Why are NASA's engineers concerned about gusts affecting the X-57?

I understand that NASA is recommending automatic torque based prop speed control to be used in flight but also in gusty conditions.

Source: https://en.wikipedia.org/wiki/NASA_X-57_Maxwell

The speed of each propeller can be controlled independently, offering the ability to change the over-wing airflow pattern to cope with flying conditions, such as wind gusts.

I understand:

  1. The original airplane being used had a wing loading of about 17lbs/sq ft.

  2. The X-57 has a wing loading of 45 lbs/sq ft with its new smaller wings.

  3. The higher the wing loading, the less an airplane is affected by wind gusts as it needs much more wind velocity to generate lift. E.g. frontal wind gust hitting one wing, then the other on a lower wing loading would create much more roll.

Source: https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-109.html

High-Aspect Ratio Experimental Wing

Perhaps the most noticeable difference from the X-57’s Modification II configuration is the development and integration of an experimental, high-aspect ratio wing. Designed by Xperimental of San Luis Obispo, California, the wing features a large reduction in area, with wing loading increasing from 17 pounds per square foot to 45 pounds per square foot.

The reduction in wing area also contributes to more efficient cruise flight through decreasing friction drag. The final Modification IV effort will demonstrate that the high aspect ratio wing with the integrated high lift motor system will allow the X-57 to take off and land at the same speed as the baseline P2006T. The aircraft will also be less sensitive to gusts and turbulence, leading to a smoother flight.

If the X-57 is less sensitive to gusts, why does it need a "gust control system"?

One possible answer is that the X57 needs that flow over the wings to generate lift.

I understand that the X57:

  1. has 14 x 24" 5 bladed props
  2. props spin at about 4,500 rpm
  3. motors generate about 14,000W of power
  4. stalls at about 60mph.

Entering that into java foil, the programs generates a prop exit speed multiplier of 1.6 or roughly 100mph.

See pics below

X57 prop specs

X57 prop exit speed Example:

10 mph headwind, gusting to 20 mph, increase is 10mph, which is only a 10% of 100mph airflow over the wings at a stall speed of 60mph, with wings that should be 300% less affected by wind due to 300% higher wing loading.

So why does it need a "gust control system"?

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    $\begingroup$ Sometimes engineers do things just because they can. (For example, does anybody actually need windshield wipers that turn on automatically when rain is sensed?) Perhaps it is being used as a test bed for proof of concept. Not an answer, but that would be my bet... $\endgroup$ – Michael Hall Aug 12 at 17:42

It would depend on the direction of gust. High-aspect-ratio wings have a lower rate of roll. Thus, an upward crosswind gust that introduces a rapid uncommanded rolling action may not be sufficiently correctable by aileron deflection alone, and may require the asymmetrical application of thrust to increase the control surface authority and lift of the lower wing.

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  • $\begingroup$ But 45lbs/sq ft is not that high of a wing loading, compared to passenger jets at 150lbs/sq ft* ( *I'm told). $\endgroup$ – Fred Aug 13 at 0:41
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    $\begingroup$ @Fred: That's comparing apples and oranges. The aspect ratio of a 747, for example, is 6.96. On the X-57, it's 15. High-aspect-ratio = Slow roll rate. Even gliders, which have a very low wing loading and very high aspect ratio, can have a problem recovering from sideslipping when a gust of wind raises one wing sharply. This is one reason they incorporate pronounced dihedral into their wing design. $\endgroup$ – Aaron Holmes Aug 13 at 1:42
  • $\begingroup$ Sorry. I read high wing loading, instead of high aspect ratio. $\endgroup$ – Fred Aug 13 at 13:32
  • $\begingroup$ Wouldn't full span ailerons or ailerons with a deeper chord be a much simpler and safer engineering solution? $\endgroup$ – Fred Aug 13 at 15:18
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    $\begingroup$ @Aaron Holmes just to benefit your good writings, adding wing dihedral makes side gust worse. The drag balance around the CG (vertically, from the side) determines crosswind roll stability. The easiest way to do it, just like a paper airplane, is to put a light "keel" underneath. This is the unsung benefit of ventral fins, and part of the Predator drone design. $\endgroup$ – Robert DiGiovanni Sep 12 at 18:47

Because all those little propellers will make their own "gust" if you change their power setting. Increased lift with a staticly stable configuration will cause a sharp pitch up if the gust is strong enough. This may be the reason glider pilots like to move the weight back a little, towards a more staticly nuetral gust behavior.

Also, those skinny wings do little to help pitch stability, and engineers these days simply cannot bring themselves to make the horizontal stabilizer a little bigger.

However, for this application, a properly functioning gust control system may be a welcome improvement.

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  • $\begingroup$ One may question what a row of props along the wing (or vortex generators), will do for cruise efficiency. I see removing the LE props and keeping the rest as a GA "flying flapjack" (with a lower AR wing). Also, because electric motors can spin up faster than gas driven (in their entire rpm range), they could program a slower rev to make "gusts" more controllable; but reducing the total number of props to 6 or 4 (or 2 or 1) and making them larger, seems to make more sense. $\endgroup$ – Robert DiGiovanni Sep 12 at 18:37

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