HP-18 Center Stick Project
The purpose of the HP-18 Center Stick project is to design, develop, validate, and manufacture a field-installed retrofit kit that comprises a conventional center stick and forward fuselage control system for the HP-18 sailplane. This control system will replace the sliding side-stick controller that is standard equipment for the HP-18 kit. The project will develop, in addition to the center stick system itself, the production tooling and methodologies to manufacture production units of the system. The production span is expected to be 35 +/- 15 units.
The primary project goals are:
- Safety - The center stick will be designed and tested to FAR part 23/JAR 22 standards. It will involve no significant increase in the number of points of potential failure. It will offer improved pilot interaction with the aircraft controls, and promote reduced fatigue while flying.
- Reliability - The center stick will use standard aerospace metals, alloys, and technologies. It will use standard off-the-shelf airframe bearings. All hardware and fasteners in the primary system will be standard AN/MS/NAS parts. The center stick will use technologies native to the HP/RS sailplane series, and will integrate seamlessly with existing control system components.
- Ease of Installation - The center stick kit will be field-installable by mechanically-inclined persons with little specific training in composites. Some drilling and fitting, a little sheet metal cutting, and other basic metalworking skills will be required. No welding will be required. Procedural, task-oriented step-by-step instructions will be included.
- Low AOG down-time - The center stick kit will require no more than 150 hours for the complete installation. Experienced aircraft technicians will require substantially less AOG time.
- Economy - The cost of the retrofit kit is being targeted for the neighborhood of $750 to $1000 US. Some features (wheel brake control, others TBD) may cost extra.
The current design consists of a welded steel platform that mounts to three phenolic pads that get bonded and glassed into the inner surface of the fuselage pod. The platform consists of a pair of transverse steel tubes that support a pitch carriage and a roll bellcrank. The platform mounts to the pads with countersunk screws that are inserted from the outside of the pod (and then filled over).
The pitch carriage is supported on lateral pivots on the platform. The pitch carriage supports the control stick on one end, and drives the pitch push-pull tube at the other end.
The control stick is supported on a longitudinal pivot on the pitch carriage. An extension below the longitudinal pivot drives a lateral pushrod, which drives the roll bellcrank. The roll bellcrank drives a longitudinal push-pull tube.
Two new push-pull tubes will extend from the horns of the center stick mechanism aft. One will connect to a modified horn on the pitch/yaw mixer on the aft face of the rear wheel-well bulkhead. The other will extend to an added bellcrank near the site of the present 18-439-16 pitch/roll decoupler.
Both the pitch and roll push-pull tubes run aft along the right side of the cockpit. This feature avoids the difficulty of either moving the landing gear lever to the right, or adding more push-pull tubes to the already-crowded left cockpit wall.
26 November 2001 - Now accepting orders
9 July 2001 - First test flight!
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28 August 2000
The center stick design/fabrication/testing program consists of the following steps (Step/Status/Description):
- Basic layout of what goes where
Done. After abandoning a design that used a molded fiberglass platform, I have worked out the welded design now in development.
- Build a wooden mockup and validate fitment and interference issues.
Done. I have used wooden dowels and fiberboard to mock up the entire cockpit system in an unfinished HP-18 pod.
- Build jigs for first article
In progress. I have finished the production jigs for the platform and the pitch carriage. I will build jigs for the other parts after validating their first article units.
- Fabricate first article
Done. On Friday 12 January I had welded steel first article parts for all three major weldments in the assembly.
- Validate first article fitment
Done. I have validated the arrangement and fitment of the -701 platform assembly, and also the -702 pitch carriage and -703 control stick.
- Test first article
Done. After building a complete first article assembly, I'll fit it into the test pod, connect it to a set of dummy fixed pushrods, and load it to 125% of the expected control inputs. The purpose of this test is to validate the ability of the stick mechanism to resist the expected loadings without failure.
- Prototype installation
Done. After testing the first article, I'll perform a prototype installation on an actual flightworthy HP-18. I am buying an HP-18 for the purposes of this phase of the program (or at least that's what my accountant would tell me to say).
- Load Test prototype installation
Done. After performing a prototype installation, I'll test the entire control system to 125% of the expected control system inputs. This will be done in two conditions: Once with the surfaces locked in neutral positions, and once with the surfaces free and positioned at their stops. The purpose of this test is to validate the ability of the entire control system to resist the expected loadings without failure, and to ensure that the new stick mechanism does not impose excessive loadings on the previously-existant parts of the control system.
- Flight test prototype installation
In progress. First test flight was 7 July 2001. After successfully load testing the prototype installation, I'll conduct a flight test program that evaluates the flight charactistics of the prototype installation. The focus of the flight test program will be an evaluation of control forces, control harmony, ergonomics, and PIO characteristics. The flight test program will be conducted in accordance with FAA guidelines for a major alteration to an experimental homebuilt aircraft.
In progress. Using diagrams from steps 1 through 4, photographs from steps 5 and 7, and flight data from step 9, I'll produce a set of documentation that will guide the installer through the installation process. The documentation will consist of assembly drawings and installation diagrams, a set of step-by-step installation instructions, a sample weight and balance worksheet, a sample letter for notifying the FAA of this major change, and a suggested flight test syllabus. Also included will be instructions for testing the finished installation to the same 125% of expected loads that the prototype endured.
In progress. After building and flight testing a prototype installation, I will use the original jigs to produce complete kits of parts. The kits will contain all of the weldments, hardware, bearings, materials (excepting paints, resins and adhesives), and documentation necessary for installation. Anticipating successful validation of the first article, I have already started cutting the steel tubes and pivots for the first five kits. It may seem like I'm getting ahead of myself here, but when you have a complicated cutting or notching setup, it is usually best to get as much done with it as you can before breaking it down to arrange the next cut. Also, I'm pretty confident about the parts that I've already cut metal for.
Not started. After starting production, I will be available to provide help to those who install the kits into aircraft.
There is no fixed schedule for this program. The timing is driven by family activities and career necessities. I work on it when I can get shop time, and I think about it when I can't work on it. I figure to have the prototype installation done by next spring, and to conduct the flight test program in early summer 2001. If it does go like that, production units will be available in mid-summer 2001.
I know that it sounds like I'm taking a lot of time with this. I also know that others have developed center stick installations in a lot less time. My reasons for taking so much time are:
- I'm developing an entire new system, not using salvaged components from other ships. The benefit of this effort is a system that more easily integrates into the aircraft.
- I'm developing a drop-in kit that can be installed in a finished ship with a minimum of downtime and modification. You won't have to retire the ship for an entire season.
- I'm developing a kit that can be installed by those not expert in fabrication, welding, or composites.
- I'm carefully developing, testing, and validating a simple, robust mechanism cabable of accomodating all reasonably expected input loads. I regularly consult with an aeronautical engineer, and will validate the design to basic certification standards.
The price of the complete center stick kit is expected to be between $750 US and $1000 US.
That may sound like a lot of money, but most of it will go towards the production expenses of steel materials, bearings, welding services, and other kitted parts. The rest goes towards recouping my investment in time and money in the development program.
Also, consider that the bearings alone would cost $313.50 if you bought them at Aircraft Spruce and Specialty.
Terms are 50% deposit at the time of order, and 50% due at time of shipment. No deposits or orders will be accepted until the completion of Step 9 of the development program.
page updated 11/26/01 all text and graphics copyright (c) 2001 HP Aircraft, LLC