Why are Helicopters with the Flettner-System so slow?

[hotzenplotz]

The title is saying it already:
.
I wonder why why Helicopters with the Flettner-System are so slow?
.
Flettner Fl 282 B : 43 kts
HTK / H-22 / K-225 Mixmaster : 62 kts
HH-43 Huskie : 104 kts
Kaman K-Max : 100 kts

[Graviman]

Hotzenplotz,

Interesting observation. This may be simply to do with the mission profile for which this type of machine is designed. Most of these machines do not have the horizontal stabiliser which forces the fuselage attitude to present a profile for minimum drag. Two hubs will suffer over one hub, although with correct attitude and fairing i gather this can be minimised.

Mart

[NickLappos]

The speed of a helo is almost always set by the stall propensity of the rotor. The less remaining thrust on the rotor in a hover, the less the amount of thrust available to overcome retreating blade stall on the "downwind" side of the rotor in forward flight, and thus the lower the Vne.

It is also a fact that rotors want to be near stall to be most power efficient, so that the early recip helos all had to be sure they squeezed out all the hover lift they could just to get airborne. In other words, the blades were purposely made with much less chord, so they had to be operated at a higher pitch angle in a hover, and therefore at a better L/D. This thin chord rotor stalls much sooner, at lower airspeed. Why just recips? Because the recip engine is so heavy for its power that a recip helo typically spent 10 to 13% of its max gross weight on the engine (vice 4% for a turbine), so it has to pinch pennies when it comes to power, or it won't carry any payload. 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. This meant that recips were always designed to be near stall in a hover, making Vne very low in airspeed. That is why the Vne of almost all recips is at 100 knots or so, and the earliest recips had Vne of 50 to 60 knots.

A second contributor is that cruise drag is overcome by the rotor by using more thrust, so that a very draggy helicopter needs even more rotor thrust to pull itself thru the air, thus making its Vne even lower. The Fletner is especially draggy, so it would eat up rotor thrust (and therefore speed) fighting the drag.

[Graviman]

Why just recips? Because the recip engine is so heavy for its power that a recip helo typically spent 10 to 13% of its max gross weight on the engine (vice 4% for a turbine), so it has to pinch pennies when it comes to power, or it won't carry any payload. 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.

Nick, i remember hearing somewhere than Kaman were the first to switch over to turbine helicopters. Then again their machines always are sold on efficiency, so they are probably using low chord blades. I don't think i've ever seen any real attempt at fairing intermeshing masts for low drag either.

Were S69 ABC blades designed with lower than normal chord? I imagine that X2 will demonstrate that rigid rotor counterrotators have good efficiency potential at all airspeeds, by simply avoiding retreating blade stall. This would especially be the case if the design is eventually able to beep down RRPM at high speed.

Mart

[Dave_Jackson]

hotzenplotz

The Flettner Fl 282 was one of the very earliest helicopters. It is reported that the maximum speed of the Flettner was 93 mph, where as the maximum speed of the Sikorsky R-4B was 75 mph.

Later, Charles Kaman left United Technologies, and with $10,000.00 he started to develop a helicopter. I suspect that he could not compete head-to-head with the advantages that Sikorsky had, and he therefore went after a niche market.

It can be argued that Kaman did not advance the intermeshing configuration when he replaced Flettner's rotor heads with simplistic teetering heads.

IMHO, Kellett (http://www.unicopter.com/0896.html) was headed in the right direction. He wanted to develop a rigid 3-bladed intermeshing helicopter. Unfortunately, the death of his test pilot, due the failure of a control linkage, apparently ended his chances of getting the one million dollars that was needed to continue this development. His own death a year later would have brought final closure to his aspirations.


This web page may be of interest. Intermeshing Configuration - Concerns (http://www.unicopter.com/B280.html)


Dave

[NickLappos]

Grav,
The concept is called "aerodynamic blade loading" and is abbreviated as Ct/sigma. Most helo aero books discuss it well, the best is in Stepniewski and Keys, frankly.

