Are traditional tail rotors here to stay?

[Velktron]

It seems that none of the proposed alternatives such as coaxial rotors, tandem rotors, synchropters, NOTARs etc. are really up to the job of completely displacing tail rotors, on both full size and model helicopters.

Sure, there are some successful applications of "alternative" configurations such as tandem rotors (Chinook...), Coaxial (Kamov Ka52) or synchropter (Kaman K-Max), but in general all those schemes entail a greater engineering complexity, or other side effects that cancel or limit whatever benefits they may have over the classic tail rotor configuration. A simple example of this is yawing rate: no multi-rotor collective pitch configuration (or single rotor NOTAR etc.) can beat the tail rotor in yaw rate and agility.

Also, the often-purpotred inefficiency and waste of power of a tail rotor configuration is cancelled out by the greater weight, air drag and/or transmission losses of a coaxial or tandem configuration, so much that the world's fastest helicopter (and the world's heaviest helicopter in service) uses a tail rotor (I'd hesitate to state that synchropters are defect-free in that matter, but there just aren't enough examples to refer to).

In the model world, the only viable "alternative" configurations seem to be the coaxial (even though in a very simplified version, with fixed pitch and inertially stabilized upper rotor, typically found in entry-level micros) and the quadcopter (which however has no notable full scale equivalent).

True there are some fully collective pitch model coaxials or even tandems, but these are typically high-cost, one-of-a-time DIY projects, and none of them has as of yet been mass-produced economically (not to mention that tandems seem to require at least 2 or even 3 gyros to be controllable, as they exhibit instability on all three axes).

Then...agility and speed once again: how long will it take to see a tandem or coaxial model heli to fly as fast and with comparable agility to the "average" 3D heli, regardless of the source of power?

[HugePanic]

"real" helicopters dont do 3d-flying (maybee loops and rolls).

they have to travel from A to B, OR lift heavy stuff to C.

when it comes to lifting, kamov and kaman showed that the advantage of counter-rotating rotors can make money.

to the Vmax:
what i really want to see is a design like the bo46 with controled swinging hinges. such a helicopter could be way faster than anything possible today.

but for 3d-flying they both are no good. to much rotating masses..

[Brandano]

Well, the problem isn't with the rotating masses. You double the amount of rotor, but also double the amount of control surface. And you get rid of a whole lot of gyroscopic effects. The real issue is with rotor strikes, and with the increased complexity of the swashplate system on coaxial rotors. Intermeshing rotors don't really have rotor strike issues, but the number of blades on the rotors is limited by the geometry, and the rotor tilt means the tips are closer to the ground than what you would ideally want. In the civilian helicopters this means that to get enough ground clearance the rotors are on top of tall masts. As far as agility goes I believe the Hokum has demonstrated to be as agile as the Apache.
[edit:] The few movies I have sen of the flettner 282 (http://en.wikipedia.org/wiki/Flettner_Fl_282) seem to show a rather agile machine as well, considering the period.

[Velktron]

Well, it's impressive to consider that after Sikorsky's essential design innovation (using a single torque countering rotor instead of whole arrays of them), no alternative has really become mainstream, despite the many "evils" and drawbacks attributed to the tail rotor.

Tandem configuations seem limited to mid-sized and larger helicopters (again, taking note that the world's largest and heaviest helicopter in service is single-rotor!), while synchropters had/have their moments of glory with the Kolibri or the K-max, but for some reason are not very commonplace (and were never used in larger designs for some reason).

For what regards the agility of the Kaman vs the Apache, yeah, it's a demonstrated fact that at that scale, it can be equally or more maneuverable (albeit much more complex mechanically), but I doubt we'll see any coaxials doing 3D in the immediate future, due to cost/mechanical reasons.

Perhaps the future of the helicopter lies in hybrid designs like the Sikorsky X2 or even a comeback of the Fairey Rotodyne (although these are heavily optimized for forward flight or going from A to B, with less emphasis on maneuvering and impressive stunts).

