Could you mast-bump a tail rotor?

[aeio540]

I was wondering if it is possible to cause mast-bumping with a tail rotor?

If I understand correctly, semi-rigid two bladed rotors are excellent candidates for mast-bumping and I think many helicopters use a teetering hinge on both main and tail rotors.

 

Could a ham-footed pilot unload the tail rotor enough to cause it to shear itself off?

[RotorWeed]

I was wondering if it is possible to cause mast-bumping with a tail rotor?

If I understand correctly, semi-rigid two bladed rotors are excellent candidates for mast-bumping and I think many helicopters use a teetering hinge on both main and tail rotors.

 

Could a ham-footed pilot unload the tail rotor enough to cause it to shear itself off?

 

I think if your tail rotor hit the main rotor mast, you have more problems then just mast bumping.

[aeio540]

I think if your tail rotor hit the main rotor mast, you have more problems then just mast bumping.

 

Okay, true, if your tail rotor hit the main mast you'd be in trouble. I'm not talking about the main mast. I'm not exactly sure of the name of the shaft that juts out and holds the tail rotor.

 

How bout we call it that from now on. The shaft that juts out and holds the tail rotor, could you unload the tail rotor during flight enough for it to smack into the side and shear itself off?

[FauxZ]

At the Robbie safety course they talked about something similar with the R44 tail rotor when they had the spherical bearings in the tail rotor. It wasn't because the rotor unloaded, it was an excessive flapping issue. If i recall correctly, it was caused by large pedal movements while in forward flight. The spherical bearings allowed for too much flapping and there was contact. They now use an elastometic bearing which provides for some dampening of that flapping motion.

 

Thinking about it, I'm not sure if the tail rotor hit it's mast, or if there was some other form of contact, but there was contact.

[RotorWeed]

Okay, true, if your tail rotor hit the main mast you'd be in trouble. I'm not talking about the main mast. I'm not exactly sure of the name of the shaft that juts out and holds the tail rotor.

 

How bout we call it that from now on. The shaft that juts out and holds the tail rotor, could you unload the tail rotor during flight enough for it to smack into the side and shear itself off?

 

What do you mean by "unload" the tail rotor? The tail rotor does not support the weight of the aircraft the last time I checked.

[aeio540]

What do you mean by "unload" the tail rotor? The tail rotor does not support the weight of the aircraft the last time I checked.

 

No but the tailrotor puts a force to counteract the torque of the main rotor, maybe I'm incorrect, but there should be some disk loading on it. Again, I could be wrong on this, but the tail rotor experiences flapping and lead-lag, vortex-ring state, etc. and so hinge type tail rotors

have similar mechanisms as the MR. So if you were to say Jam your foot all the way to the stops on any anti-torque pedal and waited till you reached some excessive rotation and then jammed your foot on the opposite pedal, I assume there would be a short moment in time when the TR is not creating any force.

[FauxZ]

So if you were to say Jam your foot all the way to the stops on any anti-torque pedal and waited till you reached some excessive rotation and then jammed your foot on the opposite pedal, I assume there would be a short moment in time when the TR is not creating any force.

 

But mast bumping doesn't occur just because the rotor is unloaded. The problem occurs when the disc moves relative to the airframe. So even if you could unload the tail rotor for a second, you'd have to find some way to move the rotor to one side relative to the chassis. That's a tall order without cyclic control of tail rotor pitch.

[aeio540]

But mast bumping doesn't occur just because the rotor is unloaded. The problem occurs when the disc moves relative to the airframe. So even if you could unload the tail rotor for a second, you'd have to find some way to move the rotor to one side relative to the chassis. That's a tall order without cyclic control of tail rotor pitch.

 

Oh okay, so without cyclic control of the TR, an unloaded rotor doesn't risk a mast bump? Hmmm.

I thought that without a load on a rotor blade, they will flap to their maximum limit and well, you know the story.

 

So something hypothetical here, you take your R22/Jetranger/Cobra and from level flight you nose it down and neutralize the cyclic, would you not have to worry about losing your MR because there is no cyclic input?

[FauxZ]

So something hypothetical here, you take your R22/Jetranger/Cobra and from level flight you nose it down and neutralize the cyclic, would you not have to worry about losing your MR because there is no cyclic input?

