How Big For Notar?

[MightyGem]

Is there a maximum aircraft size for using Notar? I'm thinking that the diameter of the fan would eventually become too large to be practical.

[ItchyRivers]

A question in kind of a similiar vein. Im an aero eng student and I want to do a CFD analysis on the cross section of the tailboom of a NOTAR heli.

Would any of the pilots out there give me a rough diameter of the boom and also a picture/description of the two slots running the length of the tailboom. Also, if anyone in the know has any idea of the velocity of air in a heli downwash that would be great.

Cheers,

[MightyGem]

Numerous pictures here, (http://images.google.co.uk/images?hl=en&q=notar&gbv=2) if they're any good.

[Graviman]

ItchyRivers, in principle there is no limit to Notar size. The biggest problem seems to be that Notar is not as effecient as either a tail rotor or fenestron. This is due to the tail rotor offering a good area, and using the front up rotation to gain extra thrust in the main rotor downwash.

A good starting point for your CFD sim is the momentum method hand calculation, as detailed by Prouty (Helicopter Performance Stability & Control).
The calculation gives:

Rotor_Induced_Velocity = SQRT ( Weight / ( 2 x Air_Density x Rotor_Area ) )

While the downwash velocity is 2 x Rotor_Induced_Velocity.

The basic concept behind Notar seems to be to try to force the coanda effect to keep the flow attatched to an unaerodynamic aerofoil (circular section). The practical installation seems to rely on the small cross section air jet to replace the tail rotor. Conventional tail rotors sometimes fit strakes to the boom, but not to produce any torque reaction. Instead these seem to provide a definate flow seperation point, to avoid buffeting in cross winds.

Hope that helps.

Mart

[RVDT]

MG,

Your assumption is probably correct, keeping in mind the escalation in proportions when you "double" the size of something. ECF did fly a Puma with a fenestron and I think it was shelved for these reasons.

Graviman,

Ref the strake, my take on it is if you look at the flow around the boom there is a possible Coanda effect working AGAINST you and the strake is there to make it stall or destroy it.

[Graviman]

RVDT, that may also be true as the strake would act as a spoiler on the "wing" top surface. Not sure in practice whether there would be circulation induced by the main rotor, since the whirl velocity gradient would be small over the height of the boom. MD had to go to some extreme solutions to generate circulation the other way - better to fit an aerofoil trailing edge methinks.

I seem to recall Nick Lappos mentioned that strakes had been tried at Sikorsky, but with no benefit found. Another Sikorsky endeavor, the S-69 ABC, suffered some hover position instability which was cured by fitting ventral strakes to force flow seperation at a fixed position. This reduction in control variation seems to offer a reasonable explanation for any advantages found in the field for fitting strakes. The machine would feel more yaw stable in crosswinds.

Mart

[NickLappos]

Itchy,
Remember that a NOTAR shows its coanda slots as its anti-torque device, but that is hardly effective in most environments, so that the other two anti-torque devices are also needed, and are part of the total picture. The tip thruster is much more capable, and is the true control device in a hover. The tail cone actually sees little downwash while maneuvering in winds as low as 6 to 10 knots, for example, so that the original Notar lost control in low wind hovers until the tip thruster can was installed.
At higher speeds, the coanda is also useless, and the tip thruster is terribly inefficient, so that the rudders are used.

In reality, the Notar is a cobbled up mix of three devices, and has more parts and more complexity that any other anti-torque device. It is, however, very safe for those outside, very quiet, and kinda cool.

[ericferret]

The MD900 FAN max output is 450 cubic feet second drawing 200 horsepower. 5412 RPM providing 1 to 1.12 psi over ambient within the boom.
Control breaks down as follows
Hover Coanda 70% Thruster 30 % Fins 0%
O-50 knots Coanda Reducing, Thruster increacing, Fins Nil
50-80 knots Coanda Nil, Thruster Reducing, Fins Increacing
Over 80 Coanda Nil, Thruster Nil, Fins Full

[NickLappos]

In a 6 knot rear wind, there is no wind on the coanda tail cone:
Coanda = 0
Thruster = 100%
Fin = 0

In a side flight regime of about 10 knots, the above is also true.

[Dan Reno]

When the Army was testing the NOTAR (Explorer model?), the only problem they voiced loudly was it's inability to counteract increasing weights. It's a novel flying machine but you can only push so much wind down it's tail.

[rotorque]

How does blade loading affect NOTARs?

Does hover performance increases with loading due to downwash velocity? Would larger models be produced with higher blade loads?

