Homebuilt Coaxial Helicopter: Ben Dixey, SW UK

Aerofoam

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I can't agree with that description. Lacking a swash plate is not the same as weight shift and doesn't mean that the control is not aerodynamic.

But you can't dismiss that you are shifting the weight under the rotor and also due to the inertia of the pendulous mass, mechanically tilting the rotor plane.
I'm not saying there isn't something aerodynamic going on, but I would love to quantify what is going on!
 

Aerofoam

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What if you took something like this Nemeth Round Wing and put it on a gimbal on a trike airframe with a triangle control bar?
Is it weight shift, tilt wing, or both?
There is no real aerodynamic enhancement going on with this airfoil, on a gyro rotor, the dihedral (coning) would provide some additional
tilting augmentation.
If you want to make your brain hurt, think about building this wing as a hollow structure, then installing a rotor bar inside with tip
weights to simulate the gyroscopic forces in a gyro plane, would it fly like a gyro plane?
 

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Aerofoam

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Here is a hybrid of both!!!
 

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Doug Riley

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The "controversy" over whether a given system of lift vectoring is weight-shift or not results from imprecise application of Newton's laws. Weight shift vs. aerodynamic control is actually a matter of degree. It is, to borrow a trendy phrase, "non-binary."

We all know that any action triggers a reaction. Picture a gyro with a Bensen-style overhead cyclic control stick. The teeter hinge and tilting spindle are functionally the same mechanism as a swashplate. When the stick is deflected, the pilot does not muscle the whole disk into a tilted orbit. That would take a LOT of muscle. Instead, the tilted spindle and teeter hinge cause a cyclic pitch change. As each blade reaches a certain o'clock position in its orbit, the blade experiences a cyclic pitch change (up or down). The blade then flies higher or lower than its original orbit during the succeeding 90 degrees of rotation. This is what tilts the disk, not the pilot's muscles. The rotor is its own servo (or power steering). But, still...

The pilot applies SOME force to the stick to move it. This force changes the AOA of each blade as the blades cycle around their orbits. The body of the gyro DOES move very slightly in the direction opposite the pilot's push on the stick. But to call this effect "weight shift" would be silly. The frame's direct reaction is microscopic. Essentially all of the control power comes from the rotor tilt. The rotor has used its OWN energy to move ITSELF to the new orbit.

A trike OTOH has the same control bar as an overhead-stick gyro. The direction of movement of the bar to achieve a given pitch or roll is the same as in the O.H. stick gyro. BUT in the trike, a great deal of the tilt of the wing IS created directly by the pilot's muscles. Since the trike pilot typically experiences higher control pressures than the gyro pilot, and since the triker is pushing against the whole wing, not merely rotating a 7" wide rotor blade a couple degrees around its pitch axis, there's more reaction in the airframe. This tilting of the trike cart during control inputs is large enough to be quite noticeable to outside observers.

But modern hang-glider/trike wings aren't pure weight-shift, either. They often incorporate wing-warping mechanisms. This creates an aerodynamic servo effect, reducing control pressures. In effect, these wings have evolved partway toward the rotorcraft approach, in which aerodynamics supplies most of the power to move the lifting surface. So modern trikes use a blended weightshift/aero control setup.

Conventional fixed-wing planes use nearly-pure aerodynamic controls. If, e.g. a FW pilot pushes the yoke forward, he/she uses SOME force, producing SOME reaction pressure against the pilot's seat back, and hence rotates the airframe. Newton's law of action-reaction always applies. But, as with the gyro, this reaction is microscopic and is of no interest. The system actually works by deflecting the elevator down, which (via another aerodynamic servo effect), tilts the entire airplane nose-down and reduces the wing's AOA.

To talk precisely about these things, it would be best to say "this trike has a 40%/60% weightshift/aero control system." A gyro has about a 1/99 system -- nearly all the power to alter the direction of lift comes from aerodynamics, not muscle. Ditto the conventional FW plane.
 

Aerofoam

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Thanks for that, the "certain O clock position" would show that at the moment of stick input, the the individual blades will have an aileron type movement within the perpendicular position to the stick movement of the rotor blades in the disk rotation...
And because they are teetering, they easily change flight plane in an aerodynamic manner.
So on a 3 bladed system, is it slightly phase shifted?, or are they fully articulated?
 

WaspAir

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Three blade systems have a pitch link for each blade that responds to the swash plate tilt to cause cyclic variation in pitch. Each blade flies to its position without regard to the others. There is a phase relationship in that the highest pitch corresponds to the fastest up-flapping speed but greatest flapping displacement is delayed and corresponds to midpoint pitch (descending thereafter with minimum pitch at the fastest down-flapping rate and midpoint pitch again at the lowest displacement). The midpoint pitch is determined by the collective setting in use at the time.
 
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btd1982

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The "controversy" over whether a given system of lift vectoring is weight-shift or not results from imprecise application of Newton's laws. Weight shift vs. aerodynamic control is actually a matter of degree. It is, to borrow a trendy phrase, "non-binary."

