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#26 2016-12-30 17:26:00

SpaceNut
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Re: Scouting Mars by Helicopter

GW Johnson wrote:

I would do your lift fan explorations as part of the generalized category retropropulsion.  I have doubts that the same configuration would work in both subsonic and supersonic slipstreams,  and I have doubts about practical levels of disk loading in "air" that thin, even at the surface,  much less 10 km+ up.  Nevertheless,  it needs a thorough look. 

GW

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#27 2016-12-30 17:31:49

SpaceNut
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Re: Scouting Mars by Helicopter

More copying of OT post on helicopter use to scout mars:

kbd512 wrote:

GW,

I agree with most of the points you're making, but I still think the best way to visit multiple sites in a single mission is to figure out how to make aircraft fly on Mars.  If it's possible to make practical electric aircraft work on Mars, then there's virtually no point on the planet that couldn't be accessed in a matter of hours.  More importantly, the same technology that would make powered flight practical also makes soft landing substantial payload tonnages feasible without resorting to dumping truckloads of propellants through a rocket engine.  Every kilogram of payload delivered to the surface of Mars has a substantial price tag attached to it, so there's a clear incentive to limit EDL mass to what is minimally required to soft land.  I could be wrong, but I think the best way to do that is a two stage supersonic compressor combined with a conventional two or three stage lift fan.

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#28 2016-12-30 17:33:35

SpaceNut
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Re: Scouting Mars by Helicopter

GW Johnson wrote:

Well,  let's just say I'm skeptical but intrigued by the notion of some sort of aircraft on Mars.  Some sort of helicopter would really help,  at least up to a few 100 km.  But the density is so bloody low,  I have to be very skeptical of any sort of airplane or helicopter projects for Mars.  You still need very,  very low landing speeds not to crash.  Almost regardless of the scenario you are looking at.

With densities that low (fraction of a percent of that here),  dynamic pressures are minute at such speeds,  and you can only build so much area before the square-cube weight growth eats you up on Mars,  even at the lower gravity.   Aero coefficients are the same there as here,  they have no density or pressure or velocity effects in them.  Those went to dynamic pressure.

I know much less about helicopters,  but small airplanes that land on dirt strips here need to touch down between about 30 and 60 mph to be reliably not crashed by your ordinary pilots.  No one has built a large dirt-strip airplane in a lot of years now,  but landing touchdown speeds on them (I'm thinking of the B-17) were under 100 mph,  even with specifically-trained pilots. 

At 60 mph here on Earth at sea level,  dynamic pressure is around 9.2 psf,  and not-quite-stalled lift coefficients on straight,  subsonic wings are about 1.1 or thereabouts,  with little or no flaps.  (Not much higher with flaps.)  Your wing loading W/S is then limited to just about 10 psf max at 60 mph touchdown.  Significantly worse in thinner air at altitude or on a very hot day.

For the same touchdown speed on Mars (a simple kinematic requirement to avoid a rough field crash,  nothing to do with actual aircraft design otherwise),  CL is no different,  and the density is about 0.7% that here.  Dynamic pressure is then about 0.064 psf.  That limits your wing loading to about 0.07 psf,  those being Mars pounds at 38% of corresponding Earth pounds,  of weight.  100 sq.ft can carry ~7 lb Mars weight,  which is about 18-19 lb Earth weight. 

Wing structure weight typically is the 0.58 power of aspect ratio.  My C-170 has aspect ratio near 7,  and its wings (without fuel) weight about 100 pounds (Earth) each.  Their total of 200 lb here is about 76 lb there on Mars.  Even if advanced carbon composite,  they would still be about 30 lb there (2.5 times lighter).  Higher aspect ratio might lower that ~10-20% before you push structural limits too far on the materials.  Looks like a losing proposition to me. No payload,  no fuselage,  no power supply,  no propulsion.  Just the wing. 

Why would a helicopter (or any other subsonic lift fan) be any different?  They're not different here in any significant respect. 

GW

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#29 2016-12-30 17:35:43

SpaceNut
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Re: Scouting Mars by Helicopter

GW Johnson wrote:

I probably should have based my numbers on ~150 sq.ft of wing area,  not 100.  But it makes no real difference to the conclusions.

GW

Unless we find means to reduce parts used in the helicopter mass we will somehow need to find a way to create more lift.

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#30 2016-12-30 17:36:47

SpaceNut
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Re: Scouting Mars by Helicopter

GW Johnson wrote:

You cannot find more lift,  unless you can tolerate supersonic blade speeds aerodynamically,  structurally,  and from a lethal noise standpoint.  The blade tips on the Tu-95 Bear are just-supersonic at full power,  and it can be heard from a shallowly-submerged sub if flying at low altitude.  A supersonic prop was tried in the 1950's on an experimental variant of an F-84 Thunderjet.  It was not found attractive at all.  The Nazis also tried it,  and abandoned it,  during WW2.

You can add extra blades,  but there's interference from blade-to-blade that starts robbing you of your improvements.  Conventional wisdom says about 6 blades is max,  but I really suspect that number is actually up around 13-15,  for a rotational tip speed under 0.9 Mach,  and a well-subsonic slipstream.  Spin faster or fly faster,  and it starts looking like ~6 blades max again.

So,  limited to subsonic tip speeds,  it is well-known that you want as much of your blade length operating as near to best L/D as you can achieve.  That gets you the most disk thrust for the power required to spin it,  something very important to a practical design.  Best L/D speed is not fastest-feasible speed,  it's usually in the lower third of the feasible speed band with more wing-like airfoils,  with the exception being the very thin-section (easily-stalled) airfoils out at the tip. 

