pulling pitch on your rotor blades

Mel that souns like WAY too much fun! We had a Bravo when I worked at Rogers helos and would pull the whole rotor off to tear it down and change seals. I WISH we had a 5 ton wrecker instead of the POS forklift with A frame rig we used. Almost nosed it over moving the rotors and head to the hanger.

I built a H model a few years ago for a customer and pulled in a few inches of collective, but not dramatic.  I had to reposition the collective slide the swashplate and redo the boot,  made new PC links, etc.  IIRC it was an ESCI head so I just cut it all apart at the bearings and mounted it all up on a rod so I could get the blades matched. I did not induce any blade twist, tho. A fair bit of work but visualy interesting and the customer really loved it. Come to think of it, that was my final commission job.

When our seals started leaking to where we had to use a bucket, they were changed out!  Actually did this on my bird during "Summer Camp" up at Ft. Chaffee, AR one year.  Two mechanics and a test pilot, couple of blade stands, M/R stand, a 5 Ton Wrecker and one afternoon of hard work.  One of my "stick time" test flights too!  I guess the "work aid" you mentioned was for when you pulled the grips to change them.

While doing some research on rotor heads I did come up with a picture of the greese mod head on the internet. It also showed the greese fittings in the grip, if you do find it look near the outboard end of the gip near the middle.  Now where is the link Maybe I can find it tomorrow at work.

Just a bit of trivia on the greese mod. Actually the Canadian helicopter operators in the mid 70's came up with putting greese in the grips, and got MOT to approve it. Bell thought the cold weather would congeal the greese too much and create high pitch change forces so they never pursued the modification but tried finding a better seal.

  If you find a copy of an air force Technical Order (T.O) you will see a work aid, two plastic buckets suspended by safety wire just below the inboard side of the grip. This was to catch the oil that leaked from the inboard grip seal.

Check out the 1/35 Panda thread.  There is a drawing posted of a B rotor head and the parts in question are 43 then 47 through 51. 

 

oh man....yuu left me in the dust with your description....i'll look into what you've said...but it flew over my head ....lol

oh man!!! spinning spinning!!!  full pedal FULL PEDAL!!  (drank too many Martoonis)

 

 

 

 

 

 

 Glad to help.  Although my huey memory is a bit rusty I can still recall a few details.  CC is right about the PC links.  As for the Oil resevoirs, the blade grips went from being oil filled to grease filled in 1985.  Used to be a clear resevoir on top of the mast end of the blade grips.  A bolt that went through the center of it was the minimum oil level line. When the modification was done the clear resevoir was removed and a cover plate put in its place.  The mounting area for it was not removed as this would require major reworking of the blade grip.  Look for a circular appendage on the top of the grips with the flat face toward the blade and you have it.  If you want to model a vietnam era bird you need to add a clear disk to the resevoir area with a solid flat end.  Visualize a cylinder with clear sides and solid ends.  The clear area on the real thing is only about 1 inch wide so use your best judgement when making the modification.  I'll have to dig for my detail photos but if I can find them I'll e-mail them to you.  Hope this helps you.

no...not bored...

I'm thrilled to find some eggbeater builders with some serious dedimucation!  :P

 

I will be adjusting the control arms that extend from the blades....but the other stuff I am clueless on....the Panda rotor assembly has some issues already I read from the Cobra Companies instructions...i.e:

"the pitch change control horns molded onto the main rotor hub should stand away from the hub and the pitch change links are not provided for the kit....after 1985 the oil fill resevoir was removed from the inboard end of the grip.."

 

these items will take a bit of research on my part and then I'll see what I can do...got any ideas or pics?

According to the information and knowledge I have I would have to say yes.  I have experience on the UH-1, UH-60, OH-58 and MD-500 series aircraft and all of them have a blade twist designed in.  To the best of my knowledge it is a design necessity.  The result of an untwisted blade is a MUCH higher lift difference at the blade tip than at the root.  Speed at any point on the blade varies with the radius or distance the blade is from the center of the main rotor shaft (mast).  The result is an extreme speed differential between the blade tip and root.  However, the lift differential between the blade tip and root is even greater because lift varies with the square of speed.  When speed doubles, lift increases four times.  If the blade were not twisted to decrease the lift toward the tip end of the blade the coning of the blades would be extremely high when weight is applied to the system.  The structure of the blades would have to be so strong (translated to so heavy) to prevent structural failure, they would be inefficient to use.

I like your idea about the razor saw.  I'll try using it on a UH-1H diorama I'm working on.  Some of the kits in my collection have the twist molded in or are molded in such a way that when the blades are mounted you can build in any pitch setting you want.  If you really want total accuracy make sure you adjust the positions of the swashplates and pitch change links as well depending on the aircraft you are building.  

Hope I didn't bore you with part 2 of the daily aerodynamics lesson.

 Keep it fun!   

for all rotor systems??

well....on one build I did ...I heated the rotor hub until I could add pitch to the blades....of course the hub did not look so good twisted...but the blades looked cool... 

I'm contemplating using a razor saw this time at a more feasible spot between the blades and the main shaft....

Charlie31 wrote:

Looking at a few pictures of Hueys, I can see something like you describe in some of the pictures.  The rotor root has more noticable pitch than the tip.  But the pictures I can see it in are all A or B models, and can't see it in later model photos.  It seems like the early models might have had some twist in the rotor, giving it that appearance.  IIRC, having a higher AoA on root than the tip delays retreating rotor stall at the cost of autorotation performance.  Just a thought.

 

I saw this one and thought I would try to assist with our aerodynamics lesson for the day.  According to U.S. Army FM 1-203 the "blade twist" is designed into the blades by manufacturers to distribute the lift more evenly along the rotor blade.  Because of the potential lift differential along the blade resulting primarily from speed variations, blades are designed with a twist.  Blade twist provides a higher pitch angle at the root where speed is lower and a lower pitch angle nearer the tip where speed is higher.  The twist distributes the lift more evenly along the blade and increases both the induced-air velocity and blade loading near the inboard section of the blade.  Thus, the twisted blade generates more lift near the root and less lift at the tip than the untwisted blade.  Blade twist also gives a more positive angle of attack in the driving region (middle portion) than in the driven region (outer portion) during autorotation. 

And now you know the rest of the story.  Clear as mud now, right?     

All the Hueys have a Collective Lever Friction Lock which is the large knurled ring just above the throttle twist grip on the right pilots collective.  It was primarily used to set friction on the collective during flight to be sure it stayed in position if you removed your hand to set switches, scratch your nose, etc.  It was sometimes used to keep it down during ground run ups.  There was a small tit on the right collective and a small yellow "T" handle that was attached to a steel cable would fit over to "lock" it down when on the ground and not running.  An unrestrain main rotor blade and improperly locked down collective has been the demise of many helicopters when an unexpected wind came up and got the rotors "windmilling". 

Looking at a few pictures of Hueys, I can see something like you describe in some of the pictures.  The rotor root has more noticable pitch than the tip.  But the pictures I can see it in are all A or B models, and can't see it in later model photos.  It seems like the early models might have had some twist in the rotor, giving it that appearance.  IIRC, having a higher AoA on root than the tip delays retreating rotor stall at the cost of autorotation performance.  Just a thought.

We were NOY suppose to tie down the rotors on a Huey with any pitch left on them, especially on the C/M models, wind could build up pressure in the hyudraualic system and if enough was put into the system, it go damage the seals, etc. Collective was to be all the way down(which would put it out of the way)