Anyway....I think this is yet another case of how certain full-scale technologies just "get in the way" when applied to scale models: e.g. turbine/turboshaft motors lose their power-to-weight advantage when used on scale models, and multi-rotor model helicopters prove quite clumsy and complex compared to the (perhaps more mature?) single-rotor and tail-rotor technology.

[mnowell129]

"real" helicopters dont do 3d-flying (maybee loops and rolls).


not entirely true, the red bull bo-105 does some pretty amazing stuff, stressed to -1 G.
http://www.redbullusa.com/en/ArticlePage.1165622311204-62961151.1/htmlArticlePage.action#page=ArticlePage.1165622311 204-62961151

[MCarlton]

I think we've stuck with tail rotors on RC for so long because it works well enough at our scales and rotor rpm's not to make it a viable and financially beneficial exercise to invest in the sort of highly complex micro engineering that the alternative would require.

The same is true for full size. If a fairly cheap, well understood technology will do the job well enough to meet the mission spec, then thats what will be chosen.

Alternatives like tandem rotors, developed to allow the same lifting capacity as single much larger rotor, without the problem of making sufficiently rigid very long rotor blades at a cost defence buyers would go for.

Some of the contra rotating designs eg kaman, kamov etc, represent the latest thinking in a field which they have followed for some time now. Early kamov helicopters used this system to allow an "open back" fuselage for easier loading and a shorter overall length for small escort carriers, frigates and the like.

Sometimes, high tech solutions to problems are not the best, but they might get more headlines, more press and more "wow" factor. Important when trying to stake your claim amongst technology superpowers.

[Brandano]

incidentally, coax helicopters were here much earlier than most people thinks: http://www.aviastar.org/helicopters_eng/breguet-dorand.php, http://en.wikipedia.org/wiki/Gyroplane_Laboratoire

[Thomas B]

For what regards the agility of the Kaman vs the Apache, yeah, it's a demonstrated fact that at that scale, it can be equally or more maneuverable (albeit much more complex mechanically), but I doubt we'll see any coaxials doing 3D in the immediate future, due to cost/mechanical reasons.


Actually, if you really study the Kaman K-Max, it is a very simple helicopter and Kaman and other sources present the case that it is among the simplest. The rotor blade mounted servo tab control surfaces help keep the controls very simple. No forest of pushrods or swash plates on or around the rotor heads. The lack of tail rotor drive and the complexity of the tail rotor also helps makes it very simple mechanically.

It is certainly the champion loadlifter in it's weight class and it has controls that are incredible effective.

These simplifications outweigh the minor added complications of the synchro gearbox and the dual rotors.

[Thomas B]

Well, it's impressive to consider that after Sikorsky's essential design innovation (using a single torque countering rotor instead of whole arrays of them), no alternative has really become mainstream, despite the many "evils" and drawbacks attributed to the tail rotor.



It is also interesting to consider that Sikorsky is abandoning that classic configuration as they seek to advance the technology of rotary wing flight.....they are going coaxial to permit very high speeds and addding a pusher prop. Have you seen the new X2?

http://www.aviation.com/technology/080428-sikorsky-x2-helicopter.html

[mnowell129]

. No forest of pushrods or swash plates on or around the rotor heads.
I think the Kaman has a swashplate. It just doesn't have feathering bearings.

[Thomas B]

I think the Kaman has a swashplate. It just doesn't have feathering bearings.

Actually, I am not so sure that it really has a swash plate. It looks more like the rotor controls come up through the shaft and exit to the side of the hub and link to the rotor in a way different than a swash plate. Kaman calls it the azimuth assembly. Compared to a normal heli head, it is a MUCH simpler arrangement. Take a look at the picture I attached and the simplicity is easy to see.

In fact, as far as the rotor head and blade holders are concerned, this is a fixed pitch heli....except for the fact that the servo tabs change the rotor angle of attack outboard of the blade holders by flexing the blade.

The pic is from the interesting link below that goes into some detail about the servo flap system:

http://www.helis.com/howflies/servo.php

[mnowell129]

It doesn't have a "traditional" swashplate, but it does have a similar mechanism to vary the pitch of the servo tabs cyclically. It also varies the tabs collectively for collective pitch.
It also has a pushrod in the blade from the hub to the servo tab, a very looong pushrod.
The kaman system is clever and has advantages, but it's not that much less complicated than other helicopter mechanisms.

