R22 Corner
Joined: Nov 2000
Posts: 452
Likes: 0
From: Vancouver, BC, Canada
Robinson: Slowing the rotor with pitch
The following posting was pilfered from rec.aviation.rotorcraft.
It is posted here for its informational value and, more importantly, it's posted in the hope of provoking thought, or, better yet, just provoking; like the good old days.
Lu,
This subject is directly related to your concerns. Your comments will be appreciated.
Posting by Steve Waltner, on rec.aviation.rotorcraft
I sent an e-mail the their [Robinson] customer service address and received this response:
=======================================
First, let's ensure everyone understands that the R22/R44 MR hub has one central teeter hinge and two coning hinges (one per blade). Teetering (a specific type of flapping) occurs primarily about the teeter hinge while coning occurs about the coning hinges. At the root-end of each main rotor blade is a spindle bearing assembly, about which the MR blade changes pitch (feathers) and which in turn connects the blade to the hub via the coning hinge.
When there is insufficient centrifugal force to keep the rotor disc flat, the blades will droop. To prevent the blades from drooping so low as to chop off the tailcone, aluminum droop stops affixed to the main rotor driveshaft are contacted by stainless-steel spindle tusks extending inboard of the coning hinge. Both the droop stop and spindle tusk can be seen thru the window on either side of the hub. Do not confuse the aluminum droop stops with the amber-colored urethane teeter stops also affixed to the driveshaft.
The arm between the droop stop and coning hinge (the fulcrum) is very short, while the arm between the droop stop and blade tip is quite long (on the R22 the ratio is about 75:1). This ratio accounts for the "Never Pull Down - Push Up Opposite Blade" sticker on the underside of the blade tip. Even when the blades are stopped, the droop stops are reacting a large amount of force, which in turn creates friction between the droop stop/spindle tusk surfaces. This friction is important in reducing rotor teetering at low rpm when there is little centrifugal force stiffening (and minimizing flexing of) the MR blades. Raising the collective (don't do it!) during shut down increases the blade angle of attack and generates enough blade lift to unload the droop stops, which removes the friction that is retarding rotor teetering. Any cyclic inputs, or wind, causes teetering which reduces clearances and allows the now-flexible MR blades to contact the tailcone (and your disembarking passengers).
Fly (and shut down) safely!
=======================================
I also asked for clarification on using the tail rotor to slow down the pitch, which received the following response:
=======================================
RHC will not recommend using the pedals to slow down main rotor. We
have an elastomeric bearing on the tail rotor assembly that will flap
and may strike the tailcone and or cause unnecessary wear.
Slow down drive system by using the rotor brake.
=======================================
Steve
It is posted here for its informational value and, more importantly, it's posted in the hope of provoking thought, or, better yet, just provoking; like the good old days.
Lu,
This subject is directly related to your concerns. Your comments will be appreciated.
Posting by Steve Waltner, on rec.aviation.rotorcraft
I sent an e-mail the their [Robinson] customer service address and received this response:
=======================================
First, let's ensure everyone understands that the R22/R44 MR hub has one central teeter hinge and two coning hinges (one per blade). Teetering (a specific type of flapping) occurs primarily about the teeter hinge while coning occurs about the coning hinges. At the root-end of each main rotor blade is a spindle bearing assembly, about which the MR blade changes pitch (feathers) and which in turn connects the blade to the hub via the coning hinge.
When there is insufficient centrifugal force to keep the rotor disc flat, the blades will droop. To prevent the blades from drooping so low as to chop off the tailcone, aluminum droop stops affixed to the main rotor driveshaft are contacted by stainless-steel spindle tusks extending inboard of the coning hinge. Both the droop stop and spindle tusk can be seen thru the window on either side of the hub. Do not confuse the aluminum droop stops with the amber-colored urethane teeter stops also affixed to the driveshaft.
The arm between the droop stop and coning hinge (the fulcrum) is very short, while the arm between the droop stop and blade tip is quite long (on the R22 the ratio is about 75:1). This ratio accounts for the "Never Pull Down - Push Up Opposite Blade" sticker on the underside of the blade tip. Even when the blades are stopped, the droop stops are reacting a large amount of force, which in turn creates friction between the droop stop/spindle tusk surfaces. This friction is important in reducing rotor teetering at low rpm when there is little centrifugal force stiffening (and minimizing flexing of) the MR blades. Raising the collective (don't do it!) during shut down increases the blade angle of attack and generates enough blade lift to unload the droop stops, which removes the friction that is retarding rotor teetering. Any cyclic inputs, or wind, causes teetering which reduces clearances and allows the now-flexible MR blades to contact the tailcone (and your disembarking passengers).