The turbine helo power to weight ratio varies from 3 to 10 Hp per pound, a recip never got more than about 1 HP per pound. Thus, the turbine engine was so light, the designer could put on on that was "too big" and reach for speed by trading down hover efficiency for cruise speed. That is why the S-58 has a Vne of 110 knots and the S-61 has a Vne of 144. That jump is purely the product of making the rotor chord match the high speed cruise needs instead of hover efficiency.

[Graviman]

The concept is called "aerodynamic blade loading" and is abbreviated as Ct/sigma. Most helo aero books discuss it well, the best is in Stepniewski and Keys, frankly.

Thanks Nick. I'll have a look in S&K this evening.

Mart

[Dave_Jackson]

Mart,

These rotor specifications on the Sikorsky S-69 (XH-59A) ABC (http://www.unicopter.com/A064.html) might help with your calculations.
_________________________

Of related interest:

Here is a web page on the little known Praga (http://www.vrtulnik.cz/praga.htm) , an early Czechoslovakian intermeshing helicopter. The pictures are easy to read. :)
Web page link thanks to quadrirotor.


Dave

[Graviman]

Understand now, Nick. C.N.Keys covers this well in "Chap III: Forward flight; Sect 6.1: Rotor Stall Limits Methodology".

Interestingly, you could almost put forwards a case for increasing single rotor RRPM with speed to keep rotor advance ratio down (ignoring compressibility). Maybe control system reducing RRPM in hover, for longer duration. I certainly understand how increasing sigma helps overcome the reducing Ct/Sigma with mu.

Still struggling to work out why hover power goes up with solidity ratio. I take it the blade section and twist is chosen to minimise high speed cruise power. This means section min drag AOA is above hover AOA @ 0.75 radius (although this implies camber - reflex perhaps to avoid divergence). Clearly good twist for high mu will be bad for hover.

----

Definately, unloading the retreating blade is the way to go. I find myself wondering how an intermesher developed with 15% effective offset hinge would perform. Certainly there would be negligable risk of blade clash, unlike coaxial on startup with "wrong" pedal applied :eek: . The other possible advantage is that, for upper advancing, you get inbuild lateral dihedral without the need of Comanche style air speed sensors.

Hmmm, maybe Dave has a point... :ooh:

Mart

[Dave_Jackson]

Mart said;
Hmmm, maybe Dave has a point... :ooh: And what point might that be? :ooh:

[NickLappos]

Grav,
Congrats on doing the studying!! (BTW Chuck Keys was on the Comanche team) And on really understanding the issues:

Nr increase with speed or maneuvering - right on, Comanche set its rotor speed automatically based on maneuver state, see patent number 4,998,202 at http://patft.uspto.gov/

The reason why the hover efficiency increases with reduced solidity (or increased Ct/sigma) is that the profile power is too high when a big blade is hovered. Things want the blade to work more than a wide chord blade must work in the hover, so skinny blades at more angle are better. See in S+K where the Figure of Merit is plotted vs Ct/sigma and note that the Ct/sigma wants to be about .125 or more at peak FM. Since most high speed helos want to have hover Ct/sigma at about 0.07 to .08, that means they throw away a significant rotor efficiency potential (about 7% thrust at equal power.)

Why not beep down in a hover and up in cruise? We do, the S76A allows that. But not all helos can work at wide rotor rpms, vibration, tail rotor thrust and shaft critical speeds all need more work than it is worth for some designs.

[Dave_Jackson]

Nick & Mart.

Nick, there is no argument with what you say. However, your statement "that the profile power is too high when a big blade is hovered" got the neurons firing.

The theme of this thread relates to forward speed and a craft with twin main-rotors. The three of us, and others on the forum, have an interest in what might be called the next generation rotorcraft. Therefore ...

During cruise, all future rotorcraft will likely slow their rotors. This reduced rotational speed strongly suggests that their chords will probably be increased over that of current rotorcraft. During hover, a larger chord will increase the profile drag (as you say), however, a slowing of the rotor speed during hover will reduce the profile drag. This raises the question as to which will win the tug-of-war over the profile drag.

