I was reading this article about how NASA will develop this new X-57 X-plane which has many tiny electric motors mounted on the wing.


Now, I wonder: How come this aircraft still has a vertical stabilizer? I thought that one of the points of having many tiny electric motors was that you needn’t worry about an engine failure (one engine fails, you shut down the one on the opposite side; you still have 12 working, no big deal). You probably don’t have to worry about lateral stability either, because those motors can probably easily stabilize sideslip oscillations (a bit like on a quadrotor, where you can maintain a stable hovering position thanks to accelerometers and fast control feedback).

Am I missing something?? If so, isn't there a way to downsize the stabilizer a little bit? How would you go about calculating that?

EDIT: I realize the problem of power failure, but couldn't it be just avoided by carrying lots of independant battery packs (instead of a central big one), so you can be sure that you statistically never lose control of all your engines?

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    $\begingroup$ Well, during a full power failure (complete battery failure) the aircraft would be a glider, which could benefit from a vertical stabilizer. $\endgroup$
    – Steve H
    Aug 4 '16 at 11:25
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    $\begingroup$ Very few finless aircraft have been built using engine thrust (and maybe spoilers) to control yaw; they were all military too I think. A tail fin really is a simple, reliable way to control most "normal" aircraft, including this one. It will work even without engines, as Steve H just said. $\endgroup$
    – Andy
    Aug 4 '16 at 11:44
  • $\begingroup$ I agree, the fin is a great solution; but a heavy and draggy one too... I just thought NASA could be a little more ambitious in their design :-) $\endgroup$ Aug 4 '16 at 12:15
  • $\begingroup$ If you want ambitious look at the B-2 Spirit. But if the controls go down, you have to eject - which has happened. $\endgroup$
    – Andy
    Aug 4 '16 at 13:21
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    $\begingroup$ So lots of spare batteries, redundant electronics and control systems is better than just having a rudder? $\endgroup$
    – GdD
    Aug 4 '16 at 13:59

Differential thrust is not a good way to control the yaw of an airplane. For one thing thrust control has to be extremely quick, if it takes even a second between a control input and the thrust response then you won't have adequate control over the airplane. For another you are entirely dependent on having engine power available for stability, if you lose your electrical system or your battery goes dead then so are you!

Another reason differential thrust is not a good idea is that at full power the only way to create differential thrust is to reduce power on one side. If I'm on climbout and I need to yaw I will lose climbing power, this means I would have to reduce my flight path angle to maintain airspeed. If I'm close to stall speed a sudden reduction in power could make me enough airspeed to stall. Not good. The only way to really mitigate against this is to increase motor power and restrict the maximum power a pilot can get. This is extra weight and cost.

Lastly, the X-57 has the engines all down the wings, this means that when the engines are going they will be producing airflow over the wing. Differential thrust means that the engines are forcing more air over one wing than the other, the wing getting more airflow will create more lift, this has the following undesirable characteristics:

  • A yaw input will have to be coordinated with reverse aileron to maintain level flight, a strong yaw input would need a stronger aileron change. This means badly coordinated controls and would not be very intuitive
  • At high angles of attack, for instance close to stall speed, sudden reduction of thrust over one wing could stall it while the other wing keeps flying, this would lead to a snap-roll and possible loss of control. In short it's dangerous.

Comparing quadcopter technologies to the technology going onto the X-57 is a bit of a misnomer:

  • Quadcopter motors are small and the blades are small compared to what will be used on the X-57, the ones on an airplane will be much bigger and will have a much larger response time as a result due to the longer moment arm
  • A quad has a higher thrust to weight ratio than the X-57 will, so less power available for differential thrust
  • $\begingroup$ Don't you think that tiny electric motors could be indeed very reactive (probably way faster than a second)? Gyrometers give you the yaw rate instantly as well... (like on a quadcopter) $\endgroup$ Aug 4 '16 at 12:19
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    $\begingroup$ These motors and propellers are quite a bit bigger than those on your average consumer grade quad copter, and even those on a large one used for TV/Movie filming, therefore, they have quite a bit more inertia than those tiny motors do. Even if the electrical impulses & computations take milliseconds, changing the actual rotational speed of the propeller(s) could take a second or longer, simply due to physics. As we all know, physics always wins. $\endgroup$
    – FreeMan
    Aug 4 '16 at 13:07
  • $\begingroup$ @HughKeller also bear in mind the yaw response of quadcopters is not controlled by the thrust of the rotors - its controlled by changing the angular momentum of the rotors. It's like having a reaction wheel. Instant response (limited only by motor+rotor acceleration). $\endgroup$
    – Andy
    Aug 4 '16 at 15:47

Another very good reason for not changing the tail surface at all is that this is a research aircraft--it is not an ideally optimized aircraft. By basing it on the Tecnam fuselage with no changes, NASA can get a baseline number for the drag reduction allowed by this small high aspect ratio wing vs. the original, much larger wing on the Tecnam twin engine aircraft. They are purposely isolating this single variable. Indeed, a paper authored by someone on this project gives an airframe drag reduction estimate of 1.5 times compared to the parent aircraft; they believe an optimized design could reduce drag further.


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