# Helicopter Question: NOTAR



## Freddy Chef (18 Feb 2005)

I first heard about *NO* *TA*il *R*otor helicopters on a documentary about Air Services Unit of the Calgary Police Service (Calgary Police fly the MD520)

Support Section -> HAWC1 & Air Services Unit:
http://www.gov.calgary.ab.ca/police/inside/frame1.html.

It sounds like a keen idea to use an internal fan and vector nozzle to do the work of the tail rotor:

http://www.kulikovair.com/Notar.htm

replacing the tail rotor and transmission system that makes it work.

Will this system work on a helicopter larger than the MD520? (ie: Bell 214 or Sikorsky S-70)


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## bossi (18 Feb 2005)

Interesting - the larger, double engine MD Explorer boasts "The MD Explorer is powered by two Pratt & Whitney Canada PW206E turboshaft engines, providing 463kW take-off power and 410kW maximum continuous power. ... The Explorer has a cruise speed of 248km/h and a range of 476km."

http://www.aerospace-technology.com/projects/md_explorer/

And, the MD 600 is referred to as a "stretch" version ... (and seems to have its' fair share of prangs initially ...)  http://home.wanadoo.nl/helicopter/600n.htm

(chuckle ... and ya gotta luv them "black helicopter" stories ... http://www.skepticfiles.org/waco/ltstory2.htm)


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## big bad john (18 Feb 2005)

MD520NS NOTAR


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## Spr.Earl (18 Feb 2005)

Ah the old exhaust directed out back!!
Supposed to  be a better control  and very good hover.


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## Kirkhill (18 Feb 2005)

I just posted this under the attack helicopter thread.   A UAV version of this aircraft.



> They have taken the standard MD 500 series LOH (which I believe is used as an armed platform by the US Special Ops Community) and taken the pilot out of it and flown it as a UAV.



http://army.ca/forums/threads/25684/post-169215.html#msg169215


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## Inch (18 Feb 2005)

I asked a few of my instructors about this one today, the question that kept popping up was "if this is so great, why isn't it in more widespread use?" 

I'll admit that I don't know a whole lot about NOTAR, but I do know that if we lose a tail rotor, we autorotate. When we get close to the ground and pull in collective to cushion the landing, it will induce some yaw in the aircraft due to the inertia of the rotor slowing down. So what happens to a NOTAR when they have an engine failure? Do they lose their anti-torque? If we have an engine failure and have to autorotate, our tail rotor still provides some anti-torque to counteract the torque of the main rotor, to put it simply, we still have yaw control as long as the rotor is spinning and the tail rotor is functioning properly. So if that's the case, then everytime you have an engine failure in a NOTAR, you would also have a loss of yaw control. Sounds like a pretty bad day to me. I'll take the tail rotor and it's potential hazards over losing your yaw control with any malfunctions of the engine. 

Just my thoughts, maybe Duey or Sam could shed a little more light on the subject.


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## Freddy Chef (18 Feb 2005)

Thanks *Inch*,

Your point about engine failure auto-rotation explains the same underlying query I had of  NOTAR's effectiveness [*â Å“why it isn't it in more widespread use?â ?*]

I guess the *â ?variable pitch fanâ ?* 







in the NOTAR system (which does the job of the tail rotor) would not be as effective in an engine failure auto-rotation situation, compared to a larger auto-rotating airfoil that is the tail rotor. 

Cheers.


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## Sam69 (18 Feb 2005)

Inch said:
			
		

> I asked a few of my instructors about this one today, the question that kept popping up was "if this is so great, why isn't it in more widespread use?"



I'll start by saying that I am not a NOTAR expert, nor have I ever flown one. But, a good friend of mine, who is a qualified Test Pilot, flew a 520N and described as an "accident looking for a crash site." Essentially he found the NOTAR, while offering durability advantages over a tail rotor, to be much less responsive and effective.  In particular, he found it very sensitive to the relative wind in the hover.

It is my understanding that, among other reasons, NOTAR is not used on bigger machines because it doesn't scale well to larger helos with greater anti-torque requirements.

Lastly, in the event of an engine failure, the NOTAR continues to provide anti-torque control because the fan that supplies air to it is driven off the main gear box. Therefore, as long as the main rotor is still turning, it is still producing thrust out the back. I'm not sure how much, if at all, the thrust is reduced by the loss of the engine.

