically connects the flaperons and airbrakes to the controls when the wings are assembled. Kuykendall studied causes of glider accidents and found that failure to properly connect the controls when assembling the wing was a common cause of accidents. The autoconnect precludes this from happening.
Smith suggested using an airfoil from a popular European glider. Since there were no published coordinates for this particular airfoil, Kuykendall took measurements directly from an actual wing, which Smith used to create a smoothed data set on his computer. They then scaled the coordinates and made fiberboard templates to validate against the original wing. And this is where the story gets interesting.
The next time they had access to that particular sailplane, they tried the templates on it. “They didn’t fit worth a darn,” says Kuykendall. “There were huge gaps. It was like the templates were for an entirely different glider. Puzzled and disappointed, we headed back to Steve’s car. Then we both stopped at the same time, turned to each other, and simultaneously asked: ‘Which wing did we originally measure?’
“We returned to the trailer, got out the other wing, and tried the template on it—a perfect fit! What we had discovered was that the airfoils of this particular glider are substantially different between the right and left wings. Tests using our templates showed this same asymmetry in every example of this glider we encountered. And these are not dog-meat gliders; they are expensive, go-like-stink gliders that fly straight as an arrow and have national championships to their credit.”
The tapered carbon fiber spars are pinned together at each end. The tubes fore and aft of the spar are lift pins to react to wing torque (pitch) forces. The flaperon and drag brake pushrods can be seen just behind the spar; their respective funnels have yet to be installed.
[Author’s note: Just to be clear, this asymmetry was not a design feature. While it is true that many aircraft use different airfoils at different span stations, the asymmetry that Kuykendall and Smith discovered was instead a byproduct of the manufacturer’s apparently loose fabrication tolerances—tolerances that nevertheless resulted in a safe glider with no apparent performance decrement.]
“On the basis of those template tests,” Kuykendall continued, “I decided that the accuracy and consistency required to make truly-high-performance sailplane wings lay within the grasp of an amateur sailplane developer such as myself. Soon afterwards, I started the sketches and working drawings for what was to become the HP-24 kit sailplane.”
The airbrake torque tube jigged up for welding. All steel parts of the glider are sent out for professional welding on precise jigs fabricated by Kuykendall.
RV-Inspired Sailplane Kits
Kuykendall’s hero, and a source of engineering inspiration, is Richard VanGrunsven, designer of the RV series of aircraft. While most kitbuilders are familiar with the RV line of kits, what is less commonly known is that VanGrunsven is an avid sailplane pilot. In fact, VanGrunsven offered hangar space at his factory to do the static load test of the prototype HP-24 wings. Kuykendall wants to do for kit sailplanes what VanGrunsven has done for small GA kit aircraft: allow builders of average mechanical aptitude to construct straight, safe planes with a reasonable investment of time and money, enabled by economies of scale and excellent customer support.
“The evolution of VanGrunsven’s designs, from essentially scratch-built to CNC prepunched, and finally, to match-hole technology, takes a lot of the stress and worry out of aluminum construction,” says Kuykendall. “The HP-24, with its composite construction, is obviously a different animal, but our design and evolution philosophy is driven by similar concerns and is based on lessons learned from problems inherent in previous kit sailplanes.”
Kuykendall says he has “personally examined almost every kit sailplane out there, including most of the composite ones. What one tends to see is companies selling long, floppy shells—fuselage halves and wing skins—that require builders to spend hundreds of hours building alignment jigs. There is a lot of time and energy that goes into addressing the worry over straightness, and this is a bigger concern on a 60-foot wing, where a small deviation inboard can be quite large at the tips, as compared to, say, the wing of an RV. By preassembling components on our factory jigs and molds, while staying within the 51% rule, we save the builder a significant amount of time and stress.”
Those jigs and molds are production-quality pieces of art and represent a huge investment of man-hours—an investment the builder avoids by doing the large-scale layups at Kuykendall’s shop during the factory-assist portion of the build.