[Velktron]

Of course I saw the X2:


Perhaps the future of the helicopter lies in hybrid designs like the Sikorsky X2 or even a comeback of the Fairey Rotodyne (although these are heavily optimized for forward flight or going from A to B, with less emphasis on maneuvering and impressive stunts).

Anyway....I think this is yet another case of how certain full-scale technologies just "get in the way" when applied to scale models: e.g. turbine/turboshaft motors lose their power-to-weight advantage when used on scale models, and multi-rotor model helicopters prove quite clumsy and complex compared to the (perhaps more mature?) single-rotor and tail-rotor technology.

[HugePanic]

In this picture the control-surfaces at the blades are visible.
In my understanding these surfaces control the AOA of the rotor blades.
There must be a swashplate to conrol these padels.
http://philip.greenspun.com/images/200606-i80-helicopter-trip/kaman-k-max-heavy-lift-helicopter-4.4.jpg

[Brandano]

A swashplate doesn't need to be outside the rotor mast. There's other more conventional helicopters where the pushrods run inside the rotor mast. The Augusta A129, for example.

[Thomas B]

In this picture the control-surfaces at the blades are visible.
In my understanding these surfaces control the AOA of the rotor blades.
There must be a swashplate to conrol these padels.


You can do trailing edge flaps to flex the blade without a swashplate. Check out this research paper and test of a small heli modified to be without a swashplate.:

http://research.nianet.org/~shenjw/webpage/ahs59.pdf

Kamans original patent had the servo tabs cable controlled through the rotor shaft.....no swash plate in sight.....:)

http://patimg2.uspto.gov/.piw?docid=US002455866&PageNum=2&IDKey=E0120F85E0EF&HomeUrl=http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1%2526Sect2=HITOFF%2526d=PALL%2526 p=1%2526u=%25252Fnetahtml%25252FPTO%25252Fsrchnum. htm%2526r=1%2526f=G%2526l=50%2526s1=2,455,866.PN.% 2526OS=PN/2,455,866%2526RS=PN/2,455,866

[mnowell129]

Kamans original patent had the servo tabs cable controlled through the rotor shaft.....no swash plate in sight.....:)

http://patimg2.uspto.gov/.piw?docid=US002455866&PageNum=2&IDKey=E0120F85E0EF&HomeUrl=http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1%2526Sect2=HITOFF%2526d=PALL%2526 p=1%2526u=%25252Fnetahtml%25252FPTO%25252Fsrchnum. htm%2526r=1%2526f=G%2526l=50%2526s1=2,455,866.PN.% 2526OS=PN/2,455,866%2526RS=PN/2,455,866

Figure 19 shows what functions as a swashplate, the upper half spins with the shaft, the lower half doesn't. The whole assembly tilts on a ball joint. It uses cables to the servo tabs rather than pushrods to the blade holders, but it's still effectively a swashplate.

[Thomas B]

Figure 19 shows what functions as a swashplate, the upper half spins with the shaft, the lower half doesn't. The whole assembly tilts on a ball joint. It uses cables to the servo tabs rather than pushrods to the blade holders, but it's still effectively a swashplate.

I think it serves a similar function, but it is not a swashplate. It transfers the control movements in a different way than using two rubbing plates.

[mnowell129]

I think it serves a similar function, but it is not a swashplate. It transfers the control movements in a different way than using two rubbing plates.
The two halves of a swashplate don't rub, they have a bearing between them just like this assembly.

[Velktron]

Well...it appears very similar to a swashplate, and since it's capable of collective and cyclic adjustments then it probably is just an undersized swashplate (as it just has to command the servo flaps, not the entire blade).

Again, by the sounds of it, this is yet another example of a technology that wouldn't bring any advantages if scaled down (model swashplates are already smaller -by taking scale into account- than their full-scale counterparts, and a model Kaman would likely have small-coin sized mini swashplates, and a more complex system of servos/pushrods.

