Hi!
I am currently developing a basic design concept for a multicopter for mars exploration within the framework of a university project. Mars atmosphere is 100x thinner than on earth while gravity is about a third of earth's gravity. This makes aerodynamic design very tricky since Reynolds numbers are extremely low.
Therefore currently I am stuck with the rotor design and hope that you guys can help me a bit.
My biggest problem so far is settling on a feasible rotor diameter. Right now I am assuming a blade loading of 20kg/m^2 which gives me a rotor radius of around 0.5m for a 20kg octocopter. At the same time my calculations give me a thrust coefficient of 0.06, chord 0.27m, lift coefficient 0.9 and Tip-Reynolds number 40000. Power requirement would be around 2500W.
Now I don't really know what to make with those numbers and if they make sense at all, i.e if this thing would actually fly on Mars...
Could you give me some hints on how to move towards an optimal and feasible design (number of rotors, rotor diameter, blade number etc.) of a multicopter mass of let's say 20kg?
Best Regards
Replies
We already know multis are inherently the least efficient means of drone flight. A conventional heli, and a fixed wing both are more efficient.
I believe a multi would be the last platform to ever fly on mars.
Factor in Reynolds numbers and kinematic viscosity on Mars and it is the equivalent to flying at around 90,000-100,000 feet here on earth. Keep in mind, due to the much thinner atmosphere and the fact that pressure still decreases exponentially with altitude (due to gravity), the effects of altitude (on an aircraft) are more drastic than here on earth. For instance a 500ft elevation change on mars may act on an aircraft similar to 5,000ft here on earth. This being said not only would a multicopter have trouble on the ground, but it would be extremely limited on any realistic altitude.
Reynolds are fairly simple. It is a theoretical number that compares the kinematic viscosity of a fluid (or air), fluid/air density, velocity, and chord.
This means you would have to develop a multi that would fly at extreme altitudes (here on earth).... On top of this it would HAVE to be extremely high rpm (lower Re's the higher the speed of sound, the less we have to worry about supersonic tips and their shockwaves.) This increases inertia exponentially making RPM control out of the question for a Multi platform. Therefore the only viable answer would be variable pitch props. Keep in mind High RPM and high inertia decreases motor efficiency, another negative factor of multi's on mars.
There are dry lake beds on mars with VERY smooth surfaces, and this has already been looked into as a viable option for heavier than air aircraft on mars. (directly from Nasa studies)
The prop would be fairly wide compared to what we are used to, with an extreme amount of twist. RPM would be very high as well.
It would not be difficult to design an aircraft very similar to the self landing fixed-wing drones we have now. The most difficult would be recovery and making sure it could get back to the platform for recharging and data transfer. This, as mentioned can be done by a tow/bot. AS long as the aircraft was within a reasonable distance, the towbot could simply tow it onto the platform.
Vtol capabilities add excess weight and decrease efficiency, especially at these low Re's.
Nasa seems to agree with my theories, as all of their most recent proposed Mars aircraft have all been fixed wing.
Search the following proposed MARS aircraft:
ARES
MAGE
AME
MATADOR
Also check out this quick article by NASA on the complexities of MARS atmospheric flight. (essentially the same content that I have been explaining.)
http://www.nasa.gov/centers/ames/research/technology-onepagers/mars...
If you want VTOL it would almost certainly have to be done with oxygen/methane fuel rockets, as these fuels can be harvested on the surface without finding Ice caps. However, this would add the complexity and weight of these rocket systems, as well as the complexity and weight of the fuel harvesting and pumping equipment at the base station. All of a sudden this makes a TowBot a very reasonable option.
In my opinion Fixed wing would be the solution. Due to the low reynolds numbers it is going to require a fairly large wing both in chord and span. The plane wouldn't be quite as familiar to us as are planes here. The prop would have a significantly large chord, and would rotate at much higher rpm. If I were to design it I would, produce a plane with a 3 piece folding wing. For example my Albatross will break into 3x one meter sections. I would do something similar, however it would fold into thirds, or smaller to save space. The craft would need mapping software, and intelligence. There is no GPS so it would have to solely navigate by terrain recognition, as well as radio signal/tracking from the GS (distance and heading to/from the GS). The capsule would be of similar build as previous mars rovers with pyramid style base, and it would have to land on some lake bed to ensure considerable amount of area to land as well as a smooth surface. The plane could land sloppy and use a rumba style bot to taxi it back onto its platform. The platform could contain all of the actual analyzing bots/software as well as data transmitters to send back to earth. It would also contain a vast array of solar panels to recharge the craft. The base station would have to have the ability to fold up its sides (back into a pyramid form) to protect the sensitive craft from high winds.
This is all theoretical, however this would be MY idea of a mars atmosphere and terrain mapping aircraft,
What about the disc load, is 20kg/m^2 realistic or is this way to high?
The propellers should be driven by electric motors which are powered by LiPos.
First off we need to figure out the kinematic viscosity of air on mars.. besides the pressure, what are the properties of the gas? This will be significant in proper airfoil design.
Secondly I believe the best design would be a thick chord blade, constant speed with variable pitch. However, since everything would have to be one-off anyways, the blades would still have twist (unlike variable pitch RC blades. Think full scale Turbo prop).
Thick chord, and high rpm will help you get the Re's needed.
As for 20kg, that might be considerably more difficult than say 10 strictly due to Re's
Klaus, I think you will face major issues with a multi-rotor in a low density atmosphere, like Mars. If you have the capital, I'd suggest investing in aero/fluvial modeling software. I say this as there will be many questions that only trial, error and inspiration will address.
On a positive note, you may find that a glider/rover hybrid based on the albatross, will be more suited to planetary exploration.
Yes, A multicopter is a brilliant but highly adapted craft that may only work on a planet with earths peculiar mix of atmosphere, gravity and climate.
Regards, Chris
Ps, check this out!! http://youtu.be/cB29pFKa4Lo This is the Carter-copter, one of the most efficient rotorcraft about, maybe an autogyro/single-copter will offer useful flight times and efficiency.