Construction Considerations
Kuykendall considered going the LSA route (for which a glider category exists), but there is simply too much paperwork and compliance for his small company, HP Aircraft, LLC, to deal with at this stage. Nevertheless, he guarantees that the HP-24 will qualify for Experimental/Amateur-Built. “I will do whatever it takes to ensure that builders are in compliance with the 51% rule. Early kits will naturally require more builder involvement, but as we move forward toward more prefabricated parts, we will work with the FAA to ensure the builder is still doing the major portion.” For example, bottom wingskins will be molded with removable panels that allow the builder to install the integral control hardware for flaperons and ailerons. This work traditionally requires lots of cure-cycle time for individual parts (i.e., needing to wait for a part to cure before proceeding to the next step), so it is more efficiently done at the builder’s home shop, rather than at Kuykendall’s shop. And for Canadian E/A-B rules, which require inspection before closing up major assemblies, Kuykendall will provide fuselage halves rather than the prebonded fuse that U.S. buyers will receive.
Bonding top and bottom wingskins. Once all internal fixtures are complete, the top skin is lowered onto the bottom skin and the two jigs clamped together for bonding. Alignment pins and in-mold bonding ensure precise alignment and straightness of the final wing.
Although Kuykendall sees a relatively-fast build time for the kit, estimated at 500-1000 hours, depending on builder skill level and familiarity with composite techniques, builder participation is paramount. Kuykendall encourages all builders to spend at least one week at his shop, laying up tail, wing and fuselage skins, and thereby learning the skills necessary to finish and maintain their sailplane.
This jig fits into alignment holes on the upper wingskin mold and allows the airbrake to be precisely located on the skin. The extensive use of such jigs in the HP-24 project means that customers will get parts that fit precisely right out of the box.
The first five builders of the HP-24 will receive a significant discount off the price of their kits. Kuykendall knows these early customers will be beta builders, working with standard kits before quickbuild kits are available. He views these builders as sharing in some of the development work of validating the kit assembly process, so he plans to reward them with $6,000 off the 2013 kit price of approximately $32,000. (This price is for the basic kit and does not include paint, avionics, or trailer.)
Down the Road
Given Kuykendall’s “open-source” mindset, it is no surprise that even with only one ship flying, he’s already thinking about HP-24 variations made possible by future technology. “Long term, I’d like to see a self-launch model, an FES [front electric sustainer] system, and water ballast options that will significantly expand the cruise-speed profile,” he explained. In a sense then, customization of the HP-24 is part of the building process.
“It’s important to me that readers understand that, in my philosophy, ‘HP-24’ is not the name of an aircraft type. Rather, it is the name of a personal dream, an epic project, and most broadly, a kit from which an aircraft can be assembled,” said Kuykendall. “In the eyes of the FAA, the person who builds and certifies an E/A-B aircraft is the manufacturer of record, and they are free to call the aircraft whatever they wish. I therefore wish to establish a tradition in which the sailplanes arising from this kit each bear a unique name. Brad Hill, builder of kit number 1, started this off by declaring his aircraft the ‘Tetra-15’; and when I finish my own aircraft, it too will have its own name. I realize this is somewhat unusual, but it reflects my belief that no two amateur-built aircraft—even those built from the exact same kit of parts—are identical. So, I guess you could say, ‘We provide the dream, you create the story.'”
Finished wing loaded to 4.4 G (2200 lbs). Future wings will feature a spar good for more than double that. Note the degree of deflection – the tips have deflected almost 4 feet relative to the support stand.
Akawhat?
Laying up the fuselage in the production molds is a team effort at an Akaflieg.