The servos are another issue too: even admitting that a servo flap would require a less powerful/lighter servo in order to operate, would that translate in an actual advantage in a model, or would it make no difference due to the lack of sufficiently small/light servos to pick from?

Which brings another question...what system did the Flettner 282 Kolibri use to command its rotors?

[Brandano]

Weeeell... since these are called Flettner tabs... :)
[edit]:
Hmm, I have to eat my own word here. The Flettner tabs are the servo tabs usually applied to control surfaces of aeroplanes, and not necessarily the servo tabs used on helicopters. The few pictures of the Kolibri I can find seem to show a plain rotor blade, so probably the entire blade's pitch was directly controlled. On the other hand, the first use of servo tabs for an helicopter seems to be on the machine created by Corradino D'Ascanio in 1930

[Thomas B]

The two halves of a swashplate don't rub, they have a bearing between them just like this assembly.

They are certainly rubbing through a bearing.....but they are still rubbing.

I suppose this discussion depends on how one defines a swashplate....if your definition is any device that transmits input from a fixed system to a rotating system, every helicopter has one.

If you define a swashplate as one device that does the trick mentioned above, doing so by the interface of two plates rubbing through a bearing...not every heli has a swashplate, per say...:)

[Thomas B]

Again, by the sounds of it, this is yet another example of a technology that wouldn't bring any advantages if scaled down (model swashplates are already smaller -by taking scale into account- than their full-scale counterparts, and a model Kaman would likely have small-coin sized mini swashplates, and a more complex system of servos/pushrods.



Actually, I think one could indeed design a small servo tab helicopter and put the entire radio system in the rotor hub and blade. The only new device needed would be a rotor postion sensor interfaced with a mixing device that could tell the servos when to move the tabs to flex the blade.,,,kind of an on board CCPM mixer. The RX and servo battery could be part of the hub, with servos at the blade roots and pushrods or torque rods or pull/pulls controlling the tabs.

An intermeshing heli could possibly be done with two such systems, with the shafts geared together.

Would such a model offer any worthwhile improvements over current model heli arrangements?...unlikely, but it would be interesting to see if it could be done.

This would certainly be a heli without a swashplate......

[HugePanic]

i saw a working model of a kaman helicopter.
but it was equiped with two swashplates and conventional blade control.
the mixing of the controls was mechanical.

i seached for pictures, but i cant find any... sorry...

[Thomas B]

i saw a working model of a kaman helicopter.
but it was equiped with two swashplates and conventional blade control.
the mixing of the controls was mechanical.

i seached for pictures, but i cant find any... sorry...

I saw that nice large K-Max model....it was done in Germany and definitly has swash plates and conventional rotor controls..no servo tabs.

A very cool model, in any case.

Link to pictures here:

http://www.ofheli.de/Fotogalerie/kmax/index.html

[mnowell129]

Actually, I think one could indeed design a small servo tab helicopter and put the entire radio system in the rotor hub and blade. The only new device needed would be a rotor postion sensor interfaced with a mixing device that could tell the servos when to move the tabs to flex the blade.,,,kind of an on board CCPM mixer. The RX and servo battery could be part of the hub, with servos at the blade roots and pushrods or torque rods or pull/pulls controlling the tabs.

This has been discussed in the autogyro forum before. Servos aren't fast enough. They would have to go from lock to lock in one revolution of the rotor. If you take a low end rotor rpm, say 1000, this is, 16 cycles per second or 33 half cycles per second or a full lock transit time on the servo of .03 seconds. The fastest servos out there are .05 seconds for 60 degrees or 0.10 seconds lock to lock, 3 times to slow, even if they could stand that kind of duty cycle.
Some other actuator might work, maybe a piezo....

[Thomas B]

This has been discussed in the autogyro forum before. Servos aren't fast enough. They would have to go from lock to lock in one revolution of the rotor. If you take a low end rotor rpm, say 1000, this is, 16 cycles per second or 33 half cycles per second or a full lock transit time on the servo of .03 seconds. The fastest servos out there are .05 seconds for 60 degrees or 0.10 seconds lock to lock, 3 times to slow, even if they could stand that kind of duty cycle.
Some other actuator might work, maybe a piezo....

