Active Blade Twist, the Tail Rotor, and Future Rotorcraft

[Dave_Jackson]

Active Blade Twist is a field of research that is being pursued by a number of institutions and companies. In addition, ABT is possibly the most likely candidate to take rotorcraft up to the next generation. An overview of Active Blade Twist. (http://www.unicopter.com/B372.html)

Ray Prouty said in Rotor & Wing - January 1999; "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." It is possible that a complete application of ABT (including the related improvements of; Reverse Velocity Utilization, Slowed Large Chord Rotor, and Higher Harmonic Control) will result in a doubling of the helicopter's main rotor thrust. This will move the maximum lift-to-drag ratio of helicopters up to 8 to 12.

It should be noted that the advantages of active blade twist cannot be applied to the tail rotor. In addition, consider that the tail rotor consumes approximately 10% of the engine's power.

-------------------------------
Mia culpa :uhoh:

Incorrect mathematical calculations removed.
-------------------------------

The bottom line is that Active Blade Twist will significantly improve the helicopter's lift-to-drag ratio but it will not double the justification for eliminating tail rotors.
http://www.unicopter.com/No_Tail_Rotor.gif

Disclaimer: All values are approximations for simplicity of explanation. :)


Dave

[Graviman]

"In addition, consider that the tail rotor consumes approximately 10% of the engine's power."

Agreed that a no torque solution is ideal, in power consumed for a given weight. But, what is ideally required is an analysis of intermeshing vs conventional from hover to high speed flight. Both should be subdivided into with and without pusher prop to maintain hub profile at lowest achievable drag. Attention should be paid to hub fairing too, for best possible results. I am still concerned by N.L.s observation that what you gain from removal of tail rotor, you loose by the drag of a second hub. It is for this reason that i still advocate outboard advancing, with feathered retreating, for the least power requirement at high speed.

Regarding Root/Tip control, i think all agree about it's performance benefits. What really needs to be understood is the performance benefits for a given complexity. Could you simplify the "options" on offer? To my mind a flexible blade with root only control, and tip aero-trimed to fixed AOA, is the least complex approach. At higher speeds, when pusher prop becomes necessary, then the full root/tip control mech can be implemented. I really am not convinced by reverse velocity utilisation, and much favour the idea of just feathering the offending blade...

Mart

[slowrotor]

The 10% loss from a tail rotor is a better deal than 40% overall hit in efficiency that comes with two main rotors(Wayne Johnson). One large rotor will lift more per horspower than two small rotors of the same disc loading.

The best lift to drag ratio is demonstrated by sailplanes (some claim 60 to 1) and they all use ONE wing. Multiple wing gliders have been built and they do not glide as well.

A 10% loss at the tail rotor is the best we have so far. Just make the tail rotor bigger if you are concerned about the power.

[Graviman]

"The 10% loss from a tail rotor is a better deal than 40% overall hit in efficiency that comes with two main rotors"

True, but i think 40% is pessamistic for intermeshers (you're thinking of tandems). The Disc loading will be close to halved for each rotor, while power requirement is above half. My guess is that intermeshers are better in hover, but lose out at some speed, due to increased hub drag. There will then come a speed where a feathered retreating intermesher becomes more efficient again, since retreating tip stall rears it's head on a conventional. The question then is does variable RRPM introduce unwanted retreating blade drag, and indeed is reverse velocity utilisation a better route?

Regarding root/tip control, i see this as a means of making an intermeshing rotor wing think it's a fixed wing. I used to fly gliders, so like the idea of efficiency. Instead of air flowing over wing, wing flows through air at optimised twist. The ideal is if the retreating blades aerodynamically dissappear. If you were really crazy you could put the rotors below the fuselage, with main landing gear on hub masts... :8 :}

Dave, do you have any evidence about efficiency in forward flight of breaststroke vs outward advancing? I think for the topics started in these threads you may need to consider some wind tunnel time.

