Q-talk 108 - The Jim-Bob 6-Pack
- Category: Q-Talk Articles
- Published: Wednesday, 23 December 2009 16:24
- Written by Bob Farnam
- Hits: 3895
This addendum to the plans will explain why and how the so-called "six pack" was developed for the Q2/Q200 series of aircraft. It will also give a step-by-step guide to adding these changes to your airplane, if you decide to do that.
First, here is some background. After many years of co-ownership and multiple tail dragger hours, Jim Ham and I decided to sell our Cessna 170 and build Q200's. We later purchased a Piper PA-22/20 Pacer to fly w hile we were building. During this time, 80's and early 90's, it was becoming clear that many people were having trouble with the ground handling of Quickie aircraft. Jim and I brainstormed and developed a series of modifications that we hoped would eliminate the problems.
As we saw it, there were several issues. First, the Airheart brakes included with the kit were not working well. Some builders replaced them with Mateo or Cleveland brakes. In 1988. Bob La Rue addressed the problem by publishing in the Quickie newsletter, Q-Talk issue 11, the redesign of the Airheart mounting system. Basically, his system substituted a balanced 4-point suspension for the caliper. This allowed it to float freely during braking so that it could follow any runout of the brake rotor.
A second important point to highlight was that the airplane also used a single, hand-operated master cylinder to both brakes. This was OK for stopping the airplane, as long as both brakes operated evenly and together, but it gave up any possibility of steering using the brakes. Brake steering is used on all other taildraggers and is an important part of control.
The third problem probably in an effort to reduce the cost of the airplane, was a non-swiveling tailwheel. Without differential brakes, a swiveling tailwheel could not be controlled anyway. In order to get a reasonable turning radius on the ground, the non-swiveling tailwheel has quite a bit of angular movement. This makes the ramp turning radius reasonably good, but makes it difficult to drive a straight line at high speed down the runway, due to its sensitivity.
The fourth issue involved the rudder cables that were run from the pedals, directly to the tailwheel and then back to the rudder. This put the considerable loads from the rudder pedals into the tailwheel bearing, thus protecting the small bearing at the base of the rudder. It did not allow for the use of tailwheel steering springs that would permit unwanted elasticity to get into the rudder control (think flutter). Also, with this design, all of the cables go slack if the tailspring is broken which is a relatively common occurrence.
Fifth on the list was the lack of tailwheel steering springs made it easier for the tailwheel to break loose (in a friction sense) from the runway and to begin to slide, thus losing its steering effectiveness. This can happen with the input of a quick steering command. It is made worse by the fact that canard taildraggers have rear wings that WILL lift at speed regardless of what you want it to do. This lightens the contact load of the tailwheel and ftirther reduces its steering effectiveness. Steering springs soften the steering input and make it less likely to slide. A further benefit of steering springs is by softening the steering effect, the airplane has fewer tendencies to dart to the side when the airplane is landed with the controls crossed, as in a crosswind.
The sixth problem was there was no way in the original airplane to get extra load on the tail-wheel aerodynamically. The factory first addressed this issue with the addition of an adjustable "T" tail. It was later replaced by the aileron reflexor. The reflexor moves both ailerons up (or down) together and alters the lift of the rear wing. The "T" tail works too, but with an increase in complexity, weight and drag. The reflexor is the method of choice now.
Seventh trouble spot was that this airplane has a tendency to float down the runway, if the approach is too fast. Since it is clean, it does not want to stop flying. Also, for the same reason, it is difficult to make a steep approach without the speed getting too high. That is why the factory designed the belly board drag brake. You do not have to have one to fly successfully, but it is one more element of control that makes the airplane easier to fly. Though the factory designed reflexor works just fine, there are several other designs that reduce complexity and/or improve the ergonomics.
1. To solve uneven brakes, put in Mateo or Cleveland brakes, if you want to spend the money. Otherwise, do the La Rue mod to the original brakes and they will work just fine.
2. You need differential brakes. They are an important part of the steering control on any taildragger. Either put in hand-operated (two lever) brakes, or add toe brakes. I chose toe brakes because they felt more natural to use. At one point, I made the mistake of reducing the sensitivity of my brakes. I was concerned that it would be too difficult to control direction during heavy braking. Unfortunately, I made them so weak that I could barely stop the airplane. The brakes were made more powerful by changing the lever ratio at the master cylinders. After many adjustments, I arrived at brakes that were 6 times more powerful than the originals. Interestingly, each time I made a change toward more power, the airplane got easier to steer. I now had more touch, since it was not necessary to mash on the brake pedal and I was getting more practice. See Fig 3.
3. Install a full swiveling tailwheel. I like the one from Aviation Products in Ojai, CA. Be sure to retain the originally specified ground angle of attack of 7.5 to 8 degrees. Do not use the original 5/8" round glass spring without a lot of beefing up. They break. I have successfully used a Dragonfly glass spring, which has an oval cross section 1" wide x 5/8 high This is stronger, and stiffen especially to a sideways load. It still needs the BID wrap for torsional strength. Do not shorten it more that the factory specified length. If you do this you will reduce its springiness and its ability to absorb energy. Others have successfully used a round spring steel tailspring. They work too, although they weigh more that the glass.
