Helos vs. Gyros: Dissymetry of lift

[magilla]

Hey guys,

I'm going through my helo aerodynamics books, and I have a question as it pertains to gyros:

Do gyros experience blowback?

More specifically, since the tip path plane of the gyro is changed by cyclic input directly to the head, resulting in a non-rotating cyclic input being manifested in a non-rotating rotor head movement (double roller bearing) there is therefore no resultant "cyclic feathering" that is compensated for in a helicopter.

In a helo, non-rotating cyclic inputs are CONVERTED to a rotating input to the rotor system, and cyclic feathering is built in. This compensates for gyroscopic precession and blade flapping by automatically changing the pitch of each rotor blade through 360 degrees of rotation. Most pitch applied at 12 o'clock position, with gyroscopic precession acting on the retreating blade at 9 o'clock position (the downflap blade) and least pitch applied at 6 o'clock position, with gyroscopic precession acting on the advancing blade at 3 o'clock position (the upflap blade) with it sequentially going to no pitch input at the 3 and 6 o'clock positions.

As you accelerate a helicopter, the advancing blade has more lift through increased relative wind, which is compensated for by blade flapping and cyclic feathering. If you don't have a cyclic feathering mechanism, you experience "blowback," especially if you do not compensate for this dissymetry of lift by applying a hefty dose of right cyclic...

In other words, as you accelerate, the advancing blade side generates more lift, which causes the FRONT of the tip path plane to rise (gyroscopic precession 90 degrees in direction of rotation). To stop blowback, you MUST apply right cyclic with forward cyclic to compansate for dissymetry of lift. Even with cyclic feathering, in a UH-1 you still need to apply right cyclic as you accelerate.

Damn! I made this difficult, and didn't intend to.

Bottom line, when you all accelerate into forward flight, do you need to apply right cyclic while accelerating to prevent blowback in a gyro?

Or am I totally mistaken? As I think about this, since forward thrust is developed by the prop in a gyro, the rotor only generates lift... and the dissymetry of lift is compensated for by blade flapping alone. Is that correct?

Please help me understand this more....

I get the difference between helo rotors and gyro rotors - helo rotors are driven by the mast and forward thrust is generated by the rotor system alone. Gyro rotors only generate lift through relative wind, and forward thrust is provided for by the prop.

I do believe that if you had a partially driven rotor (goped or what have you) you would experience blowback to a lesser degree. How about if you pre-rotate prior to take off?

Gyro auronatical geniuses please speak up!!!

Also, does this phenomenon manifest itself in gyros:

Gyroscopic precession explains some fundamental effects occurring during various helicopter maneuvers. For example, the helicopter behaves differently when rolling into a right turn than when rolling into a left turn. During roll into a right turn, the aviator must correct for a nose-down tendency to maintain altitude. This correction is required because precession causes a nose-down tendency. During a roll into a left turn, precession causes a nose-up tendency. Aviator input required to maintain altitude is different during a left versus right turn as gyroscopic precession acts in opposite directions.

Could this explain some issues we have with making downwind turns?

 

[Rotor-Head]

I'm not sure of this answer, but I'd love to know. That one thing I have observed in my gyro is that if all the linkages are set the same, my machine flies forward with a slight right stick, so I have my linkages from my stick to my rotor head set so that when I am not flying and the rotor head is level, my stick sits off to the left a bit.. In other words, when I am flying, my stick is centered, but my rotor head is leaning slightly right to compensate for dissymmetry of lift.

Probably not what you were looking for.... sorry.

 

[C. Beaty]

Blowback refers to the rearward tilt of the rotor dixc relative to the rotorhead axis in forward flight. The article below appeared in the PRA magazine several years ago.

Rotorblade Motion and Control

Does it flap?

What is the axis of rotation of a rock being twirled on a string? The twirler’s forearm? His wrist? His thumb and forefinger?

Actually, none of the above. The rock clearly rotates about the axis of the circular path it travels.

So it is with freely hinged rotor blades.

The early fixed rotorhead Autogiros that were controlled by tilting the complete airframe via ailerons and elevator certainly presented the appearance of flapping rotorblades to the view of a stationary observer. The advancing blade ascended and the retreating blade descended relative to the rotorhead.

