Question for Chuck B or whoever

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May 11, 2005
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Burlington Wi
HI Chuck and the Guys

I am mounting up a set of NACA 23012 blades for a helicopter. Since they are asymetrical the chord line is not in the center of the blade like the symetrical blades are. it is lower. should the feathering axis be in the center of the blade or should the feathering axis be in the chord line of the blade.or does it even make a difference.

Doug S
 

C. Beaty

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Doug, the dot just below the mean line at ~25% of chord is the location of the aerodynamic center of the 23012 and ought to be the CG of the blade section. But nothing is so critical that it must be exact.

The most appropriate way of mounting the blade is to locate the feathering axis on the airfoil center at ~25% of chord.
 

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Burlington Wi
another one for chuck

another one for chuck

Hi Chuck/Guys

We know that a rotor blade needs a chordwise balance of 25% to be aerodynamicly stable. now we add on doublers a drag hinge and brackets now the rotorblade is now trailing edge heavy.But it is trailing edge heavy at the root end of the blade.We know that on production helicopter blades they shift the tip weights fore or aft to track the blades. aft makes the blade climb. fore makes the blade dive. On a Bell 206 helicopter they also use A weight on the trailing edge of the blade, but it is at the root.If I understand this correctly
is also used for blade tracking.Going back to all the extra weight we have added to the trailing edge of the blade, is this extra weight we added to the root end of the blade going to affect the tracking of the blade.With all the extra weight just added to the root trailing edge of the blade going to have any effect on the tracking of the blades. we also know that a blade with more than A 25% chord balance can become unstable and flutter. With the extra weight we added to the root of the blade, our blade now has a balance point of approxamatly 35% Even tho the bare rotorblade without any attachments still has a balance point of 25%.could this now 35% balance point at the root end of the blace cause any problems such as blade flutter

Doug
 

brett s

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Weights aren't used for blade tracking on any helicopters I've ever flown or maintained - pitch change links and/or blade trim tabs are adjusted, depending on the exact issue.

Blade tip weights are normally used for balance adjustments, and most are pretty close to that 25% chord point (and way too light to affect the chordwise balance much anyways). Weights at the blade grip end are used to balance out collective forces (usually mounted on a small arm of some sort) or sometimes to balance the rotor head assembly - depends on the type, but not tracking.

Some designs use heavy fixed blade tips weights to add inertia, adjust the chordwise balance, or both.

Getting back to your question, tail heavy blades can cause all sorts of bad things to happen - the less torsionally stiff they are, the worse it'll be.
 

magilla

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Track AND Balance

Track AND Balance

Weights at root are used to balance the rotor system;

Weights on the blade end are used primarily to balance the blades (static balance)

Trim tab and PC link adjustments used to adjust blade track, depending on how the whole airfoil flies.

Some blades don't fly straight - they curve up or down, depending on trim tabs. I've seen graphs showing blades that make a U shape or inverted U depending upon the trim tabs and strength of the main spar.

You adjust the PC rods, and the whole curve moves up or down toward the mean, and you adjust the trim tabs to get the blade to fly straighter.

Sometimes you have to accept vibes at one airspeed to smooth out at another airspeed.

The point is, with any change to the flight of the blade, you will affect the centrifugal arm; The arm gets shorter the further you move away from the center, which then affects the balance of the rotor system.

The key being that blade track affects balance, but balance doesn't affect track. Weights added at the tip that make the blade fly higher or lower will affect track AND balance; Weights added at the root will only affect balance.

In the helo, blades that are out of balance will give you a side to side 2-per-rev vibration, and those that are out of track will give you a up and down 2-per rev vibe. Thos out of track and balance will give you both (rough ride and stick shake!!!)

Anyway, hope that helps.

Just think of weights added to the end of your ceiling fan versus the inside edge.
 
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