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A 2022 March 15 NASA press release says:

A recent software change already on the rotorcraft frees Ingenuity from its previously programmed maximum altitude of 50 feet (15 meters). The altitude gains could result in incremental increases in both air speed and range.

Why would flying higher increase airspeed?

  • Air density, temperature, and composition hardly differ over the range of heights Ingenuity could theoretically reach in its brief flights.

  • Winds aloft might be faster, but NASA says this is airspeed, not ground speed.

  • Being farther from terrain doesn't affect how fast visual features progress past the navigation camera.

Maybe because the navigation camera can see more terrain and thus more large visual features, for a flight where small details don't help navigation?

Maybe for flights over rougher terrain, such as what's planned for the next few flights?

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  • $\begingroup$ Maybe can accelerate for longer times before hitting a limit? $\endgroup$
    – UVphoton
    Commented Mar 15, 2022 at 23:45
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    $\begingroup$ I find the words "could" & "incremental" intriguing in the quotation. $\endgroup$
    – Fred
    Commented Mar 16, 2022 at 2:10
  • $\begingroup$ The article mentions a delta that rises over 130 feet. I imagine that would be a longer more complex climb if you were carefully maintaining a maximum altitude of 50 feet along the way. $\endgroup$ Commented Mar 16, 2022 at 3:56
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    $\begingroup$ My feeling is your 3rd point - They need to keep objects tracked by the camera at a certain angular speed. There's also ground effect, but likely this is not much and shouldn't affect airspeed. $\endgroup$
    – asdfex
    Commented Mar 16, 2022 at 8:46
  • $\begingroup$ Ground effect is half a wingspan, so, no. :) $\endgroup$ Commented Mar 17, 2022 at 2:02

2 Answers 2

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I think the use of "airspeed" might be just a PR fumble mainly because Ingenuity has no way to measure airspeed.

Ingenuity navigates visually-inertially with the following sensors [1]:

  • a Bosch Sensortech BMI-160 inertial measurement unit (IMU), for measuring 3-axis accelerations at 1600 Hz and angular rates at 3200 Hz
  • a Garmin Lidar-Lite-V3 laser rangefinder (LRF), for measuring distance to the ground at 50 Hz
  • a downward-looking 640 × 480 grayscale camera with an Omnivision OV7251 global-shutter sensor, providing images at 30 Hz
  • a muRata SCA100T-D02 inclinometer, for measuring roll and pitch attitude prior to flight

Ingenuity's flight log gives flight stats in "Max. Groundspeed":

ingenuity flight log

The MEDA instrument(s) on Perserverance can support ~1 Hz rate pressure, temperature, and wind measurements [2] that might allow a very crude (and not at all useful) reconstruction of airspeed. The rover is too far from the helicopter for this data to be of any use other than pre-flight weather reports.

Additionally, being farther from terrain does affect how fast visual features progress past the navigation camera. The navigation camera is extremely wide angle: 133°H x 100°V [3], ~3 mm full frame focal length equivalent, which adds some non-linearness to this effect. Consider this animation I made showing a fixed "feature" on the "surface" and the NAV cam's (horizontal) field of view moving past at a constant speed (5 m/s):

Nav cam visual GIF

(Personal work, warning: aspect ratio is not 1:1)

A given surface feature is in view longer when flying higher and its apparent motion from one frame to the next is lower (of course some motion is desirable to tell how far it has moved).

Maybe for flights over rougher terrain, such as what's planned for the next few flights?

This would represent a change in tactics compared to previous flights over rough terrain, specifically Flight 9 (pre-flight blog post, post-flight blog post). In Flight 9, Ingenuity entered vertically varying terrain slowly to mitigate accruing heading errors early in the flight. This came at the cost of flight duration (Flight 9 remains the second longest flight to date).

However, as shown above, the apparent motion of a surface feature in the navigation images is slower at higher altitudes. Therefore, at a higher altitude, an equivalent (& safe) "feature motion rate" can be achieved at a higher groundspeed.

References:

  1. D. S. Bayard, D. T. Conway, R. Brockers, J. Delaune, L. Matthies, H. Grip, G. Merewether, T. Brown, and A. San Martin. Vision-Based Navigation for the NASA Mars Helicopter. In AIAA Scitech Forum, 2019. (Link)

  2. Rodriguez-Manfredi, J.A., de la Torre Juárez, M., Alonso, A. et al. The Mars Environmental Dynamics Analyzer, MEDA. A Suite of Environmental Sensors for the Mars 2020 Mission. Space Sci Rev 217, 48 (2021). https://doi.org/10.1007/s11214-021-00816-9

  3. Balaram, J. B., Canham, T., Duncan, C., Golombek, M., Grip, H. F., Johnson, W., Maki, J., Quon, A., Stern, R., and Zhu, D. “Mars Helicopter Technology Demonstrator,” AIAA Science and Technology Forum and Exposition (AIAA SciTech), AIAA 2018-0023, Kissimmee, Florida, 2018. (Link)

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It's about control and safety, not power or aerodynamics.

Helicopter flight is a constant management of the trade-off between airspeed and lift - in layman's terms, the more air it pushes backwards to accelerate the less air it pushes downwards, to fly. And adding power (spinning the rotors up) to compensate is quite sluggish.

50ft is an awfully narrow range to manage altitude and vertical velocity which will definitely begin "misbehaving" during a rapid burst of horizontal speed. The extra 100ft give time for the power to compensate for the lift loss and keep the altitude stable without risk of hitting the ground. In other words, the flight path can be far more sloppy and still remain safe.

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    $\begingroup$ Ingenuity's electric motors are almost certainly a lot more responsive than a gas turbine powered helicopter. Rotor RPM is essentially constant, though the power required changes with rotor blade pitch (drag). I highly doubt that the rotor RPM control responsiveness needs to be compensated for with extra height $\endgroup$ Commented Mar 16, 2022 at 13:45
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    $\begingroup$ Have height excursions or horizontal accelerations exceeded NASA's expectations yet? The published videos suggest not. $\endgroup$ Commented Mar 18, 2022 at 2:56
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    $\begingroup$ @BrendanLuke15 This is all true within a certain band of flight parameters. You don't run the rotor at max possible RPM at all times, because the economy of it would be abysmal. You don't keep RPM constant "economy speed" if your tilt causes sufficient lift loss you start losing altitude. And don't forget this is 0.06 bar of Mars, which besides lousy lift also means poor air cooling. The flight time is currently limited by the motor heating, not the battery capacity. $\endgroup$
    – SF.
    Commented Mar 18, 2022 at 10:46
  • $\begingroup$ "The rotors are designed to operate at speeds up to 2800 RPM. The speed is fixed for the duration of flight" H. F. Grip et al., “Flight Dynamics of a Mars Helicopter,” in 43rd European Rotorcraft Forum, 2017. If lift decreases you increase collective, not rotor RPM (though power must increase because of increased drag from the rotor). $\endgroup$ Commented Mar 18, 2022 at 12:52
  • $\begingroup$ @BrendanLuke15 Your paper is all regarding nominal operation, at nominal speeds and up to nominal altitude, for the tech demonstrator purpose. What is being done currently is all about going beyond nominal and pushing the limits. $\endgroup$
    – SF.
    Commented Mar 18, 2022 at 14:05

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