4. To get around the cable problem, we designed a bell-crank mounted in the floor of the tailcone, just behind the FS120 bulkhead. The rudder cables run from the pedals to the bellcrank, with a disconnect at the tailcone split. From the bellcrank, two sets of cables run aft. One set, which is set up so as not to change the rudder pedal/rudder ratio, runs though turnbuckles directly to the rudder. The other set runs from the bellcrank through tailwheel springs and then to the tailwheel control horn. The ratio of rudder angle versus tailwheel angle can be adjusted at the belcrank by changing the attach point of the cables to the tailwheel. The rudder, which moves +/- 28 degrees, should allow the tailwheel assembly to just reach the release cam point, which allows the tailwheel to full swivel. Now, if the tailspring breaks, you still have the rudder. The rudder pedals stay in position and you can use the toe brakes. The steering on the runway is less sensitive. You can lock one brake and pivot around the locked wheel for tight maneuvering at slower speeds on the ground. Major rudder pedal loads are now taken by the bellcrank bearing, not by the light bearing at the base of the rudder.
5. If you install the reflexor, you will greatly enhance the control you have over the airplane. The reflexor is a very powerful pitch device that has several uses. One important use is to help increase the load on the tailwheel immediately after landing. Another use is to slow the airplane on final approach without using up much of the elevator travel, thereby leaving plenty of elevator authority in the stick for final flare and landing. A third use is to preset the relative lift of the two wings before takeoff to compensate for e.g. location This way, it is possible to make the airplane lift off with the same stick feel regardless of where in the e.g. range it may be loaded. The fourth use is to reduce cruise drag for increased speed. I can not vouch for other airplanes, but in mine, if I reflex the elevator up a little bit (3/8" or so at the trailing edge), the airplane will pick up 3-4 knots. The spring trim system will let you hold the elevator in this position. The reflexor will let you re-trim the airplane for level flight. Some have actuated the reflexor with an electric trim system. I do not recommend that for a couple of reasons. First, unless it is a two speed unit, if it is set up slow enough to be reasonable to adjust for level flight without undue sensitivity, it will be slow to move back into trim if you happen to take off without remembering to reset after landing. Do not laugh -I have done that. Did you ever see a Q do the cobra maneuver? Also it will be slow to reflex after landing. Third, of course, is it adds complexity and maybe some weight, plus it relies on electric power. I changed my manual system somewhat from the factory setup by putting it in the left arm rest and using a lever which works with the same sense as a stick - that is, pull back for pitch up and push forward for pitch down. I like controls in the airplane to work in natural and intuitive ways.
The factory belly board is another device that is not strictly needed, but adds more control. It allows a steeper approach angle, better view over the nose, easier speed control and less floating on the runway, making it easier to get into short runways. Make sure you have a secure up latch. Mine deployed (un-commanded) once at about 150 kts and left a bruise on my shoulder that lasted for a couple of weeks. The airplane may be flown up to 130 kts with the belly board down and will fly and climb just fine with it down.
I have included with this write up several drawings that may be of help.
A drawing of one way to build toe brake pedals which will mount on the factory rudder pedals. Also shows the location of the brake master cylinders. (See figs. 3 & 4) A drawing of the bellcrank and bellcrank mount in the tailcone. See figs. 5 & 6)
A schematic drawing of the cabling, which I used for the rudder and tailwheel. (See fig. 6) A drawing of the way I modified the Aviation Products tailwheel to mount on the Dragonfly spring. (See figs. 1 & 2)
Belly board and reflexor drawings are part of the original plans addendums. The La Rue modifications to the brakes are available in issue 11 of Q-Talk.
La Rue mods.?Make up the parts per the La Rue article in Q-Talk issue 11. It is possible to make the mounting plate so that it uses the original bolt locations so you will not have to drill new ones.
Brake pedals- Make up the pedals as shown on the drawing (fig 3). They slip onto the top bar of the original rudder pedals. Drill cotter pin holes in the top bar of the rudder pedals so that you can install washers and cotter pins to keep the pedals centered in place.
Master cylinders and reservoir- I used Mateo cylinders with a remote reservoir. Use the shorter of the two models. The remote reservoir can be mounted as high as possible on the forward side of the firewall above the pedals. If you use integral reservoir master cylinders, you will hate yourself. They are terrible to service down in the "hell hole". You will have to make up phenolic mounts similar to the mounts used for the rudder pedals to hold the bottom of the master cylinders and let them pivot for and aft. Keep the mount veiy low and close to the canard. Put a 2" x 4" pad of three BID under the mounts where they bond to the top of the canard to spread the load. There is considerable down force on the master cylinders during heavy braking. Make sure you do a good job of glassing the mounts to the canard. I managed to break one of mine off years ago. Fortunately, it was while taxiing, not upon landing. I have my master cylinders pivoting on a 3/16" clevis pin for easy removal. Make sure you put a substantial loop in the brake lines to keep them from pinching or fatiguing during normal use. I have used Nyloseal lines successfully.
