Originally Posted by
paco
"2-bladed rotors are usually teetering and therefore are not susceptible to GR"
Teetering heads don’t have dragging hinges because they are actually the most susceptible to ground resonance. It is because articulated rotors have dragging hinges that they become a problem.
Apologies - I was imprecise with my wording. Meant to say that 2-blade teetering rotors don't have lag hinges (I don't know of any examples), and therefore, because of no lag hinges, do not experience ground resonance.
I appreciate some of the ideas thrown around here. Just to address a few points above for interest, 2-bladed articulated rotors at full scale are rare - but they have existed. Most were of the autogyro variety back in the early days of rotorcraft development. The Pitcairn PA-22 is an example. A photo is included in a NASA report discussing the history of the autogyro (NASA/TP-2015-218714) if you are interested - I tried to include a screenshot here but wasn't able to. You can also google it, but you'll see both 2 and 3 blade variants. That being said, at RC scale, there are plenty of 2-blade articulated designs, and I've also begun to see more and more in the e-vtol space at "small/medium" scale also with no lag dampers. By small/medium, I mean in the 50-500 lb weight class with rotor diameters anywhere from ~5-15 feet.
It seems there are more ground resonance events in the RC space than I'd been aware of, but they also don't look to usually be catastrophic. Complete guess here, but I'd assume that the reason for not including dampers at the RC scale is that:
(1) If GR occurs, its probably not going to be a safety hazard
(2) its less likely to destroy the helicopter instantly (smaller scale things are usually more robust than larger things due to size/scaling effects
(3) adding damping adds cost/complexity, and if #1 and #2 are true, no reason to add cost/complexity to the product.