Cora – Kitty Hawk's prototype air taxi [video] • Hacker News Board

Re: Cora – Kitty Hawk's prototype air taxi [video]

#261

Post by SlowBro » Tue, Mar 13, 2018, 6:53 PM UTC

mannykannot wrote: post #67

SlowBro wrote: post #58

Earlier quoted context omitted.

Would a classic helicopter rotor overhead be a better solution, paired with those classic wide horizontal wings? Lift with the rotor, then thrust with the rear prop, using both wing and auto rotating overhead rotor combined for maximum lift. Requires a tail rotor or similar. Perhaps the rear thrust motor or prop can be rotated sideways for takeoff? Now we’re getting complex, but not extraordinarily so. Ape4 noted wou…

There have been several experimental aircraft like that [1] [2], but they are more like augmented helicopters than fixed-wing aircraft with VTOL. Aerodynamically, the arrangement is a biplane, and biplanes are rather inefficient because interference between the flows over the two wings reduces the overall efficiency. Note that in the case of the Sikorsky S-72, the intent was to stop the rotor in cruise. [1] https://e…

Thank you. Looks like what I have in mind is called a Gyrodyne[1] or Heliplane[2]. I would expect it would have almost the same range and speed as an airplane with VTOL capabilities. As you mentioned, the interaction between the two flight surfaces may be too inefficient, but the X3 you linked to broke the unofficial speed record, so there's that.

[1] https://en.wikipedia.org/wiki/Gyrodyne

[2] https://en.wikipedia.org/wiki/GBA-DARPA_Heliplane

Re: Cora – Kitty Hawk's prototype air taxi [video]

#262

Post by schiffern » Tue, Mar 13, 2018, 7:30 PM UTC

pc86 wrote: post #227

DennisP wrote: post #221

Earlier quoted context omitted.

He specified "general aviation aircraft not designed in the 1950's." The Cessna 172 was first flown in 1955: https://en.wikipedia.org/wiki/Cessna_172 Also, it may be worth taking into account that airplanes travel in straighter lines.

I'm talking about overall miles traveled so I'm not sure the straight line thing matters. Yes for the same trip you will travel a shorter distance in a plane. The plane will also be running for 20-30 minutes prior to takeoff and 5-10 minutes after landing. If you exclude GA aircraft not designed in the mid-20th century, you exclude 90% of GA aircraft and 99% of certified GA aircraft. We can make that distinction if y…

>The plane will also be running for 20-30 minutes prior to takeoff and 5-10 minutes after landing.

That's been true up until now, but this vehicle doesn't need to do that.

Also, most e-planes have a small motor in the wheel for taxiing.

>I'm talking about overall miles traveled so I'm not sure the straight line thing matters. Yes for the same trip you will travel a shorter distance in a plane.

I think you just answered your own question.

Re: Cora – Kitty Hawk's prototype air taxi [video]

#263

Post by jessriedel » Tue, Mar 13, 2018, 7:52 PM UTC

ransom1538 wrote: post #70

Small planes have had parachutes for quite some time [1] now and have saved many lives. Why on earth would this company put people up into the air unmanned without one? [Personally, if a parachute was onboard I wouldn't mind jumping into one.] https://www.youtube.com/watch?v=kQyrPVIIQdE

As mentioned in another HN thread, the Cirrus system is certainly spectacular, but it is heavy, requires a $15k repack every 10 years, and doesn't prevent a large fraction of accident types (e.g., an uncontrolled spin). They are most necessary for private (human) pilots who are the most dangerous kind. It's plausible that the increased safety from an autonomous system will make parachute systems unnecessary, but you'd have to dig into the statistics.

Re: Cora – Kitty Hawk's prototype air taxi [video]

#264

Post by schiffern » Tue, Mar 13, 2018, 8:08 PM UTC

syntaxing wrote: post #118

danielvf wrote: post #47

Contrary to almost every comment here so far, this is both innovative and energy efficient. Let's talk energy efficiency first. Once it's up in the air, it's has a similar profile to a normal single engine airplane. A Cirrus SR22 can go 1,200 miles on 80 gallons of gas. That's roughly speaking 15 miles per gallon - better than many pickup trucks or large SUVs. And that's while going 180 mph. Slower is more fuel effic…

I don't get how it is physically possible that this plane is energy efficient? The smaller prop, the less efficient the thrust conversion. You also need laminar air for rear props to be efficient. The drag on the wings should create a ridiculous wake behind the aircraft. I would love to learn how KittyHawk are trying to mitigate these problems (which are huge constraints even for major players).

