X-Cell Custom Flight Test Program
1997 Flight Season
While the Winter throughout the Northeast turned out milder than usual, few weekend days presented perfect flying weather--it was either in the mid 40's with stiff winds, or rainy. The few good, warm days we did experience often came with a price of high winds as well, often making standing (not to mention flying) nearly impossible. Fortunately, this forced me to put the X-Cell on the bench and thoroughly check all parts prior to the start of the flying season multiple times. Some of the things I checked at this stage were:
- Main Rotor Balance
The main rotor assembly was completely removed from the main shaft and placed on the high-point for a static balance check.
- Main Gear to Tailrotor and clutch shaft alignment
This essentially amounted to loosening the clutch shaft bearing holders between the frame halves as well as the engine mounts and realigning the gear mesh with a simple piece of paper pushed through the two gears. I carefully pushed the gears together to produce a completely perpendicular mesh and then double-checked clutch to engine mainshaft clearance to assure that the entire system was in balance. Lastly, I followed up with a couple of small bubble levels placed on the clutch bell (as per Minature's instructions) to verify alignment.
- Main rotor tracking/Tailrotor Tracking
I rechecked the height of each main rotor blade as it passed near (or just over) the tip of my transmitter's antenna, and adjusted the tracking screws on the head block. This was done primarily to verify correct reinstallation of the main rotor head following the balancing effort.
- Examination of all systems for cracks
After last season's workout with a shaky engine, I felt it best to get some mineral spirits and a can of compressed air to clean the surface of the engine and lower frames in an effort to reveal any near-obvious cracks. Then, I removed the blades (particularly the stock plastic tail blades) and flexed each _very_ gently to reveal any possible stress cracks. Fortunately, all systems checked out.
- Examination of all ball links
Fairly self-explanatory. Some pilots consider a good link one that requires only minor force to move the link around its pivots. None of my links were yet to that point. In fact, most were fairly stiff by those standards, but since I didn't notice any difficulties in flying the ship, I considered it better to err on the conservative side and stick with slightly firm links.
The highlights of this timeframe included the eventual replacement of my Miniature Aircraft Supply #3966 aluminum header with a stainless model and the arrival of my painted canopy and fins. I did manage to take the aircraft outside for a brief run in early March in a simple attempt to prevent engine corrosion, but winds prevented any inclination to break the skids free of earth's grasp. By the end of March, I was wondering whether I'd ever get to fly again, but sure enough, I did in April.
One point regarding fuel storage--by the end of the 1996 flying season I had used three gallons of 15% Cool Power. Beginning with the 3rd gallon, I began pulling my Six-Shooter pump assembly from the bottle and sealing it with the original plug in order to prevent moisture from contaminating the fuel. Fortunately, this practice, combined with a cool, dry storage location has helped preserve the fuel such that it has survived nearly 10 months with no noticeable performance problems. I think this should put to rest the notion that fuel spoils in a relatively short period of time. Of course, if you don't use Cool Power or don't have a thermally-stable basement in which to store your fuel, your mileage may vary.
First flight this season came in the afternoon on April 26th. The ship started normally and responded well to inputs. After a slightly extended hover session to ensure the engine's health and general shape of the ship following disassembly, I pulled the ship up into a 10' hover and began to work my way around the field, primarily in a side-to-side motion, since my backyard prevented much of a circle.
Hover RPM didn't seem to climb much above 1150 (est from pre-hover ground tachometer readings), so I enrichened the mid-mixture slightly and checked the high mixture needle position. This change, combined with a slight pitch curve change near the hover region, produced a steady 1300RPM "hum" that brought the neighbors over for a peak. :-) Further exercise of the ship revealed a tendency for high cyclic stability, so between tanks I tweaked the aileron and elevator channel ATV from 70% to 80% volume. This brought the ship more in tune with my NHP simulator aerodynamic model and made a significant improvement in the ship's response. This was the first significant change to the model since the original setup and roughly on schedule with my skills development estimates.
Current Mixture Settings (OS61SX-Cool Power 15%)
- Low: 1.75 in from full out
- Mid: 1.25 out from full in
- High: 1.25 out from full in
I believe I may need to further enrichen the high mixture needle as I begin to experiment with slow-forward flight and spend more time at greater than 1/2 throttle, but these settings have worked quite well for me since they were finalized after approximately one gallon of fuel. OS61SX owners should not, by the way, expect the mixture requirements to stabilize much before that.
One highlight of this month was the discovery of a school-owned field not too far from my residence that would surely help learning slow-forward-flight<->hover transitions...and not a moment too soon.
First trip to the field!
The weather turned out to be beautiful this day, but the winds were a bit high following a low pressure system that had pulled some severe thunderstorms through the area the previous afternoon, so I was forced to wait until about 5PM to put my ship in the car and make the short hop to the field. I was all too familiar with the pressures associated with having an audience, so I was a little anxious about flying when I found a ball game in progress at the field adjacent to the one I'd chosen, but things turned out well. While a small crowd gathered to watch this novice heli pilot do lots of boring circles, all kept their distance (which is a good thing for people at my stage of learning...the pressure to perform can be a fledgling heli-pilot's undoing--one split second of inattention can ruin months worth of work and progress).
Most of the flying went as planned and I was able to circle the ship around myself in a level, near-constant radius turn. For a short while I began to sense the subtle transition between the familiar hovering mode and the influence of ETL (effective translational lift), or the point at which forward speed produces a change in the aerodynamic qualities of the rotor systems. In particular, I confirmed the need to lower throttle/collective as forward speed built up in order to prevent the ship from gaining too much altitude, as well as the need to raise throttle as I brought the ship through a descent-to-hover landing transition while pulling back the cyclic to gradually reduce forward speed.
