Desperately Seeking SUSAN

[Dave_Jackson]

Synthesized Understanding of Symmetrical Aerodynamic Nuances

Did Charles Kaman foretell the future when he said;
"You only have one pure helicopter development program in the country, the Boeing-Sikorsky Comanche, and you have one convertiplane, now called a tiltrotor, that's the Bell-Boeing Osprey. That's all there is and there are no others in sight. None on the horizon, so what's being bought was developed a long time ago, 20 years or more, or variations are being bought.
So the verdict is clear, there isn't any other thing that's going to happen on the helicopter front."

Maybe Yes; Maybe No.
Who has the technological foresight and the financial ability to optimize the rotor blade's lift; at every blade segment, at every azimuth, and in every flight mode?

Or, to put it another way, who, how, and when will a concerted research effort be put into unifying the rotor attributes of;
~ Active Blade Twist.
~ Higher Cyclic Control.
~ Slowed Rotor Speed.
~ Reverse Velocity Utilization.
, and then combine two of these rotors in an efficient laterally symmetrical configuration
c/w with separate propulsor(s).


Prouty has stated that;
"The overall airplane lift-to-drag ratio can be 10 to 30, depending on the configuration, whereas the maximum a helicopter can do is 4 to 6."

IMHO, the gap between the helicopter and the airplane can never be meaningfully reduced until the above features are developed and unified.

Any arguments?

Dave

[Graviman]

Dave,

Higher harmonic control can be accomplished by putting gyros and accelerometers in the control system feedback. This means that all fly-by-wire machines effectively already have higher harmonic control. Cyclic and collective will vary above 1P to maintain constant acceleration or roll rates.

Reverse velocity utiliation will probably never happen, since the X2 is reincarnating the ABC principle. This will offer better aerodynamics.

Active blade twist is something which offers advantages by optimum twist over different flight conditions, including autorotation. At 250kts proposed by X2, this means a root swashplate/spider and tip servos would work best. I am not aware of any development.

Mart

[slowrotor]

Dave,
It is the circular rotation of the wing that concentrates the lift near the tip that gives the poor lift L/D. Also, the high tip speed.
For optimum L/D, you would need to invent some type of direct lift other than the circular rotating wing.

[Dave_Jackson]

Mart,

You mention "Higher Harmonic Control" Perhaps, I should have said, 'Independent Blade Control', not 'Higher Cyclic Control'.
As for Higher Harmonic Control, one report states "The results show that properly phased 2/rev inputs can reduce the rotor required power by up to 3.8%. The improvements are very small."
For Independent Blade Control, any appropriate types of sensors and actuators might be employed.

Reverse Velocity Utilization should be extremely compatible with ABC. A report by Sikorsky in 1980 summarizes the features and future improvements to the ABC rotor. It discusses reverse velocity utilization. It even shows the profiles of proposed airfoils.

In fact, virtually, all of the X2's currently known features were mentioned in the above, and other, 25 to 40 year old reports.


Also consider that; ~

If your mention of "tip servo" is in reference to a trailing edge servo flap, I would be concerned. The sequence of events that must take place for the above flap to change the lift of the blade may be too slow for a high speed craft.


Slowrotor,

It is the circular rotation of the wing that concentrates the lift near the tip that gives the poor lift L/D. Also, the high tip speed.

You are absolutely correct. This is where utilizing a [Slowed Rotor Speed] during cruise is an advantage. The [Advancing Blade Concept] places most of the lift on the advancing blades. By slowing the rotational speed of the rotor, the airspeed differential between the advancing tip and the advancing root is reduced. A stopped rotor, if it was possible to do, would totally reduce this airspeed differential.

In addition, the slowed rotor speed allows the advancing tip to retain the same Mach number, but replace the reduced rotational speed with an increased aircraft speed.


Dave

[Graviman]

Dave,

Originally Posted by DaveJackson

mention "Higher Harmonic Control" Perhaps, I should have said, 'Independent Blade Control', not 'Higher Cyclic Control'.
As for Higher Harmonic Control, one report states "The results show that properly phased 2/rev inputs can reduce the rotor required power by up to 3.8%. The improvements are very small."
For Independent Blade Control, any appropriate types of sensors and actuators might be employed.

How do you distinguish Independant Blade Control from Higher Harmonic Control? 3.8% fuel saving for higher harmonic is not to be sneezed at - remember we were disputing 15% payload capacity in Nick's Ballistic Recovery System thread. Agreed that the more control you can give a blade the better it will slip through the air.

Originally Posted by DaveJackson

If your mention of "tip servo" is in reference to a trailing edge servo flap, I would be concerned. The sequence of events that must take place for the above flap to change the lift of the blade may be too slow for a high speed craft.

I do not know what success or otherwise Kaman had with tip servos. I imagine that as long as they were large enough to overcome the tip rotational inertia, response would be fast enough. The possibility is to have the tip self optimise to the local airflow, with the control system gently keeping the pitch/roll moment tally around the azimuth.