[NickLappos]

Sure you could make the coanda somewhat more effective by increasing the disk loading. This has the double effect of making more downwash on the tail cone, and reducing payload even further than the Notar already loses (since increasing disk loading is a very good way to waste engine power)!

Nice trade, after a while, you could leave the whole helicopter home, and just have a keen Notar sitting on the pad!

The Notar suffers from what we could call the Discovery Channel syndrome - where old PR baloney is recycled so that new generations of viewers think it is a good idea.

For the record, a Notar consumes more power, leaves home passengers, costs more to make and more to operate, is less mechanically safe than a tail rotor or a fenestron. But its concept sounds so great - something for nothing - that folks just run up and hug it.

[rotorque]

Thanks Nick...

You know as well as most that the PR machines of big buisness are the ones who drive industry.

If the product 'looks good' it will sell, .... put a pretty blonde at the controls of an R22 and even I could be persuaded to buy one, add the catch phrase 'safety' to it and bingo, you have a seller.

What I was aluding to above, was the fact that if someone is going to purchase a NOTAR, for whatever reason, (whether based on a 'safety' sales pitch, because it looks good, or because they have always wanted one) then putting in larger engines etc to increase hover performance may be an option to the would be buyer.

With that in mind, I guess really big NOTARs are possible...

[Graviman]

The Notar suffers from what we could call the Discovery Channel syndrome - where old PR baloney is recycled so that new generations of viewers think it is a good idea.


Discovery Channel synrome? Sounds nasty, you'd think there would be an inoculation or something. Maybe exposing the public to small doses of real knowledge. ;)

Mart

[NickLappos]

rotorque,

I don't think there is anything stopping a Notar from being scaled up, except that a big helo has its tail rotor perhaps 7 to 10 feet from the ground, thus making the Notar's best virtue (and a very significant one) - safety to ground personnel - moot.

[ericferret]

Why is a notar "less mechanically safe than a tail rotor or fenestron" ?

[NickLappos]

Three times the number of critical parts, including a fully controlled tail rotor buried inside the cone, a rotating can for primary yaw control, and a controllable rudder for high speed control. You get three yaw controls because the coanda is so simple!

[Graviman]

Just one thought, Nick:

One of the disadvantages of a tail rotor is the need for a long driveshaft, and additional gearboxes. Indeed this was my main concern with Dave's side-by-side concept layouts - i know how overlooked driveshaft installations often are. Albeit treatable as an engine failure, a tail rotor failure is not a nice event. Even twin engine aircraft still have one TR driveshaft, albeit suitably unstressed.

There must be some advantage in having the driving fan close to the main powertrain? This way "TR" only failure is less likely, since only modes losing MR drive would affect it. The coanda effect boom is a non-starter for me, since blending stabiliser into boom aerofoil makes more sense - and even this doesn't work with crosswinds or tail winds.

So howabout a mixer nozzle, for ItchyRivers to consider? It would use the Notar nozzle concept, but the coanda effect this time would be used to ingest as much surrounding air as possible. Possibly installed as if it was a fenestron. I doubt it would achieve tail rotor efficiency, but it might have applications for mid-size machines...

Mart

[NickLappos]

Grav,
Good comment, and part of the choice you make. The shaft for the Notar's tail rotor is a very short one, and it is virtually a direct connect to the MGB. But there are two separate, long and flimsy controls for the rudders and the rotating can.

Horses for courses, but let nobody tell you coanda is simpler!

[rotorfossil]

When the MD 520N first came out, I did a flight test on it for an article. As it was directly comparable at the time with the MD 500, a pencil and fag packet calculation showed that it was heavier, slower and also had one or two odd directional handling characteristics in the transition phase between the hover and cruise flight and back again. I grant that it was externally a lot quieter, and if you wanted to stick your tail into a tree, safer. It seemed a lot of effort for not a lot of benefit. Rather that someone had an idea, spent a lot of money and didn't bin it before doing a cost/complication benefit analysis.

[quichemech]

Nick, 2 long and flimsy controls? What about 2 steel cables running the whole length of a very long tail boom, fretting their little hearts out on the fairleads:rolleyes:

You're right, but no tail system is perfect, the notar can take a lot of damage as well, which a normal system might well not absorb, but it is also very easily damaged. It is very much a case of horses for courses.

But as you said earlier, they do look pretty cool.

[IFMU]

Nick, 2 long and flimsy controls? What about 2 steel cables running the whole length of a very long tail boom, fretting their little hearts out on the fairleads
Which, if I'm not mistaken, you can cut either one of them on most Sikorsky designs and still have full control.