The following results are calculated from Prouty's 'Combined Momentum and Blade Element Theory with Empirical Corrections', in the chapter 'Aerodynamics of Hovering Flight'. It consists of incremental changes to the blade's chord and then finding the RRPMs that give matching thrusts.

The constants are: Pitch = 8º. Taper = 0º. Twist = 0º. Blades = 4. Profile = NACA 0012

http://www.unicopter.com/1090.gif

Interestingly, after excluding the first column, the horsepowers do not vary much. This appears to imply that on high-speed rotorcraft, wide chord blades will not be detrimental to hover.


Dave

[NickLappos]

Dave,

if you can, rerun your chart to add a column for Ct/sigma, and you will see that the wide chord blades are running at the same Ct/sigma ratio, because you are running them slower. The point is that they have to run at a significant angle of attack, so wider blades must be run slower to keep the ratio of profile to induced power in the optimum range. If you slow all blades down and look for which blade gives the least power for thrust, it will probably be the skinniest blade run at a somewhat higher speed.

Also, take just one blade and vary the tip speed to see what the power does (again, show Ct/sigma) and you will see that the blade uses less and less power as you slow it down until Ct/sigma gets to about .12

Also, use some parametric for blade weight (perhaps blade volume would be a good first approximation?) so that you can see the influence of blade system weight on a designer's task. My guess is the fat blades weigh far more than the skinny ones, which mean the head, shafts transmission housing and mounts all weigh more to support the higher loads.

[Dave_Jackson]

Nick,rerun your chart to add a column for Ct/sigma, and you will see that the wide chord blades are running at the same Ct/sigma ratio, because you are running them slower.The program has been rerun and rows for [Ct] and [Ct/sigma] have been added to the above chart.
Actually, the Ct/sigma ratio varies with changes to the chord + rotor speed.

I agree with your latter comments, as they apply to today's conventional rotors.

My previous posting should have been clearer in emphasizing that it applies to future rotorcraft. Rotorcraft such as Sikorsky's X2 (http://www.unicopter.com/1465.html) and my Intermeshing UniCopter (http://www.unicopter.com/UniCopter.html) and Interleaving (http://www.unicopter.com/1121.html)concepts. Their rotors will turn slower during high-speed cruise and the craft will be primarily supported by the advancing blades. This will necessitate wider chords.

Their greatest demand for power will be during high-speed cruise and they will therefore have plenty of excess power for hover. I just wanted to see if the wide chord would be detrimental during hover.
It appears that it will not be detrimental.

As you say, the weigh will obviously be greater for the wider chord, but, it will probably be less than adding separate wings, which the compound helicopters must do.


Dave

[NickLappos]

Interesting! Can you run the 1' chord blade down in speed, increase collective to keep Thrust constant and see what happens as you make that balde go to .12 Ct/sigma?
Thanks!

[Graviman]

Dave, seriously good effort! :ok:

Maybe keeping the pitch of the NACA 0012 at 8º explains why the power is varying as it does - higher speed means more drag. Normally the wider chord would operate at less pitch, at the same RRPM in hover as cruise (except in S76A). I imagine figure of merit suffers for wider chord blade with camber, operating below optimum pitch. :confused:

Nick, thanks for explaining this. :ok:

Couldn't find diagram in Keys (looked pretty hard), so am referring to
Prouty "Helicopter Performance Stability & Control"
Figure 1.12 - Figure of Merit vs Average Lift Coeff <Cl>
(Where <Cl> = 6* Ct/Sigma).

...note that the Ct/sigma wants to be about .125 or more at peak FM. Since most high speed helos want to have hover Ct/sigma at about 0.07 to .08, that means they throw away a significant rotor efficiency potential (about 7% thrust at equal power.)

For maximum ideal twist:
Peak FM of 0.8 gives <Cl> of 0.76, or Ct/Sigma of 0.127
Hover Ct/Sigma of 0.075, or <Cl> of 0.45, gives FM 0.7.
So hovering wide chord blade rotor operating at 88% of what the "skinny blade" ideal could achieve.

