NASA Will Send Helicopter To Mars To Test Otherworldly Flight (bbc.com)

NASA is sending a small, autonomous rotorcraft to Mars via the agency's Mars 2020 rover mission, currently scheduled to launch in July 2020. NASA says the goal of the mission is to "demonstrate the viability and potential of heavier-than-air vehicles on the Red Planet." BBC reports: Its design team spent more than four years shrinking a working helicopter to "the size of a softball" and cutting its weight to 1.8kg (4lbs). It is specifically designed to fly in the atmosphere of Mars, which is 100 times thinner than Earth's. NASA describes the helicopter as a "heavier-than-air" aircraft because the other type -- sometimes called an aerostat -- refers to aircraft like balloons and blimps. The helicopter's two blades will spin at close to 3,000 revolutions a minute, which NASA says is about 10 times faster than a standard helicopter on Earth.

Comments

Re:Wow

[molarmass192]

I know I'm pedantic here, but the near zero Kelvin temps apply to interstellar space, within solar space it's significantly warmer (>150 Kelvin), but you're still going to need to pack a jacket.

Helis would. Here's why

[raymorris]

I'm not the one who said that, but I do have enough understanding of the physics of props to explain why it should likely be largely true for helicopters. "Drones" (quadcopters) would likely run into a problem.

You have probably played around with sticking your hand out a car window and angling it so that it flies up. You've probably noticed that if you angle it too much toward the vertical, the airflow pushed your hand backward with considerable force. Props are of course angled similarly, so that some of the oncoming air is forced downward, creating lift, and the greater the angle, the more drag, or backward force is created. It is the job of the motor to overcome that drag. A more dense fluid has more drag, and so requires more motor power to maintain the same speed. So the required motor power is equal (and working opposite to) the drag.

You may have noticed that the same thing happens in water. Because water is more dense and vicious than air, water creates a greater force at a given angle - both greater lifting force and greater drag force.

We call the drag created by the angle "induced drag". There are complex formulas for drag and lift, which involve something called the Reynolds number, but as it happens when calculating the ratio of drag to lift the other numbers cancel out and drag is directly proportional to lift for a given foil, as density and speed changes. You can intuitively imagine that if you hold a flat board in a flow, at exactly 45 degree angle to the flow half the force will be up and half drag backward. That isn't exactly correct due to flow separation and complex stuff, but the intuitive understanding is that the magnitude of the forces is related is true. Something more dense, like water, will have more force - more drag and more lift. Lower density will give less drag and less lift.

Lower density means less lift AND less drag, proportionately. The required motor power is exactly equal to the drag. So the required motor power is reduced as drag and lift are reduced. Thus required power is directly proportional to lift - the thin atmosphere gives less lift, and needs less motor to overcome drag.

So just moving a helicopter from Earth to Mars we find that the lift (at a constant RPM) is too small, and the Earth motor is way oversized for the need. To make it fly on Mars, we need the prop to generate more lift BUT we don't mind a lot more drag - we have plenty of motor to overcome drag. We can easily generate more lift (and drag) by making the prop larger, spinning it faster, or increasing its "pitch" (the small dimension of the prop, from top to bottom on a helicopter). In particular, larger props generate a lot more lift and So it's easy to get lift equal to the weight of the craft - our motor was designed to overcome the proportional drag. drag. Larger props are also considerably more efficient, having a better radio of lift to drag.

Quadcopters have a problem when you want to make the props bigger. Quads of course have four props, and if they are large enough to be efficient the ends of the props nearly touch at the center of the craft. You can't go larger without the ends of the props hitting each other. To put much larger props on a quad, you'd need them at different heights, going over and under each other. Even if you do that, the prop can't be longer than the width of the frame, because the tip of one prop would hit the axle of the others.

This is just a first approximation to show it is generally a reasonable idea to simply put bigger or props on for lower density atmosphere, or just let the same props spin faster since they have less drag. Practical issues arise such as building the prop to be stiff enough given the extra length.