Why Is It Called A Helicopter?

Not too long ago I happened to have my son (who is six) and his cousin (who is also six) in the back seat of my car as I was running some errands. Which, as you may have noticed, is how so many of these articles start. The car, it turns out, is a wonderful place to start getting questions. Why? Maybe watching the world roll by makes him ask questions. Or maybe it’s the science and history podcasts I listen to. Or maybe he’s just six, and questions get asked.

Anyway, back to the story. I’m driving, and they’re in the back seat chatting and playing and fighting as six year olds tend to do. Then my son says “Look! A helicopter!” They both ooh and aah and stare, watching it fly overhead. “Why is it called a helicopter?” my nephew asks.

Helicopter?

I have no idea, of course. I think that, way back when I realized that English words were often made up of other words, I constructed an etymology in my head that derived it from the Greek word for ‘sun” – which I was pretty sure was “Helios”, because a little knowledge is a dangerous thing. But I’ve never actually bothered to look. Also, in the interests of fair and full disclosure, it turns out that I’ve been spelling it wrong my whole life. I always thought it was “helecopter”, when it’s actually spelled “helicopter”.

So, let’s check it out. The Online Etymology Dictionary gives the following derevation for helicopter: “1861, from French hélicoptère “device for enabling airplanes to rise perpendicularly,” thus “flying machine propelled by screws.” From Greek helix (genitive helikos) “spiral” (see helix) + pteron “wing” (see ptero-).”

Clearly, nothing to do with the sun at all.

Why ‘spiral wing’?

Because of Leonardo da Vinci.

leonardo-da-vinci-helicopter

Leonardo wasn’t the first to come up with a device that could take off vertically. That credit goes to the creator of the bamboo dragonfly or Chinese top, a toy created around 400 BCE that consists of a propeller on a stick. You spin it the correct direction, and it leaps into the air. However, Leonardo is credited as being the first to try and design a device that could carry a human being (although he never constructed it, and since it would have been human-powered it would never have worked), and his design used a spiral-shaped wing – hence the name “aerial screw” and the origin of the word we now know as “helicopter”.

How Do You Get To Space?

We’ve got another contextless question here. Just the question “how do you get to space?” listed in my notes. I strongly suspect, though, that it has to do with my son’s current obsession with Star Wars. There’s a lot of space ships in those movies, after all, and he’s got a toy Millennium Falcon that he abuses in true five-year-old fashion. So he thinks about space – or at least movie “space” – a lot.

What Is Space?

Generally speaking, “space” is “outside Earth’s atmosphere”. Specifically, though, this gets problematic. There’s six different layers of the atmosphere, after all:

  • The troposphere, which averages about 7 miles (11 km) thick, and ranges from 8 km thick at the poles to 16 km at the equator. This is what most of us think of as “the atmosphere”, with breathable air and weather and most of clouds. The temperature drops with altitude in the troposphere.
  • The stratosphere, which sits on top of the troposphere and extends upwards to 30 miles (45 km) above the Earth’s surface. You find the ozone layer here, as well as temperature increasing slightly with altitude – up to a high of 32 degrees Fahrenheit (0 degrees Celsius).
  • The mesosphere, which sits on top of the stratosphere and extends upwards to about 53 miles (85 kilometers) above the surface. Temperature begins dropping with altitude again in this layer, reaching a low of -130 degrees Fahrenheit (-90 degrees Celsius), and it’s the atmospheric layer in which meteors begin to burn up as they fall towards Earth.
  • The thermosphere, which sits on top of the mesosphere and extends upwards to about 372 miles (600 kilometers) above the surface of the Earth. Temperatures can reach thousands of degrees (Celsius or Fahrenheit), but it’s measured in the energy of the molecules of gas in this layer and there’s not a lot of molecules that high up (the average molecule would have to travel 0.62 miles/1 kilometer to collide with another molecule), so it doesn’t feel hot.
  • The exosphere, which sits on top of the thermosphere and extends upwards to about 6,200 miles (10,000 km).
  • The ionosphere, which starts 30 miles (48 km) above the surface and stretches to 600 miles (965 km) above the surface. That’s an average figure, though, as it grows and shrinks based on solar conditions. It’s also divided into sub-regions, based on what wavelength of solar radiation it absorbs. Note that the ionosphere, although considered a seperate layer of the atmosphere, overlaps the mesosphere, the thermosphere, and the exosphere.

So, where’s space? The definition “outside Earth’s atmosphere” could mean that you have to exit the exosphere to be “in space”, but this would put us in the ridiculous position of having to consider the International Space Station (which orbits at an average height of 249 miles) within the atmosphere. Clearly, that’s ridiculous.

