Helo Speed Records

This is becoming an aerospace video blog. 🙂

The eighties

Versus the teens

Westland Lynx only reached 90 m/s while X2 is doing 130 m/s. The ability to let the rotor spin slowly since only the advancing blade needs to lift in a counter rotating rotor allows the X2 reach a high speed.

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Sankrithi Channels Wallis

Most people with interest in aerospace history know of Barnes Wallis’ geodetic structures, most famously used on the Vickers Wellington. He first invented them for the R100 airship, basically a weave of thin aluminum shapes going in different directions, forming a grid. No bulkheads or even wing spars needed, but it was quite complicated to build. Fabric was used as a cover, first linen and later thin steel wire mesh. It became outdated when airplanes were pressurized and moved to aluminium monocoque structures.

Well, now, airplane manufacturing technologies are changing again, after some 70 years of riveted aluminium sheets, bulkheads and spars. Composites laid by robots enable fancy shapes, and optimizing the strenght carefully in various directions. Boeing has been looking at a 737 replacement. So far, pressure hulls have had to be cylinder or ball sections – only that way the thin skin can be in pure tension, the only force it can really resist. Often a double bubble has been used, with the cabin floor dividing the lower and upper half into two circular sections and also keeping the left and right side together at the same time. A circular frame is not very space efficient for humans though, so Sankrithi et al at Boeing figured out how to put fibers in a grid to enable a roughly elliptical shape that is wider than it is tall. The advantages are not entirely clear to me from their description, since they seem to say the ratio of seats per circumference is similar to a circular frame, but it is nevertheless interesting. (Also, it’s strange that Free patents online shows that the patent was filed in 2009, yet Google shows it was filed in 2005, yet they seem the same at first glance).

This has some relation to the X-33 where they also attempted non-cylindrical composite pressure vessels. The technology has advanced since though.

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Humans Fast, Machines Slow

Rand Simberg talks about impedance matching. So I’d like to make a post of my comment there (I’ve always wondered why this obvious alternative gets mentioned so little…)

What to do when you arrive at Mars or Earth with your solar electric propelled vessel?

So, the problem with most low fuel demand velocity change schemes is that they only give slow accelerations. Low fuel high velocity change means solar or nuclear electric propulsion and aerocapture mainly.

High delta vee aerobraking is hard to do in one pass – it gets dangerous because of atmospheric variability and potentially other reasons.

Simple: detach a small capsule with the humans that goes directly to the surface (with only days of life support) and leave the untended craft to do multi-pass aerobraking. Hitting van Allen belts a few more times or taking a long time doesn’t matter that much with no humans onboard.

You could also potentially ultimately leave the long distance craft at some Lagrange point instead of LEO. (Cue some clever and complex maneuvers to save fuel – maneuvers that take long.)

Something similar could also be done when a long distance stack is assembled in LEO: send the humans there only after it’s through the belts. They can go with a smallish capsule again. Potentially at some Lagrange point, or in space without any fixed reference, just along the way. It could be dangerous though if the capsule doesn’t have much life support.

Many of these things have potential delta vee penalties as well as timing inflexibilities, but they could have enough other benefits that they should be considered.

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The Swiss Monotracer Two-Wheel Car

If you speak german, this video is nice. They show it driving, in construction and design.

Though it is complex, heavy at 450 kg and expensive at 62,000 euros. Uses a BMW motorcycle engine and sits two people in tandem and looks like an airplane cockpit. It could serve as an interesting datapoint if more fuel efficient non-mass transport vehicles are to be developed for single commuters. The E-tracer version has had success in the automotive X-Prize. It’s no small deal, since that includes handling tests.

Overview of the Monotracer in english:

Lane change handling test at automotive X-Prize:

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The Last I4

It doesn’t have the same sound as “The Last V-8” now does it? When you look at what’s happening in the world of automobiles, you get some idea of a change. It is always slow, yet I predict that when it happens, probably starting before the end of this year, people are taken by surprise.

Why Would Anyone Buy A Hybrid Car?

It doesn’t have that much better fuel economy than a modern petrol or diesel engine if you drive out of the city, and it costs some more and is complicated. New turbocharged and variable valve engines can do pretty well because they can be built to opearate in a flexible manner. BMW has even introduced a technique that could be called a “virtual hybrid” – where the aircon compressor and battery charger are disconnected when the gas pedal is pushed to the bottom, resulting in extra power available for traction – allowing a smaller engine to achieve the same acceleration.