Bellcrank- Make up a bellcrank as shown in the drawings. (Fig. 5) You can substitute bail bearings for the bushing, but it should be double row. A single row bearing of this size does not have enough load rating. The bellcrank is made of 1/8" thick low carbon steel w ith a boss welded or brazed on for the bearing. Make up the phenolic block and drill and counter bore the bottom side to take the bolt mounted upside down. Cut through the inner skin of the tailcone and remove the foam in the cutout down to the inside of the outer skin. The phenolic block is going to sit in a bed of flox against the outer skin. The flox will keep the head of the bolt from turning. After putting the block in place, glass over the top to the inner skin with 3 BE). Use a generous fillet and overlap onto the inner skin at least an inch in all directions. Use Nylaflow fairleads to guide the cables from the rudder pedals to the bellcrank. Make sure that you use a drilled bolt for the pivot so that you can use a castle nut and cotter pin to hold the nut in place when assembled. Self-locking nuts are not to be used when there is relative motion against the nut.
Rear cables- You already have fairleads for the cables to the tailwheel. You will need to get a long drill and install new fairleads for the rudder cables, which lead directly to the rudder control horn. Use the same installation technique that is used for the original rudder cables. The rudder cables should have turnbuckles (fig. 5) to allow small adjustments in the rudder position and remove slack. The tailwheel cables will have tailwheel springs in them somewhere. Mine are outside, next to the tail-wheel. Others have mounted the springs inside the tail-cone, out of sight. Outside is easier to inspect, but inside is cleaner and better looking. You will have to decide.
Tailwheel mount and tailspring- I have included a drawing (fig. 1 & 2) of how I modified the Aviation Products tailwheel to mount on the Dragonfly spring. You may need to remount the spring to maintain the ground angle of attack with the new wheel, that will otherwise hold the tail higher in the air. If you use a 5/8 round steel spring, you will not have to modify the tailwheel. You can just drill it for a retaining bolt. You will, however, have to figure out how to mount it in the tailcone so that it is secure and won't twist. The tailwheel control horn can be used as is. You want less angular movement of the tailwheel anyway. Adjust the throw by choosing the radius on the bellcrank where the tailwheel cables connect. The drawing shows the dimensions of mine, but you should confirm that it is right for your bird. You want enough throw so that when the rudder is at full travel (+/- 28 degrees), the tailwheel cam is just at the disconnect point. For either tailspring, do not shorten it more that the plans show. You will stiffen it, make it less able to absorb energy, and possibly move the failure point into the tailcone. Note that the fiberglass spring still needs the BID wrapped around it per plans to strengthen it for torsional loads.
Reflexor- The plans reflexor works fine and there are other successful designs around. All of the ones I have seen work on the eccentric principle. I recommend mounting the control in the left armrest so that you can keep your right hand on the stick for landing. Mine is a short lever, which works like a little stick. The eccentric does not back drive easily. You probably will not have to add much friction to keep it in place.
Belly board drag brake- The factory design works well. I did change it a little and put my actuating lever inside the center armrest instead of outside. It sticks up through a slot. My reflexor will move the trailing edge up about Vi" and down about %". How it ends up being used will depend upon the relative angle of incidence of your canard and wing. I do not use much trailing edge down movement.
This group of changes constitutes what has come to be called the "Jim-Bob 6-Pack". All of the airplanes in Livennore, CA (6-5 flying) have these features. One of the 6, Geoff Rutledges' airplane that was built by A1 Kittleson, uses another equally good approach. Instead of a bellcrank and split cables, it runs the rudder cables to the rudder, then through steering springs to the tail-wheel. A1 beefed up the lower rudder bearing to take the loads. Either approach allows the use of tailwheel steering springs and preserves the rudder control and brakes in case of the loss of the tailwheel spring. All of these airplanes are well behaved on the ground and none has suffered a loss of control. Some have the Gall alignment, some do not. I can not easily do the Gall alignment because I use standard 5.00 x 5 aircraft tires and there is not enough room in the pant to tilt the wheel. Without the camber fix, my tires wear on the inside of the tread. To solve this problem, just turn them around midway through their life. The aggregate experience of the Liv-ermore pilots is that the Q as modified is a fine taildrag-ger with plenty of steering control. My airplane has been flying with these mods since May of 1998 and has around 500 hours on it now. Jim Patillo has been flying for many years and also lias 500+ hours on his Q200. We are satisfied that this delightful airplane is FIXED and now lives up to its original potential as one of the most efficient and nice-to-fly airplanes in the world.
Ed Note: The Q community owes a huge debt to Bob Far-num for all the time he has taken over the years in designing, implementing, evangelizing and documenting these corrections to some nagging problems with the Q2 and Q200's. This is one of the finest examples of how "Together we build better planes." Thank you, Bob!
You can order a printed copy of Q-talk #108 by using the Q-talk Back Issue Order Page.