What Exactly Is Mast Bumping
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What Exactly Is Mast Bumping
Hope you don't mind me invading your forum. I'm a fixed wing PPL and aero engineer with a passing interest in helicopter flight, mainly from a technical point of view.
Quite simply, I've been trying to get my head around exactly what's going on during 'mast bumping'. I understand that it's associated with loss of control of the rotor disk during low-'g' flight, but I can't visualise exactly what part of the rotor head is coming into contact with the rotor mast to the degree that the mast can be sheared off.
Can anybody help with an explanation?
Thanks
WUT
Quite simply, I've been trying to get my head around exactly what's going on during 'mast bumping'. I understand that it's associated with loss of control of the rotor disk during low-'g' flight, but I can't visualise exactly what part of the rotor head is coming into contact with the rotor mast to the degree that the mast can be sheared off.
Can anybody help with an explanation?
Thanks
WUT
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Look at (or visualize) a 2-bladed rotor system. It is known as a "teetering" system because the rotor hub is attached to the rotor mast at a single point - the teeter hinge - and it is free to pivot about that point to allow the blades to flap. If you look at a helicopter from the front, with the blades running laterally (like the wings of an airplane), if you pushed the left blade up, the right blade would go down, just like a teeter-totter.
A design feature of teetering rotor systems is that the hub is "underslung", meaning the rotor blades' plane of rotation is below the pivot point when viewed from the side. Imagine a steel rod with a hole through one end standing vertically (rotor mast). Now make a steel box with a corresponding hole through the sides. Put the box over the rod, put a bolt through the holes, one model teetering rotorhub!
Rotor viewed from top:
blade ........... rotor hub .......... blade
============[()]============
teeter hinge axis is up/down in this diagram,
running between the ellipses (rotor mast)
Obviously, the hub can only have a certain range of teetering before the inside of the hub will hit the rotor mast. In normal flying, the only time these limits might be approached is in a slope landing, where the fuselage and rotor mast are at an angle while the rotor disk remains perpendicular to "true vertical".
However, in a low-G condition inflight, the angle between the rotor disk and the airframe can very rapidly exceed the flapping limit of the rotor hub. The hub hits the mast and in short order, the mast can separate. Bad thing.
There's a lot more aerodynamics, geometry, and physics involved, but that's the gist. For a little more info on heli-flight overall, may I suggest the Dynamic Flight website
John
A design feature of teetering rotor systems is that the hub is "underslung", meaning the rotor blades' plane of rotation is below the pivot point when viewed from the side. Imagine a steel rod with a hole through one end standing vertically (rotor mast). Now make a steel box with a corresponding hole through the sides. Put the box over the rod, put a bolt through the holes, one model teetering rotorhub!
Rotor viewed from top:
blade ........... rotor hub .......... blade
============[()]============
teeter hinge axis is up/down in this diagram,
running between the ellipses (rotor mast)
Obviously, the hub can only have a certain range of teetering before the inside of the hub will hit the rotor mast. In normal flying, the only time these limits might be approached is in a slope landing, where the fuselage and rotor mast are at an angle while the rotor disk remains perpendicular to "true vertical".
However, in a low-G condition inflight, the angle between the rotor disk and the airframe can very rapidly exceed the flapping limit of the rotor hub. The hub hits the mast and in short order, the mast can separate. Bad thing.
There's a lot more aerodynamics, geometry, and physics involved, but that's the gist. For a little more info on heli-flight overall, may I suggest the Dynamic Flight website
John
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Thanks very much John. That's the first explanation I've seen that makes real sense - it's the fact that the teetering rotor is underslung, as you pointed out, that makes all the difference.
Thanks again
WUT
Thanks again
WUT
Wind Up:
Next time you see a JetRanger, Bell 47 or R22/R44 ask if you can have a look at the rotorhead - the clearances are minimal, but enough to prevent bumping except in abnormal circumstances.
Next time you see a JetRanger, Bell 47 or R22/R44 ask if you can have a look at the rotorhead - the clearances are minimal, but enough to prevent bumping except in abnormal circumstances.
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Hopefully I'm not going to confuse things by pointing out that the helicopter hangs from that hinge.
It's so obvious when you're sitting on the ground that an object weighs something, that it's easy to miss; but its weight is what makes it hang. Flying along straight and level, it still weighs the same, being subject as it is to one g. However, push the stick forward (or better still, get a good speed up, pull to the up 45 and then give it a good push) and the aircraft will become weightless underneath that hinge.
