ADDITIONAL TIPS -- HP-18
The following are some additional improvements for HP-18s.
1. AILERON PUSHRODS The alignment of the Teflon pushrod guides in the wing trailing edge channel can impact aileron smoothness. I built and located the blocks per the plans. After flying for several hours, I noted that the aileron pushrod tubing was not running in the cen-ter of the holes as seen by the wear pattern. I assembled the airplane and blocked up the wingtips to take most of the weight. Lowering the flaps all the way, I removed the machine screws that hold the pushrod guides to the wings. What a revelation! The guides near the root wanted to be 3/8" higher and 3/16" away from the rear spar. Due to the flexibility of the push rod tubing, the guides near the mid span are generally ok without any changes. I recommend leaving one mid span guide alone and shimming and adjusting the rest to fit. Figure the adjustment for each guide, make the modifications required, and recheck the alignment. Near the wing root and the last guide before the aileron, set the guides with the ailerons in neutral. Under no circumstances increase the size of the hole in the guide. Here are four ways to adjust the guides:
1. Unless all your guides are identical, you may be able to do some swapping around.
2. Shims placed behind the guides will resolve the alignment fore and aft. Be sure and use longer machine screws if necessary. Check for interference with the flaps in the full negative position.
3. New guides can resolve small misalignments.
4. For major relocations, without changing the nut plates, see sketch #1. Use Pan head or countersunk flat machine screws to mount the bracket where the head may interfere with the pushrod. The screws and nuts holding the guide to the bracket must be reversed and inlet as shown to prevent interference with the flaps in the -10 degree position. Check to be sure.
2. CONTROL STICK GUIDE
The control stick guide is a metal to metal rub. I built a new part out of steel with a riveted 3/8" Teflon guide. Note how one corner is rounded off to allow the trim cable to move. The second guide, just behind the pilot seat, was remade with the 3/8" Teflon replacing the 1/8" original. Now the stick floats in Teflon, and the drag is almost nil, if you do the next modification.
3. SQUARE TUBE SLIDER
Dick came up with a very clever way to separate the rotational and translation movements of the control stick. Per the plans, shimming is required to get the alignment necessary for this to work smoothly. There is also a way to change the square aluminum shaft to lower the friction considerably. Just file away part of the square block, leaving 1/8" ridges all around every 1/2" or so. Also, if you didn't put a vent hole in the bottom of the tube, add one. Otherwise, with a little grease, the assembly becomes a hydraulic cylinder. The trim springs will return my stick to neutral after deflection. That is how the system should be.
4. FLAP LOCK
I have seen several versions of a flap lock. Unless you have three hands, this is a necessity. The one in the sketch works fine for me, and uses the original parts that came with the ship. The flap crank rotation for the flying range is less than 180 degrees, so my locking plate is only a half circle. The brass bearings and collet were machined for me at minimal cost. Use a minimum of two set screws in the collet and Locktite. I used a soft spring that acts only to retain the 1/4" rod in the locking plate. When landing, the soft spring makes it easy to avoid locking up the flaps in a hole.
5. TRIM SYSTEM/STICK
I have given up on the push button on the stick to set the trim. The system will work if you want to spend the time setting it up properly, however I found it unnecessary. My ship, with the C.G. towards the aft limits, is essentially neutral at all airspeeds above 60 knots. When thermaling, however, I have some backpressure. I just reach over with my left hand and give the trim cable a tug to adjust the system. In addition, I use a large diameter bicycle handle grip on the stick because with the small diameter stick my hand would cramp badly after a couple of hours. See below for what to do with the stock brake setup. Hint, throw it away.
The headrest mounting plate is a potential spear aimed at the back of your head. The design has the headrest pivot tube glued to a flat plate. See sketch. In a crash, the first impact will cause the glue to fail, the second impact will drive the mounting plate into the back of your head. The answer is to weld the tube to the plate. I made the plate out of steel and welded a tube.
Dick put a lot of effort in reducing frontal area on the -18. As a result, us western pilots who can't fly without oxygen have stuffed bottles in interesting places. In my ship, the battery is mounted behind the spar on a plate and a 22 cf. oxygen cylin-der in front, behind my head. I put a lot of fiberglass where the cylinder brackets mount. In a bad crash the bottle may stay in place, but I am not sure. On my new fuselage, I have installed the oxygen cylinder in the tail cone, mounted on rails just behind the first bulkhead. A 7" dia. hatch in the top of the tail cone, with appropriate doubler glued and riveted to the skin, provides marginal access. A remote fill is included. This moves the C.G. back a bit, and will get some of the lead out of my tail. The battery is going under the seat. Now I won't have to un-tape the wing cover canopy after each flight, there is room for water dump valves, and the prospects of damage in a crash due to flying objects is reduced. (All that is missing is the time to complete the new fuselage.)
Lubriplate makes a snowmobile grease that is good to -50 degrees, or so. It is excellent for control system lubrication, and will not turn solid at 35,000' in the wave.
You can tell a dedicated HP-18 flyer by the skin missing from their right elbow. I padded my fuselage and soon wore the pad-ding down to the fiberglass. Now my wife makes me a thick elbow pad about 3" wide at the start of each season. On a good year, I wear it to shreds.
I love the completely reclining position in the -18. However, being 6' 4", and too many pounds, I can't move much when flying. (I raised the canopy 3" by gluing balsa to the fuselage rails, tapered to nothing at the front, and built a new wing cover canopy to fit.) Much worse, there is no way to use a baggie, in the traditional soaring way. This limited my first flights to under two hours, because dehydration is a point of paranoia with me. The answer is external catheters, available at any medical supply place for about $2. I installed two tube fittings on the lid of a bottle, one attaches to the catheter, the other has a 8" rubber tube which acts as a vent. When full, or landing, the vent plugs into the other fitting, making the bottle tight. After hitting a thermal while trying to empty the full bottle into a baggie, I carry three bottles set up for use.
With 90 degree flaps and sufficient experience, the -18 can be touched down at about 35 mph. As a result, the original drag brake works on most flights. There are a lot of circumstances where a faster landing speed is a good idea, in which case the brake does not do a lot. I added a disk brake, brake cylinder, and all other related parts to my ship shortly after the first 10 hours were flown off. (I do not have any problem explaining my high empty weight.)
Another HP-18 owner and I went together and purchased a set of Cleveland 5" wheels and disk brakes. (Aircraft Spruce would not sell them individually). Installing the system requires offset-ting the main wheel about 1/2" off center. A bracket is welded to the front fork to mount the brake. I put the brake cylinder in front of the instrument panel behind the rudder pedal assembly. A lever system and rod through the instrument panel, below the tow release, makes it easy.
Now it is very easy to stand the glider up on its nose and grind off a lot of fiberglass. This is a very rapid stop. Most landings the brake is not touched, however it has prevented serious damage in at least two occasions.
Once again, call if you have any questions. Also be sure to check your work with the A&P, etc.
Bruce Patton (805) 544-1052