We all know that any action triggers a reaction. Picture a gyro with a Bensen-style overhead cyclic control stick. The teeter hinge and tilting spindle are functionally the same mechanism as a swashplate. When the stick is deflected, the pilot does not muscle the whole disk into a tilted orbit. That would take a LOT of muscle. Instead, the tilted spindle and teeter hinge cause a cyclic pitch change. As each blade reaches a certain o'clock position in its orbit, the blade experiences a cyclic pitch change (up or down). The blade then flies higher or lower than its original orbit during the succeeding 90 degrees of rotation. This is what tilts the disk, not the pilot's muscles. The rotor is its own servo (or power steering). But, still...

The pilot applies SOME force to the stick to move it. This force changes the AOA of each blade as the blades cycle around their orbits. The body of the gyro DOES move very slightly in the direction opposite the pilot's push on the stick. But to call this effect "weight shift" would be silly. The frame's direct reaction is microscopic. Essentially all of the control power comes from the rotor tilt. The rotor has used its OWN energy to move ITSELF to the new orbit.

A trike OTOH has the same control bar as an overhead-stick gyro. The direction of movement of the bar to achieve a given pitch or roll is the same as in the O.H. stick gyro. BUT in the trike, a great deal of the tilt of the wing IS created directly by the pilot's muscles. Since the trike pilot typically experiences higher control pressures than the gyro pilot, and since the triker is pushing against the whole wing, not merely rotating a 7" wide rotor blade a couple degrees around its pitch axis, there's more reaction in the airframe. This tilting of the trike cart during control inputs is large enough to be quite noticeable to outside observers.

But modern hang-glider/trike wings aren't pure weight-shift, either. They often incorporate wing-warping mechanisms. This creates an aerodynamic servo effect, reducing control pressures. In effect, these wings have evolved partway toward the rotorcraft approach, in which aerodynamics supplies most of the power to move the lifting surface. So modern trikes use a blended weightshift/aero control setup.

Conventional fixed-wing planes use nearly-pure aerodynamic controls. If, e.g. a FW pilot pushes the yoke forward, he/she uses SOME force, producing SOME reaction pressure against the pilot's seat back, and hence rotates the airframe. Newton's law of action-reaction always applies. But, as with the gyro, this reaction is microscopic and is of no interest. The system actually works by deflecting the elevator down, which (via another aerodynamic servo effect), tilts the entire airplane nose-down and reduces the wing's AOA.

To talk precisely about these things, it would be best to say "this trike has a 40%/60% weightshift/aero control system." A gyro has about a 1/99 system -- nearly all the power to alter the direction of lift comes from aerodynamics, not muscle. Ditto the conventional FW plane.
Thanks for this. I had read this explanation before but couldn't visualise it until now, it was the information about the pitch change happening 90 degrees before which made the difference.
 

btd1982

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If it requires excessive forward stick, It would seem evident the CG is off and the rotor head, or seat needs to to be adjusted, assuming you built in that adjust-ability.
As far as "tilt rotor, weight shift, or combination of the two" I have been asking myself that question since I started building RC gyroplanes back in the mid 90s. The gyro plane community refer to "cyclic" which I would associate with a fully articulated rotor head changing the pitch of individual blades causing the the disk to aerodynamically fly in a different plane. But with a gyro plane, fixed blades, you are literally shifting the under carriage through the push tubes, if you swapped the old style drop bar for a triangular frame control bar, it would be identical to a weight shift hang glider.
The part that makes it slightly different is that the rotor doesn't have a lot of mass, so the "Combination of the two" idea comes to light.
I am assuming the control input shifts the disk plane as well as shifts the weight, the ratio would be interesting to find out.
This discussion also reminds me of a hovering platform project I was on the peripheral of in the early 2000s. I may be opening a can of worms, but It brought to light that a rotor with the payload above the disk can be more stable than payload in a pendulous configuration. That one is counter intuitive.
The CG is ok, it hangs pretty level from the centre of the mast with my hand off the cyclic. To get it to fly level it needs the 1.8 degree nose heavy attitude because of the rotor drive assembly pulling the mast 0.7degrees aft in flight. I'm pushing the cyclic against this mast weight to keep the mast vertical. Because the control rods push up on the mast from a rear part of the airframe it lifts the nose. I've built a frame to balance the mast housing the battery while I'm at it, this will sort the cyclic pressure and I won't need the 1.8 degree nose heavy attitude anymore. Perhaps easier to explain in the next video.
 

Doug Riley

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I believe that precession lag in a system with either flap or teeter hinges results in maximum displacement from the initial orbit 90 deg. after the cyclic input. IOW, there's no difference between a semirigid seesaw arrangement and separate flap hinges, in terms of the lag angle.
 