I'm sure Duey can explain this better. After all, he is used to flying without a tail rotor.  ;D

Sam


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## SeaKingTacco (18 Feb 2005)

> Do they lose their anti-torque? If we have an engine failure and have to autorotate, our tail rotor still provides some anti-torque to counteract the torque of the main rotor, to put it simply, we still have yaw control as long as the rotor is spinning and the tail rotor is functioning properly.



Whoa.   I specifically recall at 406 Sqn being taught that, in the event of an autorotation, there is not a huge concern about countering torque.   The engine power that causes the torquing force (that whole "action-reaction" thing) on the airframe is no longer present and that the aerodynamic forces will more or less keep the helicopter straight.   Admitedly, us Navs only get bare-bones helo theory of flight but, was I taught wrongly?   Did I remember wrongly? Sam? Inch? Duey?

Cheers.


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## bossi (19 Feb 2005)

Once again, I have to express my thanks and appreciation to my rotorhead comrades
(from my perspective as cargo in both rotary and seized wing aircraft)!

When I found that website and noticed all the NOTAR choppers that had crashed, I wondered if there had been an inherent design "wrinkle" ... especially since so many of the crashes had happened nearer to the beginning of their production run ...

Similarly, my dream machine (the Osprey - in my fantasy world, descended from Canada's CL84 forty years ago) has had its' share of bad luck - I'm sure looking forward to when they finally get it right ...

Canadair VTOL CL84 VTOL


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## Inch (19 Feb 2005)

I understand your concern SKT, for all intents and purposes (    for you Sam), there is no torque to the head since it's autorotating. I've never done a tail rotorless auto so I can't speak from experience, but in discussing the situation with my instructor my understanding is that without a tail rotor you will yaw at the bottom of the auto when you pull in collective, not to mention that you have to be perfectly into wind or you're not going to come straight in without some use of pedals. We discussed it yesterday morning, then I went flying in the afternoon and as luck would have it, we did autos. I payed extra attention to how much I was actually on the pedals, keeping the ball centred is all pedals and you need a lot of right pedal as soon as the torque is gone from the head on entry. 

Now, since the tail rotor yaws you left and doesn't reverse flow, it just reduces it's thrust to get you to go right, there has to be something still wanting to yaw the helo to the right because you can still turn both directions in an auto (my guess would be the wind since in an auto, the wind, instead of the engine, is turning the rotor so there should still be some torque to the head), if there wasn't something yawing you right you'd be relegated to doing a left hand turn in an auto and that's not the case. 

It's been over a year since my helo aerodynamics lectures but that's my understanding of it as well as what I gathered from my IP on Friday.

bossi, there's a Marine Corps Osprey here in Shearwater right now. Not sure what they're doing but it's a pretty cool looking piece of kit, it sure does have a distinct sound from those 2 rotors.


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## bossi (19 Feb 2005)

Inch said:
			
		

> bossi, there's a Marine Corps Osprey here in Shearwater right now. Not sure what they're doing but it's a pretty cool looking piece of kit, it sure does have a distinct sound from those 2 rotors.



Cold weather/icing trials (... just like HMS Albion ... sigh ... another of my favourite CEF fantasies ...)


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## Inch (19 Feb 2005)

bossi said:
			
		

> Cold weather/icing trials (... just like HMS Albion ... sigh ... another of my favourite CEF fantasies ...)



That's what I figured but it's not painted orange like the RN Merlin that's here doing icing trials. Most a/c I've heard of doing icing trials are painted orange, the critical parts to flight that is (wing tips, tails, etc), I believe this is because ice is easier to spot on the orange vice the grey or green.


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## Good2Golf (19 Feb 2005)

Quick point about torque and the autorotation...there is still bearing drag on the main mast which, while small, is still present during autoroatation.   When you pull in pitch at the flare, the increased loading on the disc to stop descent also increases overall friction on the mast and the nose of the helo will twitch in the same direction as main rotor rotation.

Regarding the NOTAR, I figure some of the attitudes about it I've heard from chatting with A/MH-6J drivers with the 160th sums it up..."piece of crap", "it's like having an inch of slack in the pedals", "If you don't know to keep your tail out of a tree, you shouldn't be flying", etc...   Boeing (then MD) tried to get the 160th to convert to 520Ns and 600Ns in the mid-90's.   The guys hated the machines.   Interestingly, this also resulted in the 160th upgrading the 6-shooters from a 5-blade main rotor/2-blade tail rotor to a 6-blade main and 4-blade t/r.   The boys didn't find the aircraft very stable (heading-wise) in the hover or low-speed flight....exactly NOT what you want when you're trying to point your little beasties rockets in the right direction during a strifing run, hover shot, etc...   