And absolutely none of that known propeller technology is compatible with more than about a 0.8 Mach slipstream speed.  Remember,  we've had dive experience to just-supersonic speeds with propeller airplanes since the P-38 just before WW2.  Props that work sbsonically simply quit working at all as effective thrusters as you hit the sound barrier,  because you are spinning subsonic airfoils at vector-addition supersonic speeds,  even if the rotational component is still subsonic. 

As far as I know,  there is no way around that.  That "barrier" is precisely why jet engines were invented.  They make supersonic flight possible,  not some "trick" propeller design.

GW

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#31 2016-12-30 17:41:42

SpaceNut
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Re: Scouting Mars by Helicopter

RobertDyck wrote:

The objective here appears to be bringing the vision from the TV show to life. NatGeo showed a swarm of drones covering vast area quickly to look for resources. If helicopter blades don't work on Mars, could a jet engine? If there's an issue with the CO2 atmosphere of Mars, could a ducted fan?

aeo-rc-30mm-ducted-fan-combo-w-9000kv-brushless-motor-9.gif

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#32 2016-12-30 18:20:49

SpaceNut
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Re: Scouting Mars by Helicopter

GW Johnson wrote:

The lift fan or helicopter problem looks an awful lot like the static performance problem for the propeller.  The stream tube is very wide at low speed from which the disk draws air,  trending from wide to disk size at the disk plane,  to about half the disk diameter at the achieved final exit jet velocity,  several disk diameters downstream.  That's from actuator-disk theory.  Interrupt that exit jet (as with the surface,  and you get less thrust. 

There's some,  but not a lot,  of stream velocity at the disk plane.  This stream velocity adds vectorially to the rotational tip speed,  for an absolute tip velocity that really needs to stay subsonic.  You can get away with up to about 0.9 or 0.95 Mach,  because the sections there are so very thin.  This thinness delays the transonic drag rise to such high speeds. 

None of that depends on anything but speed,  and speed of sound.  The lift and drag forces per unit blade area depend upon dynamic pressure,  and the lift and drag coefficients.  These get integrated root-to-tip along with the chord length and twist distributions to get the net blade lift and drag.  Thrust comes from the axial component of lift.  Torque comes from the vector addition of the other lift component,  and both components of the blade drag.  It's just the geometry,  but it gets a bit complicated to book-keep.  That's blade element theory. 

Technically,  you need to simultaneously solve the blade-element and actuator-disk models,  to get a reliable solution. 

Ignoring any blade-to-blade interference problems,  you just add this up for your total number of blades.  That thrust,  distributed over the circular disk area,  is the "disk loading".  (Disk loading is what you converge to make the two models solve together.)  At max feasible pitch,  that's all the lift there is,  and it's usually associated with a lot of power (torque),  because the max-thrust aerodynamics are not the most efficient.  Murphy's Law,  but a very real effect.  Your efficient disk loading is less;  that's where you usually want to fly. 

Disk loading will be proportional to some reference dynamic pressure.  That means for Mars,  at the same speeds otherwise,  disk loading is proportional to density,  which on Mars is around 0.7% that here on Earth.  For a given disk diameter,  area x disk loading is all the Mars weight you can hold up.  That's Mars weight units,  which would be a larger number of Earth weight units,  due to the 38% gravity.  Earth weight is weight on Mars / 0.38. 

What that really means is a disk design that would load to "X" weight/area disk loading here on Earth,  will only load to 0.007 "X" /0.38 weight/area on Mars.  The low density factor greatly overwhelms the reduced gravity factor.  The resulting reduction in disk loading is about 0.02 that on Earth,  or about factor 50 times less.  That's just way out of the ballpark for what you can compensate with high strength/weight composite materials,  which is nearer 2.5:1. 

The 48 foot diameter 2-blade rotor on a Bell model 205 helicopter picks up a max gross weight of 9500 lb here on Earth.  The same rotor could only hold up about 190 lb on Mars.  If you can design around that kind of lowered disk loading (and you'll need a lot bigger rotor diameter and more blades),  then helicopters on Mars are feasible. 

People looking at little drones flying around here on Earth,  then at how similar the landscapes look on Mars to some here on Earth, and then just make the leap to thinking that those same kinds of drones could easily fly around on Mars.  They know about the lower gravity making things easier.  But few realize just how close to vacuum that "air" really is.  It only has significant dynamic pressures at supersonic and hypersonic speeds. 

To get the same dynamic pressures we use here,  wind speeds have to be just about factor 12 larger there.  A 60 mph speed that produces 9.2 psf here at sea level,  requires a 720 mph wind speed there. And THAT's the problem with helicopters,  airplanes,  or lift fans,  on Mars.  And parachutes. 

GW

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#33 2017-01-06 23:40:03

SpaceNut
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Re: Scouting Mars by Helicopter

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#34 2017-05-17 22:48:22

SpaceNut
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Re: Scouting Mars by Helicopter

No forgotten yet.... How a Helicopter Drone Could Fly on Mars

We've gotten pretty smart at telling rovers what to do when they're working on Mars. NASA has more than a decade's experience in directing these machines on the Red Planet, asking them to image rocks, drill into the surface, or drive over varying surfaces. The Curiosity rover is so smart that in some cases, it can identify targets by itself to analyze.

But rovers have a big limitation: They stay on the ground. Aerial imaging is only available through satellites that orbit several miles above a rover. While this can provide a large overview of the site, it makes it difficult to anticipate what's just over the next hill or crater.

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#35 2017-06-07 18:56:20

SpaceNut
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Re: Scouting Mars by Helicopter

Helium or hydrogen lift would be a way to augment the blades lifting capability.

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