By its very nature, the HP-24 project has drawn quite a few people into its orbit—designers, pilots, prospective builders curious to see if this could be their next project, even engineering students. Most get first-hand involvement via the “Akaflieg,” a term Bob Kuykendall borrowed from the German “Akademische Fliegergruppe.” These are groups of aero-engineering students at German technical universities who typically design gliders. Kuykendall hosted his first Akaflieg in 2005 and had just wrapped up AF15 as this article was going to press.
The Akafliegs are a great way to learn composite construction skills, network with like-minded folks, and get an idea of what building an HP-24 kit involves. The AF15 crew was composed of Kevin Christner, a venture capitalist and glider historian with 500+ hours in more than 30 glider types; Joshua Daisey, an aviation industry business manager with 75 hours in his RS-15; Doug Gray, a retired Silicon Valley engineer with 40+ years in gliders; Craig Wexelberg, a 29,000-hour captain with US Airways; and myself. Our work mostly centered on laying up the flaperons for Gray’s ship, HP-24 kit number 2.
The finished fuselage half, ready to meet its mate. All major components for the HP-24 will be molded at the HP Aircraft facility. Participation by the builder will be part of the package.
For Christner, Daisey and Wexelberg, the Akaflieg was an opportunity to learn more about the HP-24 before actually purchasing a kit. “I was impressed with Bob’s referencing experts in critical areas and then applying his own manufacturing techniques,” said Daisey, a graduate of Embry-Riddle with both an A&P rating as well as an MBA. From Christner’s perspective, “there are many ways to construct a glider to fly a particular profile. What is more important are the planform, convenience, and features like autoconnects—and these are covered well by the HP-24.” Christner has a particular need for tip extensions and water ballast, and Kuykendall’s willingness to customize kits to customer’s requirements was a big plus for him.
In contrast, Wexelberg was more focused on ease-of-build, as well as a kit offering an FES (front electric sustainer), issues which he was able to discuss face-to-face with Kuykendall. The week allowed him to learn new skills, and also realize that he’d probably have to wait for the kit to mature a bit more before he’d be ready to build.
Carbon fiber produces the lightest structure in a compact and aerodynamic package.
Gray has been with the project since its inception; Kuykendall, in fact, crewed gliders for Gray as a youth. Gray emphasized his confidence in Kuykendall’s design but admitted that beta kits are still at a preproduction level. “I’m satisfied with Bob’s engineering design ability—he hasn’t tried anything radical, and all production is under his oversight,” Gray explained. “But much of it is still in his head. Manuals and plans have yet to be formalized, and the HP-24 components require a higher level of prefabrication compared with the typical kit plane, because so much is bonded together. Safety-critical parts need to be completed at the factory.” Gray also stressed the importance of prospective builders attending Akafliegs: “Builders need to have an idea of the construction of critical parts, and what and where you can sand without causing damage.” Gray is aware that, statistically, only a small proportion of homebuilders finish the job. To succeed, he said, “you need time, other planes to fly while building, and preferably, a big shop with the necessary tools.” Sounds like the ideal starting point for any type of kit.
—E.S.
Summer 2013 Akaflieg participants, L to R: Doug Gray, Craig Wexelberg, Bob Kuykendall, Kevin Christner, Joshua Daisey, and the author.
A Carbon Pussycat—HP-24/Tetra-15 PIREP
I’ve been flying sailplanes for 35 years now, and the HP-24 is my third homebuilt sailplane. In the late 1990s, I was the first builder of the Russian AC4K, which I flew for several years before purchasing a Slovenian Apis13 kit. I was also the first one in the air with that model, so I’ve managed to gain quite a bit of experience and knowledge regarding the techniques used to build composite sailplanes.
The cockpit is typical of a modern sailplane with room for necessary instruments and computers.
