Rotor dynamics: Reverse Velocity Utilization by Traversing Root Pitch Axis

[Dave_Jackson]

This posting is to place the following invention into the public domain; so that anyone may use it.
http://www.UniCopter.com/1516.html

_______________________

Two rotorcraft concepts that might make use of this invention are;

Helicopter - Intermeshing - R/C Electric - UAV
http://www.UniCopter.com/1515.html

Helicopter - Interleaving - R/C Electric - UAV
http://www.UniCopter.com/1512.html


Suggestions or criticisms appreciated.

Dave J.

[MD900 Explorer]

Dave Jackson

To provide a simple means for the root end of a blade to utilize the reverse airflow, when it is on the retreating side during forward flight

I am not very theoretical in this area, but can i deduce from what you are saying in your propsals that you wish to design a blade that will behave the same characteristics in an advancing blade as to a retreating blade, therefore not causing any flapping in the relative angle of blade movement?

Would not your design, with the formula for lift, not give extensive vibration due to the moving "Spar", even with dampers? The shape does not even look close to a normal aerofoil, so how can the inflow angle produce enough lift to make the blade productive enough even in your coaxial example?

MD :(

(a mere tadpole in the atlantic....:eek: )

[Graviman]

Well, i think this must be the Dave Jackson School of how to make a simple thing complicated! :E

MD900 Explorer, the shape is to allow the blade root to slip both ways through the air. It means that the reverse velocity portion at the root of the retreating blade, in a high speed heli, does not present unwanted drag. The application is for a counterrotating helicopter so there is no need to maintain symetrical lift over advancing and retreating sides (intermeshing or interleaving in this case).

Dave, from the reliability point of view a simpler idea is to have the main spar in the centre, and design a torsionally stiff blade to avoid flutter. If independant root&tip is the objective, this allows a simple connection to tip. My worry is that you are introducing shear flexure in the blade plan, in addition to the shear required for torsional flexing. Fatigue is an engineers worst knightmare, since you don't know it's there until it's too late. FE, goodmans curves, and knowledge of true fracture stress help but the only real proof is durability testing to design life (ie LOTS of rig testing).

Again, the real problem here is what happens at the zero velocity circle. The blade would ideally take on a kink to present the correct spar position to the air flows either side of the circle. This introduces a lot more strain than simply designing the blade to ~linearly twist over it's length. This does not begin to cover the blade flexure eigenmodes you may be introducing, or the wear that you may experience as the blade "bangs" from side to side. A coffee breaks worth of cost/benefit analysis tells me it would introduce more problems than it would solve - incidentally these types of difficulty are why i favour feathered retreating...

Still, build it and prove me wrong! :ok:

Mart

[MD900 Explorer]

Graviman

Thanks for the explanation. Still way over my head though.

Dave

Good luck building it. :ok:

MD :oh:

[Dave_Jackson]

MD900 Explorer;

Your concerns and those of Mart are valid.

The intent is to improve the lift of the retreating blade, however, not to the point where it equals that of the advancing blade. The actual shape of the airfoil would not be symmetrical as is shown in the sketch. It would be shaped to favor the advancing side. Perhaps similar to this.http://www.unicopter.com/1485-3.gif

Yes vibration may be a serious problem. My thinking (or hoping) is that the individual blade will experience vibration but when it is combined with 3 or 4 additional blades on an 'absolutely' rigid rotor, the vibration at the rotor's mounting point will be significantly reduced. Further vibration reduction should come about by the combining of this rotor with a counter-rotating second rotor. The vibration that gets through to the fuselage might be very small.


Mart,

An intention is that the cyclic and collective only control the tip end of the blades. The pitch at the root end is determined solely by aerodynamic drag. The twist will be linear. This may not allow for autorotation, but in the following application autorotation will not be required;--

This idea may only be good (if it is any good at all) for small UAVs; not for manned craft. A couple of reasons for its possible viability in UAV's could be the high frequency of the vibration and the reduced safety factors that will be necessary.

The future will probably see large quantities of small semi-autonomous UAV's. UAV's that do not fire weapons but are the weapon, and are expendable. With quiet electric drives and high-speed flight, they will explode at the desired location and the components of the craft will be part of the shrapnel.


Dave

[Graviman]

An intention is that the cyclic and collective only control the tip end of the blades.

Wowsers, that makes things harder aerodynamically - I can see the point though. Are you so convinced that a double swash plate is unfeasible? I envisioned the pitch control system as a sum and difference design, with pilot controlling the sum (ie root&tip average) and control system deciding the difference. Control system is the hardest aspect, but i agree that a simple mechanical solution should be possible.

One suggestion might be pilot collective controls advancing root, and cyclic controls advancing&retreating tips. Retreating root is either feathered or pitched opposite to advancing root - feathering sounds a lot easier in practice, but here is where the control comes in. High speed flight then just involves lowering collective, the same way you trim forwards in fixed wing - in fact you could think of the tips as ailerons.

Plenty of research for somebody (probably based in NY ;) )

The future will probably see large quantities of small semi-autonomous UAV's.

Interesting, and for this application i actually get the electric powertrain for short range usage. UAVs are never as elegant as manned flight, but whatever pays...

