Helicopter Rotor head Under sling

DASBUR

Member
Joined
May 11, 2005
Messages
202
Location
Burlington Wi
Hi All

Composite blades are being tested on a Rotorway helicopter. I spoke with the blade designer/manufacturer a Oshkosh. He stated that the blades are overbalanced at the tips. So there is less weight at the root end of the blade. This puts the span CG further towards the tip of the blade. They made an under sling change on the rotor head. Common sense tells me that the under sling should be increased because the blade cg has moved out. They have decreased the amount of under sling.
The under sling change was a calculated change when the blades were installed. They knew it needed to be changed before the testing began. It was not a we have a shake I guess we will need to change the amount of under sling. Again common sense tells me that the under sling should be increased but they decreased it. Never thought to ask them why, it's bugging the hell out of me. Any ideas???

Thank you Doug
 
All things being equal coning angle will decrease as the blades cg moves towards the tip because of the increased centrifugal force trying to pull the blade flat against lift. A smaller coning angle would require a decrease in under sling.
 
Doug,
You probably know this but another variable that affects undersling amount is how a particular blade design is predicted to ACT in flight.
A tip weighted composite blade with more inertia will behave differently in the following ways (by all means not a complete list).
* Will be characteristically stiffer due to inherent elastic properties of carbon being significantly less
* Will be centrifugally stiffer for obvious reasons
* Will have a smaller total flapping range in degrees because more mass equals more rigidity in space
* May be easier to keep track & balance settings more consistent and stable in flight.

All that could require an undersling change. Also, keep in mind...A lot has been learned since the last time the Rotorway was revised. Perhaps the undersling distance could have been better (from a decrease-the-vibrations point of view) than it was and really needed changed. I know when Fetters lessened the undersling distance and pre-cone amount on the Bravo Mini-500, it made rotor vibrations significantly lower.
 
Doug,
You probably know this but another variable that affects undersling amount is how a particular blade design is predicted to ACT in flight.
A tip weighted composite blade with more inertia will behave differently in the following ways (by all means not a complete list).
* Will be characteristically stiffer due to inherent elastic properties of carbon being significantly less
* Will be centrifugally stiffer for obvious reasons
* Will have a smaller total flapping range in degrees because more mass equals more rigidity in space
* May be easier to keep track & balance settings more consistent and stable in flight.

All that could require an undersling change. Also, keep in mind...A lot has been learned since the last time the Rotorway was revised. Perhaps the undersling distance could have been better (from a decrease-the-vibrations point of view) than it was and really needed changed. I know when Fetters lessened the undersling distance and pre-cone amount on the Bravo Mini-500, it made rotor vibrations significantly lower.
I am testing composite blades On my Helicycle The manufacturer did a calculation for A Helicycle and the teeter height change would be needed from 2.250 to1.0 inches. and no change to the cone angle. stock is 1.2 degrees the problem with the composited is they droop more and on a run down when the wind was high the main rotor did ever so slightly contact the tail rotor blade no damage done other than a slight paint chip. there report was in polish I translated over to english there may have been a mention of cone angle change to 1.7degrees I am waiting clairification on that now. Is there a program that you can plug all your specs in and wa la get the cone angle and teeter height. I all ready knew the teeter height was going to be too much the question now is the cone angle seems to me to to low any one know the teeter height and cone angle on a CH7

Doug
 
I am testing composite blades On my Helicycle The manufacturer did a calculation for A Helicycle and the teeter height change would be needed from 2.250 to1.0 inches. and no change to the cone angle. stock is 1.2 degrees the problem with the composited is they droop more and on a run down when the wind was high the main rotor did ever so slightly contact the tail rotor blade no damage done other than a slight paint chip. there report was in polish I translated over to english there may have been a mention of cone angle change to 1.7degrees I am waiting clairification on that now. Is there a program that you can plug all your specs in and wa la get the cone angle and teeter height. I all ready knew the teeter height was going to be too much the question now is the cone angle seems to me to to low any one know the teeter height and cone angle on a CH7

Doug
I can take some measurements for you on the weekend.

wolfy
 
Add mass to the tip flattens the coning and thus reduce the undersling. Nothing more than that.
 
I am guessing you are talking about gyro tech blades?

wolfy
Hi

Yes Gyro tech blades. Once I got the blades I was trying to calculate out the proper coning angle and undersling. the company figures showed a 25mm undersling and no change to the coning angle. I thought that was odd because the helicycle has only 1.2 degrees. There blades supposably flew fine on the ch7 Thats why I was interested in what the ch7 had. With 1.2 the blades can flap onto the tail rotor the 2.0 would help. They are now making up a new set of blades for the new Helicycle factory that the weight distribution through the blade will better match the aluminum Helicycle blades so no rotor head changes should need to be made.

Thanks Doug
 
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