Scott--

The portion of your recent post  on the pressure spike topic (which I have highlighted in red, below) has my head spinning. The reason is that I have spent the last two weeks re-rigging my hydraulic cylinders because I did it wrong the first time around. I had previously rigged the cylinders to keep pressure on the system when the gear was retracted, and had the cylinders bottom out just after the gear was down as you implied they should.  According to Pg. 292 of the plans, the way I did it was exactly backwards, and that scheme seems to have actually bent one my cylinder attach bolts. If you go to Pg. 292 of the Builder's Manual Ch. 11 , it emphasizes that the cylinders must bottom out internally (or against the supplied aluminum spools) when the gear is retracted. But paragraph 6 goes on to state: "The gear down side, due to reduced geometry, does not require that the system be bottomed out." That's why I posed my original question about going to 800 PSI rather than 500 PSI.

So I've rigged my cylinders per the instructions on Pg. 292: Cylinders bottomed out against the spool pieces when the gear is retracted, but NOT bottomed out when the shaft is extended and the gear is down. Therefore I have low-side pressure pushing on the over-center link when the gear is down. Is this wrong? Would it even be possible to rig the cylinders so they are bottomed out both with the gear up and the gear down?

I note that the plans describe the original placement of the main gear cylinders back on Pg. 168 in Chapter 5, but the details of their action and adjustment comes 124 pages later in Chapter 11. It's almost like Lance added a page due to some confusion on this issue.  (I know I'm confused.)

Help!

  --John

At least for LNC2's the down actuators are to assist gravity and the nose gear gas spring in order to extend the gear.  The infamous rat-trap springs on the main gear are to merely lock the over center.  Thus, not much hydro pressure is needed to extend the mains and nose even at 122 Kts (max extension speed).

 

When the gear is extended the actuators under pressure are merely used to keep the over center links locked (kinda like a backup system).  The actuator down stops must be set to keep hydro pressure off of the links.  "Why?" You might ask.  Well, examine the geometry - the force triangle is from the pivot to the link to the link-spar attach point and back to the pivot along the spar.  If enough hydro force is applied to the actuator, the rod may become bent or the now excessive force present at the link-spar connection can delaminate it from the spar.  Hard landings with some side loads have caused failure there and that is why a service bulletin recommends putting a reinforcing plate between the link-spar attach point and the upper spar.

 

Yes, excessive pressure without proper down stop rigging can cause problems.

 

Scott Krueger