(Originally posted on NOVAerial Robotics Blog)

NASA is now planning on using a UAV in addition to the Rover in order to explore Mars.  The reasons are very simple, and parallel the desire to use UAV’s on earth.  Having a camera in the air, instead of being stuck on the ground, allows for better perspective to see details of the terrain.  And also, avoids the problem that rovers have with negotiating difficult terrain.

But what is particularly interesting with this story, is that they have decided to use a coaxial helicopter instead of a quadcopter.  Why?  Multirotors on earth, already have enough trouble with stabilization due to the need to accelerate and decelerate the propellers to control their flight.  While small quadcopters do fly very well, as they get bigger, and the rotors get bigger, stabilization gets harder.  This is because the rotor inertia increases exponentially with diameter.  It takes much more energy to change the speed of a large rotor than it does smaller ones.  This is why you do not see very large multirotors flying very dynamically.

On Mars, the problem is even worse.  Due to the thin atmosphere, the rotor must be very large to be able to generate enough lift to fly. It would require a very large and heavy rotor to achieve lift, and the power required to change their speed quickly enough for flight control, would be much too large and heavy to fly.

Helicopters do not have this problem, because they use a swash plate to actuate variable pitch on each blade for flight control.  The motors do not need to be large enough to accelerate the rotors quickly for roll and pitch control.  So you can have a tiny motor spinning a large rotor in the thin Martian atmosphere.  Roll and Pitch control is achieved by cyclic pitch.  Yaw control of a coaxial helicopter is achieved via motor torque, and thrust can be controlled either by variable collective pitch on the rotors, or increasing the rotor speed.  I expect they used the former in this case, for the same reasons why fixed pitch props don’t work for pitch and roll control.  All of this also explains why large UAV helicopters fly so much better than large multirotor drones.

One interesting thing to point out, is that all of this also holds true for UAV’s flying at high altitudes on earth with thin air.  Multirotors will suffer much more than Helicopters will.  We have flown UAV’s in Colorado, both multirotors and helicopters.  The multirotors required installation of larger diameter propellers to achieve enough lift, in our case, changing from 10″ props to 12″. This negatively impacts the flight control, as the propellers are noticeably heavier.  The helicopter however, simply required a change to the motor controller to run the rotor slightly faster to make up for the thinner air.

We are excited to see that NASA is taking a novel approach to exploring Mars.  Here is a 37 minute video which goes into deep detail the system.

http://1080.plus/Mars_Helicopter_Scout/w3y7iJEe7uM.video

Views: 1779

Comment by Joe Renteria on September 2, 2016 at 11:26am
If that rendering is accurate, it also looks like they are using basically flat rotors. Lift must be pure planing and not through airfoils...
Comment by Gary McCray on September 2, 2016 at 2:39pm

Excellent Post Rob,

A coax heli makes perfect sense for Mars, even though the gravity is less (38% of Earth), the atmospheric pressure is still proportionately a lot less than earths 0.6% in fact (don't know why they said 1% in the video).

So approximately 1/200 earth atmospheric pressure at sea level. (= approximately 20 miles up on earth)

Amazing they can design even a heli for that - lots of rotor not much heli I suppose.

Solar array on top of rotor charges battery for for a single two to three minute flight per day.

It seems to me, possibly a retractable tethered heli that was powered from and docked with the Rover by retracting the tether might have been a better option.

It could be much lighter and more secure and not subject to bad landings.

Just a thought.

And of course in the video they show a large no fly zone around the Rover, so my guess is they don't want the heli anywhere near their Rover.

In the video they also said 5 flights is considered a success and that expected termination will be from an out of norm landing.

I'll bet the main thing that the Rover will be doing during those first 5 flights is watching the heli.

Which at that time will, including delivery charge, probably be the most expensive UAS in history - by a considerable margin.

Best Regards,

Gary

Comment by JB on September 2, 2016 at 8:40pm

So much for being the best VTOL platform on earth.... ;-) 

Comment by Laser Developer on September 3, 2016 at 12:58am

Very interesting Rob. We've been asked to get involved with making a small ground mapping LiDAR for one of these Mars missions. It's a bit beyond our normal scope of work so probably won't happen but it's fascinating to see terrestrial technology transferred to other planets. As exciting as putting people on Mars may be, robotic vehicles will increasingly be used for hazardous exploration.

Comment by robert bouwens on September 3, 2016 at 1:19am

rises the need for do-178 compliant firmware :-)

Comment by Laser Developer on September 5, 2016 at 3:24am

@Rob - I think I'm becoming a 'cotper convert. Did you see this video from CybAero? Such stability!

Comment by Rob_Lefebvre on September 6, 2016 at 5:39am

I haven't seen that video.  Yeah, helicopters fly much better than mulirotors, particularly as the size grows, but I didn't even see anything in that video that really shows it.  I shot this a couple years ago. This is a small heli, just about 1.6kg.

Comment by Laser Developer on September 6, 2016 at 5:44am

Wind? What wind! Would love to see a quad doing the same to get a comparison.

Comment by Rob_Lefebvre on September 6, 2016 at 5:47am

Yeah, I'm waiting for a sufficiently windy data to try something like that.  Just haven't had the stars all line up.  High wind, no precipitation, not below freezing. :) 

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