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Comment by Feneric | Hacker News Reader

It's surprising that even with the energy it takes to generate a plasma strong enough and in enough quantity to achieve the desired effects, it can still result in an estimated 25% energy savings. It goes to show how much energy just deals with drag in the current system.

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>> It's surprising that even with the energy it takes to generate a plasma strong enough and in enough quantity to achieve the desired effects, it can still result in an estimated 25% energy savings. It goes to show how much energy just deals with drag in the current system.

You've made an assumption that they are taking the energy required to create the plasma into account in their aerodynamic efficiency calculations. I didn't see any indication of the power required to produce the plasma, much less that number of power already required to fly. I did see the suggestion of using it on electric planes or wind turbines where larger amounts of electric power are readily available - one can interpret that availability as a convenience (high voltages and power are already there meaning less complexity) or an oversight (we're just neglecting the energy required). Nowhere in the article is this directly addressed. It would not surprise me if the truth were somewhere in between - it takes a lot of power, but saves even more.

I've seen a similar situation in the hybrid car world when making certain comparisons.

Do we know if there's a lot or a little lost electricity that is generated as a result of the main engines running? I know cars waste a bunch of electricity (which is how hybrid batteries charge themselves when driving).

Cars and planes don't "lose" electricity when their engines are running; otherwise their batteries would be constantly overcharged. Hybrid cars just have more powerful alternators.

Hybrid cars don't necessarily have an alternator in the sense that an all gasoline car does-- for example, Toyota's hybrid system uses a pair of motor/generators (electric motors which can be configured to generate power too) connected to each other and to the gasoline engine through a planetary gearset.

Electric motors and alternators are mostly equivalent, the difference is that the diode assembly on a car alternator would stop you from using one as a motor. Otherwise, they are roughly identical.

That you don't find a 'traditional' alternator in an electric or hybrid car is simply because the electric motor doubles in that role when required.

Almost all electric motors when spun will generate power, you'd have to do some work to get one not to generate power when driven.

AC induction motors are the most common type of motor for anything other than small electronics and turning one by hand will not generate any output power. They are definitely not identical to an alternator. The armature of an induction motor doesn't have an associated magnetic field so turning it won't generate any power in the windings.

Permanent magnet motors are the only motor that will generate power if you just pick one up and turn it by hand, and they are quite rare (and expensive) at scales larger than a RC toy motor

Sorry, but you're wrong there. Remanent magnetism will allow you to use an AC induction motor as a generator just fine.

Absent any remaining magnetism you can 'flash' the motor core with a short DC pulse and then start rotating it with your chosen driver afterwards.

Just one sample (of many) of people using this trick for all kinds of interesting small RE applications:

http://www.smallhydropower.com/induction.htm

Interesting reading, thanks. There is always somebody with another clever trick up their sleeve.

> Do we know if there's a lot or a little lost electricity that is generated as a result of the main engines running?

That's not how it works. Car use an alternator instead of a generator because it lets them control how much electricity to make regardless of the speed at which the alternator spins. They basically vary the strength of the magnet in the alternator.

If they didn't the effect of "extra electricity" is a higher voltage, which would obviously be bad.

> I know cars waste a bunch of electricity (which is how hybrid batteries charge themselves when driving).

Hybrid batteries mostly save energy by capturing wasted energy when braking and when going downhill. Charging batteries from the engine makes the engine consume more gas.

> Hybrid batteries mostly save energy by capturing wasted energy when braking and when going downhill

Only for some workloads. A remarkable portion of the efficiency gain in hybrid vehicles is due to letting the ICE run at only the most efficient portion of the torque and power curves. In my experience (10 years in a Prius) regenerative braking is a distant second.

The waste energy in a regular car is not electricity, it is mostly simply heat. Instead of heating up a brake-disc by clamping it with calipers and pads an electric car or a hybrid will engage the motor by allowing it to be driven by the car instead of the other way around. The resulting energy at the output terminals of the motor is rectified and used to charge the batteries.

So when you brake your regular car you lose some heat to the environment, you don't 'waste a bunch of electricity'.