 

The "instigator," for lack of a better word, is the roll induced by the tail rotor thrust when you unload the rotor. If you were to do nothing, you'd roll the machine upside down and then you'd have a whole new interesting set of problems to deal with. The mast bumping occurs when the pilot ACTS incorrectly by applying lateral cyclic and tilting the rotor disc while it is unloaded. Notice, it is the pilots incorrect action that turns it into an emergency. However, pilot inaction will eventually lead to the same problem as you roll upside down.

 

FYI, while we're on the topic. I spoke with a friend who has had this demonstrated to her by a competent pilot some time ago. In training, we always drive home the point of "you can roll as fast as 100° a second!!" and your gut reaction may indeed be wrong. However, during her demonstration, she was shown just how SLOW you can actually roll. To paraphrase, "If we weren't demonstrating it on purpose, I would not have even thought of it being low g. Everything happened very slowly and smoothly. Even knowing what was happening, my gut wanted to push left cyclic"

 

Food for thought, not exactly on topic.

[RotorWeed]

Oh okay, so without cyclic control of the TR, an unloaded rotor doesn't risk a mast bump? Hmmm.

I thought that without a load on a rotor blade, they will flap to their maximum limit and well, you know the story.

 

So something hypothetical here, you take your R22/Jetranger/Cobra and from level flight you nose it down and neutralize the cyclic, would you not have to worry about losing your MR because there is no cyclic input?

 

Mast bumping or droop stop pounding or what ever you wish to call it can be caused by any strong, or rapid cyclic inputs into a rotor disc. If you rapidly push the cyclic forward you will unload the main rotor disk from the weight of the helicopter and this phenomenon might occur. (you will know it because of the rapid roll rate that devlops)

The tail rotor doesn't support the weight of the helicopter.

 

Why are you asking this question BTW?

[aeio540]

The tail rotor doesn't support the weight of the helicopter.

 

Except in blackhawks?

 

Why are you asking this question BTW?

 

Oh I was reading about some helicopter crash and they attributed it to mast bumping or something similar and all the input people had was that it had an underslung rotor, which has a teetering hinge. Well, I looked at countless rotor heads and diagrams and low and behold, the tail rotor system to many helicopters have a similar design to the main rotor: two bladed and hinged.

 

Okay, I think there is some confusion(dizzam internets!), when I mean that the tail rotor is unloaded, I don't mean that it's carrying any weight, but the tail rotor is in fact loaded as long as it is pushing air in the opposite direction of the helicopter's yawing tendency. If it wasn't loaded with counteracting the force, then there would be no need for a tail rotor.

 

I'm not a helicopter pilot, atleast not yet so I wasn't in a helicopter and heard a loud bang and ignored it.

[nbit]

If tail rotors couldn't "unload", a helicopter never be to be able to yaw but one direction during an auto... Think about it...

 

Because a tail rotor doesn't have a cyclic type input, the designers and engineers of a two bladed tail rotor with a teetering hinge have some options to prevent mast bumping that cannot be used on a semi-rigid main rotor with cyclic control.

 

One is that the designer/engineer can put a much higher delta angle in the pitch change mechanism that will cause the tail rotor to have high tendency to re-center or remain close to a plane perpendicular to the tail rotor drive shaft, thus avoiding "mast bumping". If a main rotor had such a high delta angle, it's cyclic wouldn't be very effective, but it doesn't apply to the operation of the tail rotor.

 

The other option is that the design of the tail rotor can more easily include a "shock absorber" and/or elastomeric bearing that provides a fair measure of protection to the tail rotor drive shaft.

 

There may be other options available, but those are the ones I have heard of...

 

Regards...

[RockyMountainPilot]

At the Robinson factory course they used to show a low-g demo and roll the thing to about 80 degrees in a blink. They stopped doing this as they realized they were teaching recovery instead of avoidance. Avoidance is always better.

 

Look at the R22 or R44 tail rotor and swing it side to side. You will see the very large pitch change due to the delta hinge. Even if you unload the tail rotor, the delta hinge will keep it centered in most situations. If you had low rotor RPM and stalled the TR in forward flight, it would probably bump its mast, but you have much bigger things to worry about if that happens.

[aeio540]

Okay, so most helicopters come with a design to prevent it. And I guess if it does happen, there are other things going on at the moment that are much much worse.

[FLHooker]

Ok, I've kinda just read the thread from the sidelines.. and am going to attempt to put my 2 cents in...

