Can I Build an HP-Series Glider From Scratch?

The answer to that question depends absolutely on who you are, and how you define "from scratch." If it means that you buy a few Essential pieces from HP Aircraft, LLC and make the rest yourself, then yes, building an HP from scratch is within the reach of most amateur aircraft builders of moderate skill.

If building from scratch means starting with raw materials and proceeding from there, then the question becomes a little more complicated. If you are comfortable with the thought of commissioning thousands of pounds of custom aluminum extrusions from Alcoa or Reynolds Aluminum, then the answer is an unqualified yes. If you are comfortable with the thought of chucking up a bar of 1" x 2.5" 7075-T651 aluminum on your twelve-foot mill table and hogging out a spar cap, then the answer is also an unqualified yes.

Having said all that, keep in mind that you don't really have to build an HP from scratch; virtually all of the components for the HP-18 and RS-15 are available from HP Aircraft, LLC.

The Resourceful Builder

Whether or not it is possible to build an HP at all depends on whether or not one is what I have come to call a "resourceful builder." This term probably conjures up visions of the sort of folks who read Popular Mechanics and built their own submarines from plans advertised in the back. And well it should - the HP series of gliders have always been somewhat of a challenge to construct. Compared with Dick VanGrundsven’s RV-series airplanes, the modern standard by which all sheetmetal kitplanes are measured, the HP kits are far less intuitive, and offer far more opportunities to go astray.

To build an HP kit, you need to do more than read step-by-step procedures. You need to interpret and understand the intent of the blueprints and instructions. Everything you need is there; you just have to look deep and find it. The resourceful builder’s most important resource is between their ears.

Plans

HP Aircraft, LLC currently has plans in stock for the HP-18 and RS-15 series sailplanes. Other plans sets, such as for the HP-11, HP-14, the T-tail, and the Schreder trailer, will be available soon.

This is good news for those who have completed HPs but no drawings; the plans are indispensable for repair and maintenance information. For the scratch builder, however, the plans are only useful if you can procure the necessary Essential Parts (or suitable substitutions) described later.

Regarding the HP-18 (and probably true regarding the RS-15), Dick wrote:

"It should be emphasized that the '18 can't be built from the plans alone. Detailed dimensions are not included for parts that are already made or that have been scribed on material in the kit. The prints are intended to help in assembly only."

While this is absolutely true for Essential parts, the HP designs are generally so completely straightforward that the resourceful builder should be able to obtain dimensions for the merely Troublesome parts by careful inspection of the plans or by measuring the parts of an existing aircraft.

Essential and Troublesome Parts

All of the common HP-series gliders are based on a handful of essential parts that are pretty much indispensable, i.e. you can't really build the glider without them, and you can only get the parts from HP Aircraft or from an unbuilt kit.

There are also some of what I call "troublesome parts"; these are parts that are somewhat difficult to make or obtain, but can be obtained or made by the resourceful builder.

The following sections describe each of the common HP gliders in terms of essential and troublesome parts. Starting from the top:

HP-18 and RS-15:

The HP-18 and RS-15 both use aluminum wing spars, PVC foam ribs, and a bonded wing skin. Both have fiberglass fuselage pods with an aluminum aft fuselage; but the RS-15 uses an tubular aluminum tailboom while the HP-18 uses a conventional aluminum monocoque tailcone. Because of the remaining stocks of wing spar caps and fuselage pods, these are probably the most buildable HPs left.

Aesthetically, I find the HP-18 and RS-15 to be among the most pleasing and well-balanced homebuilt aircraft ever. With the sleek flowing lines of the narrow-waisted fuselage, and the smooth bonded wing skins, they were among the first homebuilt aircraft ever to not look like they were built in garages. Dick Schreder deserves a lot of credit for coming up with those two designs.

Essential Parts:

• Machined Spar caps - The 12-foot-long wing spar cap sections are precision machined from solid 1"x2.5"x12' bars of 7075-T6 aluminum, and without them you are pretty much stuck. HP Aircraft has several full sets of HP-18/RS-15 spar caps in stock.