With nothing making it hang, the other forces that are usually submerged in the background come rushing to the fore - mainly the side thrust from the tail rotor. Pushing lustily sideways, below the line of the hinge, it makes the helicopter roll sideways to the point where the flapping stops can go rat-tat-tat on the top of the mast. Knocks it clean through in a very short time.
It's so obvious when you're sitting on the ground that an object weighs something, that it's easy to miss; but its weight is what makes it hang. Flying along straight and level, it still weighs the same, being subject as it is to one g. However, push the stick forward (or better still, get a good speed up, pull to the up 45 and then give it a good push) and the aircraft will become weightless underneath that hinge.
With nothing making it hang, the other forces that are usually submerged in the background come rushing to the fore - mainly the side thrust from the tail rotor. Pushing lustily sideways, below the line of the hinge, it makes the helicopter roll sideways to the point where the flapping stops can go rat-tat-tat on the top of the mast. Knocks it clean through in a very short time.
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Mast bumping on a Bell 47???
To: The Nr Fairy
The only common thing relative to the 206 and the model 47 is that their rotors teeter about a hinge.
Unless they have changed the design of a Bell 47 rotorhead since I last worked with them you can not get mast bumping on a 47. There are two Sprague cables that limit the movement of the blades on the teeter hinge. You will reach the limits allowed by these cables before you can hit the mast. Excessive cyclic movement (range wise) will cause the cables to reach their limits and the pilot will be aware of this due to heavy pounding resulting in his/her pulling back on the cyclic.
Next time you see a JetRanger, Bell 47
Unless they have changed the design of a Bell 47 rotorhead since I last worked with them you can not get mast bumping on a 47. There are two Sprague cables that limit the movement of the blades on the teeter hinge. You will reach the limits allowed by these cables before you can hit the mast. Excessive cyclic movement (range wise) will cause the cables to reach their limits and the pilot will be aware of this due to heavy pounding resulting in his/her pulling back on the cyclic.
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A quick deviation off topic.
Lu,
Can you shed any light on the reasoning behind the incorporation of the second hinge (at 90-degrees to the teetering hinge) in the 47's hub?
Teetering rotors are referred to as being a Universal joint, a Hookes joint or a Cardan joint. This type of joint has two hinges, but the Bell 47 is the only teetering rotor head, that I know of, which actually has both of these hinges.
Can you shed any light on the reasoning behind the incorporation of the second hinge (at 90-degrees to the teetering hinge) in the 47's hub?
Teetering rotors are referred to as being a Universal joint, a Hookes joint or a Cardan joint. This type of joint has two hinges, but the Bell 47 is the only teetering rotor head, that I know of, which actually has both of these hinges.
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Mast bumping on a fully articulated rotorhead. (Not)
To: Russell Sprout
The answer is no. In a conventional fully articulated head the rotorhead is rigidly mounted to the mast and the blades are isolated from the rotorhead by hinges or bearings. In some heads that use elastomeric bearings this bearing serves as an offset hinge and offers three degrees of freedom. Although you can't get mast bumping the blades are limited in their vertical travel by adjustable stops. If the pilot inputs excessive cyclic pitch there is a possibility that the blades can contact these stops. This contact although noted by the pilot as a severe beat it is not as severe as if it were mast bumping which can cause loss of the rotorhead.
The answer is no. In a conventional fully articulated head the rotorhead is rigidly mounted to the mast and the blades are isolated from the rotorhead by hinges or bearings. In some heads that use elastomeric bearings this bearing serves as an offset hinge and offers three degrees of freedom. Although you can't get mast bumping the blades are limited in their vertical travel by adjustable stops. If the pilot inputs excessive cyclic pitch there is a possibility that the blades can contact these stops. This contact although noted by the pilot as a severe beat it is not as severe as if it were mast bumping which can cause loss of the rotorhead.
Last edited by Lu Zuckerman; 6th Mar 2004 at 07:55.
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Lu :
The sprag cables dont seem to be necessary on the 47 heads anymore. I havent seen one with them fitted since the late 70's. I think a bulletin allowed their removal.
There are two Sprague cables that limit the movement of the blades on the teeter hinge. You will reach the limits allowed by these cables before you can hit the mast.
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I wish I had a time machine.
To: Dave Jackson
Dave I have limited number of functioning brain cells and very few of them are allocated to long term memory. You are asking me to go back over fifty years. If I remember correctly the hinge you alluded to allows the blades to rock. In effect when the pilot inputs cyclic pitch the blades do not change pitch individually. The entire rotorhead rocks on the hinge and one blade has decreased pitch and the opposite blade has increased pitch. The stabilizer bar which is at 90-degrees to the rotorhead acts as a rigid element (gyroscopic rigidity in space) creating a reaction force keeping the blades rocking in the correct direction as the blades rotate. The swash plate also assists in this change in pitch (rocking).
