- All current blades are not a constant-pitch helix. This would be needed for them to fit inside each other.
- Careful thought would need to be applied to balancing the propellers given the slight difference in the sizes of the pairs of opposite blades. The masses need to match, obviously, but so too would the moment of inertia, lift and probably drag.
- The blades could not be variable pitch, except when fully extended.
- The blades would have to have a constant cross-section.
- Your [it-won't-work-because] goes here...
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Thursday, 14 June 2018
This is a couple of Augusta Westland AW609s. They are vertical take off and landing aircraft that rotate their engines and propellers when up in the air to fly horizontally. There are quite a few other VTOL aircraft that use this principle.
If you look, you can see that the propeller blades twist like a helix (all propeller blades do this; it compensates for the fact that the tip is moving faster than the middle). The blades can also be twisted as a whole, which is called variable pitch.
In a helicopter with just one rotor, variable pitch is essential for forward flight because the blade that is moving forward with the direction of flight is going fast into the air, and so generates more lift, whereas the one on the other side of the rotor that is going backwards relative to the air generates less lift. Without the blades twisting every half-rotation using their variable pitch to give more lift on the back stroke, the helicopter would simply tip over and fall out of the sky.
But this effect is neutralised with two rotors like the AW609, one on the left and one on the right of the forward direction, as long as one rotates clockwise and the other rotates anticlockwise. Then the forces balance, and the blades don't need to flap with each half-revolution.
Given the lack of need for variable pitch, this could be made to work with four-bladed propellers (rather that the three you see in the picture), or, indeed, propellers with any even number of blades. The blades would be hollow, with one very slightly smaller that the other. To reduce the propeller diameter the blades would be drawn through the hub and the smaller one would slide inside the slightly larger one opposite. They would also have to twist as they did this, to accommodate the helical blade shape.
There are a few problems with this idea, but I don't think they are insurmountable:
I don't know if the aerodynamic compromises needed to accommodate the above list (plus the things I haven't thought of) would nullify the increased speed and range that would come from having a more-or-less conventional sized propeller for horizontal flight.
But it would be interesting to do some experiments and calculations...
Wednesday, 11 April 2018
This is an edited version of a letter that was published in the London Review of Books Vol. 39, No. 11, 1 June 2017.
Driving speed is easily controlled by self-funding radar cameras and fines; in contrast, MP3 music sharing is unstoppable.
Every technology sits somewhere on a continuum of controllability that can be adumbrated by another two of its extremes: nuclear energy and genetic engineering. If I want to build a nuclear power station then I will need a big field to put it in, copious supplies of cooling water and a few billion quid. Such requirements mean that others can exert control over my project. Nuclear energy is highly controllable. If, by contrast, I want to genetically engineer night-scented stock to make it glow in the dark so it attracts more pollinators, I could do so in my kitchen with equipment that I could build myself. Genetic engineering is uncontrollable.
We may debate controllable technologies before they are introduced with some hope that the debate will lead to more-or-less sensible regulation (if it is needed).
But it is pointless, or worse damaging, to debate an uncontrollable technology before its introduction. Every technology starts as an idea in one person’s mind, and the responsibility for uncontrollable technologies lies entirely with their inventors. They alone decide whether or not to release a given technology because - if they put the idea up for debate - its uncontrollability means that people can implement it anyway, regardless of the debate's conclusions. (Note in passing that - all other things being equal - an uncontrollable technology will have greater Darwinian fitness than a controllable one when it comes to its being reproduced.)
In my own case I classify technologies I invent as broadly beneficial or damaging. The former I release online, open-source. The latter I don’t even write down (these include a couple of weapons systems at the uncontrollable end of the continuum); they will die with me.
I may be mistaken in my classification, with consequences we may regret. Other inventors may act differently: we may regret that too. But we shouldn’t make the mistake of indulging in (necessarily) endless discussion of what to do about a technology if it is uncontrollable. The amount of debate that we devote to a technology should, inter alia, be proportional to how controllable it is.
Technological changes have unforeseen and occasionally negative social and political consequences. This is inevitable when something powerful impinges on things that are relatively weak like regulation; the same applies to the benefits. Fortunately the vast majority of people are well intentioned, and technology amplifies the majority along with its complementary minority. Much happens faster and more spectacularly, but the ratio of more good to less bad stays about the same.
Monday, 12 March 2018
Castaway, the first British reality TV show nearly two decades ago, dropped a group of about thirty people on the remote Scottish island of Taransay and filmed them as they argued with each other and fell out brutally and in a psychologically damaging way over the following weeks.
I watched the opening episode, which had the whole group in a room in London before they set out discussing what they would do and how they thought things would work, and I predicted to anyone who would listen (i.e. my family and the cat) that the whole thing would be a social and emotional disaster for most of them. And so it was.
The problem was that - in that London room - they were all talking with each other excitedly and at length in a friendly, convivial, and engaging way.
Think of two island fishermen in their fifties who have known each other since childhood. On a Monday their total day's conversation as they pass each other on the quayside might be:
And similarly every day of the week, with - perhaps - on the Friday:
"Morning. Storm's coming."
They, and the rest of their island community, have evolved a peaceful system of friendship and cooperation an essential component of which is not annoying each other with their personal views, history, random thoughts, and chatter.
Our two friends sit together all evening in the pub in silence, their pints of beer in front of them on the table, taking a sip every minute or two and thinking their own thoughts. If something needs to be communicated (like a storm) they mention it, then shut up. Occasionally the whole community all gets very drunk and sing and play the pub piano and talk nonsense for hours then, the following morning, their hangovers enforce a return to their normal reservation.
A lot of folk anthropology consists of just-so stories about how we are adapted to life in a hunter-gatherer village and how we carry that inheritance over to modern global civilised life. Sometimes, it is claimed, conflict results; one obvious example is xenophobia. But one thing we have certainly not carried over is the circumspect reservation that we can observe today in isolated small communities. Every communications technology we have created - printing, the telephone, radio, television, the internet, social media - works against that reservation, and we embrace them all with delight.
And we wonder why we don't get on as well as the two fishermen.