Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Control Authority
#1
I'm a former USMC Helicopter mechanic......

Will the Mosquito reach it's teeter stops within flight?

I'm used to multi bladed articulated heads ....so this question may not be relavant.

Just wondering if a mast bump could occur.

James L
Reply
#2
ANY...repeat...ANY two blade semi-rigid teetering rotor blade system can and will encounter mast bumping if the controls are mis-managed. Rapid stick movements at varying speeds and rotor loadings will create mast bumping. This is a condition to be avoided at all times, in all two blade helicopters. Low rotor RPM and rapid stick movements will create the most likely scenario. So...keep your rotor rpm in the green, high side, and make gradual and small stick movements at all times. The rotor can only precess so fast and slower RPM makes for slower precess rates. And large control stick excursions are always a no-no in a two blade lightly loaded helicopter, such as the Mosquito.
Also, on the other end of the spectrum, high speed flight, teeter stop hammering could occur when retreating blade stall occurs. The Mosquito, as all helicopters, has a VNE. Never test VNE, even in a dive. That is for test pilots. Fly Safe, Live Long Time.
Ancient Chinese Secret.
Flappingly, Mike Driggers aka emdeedee
Reply
#3
Well alright. Helicopter 101.

Mast bump? Someone care to explain this one? Detailed or in general.
Teeter stop hammering?
VNE?

Thanks
Rick
Reply
#4
OK.....Helicopter 101

The rotorblades teeter on the top of the main shaft. This let the helicopter fly in forward flight. Remember the air for the retreating blade is not moving as fast (relatively) as the blade advancing on the other side. So to balance out the differences in lift (speed) the rotors will teeter (like a see saw). All helicopters have a Vne (Velocity to never exceed.) This is the speed that if you fly faster than it.....you are tempting fate. The retreating blade starts to match the air moving past you. This results in the retreating blade starting to stall. When the blade starts to stall.....it looses lift. When it looses it's lift it will let the rotor head teeter too far, to it's teeter stop. The head can only teeter so far....as with any mechanical device. This is not a drawback to the mosquito....it is a fact of life for helicopters.

This causes the rotor system to hit the teeter stops. When this happens, the hammering will actually shear off the rotor head.

The reason I ask the question. I was wondering if the Mosquito was more prone to mast bumping than any other helicopter. It is not relevant for a pilot with time...but this is relevant for students. A student tends to move the stick in rapid movements or jerky motions. This probably from nerves......or some fright.

I actually viewed a Huey do this one time....it was not pretty. I'm not sure why they entered mast bumping (Vne, rotor head failure....?) but the result is unfortunately usually the same if the condition is not corrected fast.

I'm not trying to scare anyone.....this is a rare thing that happens.....unless you fly your aircraft beyond it's limitations.

James L
Reply
#5
Thanks James!

So with more than 2 blades on the rotor is this mast bumping less of a problem? More blades the more the lift is spread around the rotorhead?

Thanks
Rick
Reply
#6
On multi blade heads the head does not teeter. Each blade has stops that are retracted with rotation (centrifugal force) These stops prevents the blades from drooping when the head slows down. Each blade is independently (to an extent) hinged or grommeted on an elastromeric head (yes the rotor blade is actually mounted into a rubber grommet.) They are hinged up and down independently of each other and flap (forward and back) on a hydraulic type system (well on the helo I worked on). It was possible to reach the upper and lower limits of the blade movement....but much harder. I actually had a video one time of the CH53A doing barrel rolls and a very elliptical loop. So this 24,000 lb helo was like a Ferrari.

The more the blades the more complicated the rotor head. Also more weight and more things to check and go wrong. Also the helo is much more "touchy" to fly. So there are trade offs for multi bladed systems.

I must say...I'm speaking from a (Crew Chief)mechanic point of view...I have fewer than 9 hours in the seat. We had to be trained to fly in an emergency.....so we never got hover, or take off training. Just Straight forward flight. Seemed sort of stupid to me, what would we do if we needed to land???!! I guess they thought we would figure it out.....if the time came.