Seeing Dave's data leave's me more confused, but i'll wait until Ct/Sigma = 0.12 data.

Mart

[Dave_Jackson]

Nick,

As requested.

The constants are: Chord = 1 ft. Aspect ratio = 20:1. Sigma = 0.064. Taper = 0º. Twist = 0º. Blades = 4. Profile = NACA 0012

http://www.unicopter.com/1090-A.gif

Dave

[NickLappos]

Dave,
As predicted, the HP drops as Ct/sigma rises to .12 If you keep running the Ct/sigma up, the power will start back up again.
Note that the 1 foot chord at Ct/sigma of .12 uses about 9% less power than any of the other cases use.

[Dave_Jackson]

All very interesting.

Nick, your statement " As predicted, the HP drops as Ct/sigma rises to .12 If you keep running the Ct/sigma up, the power will start back up again.' is certainly not being questioned. However, you picked a specific chord size and this changed the chord from a variable to a constant.

For the fun and the knowledge, another comparison is made. The turquoise is the [Chord: = 2.5'] column in first chart above. The yellow column is the same as the turquoise except that the Blade Pitch has been changed to 13º. This 13º is the pitch in the second chart where the [CT/Sigma] = 0.121.

The constants remain the same.

http://www.unicopter.com/1090-B.gif

This third chart shows that changing the chord from 1' to 2.5' drops the horsepower further, from the 358 hp in the second chart, to 260 hp.

This suggests that a low solidity ratio is good for existing helicopters, for a number of reasons. One is the utilization of centrifugal force.

However very, very early attempts at hovering, were best served by a very high solidity ratio, to work with their slower rotational speed. Of course, to keep the weight down they used cloth for the airfoils and guy wires for the strength.

It appears that future helicopters will move back toward the earlier ones in that they will have large chords and slower speeds. Of course, the cloth and guy wires will be replaced by composite construction.

Any thoughts?


Dave

[NickLappos]

Congrats, your data is great! Now run that super-efficient 2.5 foot chord case (260 HP) at 80 knots and see what its stall behavior is. This proves my point, I think. The low speed of Vne is a necessary tradeoff against the hover efficiency.

Also, don't think the 2.5 foot chord is better than the 1 foot chord, until you run the total system weight for it, and a thinner chord blade set. My guess is that the helo with 2.5 foot case has an empty weight of one or two hunderd pounds more than the 1 foot case.

[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. :ok: 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]

Dave,

http://www.1940.co.uk/history/article/wallis/wallis.htm
http://www.effingham.co.uk/kgv/wallis.htm
http://www.largemodelassociation.com/john_greenfield_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=nanotube&hl=en&lr=&sa=X&oi=images&ct=title

[Graviman]

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! :eek:

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/showthread.php?t=252205&page=2 and most of the following posts on that thread should have been posted on this thread. :uhoh:

________________________________________________________



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 (http://www.unicopter.com/B263.html#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]

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 (http://www.unicopter.com/B263.html#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:
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 (http://www.unicopter.com/1090.html#Calculations) suggest the preference is single-speed rotors.

_______________________________________________________

And furthermore :bored:

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]

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 (http://www.unicopter.com/0891.html#Technical_Documents) 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. (http://www.unicopter.com/0949.html) '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. :O

[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 (http://www.unicopter.com/1096.html). 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? http://www.unicopter.com/Think.gif


Dave

[Graviman]

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.


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.


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]

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

[Dave_Jackson]

Re Speed:

Here's a picture of an enclosed Flettner FL-282. This picture was taken 1941, a year before Sikorsky came out with the 'boxy' R-4.


http://www.unicopter.com/Temporary/Flettner-GermanBook3.jpg


http://www.unicopter.com/GreenAndBlackStripe.gif


Sometimes misconceptions are born out of misrepresentations (politely called marketing).

