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Tail Rotor Authority at Idle


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If the main driveshaft breaks, there will be nothing connecting the transmission to the Tail Rotor,...right?

 

As for Tail Rotor authority at idle, in an auto. I have made 90 degree pedal turns in the R44 during autorotation,...if that's what you're asking about?

:huh:

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There is plenty of tail rotor authority at idle. If the ground or surface is the least bit slippery, you would be able to pedal turn at flat pitch on the main rotor. It probably would put more strain on the tail boom than it is designed for. As for driveshaft failure, the jetranger is designed where the engine accessory gearbox drives the tail rotor driveshaft.

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If the driveshaft fails, you will have good TR authority as long as you do not reduce the throttle in autorotation. The MR is no longer driving the TR in the auto since the shaft has broken. We are taught: left (yaw), lower (collective) leave it alone (throttle) since when you enter the auto with a broken dirive shaft and roll off the throttle, you will be reducing the tail rotor RPM to idle RPM (or 0 RPM if you close it). This is assuming the governor will maintain proper TR RPM with very little load on the engine. This is something that has always concerned me. Never had to find out for real. It's the same procedure for an engine failure since rolling off the throttle will have no effect, of course. The Bell 206 RFM only states in the event of a drive shaft failure: Accomplish autorotative decent... and NOTE "To maintain tail rotor effectiveness do not shut down engine".

Edited by helonorth
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This is my fault for being too vague. This question stemmed from a discussion we were having while talking about FADEC failures. Our EP for a main driveshaft failure is this:

 

In the event of a main driveshaft failure:

 

AUTOROTATE - Throttle full open.

 

If FADEC failure is annunciated:

 

Throttle - Reduce to idle detent.

AUTO/MAN switch - MAN

Throttle - Adjust to maintain 100% NP, if time permits.

 

After landing:

 

EMER SHUTDOWN

 

As said above, in the event of a main driveshaft failure the accessory gearbox will drive the tailrotor.

 

The general consensus among the guys is that if FADEC does fail while in the auto, after going to idle and switching to manual control, they would leave the throttle alone and complete the autorotation at idle RPM. My question is regarding how much directional control you would have at idle. Is it enough directional control to safely accomplish an autorotational descent and landing? How much benefit would adjusting back to 100% NP actually provide?

Edited by SBuzzkill
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Sounds like test pilot stuff. What aircraft tells you to go to idle and autorotate for a FADEC failure? Holy smokes! I fly a 407. If the FADEC fails, there is no switching to manual. The FADEC has failed, you're already in manual! No more FADEC. The FADEC switch only turns off the horn/tone thing at this point. Never heard of such a thing. As far as the RPM, might as well go to 100%. What do you have to lose?

Edited by helonorth
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As a turbine engine mechanic, My guess (work fighters not helicopters) is that the FADEC can go into a hybrid mode. That is what the DEC (GE) or DEEC (PW) on the fighters do most of the time. It is a fail safe for the engine to prevent compressor stalls and it limits the inputs the pilot has to control the engine. Switching to Manual would give the pilot more manual control of the engine to get safely on the ground. (hopefully before the he/she compressor stalled or burned up the turbine.

 

As far as TR authority on a Kiowa after a drive shaft failure... I don't know, I would love to fly one, but I wouldn't want to be in the position to learn from experience what the outcome would be.

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Buzz flies the 58D Kiowa, as do I. That is our EP for main driveshaft failure.

Your EP for the drive shaft failure is probably pretty standard. As far as the FADEC failure, I don't understand why you have to go to idle, auto, then (if time permits!) roll the throttle back up. Flying as low as you guys often do, you could very well be auto rotating at a very inopportune time, just for a FADEC failure! The 407 EP is either to adjust the throttle to match your NG or adjust the collective to 92% or so NG, (MUCH easier), adjust throttle as necessary to maintain RPM and land as soon as practical. Hardly an emergency. All our aircraft are also equipped with the revisionary governor, which is basically a FADEC back up. If I was flying an OH-58D I would much prefer an old fashioned fuel control, as FADEC failures, while rare, do occur. Depending on where you are, you could have a real problem if you follow your EP. If the engine is running (it will be) and you can maintain NR (I don't understand why you couldn't), negative on the auto, sir! Don't you practice flying it manual? Does it not fly? I can also pretty much guarantee you will also overspeed the hell out of it by rolling back up if you're in a hurry (you will be with the ground coming up fast). I would think the army would be able to update to a computer that won't send you into an auto!

 

Sorry, I never did answer the original question, but there are probably very few people that could give you an answer, at least in the civilian world. If your TR RPM was at idle speed in an auto, your MR speed would be....you're probably dead. There actually are probably a few civilians that auto'ed with it at idle for a drive shaft failure. Those of us that trained as civilians often roll the throttle off automatically when faced with an autorotation. I hope I'm cured!

Edited by helonorth
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I think you're a bit confused here. This is not our EP for a FADEC failure.

 

What I listed above is the EP for main driveshaft failure, the FADEC stuff is part of the procedure if the FADEC fails while in your autorotation.

Edited by SBuzzkill
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I was kind wondering if that's what you were getting at, but the possibility is so remote you may as well ask what you should do if the tailboom falls off after your FADEC fails. Your luck is that bad. You actually have a procedure for a FADEC failure while in an autorotation? Wow. Now that's a bad day. But really, so what if your FADEC fails while in an auto? You will have other things more pressing to worry about. I guess you could ask the pilot not flying to check it out on the way down (if time allows)!

 

I guess you could have asked "what do you do if the FADEC fails while in an auto rotation after a drive shaft failure." I would have answered "hell if i know, probably not much!"