>The smaller prop, the less efficient the thrust conversion.

Right, because the actuator disk loading[1] goes up, meaning that you're "throwing air downward" faster, with thrust scaling as v (momentum) while power scales as v^2 (kinetic energy). This is why propulsive efficiency decreases as exhaust velocity increases (for rockets this is simply exhaust velocity, while for air-breathing engines it's the change in velocity caused by the propeller/turbofan relative to the original airstream velocity).[2]

So bigger props are more efficient, because the actuator disk has more area. But the other way to add area is to add propellers. This has 12 props, so the disk loading is 1/12th as much.

The equation for hovering power is:

  P = (mg)^3/2 / sqrt(2 A ρ)

So twelve rotors only needs 1/sqrt(12) = 29% as much power for the same thrust as a single rotor of that size. Or equivalently, having 12 rotors is like having a single rotor that's 3.5x as large in diameter.

>You also need laminar air for rear props to be efficient. The drag on the wings should create a ridiculous wake behind the aircraft. I would love to learn how KittyHawk are trying to mitigate these problems (which are huge constraints even for major players).

They showed CFD in the video, so apparently that combined with test flights and scale model flights (both shown on their website).

Notice that the propellers are rotated slightly inward, but the outer propeller (that rotates in the opposite direction) is angled outward. I suspect they're managing the vorticity of the propeller wakes, using it to lower induced drag (effectively creating a longer "virtual wing").

Also, it looks like they transition to horizontal flight and then shut down the rotors (or at least, dramatically lower the power). The video shows the hovering system shutting down and self-aligning the propeller blades into the airstream.

[1] https://en.wikipedia.org/wiki/Disk_loading, https://en.wikipedia.org/wiki/Actuator_disk

[2] https://en.wikipedia.org/wiki/Propulsive_efficiency

Re: Cora – Kitty Hawk's prototype air taxi [video]

#265

Post by ethagknight » Tue, Mar 13, 2018, 8:28 PM UTC

danielvf wrote: post #94

ethagknight wrote: post #71

Earlier quoted context omitted.

Agree with most of what you say, but two open questions- debt alone on $500k amounts to $180/day (10yr am, 6%). Then there’s cost of energy, dead time during recharge, dead leg trips to the next customer, dead let to charge port, maintenance will be rigorous even if the plane doesn’t need it because FAA. Landing fees while be high for urban landing points, plus ground crew at some locations. “Max $100/hr” is way low,…

I'd love a link to that Uber cost number - I'm having a hard time wrapping my head around a car costing more than $100/hr to operate. Even a New York City cab, rented to a driver from a profit making cab company and including the cost of the medallion was $120 for twelve hours of driving. A chunk of a GA aircraft's operational cost is annual maintenance. By flying more per year, the operational cost of an aircraft go…

Thanks for thoughtful response!

- I was just looking at the cost for me to take Uber across town. My experience has a lot of 30 minute trips in different cities, which has equated to $50 rides on UberX, and that is always between urban centers and airports (i.e. ideal connecting points). My assumption would be that Uber gets as close to operating cost as feasible, if not cheating on the backs of the drivers, plus Uber hasn't actually made money (but thats a different discussion!). So thats my $100/hr for an Uber. Never mind the fact that personal car ownership is far cheaper.

- With very limited experience with general aviation, I have chartered a few private flights- for around $4000 for a day trip on a late model KingAir. Not worth much for this comparison. But thats why I looked to Uber as the absolute low end, because the best economics of air travel will probably only approach the best economics of surface travel.

- I just don't know why Cora's aerodynamics wouldn't look a heck of a lot like a quadcopter. My background is Civil engineering (im a static kind of guy!) so i dont know much here. All those extra surfaces add a lot of drag, no way around it. I wonder if they could turn the props and feather them up so they all align and provide lift with forward flight? I'm sure Eric Allison (engineering lead) is on top of it.

Dont get me wrong, I think its great stuff and I look forward to its development. However, there are a lot more problems to solve than just perfecting the actual aircraft. Its a whole ecosystem, not just an air shuttle. Recent history has given us a few great entrepreneurs who have overcome a multitude of problems in their space, but i definitely wouldn't bet too early on something like this without a little more to show for it. The New Zealand partnership will be a good prooving grounds.

