# What would be the stall speed of Solar Impulse?

In most of the tables, only the take off (22 mph) and cruise (43) speed of the Solar Impulse are listed. It is an experimental plane, slow, with very huge wing for solar batteries.

The take off speed may not be the slowest speed the aircraft can fly, for instance, when landing. Is it possible to infer somehow the expected slowest speed of this airplane?

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 constant, which makes it very easy to give a lower bound for the minimum speed.

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. If we assume standard day conditions at sea level (density $\rho$ = 1.225 kg/m³), we find vor v$_{min}$:

$$v_{min} = \sqrt{\frac{2\cdot m\cdot g}{c_{l_{max}}\cdot S\cdot \rho}} = 9.24 \;\small{\frac{m}{s}} \;\text{or}\; 33.27\; \small{\frac{km}{h}}$$

For those of you from metrically challenged cultures: This is 18 knots or 20.67 mph.

• So very close to take off speed. I assume this is because it has no flaps or anything else to configure it differently for landing.
– h22
Jun 30, 2017 at 7:38
• @h22: Exactly right. It is a one-point design. Jun 30, 2017 at 10:38
• The root is rather the absence of fuel, isn't it? If you need the same lift for take-off and landing (because same weight) then you don't need a different configuration. it could have flaps to adapt cruise/low speed. But even if it had flaps, it would use the same flap configuration for landing and take-off. Jun 30, 2017 at 14:14
• @SylvainBoisse: No, look at gliders, ASW-20 or ASH-25 for example. They also don't change mass during a flight (OK, except for water ballast) and profit immensely from flaps. They have a setting for slow flight and one for landing. The slow setting is usually used for takeoff, but never the landing setting because it creates too much drag. The root is that Solar Impulse flies at one speed mostly, just above stall. Jun 30, 2017 at 14:33
• @PeterKämpf Good point. I've never looked much into gliders, but you're right they are the closest thing to an electric plane. I'll look into this on SO. Thank you Jun 30, 2017 at 14:36