The commonly accepted altitude definition for space is the Kármán line, which is 62 miles (100 km) above sea level. This line, sitting in the lower reaches of the thermosphere, is approximately the altitude above which wings no longer provide lift and below which atmospheric drag makes orbital paths fail without forward thrust.

Ways to get to space

Clearly, then, to get into space “all” we have to do is travel at least 62 miles (100 km) straight up. Simple, right? Well, maybe not. At present, we have only a handfull of ways to leave the surface of the Earth: balloons, really big guns, rotor wing aircraft, fixed-wing aircraft, and rockets. Each has some advantages and disadvantages.

Balloons

For balloons, the current record-holder for unmanned flight is the Japanese BU60-1, which reached an altitude of 33 miles (53 km) and carried a 10 kg payload. The altitude record for a manned balloon was the StratEx, which reached an altitude of 25.7 miles (41.4 km). Both were impressive achievements, but neither made the Kármán line.

Balloons function because of Archimedes’ Principle:

When a body is fully or partially submerged in a fluid, a buoyant force Fb from the surrounding fluid acts on the body. The force is directed upwards has a magnitude equal to the weight, mfg, of the fluid that has been displaced by the body.

In other words, if an object is submerged in a fluid but is lighter than that fluid, the fluid pushes it up until the object it is the same weight as the surrounding fluid. And while we don’t tend to think of it in this fashion, our atmosphere behaves like a fluid. We don’t float in the air because we’re denser then the air. A balloon inflated with air doesn’t float, because it’s actually slightly more dense than the atmosphere (because you forced more air in to inflate the balloon). But balloons filled with hot air, or with a gas that is less dense than our atmosphere (hydrogen or helium, say) will float.

Because of this, you could theoretically float a balloon into space – all you need is for the balloon to be less dense than the surrounding environment, after all. There’s a catch, though. The material of the balloon needs to be strong enough to not burst if the interior is pressurized, strong enough not to be crushed by the denser exterior, and light – because the mass of the balloon is added to the mass of the interior to determine if Archimedes’ Principle will lift it. A vacuum would be the ideal interior, but so far we don’t have anything simultaneously strong enough to keep the atmosphere from crushing a vacuum balloon and light enough for the air to push it up.

Big Gun

Jules Verne proposed this in From the Earth to the Moon way back in 1865. All you need is enough power, and you can launch something (or someone) into space from the ground. How much power? Well, using a ballistic trajectory calculator and assuming that the projectile is fired straight up from sea level, you would need a muzzle velocity of 4,595.52 feet per second (1,400.715 meters per second) to launch something 100 kilometers into the air – ignoring the effects of atmospheric drag. If you’re curious, that’s 142.78 times the force of gravity.

This is clearly not a good way to put people into space, but it would work well for launching non-fragile items. And in 1966, Project HARP demonstrated that it would work. A 16-inch (and 119 feet long) gun constructed by the US Department of Defence in Yuma, Arizona fired a 165 pound shell 590,000 feet (179,832 meters) into the air on November 19, 1966 – that’s 111.74 miles (179.8 kilometers), putting it well past the Kármán line

Plane

Planes seem an obvious choice, right? We all know what they are, they take off and land, so why not fly into space? Well, there’s a good reason for that – the Kármán line (meaning that by the time you reach space your wings aren’t working) and the propulsion (both jets and propellers require air to function). So, any plane that could reach space would need to be a rocket as well as an airplane. Right now, the world altitude record for a airplane was set on August 31, 1977 by Alexander Fedotov, who reached 23.4 miles (37.66 kilometers) in a MiG-25.

Rocket

Right now, this is the way we get to space. A rocket engine carries stored fuel and utilizes Newton’s third law by expelling that fuel in one fashion or another from one end of the craft to push the opposite direction. Most rockets in operation are combustion rockets, meaning that the fuel is ignited and burned in some fashion. These can be quite expensive, as the rocket has to lift all of its fuel at the time of launch. However, with sufficient fuel and engineering and money, you can build a rocket capable of reaching any altitude.

Other Strategies

Any number of launch methods that do not rely on rockets have been proposed – the space gun technically is counted in this category, but it differs from the others in the fact that it has actually been constructed. Most of the others have a US Department of Defense technology readiness level of 2, meaning that they are dependent on the invention of the materials and technologies needed to support them, and may not actually be feasable. They include:

  • Space tower: a tower that reaches above the Kármán line, possibly as far as geosynchronous orbit (22,369 miles or 36,000 km).
  • Skyhook: A satellite that lowers a lift cable (or the equivalent) and then reels in the payload.
  • Space elevator: A tether attached to the Earth at one end and a geosynchronous satellite at the other, which can then allow vehicles to climb into space.

How do you stay in space?