There is a huge gap between ~600mph vs much lower speeds of electric planes or windmills. As to energy requirements, it's likely fairly low in large part due to the expected altitude of 30,000+ feet. But also because the goal is stabilizing the airflow.

PS: Drag force is velocity ^2; energy lost to drag Drag force * Distance or v^3.

Power, not energy.

Could you justify the downvote for a factual correction?

> It goes to show how much energy just deals with drag in the current system.

All of the energy used during cruise flight (straight and level, accelerated) goes to drag.

I am not a physicist, but, doesn't fighting gravity by way of lift mean that at cruise altitude the plane is still fighting gravitational acceleration constantly? Drag is friction, right?

It's true that at least some energy must be expanded to lift the plan, even were there no drag; otherwise planes would lift themselves.

Drag is more like vacuum than friction.

Keeping an object at a constant altitude requires force, but it doesn't necessarily cost any energy. For example, there's no energy being expended keeping my coffee cup elevated, just a table exerting a force.

From my admittedly limited understanding of aerodynamics, a plane's engines are only fighting against drag to keep the airspeed up, and it's the airspeed passing by the wings that generates lift -- if engines are necessary to generate lift, gliders and kites wouldn't be able to work at all.

There's more too it than that. The airfoil causes a net downwash, and via newton we know the acceleration of that mass of air will cause an upward force. The finer details of this are something a lot of textbooks get wrong. Wikipedia's article about it is pretty good.

With a kite the wind is the engine, the string allows the kite to use it. Or you can run on a windless day.

With a glider the tow plane or ground tow rope provides the initial energy to get to altitude, giving the glider potential energy. As it glides that potential energy is converted to kinetic energy. The pilot uses their knowledge and skill to glide to places where they can gather more energy from updrafts of various sorts. I think it's really amazing how after that initial injection of energy, it's just all just skill and ambient energy.

The power required to fly at constant altitude and speed is velocity multiplied the force of drag.

Energy is consumed to exert a force over a distance, but we're not moving against gravity ("constant altitude"), so no energy is directly expended to fight gravity.

Now, that argument cheats a little, because there is a relationship between lift and drag: compare induced drag (drag created as a result of producing lift) to parasitic drag.

Consider a helicopter nearly hovering, but moving forward at a walking pace.

To maintain the hover and prevent the helicopter from falling out of the sky, the engines are consuming large amounts of power. To move it forward at a sedate pace only requires a small expenditure of energy to overcome drag.

If it was on wheels, a human could push it across a hangar with little effort. A human definitely could not hold a conventional helicopter in the air by lifting or by pedaling to turn the rotors.

The reactive force lifting the helicopter should not be thought of as drag.

Are you actually claiming that a helicopter doesn't use any energy to hover in place? Your argument describes the work done on the helicopter (or plane), but not the work the helicopter does on the air, which it pushes downward considerably. So, too, does an airplane's wings 'push' air downwards. Contrast to the vehicle sitting on the ground, where the ground is incompressible and no work is done on it.

The relationship between drag and lift is complicated. They're ultimately both the same phenomenon: pressure differentials introduced by dynamic fluid flow. You cannot have lift without drag, and you cannot have drag without lift [1].

That being said, this is fluid dynamics, where nothing is simple. Some of drag could be loosely described as friction, but not all of it. Think of what you feel while you're swimming, or sticking your hand out of a car window. It's like something is actively pushing against you, like you're catching a ball or something -- which you wouldn't normally call "friction". On top of this there are temperature effects, turbulence, ... and so on. And, most of these are actually at least somewhat coupled to each other.

Anyways, though at the end of the day it may be technically accurate (in certain contexts) to say that all of the power consumed at level flight is going to drag, it's also disingenuous; a bunch of that drag is the direct result of needing to generate lift to fight gravity.

[1] Admittedly this is a somewhat loose interpretation of the word "lift" but when you get down to the nitty gritty details like this I don't think "lift" is any more than a semantic construct to denote "useful drag". But the lift created to help control Apollo command capsules during reentry is a good example of this: by altering the angle of attack, thereby introducing highly asymmetric drag, the capsules "generated" lift to ease reentry angles.