Mart

[Dave_Jackson]

Graviman;I am still concerned by N.L.s observation that what you gain from removal of tail rotor, you loose by the drag of a second hub. This subject has been discussed previously on PPRuNE. In summary, there are reports stating that two small hubs produce more drag than one large hub. There are also reports stating that drag is significantly reduced when the hub(s) are faired into the fuselage. The greater the rigidity of a rotor, the closer it is located to the fuselage.Could you simplify the "options" on offer? Hell No. :D IMO, one should work towards a theoretically optimal rotorcraft; obviously taking into account the current and near-term developments from outside the realm of rotorcraft. Then, and only then, should one start to tackle the constituents from a lower level and more practical perspective. If alternative A fails then go to alternative B, but never loose site of the principal objective. Simply put, it is a reiterative top down approach.


Slowrotor;
The 10% loss from a tail rotor is a better deal than 40% overall hit in efficiency that comes with two main rotors(Wayne Johnson). What book were you reading:confused: :confused:

The following two quotes are from his bible 'Helicopter Theory'.

"This configuration [twin main rotors] automatically balances the torque without requiring a power absorbing auxiliary rotor. The rotor-rotor aerodynamics interference losses absorb about the same amount of power, however." ~ page 9

"There are aerodynamic losses from the interference between the main rotors and between the main rotors and the fuselage; these losses reduce the overall efficiency of twin main rotor configurations to about the same level as for the single main and tail rotor configuration." ~ page 317

Wayne Johnson provides an excellent coverage of the rotor, however I don't think he delve into related subjects, such as blades and configurations.

IMHO, Stepniewski was the most knowledgeable person when it came to considering alternate configurations. Just before he died a few years ago, he produced a report that evaluated three potential configurations for future rotorcraft. These configurations were; (1) the cold-jet-driven single rotor, (2) the compound, and (3) the ABC intermeshing. The ABC Intermeshing, was his choice. :ok:


Dave

[Graviman]

"Stepniewski ... ABC Intermeshing, was his choice."

I'd say we were agreed there, Dave. What i'm puzzled about is why do you keep raising reverse velosity utilisation? Everything in your last post favours outboard advancing feathered retreating intermeshers. Did Stepniewski have a rotation preference?

"...two small hubs produce more drag than one large hub... drag is significantly reduced when the hub(s) are faired ... rigidity of a rotor ... closer... to the fuselage."

Good point, well made. In fairness a rigid single rotor would still have less drag, but perhaps not significantly so.

Mart

[Dave_Jackson]

Graviman;

There is no conflict.

Twin main rotor configurations, Advancing Blade Concept, Higher Harmonic Control, Active Blade Twist, and Reverse Velocity Utilization are all compatible. Very compatible.

Dave

[slowrotor]

Dave,
Read Wayne Johnsons book page 118 "It follows that by operating the rotors coaxially the induced power required is increased by a factor of ..... 41%. and again on page 119.... "As the separation decreases, the increase in power approaches the 41% of coaxial rotors."

I take that quote to mean that the coaxial configuration requires 41% more power than twin tandem. I can not find any other better reference on the difference between configurations. Several references state that coaxial has double the disc loading, so that should explain the efficiency loss of coaxial.
An intermesher would only be slightly better than coaxial in power efficiency in my view.

P.S. induced power is quoted above, but I bet profile power and tip loss is more also with extra blades.

[Dave_Jackson]

slowrotor,

Sorry. You were reading 'Helicopter Theory'. :O

Adding more blades or increasing the disk loading will increase the power-to-thrust ratio. On pages 118-21 Johnson is using only Momentum Theory, so let's forget anything to do with the blade count.

The 41% (root 2) increase in power is the result of 'high' disk loading. The disk loading is based upon the method that is used to determine the disk area. As two independent disks are moved closer together, both horizontally and vertically, the area of overlap will increase. When they are vertically aligned (coaxial) and are very close to each other, the calculations are based on half the disk area of two identical but independent disks. Therefor the disk loading, in this situation, is twice as much, and the power requirement is 41% greater.

