peterw

I'll put in what I have found in regards to hub bars and heavy blades. This is what was happening to the rotors I used to fly on. It may also apply to other heavy rotors that do not use tip weights. The rotors I was using were a 8 - H- 12 profile extusion with no tip weights. To run at the required rpm for the centrifugal force to match the lift and hold the cone angle at 2 degrees the blade pitch had to be set at 3/4 to 1 degree.
As the blade pitch is increased the rotor rpm and centrifugal force goes down although the lift has stayed the same. This changes the ratio of centrifugal force to lift and the coning angle will increase. My hub bars are one inch thick and have plates added each end with a 2 degree coning angle milled into these plates.When drawing centrelines from halfway between these plates to the centre of the hub bar the lines cross below the teeter point slightly up from the bottom of the hub bar. This would mean that if the centrifugal force to lift ratio is holding the blades at 2 degrees they will be putting mainly a tension force on the hub bar similar to pulling on a rope, also with a very small bending force trying to bend the hub bar around the teeter block. If more coning angle is added, to say 4 degrees the lines will intersect about 7mm below the hub bar and increase the bending force.This is if the end plates are milled at 4 degrees. If the end plates are still at 2 degrees the intersection of the lines will be a lot lower.
The length of the hub bar between the end plates is 360mm. The cross section is 2 1/2 inches wide x 1 inch thick. Because of the length to thickness ratio there can be very little deflection before the metal is stressed and is therefore fairly rigid. The section where the steel plates connect the hub bars to the rotors is very rigid so it is the rotors that have to flex to get the 4 degree cone angle. Lines running down the blades so there is only tension and no bending forces will intersect well below the hub bar. There is now a very large increase in the bending force. This is strongest at the centre under the teeter block. When metal bends there is very little compression of the metal on the inside of the bend. The metal on the outside stretches. If the bend is within the elastic properties of the metal it will return to shape. With constant bending it will lose this elasticity and work harden. The closer the bends are to the limit of its elasricity the less bending and straightening cycles are required to work harden the material. If the bar is uniform width and thickness the bending and stretching will be strongest under the teeter block. this will taper away to either side. If there is a section of bar with less material such as at the teeter block holes this metal will have to stretch further to hold the same tension as the rest of the bar. This metal will work harden a lot quicker than the material around it.
To get a coning angle of one degree the centrifugaal force near the tip of each blade needs to be 57 times the weight of the aircraft.
This is a ratio of 57 - 1
two degree requires 28 -1
three degree requires 19 -1
Due to there being less difference in the ratios as the coning angle rises, a rotor with a coning angle of 4 degrees when coming under load will increase the coning angle more dramatically than a rotor set at 2 degrees under the same load. This means that the hub bar already subjected to high bending forces then has to handle severe bending cycles. This hub bar will fatigue and crack at very low hours. It is not if but when it will break.
I have 3 sets of rotors. The first is set at 1 degree pitch and cone at 2 degrees. The second is 1.25 degreepitch and cone at about 2.5 degrees. The third is 1.5 degree pitch and cones at 3.5 degrees. All are made from the same materials and profile. They are all machined for 2 degree cone.
From experience the 8 - H -12 profile will use a lot less horspower to fly as the pitch angle is raised from one degree. According to a lift/drag chart that I have seen this profilehas the best L/D ratio at 3 -4 degrees. This will make it tempting to ask the manufacturer to put more pitch on. To do this without a hub bar designed to handle the higher coning angles or tip weights to keep the angle down is very dangerous.
Although I was told there would be more stick shake I was not warned about the extra stress on the hub bar. It was 2003 that I bought the last set of blades at 1 1/2 degrees. I did not fly them much because of the stick shake. When recently researching trhe forces acting on the rotors so I could sort out the shake I noticed that extra stress would be put on the hub bar by the extra coning angle. The main cause of the shake was that the teeter point was too low for the high cone angle.
The ratios of weight to centrifugal force may not be exact but I think they will be close. I think the overall theory is correct so I have put it to the test by placing it in the forum for comment.

Gyrodes

Hi Peterw,
Just to put a little more info with the above, what is the AUW of the machine in flight? This is so that the numbers can mean something to some of us.
Very informitave post, Makes one realise the stresses we are dealing with.
Thanks Des Garvin

__________________
What you focus on grows. Des Garvin

peterw

AUW is 320 kgs but not including the weight of the rotors.
320 kgs is the weight hanging off the hub bar.
The main point of this theory is that a slight change in rotor pitch will make a big change to the forces and stresses acting on the hub bar.