NOTAR in auto

[HMM]

Does anyone know how the NOTAR system works in autorotation?

[BlenderPilot]

Of course in Autorotation there is no need to counter torque, so yaw control requirements are minimal, and also for obvious reasons the coanda effect is not going to be present during an auto, but . . . . . .

The variable pitch fan is transmision driven so it keeps turning as the long as the M/R turns, I am guessing that this air coming out of the rear "nozzle" combined with the the movable vertical fins give you a pretty decent amout of yaw control in case you need it.

All of the above are the product of guesswork, so correct me if I am wrong please.

[FloaterNorthWest]

Blenderpilot,

You aren’t far off.

You need to look at control authority speeds and looking at it from the 902 you get the following.

Above 80kts the thruster and coanda effect on the tail boom (derived from the NOTAR fan) have little or no control authority, directional control is from the vertical fins via the VSCS system. So in auto above 80kts the fins are driving the aircraft to the left (to counter the normal turning effect to the right of torque reaction) so a lot of right pedal is needed in autorotation. The fins driving left means it is also easier to turn to the left in auto than to the right.

At between 50 and 80kts directional control is shared between the thrusters and fins.

50kts down to translational lift the fins are not working, the thrusters authority is decreasing and the coanda’s authority is increasing.

Below translational lift the coanda is giving you 70% authority and the thruster 30%.

You are correct in the fact that the fan is still driven by the rotor so works in the same sense as a normal tail rotor.

FNW.

[tbc]

All I would add is that if there is no air coming out of the Thruster Can or you cannot adjust said Thruster Can then an autorotation is not something you would want to do.

So, if the NOTAR drive is broken (no air from the thruster) or the thruster is fixeded (jammed pedal) then doing an autorotion is probably going to make a bad day worse.

In a typical autorotation (power on), you might need to use up to full right pedal and vary it in order to control the descent path.

[TheFlyingSquirrel]

Do Fenestrons, when compared to conventional T/R's require much more pedal input during autos?

[HeliMark]

A question on tbc's comments.

What is the difference of a tail rotor control failure in one with a tail rotor and one with a thruster can? Both are, depending on where they are fixed at providing some thrust.

The NOTAR system provides the same authority as the tail version, although a bit slower in response.

A stuck pedal in a 350, although there are ways to reduce this, may mean a 70 kt adventure down the runway. A NOTAR with the same situation at least is 20-30 kts slower on the procedure.

[quichemech]

TBC

read the comment from floaternorthwest.:ok:

[SilsoeSid]

I would suggest tbcs post is re-read by some!:ok:

[Thomas coupling]

What is the procedure, then for a 'jammed pedal' scenario in a 902?

Helimark's response sounds fine to me, whats the difference overall?

[SilsoeSid]

Left or Right pedal setting?

[Thomas coupling]

so you don't know either???

[SilsoeSid]

In answer to that TC, all I would need to do is look down at the pedals to see if the right or left pedal was forward!
(different procedure for left or right setting! ;) )

To come back on line a bit;

"Does anyone know how the NOTAR system works in autorotation?"

Autorotation = RoD airflow.
NOTAR system utilises the Coanda effect which relies on downwash.
(:ooh:downwash, rate of descent!!!:ooh: )

Hence tbc says, "In a typical autorotation (power on), you might need to use up to full right pedal and vary it in order to control the descent path."


FloaterNorthWest's figures are all well and good, if tbc's fan drive doesn't break. HeliMarks comparison is also well and good if you forget about the coanda effect. As for fenestrons, I believe the shaping of the fin will assist in directional stability somewhat, which will decrease however as the airflow over it decreases, so in answer to Flying Squirrel, no.

:ok:
SS

So ultimately the answer is, yes, by using the thrust produced from the drum
edited to actually give an answer!!

[TeeS]

Hi Silsoe

When in autorotation, is there any effect (ie thrust) created by the relative airflow up over the tailboom? I would have thought that the airflow round the tail would still be modified by the fan efflux, even if the effect was not large.