There is no universally accepted standard for determining the disk area of twin main rotor configurations, but this page will give a little more insight into the subject. (http://www.unicopter.com/0949.html)

The optimum configuration for minimizing the power-to-thrust ratio MAY(*) be a 1-blade or 2-blade single rotor. However, other performance characteristics are being demanded from helicopters and this will necessitate three or more blades.

A coaxial rotor with 2-blades per rotor will normally have a better power-to-thrust ratio than a 4-blade single rotor, if all three rotor diameters (not total disk areas) are the same. This is because the tail rotor will consume more power than the interference (less swirl recovery) of two main rotors will consume.

(*) Large slow turning coaxials have been used in an attempt to achieve man-powered flight.

Dave

Edited to clarify, and to add sub note.

[Graviman]

"Twin main rotor configurations, Advancing Blade Concept, Higher Harmonic Control, Active Blade Twist, and Reverse Velocity Utilization are all compatible."

OK Dave, but my concern is to avoid a potential FMECA (Failure Modes Effect and Criticality Analysis), or even dynamic, knightmare. HHC can be done by servo trimming of basic control input, ABT can be be developed independently - but all on one project sounds very risky, especially before the basic ARR concept is proven! If this were a research project for a heli company, i am convinced they would want it broken down into solvable chunks - not that it wouldn't get funded. :oh:

Still, regarding the theory i remain unclear as to whether Stepniewski had a prefered intermeshing rotation direction (at least i couldn't find it). Also if there are adverse interference effects from rotors in close proximity, why use the retreating blade to upset the air for the advancing blade? The reverse velocity utilisation should surely only be used to allow dragless passage of the retreating blade through the downwash (ie no lift). I appreciate your velocity diagrams, but would be much more convinced by either tunnel work or CFD:

http://www.cfdrc.com/bizareas/aerospace/aeromechanics/rotorcraft_predictions.html


BTW the animation is pretty neat...

Mart

[Dave_Jackson]

Mart,but all on one project sounds very risky ...... i am convinced they would want it broken down into solvable chunks It is broken down into chunks. The most difficult (but theoretically best functioning) method of achieving the most difficult feature (http://www.unicopter.com/UniCopter_Blade_NACA00xx.html) is currently being tackled. If it fails then the next best method for achieving the most difficult feature will be attempted (http://www.unicopter.com/1371.html)

After this, the second most difficult feature will be tackled. Top down.Still, regarding the theory i remain unclear as to whether Stepniewski had a prefered intermeshing rotation direction (at least i couldn't find it)This question was answered in a reply to your post, 15 days ago. Intermeshing Helicopters thread posted 8th May 2005 (http://www.pprune.com/forums/showthread.php?s=&postid=1877510&highlight=breaststroke#post1877510)

Reverse Velocity Utilization is little more than a very large Active Blade Twist. (simplistically speaking)


Dave

[quadrirotor]

What do you think of this configuration, for recreational purposes, without the wings and their rig, and a synchromesh rotor with only actuation of the collective pitch of each rotor?
http://www.rotaryforum.com/forum/attachment.php?attachmentid=4618

[Graviman]

OK Dave, another evening spent reading up on choppers! :rolleyes:
But i keep my weekends for engine design. ;)

"It is broken down into chunks..."

OK. Looking through 1st method i'm guessing you've done quite a bit of high tech boat building? I can't really comment since composites to this level are outside of my expertise - willing learner though. I say only: measure twice cut once, but test three times!

Can i suggest another clawback option: "the floating tip method". Basically as BERP approach, in that trailing edge forces tip to fixed AOA. This still gets the torsional flexure development, but avoids complexity of elastomeric bush along blade.


"This question was answered in a reply to your post, 15 days ago."

Doh! := Does this mean you are unconvinced about alternative means to make outward-advance have breaststroke stability?