However, viewed from the real axis of rotation, the tip plane axis, the blades don’t flap, don’t speed up or slow down and the only thing that might seem strange to a casual observer is a cyclical variation of pitch.

Axis of view

Gyroplanes controlled by tilting the rotorhead have two axes from which the rotor may be viewed; the tip plane axis and the rotorhead axis. Swashplate controlled helicopters have three possible axes of view; tip plane axis, powered shaft axis and swash plate axis.

Any view except along the tip plane axis is quite complicated and requires some fancy mathematics for analysis.

Viewed from the rotorhead axis, the blades appear to flap and with a coned rotor, the CG of the upward flapping blade moves nearer to the center of rotation and must speed up to conform to conservation of energy law. The retreating blade must slow down since its CG moves away from the center of rotation. The same law that governs pirouetting ice skaters as they tuck in or spread out.

Cierva’s rotor analyses always used the rotorhead frame of reference, perhaps to bedazzle and befuddle his competitors. When the rotorhead reference frame is used, to make the math work, Coriolius theory must be applied to explain the need for drag hinges on rotors with three or more blades.

The NACA as well as textbook authors picked up Cierva’s analysis and ran with it. Gessow and Meyers (“Aerodynamics of the Helicopter”), to their credit, point out that Coriolius forces are in the eye of the beholder.

Drag Hinges

Viewed from the tip plane axis, the blades don’t speed up or slow down and drag hinges are a kinematic rather than a dynamic necessity.

Flap and drag hinges serve the same purpose as the universal joint of a socket wrench as illustrated in figure 1. With only a flap hinge, tilting the rotorhead of a coned rotor would require swinging the rotorblades through an arc, rendering control by human muscle power impossible.

With both flap and drag hinges, tilting the rotorhead can only rotate the blades about their feathering axes. Flap/drag hinges also permit the rotorblades to rotate at uniform angular velocity about an axis that differs from the rotorhead axis, like the rock on a string.

The Autogiros of Cierva employed a rotorhead configuration similar to that depicted in figure 1 with the flap hinges crowded as close together as possible; but nonetheless, very high stick force was required for cyclic control. With several tons of centrifugal force acting on each blade, there is high resistance to rotorhead tilt; the “T” bar effect.

The Sikorsky S-51 helicopter rotorhead employed centrally located flap hinges as shown in figure 2. Any modern tilt head 3-blade gyro should utilize a similar scheme.

Teetering Rotors

Cierva experimented extensively with 2-blade rotors but experienced little success. The vibration problem appeared to be insurmountable. There is a 2/rev aerodynamic drag variation that can’t readily be solved through the use of drag hinges that must be located far enough outboard from the center of rotation for centrifugal force to keep them in alignment. Then the “T” bar effect comes into play and centrifugal force doesn’t permit sufficient drag hinge motion to accommodate aerodynamic drag variation. In the case of 3-blade rotors, the periodic aerodynamic drag force of each rotor blade, when added together, becomes a steady force.

Arthur Young, the designer of the Bell-47 helicopter, was the first to solve the vibration problems of 2-blade rotors; at least partially.

Young’s solution was to undersling the rotor so as to locate the teeter bolt at the CG of the coned rotor and to allow the periodic drag variation to be accommodated by providing the softest possible mounting of the rotorhead. With soft mounting, the rotor is free to move fore and aft relative to the airframe. The rotor doesn’t actually move fore and aft relative to the airstream; it merely has a 2/rev speed variation.

Another requirement is for the mass of the rotorhead and stuff mounted thereupon to be as small as possible. Mounting electric starter motors, batteries and the like at the rotorhead may provide a ride with less shake of the airframe but increase the periodic in-plane flexing of the blades and hub. Some attempts at utilizing crisscrossed seesaw rotors have resulting in cracks developing in the rotorblades.

Teetering rotors combined with a rigid rotor pylon can’t provide a smooth ride and can even be dangerous due to the increased stresses imposed on the blades and hub.

Cyclic pitch control

The traditional explanation for equalization of lift between advancing and retreating sides of the rotor disc is that the advancing blade flaps upward and the retreating blade flaps downward; the vector sums of rotational velocity, forward velocity and flapping velocity reduce the angle of attack of the advancing blade and increase the angle of attack of the retreating blade. True, but needlessly convoluted.