I will qualify this with a very large I think
Can you shed any light on the reasoning behind the incorporation of the second hinge (at 90-degrees to the teetering hinge) in the 47's hub?
I will qualify this with a very large I think
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i have never seen a cable on the rotor head of any 47 that i have flown nor seen or heard any sign of mast bumping on the type.
the cables were removed from the head of the 47about 1974/5.
they were attached to the stab bar and went up to the gimbal.
you will run out of cyclic travel before the head can touch the mast and the rabbit ears also will stop the gimbal from reaching the mast. the rabbit ears act as dynamic stops as well as static stops.
the cables were removed from the head of the 47about 1974/5.
they were attached to the stab bar and went up to the gimbal.
you will run out of cyclic travel before the head can touch the mast and the rabbit ears also will stop the gimbal from reaching the mast. the rabbit ears act as dynamic stops as well as static stops.
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Recollections are like erections. They fade after time.
To: imabel and sprocket as well as whatsarunway
As I had mentioned to Dave Jackson my recollections had to go back over fifty years ago when I was maintaining an early Bell 47 (HTL-1). The last time I had anything to do with a model 47 (J-2 Ranger) was in 1966 or thereabouts. At that time the Sprague cables were still installed.
Regarding the onset of Zero G / Mast bumping it was not only happening on the Hueys in Vietnam it happened to the early 206s when they first went into service which was sometimes towards the end of the Vietnam conflict or thereabouts.
Back in 1965-66 I was working on a simulator related to the Saturn Apollo program. The facility I worked at was in Huntington Beach, California and my job was being constructed in Santa Monica. My position allowed me to fly in one of our companies two J-2 Rangers. On one particular flight back to Huntington Beach I was flying in a ship that had just come out of a major check. The pilot was complaining of a severe beat whenever he displaced the cyclic any more than an inch or so.
When we got back the helicopter was taken into the far end of the manufacturing facility. I borrowed a flashlight and went underneath. I found the Sprague cables on the engine, which were adjusted so there was no allowed movement of the engine. This would not account for the beat although it was related to it. I then checked the Sprague cables on the rotorhead and I found the attaching clevis was 90-degrees out of position, which limited the teetering of the rotorhead. I had the pilot ground the helicopter and it was returned to the contract maintenance facility on a flatbed truck.
What happened was when the rotorhead was restricted from teetering it would couple up with the mast which would try to displace the engine. The engine movement was minimized by the mal adjusted Sprague cables and there would be a severe beat, which was felt throughout the airframe.
As I had mentioned to Dave Jackson my recollections had to go back over fifty years ago when I was maintaining an early Bell 47 (HTL-1). The last time I had anything to do with a model 47 (J-2 Ranger) was in 1966 or thereabouts. At that time the Sprague cables were still installed.
Regarding the onset of Zero G / Mast bumping it was not only happening on the Hueys in Vietnam it happened to the early 206s when they first went into service which was sometimes towards the end of the Vietnam conflict or thereabouts.
Back in 1965-66 I was working on a simulator related to the Saturn Apollo program. The facility I worked at was in Huntington Beach, California and my job was being constructed in Santa Monica. My position allowed me to fly in one of our companies two J-2 Rangers. On one particular flight back to Huntington Beach I was flying in a ship that had just come out of a major check. The pilot was complaining of a severe beat whenever he displaced the cyclic any more than an inch or so.
When we got back the helicopter was taken into the far end of the manufacturing facility. I borrowed a flashlight and went underneath. I found the Sprague cables on the engine, which were adjusted so there was no allowed movement of the engine. This would not account for the beat although it was related to it. I then checked the Sprague cables on the rotorhead and I found the attaching clevis was 90-degrees out of position, which limited the teetering of the rotorhead. I had the pilot ground the helicopter and it was returned to the contract maintenance facility on a flatbed truck.
What happened was when the rotorhead was restricted from teetering it would couple up with the mast which would try to displace the engine. The engine movement was minimized by the mal adjusted Sprague cables and there would be a severe beat, which was felt throughout the airframe.
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it would be very unlikely if not impossible to get mast bumping in a 47, there used to be sprung cables from the gimbal to the stab bar but these were removed about 1975. they were more trouble than they were worth.
this first photo shows the head static with the rabbit ears up.
this photo shows the rabbit ears in the dynamic position there is a secondary stop that would not allow mast bumping even at full cyclic travel.
this first photo shows the head static with the rabbit ears up.
this photo shows the rabbit ears in the dynamic position there is a secondary stop that would not allow mast bumping even at full cyclic travel.