[BTW was my explanation of breaststroke stability, in that same thread, any use?]


"Reverse Velocity Utilization ... Active Blade Twist."

OK, but my point is why introduce additional vortices from retreating blades into advancing blades (assuming Stepniewski rotation). For Stepniewski's ABC there is a gap between hub and fuselage, while there is none in your intermeshing concepts. Therefore doesn't retreating blade downwash either produce fuselage download or advancing blade vortex slap?

[BTW please note i altered my last post in thread:

http://www.pprune.com/forums/showthread.php?s=&threadid=87769&perpage=15&pagenumber=4

I reread it and decided i was being unecessarily critical]

---

"What do you think of this configuration..."

Looks pretty cool Quadrirotor, but i'll bet it guzzles fuel like there's no tomorrow! Probably easy to fly, but likely to need lots of mechanic time after high hours. I have wondered the same about the Moller SkyCar. Lots of threads on this forum about failed driveshafts and gearbox input/output bearings...

http://www.moller.com/

Mart

[Dave_Jackson]

quadrirotor,

It might be an ideal "configuration, for recreational purposes". In fact, here are some prospective customers looking it over. (http://futureflight.arc.nasa.gov/) ;)

Seriously;
Here is some information on it (the Vanguard Omniplane), and on the similar GE-Ryan XV-5A (http://www.aiaa.org/tc/vstol/augmented.html)

Dave


[German stuff moved to a more appropriate thread.]

[Graviman]

Go on Dave, you can't just duck out when the questions get too hard! :O

Any concerns i raise are not intended to poke holes, but just to understand how your project will deal with aspects that might otherwise cause problems. Having degrees in engineering and physics, i've made a career in thinking outside of the box. I think your basic concept is sound, but (like any project) will need to keep evolving during it's course - you've even convinced me of a few things (like rotor effective dihedral) along the way. If you can defend you project at this stage, it makes it a whole lot easier later on - believe me i know... :ok:

Best regards,

Mart

[Dave_Jackson]

Graviman,you can't just duck out when the questions get too hard! I assume that you are talking about the edits to your previous post.

In addition, it is difficult to answer a 'shotgun' of questions. Fewer questions per posting and better-focused questions are preferred.
Can i suggest another clawback option: "the floating tip method". Basically as BERP approach, in that trailing edge forces tip to fixed AOA. Please elaborate on what you mean by "fixed AOA". The angle of attack at the tip must vary around the disk.Does this mean you are unconvinced about alternative means to make outward-advance have breaststroke stability?:confused: :confused: "Reverse Velocity Utilization ... Active Blade Twist." Therefore doesn't retreating blade downwash either produce fuselage download or advancing blade vortex slap? As I am sure you are aware, Reverse Velocity Utilization means that a large portion of the blade, which is experiencing airflow from the 'trailing' edge to the 'leading' edge, has a negative pitch. This portion of the blade can now also produce lift. A preferred way to do this is to have the ABT have a large range of twist.

The UniCopter is not an ideal candidate for Reverse Velocity Utilization, because the span of the reverse velocity region on the retreating blade is relatively small. However, the speed of the craft might be increased in the future. In addition, it's fun to work on futuristic ideas.

Regarding fuselage download; to have control over the retreating blade's pitch that is located directly above the fuselage will help to; reduce negative lift, minimize vortex generation and help feed air to the upper segment of the pusher propeller.


Dave

[Graviman]

Dave,

"Fewer questions per posting and better-focused questions are preferred."

OK, point well made. This is very difficult when the original post seems to be intended for debate, while i am still very unclear of ultimate project objectives.

Since you mention "no tail rotor" in the first post, i assume you mean intermesher applications. In the previous post on intermeshers, i was still very confused as to why you prefer to have Synchrolite rotor rotation opposite to Unicopter (i see one as a development of the other).

---

"Please elaborate on what you mean by "fixed AOA". The angle of attack at the tip must vary around the disk."

