Why do smaller RC helicopters have visible twist in their rotors, and slightly bigger ones do not?

Question

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?

Answer 1

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.

Answer 2

Everyone seems to be saying it has to do with the control scheme and presence or lack of a swash plate. The twist along a rotor blades' length is called washout. The property of getting skinnier as it goes out is called taper. Small RC fixed pitch helis usually have lots of taper and washout, and it makes sense why, since the tip of the blade is going much faster than the root, and needs much less pitch and size to make the same amount of lift. Its a more efficient design if you add taper or washout. Thing is, full scale helis have washout and sometimes taper too, it's just much more subtle on higher speed and larger rotorheads which take advantage of the efficiency of longer skinnier blades. Some micro helis like the scout CX have relatively straight and square blades, and some with a single head, no flybar and a swash plate (such as the blade 120 s2) have lots of taper and washout. I think fixed pitch helis just tend to be smaller and operate at lower head speeds where washout and taper help a lot. Obviously for 3D flying you'd want a symmetrical blade but could still utilize taper.

Answer 3

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

Answer 4

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?