Ground Resonnance

[Victor Duarte]

hi,
some started to talk about ground resonnance in another thread.
i feel that the 3 or more bladed rotors are in the mood, but that they have very specific problems that are avoided in teetering rotors, like ground resonnance.

isn't there several kind of resonnances packed in the term "groung resonnance" ?

- an in-plane resonnance dur to the lead lag hinge without damper ?
- a vertical resonnance, due to interaction with tyres and the blades bending ?
- a combination of these two ones that lead to a "tango" oscillation ?
- some resonnances due to the coupling of : engine vibes, tail rotor vibes, mast damping ?

I, and others, i guess, would love to read more from knowledgeable people and their experience.

thanks a lot

[Rotor Rooter]

zeeoo,

The page Flight Dynamics - General - Ground Resonance (http://www.unicopter.com/1246.html) should have some of the information that you are looking for.

If you use the Teetering Rotor with 3-blades, Constant Velocity Joint and Hub Springs (CVJ+HS) (http://www.unicopter.com/1377.html) you should be able to significantly reduce, or eliminate, the lead/lag. ;)

Dave J.

[Victor Duarte]

Thanks dave,
your CVJ head is really interesting (like all your work), but, IMO very complex to do for a non-pro building.
Chuck Beaty's attempt to build a 3 bladed rotor is really much simpler : the hub is "suspended" by 3 rubber bushings, so, the hub is aligned with the plane, almost the role of your CVJ head.

your links about ground resonnance are very interesting.
Thanks

[Rotor Rooter]

Hi zeeoo,

The following is submitted for the fun of discussing rotorheads. The comments are strictly theoretical and may be open to argument.

You said;
"Chuck Beaty's attempt to build a 3 bladed rotor is really much simpler : the hub is "suspended" by 3 rubber bushings, so, the hub is aligned with the plane, almost the role of your CVJ head."


I found posts by Chuck Beaty on the thread 'Ideal gyro blade design' and I assume that this is what you are referring to.

He said;
"When the hub can float so as to self align with the rotor tip axis, flap and drag hinges aren’t required.

The hub wasn’t free floating; it was attached to the rotorhead via rubber bushings, providing control power similar to offset flap hinges."

His first sentence is quite clear. Restated in different words: When the hub (mast) plane and the tip path plane are parallel, there is very little lead/lag between the blades because cyclical Coriolis is not taking place.

The second sentence needs a little elaboration, since this is where there may be a misunderstanding. Chuck mentions that the rubber bushings act as offset flapping hinges. These rubber bushings can also be referred to as hub springs (http://www.unicopter.com/B329.html#Hub_Spring ). As such, they resist the tipping of the disk and thereby cause the blades to 'flap' (bow). This results in the hub plane and the tip path plane not being parallel; and this results in cyclical Coriolis; and this results in lead/lag. :eek:


Dave J.

[Brian Jackson]

Interesting discussion. I'm hesitant to interject here because I know so little of the subject. But if I may ask, what is the primary purpose of Lead/Lag hinges? My current understanding (perhaps incorrect) is that they allow each blade independently to be dampened in its fore/aft movement when acted upon by aerodynamic forces. As such, when one blade is out of step with the rest at any given moment, vibration is introduced because their collective center of gravity is not in-line with the hub.

If that is indeed true I can see why it's so destructive when the heli is on the ground because the airframe isn't free to twist about its CG to compensate (equal and opposite), and the gear takes all the stress of the blade imbalance. So the system fails when a harmonic gets amplified (compounded) by subsequent vibrations at the same frequency.

If I'm way off please correct me... I'm just a newbee trying to wrap my head around this subject.

Thanks,
Brian Jackson

[C. Beaty]

Here’s what lead/lag hinges, in conjunction with flap hinges do:

[Al_Hammer]

Below is an animation showing the coupling between two shafts running at an angle to one another. This represents the situation in a two-bladed teetering rotor where the rotor blades and rotor head can be at an angle during flapping.
WIth three or more blades, this joint will not suffice, and lead lag hinges need to be incorporated if the rotor is to move in a plane different from the rotor head. It's just a question of kinematics.

In modern designs, lead lag hinges can be eliminated. The Bk117 and B0105, as an example,use a forged titanium head with no hinges except feathering hinges. Pendulum dampers are used to absorb vibration, but there are no lead-lag dampers. Note in the photo that the blade is bolted with two bolts and is not free to swivel. Elastomeric materials are used to absorb the movement. Here's another example: http://www.b-domke.de/AviationImages/Rotorhead/4235.html

[BenMullett]

Great posts - I found the NACA paper (TN 1184) at http://naca.central.cranfield.ac.uk/report.php?NID=2707

But it is a bit dry! ;)

What I recall is that the landing gear is part of the resonance,
and it is vital to maintain any oleo struts as well as the hub
dampers - eg on the Air & Space 18A.

The crude sketch shows the principle - a pair of the three
rotor blades is closer together than 60 degree spacing, and the
CG of the rotor is offset. When it rotates the vibration flexes
the landing gear. The wrong combination of flexure and
natural frequencies leads to a whopping resonance that can
destroy the aircraft beyond economical repair. I saw some
accident reports of an 18A being ruined this way, years ago.

Dampers help prevent the resonance build-up.

Hope that helps, Ben

PS X-Plane will enter a brief resonance on some gyros when performing a
one-wheel landing in a crosswind.