TAIL WHEEL IMPROVEMENTS
LANDING GEAR IMPROVEMENTS
The HP-18 landing gear and retraction mechanism is very similar to the earlier HP designs. Dick made a good thing and left it alone. I made some changes on mine before the first flight, others later. In addition, I have learned some lessons to pass on in the six years my ship has been flying. None are essential for safety, but make the system just a little better. Some of these suggestions will apply to all of the HP series.
ASSEMBLY BOLTS/GEAR LOCK UP AND DOWN
There are three 3/16 bolts in the landing gear upper frame and landing gear spring bellcrank (the rear pivot tube). Two of these bolts must be perfect fits to get the best gear function. The plans specifically mention drilling and reaming these holes with care. Tightening the bolts will not help if the holes are oversize. The fact that AN-3 bolts are between .002-3 undersize can be enough to cause some slight problems.
With the gear locked down it should be impossible to get any movement at the shock strut knuckles. You should adjust the actuating rod length to remove any movement caused by oversize holes or undersize bolts. Some report the gear does not retract all the way, or extends on high "G" loads. If the retraction rod does not lock in the gear up position, or if there is some looseness in these bolts, the gear will have a lot of movement when retracted. A little play translates to a lot of rotation in the retracted position.
In addition, the bolt that holds the spring bellcrank is heavily loaded when the bellcrank goes over center, and any looseness will get worse quickly.
When I drilled the holes the drill grabbed. The reamer went thru one hole without touching. A trial assembly proved you can't just tighten the bolts to remove the slack. The solution was to replaced all the bolts with Brown and Sharpe taper pins. To make life easier, I removed the whole assembly and did the drilling and reaming on the bench, using my slightly oversized holes for alignment. You have to watch clearances between the taper pins and the fuselage bulkhead. My taper pins are vertical with the gear down, and the nuts on the bottom. This way the rotation moves the nuts away from the bulkhead.
LANDING GEAR RETRACTION ROD AND LOCK
I spent a lot of time bending the landing gear retraction rod to get it to operate. Then I had to adjust the length to provide a positive lock-down after completing the notches in the channel that it locks down in. I resolved all my problems by cutting off the end, welding on an AN-490HT16P fitting and adding a 3/8" rod end (HF-6), lock nut and restraining washers. Now I can easily adjust the length to get a positive lockdown, and the rod end helps with the alignment.
This summer, on a normal landing rollout, the landing gear retracted without warning. Fortunately, it was on a sandy field and no damage was done. The retraction arm had failed and I thought it was caused from a bad weld from a previous repair (see below). The next day I developed a major bruise under my left thigh. The handle had come out of its locked position and stopped against my leg. My leg then restrained the gear lever until the retracting arm failed. I riveted a "hump" to the landing gear guide channel as shown on the sketch. Over time, the edge that the handle locks against had worn, allowing the thing to jump out. The hump will prevent this problem.
SHOCK RESTRAINT CABLE
Outlanding in the Central Valley Soaring Associations contest in 1986, I picked an abandoned field without a fence. It was abandoned for a good reason. The dirt was hardpan and the 2" ruts might as well have been concrete curbs. The shock adsorbers went into action, and about the fifth major bump, the restraint cable failed. With 400 psi. in the shocks, the gear rapidly disassembled itself, the wheel retracted, and the last 15' were on the belly.
No damage, except to the gear actuating arm, doors, paint and nerves. At that point I had about 90 flights in the HP-18. Due to problems in the struts I would occasionally fly with shock pressure slightly low, and the restraining cables would get a work-out even on normal landings.
I replaced the cables with the next larger size, however that is not really the correct answer. On inspection the failed cable showed some old damage. The correct answer is to inspect the restraining cables very carefully at your annual. This means let the pressure out of the strut, disconnect the cable ends, loosen the cable from around the shaft and do an inch by inch inspection. Replace if any strands are damaged, or if the cable is flattened were it wraps around the shaft. If you keep your shocks blown up so high that they never work, or if they are always flat, look at them anyway. If this cable fails, you don't need a tow to get off the ground.
LANDING GEAR SPRING PULLEY
The spring pulley caries a heavy load. The fiberglass bulkhead did not, and the pulley deflected and rubbed, creating a lot of friction. The retraction springs were not effective, and getting the gear up and down was a struggle. I lived with this for the first few hours. I then built and installed the bracket shown in sketch 1, and now the pulley doesn't rub, and the gear retracts easily.
My biggest mistake was to buy a Strut Pump. Next was a surplus oxygen bottle and fittings (filled with the strut pump) to blow up the strut in the field. Now I have so much invested I can't afford to buy a Nitrogen bottle and regulator. Over the years I have removed the strut at every annual, replaced the "O" rings, honed the lower struts and replaced the hydraulic oil with something else. About six months later, the system would need recharging, and would be ok for either another week, month, or day. Some people report no problem at all with this thing, I have been fighting it since day one.
The primary problem is the use of hydraulic fluid, which attracts water like mad (hygroscopic). This is ok if you use dry nitrogen to charge the strut. The moist air I uses causes oxidation, and bad things happen to "O" rings. Synthetic silicon based brake fluid is the worst.