Troublesome Parts:

• Fuselage Pod - The forward fuselage pods for the HP-18 and RS-15 are fiberglass layups utilizing heavy-weave fiberglass cloth and a conventional polyester resin. I originally listed the pods as essential parts, but at least one Resourceful Builder (a man named Robert Leonard) has pulled a plaster mold from an existing HP pod and built their own copy. Fortunately, isn't really necessary. Like the spar caps, HP Aircraft has several each of the HP-18 and RS-15 pods in stock.
• RS-15 Tailboom - The RS-15 uses a 6" diameter aluminum tube for the tailboom. Although difficult to obtain, aluminum tubing of this size can be found if you know where to look. Fortunately, HP Aircraft has several RS-15 tailbooms in stock.
• Shear Web Layout - The height of the shear webs that separate the spar caps determines the depth of the wing spar. While the resourceful builder can determine the outlines of these parts by measuring an existing aircraft or by working backward from the profile of the FX 67-K-150 airfoil, it would be easier to just order a tracing of the shear web pattern (or even the full shear web set) from HP Aircraft.
• Spar root fittings - The spar root fittings consist of a set of tie plates that mate the right and left spars, and some other odd close-tolerance machining. I've made all of these parts myself, so I know it's possible; I also know that it's not easy. The dimensions can be deduced from the assembly drawings, and any competent machinist can produce them. Fortunately, HP Aircraft has several sets of the root fitting parts in stock.
• Wing Ribs - Except for the formed aluminum ribs at the root and tip, all of the ribs are cut out of 3/8" thick rigid PVC foam of 4.5 to 5 lbs/cubic foot density. HP Aircraft has patterns for the ribs, and is also working on a minor refinement of the original airfoil that should have better stall and bug-handling characteristics.
• Fiberglas ancillaries - There are a bunch of minor fairings for the wing root, the center canopy (some call it the turtledeck), the stabilizer roots, and the tailcone "stinger." These are available from HP Aircraft, but wouldn't be particularly difficult for the resourceful builder to replicate.
• 5’-Wide wing skins – HP-18 and RS-15 wing kits included inboard wing skins cut from 5-foot wide coils of 0.025" 2024-T3. Apparently, Dick Schreder ordered literally tons of aluminum in order to get aluminum in the 5’ width. You can build an HP without it, but it means an extra skin splice at the inboard aft corner of the lower surface of the wing. Fortunately, HP Aircraft now has this material in stock.
• Other Parts - Pretty much everything else on the HP-18 and RS-15 falls within the skills of the typical A&P mechanic. Of course, there's a bunch of work to be done with an 8' sheetmetal brake (or 12' if you can find one), a bunch of formed aluminum parts, and about 25 lbs of various steel weldments. Most of these are shown in the drawings with enough detail to work from; for some others you may have to take dimensions off of an existing aircraft. Also, there's things like the canopies, wheels, brakes, and other tidbits; fortunately there are commercial suppliers for most of this stuff. HP Aircraft has a wide assortment of these parts in stock, and we have the drawings and tooling to manufacture virtually any HP machining or weldment that might be required for a kit or a special order.

HP-16

The HP-16 comes in two different flavors: Early, and Late. The early birds used an aluminum I-beam spar where the whole spar was milled from a single huge chunk of 7075-T6 aluminum. Actually, it came in two pieces, since there's a splice at the 12' line. However, Dick didn't like that arrangement very much, and so later HP-16s used the same wings as the HP-18/RS-15. The following information applies to the later-type HP-16.

The HP-16 generally seems to perform about the same as the HP-18 and RS-15; due largely to the similarity between the wings of all of these aircraft. However, the HP-16 seems to find more favor with larger pilots because of its more generous cockpit. The -16’s greater girth probably makes for some penalty at higher airspeeds, but the difference is seems to be pretty subtle.

Essential Parts:

• Machined Spar caps - See description above for the HP-18 and RS-15

Troublesome Parts:

• Fuselage Belly skins - The forward fuselage of the HP-16 used the same sort of compound-curved aluminum belly skins as the earlier HP-11, -14, and pretty much every other HP up to the HP-18 and RS-15. Dick made these himself by duplicating prototype skins using special hydroforming tooling. Dick said that the prototype skins were made by a local aluminum fabrication artisan (Actually, it was Morry Hummel of Hummelbird fame) by the laborious process of bumping and shot-bagging. HP Aircraft does still have one HP-16 belly skin in stock.
• Shear Web Layout - See description above for the HP-18 and RS-15
• Spar root fittings - See description above for the HP-18 and RS-15
• PVC Foam Wing Ribs - See description above for the HP-18 and RS-15
• Fiberglass ancillaries - See description above for the HP-18 and RS-15
• 5’-Wide wing skins – See description above for the HP-18 and RS-15
• Other Parts - See description above for the HP-18 and RS-15

HP-14

The HP-14 is pretty much the definitive HP-series glider; all metal and All-American (well, North American, eh?). The fuselage lacks the pod-and-boom look of most other HPs, and the wings have tons of area that make it a great ship in most conditions. Dick Schreder won the 1966 nationals in Reno in his new HP-14. Also, the late Les Sebald flew his all over the high parts of California and Nevada, and accomplished some incredible feats with it. Once, when he was diving downwind off the top of a Sierra wave, Oakland Center clocked him on radar with a groundspeed of over 300 knots. Subtracting the 100 kt tailwind and correcting for altitude brings it below the HP-14’s Vne, but still… And to answer your next question, yes, Les was one of the first to regularly carry and use a transponder in his glider.