WaspAir

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Yes, but you don't need precession to explain it (an instructional pet peeve for me). It's just simple harmonic motion. Look at a graph of a simple sinusoid (sine or cosine) and you'll see steepest climb one quarter cycle before the high point and steepest descent one quarter cycle before the low point. Many situations with repetitive motion encountered in normal life work that way.

Importing gyroscopic ideas (torque, precession, vector cross products, etc.) where they don't really fit unnecessarily complicates things, in my view. The articulated rotor gyros I've owned showed none of the stability that one would expect of a gyroscope, and the analysis based on such a comparison just misleads students.

Thanks for tolerating my rant.
 
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Aerofoam

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Yes, but you don't need precession to explain it (an instructional pet peeve for me). It's just simple harmonic motion. Look at a graph of a simple sinusoid (sine or cosine) and you'll see steepest climb one quarter cycle before the high point and steepest descent one quarter cycle before the low point. Many situations with repetitive motion encountered in normal life work that way.

Importing gyroscopic ideas (torque, precession, vector cross products, etc.) where they don't really fit unnecessarily complicates things, in my view. The articulated rotor gyros I've owned showed none of the stability that one would expect of a gyroscope, and the analysis based on such a comparison just misleads students.

Thanks for tolerating my rant.
That make complete sense, the gyroscopic forces are not the prime mover in this situation, thanks for the clarity.
 

Aerofoam

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Three blade systems have a pitch link for each blade that responds to the swash plate tilt to cause cyclic variation in pitch. Each blade flies to its position without regard to the others. There is a phase relationship in that the highest pitch corresponds to the fastest up-flapping speed but greatest flapping displacement is delayed and corresponds to midpoint pitch (descending thereafter with minimum pitch at the fastest down-flapping rate and midpoint pitch again at the lowest displacement). The midpoint pitch is determined by the collective setting in use at the time.
So, are all 3 blade systems articulated, or do some have a simple tilt mechanism without a swash plate?
I ask because in the RC world, the advent of semi flexible hinges made 3 bladed hubs with no swash plate standard and simple.
 

Doug Riley

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Wasp, based on the erudite utterances of Dr. Chuck Beaty, I believe you. Chuck observed that a teetering rotor is a resonant system. Trouble is, I have zero feel for the concept of harmonic motion, beyond the strings on my guitar. Serves me right for being an econ. major.

The 90-degree precession lag thing made no intuitive sense to me, either. I hung a piece of 2x4 from the ceiling with string tied to its middle, modelling a teetering rotor, got it spinning and whacked it from underneath to see what would happen. Sure enough, its precession from its original plane of rotation was maximum at 90 degrees post-whack.

Even very simple people can sometimes be convinced by seeing with their own eyes.

And, often, not otherwise.
 

btd1982

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Is it true then as long as a rotor can teeter freely no gyroscopic forces can be present?

I did a test with two bicycle wheels on a shaft spinning in opposite directions, the gyroscopic forces cancelled each other out and it behaved as if the wheels weren't spinning at all. The forces are being transmitted through the shaft from one wheel to the other. If I did the same test but spun the wheels in the same direction but added teeter hinges would the results be the same?
 

WaspAir

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Gyroscopic descriptions aren't necessarily wrong, but they are unnecessarily complcated and can suggest behavior that isn't there. Gyroscopes are known to be stable in space, but on my Sikorsky S-52, McCulloch J-2, Air&Space 18A, Bell 47, and the many Robinsons, Enstroms, Jet Rangers and Schweizer/Hughes ships I've flown, you'd swear that no gyroscopes were involved moments after letting go of the cyclic. Stability is not the hallmark of most rotorcraft. We talk about the rotor "disc", but that's just a set of points in space, not a real physical rigid disc or even a rigid ring like a bicycle wheel rim.
 

Aerofoam

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I think some redneck wind tunnel testing is in my future.....
This is a great thread...
 

WaspAir

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Doug, one easy way to look at it is to imagine running back and forth between two pylons, or cricket wickets, or something else that suits your fancy. Think of the starting point as A and the distant point as B. When you start, your speed is zero. You speed up and hit your max speed somewhere between A and B but then you have to slow down to avoid overrunning B. At B you are stopped for an instant, and then reverse directions, hitting the max backward speed somewhere in the middle of the path back to A. At A, you have to stop again.

Your max speed, and your max distance, are out of phase by 1/4 of the full round trip (90 degrees, if you will). Max speed in the middle of a leg, and max distance when the speed is zero. No gyroscopes were harmed in the making of this example; it's just the natural behavior of repetitive motion. Consider the flapping motion of a blade going from low A to high B and that's all you need.
 
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WaspAir

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So, are all 3 blade systems articulated, or do some have a simple tilt mechanism without a swash plate?
I ask because in the RC world, the advent of semi flexible hinges made 3 bladed hubs with no swash plate standard and simple.
All the 3 or more bladed systems I've seen, even the hingeless ones with elastomerics to absorb the lead-lag forces, are articulated.
 
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