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Why do toy (RC) helicopters have curved flat surfaces and larger rotors on true helicopters have a solid aerofoil shape?

If the rotor was mechanically strong enough to lift the desired weight, is there any (aerodynamic) advantage to the solid aerofoil over the simpler curved surface?

Surely toy manufacturers could "close the bottom" of the rotor (equating to the aerofoil shape) even if it was hollow with negligible increase in weight.

I understand the Bernoulli effect does not necessarily apply to the RC rotors and it may be simply a Coanda effect but they do seem to lift well with just electric motors.

I have seen discussion that there is a "scale up" effect but why and if so at what size does the simple rotor start to fail... one foot, three feet, ten feet? Is there any data to indicate when the transition from this simpler design to a fully extruded solid rotor is required?

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    $\begingroup$ Welcome to aviation.SE! I think I understand your question, but adding images (or links to some) would be very helpful $\endgroup$ – Pondlife Apr 4 '17 at 18:21
  • $\begingroup$ One factor that comes to mind is that in a full-sized helicopter the leading and trailing rotors have to have different angels of attack depending on the helicopters velocity. While that may be possible to accomplish on an RC helicopter, the cost would be prohibitive. $\endgroup$ – BillDOe Apr 4 '17 at 19:24
  • $\begingroup$ Thank you for your feedback. The first and second paragraphs are really the same question from different aspects. Why does the typical curved plastic blade of an RC helicopter not scale up to larger helicopters or conversely why don't RC helicopters not have aerofoil shaped rotors like larger helicopters of they are the most efficient shape. If the explanation is that there is some effect that "kicks in" as the scale increases, what might that be and when does it kick in? $\endgroup$ – C Aerospace Apr 4 '17 at 19:28
  • $\begingroup$ Thank you I agree there would be an issue of differential lift if the RC helicopter moved forward quickly enough in a single rotor design but it is not the shape of the rotor that causes that. Many are coaxial to solve the problem just as with some full scale helicopters. $\endgroup$ – C Aerospace Apr 4 '17 at 19:34
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If the rotor was mechanically strong enough to lift the desired weight

It wouldn't be. Because of the square-cube law, lift and mechanical strength grow with second power of linear size, but weight grows with third power. The result is that at RC-scale, everything has plenty of power and strength even if made sloppily (and thus cheaply) from common materials while human-carrying-scale is really stretching capabilities of the materials and engines we have.

A highly cambered airfoil would not be strong enough for full-scale helicopter blade.

is there any (aerodynamic) advantage to the solid aerofoil over the simpler curved surface?

I believe there is. No modern aircraft uses highly cambered airfoil.

Surely toy manufacturers could "close the bottom" of the rotor (equating to the aerofoil shape) even if it was hollow with neglible increase in weight.

No. Efficiency is not that important at RC scale and hollow plastic is much more difficult to make (remember, plastic is made by injecting it into a mold).

I understand the Bernoulli effect does not necessarily apply to the RC rotors

Airfoils is an airfoil whether it is flying straight or rotates. And since Bernoulli's principle is just conservation of energy for fluid flow, it always applies to it. It does not, however, mean concave lower surface would mean higher flow speed and lower lift, because the length of the path has nothing to do with lift. The air above the wing is faster—and reaches the trailing edge long before the air below—for completely different reasons. Highly cambered wings do have higher lift. They just have even higher drag.

it may be simply a Coanda effect

Coandă effect is about a fluid jet surrounded with still fluid around it, but there are no jets of air involved here, so it can't be Coandă effect.

but they do seem to lift well with just electric motors

That's the square-cube law again making the aerodynamic performance so much better relative to weight at RC scale.

I have seen discussion that there is a "scale up" effect but why and if so at what size does the simple rotor start to fail

It is not just scale up (square-cube law):

  • Quad-copters with fixed pitch rotors (typical of the toy ones) are not controllable with failed engine. Human-carrying craft have to be.
  • Fixed pitch rotors deal poorly with horizontal speed, because the lift asymmetry creates big loads. Large rotors need to lead-lag and flap to compensate for this.

Both making quadcopters impractical at large scales.

Usually, quad-copters are made up to low tens of kilograms maximum weight. But where exactly is the limit might be hard to tell since larger ones may not be made simply because there is not much use for them.

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  • $\begingroup$ It should be taken into account that the centrifugal force adds rigidity to the blades. In the early blimps, sometimes the propeller blades were made of canvas, and they assumed its 'working shape' only when spinning... $\endgroup$ – xxavier Apr 4 '17 at 19:44
  • $\begingroup$ @xxavier, I don't think that would have effect on the cross-section of the blades (on helicopters) though. $\endgroup$ – Jan Hudec Apr 4 '17 at 19:46
  • $\begingroup$ No, but it's not necessary to build the blade of any highly resistant material, as it will work in tension only. $\endgroup$ – xxavier Apr 4 '17 at 20:00
  • $\begingroup$ Thank you again. Do we know at what point we have to transition from the highly cambered aerofoil to the solid rotor? Also, not trying to be contraversial, but when I was at NASA I saw some compelling data that the Bernoulli effect is in fact minor and it is actually the air "thrown down" by the Coanda effect that actually causes lift in almost all wing shapes. If so, then there would be no advantage to extruding a plastic wing over making a simple curved blade. I agree with the comments about fixed pitch but again that does not relate to rotor shape a fixed aerofoil would suffer the same. $\endgroup$ – C Aerospace Apr 4 '17 at 20:03
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    $\begingroup$ Unfortunately, the paper I'd reference for this is behind a paywall, but there has been a significant experimental and computational study done showing that, under a Reynold's number of ~80,000 or so (which the rotors on some of these quadrotors fall into), a 3-6% thick, cambered flat plate is actually substantially MORE efficient (in terms of lift to drag ratio) than a traditional shapes. However, I don't think this has been the actual design motivation. The paper is here: vtol.org/store/product/… $\endgroup$ – Marius Apr 4 '17 at 22:39
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The two main differences are disk loading (lift per rotor disk area) and Reynolds number. The Reynolds number is the ratio between inertial and viscous forces.

Model helicopters operate at Reynolds numbers typical for small birds. If you look at their wings, they have a thin, cambered cross section. At this scale, loads are low and so they can afford to have thin wings. This reduces the acceleration of the flow from the displacement effect and helps to delay flow separation because the pressure gradients for the same lift are lower. At the low Reynolds number of small model helicopter rotors, the boundary layer will be fully laminar.

Full-scale helicopters, on the other hand, need much stronger rotor blades because of their higher disk loading and centrifugal loads. They cannot afford to use the thin airfoils of small birds, and the boundary layer can tolerate much higher pressure rises because the operating Reynolds number of a full-size helicopter rotor is large enough to ensure a turbulent transition.

Compared to wings, rotors enjoy a huge advantage: Flow separation is almost impossible. Centrifugal forces will push any slowed-down boundary layer out, where the higher circumferential speed will ensure that the flow at the wall never comes to the standstill required for flow separation.

Another reason for the flat shape of especially cheap indoor model helicopters with injection molded blades is the lower material use and shorter cycle times possible with a thin blade - they are much cheaper to produce.

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  • $\begingroup$ Thank you. Some work done at Langley in the 80's showed that the Reynolds number is always above critical ( millions) in rotors but that scaled models could be used to predict full scale very accurately. The question I have is where do we transition from Hummingbird wing to Blackhawk rotor. $\endgroup$ – C Aerospace Apr 5 '17 at 17:30

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