There are two systems working on a NOTAR to counteract M/R torque; vented fan exhaust with guide vanes and a thin slot along the boom to exhaust a thin film of air along the boom, producing a sidewards force from the "Coanda effect"   http://www.rexresearch.com/coanda/1coanda.htm.

Interestingly, you will never hear NOTAR proponents say the the NOTAR is more efficient than a conventional tailrotor...in fact it is not and in some cases, actually uses more power.   Where aircraft are power limited and there is little concern for hitting trees or bushes with the tail rotor or the higher noise levels of a conventional tail rotor, the NOTAR does not have any appreciable benefit over conventional t/r's.

Cheers,
Duey


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## SeaKingTacco (19 Feb 2005)

Inch- Thanks for the refresher on autos.   What you said jives with what I have observed from the back-seat on what is my second least favorite trip in a Sea King (that is to say auto-rotation training).   My least favorite trip is just about any LP on a frigate with a Nugget Pilot...ugh   :-X  
BTW, who is your IP?   Balls?   Dany?

Duey- with the 6-blade main/4-blade tail rotor arrangement that you mentioned, do you get slower tailrotor speeds?   Does the extra mass in the rotor head give you less droop?

Cheers.


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## a_majoor (19 Feb 2005)

I'm a little confused about the discussions on auto rotation. In the 1920s and 30s, a popular type of aircraft called an autogyro used a large unpowered rotor to develop lift, and required no tail rotor whatsoever. Since there was power (usually in the form of a radial engine and two bladed prop), these things could climb and fly almost as well as normal planes. If the engine should fail, the spinning rotor developed enough lift for the pilot to make a safe, controlled landing.

I am assuming this is what is being talked about when a helo autorotates as well, so having a tail rotor or NOTAR should make no difference at all in that situation (?)


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## condor888000 (19 Feb 2005)

From the following...

[quote author=autogyro info]
Autogyros are notably safe. If the engine should fail, the autogyro does not stall or spin. Instead, it begins to settle like a parachute. The pilot can usually maintain some directional control by slipping the rotor.[/quote]

...I would think that they would become more of a glider that an autorotating helicopter. 


[quote author=autogyro info]
If forward airspeed becomes zero, the autogyro will slowly drift to the ground, rotor still spinning. A vertical landing in this manner will not critically damage most autogyros. [/quote]

Strikes me that what would happen is that the rotor would keep spinning and the pilot would have enough control to conduct a safe landing.

Info from: http://www.answers.com/topic/autogyro-2

EDIT: Would like to point out that I'm not an expert so if I'm wrong please point it out.


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## Inch (19 Feb 2005)

a_majoor said:
			
		

> I'm a little confused about the discussions on auto rotation. In the 1920s and 30s, a popular type of aircraft called an autogyro used a large unpowered rotor to develop lift, and required no tail rotor whatsoever. Since there was power (usually in the form of a radial engine and two bladed prop), these things could climb and fly almost as well as normal planes. If the engine should fail, the spinning rotor developed enough lift for the pilot to make a safe, controlled landing.
> 
> I am assuming this is what is being talked about when a helo autorotates as well, so having a tail rotor or NOTAR should make no difference at all in that situation (?)



I'll admit that I don't know a whole lot about autogyros, but after a quick read online the big difference is that a helo has a collective while autogyros don't, also, helos aren't made to autorotate at all times since we also use the rotor for thrust, when an engine failure occurs, a change in aircraft attitude is required in order to preserve your rotor RPM (Nr). 

Use of the collective requires a pedal input to counteract the torque loss or increase.   When we get close to the ground we flare to slow our descent and then we pull in collective to cushion the landing. I did over a hundred autos to touchdown while I was on the Jet Ranger course without ever bending an aircraft. Without a collective, there would be no way to cushion the landing, and believe me, you need pedal inputs to keep it pointed in the direction you want to go. If you pull in collective too soon or don't bottom the collective as soon as you get an engine failure, the Nr will decay to the point where it will no longer autorotate. The autogyros wouldn't have had this problem, however they would require forward speed right to the end to keep the rotor turning with no way of slowing the descent close to the ground. Not a huge problem for an aircraft under 1000 lbs, but take a 20,000 lb helo in a high density altitude and you'll want to slow that descent rate as much as possible, (FYI, a Sea King will descend at around 2000 ft per min in an auto, if the DA is over 1000ft we can't practice them if we weigh over 17,000lbs, over 2800ft DA and we can't even practice them due to the high rates of descent).   

Also, since the autogyros didn't have powered rotors, they didn't have a real need for a tail rotor to counteract torque, a tail plane would provide the necessary directional control.

Condor, you are correct, since as I stated above, a helo is not designed to autorotate at all times, we require forward airspeed with the collective bottomed (lowered all the way) in order to maintain Nr and make a safe landing. Only in the short strokes (150ft above the ground) do we flare and increase collective (50ft) thereby trading our forward speed and Nr for a nice soft and controlled landing. In the Sea King, we autorotate at 70 kts, any slower and we won't have enough energy to flare and cushion the landing. Flare too soon and you crash, flare too late and you'll land at a high speed and rate of descent, cushion too soon and you'll decay your Nr and have a hard landing, cushion too late and you'll hit with a high rate of descent.  As you can see, it's possible to land a helo without an engine, but quite difficult which is why we practice them so often.


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## Good2Golf (20 Feb 2005)

SeaKingTacco said:
			
		

> Inch- Thanks for the refresher on autos.   What you said jives with what I have observed from the back-seat on what is my second least favorite trip in a Sea King (that is to say auto-rotation training).   My least favorite trip is just about any LP on a frigate with a Nugget Pilot...ugh   :-X
> BTW, who is your IP?   Balls?   Dany?
> 
> Duey- with the 6-blade main/4-blade tail rotor arrangement that you mentioned, do you get slower tailrotor speeds?   Does the extra mass in the rotor head give you less droop?
> ...



SKT, no...the -J's driveline operates at the same speed as the ealier A/MH-6's, so the tail rotor actually runs a little less pitch to maintain the same anti-torque thrust.  The additional 6th main blade does increase the rotor mass a bit, but more importantly lets the 6-J's increase AUW (as they have an upgraded C250-C40/C47 engine, up from the current C250-C30 engine rated 650 shp).  The droop might be altered somewhat but there is more effect to adjusting for droop from the C250's AFCU.  I don't know if all the little birds have been upgraded to the 6/4-rotor config from the 5/2, but the end goal is to have all Little Birds modified with blades, tranny and engine.  Interestingly, the A/MH-6J's still have no hydraulic boost on any flight control.  I bit more challenging to fly, but less risk of one more thing to fail when you're being shot at...  

Cheers,
Duey


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## Freddy Chef (20 Feb 2005)

Thank you gentlemen, helicopter pilots and crews, et al

So, compared to a conventional tail rotor, NOTAR uses more energy, and provides less yaw performance and stability? Insufficient yaw performance/stability for an attack/a military helicopter pilot, but sufficient for a Calgary Cop chasing a stolen car? [I'm sure people living in Calgary like the reduced noise aspect when their cops are patrolling their skies.] OK. When I first heard about NOTAR, it was advertised as less maintenance than a conventional tail rotor: shaft and gears connecting the tail rotor, etc. Sounds like cops in the civie world can afford the drop in yaw performance/stability.

On the note of tail rotors, helicopters with tail rotors that are imbedded in the tail; 

Eg:

Aerospatiale/Eurocopter SA-365
http://www.pilotfriend.com/photo_albums/helicopters/images/h65_5.jpg

or

Boeing/Sikorsky RAH-66
http://www.heliport.de/helimg/comanche.jpg

How well do they perform compared with helicopters with conventional/exposed tail rotors?

Cheers.


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## Good2Golf (20 Feb 2005)

Freddy, aside from the NOTAR, the "fenestron" shrouded tail-rotor (IIRC, first used by Aerospatiale, now Eurocopter) is the next quietest tail-rotor system in use today.  They have very similar authority/effectiveness to an open tail rotor.  I think you're right, the quietness of the NOTAR seems to be its biggest benefit...

Cheers,
Duey


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## SeaKingTacco (21 Feb 2005)

Thanks, Duey!


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