Subject: VTOL Simplified hub and blade
From: Rotary Engine
Date: 10/2/2007, 7:31 PM
To: AARotary Engine





Steve, What do you think of this idea.

Since we will not be flying forward in helicopter mode for long or at
high speed
the leading and lagging hinges can be done away with. Same goes for the
flapping
hinges. I think we can use GRP for the hub. Same as several European
helicopters
use. GRP has high tensile strength and a low modulus of elasticity
allowing it to
flex without failing. Same as the Long EZ main landing gear. The
innovative feature
of having a one piece carbon fiber blade with internal outer bearing race
and pitch
change arm is also a major weight and cost saving feature. The bearing
balls are
Torlon which are widely used in off shore sail boat racing. These are
subject
to the worst possible environmental conditions and survive. The
connection
between blade and hub is achieved by the bearing balls and they are fed
into the races by one access hole on each side. We have used this
feature in the past of an aircraft propeller and it works well.
-- 
Paul Lamar ...No rotor no motor.


Go one step further and use a standard prop internal pitch change
mechanism.
Milo

Care to make a 3D dwg for me? I can't see how it would work with counter
rotating
rotors?

Paul Lamar

Forgot about that! Milo

That is why I went with the tried and true helicopter pitch change.

-- 
Paul Lamar ...No rotor no motor.



I think it would start shaking pretty good as soon as you started getting
some translational lift.  Did you look at that website I sent?
http://www.rotor-international.net/

It uses co-axial two-blade rotors and they appear to be underslung with
short beams that twist for feathering, flex for coning and may be servo-tab
controlled.  Look at how far apart the rotors are.  I think if you want the
flexure to to accomplish lead and lag, the flexible portion should be
outside the feathering bearings.  The stiffer the system is, the more moment
applied to the hub and the more vibration.

I think on the model you've shown, with one bearing race, when a moment is
applied to the assembly the loads on the bearings could get pretty high.  A
second bearing race inboard would help, especially if it were an angular
contact type that could take high thrust loads and it was located far enough
inboard to resist to the applied moment.  I'm not sure about those Torlon
bearings being strong enough, but with two races, they might be and you
would avoid the shear load that would occur with your model and the loads
would be all compressive.  The slight elasticity of the Torlon may help
distribute the load which would be a good thing as well.

Steve

I don't think so. We don't do much translational lift. It is an arcing
takeoff with
considerable vertical velocity for only a second or two.

I looked at that web site but  I saw no details of the hubs. If you see
something
there in detail please do a screen capture and email that.

All constant speed props are built with only one bearing. The negative
bending
moment on the blade hinge due to centripetal force acting on the blade CG
counteracts
the positive bending moment due to thrust. Therefore it only needs one
bearing.

If the constant cord blade weighs 20 pounds
and it moves up one foot at it's CG radius position that is a negative
bending moment
of 100,000 foot pounds. The centripetal force on the 20 pound blade is
100,000 pounds.
Weight/32.2 x FPS^2.  The positive bending moment due to thrust on the blade
is only
6.5 ft x 300 pounds (each of four blades will need to generate 300 pounds to
lift the
1200 pound craft.) or about 2000 foot pounds.

Obviously the rotor blade CG is not going to move up one foot. The upwards
deflection of the blade at its CG position will only be about .2 feet or 2.4
inches
balancing the thrust moment. That puts a negligible bending load on the
bearing compared
to the centripetal load. All props and rotors work this way. If not rotors
and blades
would break  off at the hub because they do not have the bending strength on
their own
to support the weight of the helicopter.

The bearing consists of 22 half inch dia. balls. In effect they act like 22
1/2 inch
dia. bolts in single shear. The centripetal load is 100,000 pounds so each
half inch
bolt only needs to accept 4,500 pounds of shear roughly. A 1/2 inch bolt has
an area
of about .2 square inches. So the shear stress on the bolt will be 22,500
psi
roughly. A piece of cake for a steel ball but maybe marginal for a Torlon
ball. Consequently I will try to get the carbon fiber blade weight well
below
20 pounds each.

As you can see the centripetal force is orders of magnitude higher than the
bending loads on the blade hub. It is all about numbers.

Paul Lamar ...No rotor no motor.

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Paul,

I agree with you about that Hartzell prop, but look at the cut-away: the
bearings are loaded in compression.  On the model you posted, the bearings
are loaded in shear.  The rotors on that co-axial helicopter are spaced that
way because they are teetering, semi-rigid type.  You might get away with
fixed propellers as long as you minimize translational lift.  I would try it
with a big model first.  With any extended translational flight, maybe even
high angle of attack, it might start vibrating excessively.  The rotors on
the V-22 are fully articulated rotors, not propellers.  The blades on a
helicopter move a lot more in flap, lead and lag than people think.  On a
two-blade teetering type, they do it all with a flapping motion.

The VTOL prototypes in the fifties, I think, used propellers.  Maybe a good
answer would be to use more power and a bit less diameter on the propellers,
maybe use two three-blade props and make the system stiffer.  This might be
an application for two engines with a common transmission.  Is somebody
going to build this thing?  It's not impossible, Sean Tucker hovers his
airplane all the time and if he had gear, he could land it on its tail.

Steve

We have not made the races  yet so we can load them in compression.
FRP hubs do work as they are used on certain European helicopters.
It is easy to add lead and lag dampers. The problem with
teetering is mast slap.
-- 
Paul Lamar ...No rotor no motor.

The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://www.rotaryeng.net
Copyright 1998-2006 All world wide rights reserved.