From 'IGOR I. SIKORSKY-THE MAN - His Aviation Firsts' (http://www.sikorsky.com/frames/external_links/page/1,9607,CLI1_DIV69,00.html?extlink=http://www.sikorskyarchives.com)

1943 ~ R-4 First helicopter to land on a ship - Bunker Hill. . . . . . September 1942 ~ The FL-282 landed on the M.S. Greif. (http://www.vrtulnik.cz/flettner282.htm)

1943 ~ R-4 First mass produced helicopter
1945 ~ R-4 is the only helicopter to serve in World War II' ' ' ' ' ' ' "The Kolibri "Humming Bird" was the first helicopter put into mass production" [10 BEFORE 1943; plus 14 during 1943 and 1944] "and the only helicopter to make any significant contributions in World War II." (http://www.warbirdsresourcegroup.org/LRG/fl282.html) In addition, the Focke Achgelis FL-223 was also mass produced [3 BEFORE 1943; plus 20 during 1943 and 1944] and it saw limited use in World War II.


Who're you going to believe?
I'll go with the very researched and detailed book 'Helicopters of the Third Reich' by Steve Coates.


Dave

[NickLappos]

Just as with your ability to bend aerodynamics to suit your proclivities, you can sometimes embrace new versions of history that suit your need for "symmetry" in your life (and also that the forces of evil hold back the clearly superior designs that you love so much)!

The truth is not quite so boldly deceptive as you would wish (that web site you use as "proof" is a Luftwaffe web site that goes out of its way to show how wonderful everything was back then when Nazis ruled the known world):

The Flettner Fl 282 was very successful, and about 24 were built. It did operate from ships in tests in 1942, when only 3 experimental prototypes existed. That limited trial is what your web site photo shows. These sea trials were possibly before an R-4 landed aboard ship in 1942, I cannot find dates for either right now.

Regarding "rate production" I believe the story is also not so clear as your revisionist theorists would believe. Thanks in part to US 8th Air Force bombardiers, starting in 1942, a total of 24 282's were produced before production ended sometime around mid 1943. During that time, 130 R-4's were built and delivered to the US and British military from the Bridgeport plant. It could be that the R-4 was earlier, or that the two were contemporaries. By war end, over 400 R4's were delivered.

No doubt, the Flettner was very maneuverable, safe, and effective, and if its factory had not been bombed, it might have given the R4 a run for its money. As it is, the story is not as clear as you seem to believe.

http://www.centennialofflight.gov/essay/Rotary/flettner/HE6.htm

[Graviman]

Dave, why are you basing your arguements on machines that flew over half a century ago? Outside of the historical interest, i couldn't care less whether R4 or FL282 are technically better or which did what first. Both are good pieces of engineering, both had flaws which have needed to be overcome.

There is only one way for a good idea to survive, and that is through technical merit. This means that a specific problem must be identified, in order to start the process of finding a solution. If the problem is retreating blade stall, you will get no arguement from anybody that counterrotating is the solution. If the problem is just aesthetics, you will get no support from anybody. Engineers just don't think like that, and rightly so. When sales have asked the impossible, finance has offered a pittance, and service just want something that works, it it the engineer that has to dig his way out of the resulting hole. Aesthetics just are not part of the decision process.

You frequently site nature as being the inspiration of technology, so let me ask you this: Did nature make it to the Moon?

Mart

[Dave_Jackson]

Nick,
You say;
The truth is not quite so boldly deceptive as you would wish (that web site you use as "proof" is a Luftwaffe web site that goes out of its way to show how wonderful everything was back then when Nazis ruled the known world): :=
Not quite "the truth".

The picture in my posting comes from the book Hubschrauber und Tragschrauber (http://www.amazon.com/Hubschrauber-Tragschrauber-Entwicklungsgeschichte-internationalen-Gemeinschaftsentwicklungen/dp/3763761152/sr=11-1/qid=1166650065/ref=sr_11_1/103-4266018-7542269)

The production statistics come from 'Helicopters of the Third Reich' (http://www.amazon.com/Helicopters-Luftwaffe-Classic-Luftwaffe-Classics/dp/1903223245/ref=dp_return_1/103-4266018-7542269?ie=UTF8&n=283155&s=books). Appendix II lists and provides information on everyone of the Fl-282s and Fa-223s. Oh, by the way it's author is Steve Coates, who lives in England and published the book in 2003. You may wish to read the Editorial Review and the Customer Review if you appreciate facts and detail.

