Does a "take-off" assist make sense for electric aircraft?

Question

There is already a good answer that talks about how a catapult provides virtually no benefit for fuel-based aircraft.

However, I'm wondering if the same answer is true for electric planes? For example, I see here:

The high-lift [leading edge] propellers are all used during takeoff…. Once you get to a comfortable margin, say 30 percent above stall speed, then it will just run off the cruise [wingtip] props and you can fold the [blades on the] lift propellers.

This suggests to me that the aerodynamics, complexity, and weight of (at least that design of) electric aircraft could be improved if there were some other means of getting planes up to speed.

While a catapult is a pleasingly high tech solution, there are a number of reasons (some mentioned in that other answer) that make it impractical.

But what about some lower-tech options? A truck with a tow rope? A specially designed cart that the plane sits in that accelerates it up to take off velocity?

I know, that's not going to be practical for jumbo-jet sized craft. But for planes that seat (say) a dozen passengers, could this be a part of the solution? Or does the math work out essentially the same?

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EDIT in response to GdD's question What problem are you trying to solve?

The biggest obstacles to adopting electric aircraft are the limitations imposed by today's battery technology. My question is intended to determine whether take off assist could address that limitation both by alleviating the high demand for power at take off, and by reducing the weight, complexity, etc of equipment that is only needed at take off.

So, the first part of the question is: Would any type of "take off assist" make useful contributions to the operation of electric airplanes?

Peter Kämpf's answer is "no" for existing aircraft. But does electric make things different? Especially given that electric airplanes are currently being designed from scratch?

The second part of the question is: Would any useful "take off assist" for electric planes be viable?

While a steam-powered, extended-length catapult might be both possible and beneficial (and cool), it's hard to believe it's ever going to be practical. But if a truck with a tow rope allows you to extend the plane's range by 25% while also reducing cost and maintenance, that seems interesting.

To address jamesqf's other comment, my target family of aircraft is something along the lines of Eviation's Alice.

I guess I'm just wondering if electric airplane designers are failing to consider it just because that's how existing planes work? Or have they done the math?

Answer 1

A catapult launch makes proportionally more sense for electric aircraft. Proportionally to their lower range and speed, that is.

With current battery technology, only short flights at low speed are feasible. The best one can hope for is to fly a GA-type aircraft for maybe 250 nm at 90 KTAS if the batteries weigh ⅓ of the whole airplane. For this, @Finbar Sheehy's answer assumes an energy density of 250 Wh/kg of the batteries and a discharge to 75% of capacity so we don't abuse the battery.

Now lets further assume the launch will provide the aircraft with all the acceleration to cruise speed, which is 46.3 m/s in metric units. How much of the whole energy used, which is 0.75$\cdot$250/3 = 62.5 Wh per kg of aircraft mass, is that? The kinetic energy the launch provides is 0.5$\cdot$46.3² = 1072 Ws per kg or 0.2977 Wh per kg of aircraft mass. This works out to 0.0047637556 or 0.476% of the trip energy.

That is already quite a bit better than the 0.1385% which were the calculated savings for an airliner, but again not enough to make the case for a catapult compelling. Using a catapult would only save as much energy as covering 1.19 nm at 90 knots. This could become 2 nm if the propellers are used as windmills and the motor as a generator, if the batteries were not full already at takeoff. The most efficient motors, however, make poor generators.

As far as possible savings in installed power are concerned: Electric motors offer much bigger multiples over their rated power for short-time operation - this is how Tesla's "ludicrous mode" is possible, after all. Therefore, it is very unlikely that big savings on the engine are possible. The same, by the way, goes already for airliners: They, too, cruise at or near their maximum rated power in order to fly as high as they do and will not be able to use smaller engines when launched by catapult. The only savings will be in the length of runways – if we find a way to reduce the runway requirement for landings, that is.

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If vertical take-off like in the GL-10 demonstrator (to which the question links) is desired, the installed power must be approximately 3 times larger than what horizontal take-off requires. Now the design needs to add propellers and engines which will be deadweight for the rest of the flight - except when a vertical landing is also desired. It makes sense to have both (vertical take-off and landing), so using a catapult will not avoid the additional propellers and engines in most practical cases. Using a catapult will add extreme accelerations if the take-off distance is to be kept short. This will add inertial loads which require a strengthened structure, and again we will get a trade-off which will cut the possible savings to insignificant numbers.

When the V-1 buzz bomb was being developed, the first dozens of launches resulted in many crashes. Since the V-1 was unmanned and the test articles were destroyed in the crashes, it was exceedingly difficult to find out what happened with the primitive instrumentation and telemetry 70 years ago. In the end, it was found that the acceleration on the steam-powered ramp caused the wing attachments to deform and the V-1 became uncontrollable once it left the ramp. Subjecting human passengers to such loads will not be practical, so a catapult does not really help when a near-vertical take-off is desired.

Answer 2

Yes it would, for fuel based aircraft and for electrical ones. The higher the MTOW, the larger the benefit.

This article makes a case for installing catapults on runways for commercial aircraft: the engines are dimensioned for the take-off, and an assisted takeoff means lighter engines with the associated fuel burn savings. Take-off is particularly demanding on the power installation because it needs to acquire a lifting velocity from standstill.

Other power-hungry flight stages:

• TOGA for go-arounds is applied when the aircraft is at approach speed. The more the better of course - but why should GA power be max TO power for the highest payload ever, on the hottest day ever, on the shortest runway ever. This is what the engines are dimensioned for. From the wikipedia article:

  ..the engines then increase to their computed take off power. Modern aircraft flight management computers will determine the power needed by the engines to take off, based on a number of factors such as runway length, wind speed, temperature, and most importantly the weight of the aircraft.

• Climb power is set after TO: a lower thrust/power setting is selected after the TO. When reaching cruise altitude the thrust/power setting is reduced further. So the engines would be dimensioned for the climb stage.

Aeroplanes certified under 14CFR part 25 must be able to climb after a single engine failure after $V_1$: twin engined aircraft will be dimensioned for this case, if the launching device accelerates the aircraft to a high velocity such as $V_4$ there will still be an engine size saving.

A steam catapult will do the trick, or any of the options that OP states. Of course, the take-off run should not be the couple of metres that an aircraft carrier allows, but should be longer to limit the accelerations that the structure is subjected to.