I mean that the angle of attack of the tip relative to the airflow remains at an optimum constant. This means that angle of attack relative to tip path plane will vary cyclically, depending on overall airflow. Root would still be swash-plate controlled.

---


"The UniCopter is not an ideal candidate for Reverse Velocity Utilization, because the span of the reverse velocity region on the retreating blade is relatively small."

OK, so again is the project objective R.V.U or the Unicopter? I am very confused now, since you seem to be tooling up for a rotor system - for what aircraft?

"In addition, it's fun to work on futuristic ideas."

Agreed, but as an engineer i really struggle when there isn't a specific focus.

---

"Regarding fuselage download; to have control over the retreating blade's pitch that is located directly above the fuselage will help to; reduce negative lift, minimize vortex generation and help feed air to the upper segment of the pusher propeller."

OK but, bearing in mind my utter confusion about what aircraft this rotor head is being designed for, an intermeshing configuration would neatly achieve this by feathering retreating blade to local airflow. Hence my suggestion of "fixed AOA" tip.

Mart

[Edit: Typos only]

[Dave_Jackson]

Graviman,

The objective of this thread was to invoke a discussion about what the future of rotorcraft might be as significant developments are applied to the main-rotors. The intent was to consider the probable end of the tail-rotor and the potential for all counterrotating twin-main-rotor configurations.

I think that this thread has run its course.

Dave

[Graviman]

Dave,

OK, but i'm disappointed that every attempt to better understand the project objectives leads to the same impasse. :(

From this thread:

"Simply put, it is a reiterative top down approach"

In engineering terms "top down" means you have established what the final concept will look like, and are developing complete systems specifically for that concept. Bottom up focuses on specific components for general applications...

Mart

[NickLappos]

Tail rotors cost somewhere between 3 to 6% of the power needed to hover, and about 1% in cruise. Dave sells soap very well, and he needs horrendous tail rotor losses (he says 10% when he started this thread) to sell this particular brand of soap he is hawking.

That being said, active on-blade control is the future of helicopter technology. Of course, we could ask dave why that is so tied to the old warhorse he drags out on the forum every 6 weeeks or so, the perennial loser, the syncropter.

[slowrotor]

Good points.
I think there is some desire to find an alternative to the tail rotor but the thinking is usually for preventing tail strikes to passengers and other objects. The alternatives are less efficient but have a reason for use.
If Dave is concerned with power efficiency (and he should be with piston engine) then he could improve the efficiency by a much greater amount with low disc loading.

Back in the 1980's the state of Alaska put out a RFP (request for proposals) for a
"Tailrotorless" helicopter. Preventing strikes was the reason stated in the RFP.
No bids were received from any major helicopter company. Only two proposals came in, one was from me. The state decided to abandon the scheme after a news reporter determined that the real purpose of the RFP was to transfer state funds to some friend of a legislator.

[Graviman]

Nick,

"...active on-blade control is the future of helicopter technology."

Absolutely agreed absolutely.

"...the perennial loser, the syncropter."

For the normal speed range of military helis i agree, although i suspect the synchropter to be more efficient in hover (but perhaps heavier). In truth for the lower disk loadings of civil helis, which are still fuel guzzlers, i can believe the figure gets to 10% (lots of left foot on an R22).

My interest stems from the work done by both Lockheed in the 186/AH-56 and your old workhorse Sikorsky's ABC - both had the posibility of seriously pushing up top speeds. To avoid retreating blade stall drag my gut feeling is that synchropter (outboard forward) offers less drag than coaxial. My frustration is partly that Dave will open up a discussion, but won't give particulars of the application - as a specification this makes life very difficult...

---

Slowrotor,

"...the real purpose of the RFP was to transfer state funds to some friend of a legislator."