Fly (and shut down) safely!
=======================================
I also asked for clarification on using the tail rotor to slow down the pitch, which received the following response:
=======================================
RHC will not recommend using the pedals to slow down main rotor. We
have an elastomeric bearing on the tail rotor assembly that will flap
and may strike the tailcone and or cause unnecessary wear.
Slow down drive system by using the rotor brake.
=======================================
Steve
Joined: Feb 2001
Posts: 103
Likes: 0
From: the other America
This is true of most helicopter types.
Never, Never, Never use collective to slow/stop the blades during shutdown.
Blades will flap & can/will flap beyond limits (generally taking off the tailboom)
Never, Never, Never use collective to slow/stop the blades during shutdown.
Blades will flap & can/will flap beyond limits (generally taking off the tailboom)
Joined: Nov 2000
Posts: 2,018
Likes: 73
From: Pewsey, UK
Dave :
Check this AAIB report to see why raising collllective to slow blades is one way to empty your pockets !
Check this AAIB report to see why raising collllective to slow blades is one way to empty your pockets !
Iconoclast
Joined: Sep 2000
Posts: 2,132
Likes: 0
From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
The beast has risen yet another time.
To: Dave Jackson
I found the information above very interesting. Bell single rotor pilots have for
a long time slowed the rotor by increasing collective pitch. However with
the new designs (multi blade rotors) they can't do it anymore for the same
reasons that pilots of articulated rotor system helicopters including the
Robinson's' are restricted from this activity.
On blades that can flap the pilots are restricted from pulling collective to
slow the rotor down because of blade stall. As the blades slow down they
lose the velocity necessary to generate the lift associated with increased
pitch. This will cause the blade(s) to stall out and drop down and hit the
fuselage. On most Sikorsky systems they have both droop stops and anti coning
locks. These devices are spring loaded and are weighted. When the
rotational velocity of the blades slows the springs overcome the centrifugal
loading and bring the stops into the static position. The droop stops come
in first and then the anti cone locks. This must happen in the proper
sequence or, the blade(s) can drop down far enough to contact the fuselage
or possibly ground personnel.
In the response by Robinson (which was technically correct) they in fact
supported many of my arguments on the PPRuNe forum. They stated that with
the mechanical advantage between the blade span and the tusk length (75:1)
you could exert a great deal of force on the contact point if you pulled a
blade tip downward. The force would be reacted by the tusk stop and would
then act on the teeter friction possibly damaging the stop. If you take
that point further imagine the force reacted by the stop when the rotor has
gyroscopic rigidity and the blades flap and the tusk hits the stop.
1) The reactive force necessary to move the disc out of its' rotational
plane and possibly cause mast bumping is sufficiently strong as to cause the
tusk to fail. This can result in blade incursion.
2) If the tusk does not fail the rotor can be made to teeter to a point
where you encounter mast bumping.
3) Consider the forces involved and the reactions with both blades flapping
to the point where the tusks are contacting the stops (see 1&2 above).
It all boils down to the blades flapping excessively. What causes this
flapping to the maximum of the blade movement? As the Robinson response
indicated pulling pitch to slow the rotor down can cause flapping (due
mainly to loss of lift) and according to the FAA statement in the Robinson
POHs excessive flapping excursions can be caused by:
1) Incorrect recovery from a Zero G encounter.
2) Side slipping.
3) Flying out of trim.
It was for this reason that Robinson pilots are cautioned from performing
any of the above maneuvers.
My total argument against the Robinson design was the design of the
rotorhead. It was my conclusion that all of the problems would go away if
they had a three-blade rotor system like the Schweizer 300 series
helicopters.
I found the information above very interesting. Bell single rotor pilots have for
a long time slowed the rotor by increasing collective pitch. However with
the new designs (multi blade rotors) they can't do it anymore for the same
reasons that pilots of articulated rotor system helicopters including the
Robinson's' are restricted from this activity.
