Why are Helicopters with the Flettner-System so slow?

[slowrotor]

Dave,
Dacron fabric is a modern material and should be considered if weight matters to you. And weight always matters doesn't it?
Another idea is "geodetic skin" like the old Wellington bombers, for the aft part of the blade.

[Graviman]

I am learning a great deal from this exercise! Nick, serious thanks for sharing your knowledge and experience. That Comanche control system looks, at first glance, like a pretty complex program. I was suprised not to see Chuck Keys on the patent (knowledgeable chap). Will read more thoroughly this weekend.

Here is my understanding of what is happening. Looking up the NACA 0012 section in Abbot & Doenhoff, shows that the Cd vs Cl curve has a tangent to the origin at about 0.8 Cl for standard roughness. This corresponds to 8 deg AOA, which is where stall begins. This means that, having no camber, this section offers best lift/drag at high angle of attack, just before stall. For a helicopter this means running the blade at pitch just on the verge of stall requires the least power (Cp/Sigma).

The problem in a conventional helicopter is that the retreating side blade suffers reduced airspeed. If the blade is near stall in hover it will be stalled at speed, so a lower pitch is required for hover. This means a lower than ideal 0.125 Ct/Sigma for hover, so more hover power. This can be offset by beeping down Nr in hover, like S76A. The advancing side will be at an even less optimal AOA, placing additional requirements on cruise power (made worse by Glauert-Prandtle compressibility equation).

In a counterrotator the retreating blade can be unloaded, since advancing regions balance roll torque. This means that the retreating blade no longer constrains hovering blade to Ct/Sigma < 0.125. The ideal for power is if the advancing blade maintains the same pitch, hence airspeed, across the heli speed range. For this reason a counterotator works best by beeping down Nr with speed. Clearly this is limited by acceptable root stresses that the lower centripetal force, hence centrifugal acceleration, will cause. In practice this means that the advancing side will be below optimum AOA, but less so than the conventional.

Practical upshot is X2 should be more efficient in hover and at speed than conventional. So the answer to your question, Hotzenplotz, is: "because they need development"...

Marks out of 10? (Bet i don't remember any of this in the R22 next w/end).

Mart


Note: Laminar flow sections like 63-015 (R22) have low Cd "bucket", to extend lift/drag for better "penetration" to higher speeds. This will help figure of merit in hover, while extending high speed (ish).

[Dave_Jackson]

Nick,

When applied to today's helicopters; I agree with both of your points

However, when applied to tomorrow's high-speed helicopters(*), these points will no longer be valid, IMO.

Tomorrow's helicopters(*) will incorporate, at a minimum;
~ twin main rotors that are located in the same lateral/vertical plane
~ the Advancing Blade Concept
~ a propulsor that is distinct from the rotors.
These features will overcome the retreating blade stall, and the weight increase is an unfortunate reality that will be necessary for the high forward speed.

(*) 'Tomorrow's helicopters' refers to those craft which utilizes the rotors totally (or primarily) for lift, An example is Sikorsky's X2. It does not refer to tilt-rotors or wing&rotor&propulsor compound helicopters.

_________________

slowrotor,

Dacron fabric maybe too light
Could not find any reference to "geodetic skin"

_________________

Mart,

You got it. I typed the above offline before reading yours.



Dave

[slowrotor]

Geodetic structure shown here on the Wellington bomber; http://www.bomber-command.info/blwimpy6.htm

[Graviman]

Geodetic structures.

Dave,

http://www.1940.co.uk/history/article/wallis/wallis.htm
http://www.effingham.co.uk/kgv/wallis.htm
http://www.largemodelassociation.com...wellington.htm

On the other hand that is an amazing site, Slowrotor!

Mart

[Dave_Jackson]

Got it. A big nanotube.


http://images.google.ca/images?q=nan...mages&ct=title

[Graviman]

Crossing the "i"s and dotting the "t"s...

Dave,

This thread left me with an uneasy feeling that we might still be talking at cross-purposes. I'm probably doing my usual trick and tieing my shoelaces together, to stop myself running away...

Prouty say (P17, Chap 1):

Ct/Sigma = Thrust / ( Air_Density * Blade_Area * Tip_Vel^2 * Radius^2 )

Me say:

This means that if the blade area is increased for a larger span of same radius, the tip vel will come down as expected. So even for a future counterrotating rotorcraft Ct will still be optimum at 0.125.

That means that the FM vs Ct/Sigma curve still peaks at Ct 0.125. This means that rotor power is not a function of bladespan.

Is this right, or did i miss something? Don't want a Comanche bearing down on me to have the last word!

Mart

[NickLappos]

disk loading, Mart, will rise with reduced radius, and power requirements will go up!

[Graviman]

Yeah - brain error there, Nick. You'll notice i gave up trying to be smart and went back to FM vs Ct/Sigma curve. Optimum Ct/Sigma is 0.125 regardless of bladespan.

