Why don't we have jet helicopters?
June 9, 2014 2:57 PM   Subscribe

Inspired by this post. So we actually already have flying cars, they're called helicopters. But why aren't there any helicopters that use jet turbines instead of rotors for lift? I'm not talking about turboshafts or tip jets, but actual turbines like in a jet plane. They would look more like a car and people would shut up already about this flying car nonsense. Thanks.
posted by Tom-B to Travel & Transportation (17 answers total) 1 user marked this as a favorite
I'm not sure what you have in mind as far as "jet helicopters." The closest thing to that in real life are vertical and/or short take-off and landing (V/STOL) aircraft like the Harrier.

They don't have quite the same flight characteristics as helicopters, though, and they are quite expensive to operate and maintain relative to conventional jets.
posted by AndrewInDC at 3:01 PM on June 9, 2014 [1 favorite]

Well we have turbine powered VSTOL aircraft, like Harriers. The reason they aren't prevalent is because they are expensive, complex and helicopters work just fine -- there's no actual benefit (outside of specific military requirements) to justify the extra costs.
posted by wrok at 3:03 PM on June 9, 2014

Are you thinking of something like the Moller Skycar? It has four tilting turbines for both vertical and horizontal flight. Moller has been releasing similar designs for 5 decades and it still hasn't, er, taken off.
posted by zsazsa at 3:06 PM on June 9, 2014

Response by poster: Not talking about v/stol, where apart from takeoff and landing the lift is provided by the wings.

What I want to know is: why are jet turbines used for thrust (in planes), but not for lift (in copters?)

Even in the Moller skycar... it seems that lift is provided by wings?
posted by Tom-B at 3:09 PM on June 9, 2014

Perhaps safety is the answer: if an engine fails, wings continue to provide lift. A jet engine would not.
posted by dcjd at 3:15 PM on June 9, 2014 [1 favorite]

Mostly because it would be really inefficient. In thrust, you're mostly working against drag. In lift, you're going against gravity. That's the whole point of wings (aerodynamic lift)- while providing thrust, you get the lift for "free". (Not really, but you get my point). Helicopter rotors are wings that provide thrust for "free". If you used jets for lift, you'd need additional ones for thrust.

What you're basically asking about is jetpacks! (We were promised those, too!). And those have almost always proven to be inefficient as well.
posted by thewumpusisdead at 3:21 PM on June 9, 2014 [5 favorites]

Uh, fuel?

Helicopters only need enough fuel to power a motor that spins some blades. A helicopter can idle and hover without consuming all its fuel at an exorbitant rate.

"Jet helicopters" would require very costly, complex, and fuel-hungry jet engines. Even while just hovering, they would guzzle and burn fuel like crazy!
posted by General Tonic at 3:23 PM on June 9, 2014

If anyone can do it....
posted by stinkfoot at 3:32 PM on June 9, 2014

This skycar has been in dev for a loooong time.

Edit: sorry, didn't read enough
posted by stinkfoot at 3:35 PM on June 9, 2014

So you want a vehicle that provides lift from a rotor and thrust from a jet?

I think you mean a gyro-copter with jets instead of a the more common propeller.

A google search for jet gyro-copter doesn't turn up much except this non-functional prototype.
posted by Confess, Fletch at 3:44 PM on June 9, 2014

Best answer: You mean like the SpaceX Falcon 9 Reusable?

I believe that turbofan engines are most efficient when they can pull in already moving air, where the forward motion of the aircraft can provide additional pressure into the combustion chamber. With an aircraft mostly focused on vertical motion, you don't have a whole lot of forward motion, so it's horrendously wasteful to generate lift without the additional compression.

Aaaand, what others have mentioned about failure modes: In a helicopter, if the engine fails you can autorotate to ground, using the kinetic energy in the blade system to glide and give yourself a little bit of last-minute (second?) lift. If you're standing on that single jet engine and that engine fails, you're screwed unless you can convert to some other form of flight.

The n-copter folks do this by throwing more motors at the problem, my friends who fly drones have octocopters that can land safely with the failure of several motors, 2 adjacent, but... building flight control systems like that certified for human flight is freakin' expensive, and the possibility of failure goes up as the number of motors goes up.

