General Rc Blades And Speed

[Alex Evans]

I am new here, but coming from a background of full size....

Let’s use a Trex-600 ESP as an example, and I am using round numbers here

A main rotor blade is 60cm long, Mid blade is 30cm from the root

At an RPM of say 1,500. The midpoint travels a total distance (in one minute) of

30 x 2 x 3.14 x 1,500 = 282,600 cm or 2,826 meters, or 2.826 Km, therefore x 60 = 169.56 Km/h

At the same time, the blade trip is travelling:

60 x 2 x 3.14 x 1,500 = 565,200 cm or 5.652 Km, therefore x 60 = 339.12 Km/h (twice the speed!

No point here to get into the exact amount of lift generated (too many parameters), but one thing is clear, the lift is increased by the power of 2 for each airspeed.

Then, why in full size helicopters, at zero pitch when collective is down at the blade root is at zero degrees, the blade tip is at MINUS 6 degrees (yes, the blade is manufactured with a twist) but in our sport of RC, this is ignored….

Just wondering....

 

[Smoggie]

Then, why in full size helicopters, at zero pitch when collective is down at the blade root is at zero degrees, the blade tip is at MINUS 6 degrees (yes, the blade is manufactured with a twist) but in our sport of RC, this is ignored….

Just wondering....

Interesting question.

The answer needs an understanding of some aerodynamic principles. For optimum efficiency the lift distribution over the rotor span should be equal, so that the air is being accelerated downward equally over the full area of the rotor disk. As you point out, the tip travels faster so would tend to produce far more lift. To try to equalise the lift distribution, full size helis use some twist so that the tip has a lower angle of attack than the root. As a matter of fact you will see the same thing in most fixed pitch RC helis (and on RC plane propellers) for the very same reason.

Collective pitch RC helis are a different beast. Unlike full size helis or RC fixed pitch, they are designed to be able to fly upright or inverted. If you had twist on the blades to be optimum for upright flight then when inverted the twist would be the 'wrong way'. The best compromise for a heli that can fly both upright and inverted is 'no twist', so that's what we have. If you wanted to make a blade that gave optimum efficiency and give maximum flight time but was not suitable for inverted flying it would be much like a full size heli blade, with twist.

 

[murankar]

Glad he got it answered for ya. I was lost on Some sauce on that one. I probably would have figured it out (like in 10 years).

 

[Alex Evans]

Thanks Smoggie,
As a Tame flyer, I don't think about inverted flight, ever...
I was a Helicopter flight engineer, I know th etheory well and was just wondering
Now I see the point.

Cheers

 

[D.O.G.]

You guys got me lost