adjustable prop bearing

[Joe]

For all you prop gurus out there...

A ran across this drawing and something struck me as odd... why is the
prop blade only being held in with one bearing? Wouldn't it want to
rotate about this bearing slightly and put a lot of force on the pitch
change mechanism?

Or in other words, why isn't the blade held rigidly?

http://www.mt-propeller.com/pdf/datsheet/mtv-23.pdf


Joe

[Rip]

The big, single bearing allows the blade to rotate in pitch while
keeping centripetal acceleration from slinging the blade across the
airport. The force on a a prop blade this size is, very roughly, about
20 tons.
On the other hand, the force required to change the pitch of the blade,
at least on a comparable Hartzell prop, is about 20 pounds.

Joe wrote:
> For all you prop gurus out there...
>
> A ran across this drawing and something struck me as odd... why is the
> prop blade only being held in with one bearing? Wouldn't it want to
> rotate about this bearing slightly and put a lot of force on the pitch
> change mechanism?
>
> Or in other words, why isn't the blade held rigidly?
>
> http://www.mt-propeller.com/pdf/datsheet/mtv-23.pdf
>
>
> Joe

[Jim Stockton]

Have a look at:http://www.mt-propeller.com/pdf/manuals/e-118.pdf
which has better illustrations. The bearing itself is a thrust bearing with
a split inner race to prevent blade loss. The majority of forces on the
bearing would be outward. The pitch mechanism appears to be a lead screw
moving an inner sleeve to change the pitch. In the manual it says that it
takes about 1 minute to go from feathered to flat pitch implying a large
number of turns of the motor. That would pretty well secure the blade from
rotating about it's axis (pitch wise) because of the locking action of the
thread.
My 0.02 anyway.
Jim Stockton
"Joe" > wrote in message
om...
> For all you prop gurus out there...
>
> A ran across this drawing and something struck me as odd... why is the
> prop blade only being held in with one bearing? Wouldn't it want to
> rotate about this bearing slightly and put a lot of force on the pitch
> change mechanism?
>
> Or in other words, why isn't the blade held rigidly?
>
> http://www.mt-propeller.com/pdf/datsheet/mtv-23.pdf
>
>
> Joe

[Joe]

I guess what I don't get is what takes the loads that go forward and
backward (thrust) and also side to side (torsion). Does the blade
"pivot" off of the big bearing (and hence put big loads on the pitch
change mechanism)? IOW why isn't a support further out on the hub
needed.

It would seem like you could grab the end of the blade and just push
it around and the relatively sharp corner on the hub between numbers
25 and 26/27 (looks like an o-ring and the spacer/pre-load shim and
snap ring) would just gouge into the hub sleeve.

Thanks for the comments. It's kind of fun figuring out this
mechanism.

Joe

[Morgans]

"Joe" > wrote in message
om...
> I guess what I don't get is what takes the loads that go forward and
> backward (thrust) and also side to side (torsion). Does the blade
> "pivot" off of the big bearing (and hence put big loads on the pitch
> change mechanism)? IOW why isn't a support further out on the hub
> needed.
>
> It would seem like you could grab the end of the blade and just push
> it around and the relatively sharp corner on the hub between numbers
> 25 and 26/27 (looks like an o-ring and the spacer/pre-load shim and
> snap ring) would just gouge into the hub sleeve.
>
> Thanks for the comments. It's kind of fun figuring out this
> mechanism.
>
> Joe

The whole key is the fact that the forces you mentioned are insignificant,
compared to the force pulling outwards on the spinning blade.

Lets say that your engine can produce 600 foot pounds of force. Not real
numbers, but nice and round, and not off by one magnitude. <g>

Now divide that number by three, for each blade. Now you have each blade
soaking up 200 foot pounds of torque.

Let's say your prop has a 3 foot radius, and the center of pressure is two
feet out from the center.That means you are pushing with 100 pounds of force
two feet out. If the bearing is 4 inches across, that gives a fulcrum of .3
feet, compared to 2 feet, so a mechanical advantage of 6 to 1. That means
you are trying to lift the one side of the bearing with a force of 600
pounds.

Compared to the outward force of 20 tons (if you believe that is about
right) pulling out on the blade, your 600 pounds of lift, or rocking of the
bearing, is not going to do *very* much. <g>

Disclaimer: These are cocktails napkin figures, only. If anyone would like
to refine them, knock yourself out! :-)
--
Jim in NC


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[smjmitchell]

Jim is right .... the centrifugal forces dominate.

Consider a helicopter with an articulated head. There is a pin at the root
about which the blade rotates in a flapping motion. The centrifugal force
holds the blade flat and relieves the bending loads in the root area. If the
centrifugal force wasn't there the blades would simply fold up.

Many props with adjustable pitch have similar bearing arrangements.





"Jim Stockton" > wrote in message
...
> Have a look at:http://www.mt-propeller.com/pdf/manuals/e-118.pdf
> which has better illustrations. The bearing itself is a thrust bearing
with
> a split inner race to prevent blade loss. The majority of forces on the
> bearing would be outward. The pitch mechanism appears to be a lead screw
> moving an inner sleeve to change the pitch. In the manual it says that it
> takes about 1 minute to go from feathered to flat pitch implying a large
> number of turns of the motor. That would pretty well secure the blade from
> rotating about it's axis (pitch wise) because of the locking action of the
> thread.
> My 0.02 anyway.
> Jim Stockton
> "Joe" > wrote in message
> om...
> > For all you prop gurus out there...
> >
> > A ran across this drawing and something struck me as odd... why is the
> > prop blade only being held in with one bearing? Wouldn't it want to
> > rotate about this bearing slightly and put a lot of force on the pitch
> > change mechanism?
> >
> > Or in other words, why isn't the blade held rigidly?
> >
> > http://www.mt-propeller.com/pdf/datsheet/mtv-23.pdf
> >
> >
> > Joe
>
>