Since a hinged rotor system permits the rotor plane to have an axis that need not be aligned with the rotorhead axis, the concept of lift equalization is simple and straightforward when the rotor is viewed relative to the rotor plane.

In forward flight, the axis of the rotor plane tips rearward with respect to the rotorhead axis; the amount dependant upon airspeed.

Referring to figure 3, blade pitch is fixed relative to the teeter bolt. The blade on the near side (advancing side) has less pitch than the blade on the retreating side. Lift is thus equalized without jumping through vectorial hoops. It is important to understand the equality of cyclic “flapping” and cyclic pitch.

The rotorhead of a tilt rotor system is an exact equivalent of a swashplate controlled feathering rotor system minus the collective pitch capability. A tilt head gyro in no way is, as some have speculated, a kind of weight shifter like a trike. There is no way the rotor could be tilted against its own inertia without cyclic pitch control.

************
The sideward tilt is a rate effect, called wee-waa by Robinson. As a rotor is tilted, nose down for instance, the forward portion of the rotor disc has to push down on air while the rearward portion moves some air upward. Gyroscopic precession causes the rotor to tilt sideways but you won't likely notice it on a gyro.

**********
Shawn, the off center stick is the result of propeller torque. Propeller torque attempts to rotate the airframe in a direction opposite to prop rotation and to balance it, opposite stick must be held. A tall tail or differential tailplane incidence pretty much eliminates this effect.

 

[All_In]

Thank you Chuck and good question!!!
I love to learn.

 

[WaspAir]

back to the question

back to the question

Magilla, I've read the discussion so far, and maybe I missed it, but I still haven't seen a straight answer your original question as I understood it.

Let me pose a question of my own to be sure we're on the same page. Are you talking about the initial acceleration on departure from a hover in a helicopter, and the control inputs that are needed around the time you reach effective translational lift? That's the "blowback" effect I thought you had in mind, and that all the helicopter training texts describe. You describe a need for "hefty right cyclic". I've never flown a Huey, but in Robinsons, JetRangers, Bell 47s, Schweizers, Enstroms, and Sikorskys, I have never been consciously aware of a strong right cyclic input at that time (maybe I just do it naturally and don't notice, but I certainly haven't noticed), only forward cyclic (and a bit of pedal adjustment as airspeed starts to help with anti-torque). I just don't recall a left rolling tendency that would need correction with strong right cyclic.

In gyros, most will already be at translational lift speed from the ground run before take-off is attempted, so the airspeed transition of interest wouldn't be experienced in flight. If you do a jump take-off in calm conditions, there will be an acceleration from zero through 15 knots forward speed that might be comparable to the helicopter take-off scenario. Things are a bit messy at that time, however; you're also transitioning to autorotation, coping with propeller torque, and you won't have the pitch forward that a helicopter will go through. It's hard to sort out any "blowback" effect from all that's happening then.

 

[magilla]

Waspair, you nailed it. That's the blowback effect I was talking about. Since all the helicopters you mentioned have cyclic feathering built in (the swashplate changes the pitch of the blades through 360 degrees of rotation), the effect of blowback is reduced considerably. However, it is still present, and the natural pilot reaction is to add a 1/2" to 1" right cyclic on acceleration through 16-24 KIAS. After a while, you just don't notice it anymore.

But Chuck's post confirms my hypothesis about the difference between gyros and helos. I also happen to disagree with the statement "The rotorhead of a tilt rotor system is an exact equivalent of a swashplate controlled feathering rotor system minus the collective pitch capability." Gyro blade pitch is FIXED through 360 degrees of rotation, whereas a true swashplate will feather the pitch of a helicopter blade from +22 degrees to -6 degrees as it goes in a complete circle, and this is built in to reduce blowback.

Since that is the case, the biggest compensation for dissymmetry of lift in a gyro must be blade flapping. Cyclic feathering just reduces pilot workload by compensating for gyroscopic precession automatically, instead of the pilot having to compensate for it himself.

But in gyrocopters, the rotor system is not developing forward thrust, only lift. As stated above, the tip-path plane tilts more aft the faster you go because gyroscopic precession tells us that the increased lift from the advancing blade manifests itself as a tilting up of the rotor in the front. The faster you go, the more dissymetry of lift, and the more the rotor system tilts backwards.

