Helicopter Rotor head Under sling

Joined
May 11, 2005
Messages
149
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
 

Alan_Cheatham

Member
Joined
Nov 14, 2003
Messages
1,972
Location
Dallas, Texas
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.
 

bryancobb

Junior Member
Joined
Aug 2, 2010
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5,029
Location
Cartersville, GA
Aircraft
Owned Brantly B-2b/Fly Kitfox III/Mini-500b
Total Flight Time
1350
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.
 
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