Have just confirmed this by looking at Fig 1.13 (P28) in Prouty, which plots above for different Sigmas (Convert ordinate using Ct/Sigma = <Cl>/6 ). FM varies, but peak is always at Ct/Sigma ~0.125. Peak FM value increases with increasing Sigma, so it looks like Dave has a point about span improving efficiency.

Mart

[Dave_Jackson]

Opps!
Post #22 on http://www.pprune.org/forums/showthr...=252205&page=2 and most of the following posts on that thread should have been posted on this thread.

________________________________________________________



It was mentioned by IMFU, in a different thread, that Sikorsky's new X2 coaxial ABC will have multiple-speed rotors, as did its predecessor, the XH-59A ABC.

However, the possibility of using single-speed, low-rpm rotors with wide chord blades (large solidity ratio) for tomorrow's high speed rotorcraft appears to have significant advantages.

Two bits of information have come to light while digging deeper into this subject.

1/ A cursory review of the technical papers on the XH-59A ABC show that the 'solidity ratio' is much used in the evaluation of many variables. However, I cannot find any place where the solidity ratio, itself, was evaluated as a variable.

2/ Stepniewski considers the rpm of the rotors as a variable in his conceptual Intermeshing ABC, but it appears that this evaluation was only to pick the optimum single-speed RPM.


Can anyone suggest one or more potentially viable reasons for using multiple-speed main rotors?

__________________

Another bit of uncovered trivia, from the Spring 2001 AHS publication of Vertiflite;
"I [Glidden Doman] committed office and shop space to him [Anton Flettner ".. at a relatively advanced age"] for the construction of a unique intermesher with no hinges. When the project did not go ahead I hired his top engineer and lost contact."

Apparently, Kellett was not the only person at that time to consider an intermeshing configuration with rigid rotors.


Dave

[IFMU]

Originally Posted by Dave_Jackson

It was mentioned by IMFU, in a different thread, that Sikorsky's new X2 coaxial ABC will have multiple-speed rotors, as did its predecessor, the XH-59A ABC.
However, the possibility of using single-speed, low-rpm rotors with wide chord blades (large solidity ratio) for tomorrow's high speed rotorcraft appears to have significant advantages.

I found this Nick Lappos quote:
It pulled 2.5 G's at 25,000 feet and cruised at over 250 mph, as the thrust engines pushed it along in autorotation (it was an autogyro at high speed!) Not too shabby.
You can find that quote in the middle of this page:
http://www.synchrolite.com/0891.html
And, as Nick said earlier in the thread:

Originally Posted by NickLappos

A turbine helo has way too much blade chord for hover (it throws away power) because it needs that chord for higher speed forward flight.

So, if the S69 was an autogyro, its main rotor was not absorbing much power. So what would wide chord blades do to to help it speed along?
Then again maybe it gets solidity ratio through sheer number of blades rather than chord.
-- IFMU (not IMFU, that's my evil twin)

[Dave_Jackson]

IFMU

Thanks for considering the question of one-speed versus multiple-speed rotors.

I can agree that a multiple speed rotor will allow a system to be 'tweaked' to improve its efficiency, slightly. This is the intent of Karem's patent US 6,007,298 'Optimum Speed Rotor'.

However, it appears that there is not any significant justification for two-speed rotors. A couple of statements by Nick, which are mentioned in your posting, may need clarification. Nick can correct me if the following is wrong.

Originally Posted by NickLappos
It pulled 2.5 G's at 25,000 feet and cruised at over 250 mph, as the thrust engines pushed it along in autorotation (it was an autogyro at high speed!).

I believe that the craft was not in autogyro mode, nor was it in helicopter mode. It was probably in a mid-mode where the two internal turbines were driving the rotors for lift and the two external turbines were propelling the craft forward.

Originally Posted by NickLappos
A turbine helo has way too much blade chord for hover (it throws away power) because it needs that chord for higher speed forward flight.

I believe that the above statement refers to a conventional rotor. In this situation the rotor is the sole provider of lift AND forward thrust. Therefore, the rotor must provide more thrust in high-speed cruise than in hover. To achieve this greater thrust the chord was increased, because the RRPM could not be increased.

These new ABC helicopters will have rotors and propulsors (propellers etc.). The propulsors will provide all or most of the forward thrust. The rotors will basically provide only the lift and this 'lift' component should not significantly vary between hover and high speed cruise.

If this lift does not vary (climb & manuvering excluded), then there is no reason why the RRPM or the blade area should be varied. The Calculations re Constant Speed Slowed Rotors suggest the preference is single-speed rotors.

