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I am puzzled by the spikes on Ingenuity's blades (marked by a red box in the image below).

  • What is their function ?
  • Are these special to Ingenuity?
  • Why don't they collide during high RPM?
  • Don't they induce unwanted drag and/or turbulence?

enter image description here

I have tried searching NASA's website but couldn't get a description of these particular featuers. Answer to any part is appreciated.

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    $\begingroup$ Might be better on the Space SE: space.stackexchange.com/questions ? $\endgroup$
    – simon at rcl
    Commented Apr 11, 2021 at 14:35
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    $\begingroup$ While definitely also on topic at Space.SE, I think this question is on topic here. It is about a helicopter after all (even if it doesn't fly on this planet). So IMHO you can leave the question here. $\endgroup$
    – Bianfable
    Commented Apr 11, 2021 at 15:16
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    $\begingroup$ @simonatrcl 1 Q on two Stacks may be considered bad form, but it is fully legit since the question is on the boundary between two worlds. I think Stacks'users should focus much less on form. or, if they really have to, to focus on the form they give away knowledge or nothing, aparts from some crappy rewards such as "points". Read: meta.stackexchange.com/questions/104947/… and try to check the history of similar sites (i.e. Couchsurfing, just to mention a very close concept of build large community for free, then get the paycheck) $\endgroup$
    – EarlGrey
    Commented Apr 12, 2021 at 3:27
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    $\begingroup$ @EarlGrey - Thanks, and good to know. I'm just going on what I've seen happen. $\endgroup$
    – simon at rcl
    Commented Apr 12, 2021 at 7:48
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    $\begingroup$ @simonatrcl The better way is to hit "flag..." and "other..." and say "mod please move to <stack>.se". An author request would certainly be granted, any other would be considered. $\endgroup$
    – Harper - Reinstate Monica
    Commented Apr 13, 2021 at 21:42

1 Answer 1

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In the Mars Helicopter Technology Demonstrator paper these are refered as "Chinese weights" and the purpose stated is

Chinese weights provide a restoring force on the blade moments when under centrifugal loads thereby reducing the torque requirements on the swashplate actuators.

Searching for the term leads mostly to RC sites, for example this explanation.

Being placed close to axis of rotation (and presumably heavy but small in volume) the aerodynamic effect should be minimal compared to the rest of blade.

My assumption is that this can be quite specific to Ingenuity (and RC models) because limited torque available in electrical actuator servos compared to control system of full-scale helicopter. (But it is my guess only.)

Edit: I have borrowed an image from https://mars.nasa.gov/resources/25043/mars-ingenuity-helicopter-3d-model/ (go there and rotate the model yourself to get a better idea how counterweights are placed) and tried to sketch how the force balance looks like:Ingenuity 3D model with centrifugal forces shown

Red arrows are centrifugal forces exerted on counterweight or blade's center of mass respectively. These forces are always perpendicular to the mast (rotational axis). Then, each of them can be broken into two orthogonal components -- parallel with blade pitch axis (green) and perpendicular to it (blue). Note that one blue arrow lays above pitch axis and second under, so they produce opposing torque and can cancel each other.

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  • $\begingroup$ I thought they were blade counterweights at first, as is used on constant speed counterweight propellers since forever, but if they were blade counterweights, the ones for the upper rotor would be on top same as the lower one, since they are to provide a coarse pitch moment and presumably both rotors are lifting in the same direction while spinning in opposite directions. It doesn't make sense that the upper rotor would have counterweights that tend to make the blades go flat. Also, if they were counterweights, it's inefficient to have most of the meat at the base. $\endgroup$
    – John K
    Commented Apr 11, 2021 at 17:18
  • $\begingroup$ @JohnK if I understand it correctly then both pitching moment to be compensated and added moment through the counterweights is purely due to inertial forces (not aerodynamic and neither gravity), so it should not make (much) difference if they are upside down or not. "At the base" .. you mean of the blade? I guess it is some design compromise in play here. It is not only radial distance (which adds centrifugal force) but also off-the-blade-axis position (which creates actual pitching moment) which is important for force balance and not least aerodynamic consequences. $\endgroup$
    – Martin
    Commented Apr 11, 2021 at 17:42
  • $\begingroup$ Actually in the linked paper the diagram shows both counterweights downwards, but prototype photos have them both upwards. $\endgroup$
    – Martin
    Commented Apr 11, 2021 at 17:43
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    $\begingroup$ If they're being used for that purpose, the counterweights will be tuned to produce a pitching moment (from wanting to trail the blade) that is more or less the same as the blade aerodynamic pitching moment, and if is just right they will cancel each other out and the blade with have no pitching moment in some operating range so the servos have little resistance to work against. There my be large weights at the tips of those little fins that are not visible in the pics, but normally you would expect to see some kind of mass at the top of each one. In the video they are like the OP pic. $\endgroup$
    – John K
    Commented Apr 11, 2021 at 17:59
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    $\begingroup$ Yes, I'd expect weight located in the tip. Better view of the whole assembly: mars.nasa.gov/resources/25043/… -- in the model there are counterweights about the right position to balance mass of each blade -- if you imagine cetrifugall force on counterweight and large area of each blade and its component perpendicular to pitch axis, these will cancel each other regardless which direction specific counterweight points. For aerodynamic forces, I guess you can get away with aligning pitch axis so that it intersects center of pressure. $\endgroup$
    – Martin
    Commented Apr 11, 2021 at 18:16

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