James L
Fatman139 - 12/24/2006 2:01 PM

Thanks James!

So with more than 2 blades on the rotor is this mast bumping less of a problem? More blades the more the lift is spread around the rotorhead?

Thanks
Rick
Reply
#7
Hi James,
Not all multiblade rotor systems have elastomeric bearings. For example, the Schweizer/Hughes 300 series of helicopters have metal bearings instead of elastomeric bearings in the three blade rotor system. The Astar 350 is also a three blade helicopter, but has a fully elastomeric rotor head system.The physics of the blades and rotor heads is, however, the same. The blades can flap up and down, lead lag forward and backward and change pitch in the feathering axis. In addition, there is flexure in the rotor blades.
Also, I must point out that the larger number of blades in rotorheads does not make them harder to control. For example, the Robinson R22 and R44 helicopters are two blade rotor systems with the unique feature of having a teetered head with individual flaping hinges on each blade. These helicopters are the most difficult to fly compared to other helicopters.
No matter what rotorhead configuration, the flaping up and down,either by teetering, flaping hinges, or elastomeric flexure is designed in rotor systems to compensate for unequal lift between the advancing and retreating rotor blades up to the point of Vne.

Later,
Eddy Thompson
Reply
#8
Thanks James. I appreciate the responses you gave. They were informative and enjoyable to read.

I've got just a touch of reading to do on the whole about helicopters. Ignorance can be treated. Stupidity is something some people are just stuck with. No sticky's for me please.

Thanks
Rick
Reply
#9
You are right ....not all multi bladed systems are elastomeric. The helo's I worked on had two different heads...elastomeric and a wet hinged system. The flight profiles were actually close to the same.....the elastomeric system was smoother and could fly faster.

Thanks for clarifying my statements further. I sometimes forget to put things in plain English.

I wonder how many aircraft have droop stops....and have stops that retract with rpm. I'm sure there are not many. That complicates the head a bunch.

James L

GASMAN - 12/25/2006 8:32 AM

Hi James,
Not all multiblade rotor systems have elastomeric bearings. For example, the Schweizer/Hughes 300 series of helicopters have metal bearings instead of elastomeric bearings in the three blade rotor system. The Astar 350 is also a three blade helicopter, but has a fully elastomeric rotor head system.The physics of the blades and rotor heads is, however, the same. The blades can flap up and down, lead lag forward and backward and change pitch in the feathering axis. In addition, there is flexure in the rotor blades.
Also, I must point out that the larger number of blades in rotorheads does not make them harder to control. For example, the Robinson R22 and R44 helicopters are two blade rotor systems with the unique feature of having a teetered head with individual flaping hinges on each blade. These helicopters are the most difficult to fly compared to other helicopters.
No matter what rotorhead configuration, the flaping up and down,either by teetering, flaping hinges, or elastomeric flexure is designed in rotor systems to compensate for unequal lift between the advancing and retreating rotor blades up to the point of Vne.

Later,
Eddy Thompson
Reply
#10
It really comes down to understanding the system you are flying and staying within the limits. I personally prefer a semi-rigid (Mosquito, Bell two blades) to a fully articulated (Robinson, and many others). If you have a rigid system you can do the following:





The Huey can do these as well -- but if you screw up you are dead. Except for the rigid rotor systems any helicopter can be easily made to self destruct by pilot input. Mast bumping can do it for sure, but so can overflexing the blades and taking out the tail boom. I would happily own an R-22 but would be very concious of it's limits. I have not seen the Mosquito yet, but suspect it would be slightly more tolerant of pilot mishandling than the Robbie.

BTW, droop stops were part of the Sikorsky S-55 and S-61 and I suspect other non-elastomeric fully articulated systems.

ernie
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)