In some cases, like buses that need to stop often, hybrids make great sense, but otherwise I see the improvements in ordinary direct internal combustion engine driven cars narrowing the gap quite considerably. On the highway the hybrid has no advantage.

A Seemingly Small Addition

Volvo V70 Plugin Hybrid Prototype

So, add a grid recharge capability and you have a plugin hybrid. By itself that’s not much yet though, you have to enlarge the batteries too. Since most trips for most people are commuting and errands, they’re short and can be done entirely on battery power. You also still haul the gasoline engine along and it is used on longer trips. Most problems solved right here? Sounds easy.

Chevy Volt / Opel Ampera is coming soon. And just take a look at the huge number of plugin hybrids being developed, listed at Plugin America. Most of these will be dead ends, but some might make it big.

Pure Electric Cars – The Charging Problem

The problem is, gasoline is very very energy intensive. If a car uses 8 L for 100 km and a 10 kWh worth of energy, then tankage of 40 liters gives about 50 kWh of energy. Done in 50 seconds this stream of gasoline through the hose is worth 1 kWh per second or 3.6 megawatts. High enough temperature superconductors have not yet been invented that would make a hand-attachable 3.6 megawatt car charger possible. If we generously assume 240 Volts and 40 Amperes, the charging power is only 10 kilowatts. The largest home appliances like sauna stoves and water heaters are in the single kilowatts range. They often use 380 V three phase power here, but that gets slightly impractical for a car charger. This thousandfold disparity in energy replenishment speed is striking. An optimistic 10 kW charger would charge a 100 km drive’s worth of energy in an hour. Certainly useful for commuters. Charging as range extension seems doubtful. Our summer cottage is 300 km from Helsinki. If a full battery only lasts the first 200 km, one would have to stop at a loading station for one hour before one could drive the rest of the way. Not realistic. The penalty of lugging around the heavy and complicated IC engine has to be bitten at this point of battery development.

Battery Improvements?

There exist some pretty high energy per mass battery technologies right now, but they are expensive and use rare materials like cobalt that they can never really be mainstream solutions on something that stores energy in the megawatt hour class. Hopefully with enough money now available, some cheaper and less material intensive ways to store electrical energy can be developed.

Your Local Correspondent

Naturally, Finland would be a pretty ideal place to have electric and plugin hybrid vehicles. A significant portion of the populace keeps their cars parked in a spot with an electric socket nearby – those house timers that turn on the cylinder block heater on an hour or so before leaving for work in the wintertime, meaning less fuel use and wear for the engine. It is trivial to use those 240 V outlets for electric car charging. Also, electric cars are actually manufactured in the city of Uusikaupunki, Finland by Valmet. The Th!nk City is one. Too bad because of insane tax policies, it is not actually sold here – at the moment you can only get them in Norway, Austria and Holland. The Fisker Karma electric sports car will also be manufactured in “Uki”. I think they have a long nose on the car for Freudian reasons – since it doesn’t need to hold a long block internal combustion engine – or maybe it’s just that buyers are conservative. And then there’s the e-cars now project aiming to refit old Toyota Corollas with electric motors and batteries. But there’s more.

Automotive X-Prize

There’s that going on, which is actually interesting! My favorite vehicle is the Peraves E-Tracer.

Here are the results from August 2 but I still don’t know what will change / what is coming because the pages are unclear: [EDIT: fresh info at the blog indicates some testing is still going on]

1st Place Team Lithium Ion Motors of North Carolina (125 MPGe average fuel economy for the event)
2nd Place RaceAbout Association of Finland (0.179 seconds behind the leader and 100 MPGe average fuel economy for the event)
3rd Place TW4XP of Germany (11 minutes, 36.9 seconds behind the leader and 139 MPGe average fuel economy for the event)
4th Place ZAP of California (DNF – 48 laps completed)
5th Place Aptera of California (DNF – 18 laps completed)

The Raceabout team is from Helsinki’s Metropolia university of applied sciences. They have a long background of building electric vehicles, and it’s nice to see something in the competition that looks like an actual car, yet still manages to do so well.

RaceAbout's E-RA vehicle in the Automotive X-Prize 2010 competition

Hopefully the politicians here can get something sensible done, and the super-high taxes on electric vehicles (basically, anything that isn’t gasoline or diesel is considered fishy and is taxed hugely) can be dropped so we can start seeing more of them here! The current situation is a travesty!

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I Have Seen The Future of Personal Transportation

And it is awesome and Japanese. Just put a person on top and raise the speed.