On blades that can flap the pilots are restricted from pulling collective to
slow the rotor down because of blade stall. As the blades slow down they
lose the velocity necessary to generate the lift associated with increased
pitch. This will cause the blade(s) to stall out and drop down and hit the
fuselage. On most Sikorsky systems they have both droop stops and anti coning
locks. These devices are spring loaded and are weighted. When the
rotational velocity of the blades slows the springs overcome the centrifugal
loading and bring the stops into the static position. The droop stops come
in first and then the anti cone locks. This must happen in the proper
sequence or, the blade(s) can drop down far enough to contact the fuselage
or possibly ground personnel.
In the response by Robinson (which was technically correct) they in fact
supported many of my arguments on the PPRuNe forum. They stated that with
the mechanical advantage between the blade span and the tusk length (75:1)
you could exert a great deal of force on the contact point if you pulled a
blade tip downward. The force would be reacted by the tusk stop and would
then act on the teeter friction possibly damaging the stop. If you take
that point further imagine the force reacted by the stop when the rotor has
gyroscopic rigidity and the blades flap and the tusk hits the stop.
1) The reactive force necessary to move the disc out of its' rotational
plane and possibly cause mast bumping is sufficiently strong as to cause the
tusk to fail. This can result in blade incursion.
2) If the tusk does not fail the rotor can be made to teeter to a point
where you encounter mast bumping.
3) Consider the forces involved and the reactions with both blades flapping
to the point where the tusks are contacting the stops (see 1&2 above).
It all boils down to the blades flapping excessively. What causes this
flapping to the maximum of the blade movement? As the Robinson response
indicated pulling pitch to slow the rotor down can cause flapping (due
mainly to loss of lift) and according to the FAA statement in the Robinson
POHs excessive flapping excursions can be caused by:
1) Incorrect recovery from a Zero G encounter.
2) Side slipping.
3) Flying out of trim.
It was for this reason that Robinson pilots are cautioned from performing
any of the above maneuvers.
My total argument against the Robinson design was the design of the
rotorhead. It was my conclusion that all of the problems would go away if
they had a three-blade rotor system like the Schweizer 300 series
helicopters.
PPRuNe Enigma
Joined: Feb 2001
Posts: 427
Likes: 0
From: Scotland
Ok, but since you have a rotor brake it isn't actually a problem or a design flaw, is it?
Any more than the soles of your shoes wearing out if you use them instead of the brake to slow your motorbike down
Any more than the soles of your shoes wearing out if you use them instead of the brake to slow your motorbike down
Joined: Nov 2000
Posts: 2,018
Likes: 73
From: Pewsey, UK
Lu :
- are you aware of blades that dont' ?
- so do Bell 47s, Bell 206s and the like NOT experience the same problems ?
On blades that can flap
It was my conclusion that all of the problems would go away if they had a three-blade rotor system like the Schweizer 300 series helicopters
Cracked Robbo Blades
Joined: Jul 2001
Posts: 420
Likes: 0
From: Florida
For those of you interested, here it is :
R22 Safety Alert.
Issued: 25 June 2002
Unusual Vibration Can Indicate A Main Rotor Blade Crack
A catastrophic rotor blade fatigue failure can be averted if pilots and mechanics are alert to early indications of a fatigue crack. Although a crack may be internal and not visible, it will likely cause a significant increase in rotor vibration several flight hours prior to final failure. If a rotor is smooth after balancing but then goes out of balance again within a few flights it should be considered suspect. Rapidly increasing vibration indicates imminent failure and requires immediate action.
If main rotor vibration increases rapidly or becomes severe during a flight, land immediatly.
Do not attempt to continue flight to a convenient destination. Have the rotor system thoroughly examined by a qualified mechanic before further flight. If mechanic is not sure whether a crack exists, contact RHC.
Comments?
R22 Safety Alert.
Issued: 25 June 2002
Unusual Vibration Can Indicate A Main Rotor Blade Crack
A catastrophic rotor blade fatigue failure can be averted if pilots and mechanics are alert to early indications of a fatigue crack. Although a crack may be internal and not visible, it will likely cause a significant increase in rotor vibration several flight hours prior to final failure. If a rotor is smooth after balancing but then goes out of balance again within a few flights it should be considered suspect. Rapidly increasing vibration indicates imminent failure and requires immediate action.
If main rotor vibration increases rapidly or becomes severe during a flight, land immediatly.
Do not attempt to continue flight to a convenient destination. Have the rotor system thoroughly examined by a qualified mechanic before further flight. If mechanic is not sure whether a crack exists, contact RHC.
Comments?
Iconoclast
Joined: Sep 2000
Posts: 2,132
Likes: 0
From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Robinson maintenance philosophy.