Edited by helonorth
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I suppose the difference lies in that when we are experiencing emergencies there is a high chance that we have a bunch of 200gr+ pieces of metal flying at us at very high velocities. It's not too far fetched that we will be experiencing more than one emergency at a time.

 

Anyways, my question does not lie with the possibility of this happening. My question lies with how much thrust is lost when the tail rotor is operating at idle RPM.

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It's a whole bunch. Okay, seriously, when I was flying the good old 300C, the difference between the top and bottom of the green was about 200 RPM. This 200 RPM made a BIG difference in TR authority. Now the 206/407 idles at around 63% NR. You have no torque in the equation, so you certainly do not need much TR thrust. When I'm at idle in the 407 and make a small pedal input, it does rock a little bit, so it's doing something. But as I said, if the FADEC were to fail in an autorotation, that would be the least of your problems. A drive shaft failure and a FADEC failure at the same time? To me, this question is mostly academic. Trying asking on of those L3 guys. I'd be interested to hear what they say!

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I think the ony answer you can get is "enough". You won't have a huge amount of authority, but you don't need much. Remember, the torque is gone, and all you need is enough to keep the nose straight. The 206 fuselage weathervanes heavily, so as long as you're generally into the wind it should stay straight even with little airspeed at touchdown. With no main driveshaft, you aren't going to do anything to increase torque, so not much tailrotor authority is necessary.

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Remember that with the driveshaft failed, the engine doesn't have much work to do, just drive the tail rotor. The N2 governor will be going berserk because the unloaded N2 will be bouncing off the top stop. The N1 will be driven down to idle to try to control the high N2.

The smart pilot will roll the throttle to idle, so the N2 governor will not play any further part in running the N1, and the N2 will still be plenty high enough to drive the tail rotor.

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I suppose the difference lies in that when we are experiencing emergencies there is a high chance that we have a bunch of 200gr+ pieces of metal flying at us at very high velocities. It's not too far fetched that we will be experiencing more than one emergency at a time.

 

Anyways, my question does not lie with the possibility of this happening. My question lies with how much thrust is lost when the tail rotor is operating at idle RPM.

 

In the event of a main driveshaft failure:

 

AUTOROTATE - Throttle full open.

 

If FADEC failure is annunciated:

 

Throttle - Reduce to idle detent.

AUTO/MAN switch - MAN

Throttle - Adjust to maintain 100% NP, if time permits.

 

 

You’re talking about a very remote failure. You’ve entered an autorotation due to a shaft failure (aka short shaft failure) and then your FADEC fails. Normally at that point, according to your post, you would reduce the throttle to idle, switch to manual and roll the throttle back on. That would be the textbook (RFM) response; however, you wanted to know what the tail rotor authority would be if you left the throttle at idle.

 

We can find what the authority would be by employing our old lift equation and with an understanding of the helicopters drive train. Below (as an example) I’ve diagramed the 407’s drive train along with its corresponding RPM ratios before and after the failure.

 

We need to first breakdown the lift equation, L (thrust) = (CL) x (.5rho) x S x (V squared). Since the air density, and airfoil surface area the same before and after the failure, we can then say thrust (T) is proportional to (CL) x (V squared). The coefficient of lift (CL) is a function of the blade pitch angle and since all tail rotor blade pitch angles are within the designed range, we can rewrite the equation as, thrust (T) is proportional to (V squared) for any given angle of attack. Finally, V (airspeed) is a direct function of the tail rotor RPM, so Thrust (T) is proportional to the RPM squared.

 

This all breaks down to this simple theorem, if you know the RPM reduction percentage you can calculate the reduction in rotor authority (thrust) by squaring that percentage.

 

From the figure we can see the main rotor is in autorotation at normal RPM. However, since the shaft between the MRGB and engine has failed, the only hope for maintaining tail rotor RPM is the engine. The engine is at idle 60% (3790 RPM). The tail rotor RPM is also at 60% of normal (1500 RPM). Using the theorem we square 60% or .60 to come up with .36 or 36%. So, The tail rotor, in this state, will produce only 36% of its normal thrust or authority. From the numbers below you can see why, if possible, you should roll the throttle back on per the RFM during this type of multiple system failure.

 

Examples:

 

Engine at 70%, tail rotor produces 49% of normal thrust.

Engine at 60%, tail rotor produces 36% of normal thrust.

Engine at 55%, tail rotor produces 30.25% of normal thrust.

Engine at 50%, tail rotor produces 25% of normal thrust.

Engine at 40%, tail rotor produces 16% of normal thrust.

 

Frank Robinson explains some of these concepts in the video below.

 

Scan-1-3.jpg

 

http://youtu.be/GeanRnFZDqc

Edited by iChris
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  • 2 weeks later...

I would think the FADEC failure associated with a driveshaft failure is meant to imply that after the failure, depending on the power you are pulling at the point of failure, you may have a massive engine overspeed and that may, in turn fail the FADEC. I'm not familiar with the FADEC on the Kiowa, so I don't know how a FADEC failure would affect the fuel flow at the moment of failure, but, I would think the more tr rpm the better, although I doubt the likely hood of being able to target a specific engine rpm at the time of an actual failure. I could also see the FADEC failing due to the erratic NR. Again, not being familiar with the FADEC on this aircraft, if NR is a critical input for FADEC operation, I could see it failing for that reason as well.

 

There sure is a lot to think about...

 

Being that there is no transmission torque without a driveshaft, you really wouldn't need much authority at the bottom or touchdown portion. Generally you will have a whole boot of right pedal in countering the drag of the transmission. However since the engine is sill driving the tr, throttle manipulation would effect the yaw, but in the opposite way it ordinarily would, I should think. Rolling on would cause a left yaw and rolling off a right, depending on positive pitch on the tr blades, of course. Sure is a lot to get your head around.

Edited by C of G
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