71

Because they are very lightweight and fragile. Therefore, thrust and propulsion mass must be distributed over span - a single, large propeller and motor would put too much force into the structure locally. Also, it would require a higher landing gear to give the larger propeller the required clearance. You are right about the higher losses. However, these ...


28

No, it could not fly much faster with the available energy. Lift is a question of wing area and dynamic pressure. Solar Impulse 2 has 269.5 m² wing area to carry its 2.3 tons of mass. This is a wing loading of just 8.53 kg/m²; much less than even gliders have (they start at around 30 kg/m²). This allows it to fly very slowly; if we assume it ...


14

Conventional commercial designs try to maximize kg-kms per dollar fuel cost or kg payload per dollar investment. Or minimize operating & maintenance cost. All these goals need an economy of scale that forces few large complex turbines. On the other hand, most solar planes focus on raw, dollar-agnostic performance / endurance metrics like max height ...


13

Generally, best range is at high altitude(thin air, less drag), and best endurance is at low attitude(thick air, more lift) and low power. It makes sense that at night they would have to descend to a best endurance altitude to conserve battery power. Source: solarimpulse.com


12

Don't forget the electrical advantages of multiple motors. No need to conduct all the current to a single place with long lengths of heavy wire, no need to control a large current. Also, many small motors and propellers provide redundant depth/graceful degradation in case of a failure(s).


12

Solar Impulse has no flaps and a high aspect ratio wing. Therefore, the c$_{l_{max}}$ of the wing should be no higher than 1.8, most likely 1.6. While a rigid airfoil can be designed with a higher c$_{l_{max}}$, it will not have an attractive L/D and a shorter wing would have a c$_{l_{max}}$ below that of the airfoil. Since no fuel is consumed, mass is ...


12

From here Solar Impulse Performance Solar Impulse can fly at the same speed than a car, between 36 km/h (20 Kts) and 140 km/h (77 Kts). At sea level: minimum speed of 45 km/h (20 Kts) and maximum speed of 90 km/h (49 Kts). At maximum altitude: from 57 km/h (31,5 Kts) to 140 km/h (77 Kts). From here Wired.Com article, interview with pilots ...


11

Interesting question. The ATC orders mention only two reasons that controllers have to say "at your own risk": The pilot "insists" on using a closed or unsafe runway (3-3-2) A helicopter takeoff or landing uses a non-movement area (3-11-2, 3-11-6) Obviously Solar Impulse 2 isn't a helicopter, so that leaves the closed or unsafe runway reason. I don't know ...


10

It reduces moments in the wing spar. Major points of axial drag in this design are the wings and the fuselage. You'll see that two engines are close to the fuselage, and the other two are midway along the wing. By placing two engines further out, they are also located where the lift is generated, reducing the necessary strength of the wing spar. Both of ...


8

The two inner engines are in the expected position. Keeping the engines as close to the fuselage as possible has two main benefits: Less adverse yaw in the case of an engine failure Less stress on the wing on the ground and during landing from the engine weight The outboard engines are certainly unconventional. Most aircraft with four engines, expecially ...


6

In addition to the thrust distribution, a multi motor design also allows you to turn off some motors if you run into a cloudy situation that provides too little power. One long bus tying all the panels together and each motor connected to it allows all the power to flow to a subset of motors. Once the sun comes back out, you turn all the motors back on and ...


6

Solar Impulse 1 had 2 engines right next to the fuselage, and the other 2 further out where the wing bends upwards. Solar Impulse 2 has a pair of closely spaced engines on each wing, located partway out onto the wing.


6

Solar Impulse has two means to store energy: Batteries and altitude. Weather permitting, both will be fully charged at sunset, and mostly depleted over the night. The rate of depletion at sunrise depends only on the time between sunset and sunrise, not on the direction and strength of the wind. Headwind only means that less ground is covered. Solar Impulse ...


4

"Working down" conserves battery power, as the other answers suggest. But it is also better for endurance to descend, simply because the flight Reynolds number will be higher, and the viscous drag lower. See this answer for an explanation how the physics behind this works. See this answer for the Reynolds number effect on a glider. Here the same aircraft ...


4

It is basically gliding at night. Not pure gliding but a powered glide - similar to what normal aircraft do on landing approach. Altitude is energy. We all know this from science class - height = potential energy. Therefore an aircraft that's climbing uses energy and an aircraft that's gliding gains energy (technically it converts potential energy to ...


3

Altitude is energy. A long shallow dive, from 8,500 to 1,500 meters, recaptures the energy that it cost to climb up during the day. It is completely analogous to charging the batteries with solar power during the day and drawing that energy out of the battery at night.


2

As you can see from the massive delay in its flight schedule, the Solar Impulse II has run into some problems. This is always an indication that something has been learned. Unfortunately, what has been learned is that electric flight is even harder than expected. The value of this particular airplane is the demonstration that purely electric long-range ...


1

To answer the question presented in the title, I looked to Wikipedia for details on the flight(s) of Solar Impulse. According to Wikipedia, Solar Impulse 2 (HB-SIB) circumnavigated the Earth’s Northern Hemisphere in 17 legs/flights. Of those 17 legs, 14 legs had maximum altitudes above 18,000 feet MSL. In the U.S., Class A airspace is between 18,000 and 60,...


Only top voted, non community-wiki answers of a minimum length are eligible