4
$\begingroup$

Elon Musk said that Tesla will make battery which can last for million miles. So probably in the next few years there will be a (small) airplane powered by an electric motor rather than by fuel.

If this happens, will controlling the electric airplane be easier? The CG won't shift as fuel is consumed. But the constant battery weight probably will problem to the maximum landing.

$\endgroup$
  • 11
    $\begingroup$ Just to clarify in case you misunderstood: The life expectancy of the battery will be a million miles. It will not be on a single charge, rather you can expect the battery hold itself for many charge cycles $\endgroup$ – Antzi Nov 29 '19 at 2:06
  • 1
    $\begingroup$ The first paragraph of the question is not relevant to the question nor is the relation between control and landing weight. In essence the question is viable, electrification allows designers to place motors virtually everywhere on the plane, this can aid in differential propulsion control. You should update the question! $\endgroup$ – 0scar Nov 29 '19 at 7:39
  • $\begingroup$ The question should be rephrase so seem less speculative. Something like "how difference in engine placement enabled by electric propulsion can affect manoeuvrability?" would be more specific, obviously not speculative and would cover a large part of the question. $\endgroup$ – Manu H Nov 29 '19 at 7:45
  • 2
    $\begingroup$ I think the underlying question is a good one, even if it is masked a bit by the confusion over Tesla comments. $\endgroup$ – Notts90 is off to codidact.org Nov 29 '19 at 10:18
  • $\begingroup$ Differential thrust will be more precise, responsive and efficient, so is air(engine) breaking. But conventional layout doesn't benefit from this much, except like V22 or a helicopter. $\endgroup$ – user3528438 Nov 30 '19 at 4:22
9
$\begingroup$

There are a few electric aircraft now, and because batteries hold far less energy than fuel, they're low-performance and short-range compared to normal aircraft.

If we imagine that at some point in the future we'll have batteries that can match fuel, the answer would be: Only slightly.

Back in the 60's and 70's, airliners regularly carried a flight engineer, who's full-time job was to monitor the engines and tweak various controls to keep them running properly. Now that's almost entirely automated. Pilots still have a checklist to follow to start the engines, and things to monitor during the flight, but it's a small part of their responsibilities.

Even if switching to electric propulsion completely removes this, it won't make a huge difference to the pilot. (and that's assuming that the electric system has nothing to monitor or adjust)

The aircraft handling is unlikely to be affected significantly, although there are be minor effects:

  • Fuel gets used up, so electric planes will have to be designed with stronger landing gear.

  • Electric motors are easier to scale, so it's easier for small aircraft to have multiple, counter-rotating motors to eliminate torque swing.

  • The battery is the heaviest part of an electric power system (and for one with hours of endurance, by an order of magnitude), so fitting a motor with excess power has a relatively smaller weight penalty. Electric power systems are good for providing short bursts of high power between long periods of cruising. Unfortunately this is more useful for a car than an aeroplane.

  • It would be much easier to design an electric power system for an aerobatic plane. Supplying fuel and lubrication while flicking between positive and negative G takes careful design.

  • Electric motors are quiet, but they are still transferring lots of power into the air, which is inherently noisy.

Ultimately these are all minor effects compared to the elephant in the room, which is that batteries are far heavier than fuel.

$\endgroup$
  • $\begingroup$ Actually, your point about battery-powered aerobatic aircraft is a good one. While I'm sure that liquid fueled aerobatic aircraft are currently designed with fairly small fuel tanks located as closely as possible to the CoG to eliminate CoG shifts throwing things out of whack for the pilot while performing extreme maneuvers, having batteries as a solid, non-shifting "fuel" source will completely eliminate any CoG changes no, matter how small, from being points of concern. $\endgroup$ – FreeMan Nov 29 '19 at 13:41
  • $\begingroup$ @FreeMan the problem is not so much the CG change, as that a liquid fuel sloshes around and sometimes the intake pipe sucks up air instead of fuel. And famously, the Spitfire's carburettor float-bowl cut the engine during negative G, while the Me-109's fuel injected engine was fine. $\endgroup$ – Robin Bennett Nov 29 '19 at 14:54
  • $\begingroup$ Isn't the landing gear dimensioned for an emergency landing with full tanks? $\endgroup$ – Orbit Nov 30 '19 at 19:47
  • $\begingroup$ Nice info. I was surprised by the facts that will be many restriction and limitation by the electric airplane. Not as what was on my mind. $\endgroup$ – AirCraft Lover Dec 1 '19 at 2:12
  • 1
    $\begingroup$ @Orbit - for most airliners, max landing weight is lower than max take-off weight, and they'll dump fuel if they have time. They can land at max weight in an emergency, but it'll damage the tyres and brakes, and can over-stress the airframe - but that's acceptable if (for example) an engine is on fire. aviation.stackexchange.com/questions/2854/… $\endgroup$ – Robin Bennett Dec 1 '19 at 21:09
3
$\begingroup$

Robin makes a lot of great points, but since we are talking on a purely hypothetical case lets have fun and take it to supersonic speeds.

Let's assume that somehow batteries have the required energy density to be used to run some sort of low-bypass electric turbofan to propel our aircraft to high speeds where the neutral point of the aircraft starts shifting aft on typical configurations. Additionally, since batteries do not change weight as energy is depleted, the CG stays at the same place so the static margin of the aircraft would start to increase on a statically stable aircraft as the distance in between the CG and NP increases. Hence, either the control surfaces must be sized to ensure controllability through the whole flight envelope (low speeds and high speeds), or another system must be developed to ensure that the aircraft remains controllable. The Concorde and the TU-144 used a fuel transfer system to move fuel from the center section of the aircraft to containers located in the back, as to shift the CG aft proportionally to the NP movement. Theoretically, an electric airplane could shift its battery storage, but batteries have a pre-established shape, so shifting that mass may be difficult. Also, sizing the control surfaces for high speeds, rather than slow speeds (as is usually the case), could create extra aerodynamic forces that would create extra drag and/or extra stresses being put on the body.

One solution could be to make the aircraft unstable at low speeds, and as the neutral point moves past the CG at very high speeds the aircraft would then become stable. But this would increase the complexity of flight and an advanced controls system would be necessary.

I can't wait for the day that batteries have the energy density required to propel aircraft past the speed of sound, but for the stability and control people there will be a lot of challenges lol.

$\endgroup$
  • 1
    $\begingroup$ Don't hold your breath @Michael Ibanez, the day of supersonic battery powered passenger flight will most likely not be seen by us. Current batteries can deliver approx 500Wh/kg, to match the energy density of kerosene, batteries will have to become about 20 times more efficient. By engineering standards, that's ridiculously much :) $\endgroup$ – Jpe61 Nov 30 '19 at 8:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.