Head Speed Sweet Spot - What's the ideal??

[LoopBaCK]

Just sitting here at work... not doing work... but thinking about head speeds. :cool:

Where is the sweet spot as far as head speeds are concerned in AP/V. What is the ideal (whether or not we can do it)? Would a slower head speed be the goal? And if so just how slow?

I was thinking about my Army days working on AH-1F's and the cord on those blades was huge (2.64 ft) which caused the main rotor speed to be relatively slow.

So... which is the goal: slower or faster? And to what degree?

[AZ ChopperCam]

depends on the size of chopper but for 50-90 size I'd say in the 1400-1700 range.

[LoopBaCK]

Why does it depend on size - Unless main rotor speed is directly proportional to mass of vehicle? I'm speaking conceptually - not about a specific RC heli. However I expect main rotor configuration has a lot to do with it.

Again referring back to the Cobra helicopter. Huge chord on main rotor but still underpowered (I worked on the AH-1F). Our pilots always had to trade off fuel or armament. To lift a fully fueled, fully armed cobra the engine needed to be stronger. It just couldn't maintain a high enough main rotor RPM for the necessary collective pitch for flight. The Marines took care of that by adding a second engine and BAM! Fully fueled, fully armed... let's go fly!

Overall I was just curious about using a large chord main rotor to lower the necessary RPM's and achieve flight - possibly creating a more stable platform.

[tcgliderguy]

I suspect that this all relates to Reynolds numbers.... the same "fudge factor" that differentiates airfoils and wings on fixed wing aircraft.

The "Readers Digest" version of Reynold's number is just the number of air molecules that are acting on a wing... or a rotor blade... in a given unit of time.

Fly faster (or spin the rotors faster).... and more air molecules come in contact....... Use a wider chord wing (or rotor) and more air molecules interact with the surface.... Higher Reynolds number equates to greater efficiency. (Airliners, with Reynold numbers in the hundred of millions are able to fly at wing loadings of thousands of pounds per square foot... Model airplanes, with Reynolds numbers in the thousands, fly at a few ounces per square foot....)

Does this make sense?...... In other words, it is a trade-off between rotor speed, blade area (and I suspect disk area enters into somehow too....)

-Taylor

[MLaBoyteaux]

I think you also need to consider what's attached to the heli, especially when the mount is attached with bungees. The heli and mount work as a system and each piece has it's own natural frequency. Get the headspeed in resonance with a major structure and the vibration builds.

My Condor looks silky smooth by itself at most any headspeed. But with my video mount attached, there are small oscallations except for right around 1300.

My E-Raptor is the same way. Smooth sweet spot at 1650, but above or below it and I can see a slight wobble in the gear legs, but only when they're retracted.

Moving the gear legs up to the retracted position changes the tuning of the system.