 

 

by "unloading" you are referring to Zero G's which would cause the MR to flap excessively..1 way to get into mast bumping.

 

You also need to remember that the tail rotor on the TH-67/Bell 206 is turning 6 (somewhere around there) times faster than the MR, so it would harder to 'unload' unless you are in the Loss of Tail Rotor Effectiveness ranges which do shed the lift from the tail rotor.

 

And, the tailrotor does produce some of the lift for some helo's...namely the Blackhawk... I dunno the exact amount, but i do know it does.

 

The elastomeric damper aids in keeping the anti-torque pedals from creeping/helping the pilot keep them neutral... personally, i hated them. You fought against them the entire flight.

[Linc]

You also need to remember that the tail rotor on the TH-67/Bell 206 is turning 6 (somewhere around there) times faster than the MR, so it would harder to 'unload' unless you are in the Loss of Tail Rotor Effectiveness ranges which do shed the lift from the tail rotor.

LTE does not shed lift from the tail rotor. It is certain situations where the lift provided by the tail rotor is no longer sufficient to counteract the torque of the main rotor. It is simply Not Enough Tail Rotor.

 

Tail rotors are not underslung, so no mast bumping.

[Goldy]

Tail rotors are not underslung, so no mast bumping.

 

 

And there, 15 posts later...is the answer the guy was searching for !!!

[Eric Hunt]

To answer the original question of " can a tail rotor have a mast bump?", the answer is YES, in a 2-blade system.

 

Pump in a huge input of pedal in forward flight, and the flapping of the disc will be more than the Delta 3 hinge can compensate for. The bumpers at the blade grips will contact the drive shaft, or mast. It can be catastrophic.

 

Sometimes, though, before that happens, the blades can contact the vertical fin. That can cause serious damage to the tail rotor, bits can be shed. The weight imbalance then tears the whole gearbox from the frame, huge yaw from loss of thrust, and nose drop from the change of CG, main rotor mast bump and blade separation (after passing through the cockpit), end of game.

 

I lost 3 mates in an Iroquois B model from exactly this. The huge input was caused by the T/R control cable coming off its pulley and wrapping around the drivehaft in the tail boom. No reason was found for the cable problem. Models after the B had the T/R cables in the lower part of the boom, clear of the drive shaft.

[RockyMountainPilot]

Tail rotors are not underslung, so no mast bumping.

 

Who says? Any non-rigid or hinged rotor system can experience excessive flapping and damage or destruction of the stops.

 

To unload the tailrotor, you only need to bring the pedals to neutral.

 

Eric,

 

How does applying a large amount of pedal increase flapping? Flapping increases with a reduced load.

[sky2]

Who says? Any non-rigid or hinged rotor system can experience excessive flapping and damage or destruction of the stops.

 

To unload the tailrotor, you only need to bring the pedals to neutral.

 

Eric,

 

How does applying a large amount of pedal increase flapping? Flapping increases with a reduced load.

[Linc]

Who says? Any non-rigid or hinged rotor system can experience excessive flapping and damage or destruction of the stops.

 

To unload the tailrotor, you only need to bring the pedals to neutral.

Bringing the pedals to "neutral" doesn't unload the rotor, it simply decreases the amount of antitorque thrust the rotor is producing. Neutral on the pedals is still a positive antitorque blade pitch setting. To truly unload the tail rotor, you would have to put in a rapid full right pedal input. However, the danger of the excessive flapping and tailboom contact, at least on Bell products, seems to come with rapid full left pedal inputs. And the excessive flapping never seems to shear the shaft on the tail rotor, it always seems to contact the tailboom or vertical fin. In the OH-58D a rapid full left pedal input above certain airspeeds is warned against to prevent the tail rotor from contacting the tailboom. Most likely because the excessive flapping angle will contact the tailboom well before it bumps on the shaft. I believe that the B407 carried a similar warning first.

 

My statement is based plainly on that the mast bumping phenomenon is not relegated to simple excessive flapping and damage and destruction of stops, but that it is described in relation to teetering rotor systems with an underslung design only. A design where the stop is contact between the M/R yoke and the mast. You can argue that fact, but I haven't seen any other facts to counter it.