EDIT: im happy to be wrong on all this!

Re: Cora – Kitty Hawk's prototype air taxi [video]

#266

Post by orliesaurus » Tue, Mar 13, 2018, 8:39 PM UTC

Holy sheep, new zealand looks beautiful

Re: Cora – Kitty Hawk's prototype air taxi [video]

#267

Post by ethagknight » Tue, Mar 13, 2018, 8:46 PM UTC

jgalt212 wrote: post #83

ethagknight wrote: post #71

Earlier quoted context omitted.

Agree with most of what you say, but two open questions- debt alone on $500k amounts to $180/day (10yr am, 6%). Then there’s cost of energy, dead time during recharge, dead leg trips to the next customer, dead let to charge port, maintenance will be rigorous even if the plane doesn’t need it because FAA. Landing fees while be high for urban landing points, plus ground crew at some locations. “Max $100/hr” is way low,…

Great video on the Pan Am building rooftop helicopter service to NYC area airports. https://www.youtube.com/watch?v=8nbz5VFilxY

Blade, the company at the end of that video, had a fatal crash this week.

Re: Cora – Kitty Hawk's prototype air taxi [video]

#268

Post by danielvf » Tue, Mar 13, 2018, 8:55 PM UTC

markab21 wrote: post #260

danielvf wrote: post #47

Contrary to almost every comment here so far, this is both innovative and energy efficient. Let's talk energy efficiency first. Once it's up in the air, it's has a similar profile to a normal single engine airplane. A Cirrus SR22 can go 1,200 miles on 80 gallons of gas. That's roughly speaking 15 miles per gallon - better than many pickup trucks or large SUVs. And that's while going 180 mph. Slower is more fuel effic…

Not to be overly nit-picky, you obviously know ASEL referencing the Cirrus but maybe I can add a little more of my own experience. I own a Cirrus SR20, and I'm lucky at cruise to get more than 12-14mpg running 8.5gph lean of peak at altitude. I just recently flew a NA SR22 up to Philadelphia from Tampa and averaged about 12mpg. You might hit those numbers you mentioned in a turbo at flight levels, but not low, and ce…

Thanks for some real numbers!

Re: Cora – Kitty Hawk's prototype air taxi [video]

#269

Post by endless1234 » Tue, Mar 13, 2018, 9:19 PM UTC

jhall1468 wrote: post #180

endless1234 wrote: post #92

Earlier quoted context omitted.

But how does this system scale? Thousands of these simultaneously in the air above cities? The noise must be quite something. I think it's a novel helicopter replacement idea, but not really anything for mass transportation.

These would be extremely quiet while cruising but likely pretty loud on take off. The latter is moot since that's happening at an airport.

They're supposed to be able to land and take off buildings I thought?

Re: Cora – Kitty Hawk's prototype air taxi [video]

#270

Post by evgen » Tue, Mar 13, 2018, 9:48 PM UTC

danielvf wrote: post #64

javiramos wrote: post #52

My understanding is that a larger, slower turning propeller is more efficient than a smaller, faster propeller. If so, why do drone companies/small aircraft companies have been pushing the more and smaller propellers model? Better aircraft control? Optimizing for range and battery life doesn’t seem like a bad idea.

So there are two ways to control a rotor craft. You can adjust the angles of individual blades, changing this angle multiple times each rotor sweep, as helicopters do. This is expensive, complex, things wear out, and a safety risk for the aircraft. Or you can do what quadcopters do, direct drive the rotor and just adjust the speed of the entire rotor. This gets rid of both the Swashplate system and a transmission. It…

One additional point that is worth making regarding this rotational inertia in the blades is that this is what makes auto-rotation possible. In an engine-out situation a helicopter has a lot of energy stored in these blades and by conserving this as much as possible while 'falling' the pilot can pull up on the cyclic when close to the ground to trade the energy for enough lift to survive the crash. A direct-drive drone propeller cannot auto-rotate, and if for some reason it has an in-flight emergency that interrupts electrical power to the VTOL rotors it will plummet like a brick. I am not sure about the glide characteristics of those wings, but with all of the drag from the non-operative drone rotors I am guessing it would be somewhere between 'awful' and 'lawn-dart'.