When evaluating Fact versus Fantasy, consider the factual competence of the above versus Igor Sikorsky's "Two rotors are like two women in the kitchen. You might think they would do twice as much work, but the efficiency of each is lower-ed by 35 percent." (http://www.sikorskyarchives.com/charac1.html), previously mentioned by IFMU.

Perhaps one of Sikorsky's newly hired 'Quality Inspectors' should be sent over to the Marketing department.

___________________________

Mart,why are you basing your arguements on machines that flew over half a century ago? Because the title of this thread is 'Why are Helicopters with the Flettner-System so slow?' All I am showing is that the 'Flettner-System' was not slow. The direction that Kaman took the interleaving (synchropter) configuration created the conception of slowness.

Did nature make it to the Moon?
If 'nature' means 'life' then appears that the answer is; - Yes. 'Life' rode a comet. Is not the comet an appropriate 'vehicle' for the enviroment of space?


Dave

[Graviman]

All I am showing is that the 'Flettner-System was not slow. The direction that Kaman took the interleaving (synchropter) configuration created the conception of slowness.

Good point well made.


If 'nature' means 'life' then appears that the answer is; - Yes. 'Life' rode a comet. Is not the comet an appropriate 'vehicle' for the enviroment of space?


Well, not sure i'll let you get away with that one. :hmm: What i mean is that no natural lifeform can achieve a deliberate Earth-Moon-Earth voyage without technical means of some description.

Mart

[NickLappos]

Dave,
I think you used those web sites, so I criticized them. I guess someone ELSE posted them in your comments. Those Nazis are everywhere...

Nonetheless, the only 23 Flettners ever built were built around the same time as the 130 R4's. Seems like a draw to me, at best.

The shipboard landing of the Flettner seems to have been performed first, but specific dates are hard to come by. Perhaps the Flettner experimental helos were first. Perhaps not.

It is interesting that you cannot help but grab obsolete data from Jurrasic helicopters to support your theories. With approximately 25,000 aerospace professionals employed daily in helicopters, and world helicopter expenses in the $10,000,000 every year, precisely how many dollars are being spent on your pet configuration?

[Dave_Jackson]

Nick,
The first R-4 landings on the Bunker Hill were May 6 & 7, 1943.

. . .

You say;
... approximately 25,000 aerospace professionals employed daily in helicopters, and world helicopter expenses in the $10,000,000 every year ...
I'm sorry to hear that it's so small. Where did things go so wrong? :E
Igor said;
We shall see hundreds of thousands of privately owned direct-lift machines carrying Americans about their business and their pleasures.

.

Dave :ok:

[NickLappos]

Dave, the VS-300 flew in late 1939, and did many trials during the early years. I will ask Sergei Sikorsky if he recalls any shipboard landings or boat/barge landings in that period. I have seen several photos of the test work then, including boat tow and hoist tests, but none specifically of landings, so the Flettner could be first.

Regarding everyman's helo, that can only come when we solve the problem that kills the trained professionals who fly today, probably with computer controlled/stabilized machines where the crew tells the machine where to go, and IT sees terrain, avoids traffic and harsh weather, and lands itself. With the cost of processing today, some micro-modeler will graft his dime sized control system onto a robbie and the solution will be had.

If everyman flew today's aircraft, there would be no need for birth control!

[slowrotor]

Nick,
The current list price for a "robbie" on their website for an average equipped 22 is $215,000.
Will a computer chip bring the cost down?

[Dave_Jackson]

Nick,

Your comment "Regarding everyman's helo" recalls the last two paragraphs in post #34 on this thread.