Magnetic actuators would likely work. Also, you could do an unsually slow turning rotor.

[Brandano]

The useful speed range for a rotor is limited on both ends by the forward flight performance you want to achieve. I think making a slow rotor is fout of the question. And depending on the sie of the helicopter, using an actuator might be out of the question too. On a really small helicopter they work, and they have been already tried and tested. On a larger one the weight of the system becomes quickly greater than just using a swashplate.

[HugePanic]

eurocopter did some tests with piezos in the blades. but the idea was to reduce noise, not to remove the swashplate.


(the rotor-part is written in english)
https://www.unibw.de/leichtbau/lehre/seminar/seminare/seminar2003/Seminar03MartinT0.pdf

[Velktron]

I found these interesting articles: http://www.unicopter.com/A085.html

Even if most of them are in favour of the intermeshing configuration, and some contain a hint of conspiracy theorism, they make some pretty good point: http://www.unicopter.com/B280.html

[groundfx]

Fenestron tail rotors seem like a good, practical design improvement over traditional tail rotors (safer, less noise, easier to streamline the heli for fast forward flight.)

How about pie plate notar's. :) Fine for forward flight because there is not much torque. http://youtube.com/watch?v=kvP_bZt65d0

For heavy lifting, a tail rotors really starts to suck power to counteract all that torque. That's when dual counter rotating rotors of some sort are the way to go if you want efficiency.
http://youtube.com/watch?v=Maqy-I9zceM

The good old Chinook is still a pretty good, practical dual rotor design and that's why it's design hasn't changed. It flies just fine... :)
http://youtube.com/watch?v=Hv7fayHSv1M

However, if you don't care about efficiency, you can't argue with simplicity and brute power.
http://youtube.com/watch?v=mq_ih8sj6-g

[Velktron]

Yeah...the problem is that usually simplicity translates to greater efficiency and robustness due to reduced drag, simpler rotor head, and perhaps simpler and more responsive controls (yaw control is very simple with tail rotors, but slower/more awkward with other systems).

[Velktron]

Heh...more conspiracy theories against alternative configurations:

http://www.unicopter.com/B370.html


From a 2003 Article by Tom Lawrence, senior engineer at Sikorsky Aircraft Corporation and author of an upcoming book on the history of Sikorsky helicopters:

"However, the single greatest feature was Igor Sikorsky's faith in the benefits of the single rotor helicopter. Much derided at the time, the single-rotor configuration would come to dominate the worlds helicopters."


Perhaps it's because it just works and is well understood...

[Monacle]

Tail rotor power consumption is stated at about 8% for a heavy helicopter at full gross hover in the Stepenouski book. Which is not bad. The primary purpose of dual rotor helicopters is to get lower disk loading. The limit is the torsional ridgity of blades, which on the Kaman is less of a problem since it is stablized and used a hinge. So the Kaman has the lowest disk loading and hovers at the highest altitude, but it does not go fast. The Chinook carries lots of stuff since the rotors are far apart. The tail rotor issue is not significant except for fine control and running out of control even when the main rotor is still working because the tail rotor was sized for cruise

[Velktron]

Has anyone else have data on how much power is wasted e.g. in the Chinook's transmission or on other schemes? How about when accounting for increased drag/mass etc.?

[Brandano]

Fenestron tail rotors seem like a good, practical design improvement over traditional tail rotors (safer, less noise, easier to streamline the heli for fast forward flight.)
Couldn't agree more. Try and do this with a traditional tail rotor:

http://www.youtube.com/watch?v=RSc5mRTHdVY

I saw one of these guys land on a ski slope in Austria (and I believe this one is Austrian as well). I guess here the trick didn't come out as well, and the helicopter skidded after landing. It's incredible how quiet this helicopter is too, when compared with others of the same size.
Anyway, is this brave, stupid or what? I hope the reason for this maneuver was that there was more pressing business to attend to rather than wait for an airlift (these are heli-ambulances after all)

[HugePanic]

the advance of the fenestron lies at high-speed flying, noise and "security".

at hoverin the performance is worse than a traditional rotor. (smaler blades --> less efficent)

weight is about the same.

big advantages are the reduction noise and the gain in security. both is very important for emergency rescue missions. no rotor-blades cutting away the doctors head helps with first aid... :rolleyes:

what i suspect is that the fenestron is NOT very good at high altitudes. here the bigger diameter of a convential tail-rotor might not limit the performance as much as a silent-fenestron...

