2-Bladed Rotor Ground Resonance
 

All:

I have a fair amount of career experience in helicopter dynamics and yet I'm stumped. This is in the weeds, but I'm hoping some of you can point out what I might be missing.

Helicopters with articulated rotors (usually 3+ blades) can be susceptible to ground resonance and therefore have damping elements at the rotor (lag dampers) and in the airframe (oleo dampers, etc). 2-bladed rotors are usually teetering and therefore are not susceptible to GR. However, many RC helicopters have 2-bladed articulated rotors with lead lag hinges but no dedicated blade lag dampers or skid/airframe dampers as far as I can tell, and yet GR events are rare to non-existent. Does anyone know why this may be the case?

GR should be possible on an articulated 2-blade rotor, even if the fuselage and rotor frequencies are well separated. It is surprising to me this seems not to be the case even with no dampers present. Any insight would be much appreciated!

Thanks,
Chris

It happens with low rotor rpm setups, like t-rex 600 run at 1000 min-1. It also happens in hover in that combination and a training cross.

In my opinion GR happens due to the hooke's joint effect causing the center of gravitiy of the rotor system (3 blades or more) to move around.

Really interesting question. But post #2 says it can happen on a 2 blade RC model and a quick search on internet even gives a video demo of it on a Raptor 30:


The natural frequency on the RC model looks to be quite high compared to my memory of seeing the onset of ground resonance on a full scale Squirrel many years back.

There must be mass and stiffness scaling effects that come into play when considering RC models and their full scale counterparts. For roll motion of a helicopter on the ground, natural frequency is determined by the square root of (roll stiffness / roll moment of inertia). I suspect skids of a model helicopter are relatively much stiffer than their full scale counterparts. The roll moment of inertia probably remains reasonably in proportion with the size of the model or helicopter. So that would be consistent with model natural (or resonant) frequency being higher than on full scale.
The blade grips on a model helicopter not only allow lead and lag movement of the rotors, but I guess also provide some friction damping. Didn't really old generation helicopters also have friction dampers for lead and lag blade movement?
Aside from the model blades getting stuck in their grips so they are not exactly separated by 180 degrees, there could also be imbalances in the stabiliser bar if not set up properly. If anything, I might have expected ground resonance to be more common on models than (airworthy) full size helicopters.

Can't say I know of any real 2-blade helicopters that allow any amount of lead/lag - all are rigidly fixed opposite their mates. Can't allow for shuffling out of position like 3-or-more can.

Apparently it occurs in models?

"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.

To my knowledge, you can get ground resonance from two scenarios - one being an out of balance rotor system (seized or faulty drag damper or similar) and the other being when the blade passing frequency excites the natural frequency of the undercarriage. You can of course have the two combined.

I have experienced GR due to uneven tyre pressures on a Wessex during rotor start which therefore affected the natural frequency of the undercarriage and I know of a couple of occasions when a drag damper lock wasn't removed before start up leading to a very unpleasant rotor start.

Normally the designer goes to some lengths to avoid any natural frequencies of the rotor system stimulating those of the undercarriage but I presume the same effort isn't made for RC models.

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.

R.W. Prouty - Helicopter Aerodynamics

I’m a bit embarrassed to say that quote is 99% as true today as when Prouty wrote it. Some things never change.

RVDT, it’s likely the Bob Wagner you know may have worked at SA under an assumed name, as we saw several instances of tail natural frequencies matching rotary equipment naturals or harmonics. Used to be a common flight test bit of humor.