From an aerodynamics perspective, velocity alone is insufficient to describe the flow regime experienced during flight. For the Ingenuity helicopter, what specific Mach numbers and Reynolds numbers are planned to be exercised during test flights on Mars?

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    $\begingroup$ Are you asking about the Mach & Reynolds numbers the entire aircraft will reach, or its blades? $\endgroup$ May 21 at 13:15
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    $\begingroup$ @OrganicMarble: More so the rotational mach number rather than translational, since rotation was likely orders of magnitude larger. Overall, my main motivation is to understand the flow regime experienced in flight and what factors/limitations motivated the choice in flow regime in flight. Though perhaps that should be its own separate question. $\endgroup$
    – Paul
    May 21 at 22:58

The maximum rotation rate for the blades is quoted here as $2800\,\mathrm{rpm}$. However other information variously indicates $2600\,\mathrm{rpm}$ and $2400\,\mathrm{rpm}$: the second figure coming from mars.nasa.gov.

I'll take the lower figure, which seems to be what they're actually using: I presume the higher figures were before they'd worked out what they needed to achieve in practice based on measurements of lift, actual mass and so on.

The total blade length is $1.2\,\mathrm{m}$ so individual blades are $0.6\,\mathrm{m}$ long.

So the tip speed is $2400\times 0.6 \times 2\pi / 60 \approx 150\,\mathrm{ms^{-1}}$. The speed of sound on Mars seems to be around $244\,\mathrm{ms^{-1}}$, so the tip speed will be about mach $0.6$.

The blades will spin with approximately constant angular velocity with lift being controlled by pitch adjustements, as with any helicopter. So this is the only regime they will experience during flight.

I imagine they experimented fairly extensively in the hypobaric chambers on Earth, however: there's no reason to believe that there's anything special about the atmosphere on Mars which was not reproduced in those experiments.

  • $\begingroup$ I’m also curious what factors motivated the upper bound on the mach number. Was it primarily aerodynamic, structural, or power/size/weight limitations that drove the operating point for that mach number. $\endgroup$
    – Paul
    May 21 at 22:47
  • $\begingroup$ @Paul: I don't know. You really don't want the tips to be supersonic though, and I think things get pretty weird well before you get supersonic as parts of the airflow become supersonic or nearly so. $\endgroup$
    – user21103
    May 22 at 9:09

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