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Smaller, cheap, RC helicopters generally have very pronounced twist in their blades, and also vary their chord - slightly larger, slightly more expensive RC helicopters have no visible twist or chord change in their blades (i also own both varieties, and the larger blades also have no measurable twist - the answer and comments to Why do helicopter rotors have constant section and angle of attack? posit that real helicopters have twist in their blades, so i was unsure whether it is only an optical illusion in RC helis)

The most expensive of all RC helicopters, Ingenuity, also has very visibly pronounced chord and angle changes, so it does not seem to be about price :-)

With the huge difference in airspeed over the inner versus the outer portion (In the example picture, the larger, yellow, helicopters innermost chord should only get less than a quarter of the airspeed that the outermost portion gets) i would expect a more visible twist - is this something that only happens in flight, as a reaction to aerodynamic forces, i.e. does the blade twist while in use? Does changing the chord as well as the aoa pose too big a construction hurdle for the bigger RC blades, while it is easy to do in injection molded plastic for the small ones?

larger RC helicopter with no visible twist in the rotor blades smaller RC helicopter with visible twist in the rotor blades

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    $\begingroup$ The small helicopter doesn't have a swashplate while the larger one does, this makes their controls function differently $\endgroup$ Jan 3 at 15:03
  • $\begingroup$ @ratchetfreak - do you mean to say that the controls-scheme is of interest for the twist (chord-change) of the rotor? I mean Ingenuity also has twist and their control scheme is non-standard, so i am not doubting you, but could you go into the why and wherefore? $\endgroup$
    – bukwyrm
    Jan 3 at 15:18
  • $\begingroup$ @bukwyrm: Ingenuity uses swash-plates, which is standard for coaxial helicopters. $\endgroup$
    – ymb1
    Jan 3 at 15:21
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The difference is due to the different systems of how they produce their upwards lift and is not necessarily linked to the size.

On most small helicopters (like the one you have in the lower picture) you have a so called "fixed-pitch" system. The rotor blades always have the same curvature (pitch) and how much lift they produce is controlled by the RPM of the rotor. This system is easy to produce (and therefore cheap) and it's enough for small (toy-) helicopters.

The "professional class" helicopters (they can be small but also big - usually the big ones are "pro-class") use a different lift-producing system. It's called "collective-pitch". The rotorblades are not angled, instead they are symmetric in their profile and keep 0° pitch (not angled) when the rotor is not turning. At startup, the rotor spools up and keeps a preset speed but the helicopter itself is not moving. To change the lift, the helicopter starts angeling all of its rotor blades simultaneously and they start pushing the air downwards, lifting the helicopter upwards.

Because the rotorblades have to change their angle (pitch) during rotation, this system is way more complicated to manufacture, but it allows the helicopter to actually fly complex maneuvers instead of just goining up and down and moving on one plane. Collective-pitch helicopters can even roll upside-down and adjust their pitch accordingly to fly loops, etc.

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  • $\begingroup$ While i see that argument make sense for the case QuadmasterXLII's answer touches as well, i.e flying inverted, i do not really see the relevance for helis that do not flip - collective pitch should enable any sort of blade to have zero lift - on a twisted blade the inner part would still produce lift, while the outer part would already produce negative lift, cancelling each other out. The losses incurred from always having some part of the blade at a suboptimal aoa (as is the case for a straight blade) should outweigh the slight inconvenience of having to overpitch for zero lift, no? $\endgroup$
    – bukwyrm
    Jan 5 at 12:05
  • $\begingroup$ @bukwyrm Having pre-pitched blades on collective pitch doesn't make sense because, first of all, in idle position the curvature would produce a ton of turbulence and with it drag, lowering the efficiency, and second of all, one curvature type is only optimized for a certain pitch. So having curved blades would be good at one pitch setting but suboptimal at all the others $\endgroup$ Jan 5 at 16:39
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One explanation without diving too deep is sections of bigger rotors will travel at higher speeds so they don't need high angle of attacks while smaller rotors need high AOA to get enough bite

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  • $\begingroup$ Do you have numbers on that? I have a 20cm-blade-heli with lots of visible twist, and a 30cm heli with none, while, for instance, the props of something like the Osprey are twisted a lot as well, while the Bell-206, with shorter blades, has very flat-seeming ones. $\endgroup$
    – bukwyrm
    Jan 3 at 15:27
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Historically, there are two main categories of people flying expensive RC helicopters: 3-D pilots who like to do flips and fly upside down, and scale pilots who want to look like real helicopters. The 3-D pilots need symmetrical blades for flying upside down, and the scale pilots don’t care much for gains in efficiency - they just want the same camber and twist as the real helicopter they are modelling, which is subtle. I suspect that the large, straight bladed helicopter you own has the capability to hover upside-down: is this the case?

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  • $\begingroup$ No, the straight-blade heli is not able to fly inverted, or maybe it's just me :-) $\endgroup$
    – bukwyrm
    Jan 5 at 11:57

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