Why are Helicopters with the Flettner-System so slow?
 

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

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

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.

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

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 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


Dave

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.

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

Mart

Mart,

These rotor specifications on the Sikorsky S-69 (XH-59A) ABC might help with your calculations.
_________________________

Of related interest:

Here is a web page on the little known Praga , an early Czechoslovakian intermeshing helicopter. The pictures are easy to read. :)
Web page link thanks to quadrirotor.


Dave

Those pencil scribbles must be mine, but i don't remember making them...
 

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.

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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

And what point might that be? :ooh:

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.

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


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

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.

Nick,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 and my Intermeshing UniCopter and Interleaving 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

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!

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).

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

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

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.

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

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.