I originally met Bob Kuykendall back in 2009 at a speech he was giving at the Museum of Flight in Seattle, Washington, where I had worked for Boeing for many years as a technical illustrator. Bob and I immediately hit it off; I offered to do a 3D CAD model of the HP-24, and the next thing I knew, I was at his shop in Arnold, California, helping to design and build tooling for the ship. I found myself with the time and resources to act as the beta builder, but I was working so far ahead of Bob’s schedule that, eventually, I called my ship the Tetra-15. Aerodynamically, the aircraft itself is essentially an HP-24, but the cockpit is fairly customized. So although I’ll refer to the HP-24 in this PIREP, realize that these are my experiences based on the Tetra-15.
Rigging
The HP-24 is rigged with standard rigging tools—it can be done alone or with a helper. With two people, we can go from trailer to flight ready in under 20 minutes. Because each wing weighs only 110 pounds, tip weight is minimal. One feature that I do not particularly like are the non-tapered lift pins. I understand why they were designed without taper, but it does mean guiding the wing in takes a bit of precision and finagling. Ground handling is a snap, and with a tailwheel dolly and wing dolly I am easily able to push the glider to the staging area by myself; it helps that the glider is about 100 pounds lighter than comparable ships.
Up and away—the sailplane looks more at home in the air than most flying machines, soaring on slender wings for hours at a time.
The tow hook is a Tost unit in the nose. Tost is the German gold standard for hooks, and with the hook in the nose, tows are quite enjoyable. The plane naturally wants to track, and I can retract the gear as soon as we are in the air, which keeps things quiet. If I am launching without a wing runner, I have roll control within just a few feet; there are no unusual or exceptional control inputs required on takeoff.
What a Joy!
After release, I roll right to clear the tow plane. My thought when I first flew the HP-24 at this point was “What a joy!” The HP-24 is very stable about all three axes. Roll rate with the full-span flaperons is approximately three seconds, 45º to 45º. Due to the prototype bellcrank geometry, the Tetra is set up with the following flap settings: two up, neutral, and two down. (This has been changed to one up, neutral and three down in the production HP-24 kit.) I use positive flaps for slow flight and climb, and negative for high-speed cruise (75-120 knots). Negative deflection makes a big difference—I can really feel the glider accelerate with negative flaps. I should point out that both the flap and airbrake controls are levers rather than slides, which is a departure from normal gliders. But I have to say, I actually prefer the levers now.
The elevator is very light, but has a positive stick coefficient. The spring trim is effective between 80-120 knots, beyond which I have to override. However, this is no different than production sailplanes. With flaps down though, trim override drops to about 80 knots on the Tetra, which is lower than I’d like. I also find that in this configuration, I need to trim often. On most ships, you set it and forget it. This is one area of the controls that Bob is addressing for the production ships.
The controls are well-harmonized and coordinated flight is a piece of cake. It’s easy to hold a banked turn and keep the yaw string either straight back or towards the high wing, depending on your preference. Even with full flaps and spoilers, control balance and harmony remain perfect.
The HP-24 is typical of modern sailplanes – a compact egg-shaped cockpit, minimal boom, and a T-tail for good control and ground clearance.
Mixing it up
Stalls don’t harbor any surprises, and my Tetra seems to drop off to the left. In my experience, most gliders favor a break either left or right, and the fact that mine breaks left may merely be a rigging issue with the tiplets; it’s certainly not a design feature.
I’ve only spun the Tetra once—by accident—just above incipient. I was a few knots above stall, really working some light lift to avoid landing out, and I pushed it too far. It was interesting; it popped out as soon as I relaxed stick pressure. Altitude loss was only 200 feet, and I find that comforting. To provoke a spin, you really have to exaggerate control inputs.
I am not a qualified aerobatic pilot, but early in my flying career, I did some pretty crazy stuff with hang gliders. I once managed to find myself tumbling from a loop and unable to pull out, only to regain control at about utility-pole height! Suffice to say, I now leave the aggressive stuff to competent aerobatic pilots. I have done chandelles, wingovers, that sort of thing, but no loops! Nevertheless, the Tetra-15 spars were tested to 4.4 G, and production HP-24s will have even beefier spars to accommodate the 18m tips; so loops or other aggressive maneuvers are certainly within the performance envelope of this ship.