Mart

[MD900 Explorer]

Dave

If you are not using a double swash plate, are you using the spider system of pitch control? Are you considering feathered blades or are you going to have non feathered blades?

Perhaps (And i have to say i am shooting in the wind here), but what about a single control from the cyclic like aerobatic planes, making more movement from the one control? :confused:

MD :O

(just thinking aloud) :ooh:

[Dave_Jackson]

MD,

The initial thinking was to simply control the blade tips in a conventional manner. A single swashplate or spider could be used. Rotating the spar would change the tip pitch. The root pitch would be an aerodynamic controlled positive or negative adjustment to the tip pitch, depending upon whether the root was experiencing forward or reverse velocity.

This is a crude preliminary consideration of this subject.
http://www.unicopter.com/1516.html#Pitch_Angle

What are you contemplating when you mention feathering the blades?

I am not sure what difference you are considering when you mentioning the stick in an aerobatics plane versus the cyclic in a helicopter.

Dave

[MD900 Explorer]

Dave

What are you contemplating when you mention feathering the blades?

In a rigid rotor system the blades, hub, and mast are rigid with respect to each other. In this system, the blades cannot flap or drag but can be feathered, this is what i was referring to. Probably trying to get at wether the blades will be twisted or flat? Also why have a rigid system and not allow the blades to flap and drag, why not have a semi-rigid system...?

I am not sure what difference you are considering when you mentioning the stick in an aerobatics plane versus the cyclic in a helicopter

I guess i was starting to talk out of my back end. You mentioned having two pitch links to control the root and tip of the blade, but what i lost in translation was that i thought you needed a control that had more function than a normal cyclic, so i suggested the dual role cyclic of the aerobatics plane which i believe does collective and cyclical functions. (This is starting to make me dizzy.:confused: )

MD :uhoh:

[Dave_Jackson]

MD,

There are a number of advantages in having the rotor as rigid as is possible. This is particularly true for the Intermeshing and Interleaving configuration, where the blades of the two rotors operate in relatively close proximity to each other.

Mart also prefers the 'feathering' of the roots of the retreating blades whereas I would like to put them to work.

Dave

[Graviman]

Mart also prefers the 'feathering' of the roots of the retreating blades whereas I would like to put them to work.

Dave, by feather i just mean going with the local airflow. My arguement is that in an intermesher the retreating portion is already covered by the advancing side of the other rotor generating lift/downwash. The ideal will be somewhere in the middle (with TPs providing most of the flying quality feedback) - like most things in engineering.

BTW two additional thoughts:

1) The control system could consist of little more than a paddle to respond to the local downwash air velocity (forward/rearward component).

2) Gyro rate feedback on each rotor would reduce pitch/roll vibration. The rotor assy will be decoupled through an elastomeric mount assy. The gyro would effectively cause higher order inputs to cancel pitch/roll vibration.

I have to admit to understanding why Sikorsky are tackling the X2 with FBW though...

Mart

[Dave_Jackson]

Mart,

The control system could consist of little more than a paddle to respond to the local downwash air velocity (forward/rearward component).
Here is an option for controlling the blade's traversing of the spar. In addition, it might be more appropriate for a paddle then the original idea. http://www.unicopter.com/1516.html#Option


Regarding the use of a gyro and fly-by-wire or fly-by-light;

IMO all of these will be part of the future. However, the major expenses of FBW must be the cost of the sensors and the actuators, which includes the wiring etc. etc. The cost of the CPU is probably insignificant.

The other high cost will be the programming. If the coding is written off against 100 million computers it would also be insignificant, but when it must be written of against 1 thousand helicopters it will be expensive.

IMHO, improving the aerodynamics will significantly reduces the required number of sensors and actuators etc., PLUS it will give the craft a more efficient L/D ratio.


To utilize gyros, sensors and actuators, and also improve the L/D, why not give the horizontal stabilizer a positive pitch and stick a canard (with its positive lift) out the front? http://www.unicopter.com/1482.html ;)


Dave

[Graviman]

Here is an option for controlling the blade's traversing of the spar. In addition, it might be more appropriate for a paddle then the original idea.

Well, this does make more sense. You are not dependant on highly stressed sliding contacts, and have a torsional element that controls the tip. Obviously you will need some I-spars at each end of the slot to take the main blade bending loads. High density nylon bushes along the torque tube would also make sense.

What i am still concerened about is the dynamic behaviour of this system. Potentially you are asking the blade to deform in a complex manner, which while it can be fatigue proofed may suffer aeroflexure. I am still not totally convinced about the idea of just using the tips for collective control either. By the time you include root control i just see the complexity getting too much. This is why i favour simple torsional flexure, with at least partial feathering.

-

Agree with your points about FBW. My comment really comes from the complexities required to make a counterotator fly forwards, backwards and sideways. Making the machine hover, and fly forwards efficiently is one thing. Getting it to work consistently in other flight regimes would push a mechanical system. I can just see the benefits in a coaxial ABC with independant tip&root control.

For the purposes of this dicussion a mechanical solution is preferred.

To utilize gyros ... horizontal stabilizer a positive pitch and stick a canard out the front?

Aye, complex wee mechanical beastie though...

Mart