Short answer: that is not correct for heavier than air aircraft.

https://www.grc.nasa.gov/www/k-12/airplane/forces.html

Lift requires energy, usually kinetic, like the forward motion of an aeroplane being converted to lift (and drag) via the wings, or the blades of a helicopter pushing air down, or real hot and fast gases pointed downwards (think harrier jump jet).

A blimp, on the other hand, relies on buoyancy for lift, so yeah, in that case, given an altitude at which it's stable, to maintain velocity it only needs to add enough thrust to counteract the drag created by its forward movement.

Yes it is correct. Lift always creates drag. You can think of drag as the work necessary to gain lift. When lift is generated, it produced what is called induced drag[0]. Induced drag and parasitic drag, which is the drag generated from the aircraft structure itself, are where all the energy goes in unaccelerted flight.

[0] https://en.wikipedia.org/wiki/Lift-induced_drag

I think we're interpreting op differently. No, not all the energy added to a heavier than air aircraft (thrust) in straight-n-level flight* is used to counteract drag. Yes, where there is lift, there is induced drag. But the kinetic energy being added to maintain velocity is also being used for lift.

Imagine if the airfoil on an aeroplane were replaced with a symmetrical airfoil mounted with no angle of incidence. Thrust could be reduced because there's less drag from no lift. No lift, no induced drag, only parasitic drag, and the plane starts to lose altitude. Would you agree that not all the energy added to straight and level flight goes towards counteracting drag?

*where a' and v' are zero, and where for argument's sake, the thrust vector is perfectly horizontal

edit: by a' I mean change in vertical airspeed, by v' i mean change in true airspeed.

No, that's a non sequitur. An aircraft that doesn't produce lift requires less energy input, but that's not because lift requires energy -- it's because when there is no lift, the aircraft is gaining kinetic energy by losing potential energy.

If lift requires energy, then where would that energy go?

This is a bit like the debate on whether it's the current or the voltage that kills, with lift-to-drag ratio being resistance. You are right in the physics sense in that drag alone is enough to calculate instantaneous fuel consumption, but to calculate range you already need to consider mass ratios and lift.

Interesting point, sfc (specific fuel consumption) is only really density altitude and humidity dependant, since it's only calculated on a per engine basis, but a fixed wing aeroplane's range can be greatly affected by the weight and balance of the plane, ie whether the elevators need to be adding upward or downward pressure to the tail section to maintain a cruise attitude

I think ppl are equating energy with force. Airspeed, altitude, and fuel are forms of energy, kinetic, potential, and chemical, respectively. Lift, drag, thrust, and weight are forces. We're talking about a heavier than air aircraft in cruise right now, and the contention is over whether all the energy added to the aircraft if used to counter drag.

Simple example: consider a helicopter in cruise. Fuel is burned to produce thrust. There is an insignificant component of that thrust vector pointed orthogonal to the vector of velocity. Since drag by definition acts along the same vector as velocity, not all the energy is being used to counteract drag.

Back to an aeroplane in straight and level, since that's a more interesting example. Let's assume that the direction of travel of the aircraft is normal to the plane of the propeller, so thrust is acting on the same plane as drag, in this idealized situation. Energy is added to the system in the form of thrust created by the prop. Said thrust is used to maintain the amount of kinetic energy of the aircraft. At the same time, this kinetic energy is being transformed into both lift and drag by the wings (and elevators, depending on how far aft the cog is) ergo not all the energy added to the system is used to counteract drag.

To explain it in yet another way: As long as you maintain your height no energy is used for lift, as energy is equal to force integrated over distance. Like as standing on a table requires no energy ;) However, an airplaine isn't standing on anything and the lift force is generated by pushing air downwards and this is what consumes energy.

A car or a train that drives with a constant velocity has constant kinetic and potential energy (assuming level ground). Therefore all energy that is consumed to maintain the status quo is spent to counteract drag.

A plane however pushes down on air instead of solid ground and accelerates it downwards. So not only does the fuel heat up the system due to drag, some of the energy accelerates quite a chunk of air.

Now you can argue that 'moving air' is nothing else than turbulence that takes a bit longer to dissipate and is therefore just another form of drag ;)

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