_______________________________________________________

And furthermore

If Sikorsky had been able to fix the RRPM of the earlier XH-59A ABC at its slower rotational tip speed of 450 ft/sec and then calculated the optimal solidity ratio, I believe that the craft would have experianced these attributes:
1/ Silent rotors in all flight modes.
2/ A 50% reduction in the very strong moments and forces, which are caused by the opposing gyroscopic precession of the two rotors.


Dave

[Graviman]

Originally Posted by DaveJackson

However, the possibility of using single-speed, low-rpm rotors with wide chord blades (large solidity ratio) for tomorrow's high speed rotorcraft appears to have significant advantages.


Can anyone suggest one of more potentially viable reasons for using multiple-speed main rotors?

Dave, your arguements hold water. I will adapt my response from the linked thread:

RW Prouty "Helicopter Performance Stability and Control" page 26 shows that for ideal twist:

Figure_of_Merit = 1 / (1 + ( 1.5*SQRT(3) / (SQRT(Solidarity_Ratio)*(Cl^1.5/Cd)) ))

This equation can be rewritten for hover tip speed:

Figure_of_Merit =
1 / (1 + ( 1.5*Tip_Speed / (Cl/Cd)*SQRT(2*Disk_Loading/Air_Density) ))

The results are plotted for ideal twist on page 28 (fig 1.13), and indeed show that the higher the solidity ratio the higher the hover figure of merit. For each solidity ratio curve max FM is achieved at Ct/Sigma = 0.125 (or Cl = 0.75).

My interpretation is that a helicopter runs at peak efficiency when the blades are operating just below stall, for maximum Cl/Cd. Using wide chord blades allows a nice low tip speed, by chosing a low Nr for hover. For conventional heli forward flight, Nr ideally increases due to the need to avoid retreating blade stall. For counterrotators forward flight, Nr ideally reduces to keep advancing blade airspeed constant - this depends entirely on unloading the retreating blades.

The counterotator ideal is a control system which beeps down N1 with airspeed, and up with rotor load factor. A pusher prop is better running at constant speed, or beeping down with airspeed. Rotor dynamics generally favour approximately constant Nr, or at least Nr regions to avoid. My guess is that the Sikorsky engineers puzzled over the problem and decided on a 2 speed stategy as opposed to continuously variable speed. At 250kts rotors should be regarded as a wing that happens to be rotating - regardless of whether engine or pusher system causes that rotation.

Mart

[Dave_Jackson]

Mart, you said;

My guess is that the Sikorsky engineers puzzled over the problem.....

The initial and the follow-up technical documents on the XH-59A ABC clearly show the difficulties that Sikorsky faced when calculating the specifications for the craft. This was because their existing knowledge base was predominantly about single-rotor helicopters.

An example of this limitation is 'disk area'. Even to this day there is no consensus as to what constitutes the 'disk area' of the various twin-rotor configurations. 'Disk area' is probably the most fundamental parameter. It is next to impossible to optimize the other parameters when they are based on a nebulous 'disk area'. The difficulty is obvious when seeing the large usage of various K-factors (fudge factors) throughout the calculations.

Those who think that there are difficulties understanding the aerodynamics of a single rotor might want to consider the greater difficulties understanding aerodynamically interacting twin-rotors, particularly as this interaction radically changes with the changes in flight mode.

However;

Just think how much more advanced rotorcraft would be today if the research and development over the past 60 years had been built upon the superior German twin-rotor helicopter configurations.

Just think how much more advanced 'Advancing Blade Concept' rotorcraft would be today if Sikorsky had been able to develop the ABC on the shoulders of an existing twin-rotor knowledge base.


Dave
________________________


Last May, I sent a letter to the editor of the American Helicopter Society's quarterly magazine Vertiflite. It was related to this very subject. It was not published in either of the last two issues. Interestingly, 5-years earlier they saw fit to published a letter of mine that indirectly addressed the relatively simplistic subject of 'a helicopter in every garage'.

Yesterday, the letter was e-mailed again, with a 'Request Read Receipt'. Should there be no response, the letter may be placed on PPRuNe. Members may then wish to suggest reasons as to why it was not published.

[NickLappos]

Mart,
The 2 rotor speed strategy for the ABC was a product of the fact that the rotor is a big vibration machine, and its stability, as well as the response of the fuselage to its forcing function are very hard to understand and control over a broad continuous speed range. Settling on two Nr settings, and allowing only transient operations between vastly simplifies the problem.

As an example, the entire Tilt Rotor design problem is to create a wing structure stiff enough to separate the rotor and pylon frequencies (especially the alternating tilt mode and the rotor in-plane modes) from the fuselage natural frequencies. Given the types of materials available (the stiffness especially) there are only a small set of geometries (wing box depth, span and chord) that work at all, and none that work if one does not have very fancy computer tools to predict those modes. The V22 is the porduct of that very fancy structural design.

I am sometimes amused at those (Dave?!?) who design a purely aero solution and do not account for the weight and structural impacts of the design decision. This is actually backwards in the real world.

