VTOL V STOL
VTOL capable aircraft with relatively good cruise performance tend to have one of two issues. One, they have a large thrust area (efficient hover with low disc loading), but must tilt the props or rotors forward for cruising flight, which implies additional weight, complexity and expense from tilting mechanisms. The props or rotors are of a size that cannot be safely enclosed for everyday operations close to people. Two, they have a small thrust area (inefficient hover with high disc loading); this implies the motors need a lot more power for take-off than in cruise which means a heavier, more expensive powertrain that isn't in use for most of the journey. Fans of such small size and power are noisy.
These observations do not mean that such aircraft do not have their uses. On the contrary, aircraft with very large or small thrust areas, with some of the above mentioned problems, are still very well suited for their market areas and flight requirements.
Intermediate concepts using multiple rotors or fans are a compromise on these problems, to one extent or the other. This is the design space of most eVTOL concepts and all VTOL concepts that intend to offer personal door to door transport, since low noise and shrouded blades are essential for this area of the market.
Quietness, safety and footprint are the determining factors in the door to door market. Volerian’s VTOL is designed to deliver the necessary performance and cost to allow everyone to use flying cars instead of ground cars. Some of the aircraft weight is transferred onto the ducts and fuselage when in cruise so that the lift they create reduces the energy needed to keep the aircraft airborne. This helps ensure we can deliver the required range. All moving parts are concealed and the redundancy of having multiple motors helps make the aircraft safe and quiet. The aircraft is small enough to land on a normal street, rooftop, or outside a suburban house.
The speed and range needed for short journeys, such as commuting or urban travel, can be achieved using batteries for more or less any well engineered system. There is a tendency for issues of weight, normally a huge problem for VTOL aircraft, to resolve themselves into issues of range for eVTOL aircraft. This is because electric motors have a very high power to weight ratio, taking up only a small proportion of the take-off weight, but the batteries take up a large proportion of the take-off weight. Therefore a heavy aircraft can still achieve VTOL by reducing battery weight and accepting the reduced range.
Most commutes, and city diameters, are less than 20 miles, making eVTOL aircraft ideal for these areas of travel. Even a slow aircraft can cover this distance in 20 minutes, which is a lot faster than a ground vehicle in the same environment. This performance is well within the 200 W.hr/kg battery energy density commercially available today.
STOL aircraft have an obvious disadvantage in that they require an open space or runway from which to operate. This aside, they can be built to exceed the performance of a comparable VTOL across every metric except speed. VTOL cannot take-off with as much fuel as STOL and therefore have reduced range or must conserve energy by reducing drag, and therefore cruise speed. However, high speeds can still be achieved with optimally sized wings, because they aren’t needed for take-off, and a powerplant sufficient for VTOL will also be powerful enough to overcome drag at high speeds.
Even a very short take-off run, such as might be needed from a roof top or length of road, can offer significant improvements for cruise and range; such is the advantage of gaining unpowered lift from wings instead of using powered thrust. An aircraft may not even need to use much energy gaining height, if it can leave from, and land on, a runway of similar altitude. Flying slowly reduces drag considerably (since it increases exponentially with speed) but point to point travel times can still be faster than alternative forms of travel. Therefore, reduced cruise speed and climb (within the bounds of competitive, safe travel) combined with high propulsive efficiency, and the optimal wing and powertrain sizing achievable with STOL, can make the most cost competitive aircraft. Volerian’s intercity and regional aircraft concept is being designed with this in mind. Other cost factors which involve design and engineering solutions include the crew to passenger ratio, manufacturing methods/materials and operational complexity.
In broad terms; wherever a STOL aircraft is capable of operating, it can beat VTOL on cost and performance. When VTOL is necessary it should have as large a thrust area as possible given the constraints of its mission, cost and engineering limitations.