-- IFMU

[Dave_Jackson]

There is a way to get rid of the; danger, noise, cost, weight, drag, and maintenance of the fan-on-tail and the fan-in-tail. http://www.unicopter.com\Thinking.gif

Simply; get rid of them. :D


Dave

[Graviman]

Agreed that Coanda effect design will not be mechanically simpler, Nick. My thought is that the primary modes of failure affecting conventional tail rotor could be designed out. Not suggesting the MD design is an example of this!

Son-of-Notar would have an embedded fan with collective pitch, as per conventional tail rotor. Modes of failure eliminated are TR strike and driveshaft breakage, along with reduced risk to TR pitch control. This provides full anti-torque and left yaw. There would need to be a single control going to the coanda mixer which would switch jet directions for right yaw (spring or pressure default to left). Embedded fan would vary from 0 pitch to full pitch with left or right pedal, direction switcher activated with right pedal.

Considering the mode of failure where the direction switcher breaks, then the engine fails. Initially pilot does not realise loss of powered right yaw, due to need to balance torque. Donk quits, leading to autorotation. Pilot retains full left yaw control, but realises there is a problem achieving powered right yaw. Although forced to make left turns only lands safely and walks away to post on PPRUNE (probably with some good test pilot advise for lesser mortals like myself).

Another interesting possibility would be two embedded rotors for twin engine helicopters, maybe using low bypass turbines. This means that pilot would only ever loose half yaw effectiveness with single embedded rotor failure. Looking at my 1:72 Commanche model suggests that if the aerodynamics could be developed, mixer would offer a good area for air flow.

Not suggesting the idea is a golden panacea, but maybe it has sufficient merit to warrent further design/simulation study.

Mart

[quichemech]

IFMU,
It didn't look like full control as it span around in the hover in the middle of aberdeen airport runway back around 97,the crew handled it very well! A most interesting spectacle to watch it was too! Fortunately no one was hurt.

[Graviman]

There is a way to get rid of the; danger, noise, cost, weight, drag, and maintenance of the fan-on-tail and the fan-in-tail.

Simply; get rid of them.


http://www.sikorsky.com/Images/SAC_Sikorsky_Aircraft_Corporation/US-en/X2_Demonstrator_Aircraft.jpg

;)

Mart

[ericferret]

If my memory serves me right Sikorsky modified the S76 following the loss of at least one aircraft following cable failures. Doesn't the S61 have a pneumatic cartridge that forces the tail rotor into a fixed position if a cable breaks, no control available?

In 30 years I've seen plenty of nasties with cables including one holding on with four strands. I've never seen a rod system that gave me any cause for concern, Dauphin, BO 105, Hughes 500 e.t.c.

When we are talking safety of course the prime use for MD 902 is police/air ambulance, which often land in areas with non helicopter trained personnel on the ground. Tail rotors still kill people on a regular basis.

Tail drive shafts are a problem area, remember the RAF Wessex which crashed into the lake with the air cadets onboard following a coupling failure , very nasty.

There is also the birdstrike and FOD issue for tail rotors in flight. RAF Puma that lost a door in flight (Norway) 4 killed.

I seem to remember a Hughes 300 tail rotor taking the head off the pilot when it came adrift in flight, 2 killed. Delta hinge bolt failure.

I am not aware of any incident where a notar fan has failed in a catastophic manner. Are there any examples?

Notar is new and therefore statistics are in short supply. I have difficulty believing that a tail rotor or fenestron is fundamentally safer.

[IFMU]

quichemech,

Perhaps I am mistaken. Maybe the design I saw was for the Big S's latest, and not most of its models.

-- IFMU

[PANews]

16 October 2003 MD900 Explorer N179PA operated by Papillon Grand Canyon Helicopters on behalf of the US National Parks Service in the Grand Canyon National Park. Report that the aircraft suffered a ‘Notar’ failure and crashed.

The NTSB concluded that the accident in FREDONIA, AZ had a probable cause:
the pilot's in-flight loss of control due to the fatigue fracture and separation of the force limiting control rod (an integral part of the anti-torque system), which resulted in the helicopter's uncommanded yaw/spin and subsequent collision with terrain. It was all low speed so they all walked away from it.

There was also another 900 D-HITH in Germany that was unable to overcome a gusting wind in March 2005 and toppled over, I guess that could be seen as a Notar failure.

I am not sure that Notar can be hiding behind an excuse that it is a young system anymore, depending what date you go by you get at least 15 years, more really. The development numbers are low though, something like 220-230 Notars in all out there?

