Oil Hydraulic Strut - Aircraft Products Corporation of America

Here's some helpful data for all of you Curtiss Robin restorers out there which use the "OIL HYDRAULIC STRUT" manufactured by Aircraft Products Corp. in Detroit Michigan in the late 1920's.  These struts may be used on other airplane models too, but I just don't know for sure which ones.

Finding technical data to support antique airplanes is often times a frustrating task that ends up in getting nowhere.  Amazingly, though, Google and other similar search tools on the internet have helped me many times.  We are getting ready this week to go into the landing gear shock struts on a 1929 Curtiss Robin.  The main reason is to replace the oil and seals which have been sitting dormant for the better part of nearly 40 years.  I consulted with Robin Restorer, Dick Fischer, who has done this before.  He cautioned me ahead of time that some of the parts are fragile and advised not to go into them too far, unless absolutely necessary. This just may be one of those... "If it ain't broke, don't fix it" kind of situations.  Especially if you have no manual or any way of knowing what you are going to find when you go inside.  Here's some of Dick's wise words...

"There's not a whole lot to see inside them, and not much to go wrong other than a broken spring or corrosion.  They will probably be a bear to take apart due to the fine threads at the bottom of the collar (where the slots for the spanner are).  The lower tube is also very thin wall, and can be easily ruined if you put too much torque on the collar to get them apart.  If they are seemingly OK, I'd think twice before taking them completely apart.

The struts all seem to leak out the top of the collar.  There is a graphite packing at the top of the collar which you can replace fairly easily, but it will no doubt leak even with new packing.  To access the packing you need to unscrew the "castle" nut at the top of the collar.  That's a pretty safe operation, but I don't recall whether you can remove the upper tube and spring at that point or if you have to unscrew the lower tube from the collar."

OK - I get it.  This is possibly going to be an adventure.  More than ever, after reading what Dick had to say, I really want to read up on this before diving in.  None of the books in my library collection have anything on these struts.  And after checking with a few airplane restorer friends, I was having no luck finding any information.  So in a final desperate attempt... I went to Google.

Lo and behold.... under the search terms of "oil hydraulic strut aircraft products corp michigan" I struck GOLD.  It turns out that these very struts were patented... applied for in May 1929 and patent issued in Dec. 1932.  The patent information, though written in the old school technical prose, is just about as good as any manual and with this, I now have the confidence to go into these struts.

But I still think I must heed Dick's advice and simply replace the fluid and packing seal.  Seeing how these units are constructed tells me that a good "inside-out" mineral spirits bath, fresh oil, and packing should go a long way toward getting these struts back in service.     Here's the published patent information.  perhaps this will help some other restorers in the future.