This is my first science deployment with the SIERRA UAS and of course it had to be to the arctic. We were told to expect just about anything in terms of weather. Conditions have ranged from sunny, calm and relatively warm to foggy, windy and very cold. Our last flight was launched in near freezing temperatures in 20 kt winds. Fortunately, the wind was mostly down the runway but there was enough of a crosswind component to require 1/2 left rudder stick to keep the UAS tracking down the centerline. The bird is heavy enough to ignore most gusts and there is no yaw coupling into the roll axis which keeps the workload low on the ailerons.
Usually the primary and backup pilots manually fly the aircraft for a short time in the local pattern before handing it off to the GCS operator. Not this time, though. The team had cold soaked for more than an hour on the runway setting up the aircraft. Additional time was also needed to trim the throttle actuator since it had been replaced the night before. By that time, I didn’t have much sensation in my fingers which is not good when using an RC controller. Fortunately, one of the guys had a pair of chemical hand warmers which worked great placed inside my gloves. After cranking the engine over several times, it fired up and we let it “warm up” for 5 minutes. The cylinders never did get very warm in these conditions. I pulled off my gloves, the ground crew pulled chocks and we did a normal takeoff. We used to do high speed taxi runs before takeoff, but now we position the aircraft so it is ready to takeoff immediately. Minimizing taxi is reducing the rate of damage to the leading edge of the propeller from the fine pebbles on the dirt runway surface. The startup area is also well swept prior to cranking the engine.
Getting airborne and turning crosswind, I told the backup pilot to get ready to accept control and transfer to AP (autopilot). We takeoff and land the SIERRA with a 2.4 GHz RC controller and to smoothly transfer control to the AP, we must first have the 900 MHz manual controller active and then switch to AP. The cold conditions motivated us to execute one of the faster switchovers yet. With the autopilot in control, we could scurry back to the warm hangar, sip hot tea and watch the aircraft on the moving map display.
Icing is always a concern in freezing temperatures combined with high relative humidity. The aircraft has an ice warning system which monitors these parameters and issues warnings when it determines the aircraft is in clouds or potential icing conditions. Flying out over the sea ice in visible moisture put the aircraft right on the edge of icing. In one instance, we lost the long-range (Iridium) control link and by the time we regained link the cloud and ice warning flags had been triggered. Autonomous flight in icing conditions is definitely not something you want to do. Once we got the aircraft down to a lower altitude and away from the sea ice, the icing potential dropped to a safe value. We realized the aircraft had flown through lots of moisture when water poured out of the back of the tailbooms when the nose was lifted to put it on the trailer. Doesn’t take a NASA engineer to figure that one out. Sealing the seams and holes and strategically placing drain holes has prevented further water intrusion.
Tomorrow the nasty weather pattern should break and reward us with sunny skies and 1-2 m/s winds. Conditions over the sea ice are also expected to improve. The aircraft and payload are ready and the airspace clearances are coordinated. The Norwegian Civil Aviation Authority has been wonderful to work with and our flight will be scheduled between the commercial carriers operating here.
– Posted by Mark Sumich, Chief Pilot, NASA SIERRA UAS