Electric motors open a candy box of new possibilities to jaded aeronautical designers and researchers.
As with real candy, however, there is a problem of weight gain. The best commercial batteries today are 30 times heavier than their equivalent in fossil fuels. The $80,000 Tesla Model S sedan uses 1,300 pounds of batteries to store about 87 horseÂpower-hours. These batteries take it some 200 miles — half the cruising range of a conventional car fueled with just 150 pounds of gasoline.
The arithmetically alert reader will have noticed that twice 1,300/150 is not 30. The reason for the apparent disparity is that electric motors have certain advantages that partly Âalleviate the weight of batteries.
For one thing, electric motors are highly efficient. They turn 90 percent of the energy they use into useful work, wasting only 10 percent as heat. Reciprocating and turbine engines turn only a quarter to at most a third of the energy in their fuel into work. So electric motors get around three times as much work out of the energy content of their “fuel” as heat engines do.
If you look at the problem in terms of direct operating costs, the calculus is complicated by the fact that electrical energy has to be made — you can’t just bottle lightning or harvest static from cats — and most of it currently comes from the burning of fossil fuels in heat engines. But the very large turbines used to generate electricity are more efficient than airplane engines, and the distribution of electricity is cheaper than the extraction, distillation and delivery of fuel. It turns out that at current fuel and power prices recips and turbines cost about 50 cents per horsepower per hour and electric motors about a dime. So you can understand why an electric airplane might be attractive, provided that it delivered reasonable performance, although your enthusiasm might be tempered if and when it became necessary to replace the battery pack, which gradually wears out with use: The Tesla’s costs $25,000.
Since energy storage for an electric airplane is so heavy, ways have to be found to make a little power go a long way. Some of these are obvious — careful streamlining, lower speeds, longer wings — and the first generation of electric airplanes, mostly single- and two-seaters, resemble motorgliders.
There are other possibilities, and I learned of one of them recently through Mark Moore, an aerodynamicist at NASA’s Langley, Virginia, facility. Moore is interested in a concept, reminiscent of fanciful depictions of cities of the future, called On-Demand Mobility. The idea is to use air vehicles for short-distance, perhaps pilotless travel; they would combine the convenience of taxicabs with the automatism of elevators. Electric propulsion lends itself to this sort of mission because long flight durations are not required.
The particular technology we discussed is called “distributed propulsion.” Some forms of distributed propulsion have become familiar through small quadracopter drones as well as proposed man-carrying multicopters with as many as 16 motors and rotors. The categories of distributed propulsion are bewilderingly many, but the one of interest here involves propulsors placed so as to reduce drag or increase lift by their aerodynamic interactions with the airframe.
The small size, light weight and very high reliability of electric motors allow them to be put just about anywhere. One example slated for testing at Langley is a Tecnam twin whose 160-square-foot high wing will be replaced by one of only 54 square feet. The aspect ratio jumps from 8.8 to a gliderlike 17. Replacing the twin Rotaxes of the stock airplane are as many as 20 small nacelles and propellers spread out along the leading edges of the wing, each containing an electric motor and its own battery pack. Only the two at the tips are used for cruising; they gain some efficiency from their interactions with the tip vortices.
The rest are used for takeoff and landing. The wing, immersed in their fast-moving slipstream, feels as though it’s moving much faster than it really is, and its lift nearly triples. That explains the tiny wing and the resulting turboproplike wing loading, which enables the airplane, unlike more lightly loaded ones, to cruise at its most efficient angle of attack. (All current reciprocating-engine singles and twins cruise much faster than their most efficient speeds.) High wing loading also provides a smoother ride.
A great deal of theoretical analysis is going into this project, in hopes of getting the trial without the error. Some serendipities are immediately apparent; for instance, the propellers, whose blades fold back against the nacelles when they are not in use, can have fixed pitch, since the speed of the electric motor can vary with airspeed to keep the blades operating at their optimum angle of attack.
On the other hand, the aerodynamic conditions in the wake of an upgoing blade are different from those behind a downgoing one, and it’s difficult to optimize the wing’s airfoils for both the cruising and the slipstream-blown states. The weight of cabling, too, presents a new challenge. Each motor has its own battery, but all must be linked to protect against failures. Furthermore, all that battery and motor mass distributed along a very thin, narrow wing presents difficult structural and elastic problems.
This project does not directly address the needs of On-Demand Mobility. It lands and takes off at the same speed as current airplanes. You could use the augmented lift to reduce the minimum speed, but then stability and control become problematic. NASA has also studied VTOL versions with tilting motors on the wings and tail; in such a system stability and control rely on digitally controlling the output of each motor.
The possibilities seem limitless. With luck, we will see some of them in flight in the coming years. Between novel propulsion schemes and the headlong rush into digital control and robotics, perhaps the next leap forward in aeronautical design, after seven decades of
near stagnation, may actually be in the offing.
Second Thoughts
In the Aftermath column in the August issue I described an accident in which a modified Cessna P210 encountered severe turbulence and entered a spin from which the pilot failed to recover. I said the 210 and other normal category airplanes are not required to demonstrate spin recovery for certification. I should have said “recovery from a developed spin”; they are in fact required to demonstrate recovery from a one-turn spin.
The requirement deserves comment. No experience of spins is required for any pilot’s license, and any pilot experiencing an inadvertent spin for the first time is likely to be too startled and disoriented to remember what to do about it. In other words, the one-turn recovery requirement is just window dressing; it has little bearing on likely events in the real world. Besides, a one-turn spin is not really a spin at all; it’s just a stall with complications. A fully developed spin can be a stable and predictable maneuver in some airplanes and a chaotic many-Âheaded monster in others. Some airplanes recover promptly and cleanly with standard anti-spin controls; some, like the one in this month’s Aftermath, refuse to do so.
On the other hand, although the stall-spin continues to be a common cause of fatal accidents, most of those spins take place at low altitude, in the traffic pattern, where recovery — and certainly recovery after more than one turn — is very improbable, just because of lack of room. And so real spin recovery capability — after three turns, or 10 — is very seldom useful. Since it would be prohibitively difficult to ensure that every twin and business jet could recover from a developed spin, the one-turn spin requirement at least provides that those pilots who can recognize an incipient spin and know what to do about it will have a chance to recover.
This column was published in Flying’s October 2014 issue.
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