The provocative 'Letter to the Editor' will be posted on a new thread and the responses (if any) may be interesting. :ooh:


Dave

[IFMU]

Igor said;
We shall see hundreds of thousands of privately owned direct-lift machines carrying Americans about their business and their pleasures.
Igor was a technical visionary, but I believe he fell short in some of the business vision. Part of his early business vision was that flying boats would always be more viable that landplanes that needed runways, and that many smaller aircraft would be more practical than lesser large ones. On the bright side, when the flying boat market failed, he got to get back to his dream of perfecting the helicopter. Interesting that although his technical vision started with coaxials, it matured into the conventional single main rotor and tail rotor.
-- IFMU

[Graviman]

Nick,
The current list price for a "robbie" on their website for an average equipped 22 is $215,000.
Will a computer chip bring the cost down?

Slowrotor, i began in an industry which pumps out 5000 vehicles every week for an average model. The modern car delivers reasonable quality at a good cost because of these volumes: Tooling for a panel can be $1'000'000, but each car makes a very small profit (including paying off tooling investment).

If helicopters are made easy enough to fly that training becomes less of an issue then production volumes will go up. This means that parts which are machined can be forged, parts which are forged can be ADI pressure cast and so on. It also means that suppliers are willing to risk more development capitol for system development, so more components become off the shelf.

I don't think that there will be a helicopter on every driveway, primarily since a low disk loading still requires good piloting common sense. I do think that by further increasing the versatility of this incredible machine, production volumes will at least double. A future rotorcraft equivalent of today's Robbie might be $107'500.

Besides, the next aerospace breakthrough may not be so far away. ;)

Mart

[Dave_Jackson]

IFMU said;
Interesting that although his technical vision started with coaxials, it matured into the conventional single main rotor and tail rotor.
True, the single main rotor and tail rotor has reached its maturity.

Now Sikorsky, and Bell with its tiltrotors, are moving into Generation II rotorcraft, which necessitates the use of twin main rotors.

Dave

[Jack Carson]

I believe if you look into the production cost of the R-22 it would be very similar to that of a production Cessna 172. Between 60 and 80 thousand dollars of that cost is attributed to product liability which has to cover the aircraft for its anticipated life. Unlike the automobile industry liability costs for an aircraft have to be absorbed by a relatively small production run. i.e. a few thousand vs. millions. The actual cost for materials and manufacture is only a small element of the sale price.

[Graviman]

Interesting comment, Jack. To my mind this highlights the importance of developing "various computer controlled/stabilized machines where the crew tells the machine where to go, and IT sees terrain, avoids traffic and harsh weather, and lands itself". Once developed the suppliers would have a natural incentive to aim these systems at smaller/cheaper rotorcraft.

Mart

[IFMU]

True, the single main rotor and tail rotor has reached its maturity.
Now Sikorsky, and Bell with its tiltrotors, are moving into Generation II rotorcraft, which necessitates the use of twin main rotors.
Dave
However, I doubt that the single main rotor and tail rotor configuration will go away. As you add stuff that is good for speed, like pusher props & another entire rotor system, or tilt rotors, you are adding weight that is bad for hover. There is an axiom:
"What is good for high speed flight is bad for hover. What is good for hover is bad for high speed flight."
Now granted if you can invent your plastic rotors that change shape as they sling around, then you can scoff at that statement. But to whip out one more Igor Sikorsky quote:
"To invent a flying machine is nothing. To build it, little. To make it fly is everything."
-- IFMU

[Dave_Jackson]

IFMU
However, I doubt that the single main rotor and tail rotor configuration will go away.:eek: Don't be so pessimistic.



Wow! At the top of this page (http://www.unicopter.com/B372.html) is your axiom and below it is your 'plastic rotor'.
:ok: "Great minds think alike."



Originally Posted by I.I. Sikorsky
"To invent a flying machine is nothing. To build it, little. To make it fly is everything."http://www.unicopter.com/RollLaugh_2.gif OK, I give up. Which of his helicopters is he talking about?



Dave