HP-24 Project

Update 25 August 2004: Living the recip loco

Last weekend we did some more shop cleanup, and stowed the wing plugs in the overhead racks vacated by the old HP/RS forward fuselage pods. Now we have all the pods standing on their tails against the north wall of the house. I've also got the plug for the horizontal tail out of storage, and I'm going to clean it up and get it ready for molding.

And I now have the internal design of the horizontal tailplane worked out, and I've been shopping around for bids on the first two sets of some of the custom steel parts required.

Lately I've been sketching out more details of the airbrake design, and I'm beginning to see what deep doo-doo I've stepped in by blithely deciding to use such a shallow (13.5% T/C) wing. The shallow airbrake boxes I'm allowed don't leave room for airbrakes that are both simple and effective. And wing flex will make it difficult to design the system so that the cap strips stay flush at much more than about 2G of loading. I think I've got a solution in a relatively simple two-arm three-paddle design, but it will take no little attention to make everything fit into the box without interfering with each other.

I went into this project happily thinking that I could develop a simple glider contained within relatively modern shapes and profiles. But almost everywhere I turn, the profiles have implications that refuse to stay at the surface, and reach down into the core of things.

I have a good head start on the airbrake design, since I've had opportunity to inpect and measure airbrake systems on Discus, Ventus B, and LS-4. And I've also combed the Web for service bulletins that depict airbrake systems. Those have given me hints about the issues and problems encountered, and offered several solutions. One interesting aspect was comparing the Discus and Ventus airbrakes, which use similar-looking parts in a subtly differing fashion. The Discus system drives both inboard and outboard airbrake arms with bellcranks below the arm pivot, whereas the Ventus (all other systems I've seen) trust the upper paddle to mechanically connect the two arms. I believe that the key to this drive lies in the fact that the the Discus arms are almost parallel to the plane of the wing when they're stowed. Good analogies for this mechanism are steam-powered locomotives and stern-wheel paddleboats, and also the dogbone camshaft drive on some old Ducati motorcycles. Note how the connecting rods are 90 degrees out of phase, and not 180 degrees as symmetry might suggest. The steam cylinders pull as well as push, and must provide torque even at zero RPM. It sure is interesting to see how wide the application is of what I learned from being a steam locomotive enthusiast as a child.

In a couple of weekends I will show off my full-fuselage molds, and possibly my wing plugs, at the Sailplane Homebuilder's Association 2004 Western Workshop in Tehachapi. And next winter, I'm scheduled to give a talk titled "Adventures in Sailplane Development" at the 2005 Soaring Society of America national convention in Ontario, California. My theme will probably be that experience is what you get when you don't get what you want, and that experience is the difference between development and design...

Return to HP-24 page

page updated 25 August 2004 all text and graphics copyright (c) 2004 HP Aircraft, LLC