The best answer is to use dry nitrogen to pressurize the strut. Just this year I learned about motorcycle suspension oil. It is designed to work in air and is not hygroscopic. The stuff comes in different weights, and 7 weight is the closest to aircraft hydraulic fluid. Any decent motorcycle shop carries it.
(On removing the gear from my old fuselage to transplant it to the center stick version this fall, the strut was still at original pressure. There was no sign of rust or pitting inside the lower strut, and the "O" ring was in perfect condition. This stuff is the answer.)
I have tried several types of covers to go over the struts to keep dirt out. The best was a vacuum cleaner hose and just fit over the larger diameter. I thought I found an improved cover the first annual, and the hose went in the trash can, a major error. I haven't found a good replacement yet. Since I have been rebuilding the shocks at least once per year, it hasn't been an issue (unless the dirt that gets in the thing is the problem).
(I just found something....AEROQUIP hose, 1" I.D. sold by Aircraft Spruce as a vent hose for use in engine compartments. )
Most new gliders come with the gear compartment sealed. They do this for a good reason, aside from the quiet it brings. It keeps the dirt out of the interior of the airplane, and in the case of one airport I lived at, the mice from moving in and building a nest from the headrest. I tried to seal the doors, but never got anything that would last any length of time. I then decided that sealing the whole compartment was easiest.
There are six leakage paths into the gear compartment. The top is the biggest. I fabricated a removeable fiberglass pan that allows access to the bottom of the main wing pin to install the safety pin. See sketch. While you are at it, make up small covers for the other two holes ahead of the spar. Then you can store stuff behind your head and not worry about it falling into the controls. Don't put anything back there loose that you wouldn't want to hit your head in a crash.
Four of the holes in the compartment are the front and rear pivots. They should be small leaks at best. I have done nothing with them, but a fanatic could try building up a fillet with heavy grease. Don't do this inside the gear compartment, or it will trap dirt. The last hole is the brake line. I just used silicon glue to seal it. Silicon glue will also seal any small holes that develop in the fiberglass.
LANDING GEAR PIVOTS
I have noticed some wear on the 4 landing gear pivots after about 225 landings. For my new fuselage, I built pivots plates that have oilite bearing inserts. I am not sure that it was necessary, but it will eliminate the wear, make the pivots work more freely, are more airtight, and are replaceable. See sketch.
With some metal pounding, the magnesium doors that are provided will work well. When I got the forward pod for my new fuselage, the first thing I did was locate the gear door area and make a mold. Now I have fiberglass doors that are a perfect fit, and can make as many as needed. For those who may be interested, I would consider lending out the mold for the price of shipping and a promise to return it quickly and in good condition.
GEAR DOORS AND RETRACTION
On my ship, the gear door springs are about 8" long. When extended, they hold the gear doors about horizontal. Some use much shorter springs and hold the doors vertical, or in contact with the gear. Both work fine. It is a matter of opinion as to which configuration will help keep the doors on the airplane when landing in the weeds.
I began to have problems with my gear one year after an annual, in that when retracting it would hit something half way up and lock. I would cycle it two or three times and it would be ok. I must have cycled the system 50 times on the ground trying to find what was hanging up and looking for marks caused by the impact, which was very solid. Finally, taking in to consideration the air loads on the doors, I discovered the answer. I had removed the wheel to replace the inner tube and grease the bearings. On reassemble, the axle thru bolt extended about 1/8" beyond the end of the axle housing on one side. When the gear was going up, the gear door would catch on the axle bolt and jam the gear half up. When in the air, a little side slip would prevent the problem, and when cycling the gear, there was a lot of slipping.
In addition, make sure the ends of the springs are bent so they cannot come out of the door or the gear. I flew one 200 mile plus contest day with one gear door hanging open. When going over about 70 knots, a little push or the rudder would get it to close, and air pressure would keep it closed. At the next thermal it would fall open. That was before the gear well was sealed, and it got real noisy when the door opened.
RUDDER PEDAL ADJUSTER
The rudder pedal adjustment system has a safety problem. When moving the pedals forward, if the locking piece misses the hole in the lower tube, you can shove the pedals all the way to the forward bulkhead and lose all rudder control. Being 6'-4", I fly with them fully extended and had this happen. You can pull them back and hope the lock catches, but there is a better answer. I put a AN-3 bolt thru the lower tube at the most forward point of the slider to act as a limit stop.
My original system did not work real well using the solid 1/16 wire to the lock. In the new fuselage, I revised it per the attached sketch to use a 1/16 flex cable. It works much better. (see sketch)
TAIL WHEEL SPRINGS
The two tail wheel springs allow rudder movement when the tail is on the ground, or if the tail wheel pivot sticks. I was next to tow one day, and was doing my usual positive control check. I pushed hard on the pedals, and got the expected no movement because someone was holding the elevons. The next step is to verify full movement is still available. I had to push hard on the pedal because the tail wheel was not straight, and it did not move. The other tail wheel cable went slack, and its tail wheel spring fell off. You can tell this immediately because the rudder pedals are unevenly loaded by the one spring. I pulled off line, removed everything and hooked it back up and flew. Later that day, I tied the both ends of each spring to the cable fitting and wire loop. Now they can't fall off.