[RockyMountainPilot]

Bringing the pedals to "neutral" doesn't unload the rotor, it simply decreases the amount of antitorque thrust the rotor is producing. Neutral on the pedals is still a positive antitorque blade pitch setting. To truly unload the tail rotor, you would have to put in a rapid full right pedal input. However, the danger of the excessive flapping and tailboom contact, at least on Bell products, seems to come with rapid full left pedal inputs. And the excessive flapping never seems to shear the shaft on the tail rotor, it always seems to contact the tailboom or vertical fin. In the OH-58D a rapid full left pedal input above certain airspeeds is warned against to prevent the tail rotor from contacting the tailboom. Most likely because the excessive flapping angle will contact the tailboom well before it bumps on the shaft. I believe that the B407 carried a similar warning first.

 

My statement is based plainly on that the mast bumping phenomenon is not relegated to simple excessive flapping and damage and destruction of stops, but that it is described in relation to teetering rotor systems with an underslung design only. A design where the stop is contact between the M/R yoke and the mast. You can argue that fact, but I haven't seen any other facts to counter it.

 

Linc,

 

I can see where some confusion lies in the term neutral pedals. When I say neutral pedals, I refer to 0 pitch on the blades which is about 80% full right pedal. I wasn't refering to even pedals. if you enter into autorotation, the TR becomes mostly unloaded. You actually have a slight amount of right pedal from neutral to counter MRGB friction.

 

If you go full right pedal, you simply load the TR in the opposite direction, or in the direction of MR torque. To unload the rotor with pitch like that which happens with a push over, you would need some outside force to rapidly move the helicopter in the direction of thrust.

 

I have heard of instances where the TR drive shaft of gear box fails and the TR slows and experiences retreating blade stall which causes such the TR to come into contact with the boom.

 

And I agree, underslung rotor systems are not the only system suseptable to mast bumping.

[RockyMountainPilot]

Bringing the pedals to "neutral" doesn't unload the rotor, it simply decreases the amount of antitorque thrust the rotor is producing. Neutral on the pedals is still a positive antitorque blade pitch setting. To truly unload the tail rotor, you would have to put in a rapid full right pedal input. However, the danger of the excessive flapping and tailboom contact, at least on Bell products, seems to come with rapid full left pedal inputs. And the excessive flapping never seems to shear the shaft on the tail rotor, it always seems to contact the tailboom or vertical fin. In the OH-58D a rapid full left pedal input above certain airspeeds is warned against to prevent the tail rotor from contacting the tailboom. Most likely because the excessive flapping angle will contact the tailboom well before it bumps on the shaft. I believe that the B407 carried a similar warning first.

 

My statement is based plainly on that the mast bumping phenomenon is not relegated to simple excessive flapping and damage and destruction of stops, but that it is described in relation to teetering rotor systems with an underslung design only. A design where the stop is contact between the M/R yoke and the mast. You can argue that fact, but I haven't seen any other facts to counter it.

 

Linc,

 

I can see where some confusion lies in the term neutral pedals. When I say neutral pedals, I refer to 0 pitch on the blades which is about 80% full right pedal. I wasn't refering to even pedals. if you enter into autorotation, the TR becomes mostly unloaded. You actually have a slight amount of right pedal from neutral to counter MRGB friction.

 

If you go full right pedal, you simply load the TR in the opposite direction, or in the direction of MR torque. To unload the rotor with pitch like that which happens with a push over, you would need some outside force to rapidly move the helicopter in the direction of thrust.

 

I have heard of instances where the TR drive shaft of gear box fails and the TR slows and experiences retreating blade stall which causes such the TR to come into contact with the boom.

 

And I agree, underslung rotor systems are not the only system suseptable to mast bumping.

[helisphere]

So has anyone noticed that the delta hinge is backwards on a 407? Flapping causes a pitch change that increases flapping rather than decreasing it. Anyone know why this is? Seems like an unwise design to me, might explain why the 407 had its tail rotor problems.

[aeio540]

And I agree, underslung rotor systems are not the only system suseptable to mast bumping.

 

What other systems can succumb to mast bumping?

From what I understand, if you have flapping and lead-lag joints you're basically safe.

On a similar note, I don't know if anyone has seen this video but it impressed me.

To the untrained eye (mine), I would think he is putting in some pretty harsh cyclical inputs, especially with that dive in the beginning.

 

http://www.youtube.com/watch?v=lXNCZlQcvGw