[Cory]

the advance of the fenestron lies at high-speed flying, noise and "security".

at hoverin the performance is worse than a traditional rotor. (smaler blades --> less efficent)


It's not true that hovering performance is worse. The fenestron can be designed to have very similar power requirements as a conventional tail rotor during normal flight. At high yaw rates there is a decrease in efficiency......but there are also other great increases in yaw performance. The fenestron does not suffer from quartering tailwind limitations like a conventional rotor. This greatly limits the maneuvers that a conventional can perform and also limits the conventional's ability to maintain a hover in quartering tail wind, high wind speed conditions. The fenestron can also be very powerful. I saw the S-76 Fantail Demonstrator do things that no conventional tail rotored helicopter can do when I was at Ft. Rucker in 1990. Things like spinning like a top while maintaining a heading at about 100 kts, sustained sideways flight at 80-100 kts, sustained backward flight at much higher speeds than conventional tail rotored aircraft, etc. The max yaw rate I observed was several times the rate of our Blackhawks, Apaches, and OH-58s. The reason that the RAH-66 Comanche was redesigned with a fixed gun was that the fantail was powerful enough to basically make the whole helicopter turn at the speeds one would expect from a turret, even while maintaining forward flight at a considerable speed. Control resolution was fine enough to use it for targeting.

[HugePanic]

i got my information from eurocopter. i am sure it is correct for their fenestron design.

i don't know what the guys did with the comanche, but i saw a video that was quite impressive.

but i also don`t know what a bo-105 could do if it had a flight-controller...
does anybody know a good tiger video??

[macr0t0r]

Man, don't know how I missed this thread!

I got to see the K-Max in action at Moffett Field and talk to the guys. I've studied a little on the Fenestrom (http://www.patentstorm.us/patents/7040863/description.html) tail-rotor as well while at NASA. However, that was quite a few years ago.

The major problem is that the K-Max synchropter vibrates like mad. This created havoc with some experimental autonomous control systems as we tried to filter the out the noise from the accelerometers. Also, the "more effiencent" claim is HIGHLY debatable. I figure it would have the same efficiency problems as the coax helis. You get the most performance from a blade when it's driving through clean air. That's why a larger 2-bladed propeller on a plane is more efficient than a 3-bladed propeller. It's better to grab a larger diameter of clean air than to beat the same air twice. With the 2-bladed K-Max, the efficiency drop wasn't much, but the vibration was really bad. With the 3-bladed K-Max, the vibration was reduced, but now there was a drop in efficiency of lift per blade. The inboard/intersecting region of the two blade-discs becomes a pretty thoroughly beaten, turbulent mess.

You'd think the best setup is to separate the blades like in the V-22 Osprey or Chinook. However, now you've introduced a great deal of structure requirements as you pull from the ends of a beam rather than near the center. It worked out well for the Chinook since it was already going to be designed to carry a massive load over the length of it's body, but it's a costly venture for a passenger heli.

Simply-put, things get really complex when you start having blade interference or start putting your lifting power off-center from the CG. A synchropter looks like free money, but the supposedly simple design gets dragged down by many "gotchas" from those titled off-center blades that interfere with each other. Having a tail-rotor that is kept as far away from the lifting disk is a much simpler solution.

Now, about the Fenestrom design. I love it. It's the future. Yes, it's horrendously inefficient in a hover, but that just isn't as much of an issue as it use to be. As a pilot, you won't feel it. It just consumes a bit more energy than a conventional rotor. It's quite a bit more complex, since the thing has to spin at a much higher RPM, and those fins are VARIABLE PITCH! The engineering and manufacturing of those things are amazing. However, it's perfect in forward flight (and helicopters are built to go somewhere) and the peace of mind of not having a rotor-strike is worth any price. Apparently the blades are spaced at different intervals to remove the noise that is created by evenly spaced blades. In any case, the pilots seem to really like them, but the precision required to build them means we won't be seeing too many of them in our models anytime soon. I suppose you could try a fixed-pitch fan, but....well, we all know how much we hate fixed-pitch tail-rotors. ;)

The standard main and tail rotor design is popular because it WORKS RELIABLY. Since helicopters are still finicky beasts, that detail usually becomes the deciding factor. However, I still hope that someone comes up with a better idea someday.