Cheers

TeeS

[SilsoeSid]

TeeS,

I'm sure you are more than aware how it works, however for those who would like a visual aid;

http://www.globalsecurity.org/military/systems/aircraft/images/mh-90-notar-work.gif

As can be seen by the pic, a RoD airflow is going to cause all sorts of mayhem as far as the coanda is concerned.

[TeeS]

Silsoe

No, sorry to show my ignorance but it was a genuine question. I hadn't realised that the 'slots' were so directional, your diagram solved that one for me.

Cheers

TeeS

[Thomas coupling]

Silsoe: your previous reply explaining what everyone else explained is completely japanese:confused: :confused:
What on earth are you saying???

TBC: so, how does the 902 differ from the conventional helo in a real tail rotor jammed scenario???

[SilsoeSid]

TC,

Velly solly for eny confusion. (Excuse Chinese, I can't type a japanese accent ;) )

I take it you mean - "FloaterNorthWest's figures are all well and good, if tbc's fan drive doesn't break. HeliMarks comparison is also well and good if you forget about the coanda effect. As for fenestrons, I believe the shaping of the fin will assist in directional stability somewhat, which will decrease however as the airflow over it decreases, so in answer to Flying Squirrel, no."

FNW gave us a nice breakdown of the control authority speeds of a NOTARed a/c. tbc brought up a fan drive failure, which would produce no coanda effect or drum thrust output, due to no air being pushed into the tailboom. Therefore FNWs information would alter somewhat!

HeliMark then compared the effects of a tail rotor control failure with a tail rotored a/c and a NOTAR a/c. It is true that they are both producing some thrust, however, going back to FNW's post, when you get to below trans lift (touchdown!), coanda kicks in and will possibly produce a good yaw to the right. Nice !! This does not happen in a 'tail rotored' a/c.

F.Squirrel asked if fenestrons, "when compared to conventional T/R's require much more pedal input during autos." If you look at a fenestron, the tail section is aerofoil shaped so that in flight this can lessen the trim require by the pedals (+ more pwr?) I reckon that because of this, in autorotation this effect will assist down to a speed when the effect is lost, but with a minimal yaw difference.

I suppose if you opened 2 'Windows' and read the posts and my reply at the same time it would be easier to understand.

Someone once said that if you have to explain your own joke, then it wasn't a joke! (thankfully, that doesn't apply to pprune.....or does it)

Cheers
SS
:ok:


p.s I think I may have answered the question for tbc in there somewhere!

[212man]

What you will find with a fenestron in autorotation, with a drive failure, is an increasing yaw to the right, with increasing airspeed, due to the fin camber being set to assist in powered flight. At power off Vne it is very marked.

[FloaterNorthWest]

This has taken off.

My original post was just to do with autorotation but it seems that we have moved on so I will put forward a few of my thoughts.

Failures and malfunctions with the NOTAR full into two groups; drive/control failure and stuck pedal.

The best one from the pilot point of view is stuck left pedal, this could be pedals level to full left pedal (level pedals as neutral thruster on the 902 is one inch right pedal forward). As you have the power pedal the NOTAR system is helping you so the approach is low and slow aiming for a zero/zero landing when the torque reaction is balanced by whatever NOTAR trust you have. For SilsoeSid who I believe is a 902 pilot, the aircraft will break left at the end due to coanda not right (torque reaction wants the aircraft to go right).

Total failure and struck right pedal and more of a problem. What you are aiming for in the latter stages just before touchdown is the nose to the left so that as you pull power in to cushion the touchdown the increasing torque reaction pulls the nose straight and you run on. In both cases you get the nose left by flaring the aircraft and rapidily lowering the collective. Done correctly and with a good wind from the left front quarter, you are in a position to level, check and run on at a reasonable speed.