 

[RotoPlane]

Spencer - But in gyrocopters, the rotor system is not developing forward thrust, only lift. As stated above, the tip-path plane tilts more aft the faster you go because gyroscopic precession tells us that the increased lift from the advancing blade manifests itself as a tilting up of the rotor in the front. The faster you go, the more dissymetry of lift, and the more the rotor system tilts backwards.

This may be true for one set speed....but as your speed increases, the rotor disk would need to tilt forward (cyclic forward), or you would climb....as I see it.

 

[C. Beaty]

I also happen to disagree with the statement "The rotorhead of a tilt rotor system is an exact equivalent of a swashplate controlled feathering rotor system minus the collective pitch capability." Gyro blade pitch is FIXED through 360 degrees of rotation, whereas a true swashplate will feather the pitch of a helicopter blade from +22 degrees to -6 degrees as it goes in a complete circle, and this is built in to reduce blowback.

Spencer, the Robinson helicopter does apply cyclic pitch about the feathering bearings but the Bell-47 and successors do not.

The Bell-47 has a walking beam mechanism that ties the two blades together and prevents cyclic motion about the feathering bearings.

Tilt head helicopters have been built but in the case of torque driven rotors, a component of drive torque appears in the control system, mandating servo control. Helicopters with rotors driven by tip jets have almost universally used tilt head cyclic control.

 

[Resasi]

The undersling seems an elegant solution, even with it's 2 per. I am still grappling with all the ramifications of undersling and the calculations required to optimise. If it true to say that a given undersling will only be optimum for one given set of conditions what should one be aiming for in your particular gyro. Is it your particular chosen cruise speed?

If on lift off gyroscopic precession is manifest in more back tilt of the tip path plane, the roll on lift off that a gyro experiences is 'P' factor, not gyroscopic precession?

Sorry please bear with, slow to keep up but trying. I guess we all automatically compensate but it is good to know why we are inputting these various corrections.

 

[C. Beaty]

A gyro flies at constant coning angle; the rotor rpm varies with load in such a way as to maintain constant coning angle.

The extra blowback at liftoff comes from the rotor not being fully up to speed. A gyro normally breaks ground with the rotor rpm at 85%-90% of normal operating rpm. Following liftoff, the stick will be quite far forward, but moves toward trim as the rotor reaches normal rpm.

 

[magilla]

Thanks Chuck - it's becoming more clear to me now.

Ed (Rotoplane): The way I see it, your statement is correct. In a gyro, with everything being equal, as you apply more throttle, you WILL climb, unless you put forward cyclic in to accelerate. Throttle = climb descent; Cyclic = airspeed. Coordination of throttle and cyclic give you level climbs and descents while maintaining airspeed.

As stated above, the faster you fly, the further back the tilt of the rotor tip path plane due to dissymetry of lift.

Excellent discussion. I appreciate the input.

 

[Udi]

...As stated above, the faster you fly, the further back the tilt of the rotor tip path plane due to dissymetry of lift...

Only relative to the rotor head axis. Otherwise, the faster you fly, the shallower the disc angle.

 

[C. Beaty]

Dr. JAJ Bennett, in responding to a paper read before the Royal Aeronautical Society in 1937 by Raoul Hafner stated:

“The ‘false hub’ construction has been commonly referred to as ‘direct pitch control’ or ‘direct feathering control’ but such terminology is misleading because the ‘tilting hub’ method is just as rigorously a direct feathering or pitch control as the other. When either the hub or false hub is tilted by the control stick, a cyclic variation of blade incidence is effected, because the blade tips are moving along a definite path and inertia prevents a sudden displacement from this path. The pilot does not impose a load on the control column sufficient to displace the blades against the inertia or ‘gyroscopic action’ of the blade elements, as many people seem to imagine. The control mechanism is a relay, he only causes the blades to feather cyclically. The result is a cyclic variation of lift in phase with the cyclic variation of blade incidence. The cyclic variation of lift displaces the blades from their normal path, but owing to the dynamics of the flapping hinge motion, which need not be discussed in detail here, the displacement of the blade from its normal path occurs 90º later in azimuth and so the required tilt of the lift vector is effected….”