One thing that did surprise me was a sudden leaning of the engine while in a 4' hover near the end of the main tank (my fuel system, as you can see in the photos, consists of a 16oz main tank and a 2oz header/feeder tank). The lean running caused the head speed to increase substantially (1500-1600 RPM) and made the ship extremely sensitive to input, and thus difficult to put down safely. Once on the ground, however, a quick inspection revealed no outward signs of trouble. All fuel lines appeared properly connected and without obvious damage. The header tank was still full and the main tank still had about 1/2" of fuel remaining. Most importantly, the pressure feed line from the pipe to the main tank was intact. In other words, I found no obvious reason why the engine leaned out so suddenly.
Following the inspection I decided to enrichen the high needle by one click, bringing it closer to the target baseline of 1.5 turns out and the mid needle by two clicks, closer to its target of 2.0 turns out (the mid needle on the 61SX is not very sensitive and does not produce the same result as the high needle, by the way). This produced a richer (and rougher) idle-to-hover throttle transition, but once the throttle reached about 45%, the engine smoothed out. I took this to mean that the idle mix was now too rich, but I did not alter it during this session. I cautiously began the flight with an extended low-hover session, during which the engine performed normally, brought the ship into a 15' hover, and finished the second tank of the day with some additional hover-to-slow foward-flight transition practice.
Some post-flight reading of the chapter on forward flight transitions in Ray's manual revealed the following commandments that I felt appropriate to heed at my stage of skills development. :-)
The 10 Commandments of Helicopter Flight, by Ray Hostetler:
- He who inspecteth not his aircraft giveth his angels cause to concern him.
- Thou shalt not become airborne without first ascertaining the level of thy propellant.
- Let infinite discretion govern thy movement near the ground for thy area of destruction is vast.
- Thy rotor rpm is thy staff of life. Without it thou shalt surely demolish thy bird.
- Thou shalt maintain thy orientation with thy helicopter lest its nose turns and seeks to devour thee.
- Thou shalt not make a trial of thy Center of Gravity lest thou dash thy foot against a stone.
- Thou shalt not let thy confidence exceed thy ability, for broad is the way to destruction.
- He who doeth his approach and alloweth the wind to turn behind him shall surely make restitution.
- He who alloweth his tail rotor to catch in the thorns curseth his children and his children's children.
- Observe thou these parables, lest on the morrow ye be found mourning over the remnants of thy labor.
Early this month I found some of the nicest, near-windless evenings of late spring. One particularly memorable evening at the field near my house allowed two full tanks of hover/forward flight transition practice.
Before starting this session I noticed significant discoloration of the glow plug I had used for the past gallon or two, so I decided to replace it with another $11 Enya #3 and give some serious forward flight a try. I also turned the high needle another click open and the idle needle about 1/16 of a full-turn open to richen up the extremes of the mixture spectrum slightly. These changes ultimately produced what I would consider to be the most consistant performance of the 61SX to date. The entire session went well and I truly began to feel the influence of the NHP flight simulator on my flying. I found it almost second nature to fly the heli around me in medium (25-35' diameter) circles at varying altitude (5-50 feet).
As I worked the ship around the circle and entered the upwind leg, I noticed a slight longitudinal "burble" associated with the transition through ETL, which was accompanied by a need for less throttle and (in my case), a slight tug backward on the cyclic to prevent forward speed from building too quickly. Transitions back through ETL to hover seemed to be a bit more manageable (or subtle, depending on the way you look at it) and tail rotor control seemed to come more naturally. This was the first time I flew with the newly-painted canopy on the bird and it did seem to help my orientation as well as the ship's aerodynamic characteristics (directional stability, in particular) in forward flight.
Yet another flying session later the next week revealed the need to further enrichen the high mix by another click because the new plug's coil appeared to have a white or frosty appearance after only three tanks. Oddly enough, the ship ran with a good smoke trail just like those evident on previous flights, but today's flight session was in a higher wind (about 5-7 knots) that actually put the ship into slow forward flight while near a 20' hover and I found myself exercising the throttle past 1/2 more often to counter gusts and altitude changes. A few attempts to push the ship around in a circle revealed a tendency to lose altitude during the downwind to crosswind turn, but this was easily countered with an increase in throttle and roll, as well as just a hint of tail rotor to keep the ship in trim around the turn. Rather than risk the ship, though, I figured I'd spend the remaining time in "psuedo-forward-flight" and get used to how the ship feels and sounds at higher velocities. And, if you're wondering, it feels and sounds realllly good. :-)
Later in the month I purchased my second case of Cool Power 15% in order to continue my forward-flight transition practice. I began to extend each flight session to 3 tanks because I noticed my ability to keep up with the ship had improved. With the new batch of fuel, I also began to notice the mixture had become rich enough to reduce rotor RPM on descents and increase the throttle setting required to hover, so I spent a bulk of these flight sessions tweaking mixture needles to arrive at an optimum mixture that would allow me to pull sufficient pitch as well as retain rotor RPM. The idle mixture played the most significant part in the optimization, and this was leaned approximately 1/4 turn. The mid and high needles were moved back and forth by no more than two clicks with no discernable effect.
Considering that I had recently finished my fourth gallon of fuel over roughly 50 flights, I decided to put the ship through another brief overhaul, which included a thorough external cleaning and examination, waxing of the rotor blades and canopy, and replacement of the silicone exhaust header/pipe coupler as well as the tail rotor gearbox grease (per Minature Aircraft recommendation). To replace the gear box grease I removed the entire tail rotor gearbox assembly from the end of the tail boom, split the halves, thoroughly cleaned all parts with acetone, applied a touch of blue loctite to the drive shafts where the bearings were installed to insure the inner races would spin with the shaft, and, lastly, replaced the grease with the factory specified.