Why are RC rotor blades different from helicopter blades?

Question

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?

Answer 1

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.

Answer 2

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.