To quote Randall Monroe, “getting to space is easy. The problem is staying there.” Why? Well, gravity is still pulling you down. So, you have to go ‘sideways’ fast enough that you keep missing the Earth as you fall (which is pretty much what an orbit is). Using the Earth Orbit Velocity calculator on Hyperphysics, an orbit at the Kármán line would require an orbital speed of 25,745.41 feet per second (7847.2 meters per second), which translates to 17,549.1 mph (28,249.64 kph).

Fortunately, you don’t have to maintain constant acceleration at that speed – one of the defining features of space is that it’s pretty empty, meaning there isn’t a whole lot out there to slow you down once you get going. But still, you have to get going really fast to stay there – orbiting at the Kármán line means you circle the earth every 1.45 hours.

How do you get back down?

Oh, that’s easy. You fall. Whether or not you die is a matter of how you land.

Would a Helicopter Work On Mars?

This is one of those questions that I wish I could remember the context for. But I don’t. It’s just sitting there in the master list of the questions my son has asked, sandwiched between “What’s plankton?” and “What if the oceans froze?”, and I have no idea when or why he asked me this. Just “would a helicopter work on Mars?”

Usually I try to jot down something about the context. But not this time.

So, let’s get to the question. Would a helicopter work on Mars?

The simple answer appears to be “yes”. The Jet Propulsion Laboratory, on January 22, 2015, put out a press release on that very subject. They’ve designed a small proof-of-concept drone with the imaginitive name Mars Helicopter. It’s a 2.2 pound (1 kg) device, cubical in shape, with a 3.6 foot (1.1 meter0 rotor span. As designed, it would be deployed to work in conjunction with a future Mars rover.

The helicopter would fly ahead of the rover almost every day, checking out various possible points of interest and helping engineers back on Earth plan the best driving route.

Scientists could also use the helicopter images to look for features for the rover to study in further detail. Another part of the helicopter’s job would be to check out the best places for the rover to collect key samples and rocks for a cache, which a next-generation rover could pick up later.

If you prefer visuals, JPL also has a video about Mars Helicopter. It’s primarily CGI, since they haven’t constructed a fully-functional model, but it includes a few shots of a prototype trying to fly in a simulated Martian atmosphere.

But, what would it take to make a helicopter work on Mars? Not that I don’t believe JPL when they say it can be done – they are literally rocket scientists, and their entire job revolves around building things for space and other planets – but I’m curious. Fortunatly, NASA has me covered again. Let’s turn now to a paper describing the actual experiments NASA has done on rotary-wing aircraft for Mars, titled Experimental Investigation and demonstration of Rotary-Wing Technologies for Flight in the Atmosphere of Mars.

The paper begins by describing the challenges faced in constructing a rotary-wing aircraft for the Red Planet:

The Martian atmosphere is 95% CO2 with the remaining 5% comprised of N2 and other trace gases. Mars’ gravity is slightly greater than a third of Earth’s. The atmosphere of Mars is extremely cold and thin (approximately 1/100’th of Earth’s sea-level atmospheric density). Further, a seasonal variation of approximately 20% of the planetary atmospheric mass occurs on Mars (a consequence of polar CO2 condensation and sublimation). Given the thin, carbon-dioxide-based Martian atmosphere, developing a rotorcraft design that can fly in that planetary environment will be very challenging.

After that, the article goes into a lot of cool descriptions of various hypothetical rotorcraft designs and a whole lot of math that flies right over my head. It’s all stuff I’d love to understand, but it gets into some advanced-looking physics and aviation engineering concepts that I’ve never studied. Here’s what I could pick out:

* NASA is investigating both coaxial and quad-rotor configurations.
* Experiments have been done with battery power, electricity generated by fuel cells, and engines fueled by hydrazine. At present, some variety of electrical power seems to be the best option.
* It’s really hard to test an actual prototype under conditions that replicate Martian gravity, atmosphere pressure, and composition. At present, we’ve only managed it with the pressure.
* The rotors will have to be really big and spin really fast, and the rotorcraft will have to be really light.

A reasonable layman’s description of what a Martian helicopter can be found in this quote from the second page of the paper:

From an aeromechanics perspective, Mars rotorcraft will be very different from their terrestrial counterparts. Mars rotorcraft will have very large lifting-surfaces and will be required to have ultra-lightweight construction. For example, in order to lift ten kilograms of vehicle mass on Mars, a single main rotor (at a disk loading of 4 N/m2) would have to have a radius of approximately 1.7 meters.

For added fun, check out Interplanetary Cessna on Russel Monroe’s What If? site, which answers the question “What would happen if you tried to fly a normal Earth airplane above different Solar System bodies?” It’s not specifically on the topic, but it’s close.