[Embedding seems to have problems]

http://www.youtube.com/watch#v=bI06lujiD7E

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Be Careful When Starting Large Project, Pt II

The Batillus class supertankers built in the seventies that had a larger mass (gross tonnage) than anything now were scrapped pretty soon after since they had to sit around for lack of work. The oil crisis hit at a bad time, and it probably costs quite a lot to keep such a large ship in sailing shape. They probably were not very practical either. Out of the ten longest ship classes on list at Wikipedia, most have been scrapped already. The environment is hard. The Jahre Viking / Knock Nevis tanker that was the longest ship in the world was scrapped only recently, having been built about the same time as The Batillus class.

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Streetcars / Trams / Light Rail

They’re awesome, yet problematic. In the early 1900s, Los Angeles had an extensive streetcar and light rail network (the red and yellow cars), but it was dismantled, like in many other american cities in the thirties, forties and fifties. One of the reasons was a conglomerate of car manufacturers and oil and tire companies that bought the streetcar companies, trashed their vehicles and changed them to buses. Of course, there were many other reasons as well, and it’s a subject far too large to handle here.

Turku, Finland’s old capital and currently fourth largest city, had trams as well but they were dismantled in the sixties. A large investment in the track and electricity network was lost, new buses had to be bought and the roads had to be reinforced to carry the buses. It was the irresistible zeitgeist that the automobile would be the future – ironically, only a few years before the oil crisis.

Thankfully, Helsinki never did that. There were awful plans of putting a highway overpass right in the scenic main market by the seaside and other absolutely horrible things. It is sometimes very hard to understand that time. Making a huge graffiti to a beautiful Jugend building is next to nothing compared to some of the architectural and city ideas of the sixties.

What New?

That was the past. What about now? Well, Turku has been pining for the streetcars for a long time, and now it seems the inland city of Tampere (Turku’s arch rival no less) that never had trams is actually planning to upstage Turku in building a network. Both have populations of about 200,000.

And in multiple US cities, tram networks are being brought back. Los Angeles has built it anew and is expanding it, although it’s still far smaller than what it was in the old times.

What are the issues?

Well, tracks cost some, compared to buses that can run on roads, but tram tracks are actually not that expensive since they can be laid on roads, can make sharper corners than heavier rail tracks (trains, metro) and don’t require over/underpasses. And the “default” alternatives, cars and buses need roads and affect other traffic as well, so the difference might not be large. In Helsinki, trams are actually the most profitable of the city’s transportation sectors. They cost very little to run. Trams are also more flexible than heavier rail systems in a city development timescale (5 years) because the new tracks are quite quick and cheap to lay down. You can also leave old tracks in place without them doing any harm, to keep them in reserve in case they will be used later again.

What about the utility factor problem? Buses can have a larger network and transition a bit better from line to line. But still, most vehicles stand outside the rush hour. But it’s the same issue with everything, personal automobiles included.

Technology

It’s curious that newer trams in Helsinki actually seem to be noisier than older ones. This, I gather is from different technology – the new ones have high torque motors right in the wheels, and are designed for modern international rails that have ample lead-in to corners, meaning the sideways acceleration starts slowly. In contrast, Helsinki’s tracks are old, have sharp corners with no lead in. And sometimes the tracks are even uneven because of cobblestones, like in the senate square. This means that the older Finnish trams from seventies and eighties and the recently “stop-gap” purchased old Mannheim trams actually travel smoothly while the 2000:s Bombardier low floor trams bang really hard and are in constant need of repair.

One weird thing about trams is that they are very heavy. 30 tons for a vehicle carrying 100 people is a lot. Since the investment cost is high already and it will last for a long time, wouldn’t it make sense to actually spend some extra on structures and construct them out of aluminium and/or composites? Of course, since trams are operated much longer than for example buses, fatigue issues must be taken into account very carefully. You could then do with smaller motors, less reinforced tracks and many other beneficial things that would then reduce the cost. It seems trams, like local passenger trains have some mental legacy from the old czar era steam trains when everything was constructed of mild steel and weighed absolutely humongously – so that when a freight train or a building and a passenger rail vehicle collide, the passengers survive unharmed. Yet these trams move among ordinary traffic with “flimsy” buses and ordinary motor cars (that at times are crushed like soft drink cans in collisions with the heavier rail vehicles). Hence the high impact survivability traditions make less sense for rail vehicles moving among road traffic and could actually result in less safety overall.

