"Upside down" teetering rotor

[alwynhartman]

What would the theoretical flight dynamics be if (in a two bladed head) the teetering hinge was upside down so that the pivot lies below the blade grips (as measured from the ground) rather than above it?

[alwynhartman]

Aside from the obvious complication of the rotor teetering over to its extreme angle when it slows, and lets just suppose that it can be built not to do that during start-up and shut down, how will it perform in flight?

[Nf stable]

It would get the opposite effect of the Bell/Robinson sytem in regards to Hookes joint effect (or conservation of momentum, depending on ones point of view), where the theoretical lengthening/shortening of the blades through their cycle would be working in the wrong direction.

[alwynhartman]

Maybe I dont fully understand the Hookes Joint effect but if I understand it correctly then in the Bell / Robinson system the following occurs (please correct me if Im wrong): When the advancing blade attempts to flap upwards, the position of the teetering pin causes the blade cg to move away from the rotor mast / drive shaft. This blade will try to lag (or slow down like an ice skater opening her arms in a spin) while the other blade experiences the exact opposite effect and thus attempts to lead (accelerate).
The lagging force experienced by the advancing blade will work against the force of the rotor mast trying to rotate the blades through the air while the leading force of the retreating blade will then cancel this by applying an equal and opposite force to the rotor mast (in theory right?).

If the pivot is placed lower than current designs, this arrangement will be opposite with regards to the leading and lagging forces of the advancing and retreating blades, yes, but how will this affect flight performance?

[paco]

The C of G being below the pivot point (plus preconing) means that the C of G of the blades does not move too far inboard as they flap up, ro reduce the coriolis/acceleration effect. I can't see this working upside down.

Phil

[Ascend Charlie]

You will need some seriously strong static stops, because instead of the hub hanging below the pivot and only occasionally trying to rest on the mast (when shut down but not tied down), the hub will swing down in either way until it hits the stop forcefully and stays there.

Why would you even contemplate it? It is like balancing an axe on your finger, it will get you eventually.

[alwynhartman]

I see what you are saying AC but the idea is to try fathom how flight with such a system might feel.

The idea came as a comparison of di-hedral vs an-hedral in fixed wing aircraft. Anhedral is always unstable as the rolling axis is below the dynamic cg of the aircraft. Where high wing dihedral aircraft usually have the rolling axis above the dynamic cg. Both cases are for normal level flight. Fighter aircraft like anhedral as it makes the craft quick and nimble in the air but a slow trainer would benefit from the almost auto stabilizing effects of dihedral.

Thus, could it be imagined, that a Bell / Robinson type head sports a force equilibrium similar to that of anhedral forces on a fixed wing craft? So, in effect, is the current setup not perhaps more like trying to balance an axe on you finger (during flight)?

[TRC]

I always thought that a conventional underslung teetering rotor was the equivalent to a dihedral wing.

Wouldn't an overslung teeter equate to anhedral and be inherently unstable?

Let's face it, basic unstabilised helicopters don't want to fly straight and level at the best of times - why make it even more complicated.

[alwynhartman]

I think I may have answered my own question.
Correct me if Im wrong but it seems that as long as the center of pressure / center of lift, as measured over the span of each blade, is higher than the fulcrum then the system will be inherently stable. Is this the case with most two bladed helicopters? I suppose it has to be.

If I take, for example, two pieces of string and tie two of the ends together and then tie that knot to the top of a stick with a ball of clay at the bottom then, when suspended by the two free ends of the string pair, the system will always try to distribute the dead load (stick with clay or fuselage) between the two strings equally (in this case the two free ends of the string pair represent the center of pressure as acting on each rotor blade). If the two fixed ends are now tilted then the high side will become more loaded than the low side and will thus try to rectify itself by returning to a state of equal load between the two strings.
If however I replace the string with two solid skewer sticks and fix their free ends on a plane in space lower than the point where the stick with clay attaches then the system will be unstable. As the plane in space is tilted then more of the weight from the stick with clay will be distributed to the low side. The system will still be in balance but the cg would have changed to an aggravating condition of instability rather than a self rectifying condition.

So, if I have an underslug teeter block then I will have the advantage of rotor stability during low rotor rpm. However, a self stabilizing condition will only occur in flight if the triangle of forces is, in effect, upside down forming a cone of which the origin is on the teetering pivot and the base is on the same plane as the center of pressure between the blades (which needs to be higher than the pivot).

Does this sound about right? Critique is welcome.

[hillberg]

Completly wrong.:{ You will lose the rotor system from the dynamics in opperation.:cool: Whirl mode failure . :eek:(You fly fixed wings?)

[alwynhartman]

Hillberg, would you be so kind as to clarify perhaps?

[hillberg]

No need, Look at what was designed by the industry, In use now & in history, NACA/NASA documents of just that idea. (Pop Emigh) Had a design that would "self destruct" when the rotor tip path would dip below the C/L of the gimble/teter bolts.
When the center of lift or unloaded system would flap and move beyond the center of mast rotation you would get an unballance that would rip out the TXSN structure. Pylon whirl is dampened by good design, hit the books.

[alwynhartman]

Hillberg, thanks for that! History as a tendancy to repeat itself but thanks for letting me know the research has been a documented failure! I was on my way to constructing a test piece and doing my own basic research. You wouldnt perhaps have a copy of the report on hand would you?

I feel like an idiot for not being able to see it but even Igor was laughed at for refusing to phase his swashplate through 90 degrees!

[hillberg]

Try looking in the NASA technical report server,:} It's a public avialable source of reports on anything flying.:cool: Most of my stuff is in storage at this time.:ugh: and the reports are down loadable at no charge.:ok:

[Evil Twin]

?????? WHY ?????

Reinventing the wheel here aren't we?

[alwynhartman]

Perhaps, but the scientific mind yearns to explore

[FH1100 Pilot]

The idea came as a comparison of di-hedral vs an-hedral in fixed wing aircraft. Anhedral is always unstable as the rolling axis is below the dynamic cg of the aircraft. Where high wing dihedral aircraft usually have the rolling axis above the dynamic cg.

Uhh, that's not why airplanes with dihedral are more stable than airplanes with anhedral. You need to research how real aircraft fly, not RC models. The truth is out there.

[hillberg]

Got to learn what was invented before.:ok: Old wheels are the stepping stones to New wheels .:eek: Don't want to reinvent a broken wheel.:ugh: