How Big For Notar?
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How Big For Notar?
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.
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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,
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,
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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
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
Last edited by Graviman; 24th Mar 2007 at 21:11. Reason: Tidy up for clarity of equations.
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.
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.
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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
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
Last edited by Graviman; 24th Mar 2007 at 14:45. Reason: Typo only.
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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.
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.
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
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
Last edited by ericferret; 24th Mar 2007 at 11:33. Reason: changes
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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.
Coanda = 0
Thruster = 100%
Fin = 0
In a side flight regime of about 10 knots, the above is also true.
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Army tests prove it.
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.
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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.
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.
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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...
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...
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Originally Posted by NickLappos
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.
Mart
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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.
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.
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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!
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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
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
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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!
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!
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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.