Cartercopter

At some time or other, most of us have had to "get to the airport" 'to catch a plane'. Have you ever wondered why this is? Why is an airport always so far away? Why can't the plane catch us?

Well, naturally, it's partly because an airport needs a runway, which is 'a big thing', something that big pretty much has to be out of town.

Those rich b********!

On the other hand- consider that rich people have these handy gadgets called 'helicopters' that are much more flexible; they can take off and land much closer to where they live, and they only need a comparatively tiny helipad to do this.

So, why do us poor non-millionaires have to use a runway? Well, helicopters are expensive to buy, and expensive to run, and they have poor range on a full tank of fuel. That means that taxi services have never really taken off- the ticket price is too high and we are lumbered with having to use these cheaper, but more awkward, fixed wing aircraft.

Why oh why?

...do runways have to be so big? Well, at takeoff aircraft need enough lift to cancel the weight of the aircraft and leave the ground.

Now everyone has heard that wings get lift because they form a low pressure area on top of the wing; this is true. Indeed, a high pressure area forms under the wing too. A 'side-effect' of both of these effects- which are crucial to a wing working, is that they also turn the air, so that it leaves the wing flowing downwards. In fact, a wing provides lift because of the air it throws downwards.

So, at takeoff, having bigger wings would help- big wings throw more air down for a given airspeed. So you can take off earlier and have a shorter runway. So designers make the wings as big as they can, but there are issues; once in the air they add drag and they weigh down the aircraft. So a short takeoff fixed-wing aeroplane generally has poor fuel efficiency.

Let's go back to helicopters- they don't need any runway just a pad. Helicopters have narrow skinny wings called a 'rotor', but they can get much more lift by spinning them really fast. But the trouble is, once in the air, the high-speed rotor creates lots of drag. And the maximum speed that a helicopter has achieved is only 250 mph (a ZB500 G-Lynx, manufacturered by GKN Westland Helicopters). Because of this extra drag helicopters aren't efficient; and also because of the drag, they need extremely expensive and powerful turbine engines (fixed wing aeroplanes often use much cheaper piston engines).

Now, an ideal aeroplane would have the advantages of helicopters (tiny runway, small wings), together with the high efficiency of a conventional aeroplane at high speed, whilst being cheaper to build and run than a helicopter.

Sounds impossible right? I mean we've been flying for a century and nobody has managed it yet.

Cartercopter

A company called Carter Aviation Technologies has negotiated their way through the maze and come up with a clever design for a cheap, efficient, fast, short takeoff aeroplane.

The design has a rotor a bit like a helicopter (actually it's an unpowered gyrocopter rotor) but it also has stubby fixed wings like a conventional aeroplane, but smaller. It has a pusher propeller at the back.

At takeoff the pilot angles the rotor flat and spins it up to very high speed then disconnects the engine and changes the angle of blades and the vehicle then leaps into the air. Now at this point it is unpowered, but it has enough momentum in the rotor (due to heavy weights in the rotor tips) that it can hover for a short time perfectly safely. Meanwhile the pusher propellor applies full speed and the vehicle starts to move forwards. As it does so the air gets forced backwards and upwards through the rotor at the top of the vehicle giving more lift, and spinning it faster; so the vehicle maintains altitude, gains speed, and then begins to climb into the air.

Once the Cartercopter gets up to a forward speed of about 90 miles per hour or so, its stubby, lightweight wings, take over the flying- the pilot then flattens the angle of the main rotor blades so they produce hardly any lift. The main rotor then slows down, and flattens out and produces hardly any drag.

But this is where the really clever bit comes in.

Normally a helicopter or gyrocopter cannot go faster than its main rotor tip speed. This is because the retreating rotor would more or less stop in the air, and the vehicle will fall over due to lack of lift on one side.

However, with Cartercopter, the fixed wings are keeping the vehicle at the correct angle to the horizon and providing the lift. The rotor is unloaded, so the angle on the rotor induced by the difference in air speeds is very minor. This means that a Cartercopter can actually fly much faster than the tip speed of the rotor- the rotors flap a little as they rotate, but they aren't going anywhere.

The theoretical maximum design speed of a Cartercopter is an astonishing 500+ mph, which is almost twice as fast as any helicopter has ever or can ever go (the theoretical maximum is about 250 mph). Currently the Cartercopter hasn't exceeded 170 mph (yet); although even that makes it the fastest gyrocopter ever, and respectable for a helicopter.

The engine power to reach 170 mph is just 320 hundred horsepower. A helicopter to go the same speed would need roughly twice as much horsepower. That makes the Cartercopter about twice as efficient, and keeps it within reach of piston engine technology which is cheap.

What does all this mean?

Basically, what this all means is that the vehicle outperforms helicopters on every dimension except sustained hover, and is much, much cheaper to buy and maintain; and matches the performance of fixed wing aeroplanes- but with near-vertical takeoff and landing; and at only modestly higher cost to buy and maintain than a fixed wing aircraft.

Ok, but so what?

Well, the bottom line is that it means that Cartercopters might be able to work more like a 450 mph bus or taxi- picking up people at tiny airports just 5 or 10 minutes down the road from the nearest town and flying them to other airports near to where people actually want to go, and to do this cheaply. And if you want to go a few hundred or a couple of thousand miles, you don't need any other plane. Just board it at the local air-bus stop and fly. Transcontinental, no problem. No commuting to the airport, and no extra charge.

FAQ:

Why not stop the rotor entirely, wouldn't that reduce drag further?

that would require an extra mechanism to lock the rotor down, and if the rotor stops then it becomes floppy- which would mess up the aerodynamics at best. Having it spinning around keeps it out tight and keeps it stable. Because the rotor is going relatively slowly, it doesn't produce significant drag.

Is this aircraft dangerous or unstable?

probably not, there have been some incidents during the design of the aircraft, but no deaths or significant injuries.

Have they actually gone faster than a helicopter yet?

no, not the fastest helicopter. They had issues with the prototype not having enough power to do this- and they are taking it carefully, nobody has ever flown faster than the rotor speed, but they have now exceeded 0.7 times the rotor speed- faster than anyone has ever gone, and it turns out that that is the worst point since the rotor is most wobbly at that speed. They had a small crash caused by forgetting to lower the undercarriage :-) which has set them back, but they've rebuilt it and it looks like they're going to go for it soon in the next few months or so... watch this space.

Is this the long awaited flying car?

Seems unlikely, although perhaps for some people; the vehicle is likely to be noisy and still requires a fair sized runway/takeoff apron.

How can I follow the progress?

Check out: http://www.cartercopters.com/