Another alternative to the tram is the trolley bus. You still avoid pollution and fuel cost compared to buses and avoid the need to build a track compared to tram. The trolley buses might not last as long as trams and they have an image problem though – they’re seen as Eastern or Southern European and a poor man’s alternative. I haven’t studied the subject that much.

Ideas

Use large pultruded* carbon fiber tubes to construct a triangular truss space frame, reinforced by a carry-around at the door openings. Separate the wheels from the motors with axles (jointed axle or a cardan) and use very accurately tailored suspension (possibly with active components for varying loads) to ensure very low vibration levels. Use separable high impact plastic panels on the outside and inside, attached with a large number of very sturdy fasteners.

Modern frequency converters and high torque permanent motors are a natural choice of course.

This should result in a light, quickly accelerating, silent, easily maintainable, reliable and low operations cost tram. It’s also going to cost a lot to buy, but since trams are going to be used for thirty or even fifty years, it pays itself back in a fraction of that time.

The space frame construction can be customized easily by varying the number of frame triangles, and the number of panels can be varied as well. The door reinforcements and doors need to be standard components though. They potentially need metal or in-place cured composites.

*: the pultrusion industrial process results in very straight fibers that can handle both tension and compression. A good use for the expensive carbon fiber, compared to layups where the strength is much less.

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The Martin Jetpack and Its Design Tradeoffs



Here’s the technical information. It uses a piston engine to spin two smallish propellers. At first look it seems inefficient: the propellers are so small (diameter 1.7 ft so radius is 0.26 m) so the air mass flow is low and hence the velocity of the jet must be high, meaning high power needs for the thrust. (Mind you, it’s efficient compared to a peroxide rocket or a low bypass turbofan engine that have been used for jetpacks in the past.)

But let’s look closer. Engine specs. It’s a two stroke 2.0 liter V4 that produces about 150 HP at 6000 RPM and it weighs 60 kg. The quite high RPM means it produces quite high power.

If the engine spun at a lower speed, you could use bigger props, but the engine would produce less power.

If it spun at a higher speed, you could have higher power for little mass growth in the engine and then use a gearbox to still use the same size or even bigger props (if you geared it down further). But gearboxes are heavy, expensive and often unreliable.

6000 rpm is 100 Hz (rpm is a totally weird unit for spin rate anyway, why is it always used?). Speed of sound is 320 m/s. Hence at 100 Hz the supersonic radius would be 0.5 meters (100 1/s * 2 * 3.14 * 0.5 m  = 307 m/s). At the Martin Jetpack’s 0.25 m blade radius it’s about half the speed of sound. At the 7058 max RPM it’s 180 m/s or about 60% of Mach 1 – the transonic region should be easily avoided. Maybe they could be even slightly bigger.

It’s a compromise design. With the small props you can use relatively high engine speeds so your engine stays light – and you avoid a complex expensive failure-prone gearbox. The machine also stays safe as there is no free flailing propeller. With a larger propeller (or two) you would have to give up the shroud(s) since their weight would be prohibitive. On the other hand, fuel consumption would go down and hence the range could increase. It’s a fascinating design space.

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Long Endurance UAV Design

Karoliina has some thoughts on plane design, looking at High Altitude Long Endurance (HALE) UAV:s as inspiration for high L/D craft, to ultimately cruise at fast speed with little power. I disagree somewhat, and I’m sketching out why, below.

Probably the drones, like sailplanes, want low sinkrate and high L/D is secondary, because they need to just stay aloft and not go anywhere.

Power needed is P = v*D (we assume). Since D = CD*v*v, P = v^3*CD.

Lift is L = CL*v*v so v = (L/CL)^(1/2). (Note this CL is different from the L = 0.5*rho*A*cl*v^2, so CL = 0.5*rho*A*cl. It’s more practical here.)

Power thus is P = v^3 * CD = L^3/2 * CD * CL^-3/2.

The lift equals mass, so the power needed is

1. proportional to mass^1.5,
2. proportional to the drag coefficient and
3. inversely proportional to the lift coefficient^1.5.

This means the lift coefficient CL needs to be large for the craft to be able to loiter for a long time. So long wings and somewhat cambered profiles. A little drag doesn’t hurt as much as low lift so struts are a possibility.

Instead, for a piston cruiser, the L/D needs to be maximized for a certain minimum trip fuel consumption, not per time. Basically, you want to minimize delta_E = P*delta_t = P * delta_x/v so the cost function J = v^2 * CD = L * CD / CL which minimizes at maximum L/D. The CL term is less important compared to the loiterer. As a first guess this should optimize to a less cambered airfoil and smaller or shorter wings. And no struts.

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