[Dave_Jackson]

Nick said;
"I am sometimes amused at those (Dave?!?) who design a purely aero solution and do not account for the weight and structural impacts of the design decision. This is actually backwards in the real world."

Backwards??? Excuse me!!!

Aircraft operate within an aerodynamic medium. Vibration and weight, plus the resulting structural problems, are nothing more than undesired byproducts of imperfect aerodynamics.

Improving the aerodynamics of a craft results in a reduction of ALL sources of vibration. Improving the aerodynamics of a craft results in a better lift/weight ratio.

Add-ons, such as bifilars dampers and tail rotors, are only weighty 'band-aids', which attempt to compensate for imperfect aerodynamics.

I am spending my own money and time to openly research Independent Root & Tip Control - Torque Tube Method . Obviously, improving the lift/drag ratio over the area of the disk will reduce the above two problems ~ at the source.

What have the financially endowed done to significantly improve VTOL craft?


Dave

[Graviman]

Come on Dave, give a little - these discussions are interesting.

Originally Posted by NickLappos

The 2 rotor speed strategy for the ABC was a product of the fact that the rotor is a big vibration machine, and its stability, as well as the response of the fuselage to its forcing function are very hard to understand and control over a broad continuous speed range. Settling on two Nr settings, and allowing only transient operations between vastly simplifies the problem.

Nick, I can definately understand why for what was already an innovative machine the 2 speed option was chosen as carrying the fewest risks. FEA techniques will have moved on since the S-69 was designed, with the possibility of coupled models using aerodynamics and forced modal response. With accelerometer and gyro feedback allowing vibration reduction of rotor dynamics, i would be interested to see how the X2 control system evolves.

Originally Posted by DaveJackson

Aircraft operate within an aerodynamic medium. Vibration and weight, plus the resulting structural problems, are nothing more than undesired byproducts of imperfect aerodynamics.

Dave, if it flies like a dream for 10 hours then suffers fatigue failure by wing/pylon torsional modes it is no good. I agree with the concept of perfect aerodynamics, but it has to work. In truth i don't understand why many of your concepts are so much technically better than anything else. The rotorcraft is a collection of compromises, and can never be the "perfect flying machine". Many specialists from various disciplines coming together solve the problems to improve the resulting compromise, but compromise it must always remain.

Why not prove Independant Root and Tip Control, then demonstrate some test results applied to various symetrical rotorcraft. This will push the envelope, and lead to the next set of compromises to overcome.

Originally Posted by DaveJackson

What have the financially endowed done to significantly improve VTOL craft?

When X2 does fly, it will be a significant contribution to manned flight. I dare say there are all sorts of difficulties to overcome - first flight by Nov was ambitious. In the interim SAS and FBW will significantly contribute to the reduction of accidents in marginal conditions.

Besides are you seriously telling me that the ultimate VTOL the Grumman LM didn't inspire a generation?

Mart

[Dave_Jackson]

Mart,

if it flies like a dream for 10 hours then suffers fatigue failure

Are you saying that 'fatigue failure' is not related to the problem of 'vibration', which was discussed above?

When X2 does fly, it will be a significant contribution to manned flight.

Your comment is a presumption, and hopefully a correct one.
However, it should be noted that many of the 'probable' improvements were recommended 25 years ago. Improvements such as; reduced twist, larger taper, reverse velocity blade profile, all composite blade construction, reduced parasitic drag, propellers for propulsion, etc., etc.

IMHO, the Coaxial ABC should be a significant advancement in rotorcraft. The question is therefore, 'why wasn't it's development continued 25 years ago?'


Dave

[Graviman]

Originally Posted by Dave_Jackson

Mart, Are you saying that 'fatigue failure' is not related to the problem of 'vibration', which was discussed above?

Dave, we are discussing why Sikorsky opted for a 2 speed control system over continously variable speed. My response considered the comment made about V22 having many structural challenges, being designed around an aerodynamic concept. For the S-69 analysis techniques were not far enough advanced to be certain of avoiding rotor system induced resonance, so the problem was simplified by sticking to 2 speeds. For X2 analysis techniques have moved on, but the aerodynamics is one set of criteria which will be mixed with structural and powertrain considerations in the final compromise.

Mart

[Dave_Jackson]

Mart, you say;
we are discussing why Sikorsky opted for a 2 speed control system over continuously variable speed.

This may be a presumption. The statement mentioned by IFMU is "The main rotor will be slowed during high speed flight ..."

It will be interesting to see if the rotors are 2-speed or variable-speed. It will also be interesting to see if the propeller is 2-speed, or variable-speed; or perhaps variable pitch.

Of greater interest, IMO, is the earlier PPRuNe discussion, where the need for rotors with more than one (slow) speed is questioned.


Dave