[NickLappos]

ericferret,

The Notar is a complex system that HAS a tail rotor. This making your assertion that is is somehow BETTER than a tail rotor a bit silly ("I have difficulty believing that a tail rotor or fenestron is fundamentally safer.") Additionally, a Notar has a full rudder control system, and a full thruster control system. Three critical control systems instead of one. Nice improvement, huh?

Regarding cable controls, the failure you describe is because the maintenance crew strung the cable wrong, a problem with control rods, too, kimo sabe. The S76 (and all Sikorsky production models) have unique redundant cable systems, with the ability to fly home with good yaw control with one cut cable, and fly home on a centering spring after both cables are cut.

It is not easy debating with someone who quickly produces bad data, except to ask you this: What centering spring or redundancy does a Notar have for any of its THREE yaw control systems?

[dr atkins]

PAN my old friend. Once again with the MD bashing.

The Grand Canyon crash had nothing to do with a Notar failure. It was an anti torque control rod failure as you pointed out. All helicopters have those, even your beloved Eurocopter fleet. So how was that a Notar failure? There was nothing wrong with the fan, coanda slots in the boom or thrusters can.

The German crash was caused by the pilot dropping the machine off a mobile helipad . I’m sure an embarrassed pilot would be keen to blame anything on such a mistake. It was the wind, err the Notar, err it was the sun was in my eyes. I’ve seen the London HEMS crew land their 902 on a helipad only 2 foot wider than the skids in gusting winds that I wouldn’t even want to stand up in. Mind you they are very experienced pilots.

Nick,

The Rudder system on the 902 is independent to the Notar system in every way with only a small resister on the pedals being the only input. The rest of the rudder or VSCS system is controlled by electronics which are actually more concerned with vertical movements than yaw inputs. The rest of the Notar system must be considered as one as a failure of either the fan or thruster would result in a bad day for the pilot.

As far as I’m concerned apart from being a bit quieter than a tail rotor or Fenestron the system scores hands down when you have FOD blowing around the tail. Wasn’t it one of the Devon air ambulances BO 105’s that hit the tree and crashed? I’ve also seen a Tesco carrier bag cripple an EC135 Fenestron. The last London HEMS incident resulted in a large hole in the L/H fin from FOD damage which may have been disastrous to a tail rotor or Fen.

The Notar system is not better generally than a tail rotor or Fen’ but if you want to avoid FOD issues when landing away from base then it is a good compromise. Even when the fan is damaged by FOD it has still shown to perform very well.

Just my 2c worth but then I am bias.


Dr A

P.S. The Notar may be over 15 years old but how much development has it had in that time. Err practically none. If it got the investment that the Fenestron has had I think it would be a winner.

[PANews]

Dr

You lost me there.

If the NTSB stated [not me!] that the failure was an integral part of of the anti-torque system and my understanding is that the NOTAR is integral to the anti-torque system we are talking about I cannot really see how you are seeking to disconnect them!

Equally turning around and saying it doesn't matter if Notar has been found wanting in some areas because it hasn't been developed is a brilliant excuse but of course it means you are saying there is a problem!

'... would be a winner'?

Gee I thought you said it WAS a winner!

Anyway ... ericferret asked a question and I answered it with a factual NTSB extract - that does not mean I am knocking NOTAR, it seems you are doing it quite well on your ownsome!

[NickLappos]

dr. atkins,
You are quite right, the Notar is much safer when FOD, ground strike or personnel hazards are considered. Quite a bit better than fenestrons and even more so than tail rotors.

My comments on the system safety were more in line with complexity and parts count, points that I still stand on.

[dr atkins]

Nick,

I can only answer on my personal experience here which is limited to the 902, BO 105, AS355 and EC135.

I’m not sure how much you know about the Notar fan itself but it is pretty much an identical configuration to the Fenestron with the exception that the blades are a lot wider. The pitch change mechanism is similar to the Fenestron thus so far we have equal parts’ish to the 135
Here the control splits off to the tail via a flexible cable the same as a 135. At the tail you have 2 bellcranks which convert linier movement to rotating movement to the thruster cone. Now take into account you have 2 less driveshafts with couplings and no additional tail gearbox on the Notar I would say it’s about even stevens.

PAN, you implied the Grand Canyon crash was a Notar failure or else why would you have put it on this thread. You think you understand helicopter control systems although you are just an ex plod, not a pilot and defo not an engineer. You state in your first post that the aircraft suffered a Notar failure and crashed. In your second post you state that the NTSB said it was an anti torque control failure. You then worked out (must have engaged both brain cells) that any anti-torque failure on a 902 must also be a Notar failure. No pilot or engineer would think like that. Any pilot will back me up when I say an anti torque control failure has nothing to do with the type of fan, whether it be Fenestron, tail rotor, Notar or contra rotating! The result will always be the same, bad.