It seems incomprehensible that an advisory dealing with the life or death of an operator would leave the diagnosis of the problem to the pilot and then, not tell the mechanic what to look for and how to perform the necessary tests. If the cracks are external the mechanic should be told how to identify the cracks and the extent of an acceptable crack and where the cracks are allowed or disallowed. Without this information the mechanic will not know what to look for or how to look for it. If the cracks are internal the mechanic must be given the necessary information as to how to identify internal cracks and the acceptability of cracks in a given position on the blade and what to do if those cracks are detected.
This is the same philosophy exhibited in the maintenance manual relative to the rigging of the main and tail rotors which are ambiguous, lack sufficient detail and place the mechanic in a position of making a decision that could in turn place the helicopter in jeopardy.
This is the same philosophy exhibited in the maintenance manual relative to the rigging of the main and tail rotors which are ambiguous, lack sufficient detail and place the mechanic in a position of making a decision that could in turn place the helicopter in jeopardy.
Joined: Apr 2002
Posts: 147
Likes: 0
From: Hartford, CT USA
there was procedures along with the alert, as i recall it was to inspect with a 10x mag. glass near the root of the blade, most of the cracks were forming about 3-4 inches from the root. To be done as much as possible, also there was a previous warning about overloading and such putting too much stress on the blades.
Iconoclast
Joined: Sep 2000
Posts: 2,132
Likes: 0
From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
A question as to the cause of the cracking.
Not wanting to stir the pot after it has cooled however I offer the following.
Are these cracks developing as a result of excessive spanwise bending caused by leading and lagging of the blades? It has been proven that leading and lagging are present but highly restrained by the cone hinges. The cone hinges show evidence of this in their elliptical wear pattern. Two things are present. The cone hinges react the leading and lagging and wear accordingly. The reacted loads are reflected in the blades flexing along the spanwise axis. Once the cone hinges wear the blades have very limited movement relative to their fixed position. This movement can exacerbate the loads on the blades by the shock of the movement of the blade moving and hitting the restraint of the cone hinge wear level. The greater the wear the greater the shock level.
Are these cracks developing as a result of excessive spanwise bending caused by leading and lagging of the blades? It has been proven that leading and lagging are present but highly restrained by the cone hinges. The cone hinges show evidence of this in their elliptical wear pattern. Two things are present. The cone hinges react the leading and lagging and wear accordingly. The reacted loads are reflected in the blades flexing along the spanwise axis. Once the cone hinges wear the blades have very limited movement relative to their fixed position. This movement can exacerbate the loads on the blades by the shock of the movement of the blade moving and hitting the restraint of the cone hinge wear level. The greater the wear the greater the shock level.
Joined: Nov 2000
Posts: 452
Likes: 0
From: Vancouver, BC, Canada
Lu's concern may be valid.
IMHO. All teetering rotors have a lead-lag component. A common solid yoke joins the two blades in conventional teetering rotor hubs. In this situation, one blade may help to dampen the moments that are placed on the teetering hinge by the other blade. In addition, and probably more important, is the fact that twisting of the mast absorbs this lead-lag moment. On conventional teetering rotors, this single hinge (feathering hinges excluded from consideration) is centrally located over the mast.
The fact that the Robinson's coning/flapping hinges are offset from the mast raises the possibility of greater wear in these hinges. This wear would then increase the likelihood of blade damage near the root, just as Lu has mentioned.
I have wondered why Robinson did not exclude the cone/flap hinges and put the saved two or three pounds into beefing up the blades. Perhaps he didn't want to develop 'just another Bell' ('II' or is it 'too')
Whether this is serious or not is a separate question.
IMHO. All teetering rotors have a lead-lag component. A common solid yoke joins the two blades in conventional teetering rotor hubs. In this situation, one blade may help to dampen the moments that are placed on the teetering hinge by the other blade. In addition, and probably more important, is the fact that twisting of the mast absorbs this lead-lag moment. On conventional teetering rotors, this single hinge (feathering hinges excluded from consideration) is centrally located over the mast.
The fact that the Robinson's coning/flapping hinges are offset from the mast raises the possibility of greater wear in these hinges. This wear would then increase the likelihood of blade damage near the root, just as Lu has mentioned.
I have wondered why Robinson did not exclude the cone/flap hinges and put the saved two or three pounds into beefing up the blades. Perhaps he didn't want to develop 'just another Bell' ('II' or is it 'too')
Whether this is serious or not is a separate question.