Sailplane flights start with a noisy tow to altitude, but they become oh so quiet when the towplane departs.
Contest or Cross-country?
During my Phase I testing (which I like to point out was shorter than that assigned for my previous homebuilts, so I guess the FAA was impressed with the design), I went through a variety of test cards drawn up by some of my Boeing associates. We flew the cards on relatively stable days, but there is still a fair amount of scatter in the data. Nevertheless, our conservative estimate for L/D is approximately 39:1.
[Kuykendall points out that Brad’s estimate may indeed be conservative, since Brad himself has reported gliding evenly with other gliders whose manufacturers have reported L/Ds in the 42:1 range. Whether those numbers are themselves accurate is perhaps a game of liar’s dice, but Kuykendall’s design calculations—which have been quite accurate so far—are closer to 42:1; so it is possible that, with a bit of tweaking and a professional test pilot, the production HP-24s will see those kinds of numbers.]
Due to the unexpectedly-light weight of the plane (and thus lower wing loading) penetration starts falling off around 80 knots, but this can be improved by using water tanks. I know that Bob is currently developing a set of wings for a customer with water-ballasted leading edges that will take the wing loading up to around 11 pounds per square foot, which should give some fantastic cruise speeds on strong days. On the flip side though, I can outclimb just about anything out there; the usual comment I hear in thermals is “I can’t catch you!”
As a cross-country ship, builders will not be disappointed; the HP-24 has met and exceeded all my expectations. I have flown several seven-hour-plus flights with no fatigue. The carbon wings give a super smooth ride in turbulence. The cockpit is roomy and relatively quiet; the glider has adjustable rudder pedals and can accommodate pilots up to 6 feet 2 and 240 pounds. The canopy rails are low enough to give excellent downward and rearward visibility. I have plenty of room for parachute, water, oxygen, and my flight gear. Ingress and egress are quite easy with the forward-hinging canopy and pedestal-mounted instrument panel. The landing gear, which I would call “legacy Schreder,” is robust and shock-absorbing, and the airbrakes provide excellent glide path control. I have no fear of landing out, even in a small field.
I should point out that for actual performance data, interested readers can go to www.onlinecontest.org, where I log all my On-Line Contest flights. I have several times been Region 8 OLC Champion, and occasionally have ranked in the top one or two worldwide. Of course, this is based on who logs their flights, but there are several thousand pilots competing, so that’s not too bad. In addition, my YouTube page, apisdude, has quite a few HD videos shot from the tail of the Tetra-15.
Final Impressions
So, how does the HP-24 stack up against German glass? Well, it stacks up great, if you compare apples to apples. By that I mean, current top-of-the-line German ships are not a fair comparison: they are probably triple the cost and use both high-tech aerodynamics like active boundary layer control and high-tech tooling that just isn’t available to homebuilders. To design something of that nature was beyond the scope—and the acceptable risk levels—of this project.
That said, compared to a 1980s-vintage Ventus or LS6, which are in a similar price/performance range to a new HP-24, the HP does well. I fly in a lot of competitions. And although I can’t keep up at certain cruise polars, I can outclimb just about anyone; and the ship seems to be holding its own pretty well. I’m not a seasoned competition pilot though, and that is one thing Bob would like to do—get an HP-24 into the hands of a professional and see what it can really do.
Because the HP-24 has no vices, and because the controls are so well balanced and harmonized, a student with 100 hours in a fiberglass trainer would have no problem flying one. As far as building, it has been designed and laid out so that a first time builder will find it easy to build. And not only fun, but educational as well. I’ll reiterate what Bob has said: “You build it yourself, and you can fix it yourself.” I literally touched every part of this plane during the build. It was a fascinating experience.
—Brad Hill
How can I get in Involved
I see that HP has given up the LLC status. Are they still operating?