Oh the joys of high tech engineering! :ugh: I like the construction equipment (truck) industry for this sort of reason. There is less b*llsh*t than the UK auto industry, and the customer just wants a machine that effectively does a job. I wouldn't mind the transition to aerospace, but not if it means starving! :}

Mart

[Edit: typos]

[Dave_Jackson]

Nick, I think you just slipped on your own bar of soap. ;)

Excerpts from previous threads, which you participated in;In opposition to the western perspective ~ " .... the single-rotor helicopter's tail rotor power consumption accounts for 10-12% of total power." ~ Eduard Petrosyan, Deputy Chief Designer of the Kamov Company Nick, are you putting a little western spin on your percentages? ........

Would you agree that the torque of the main rotor must be offset in both hover and forward flight? In forward flight the 'angular drag' of the vertical stabilizer 'takes over' for the tail rotor?
.... In addition, the latest aerodynamic text (year 2000), by Leishman (University of Maryland) concurs with Kamov's position.

Graviman,

This is my 'top down'. Others may differ.

Top level; The Configuration.
Future commercial and military VTOL craft may involve concepts that are currently no more than dreams. Rotorcraft is subset of VTOL. Rotorcraft's near term future WILL be twin counterrotating main-rotors (w/o tail-rotor).
The intermeshing, the interleaving, the side-by-side, or all of them may be part of the future. IMO, the intermeshing is probably the best prospect for small, high performance rotorcraft.
Next level; The Rotor. The rotor is the essence of rotorcraft. Therefore, my current project is to develop and build the blade. This blade, with minor modifications, should be applicable to all of the above twin main-rotor configurations. In addition, If the current blade design does not work, the production methodology and equipment is still appropriate for the three alternative blade designs.


Dave

[Graviman]

Dave,

"Next level; The Rotor...."

OK, I see where you are coming from now. You have to forgive me for being thouroughly confused, since it was not clear to me what the actual heli project was. There have been quite a few of them...

Can you give a summary of the control system you intend to use with this rotor? I am particularly interested in how you intend to control the tip and root seperately, past the blade hub interface. Indeed, it's due to this system complexity that i suggest fixed tip tabs as a starting point. My concern is how you intend a practical control system that will cyclically alter twist with forward velocity. I presume the average root/tip twist is pilot controlled (or gyro or whatever), then a servo system alters the differential pitch? Is there any way this system can be implemented purely mechanically, for application in an inexpensive light helicopter?

I am not convinced about reverse velocity utilisation because:
1. There is a region (zero velocity circle) where upwash may still occur.
2. It does not answer the drag problems from retreating blade stall at high speed.
3. In an intermesher, use of the retreating blade to produce lift will likely be detrimental on advancing blade.
4. Although improving downwash distribution, it does not allow the optimum downwash that would come from (say) a fixed wing.
5. The reverse flow aerofoil will be at best a compromise, and at worse will suffer structural divergence.

That said, i don't see why active blade twist can't optimise advancing blade, and allow optimum feathering of retreating blade.

How about this system for an intermesher: Cyclic alters tip pitch only, with nominal tip AOA 0deg to airflow (minimise tip losses). Collective alters root pitch, but swash plate arranged to force retreating root to feathered in downwash.

Mart

[Dave_Jackson]

Graviman,

This particular thread was intended to be a general one about future rotorcraft and the features needed to finally take rotorcraft into its 2nd generation.

The UniCopter is a specific attempt to achieve the above and this is not the place to go into its details. To answer the first of your many points, the dry and boring work on this craft's flight-controls can be found here (http://www.unicopter.com/Independent_Root_Tip.html).


Want to get involved in CNC? :O


Dave

[Graviman]

"Want to get involved in CNC?"

Filament winding for blade layup or your Unicopter control system Access database? I know little about either i'm afraid. What did you have in mind?

Mart

[NickLappos]

Dave,
Of course the Kamov people exaggerate the tail rotor losses, they make coaxials! Quote them all you wish, the real tail rotor losses for a single rotor helo are about 5% in hover at MGW and about 1% in cruise.

