Are flying cars just private planes?

The ups and downs of making your car fly.

Jul 07, 2021

We all know the feeling of being stuck in traffic. The car in front of you slams on the gas, then on the brakes, then the gas again. You think to yourself, “if we all just speed up at the same time, we’ll be done with this. I don’t get it”. And if you’re like me, you’ve probably entertained the thought of taking your road trip to the sky—taking off from your freeway parking space and cruising over those gravity-bound suckers. Flying cars have always been a thing of science fiction, which has given us a litany of designs to choose from. All of which, unfortunately, have yielded next to no real-world result. Until now.

Klein Vision, a Slovakian Aerospace Startup, has just successfully tested their first intercity flight, which lasted just over 35 minutes. A single pilot flew, escorted by two actual planes, from Nitra to Bratislava—about 50 miles. After landing, the car elegantly stowed its wings and took to the tarmac. Proving in one short clip the ability of the vehicle to travel through both air and land, without altering current infrastructure (roads, airports, etc). [1]

This video is literally incredible. Seeing what looks like an average sports car soar a few thousand feet over Slovakian farmland is not a sight I ever thought I’d see, nor a sentence I ever thought I’d write. But how much of our romanticized idea of flying cars is proved by this test? That’s what we’re here to find out.

Based on my calculations, to get a Toyota Prius traveling an average highway speed limit of 60 mph off the ground, you’d need 253 square feet of wing area. That’s the size of a typical living room. This is fine if you have a runway, but you generally won’t have access to one on your commute. Your wings would protrude out of your car at least 21 feet in either direction, wreaking havoc on virtually any highway.

There are a few things we can do to make our wings smaller. The first option is to takeoff at a higher speed. This would work up to a point but you’d probably end up getting pulled over before you took to the air. The next option is to change the shape of the wings. Think about a plane putting its flaps down when landing. This is partially to slow the plane down, but its core function is to generate more lift, meaning you can land at lower, safer speeds. We can apply this same logic—giving us ultra-high lift wings, but like we just said, these come with increased drag. This means we’d have pretty awful fuel efficiency.

So alas we come to the conclusion that for fixed-wing flying cars, we’d probably need an airstrip and retractable wings to fit in with the rest of society. Sad.

But wait. What about propellers? I’m sure you’ve seen small quadcopter drones. [2]

These guys use multiple propellers to lift anything from cameras to Amazon packages off the ground, so why not people? Uber and Airbus are already pursuing quadcopter technology for individual urban air transport. The only hurdle we have to jump here is with safety. The typical American neighborhood is littered with “Drive Like Your Kids Live Here” signs. So I highly doubt they’ll be okay with metal-composite rotor blades spinning at 1,000+ rpm on their quiet street just so you can get to work a little faster. Aside from this, and if you could get past looking kind of silly on the highway, quadcopters are an extremely promising way to achieve similar maneuverability in tight spots, while still soaring above the worst parts of driving.

All this is great, but once you pay the likely absurd sticker price, how much will it cost you to operate your flying car. First of all, you’d almost certainly have to get a private pilot’s license to fly it, which will cost you anywhere from $8-14k. Additionally, you’ll incur the cost of fuel. Some flying car prototypes aim to be fully electric, but Klein Vision’s car runs on regular gasoline. Now, the mpg of their car isn’t reported anywhere I can find, and neither is the volume of their fuel tank. So, today we’re going to learn a quick way to get a rough estimate of the flying car’s gas mileage. Pay attention to units and stay with me.

We know that the car’s motor has 140 horsepower. This is equal to 104 kW. Both are units of power—I prefer metric. Power is simply energy used per unit time (i.e. 104 kW = 104 kJ/sec). When cruising along, you’re probably not going to need all that power, and 60% power is a good guess for cruise. So, we’ll say that on average the engine outputs 0.6*104 kW = 62.4 kW of power. Great. Now, where does it get this energy? Gasoline! But how much energy does gasoline have? A quick Google search tells us it has 34,200 kJ/liter. Does all of this energy go into pushing us forward? Definitely not. There are two basic efficiencies that we have to look at: thermal and propulsive. Thermal efficiency is basically what percent of chemical energy from the gas makes it into kinetic energy in the propeller (spinning). Propulsive efficiency is what percent of that kinetic energy makes it into a forward force for our car. This engine and this propeller, again from a quick Google search, have thermal and propulsive efficiencies of 25% and 80%, respectively. That means, in total, 0.25*0.80 = 20% of the energy from the gas translates to forward energy for the car.

Putting this all together we get that this engine burns somewhere around:

Over their advertised 5 hour flight, that’s 164 liters, or 43 gallons, of gas. They advertise a range of 1,000 km—621 miles. Therefore, we get a resulting fuel efficiency of 621 mi/43 gal = 14 mpg. That’s not all too great and, compared to normal driving, would cost you about twice as much. To me, though, that’s a pretty small price to pay to fly your car around, but I’m biased.

Our options here are looking pretty good, but both wings and propellers still pose the question: what actually makes them a flying car? Is Klein Vision’s test vehicle not just a private plane with retractable wings? Does driving on the road make you a car? I could totally drive a plane on I-95. Where do we draw the line? Sadly, as an Aerospace Engineer, I am unqualified to answer these questions. So I’ll leave them to you! Let me know what you think in the comments. Thank you for reading this week’s edition of It’s Not Rocket Science, see you next week!

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