Anyone want to make a better ornithopter? :D

- Jim

[Velktron]

I wonder whether the Fenestron should be classified as "better traditional tail rotor", rather then as a completely different approach like coaxial/intermeshing etc. After all, it's little more than a ducted fan placed where the tail rotor is supposed to be...maybe a contra-rotating pair of tail rotors, ducted or not, would be even more awesome?

[macr0t0r]

Contra-rotating tail-rotor would have nearly zero benefit. It's just really inefficient to beat the same air twice. It's only done to eliminate torque, but the torque from a tail-rotor is pretty small.

I always wondered about the truly bizarre rotor system that the 1/2A Cox free-flight helicopters used 25 years ago. They mounted a small motor on top of the rotor-hub with it;s own propeller. When the motor ran, the propeller supplied some vertical thrust, but it also caused a lot of torque that caused the whole rotor-hub to spin in the opposite direction. The result was that there was nearly zero torque applied to the fuselage (except due to the bearing's small drag). The motor's prop was short with a deep pitch while the main rotors were long with a shallow pitch, so there was little blade interaction. It was a pretty neat system.

This isn't really feasible on a full-scale heli. The shaft-strength to hold up a spinning motor and blades is off the scale. But, it would make for an interesting model heli design.

- Jim

[Brandano]

But on a full size heli you can drive the flow from a turbine along the blades to be ejected by tip nozzles, so that the blades are actually propelled by reaction and the only torque is that caused by bearing friction. This has been done in the past, but it is fairly complex, and requires a good rotating seal that is hard to achieve at large scales. I wonder if it would be possible to make a rotor with a central centrifugal compressor driving compressed air down the blade length and reaching combustion chambers on the blade tips. Again, complex, and you also need to account for fuel lines and control systems going through the rotor hub. one small added bonus is that you can use the compressed air to drive high lift devices on the blade itself, like blown flaps, to get better weight lifting performance or to reduce the size of the rotor.

[macr0t0r]

The Fairey Rotodyne had the tip-jets. Very noisy from what I've read, but very effective for making an auto-gyro be hover-capable without adding the weight of a gearbox.

- Jim

[Cory]

The term "performance" gets used often without defining what the design goal is. When a company such as Eurocopter says something like, "the advance of the fenestron lies at high-speed flying, noise and "security".
at hoverin the performance is worse than a traditional rotor. (smaler blades --> less efficent)", one must realize that while the statement may be true for the particular design goal of the helicopter they are referring to, it does not necessarily apply to a someone else's aircraft. The definition of good hovering performance for a passenger or cargo helicopter that needs to be "affordable" (not for me...dang! :p ) and fuel efficient and probably won't be flown in extreme attitudes or challenging weather, will probably be different than the definition of good hovering performance for a sideways snapshooting, gun toting, stealthily quiet, or, an able to accurately maintain position while rescuing people in adverse weather helicopter. I often see this same lack of definition of the word "performance" in the slope forum where I usually hang out. I'm sure the Eurocopter info is accurate for the aircraft they are talking about.

I found some interesting information about the S-76 Fantail Demonstrator here: http://whyorg.com/air/fenestron.html One of the guys who was involved with the project states, "The S-76
Fantail used exactly te same power as the original tail rotor (in a steady
hover) so the duct really earned its pay....The Eurocopter folks found out when they built a fenestron Puma that the fan doesn't scale up very well, (and bigger helos don't skunk around the trees as
much) so they abandoned it."

I wonder if there could be any "performance" advantage of a funestron (pun intended :D ) on a 3D R/C helicopter, or if it wouldn't work so well at the lower Reynolds Numbers of R/C scales?