Originally Posted by Dave Jackson

You are absolutely correct. This is where utilizing a [Slowed Rotor Speed] during cruise is an advantage.

Reduced rotor RPM is something that only becomes possible to consider with X2 15% effective hinge offset. Even then i bet Big Sky don't rush in to it. The problems are variation of blade eigenmode by RPM stiffening, and increased cone angle. Designing a blade with torsional compliance for active twist, and variable RRPM would be challenging.

Mart

[Dave_Jackson]

Mart,

Please start the engine before driving off in all direction.

[slowrotor]

Dave wrote, " A stopped rotor, if it was possible to do, would totally reduce this airspeed differential."
The stopped rotor sounds good to me. Efficient, symmetrical, no vibration and fatigue. You could call it a fixed wing helicopter.

[Graviman]

Originally Posted by Dave_Jackson

Reverse Velocity Utilization should be extremely compatible with ABC. A report by Sikorsky in 1980 summarizes the features and future improvements to the ABC rotor. It discusses reverse velocity utilization. It even shows the profiles of proposed airfoils.

In fact, virtually, all of the X2's currently known features were mentioned in the above, and other, 25 to 40 year old reports.


Also consider that; ~

Dave, i agree with your comments about ABC and reverse velocity. I suspect the actual lift achieved with RVU is very low, so it is more a question of minimising drag and avoiding blade reverse flow divergence.

Since the X2 has fly-by-wire, the fact that the lower rotor lift distribibution does not exactly match the upper rotor is not an issue. As long as the blades local AOA is optimised for airflow the machine will slip through the air, whether or not it is symmetrical. The pilot still has a nice roll rate stick proportionality in all directions.

The poor autorotation "glide" performance would by greatly addressed by active blade twist over the advancing portion of the rotor. For this reason alone it does warrant investigation.

Mart

[Dave_Jackson]

Yes, Active Blade Twist will provide many significant advantages.

Another advantage of ABT can be seen on the retreating blades in the above sketch. The root ends of these retreating blades are generating negative lift.

With ABT the tips of these blades can continue to generate positive lift (from positive pitch) PLUS the roots of these blade can be given a negative pitch, which with the reverse air flow will generate positive lift, instead of the shown negative lift.


Advantages of Active Blade Twist


Dave

[Graviman]

Dave,

You could put your drawing skills to work and sketch out a hub proposal for ABT. I'm curious how control for root and tip would package into a single hub. In particular i'm interested how the FBW actuators would link into the system. Independant Blade Control clearly needs an actuator per blade, with azimuthal control, although if i'm understanding correctly it only benefits 4+ blade rotors.

I've always been a proponent of the Lockheed mechanical system, since to me once the gyro dynamics are understood it gives a good bang for the buck. Although a fatigue hardened luddite , i accept that once active blade twist becomes a design feature maybe a mechanical system will no longer perform. Then again, with tip servos linked in to the root pitch control a flexible blade would find it's own twist...

A helicopter will never be faster or more efficient than an aeroplane, but it is more flexible. The trick is to improve the flexibility without increasing the cost...

Mart

[Dave_Jackson]

Since you asked

Mart,

Here is the 'currently' preferred rotorhub and flight-control.

Dave

[Graviman]

A bit technical - sorry folks.

Dave,

I struggled to make out the drawings (pixel resolution), but think i understand the concept. I like the idea of the pitch links seated between two pitch bearings - nice and stiff/strong. Clearly high effective hinge offset with control system development it the future of rotorcraft. The carbon fiber layup certainly looks more manufacturing friendly than say high pressure casting (to remove porosity) followed by machining - or even machine from billet .

I take it you mix actuator root and mechanical tip so you can use RVU, while pilot directly controls machine? Are you convinced that the aerodynamic benefit of independant blade control benefits the flexible swashplate complexity? My own thought is to go with conventional swashplate, then use a vertical accelerometer for collective feedback, and gyro sensors (on a chip nowadays) for cyclic feedback. The idea being the feedback varies much higher than 1P via control link actuators. This way you could probably get a three blade rotor to fly smoothly, so no need for independant blade control.

X2 uses 4 blade rotors over S69 3 blade rotors, so Big Sky must have learnt some lessons. From Prouty the usual reason is that generally a blade has fundamental eigenmode only just above 1P (1/4 wave bending), since hinge offset will be finite. This means that the next flexural mode will be at ~3P (3/4 wave bending). By having three blades the 3P modes coincide to produce a collective acceleration, while having 4 blades keep the blade resonances out of step. This is why i suggest a vertical accelerometer to adjust the collective, so the 3P eigenmode is cancelled.

This would allow HHC on a 3-blade system, without the vibrations normally produced. The roll & pitch gyros would also allow fast rates to be achieved by cancelling the vibrations from azimuth varying hub moments. It may be that 4 blade is the most failsafe soln, but i'm suggesting alternates to what could otherwise be an overly complex hub.

You might try to get hold of some rotorhead Campbell plots. I have always found them unbelievably usefull in picking up powertrain dynamics.

Mart