February 1999

by Bill Lavender

MEMPHIS, TN - There's an expanding portion of the ag aviation industry, albeit small in number, that is focusing toward the big iron, relative that is to ag aircraft. With its use in a growing worldwide aerial fire bombing fleet and high intensity pine tree fertilizing in the south, the PZL M-18 Dromader is one of the big iron.

Carl Bailey of Delta Turbines, Inc. has been in the ag aviation business for more than 36 years. He began as a spray pilot. Not satisfied with the performance of the aircraft he was using, he began modifying. One of his first modifications was a TPE331-6 engine conversion to an AT-301. I visited with Carl in the summer of 1993 and flew his rocket ship AT-301 for a day, spraying soybeans in east central Arkansas (see AAU October 1993). The Arkansas Department of Agriculture was kind enough to issue me an "honorary"/temporary ( I believe for about three days) pesticide applicator's license. That evaluation flight was probably the only really true evaluation flight that I've flown where I logged enough hours to relate to exactly how the aircraft performed. Needless to say, an AT-301 with a 480 gallon hopper and a TPE331-6 (780 shp) performed extremely well. Yea, those numbers are correct, no typos; 480 gallon hopper, AT-301 and 780 shp.

Sensing a market for a large hopper, high performance ag plane, Carl began working with the M-18 Dromader. He has always been a believer in AlliedSignal's TPE331 engines. Therefore, his first Dromader engine conversion was the TPE331-10UA (inverted air inlet and three-blade prop). I flew Carl's marvel of a conversion (see AAU July 1995), and found it to be a rather remarkable aircraft with its long 22-foot cowling, from prop to cockpit.

However, Carl soon discovered that the huge Dromader needed more than just a high-powered turbine engine. The aircraft could use servos for its control surfaces. And with the additional horsepower, a larger hopper would be in order. It seemed the more Carl worked with the Dromader the more items he felt he could improve upon. Not to mention, after a couple of spray seasons, a need for an even larger engine! Thus, the advent of the TPE331-12UA installation, an 1,100 shp powerhouse. Let's not get ahead too quickly. Carl realized if he was going to convince ag pilots the Garrett-powered Dromader was a viable option, he was going to have to make the plane lighter on the controls and increase the hopper size. Working the Dromader in his own spraying operation gave him a special insight to what needed attention and if his "fixes" worked. After several months, Carl finally was awarded his STC for servos to all three control surfaces of the Dromader, ailerons, elevators and rudder. He also expanded the aircraft's hopper top to a measured capacity of 785 gallons with a 6.5-foot lid. These mods radically changed for the better the handling characteristics of the aircraft and its ability to cover a lot of acres in a day.

Carl's work with the Dromader included removing the air scoop and the rotating beacon. These two items created a high frequency vibration that was transferred through the rudder to the rudder pedals fatiguing the pilot. With strobe lights the rotating beacon was no longer necessary. Zee Systems' air conditioning ended the need for an air scoop. Another area of technology that Carl addressed was the harnessing of an engine's torque output. He knew that no matter how much shaft horsepower an engine developed, if it wasn't swinging a properly designed "toothpick", all efforts would be mute. Carl was able to convince Hartzell to work with him in developing a three-blade prop for the TPE331-10UA. This prop developed more torque from the same horsepower than other props available for the TPE331-10's, enhancing its takeoff performance dramatically. Carl applied this same technique to the design of the five-blade Hartzell "Monster" prop. This 119-inch prop brings out the best in the TPE331-12UA for takeoff performance. That's really the only area where the -10UA is weak. Once in the air, both operate at the same shp. Only, the -12UA will be operating at a lower percentage of torque than -10UA, thus the potential for a longer, trouble free TBO.

Author's note: There exist a TPE331-11UA which is a -10 engine with a -12 gearbox, developing 1,100 shp. With a fresh CAM inspection its value is about $150,000 less than a new -12.