How you get to the flare point varies for a failure and a struck pedal. The initial speed is the same, 70-100kts, this means that the fins are giving you some directional control against the torque reaction and/or NOTAR working against you. So the initial approach for both is about 80kts.

Stuck right pedal. As the NOTAR is working in the same direction as the torque reaction, you need to reduce something. The only thing you have control over is the torque reaction so a steep approach with about 20-30% torque at 70-100kts is carried out. The theory, and it is all theory, is that you can play the tq and speed to get some degree of directional control. Then Flare, Level, Check and run on. There is some talk of chopping throttles at the last minute to control direction but I feel the old skill of controlling throttles in the modern pilot is dead, it's hard enough to get some pilots to fly manual approaches in controlled conditions.

With a drive or control failure the fins are the only thing balancing the torque reation so the speed is kept on and the approach is very flat as the tq v fins is keeping you straightish. If you did the same steep approach the fins would drive you left. That is why, and I finally get to the point, that if you have a drive/control failure you DO NOT enter autorotation UNLESS you have a double engine failure.

I was doing this with my company pilots the other day and we discussed a situation where the NOTAR driveshaft failed and took out both of the the engine input driveshafts. The only solution we could come up with is to do a zero airspeed auto and just accept the yaw due to gearbox friction as you cushion the touchdown.

As I said earlier these are all theories and any advice or ideas from other 902 drivers or instructors would be welcome.

FNW

[HeliMark]

Not sure what the 902 manual says, but for the 520/600, you do not want to get below 20 kts.

On Anti-torque failure it has a caution to not attempt flight below 20 kts. On thruster failure/fixed setting it says to use powered landing as it is otherwise unlikely that an autorotation can be accomplished.

With that, I am not sure if engine failure and thruster failure in your situation would be survivable.

[SilsoeSid]

For SilsoeSid who I believe is a 902 pilot, the aircraft will break left at the end due to coanda not right (torque reaction wants the aircraft to go right). Thanks for that, (that's what happens when you don't proof read before posting) you're quite correct and that is exactly what happened during my OPC last week! The movement to the right happens when collective is applied to cushion the landing. The trick is to balance the collective and coanda to result in a zero/zero. (left setting)
I think its a case of the coanda 'may' cause the aircraft to yaw left before yawing to the right, don't rely on it.

[MightyGem]

What you will find with a fenestron in autorotation, with a drive failure, is an increasing yaw to the right, with increasing airspeed, due to the fin camber being set to assist in powered flight. At power off Vne it is very marked.
Ahh...that could explain why a 135's power off VNE is 90kts.

[NickLappos]

Many people are victims of the mis-reporting of the Notar on the various TV shows that still circulate long after the concept has begun to move into the "once a good idea" catagory. Unfortunately, the PR types who were hyping the concept vastly oversold it. Unfortunate for them, the aircraft were never over-sold, they hardly sold at all. This is because of the poor efficiency (read that as lesser payload) of the Notar equipped aircraft. The US Army actually converted some OH-6 little birds to Notar and then paid to convert them back a few years later, they were so displeased.

Remember that the NOTAR needs THREE anti-torque systems to keep the aircraft under control, all of them part of the package:

1) A tail thruster, the principal system. It is powered by a fully controllable tail rotor inside the tail cone, with all the parts of a regular tail rotor. The thruster also needs the rotating can and its control mechanism. It uses lots of power, typically 150% of the power needed with a tail rotor, because the can is so inefficient.

2) The Coanda slots which allow some anti-torque under the limited circumstances where the aircraft is hovering in near zero wind. As little as 6 knots of wind can make the downwash leave the tail cone, and make the Coanda slot useless (but still rob power by wasting air flow). In autorotation and all forward flight situations, the slots are useless.

3) The rudders and fins which help reduce the need for the thruster (which wastes lots of power) when the aircraft is in cruise. Because of the poor stability of the rest of the system, some Notar models also require a redundant SAS that works one of the rudders.

Without all three systems working well, the Notar cannot control the helicopter within its normal envelope.