MODIFICATIONS TO THE OSPREY GP-4 MAIN LANDING GEAR
by BOB RINGER
(ed - there is a link at the bottom of the page to all the landing gear mod pics)
The GP-4 airframe is built very strong with an 8G positive and 6G negative spar. Add the rear spar, ribs, fuel tanks, wing skins and an overlay of fiberglass cloth and resin and it becomes extremely strong.
The one problem are that has occurred far too often is the main landing gear and more specifically the link arm to the main gear leg pivot bolt. I am not sure how many failures there have been but since I have started building 6 years ago I have heard of enough mishaps involving this are that I became concerned enough to investigate an alternative construction method that would add strength, yet not radically change the configuration or add to the cost. Weight was also a factor, but not a great concern.
I think I have found the solution from John Evans, New Zealand who has flown his GP-4 from a grass airfield for many years and his gear has worked flawlessly. Although John has reconfigured the gear to eliminate the troublesome bolt, he did not increase the wall thickness to add strength to bring the GP-4 more in line with the main gear ruggedness of aircraft at or near the same weight category such as Piper.
So in addition to John's great idea, several of us builders have made the wall thickness of all material in the main gear from 1/8 (.125) material which makes a very rugged landing gear and the weight penalty does not seem to be a concern. It has the added advantage of making welding easier since welding material of the same thickness is much easier.
We have also added a swivel bearing system to the trunion axle bearing blocks which allows the builder to drill the bearing blocks at 90 degrees and makes installation extremely easy since exact drilling at 86 and 82 degrees and exact alignment of the bearing blocks is not necessary to allow free movement of the gear. the bearings are cheap (under $100.00 total cost) and allows the landing gear to cycle very freely. The bearings are made by KML and the large (main gear trunion axle ) bearing is number GEZ25ES and the small (link arm trunion axle) is Number 6206-2RS. They have a groove around the outside bearing race that allows a grease fitting to work perfectly. A sleeve is installed over the trunion axles at each end and welded in place against the bearings to prevent the trunion axles from moving forward or back in the bearings. The gear moves so freely you have to be careful when the wing is upside down that it does not slip from your hand retract with a resounding crash against the Number 2 rib. This has to add to a safe, successful emergency gear deployment. Complete bearing failur3e will not prevent deployment and the gear will remain in its proper place. George likes the idea.
The first change I made to the main gear leg was to construct the outer gear leg from 1 and 3/4 by .125 material full length instead of just a 1 and 3/4 by .058 sleeve over the lower part of the 1 and 5/8 by .058 main gear leg. For those who have found that any welding on the gear leg involves a horrendous amount of interior honing to remove the distortion cause by the welding, this will be appreciated.
The bronze slide and slot arrangement to hold the gear in alignment was eliminate4d as per the latest suggestions from George and we went back to the tried and trusted scissors arrangement since the bronze slide would eventually cause the slot to expand and an exterior clamp was required. Also gear alignment to prevent tire wear was very difficult, whereas the scissors arrangement wallows wheel alignment by moving washers as in the Piper products. With the aircraft on jacks and the wheels removed, alignment can be accomplished by the use of a string stretched between the inner end of both axles and then move the washes on the scissors joint.
The retract link arm is now attached to the main gear leg with a sturdy clevis and a larger rod end bearing. The clevis is installed at or near the junction of the shorter main gear brace and the main gear. This has the effect of causing the push/pull forces to act directly against the main gear leg instead of a torsional force on the side of the gear leg. The clevis, again made from .125 material is extremely strong and I suspect the rod end bearing would fail first.
This change means the link arm must be welded on the link arm axle at a distance of 5 and 5/16 inches instead of 3 and 7/8 as called for in the plans. The hydraulic ram attachment clevises are moved to 2 and 7/32 inches on the main gear leg axle and to 3 and 7/8 inches ion the link arm axle which actually moves the hydraulic ram to the other side of the link arm. The uplock lever is moved to align with the hydraulic ram rod end cam and I made it into a clevis type of attachment to its pivot point.
That is about it for the modification. I believe the gear design as per the plans is a great working system and with the exception of the link arm pivot bolt should be trouble free.
However, the lightness of the material and the problem with the pivot bolt seems to create a situation where a crosswind or a hard landing has the potential for gear failure which can create a whole lot of damage to the aircraft. This has happened on far too many occasions and I am trying to avoid what could be a very expensive landing.
The link below leads to drawings of the redesigned main gear leg, link arm, and bearing blocks, as well as photos of the completed system. It works flawlessly and just seems to 'feel' strong. I suspect it would require a very hard landing to cause gear damage.
Bob Ringer
Hubley, Nova Scotia