Carl contacted me in December, giving me the information about his newest brainchild, the TPE331-12UA Dromader. He was really excited about it and wanted me to come to Memphis to fly it. Of course, I agreed. After Carl finished the installation, my first opportunity to stop in Memphis came during a crosscountry to Nebraska. We set the date and met at the Charlie Baker airport on the north side of town. There's more to designing an engine conversion than simply building a mount, forming a cowling, picking a prop and hooking it all together. Much of the engineering was already accomplished with the -10UA installation. However, there were some things that had to be addressed specifically for the -12UA, none the least being the inlet design and engine rigging. Carl had a strong feeling that he could improve on the inlet design for the -12UA. He knew the FOD inlet screen affected the horsepower and there should be a better way to get the air into the engine with a ram effect. He built a plenum that he thought would bring the air into the engine better with a larger inlet opening forward of the exhaust. However, after the installation was flown his sensors indicated he was not getting the ram effect he desired. So, for our evaluation flight he had reverted back to the standard inlet installation. But, Carl is confident that an inlet can be built similar to a smile-design that will give the ram air effect, improving available horsepower on high density altitude days. He also believes the dampening effect of a new style inlet will quiet the ground noise emitted by the -12UA.

I arrived at 2M8 midday. Not letting a simple evaluation flight get in the way of lunch, we headed for town. A big Sunday lunch at Morrison's Cafeteria is not the best way to prepare for an evaluation flight in an ag aircraft that is intimidating to even the most adept ag pilot. Coupled with a full stomach, Carl managed to unsettle it by

explaining that he hadn't yet quite got the rigging like it should be on the installation. Hmmm... Rigging the throttle setting and prop blade pitch on a turbine-powered ag plane is a critical adjustment. It has a profound effect on the way the aircraft lands, and is even more evident with the larger horsepower versions. During an earlier test flight, upon approach to landing, Carl pulled the throttle to idle, then slightly behind the stop. The prop went to beta and would not come out. Thinking he was going to be landing short of the runway Carl prepared by slowing the Dromader as much as possible. The slow airspeed caused the prop to come out of beta, launching the Dromader into a climb.

Carl corrected that problem by turning up the idle on the engine fuel control unit. However, Carl warned, this caused the engine to develop too much torque during landing, pulling the Dromader across the runway threshold at 100 mph. Also, because the rigging wasn't completely right, he instructed me to not move the throttle behind the stop until I flared the aircraft. He told me that when I was ready to land to ease the throttle behind the stop, being very careful to move it only a small amount. Also, I should expect a lag in the engine's response to the throttle position. Neat trick.

I won't go into detail about how I got the -12UA Dromader on the ground in one piece. I'm not sure I know how I did it. One thing is for sure, the show was on when I eased that throttle behind the stop, and it wasn't pretty; too much behind the stop.

Carl has assured me that Hartzell and AlliedSignal are working with him to remedy the rigging. Not taking into account the landing, (Carl can squeak it on) the -12UA is one phenomenal ship. Unbelievable raw horsepower! Unbelievable performance. Admittedly, the aircraft was empty without a spray system. But, it doesn't take much figuring to realize a full load, booms and a hot day could easily be managed. A maintained vertical speed of 2,300 fpm at 100 knots with 80% torque, the -12UA Dromader is climbing at an angle that will get your attention. Scooting across the field at speeds of 140-150 mph with only 45% torque, the aircraft is one fast honcho.

I flew the -12UA Dromader northwest of Memphis, across the Mississippi River, to work some of the Delta rice fields. Ferrying at 50% torque, indicating over 150 mph, I bumped the throttle to 60% torque to begin the application sequence. Of course, not before I had done a few stalls and Lazy-8's. I found I had to keep reducing the torque to keep the speed down. The first couple of runs crept up to the 175 mph mark, much too fast for me and the aircraft for that matter.

The flight was smooth except for some tail movement at the higher airspeeds. Carl developed a remedy for that as well (see pictures). Dropping the belly skin will improve airflow and help prevent center section spar corrosion.

The aileron rolls on the breakout from the field were smooth and positive. There's no question the Dromader is heavy iron. But, the servos are a marked improvement. You don't need to crowd it. The aircraft works its best with consistent, smooth turns.

I could have used a couple more days in it to really get the feel for flying it. But, that's not the way it is with an AgAir Update evaluation flight. I climb into a strange aircraft and spend a short period of time in it. The reality is that's not a fair way to evaluate any aircraft. Working the aircraft during a spray season is the only way to really form an evaluation. I'm sure Carl will have that done this season. I'm also sure that the TPE331-12UA Dromader will receive high praise, especially on a hot day working from a short strip. It is one tough hoss. Maybe the best thing about this conversion is the cost. Approximately $685,000 buys a new airframe, TPE331-12UA engine, propeller and conversion. If a -11UA was available the conversion would be about $150,000 less. Not a bad price for almost 800 gallons of turbine performance!