However, some helicopters do use ducted air, which could probably derived from the exhaust from from the main turbine engine to provide the anti-torque forces, as in NOTAR helicopters.
posted by straw at 3:47 PM on June 9, 2014 [2 favorites]

Best answer: Jet engines operate most efficiently at higher speeds. I believe this is largely because you need a lot of air going into your intake to make a lot of thrust. But regardless of the mechanics of it, it's the reason low-speed aircraft use turboprops instead of turbofans.

Remember that fuel efficiency is more important for aircraft then for ground-based vehicles, because extra fuel is extra weight.
posted by aubilenon at 3:52 PM on June 9, 2014

Best answer: What I want to know is: why are jet turbines used for thrust (in planes), but not for lift (in copters?)

because jet turbines are incredibly inefficient at low airspeed. at low speed, the most efficient propulsion is achieved by giving a small push to a lot of air (which the large area rotor of a helicopter does), while at high speed you want to give a large push to a small amount of air (like a rocket engine). take a look at Figure 3.3 here, showing efficiency of several gas turbines as a function of airspeed.

a hovering helicopter has zero airspeed, so the most efficient way to stay up there is the biggest, slowest rotor possible, like the vehicle that won the Sikorsky Prize (previously). of course, other engineering concerns make the rotor of practical helicopters smaller and faster, but in the end it's all about fuel efficiency and jet engines just aren't suited to keep something hovering in the air for any period of time.
posted by russm at 4:01 PM on June 9, 2014 [3 favorites]

IANAAE (I am not an aerospace engineer)

This has been invented as a rocket. They are unstable on the opposite axis of the lift. Lift on airplane is produced opposite of another force (gravity), and the wings are used to both stabilize AND lift. If you had only a jet lifting you, you'd not have enough stability until you were going too fast.

The only thing close to this is a Harrier, and that has already been discussed. Maybe we all don't understand the concept?
posted by bensherman at 4:02 PM on June 9, 2014

Another variation: Here are jets on the tips of helicopter blades
posted by tinker at 4:48 PM on June 9, 2014 [1 favorite]

If you had only a jet lifting you, you'd not have enough stability until you were going too fast.

stability isn't really a problem. a good pilot could manage it in the 50s, super-advanced avionics took care of it in the 60s, and these days those avionics are included in the toy quad copter your kids play with.

it really is just a fuel efficiency issue.
posted by russm at 4:57 PM on June 9, 2014 [1 favorite]

Besides the lift, fuel efficiency and stability issues (and stability and failure modes is still an issue with direct lift, it's not dynamically stable) - one of the huge issues with using direct jet thrust for flight and lift is the following:

It's extremely noisy. Even with something like a ducted fan, the noise is ridiculous. You're not going to take off from your driveway in a "flying car" without violating a lot of noise ordinances. Your neighbors will hate you. This holds true up to very large pieces of property.

Jetwash is dangerous. It will tear up asphalt and unhardened concrete, and any loose dirt, sticks or stones can be thrown at high velocities causing property damage.

A huge problem with VTOL direct lift is exhaust re-ingestion, which often leads to foreign object ingestion damage - which is really bad for a turbojet or turbofan - but also will cause re-ingestion stalls where too much hot gas and oxygen-reduced atmosphere is re-breathed by the jet causing compressor stalls, heat damage and other modes of engine failure.

Besides of all these dangers, even if helicopters or other VTOL aircraft were safe, quiet, stable, fuel efficient and affordable - the majority of the human population is not ready for jet packs or flying cars. They can't even operate cars on flat, purpose-built two dimensional roads without killing themselves and each other.

Even with advanced avionics, flight controls and glass cockpits it would be an utter mess in the skies, and when aircraft collide in mid air at any speed the results are catastrophic both in the air and on the ground due to the energy densities in fuel, mass and velocity and the inherent fragility of aerospace craft designed to be light weight and fuel efficient. It would be a blood bath.

Sure, computer controlled vehicles would be one answer, but they still haven't proven that two-dimensional road vehicles are capable of playing nice together en masse. There have only been limited experiments with self-guided multirotor toy/drone aircraft demonstrating collision avoidance and swarming/flocking behavior, and even those require a controlled environment and external video camera sensors tied to advanced, off-board computers.
posted by loquacious at 5:41 PM on June 9, 2014

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