I am stating now that there are issues with the Notar. These issues arise from lack of development, something that I hope Ms Tilton and her merry team will address. I’m not so arrogant to think the 902 is perfect, far from it. But I think it has loads of potential if someone has the guts to plough in the cash to develop it. The thruster extension is just the beginning I’m sure. There’s lots of new blood at MDHI and they all have their sights on clawing back the US market from Eurocopter( come on PAN, take the bait).

Dr A

[quichemech]

Nick, you are quite right as to the complexity issue and I am sure the good doctor will conceed that point. (Sorry it would appear not)As for the tail rotor cables, you are of course correct. An unfortunate maintenance error.
However there is no doubt as to the FOD aspect and having had the pleasure of working on all 3 tail rotor systems I do find that from a personal safety point of view a notar is a bonus.
PA News,
have a look through your course notes, the Doc might just be right, they are not interlinked if my memory serves me right. Didn't you fly it in october 2000?

[dr atkins]

No quiche, it was Oct 2001. A month after 9/11.

Dr A

[quichemech]

Of course it was, old age befuddling my memory:\

[PANews]

I go hands up on that then. If they are not one and the same I lose!
I was mislead by this statement.... 'During post-accident examination of the NOTAR control system, a fracture and separation was found in the force limiting control rod. With the rod assembly disconnected, the NOTAR fan blades assume a neutral pitch, preventing a sufficient volume of air in the tail boom to provide anti-torque control and maintain a heading.'
They said it [NOTAR] was not working properly because the rod broke.... so I just thought NOTAR might be involved somewhere along the line! Silly me!

[ericferret]

Indeed cables are prone to maintenace error and suffer from lack of maintenance which was basically my point.

Crossing control rods, not impossible hence the Enstom which rolled over at Oxford a number of years ago when the internal mast rods were crossed.
However control checks will always show up a crossed rod, whereas double crossed cables often only show when they break. Realistically crossing rods is difficult.

Misrouting cables over structure, other cables, over guard pins, wear on seized pulleys e.t.c requires vigilance at installation and maintenance.
Cables and rods all have there place in aircraft construction, but for me rod systems are safer. Cables tend to be more flexible in both senses and lighter. I suppose that you pay your money and take the choice. However I would put money on more aircraft losses are down to cable failure than control rod failure.

Last crossed cable I saw was 2 years ago. An aileron cable on a factory new Boeing 737.
The cables had been incorrectly fitted at the outboard aileron quadrant and were a long way thru. In this particular design crossing the cables at this point does not alter the control output and is extremely difficult to see as the cross point is behind the quadrant, tight against the spar. If the factory can get it wrong so can anyone.

As a matter of interest the cables I found "just holding on", were on an S76 where they exit the cabin and pass under the main transmission. No apparent reason other than wear and tear and poor inspection work. Just revisiting this if one cable had snapped the flight loads would have passed to the other (Nick Lappos)
but given the condition of both cables was equally bads would the second cable have survived for long?

[Graviman]

As far as I’m concerned apart from being a bit quieter than a tail rotor or Fenestron the system scores hands down when you have FOD blowing around the tail. Wasn’t it one of the Devon air ambulances BO 105’s that hit the tree and crashed? I’ve also seen a Tesco carrier bag cripple an EC135 Fenestron. The last London HEMS incident resulted in a large hole in the L/H fin from FOD damage which may have been disastrous to a tail rotor or Fen.


The Carrier Bag Test? Interesting mode of failure. I guess having embedded rotor intake around the periphery stops total blackage - not sure how engine intakes fare with this either. For coanda effect mass flow increaser, in Son-of-Notar tail, a quick blast of right pedal would sort machine out.

Mart

[diethelm]

Although tail rotors have less parts than Notar, the 500 series has a critical safety notice relating to the tail rotor.

In the event dual controls are installed, the second cyclic control is a risk of unhooking and removing a bra or bikini which when falling out of the aircraft, can strike the tail rotor and create a tail rotor failure. In addition, this hooking risk can cause a run away in the trim switch mechanism. This would be a tragic event and so my aircraft has the appropriate placard:

"BRAS AND BIKINIS MUST BE REMOVED BEFORE FLIGHT".

Naturally the Notar solves this problem....

[quichemech]

:D :D :D

Very good and nicely brought into perspective:ok:

[ericferret]

I always remove my bra or bikini before flight.

Wouldn't want the wife to catch me wearing her stuff!!