Essential Parts:

• Wing Spar Caps - The HP-14 wing spar cap is based on 30-foot lengths of of 1/4" thick strips of 7075-T6. This material is available if you're prepared to pay aerospace prices for aluminum, but it is hard to find and even harder to ship. HP Aircraft has enough of this stuff left for one wing, but not enough for a full spar set.

Troublesome Parts:

• Spar root fittings - The HP-14 (and many other HPs besides) have massive aluminum knuckle fittings that mate the right and left wings with two longitudinal pins. While usually spec'd to be machined from blocks of 7075-T6, I've seen two different HP-11s where they were machined out of some sort of steel. The important thing is that these are some pretty close-tolerance parts best entrusted to an experienced machinist.
• Formed aluminum wing ribs - You'll need to form about 70 different aluminum wing ribs - 35 ahead of the spar, 35 behind the spar. No, they're not identical right and left, they're mirrored. So that means you've got to make 140 different form block sets, and make them to pretty close tolerance. You might get Dick to lend you the original tooling. But perhaps not. I think that Dick formed his ribs from 2024-0 dead soft aluminum, and then sent them out to be hardened to the -T4 or -T42 condition; that's why the flanges on them look so smooth. You can probably get away with forming yours from 2024-T3 aluminum by using lots of flutes. It won't be as smooth as factory ribs, but would probably be serviceable. HP Aircraft does currently have HP-14 wing rib sets in stock.
• Fiberglass ancillaries - See description above for the HP-18 and RS-15
• Fuselage Belly skins - See description above for the HP-16. I think that HP Aircraft has one of these still in stock
• Other Parts - See description above for the HP-18 and RS-15

HP-11

As a representative example of early 1960s technology, the HP-11 is a fine balance of ruggedness and performance. Like the later HP-14, it is entirely conventional in construction, with no structural fiberglass or foam. However, it does have its share of what I refer to as Troublesome parts.

Troublesome Parts:

• Wing Spar Extrusions - The HP-11 wing spar is an I-beam section at the root, and tapers to a C-section at about the 14' line. The I-beam is built up from angles riveted to a sheet aluminum shear web. On most HP-11s (and on all of the HP-11s I’ve looked at), the angles are custom aluminum extrusions that Dick Schreder commissioned from Alcoa in the early 1960s. They're generally shaped like standard angles, but the longer leg of the "L" is curved to approximate the airfoil curve at that spot.
  
  I used to list the wing spar cap extrusions as Essential parts. However, it seems that the HP-11 spar can also be built out of standard straight-legged aluminum angles. As HP-11 builder Dick Freeman told it on the HP-Gliders mailing list on 26 October 1998:
  
  About 2 weeks ago I contacted Dick Schreder on the subject of the special extrusions he used stating that I had heard construction was impossible without them. His reply was that custom extrusions were too expensive so his first 11's used normal commercial straight extrusions with some "goop" to fill the airfoil out. He said to go ahead now and do the same thing. We didn't get into whether the "goop" was added before or after skinning but after might be easier as part of a profiling exercise.
  
  Whether the extrusions used are standard straight ones or the custom curved ones, the alloy probably has to be 7075-T6 in order for the spar to develop the necessary strength.
  
  HP Aircraft does still have some HP-11 wing spar cap material in stock. It might be enough for a full spar set.
• Formed aluminum wing ribs - See description above for the HP-14. HP Aircraft does have HP-11 wing rib sets in stock.
• Spar root fittings - See description above for the HP-14
• Fuselage Belly skins - See description above for the HP-16
• Other Parts - See description above for the HP-18 and RS-15

HP-7 through HP-10

I don't know enough about these earliest HPs to say what's inside of them. I will offer the opinion, however, that unless you absolutely must have one of these for a documentary or for a museum piece, there is little point to building one. One of these would be every bit as hard to build as a more modern HP, and would have significantly less performance.