The term, “false hub” refers to a swash plate mounted above the rotor and tilted and elevated by a control stem passing through the center of the rotorhead. The Bristol Sycamore helicopter, designed by Hafner, featured the same sort of swash plate but was called a “control spider.” (attachment)

JAJ Bennett, chief engineer of the Cierva company at the time, went on to design the Fairey Rotodyne tip jet propelled compound helicopter.

There was little love lost between Bennett and Hafner.

 

[magilla]

ARRRGH! Durf!

OK, the statement above that tilt rotors behave exactly the same as swashplate mechanisms IS CORRECT.

If you tilt the entire head forward, the pitch of each rotor blade is changed, even though it is fixed in relation to the mast. Since the mast tilts, the pitch of the blades on advancing and retreating blades DOES CHANGE on a tilt-head rotor system. A swashplate system mirrors this through a cyclic feathering mechanism to achieve the same effect. (And the Bell 47 accomplished feathering through a walking beam, since it does not have a feathering mechanism).

So, to verbalize my hypothesis more clearly:

Gyros do experience dissymetry of lift

It is compensated for by blade flapping and changes to pitch (cyclic feathering) as the rotor system goes through 360 degrees of rotation.

In addition:

The effects of translational lift are NOT really felt in a gyro, as they are in a helicopter.

Basically, because a gyro's rotor RPM is based upon lift requirements only, RRPM is governed only by generating enough lift to equal weight. A helo rotor system on the other hand maintains the same rotor rpm , and the induced flow on the front half of the rotor system as you accelerate through 16-24 KIAS is affected (less induced flow due to "clean air" on front half) whereas the rear half has not experienced clean air yet, has more induced flow (=less lift), and through gyroscopic precession manifests itself as a right roll that must be compensated for by left cyclic.

This does not happen to gyros, at least not to the same degree.

Same with blowback - I was overthinking it - to overcome blowback, the pilot has to put in FORWARD cyclic in a helicopter.

To overcome both, forward and left cyclic is required to maintain acceleration and ground track....forward cyclic to compensate for blowback, and left cyclic to account for translational lift.

Since gyros RRPM is governed by weight, it does not feel the same effects of translational lift as a helo.

 

[C. Beaty]

Pretty much there, Spencer. Just keep in mind that a gyro never has that ground effect bubble of hovering air to transition from.

One of the founding Sunstate Rotor Club members had been a Coast Guard pilot and had cut his teeth on Sikorsky R-4s. He used to relate the story of hovering one over a pier, going into translation and plopping down in the water after clearing the pier’s edge. His magic bubble vanished and those old R-4s could just barely hover to start with on a good day.

 

[ferranrosello]

Spencer, I think you are mixing two different things: the blowback, or flap back and the some strange coupling effects which happen on helicopters.

The blowback happens in both, gyros and helicopters. Because there are flapping hinges in our rotors they show blowback tendency whit airspeed. This blowback motion eliminates the lift dissymmetry and permits a normal flight without hard rolling tendencies, the very reason because first Cierva’s gyros were unable to fly.

All helicopter rotors have this blowback effect. In both kinds of aircrafts, gyros and helicopters it is necessary to compensate this flap back effect. So, when we are accelerating in take off, is necessary to make long forward movements on the cyclic to compensate the blowback and let the aircraft to accelerate.

Another very different question is because you need to move the cyclic slightly to one side when taking off in a helicopter (the side depends on the rotation sense, clock or anticlock). There are different sources for this effect. The simplest one is the combination of conning angle and forward speed. This makes the 12 O’clock blade feeling an AOA bigger than the 6 O’clock one. The precession transfers the increased front lift in a side one, making the rotor to roll towards the advancing blade.

Other explanations which make different kind of rotors behaving very differently are due to control phasing angles and flying at high density altitudes.
Ferran

 

[Resasi]

With all this in mind might I please ask if there are any commonly available rotorheads that would be good for a Bee/Hornet type ultralight?
Thanks.

 

[Doug Riley]

I sold and used the "Black Beauty" head. Starbee bought the design rights and parts inventory from us. It's a pretty standard Bensen-style offset gimbal head.

 

[Resasi]

Thank you Doug. On the list.