The S-76B tail rotor consumes 50 Hp while it hovers at 1050 Hp total power from the engines. You do the math, I am too tired of arguing with you!

[Dave_Jackson]

Kamov spin. http://www.unicopter.com/Argue.gif Sikorsky spin.
` . ` . ` . ` . `http://www.unicopter.com/Idea.gif
` .` ."two opposed main-rotors"

[NickLappos]

Dave,
As usual, you think this is a debate of words and theories, and somehow words mean nothing or everything, depending on how they agree with you.

As Lenin said, "Facts are stubborn things."

As the data I provided shows, the S-76B in a steady hover uses less than 5% of its total power for the tail rotor (and it has a particularly high thrust tail rotor). That is a fact, it can and has been measured. Your poor confused mind might think it is "spin" and your clever little icons look cute, but it is a fact, and you are simply not correct.

You are perhaps the ppruner who can least deal with facts that disagree with your opinions. You have several times accused me of "spin" which infers that somehow what I have posted is not true, or is incomplete and misleading. In all threads here and elsewhere, I try to post the facts as I know them, based on hard data and true experience. You glean half-truths from statements and try to make them into proofs for your strange and unproven assertions.

The fact is that single rotor helos use less than 6% of their hover power for anti-torque. Deal with this fact.

[Graviman]

Nick,

Please feel very free to criticise these thoughts - I am extremely interested in both your facts and opinions on this subject:


"The S-76B tail rotor consumes 50 Hp while it hovers at 1050 Hp total power from the engines."

In cruise the vertical stabiliser takes over from tail rotor for anti-torque. Tail rotor loss becomes vertical stabiliser trim drag, but total anti-torque power requirement stays at the same 5%. Result is main rotor power requirement goes up, but tail rotor power will go down to 1%. Additionally in cruise main rotor hub presents a worse aerodynamic profile, unless the horizontal stabilisers produce more trim drag to keep same attitude.

The way i think about synchropters is that the tail rotor "migrates" up to where the main rotor is. The individual hub drag stays about the same, although there will be some adverse interference effects. In hover the synchropter beats conventional for power/weight requirement (better inflow to wake contraction and no swirl), although intermeshing complexity increases rotor mass. During transition to cruise, the synchropter will begin to suffer with a higher power requirement than the conventional (twin rotor/hub interference). By introducing variable blade twist and feathered retreating blade, this disadvantage can be lessened. Again the conventional can have active blade twist, but reverse velocity utilisation never allows ideal downwash distribution (retreating blade zero velocity circle).

The real decider comes when the conventional suffers retreating tip stall, at say >200 kts. If the intermesher is operating in outboard advancing ABC mode, this will start to beat the conventional for total power requirement since conventional can only use forward and rear rotor quadrants for lift. Eventually compressibility will limit top speed, but reduced synchropter rotor rpm will help attain a higher speed. Coaxial ABC is another alternative to the same ends, but will suffer far worse drag since hub fairing is virtually impossible.

Control is, agreed, not an issue. It is very easy by use of either gyro, large stabilisers or servo control to make either helicopter handle well (at a reasonable level of cost and reliability). For this reason breastroke rotation synchropters have, at best, a limited future.


Practical upshot: Only consider intermeshing for speeds > 200 kts.


Does this sound about right, or am i missing the plot somehow? :confused:


Mart

[Edit: to make point more clearly - and resolve headache :{ ]

[Dave_Jackson]

Fortunately, a couple of unemployed soccer linesmen offered their services.


http://www.unicopter.com/Bull****.gif. . Kamov spin. http://www.unicopter.com/Argue.gif Sikorsky spin. . . http://www.unicopter.com/Bull****.gif

[Dave_Jackson]

Mia culpa

The math in the initial post on this thread was wrong.

The main-rotor improvements and the advantage of eliminating the tail-rotor are still valid. However, these improvements to the main-rotor do not proportionally increase the reasons for eliminating the tail-rotor.

Dave