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.

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A Pilot's Christmas Poem

Twas the Night Before Christmas... at the Airport.

Author unknown.

Twas the night before Christmas, and out on the ramp, Not an airplane was stirring, not even a Champ. The aircraft were fastened to tie downs with care, In hopes that -- come morning -- they all would still be there. The fuel trucks were nestled, all snug in their spots, With gusts from two-forty at 39 knots.

I slumped at the fuel desk, now finally caught up, And settled down comfortably, resting my butt. When the radio lit up with noise and with chatter, I turned up the scanner to see what was the matter. A voice clearly heard over static and snow, Called for clearance to land at the airport below.

He barked his transmission so lively and quick, I'd have sworn that the call sign he used was "St. Nick." I ran to the panel to turn up the lights, The better to welcome this magical flight. He called his position, no room for denial, "St. Nicholas One, turnin' left onto final."

And what to my wondering eyes should appear, But a Rutan-built sleigh, with eight Rotax Reindeer! With vectors to final, down the glideslope he came, As he passed all fixes, he called them by name: "Now Ringo! Now Tolga! Now Trini and Bacun! On Comet! On Cupid!" What pills was he takin'?

While controllers were sittin', and scratchin' their heads, They phoned to my office, and I heard it with dread, The message they left was both urgent and dour: "When Santa pulls in, have him please call the tower."

He landed like silk, with the sled runners sparking, Then I heard, "Left at Charlie," and "Taxi to parking." He slowed to a taxi, turned off of three-oh, And stopped on the ramp with a "Ho! Ho! Ho!"

He stepped out of the sleigh, but before he could talk, I ran out to meet him with my best set of chocks. His red helmet and goggles were covered with frost, And his beard was all blackened from Reindeer exhaust.

His breath smelled like peppermint, gone slightly stale, And he puffed on a pipe, but he didn't inhale. His cheeks were all rosy and jiggled like jelly, His boots were as black as a cropduster's belly.

He was chubby and plump, in his suit of bright red, And he asked me to "fill it, with hundred low-lead." He came dashing in from the snow-covered pump, I knew he was anxious for drainin' the sump.

I spoke not a word, but went straight to my work, And I filled up the sleigh, but I spilled like a jerk. He came out of the restroom, and sighed in relief, Then he picked up a phone for a Flight Service brief.

And I thought as he silently scribed in his log, These reindeer could land in an eighth-mile fog. He completed his pre-flight, from the front to the rear, Then he put on his headset, and I heard him yell, "Clear!"

And laying a finger on his push-to-talk, He called up the tower for clearance and squawk. "Take taxiway Charlie, the southbound direction, Turn right three-two-zero at pilot's discretion"

He sped down the runway, the best of the best, "Your traffic's a Grumman, inbound from the west." Then I heard him proclaim, as he climbed through the night, "Merry Christmas to all! I have traffic in sight."