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Rotor-blade CofG

Rotor-blade CofG

Rotor-blade CofG

We are trying to build a rotor blade from an aluminium extrusion which has the cordwise C of G at about 40%.  To achieve the 25% chordwise C of G that we are told is important would require a lot of weight in the nose section.  Can somebody explain why the 25% ballance is so critical and if not so critical what would be the effect of running with 40% ballance.

RE: Rotor-blade CofG

The center of pressure is at 25% mac and any weight behind this area will have a divergent action and will result in flutter ,the blade will osalate violently and be come a useless piece that departs the A/C NOW........
Dinamic ballance is very important....
Hit the books.....

RE: Rotor-blade CofG

Blade pitch / flap divergence and flutter stability is primarily dependent on the distance the center of gravity (c.g.) is aft of the aerodynamic center (a.c.).  The aerodynamic center is at 0.25 c (chord).  The stability boundary also depends on the pitch frequency (or control stiffness).  If the c.g. is ahead of the a.c., the blade is stable.  The c.g. can only be a short (i.e. 3% chord) distance behind the a.c. and remain stable.

Unfortunately, I don't know of a good beginners reference on helicopter stability.  If you can get a copy of Dr. Chopra's class notes, or Wayne Johnson's course notes, that is a good start.  I believe there are three rotorcraft-centers-of-excellence, and at one time they were U of MD, Penn State, and Georgia Tech. They may have short courses on helicopter design and analysis.

I would recommend that you look at the pitch flap stability, flap lag stability, and ground resonance stability before finalizing the design.  Partial list of authors on the topic rotorcraft stability: Wayne Johnson, Inderjit Chopra, Robert Ormison, Dewey Hodges, Fredrick Straub, P. Friedmann, Bill Bousman, Dave Peters, William Weller, Judah Milgram and John Vorwald.

RE: Rotor-blade CofG

optimization of helicopter rotor

RE: Rotor-blade CofG

If you are asking questions like that, Good luck!

RE: Rotor-blade CofG

I would like to get some more info on the extrusion you have. Is it like the VORTECH design? if it is you should be able to weight the leading edge internally with resin, just ensure the resin is thinned and the blade is level in all aspects before filling the chamber at the leading edge. I have made a couple of blades using the NASA foilsimII program and had great results. foilsim is free and available at http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

RE: Rotor-blade CofG

i strongly recommend you better go for FRP.  as there is design flexibility and also strength to weight ratio is very high.  if you are intersted in designing a rotor blades using FRP please contact me at hgp@lm.co.in and my mobile is +91 98441 04135

RE: Rotor-blade CofG

Where 25%C blade C of G would be important:

In a rotorblade, you have 3 factors to consider in torsion....aerodynamic forces, blade weight forces, and the structural resistance to these forces.

Centre of Twist (or sometimes called Centre of Flexure) is the point in a structure at which a coincident force results in zero twist.  For a solid extrusion, like one used by pioneering rotorcraft homebuilders, the Centre of Twist occurs near the C of G of the blade.

Many Naca airfoils have optimal performance with the C of P near 25% of chord.  By CONVENTION, aerodynamic centre is placed at 25% of chord.  Aerodynamic centre merely transforms the CofP value into a moment at 25% Chord.  It is an artificial point.

Now, if you are using a solid extrusion and a conventional airfoil that has low moment coefficients and a low variance in moment coefficients (defined at 25% chord by convention)you probably can get away with the assumption that weight balancing at 25% chord will give you good resistance to aeroelastic effects (flutter, etc).

Where this dogmatic approach can get you in trouble or limit your design capability:

Most rotorblades today are of a composite design.  They have several materials and thin shell membranes.  You have to ask a simple question:  Will the steel spar contribute to the torsional rigidity as much as the E-Glass skin.  How much weight/torsional rigidity does that planned epoxy leading edge filler provide?  To simply balance at 25% chord like a rotorcraft pioneer in the 1950s is dangerous.

You need to account for the weight and aerodynamic moments about the centre of twist.  Keeping these factors to a minimum is your priority.

But, stepping away from the dogmatic 25% chord approach to doing proper aeroelastic analysis also frees up your design. You don't necessarily need a blade where the moments are low at the Aerodynamic Centre.  You can pick an airfoil with a low variance of moments and create your structure at the points where the moments are minimal.

The short answer to your question is that the 25% chord rule is a classic simplification for solid rotorblades.  To use this method for composite blades is potentially hazardous.  Proper Aeroelastic analysis is required.

Now, I'll just stand back and await the firestorm from those with a religous fervour of the 25% chord rule :).  Debate is good.

RE: Rotor-blade CofG

don't forget then about the vibration modes spanwise..you may need to add some weight at a span station for vibration and fatigue purposes.

RE: Rotor-blade CofG

Your question has always interested me also and there seems to be some good answers here. I am unsure about helicopters and gyrocopters but I have been working on a project for years of developing a Rotorkite and the rotors that I make from wood are from 12 inches to 110 inches in length and I do not use any weighting at the front of my blades and therefore the chordwise balance point is nearer the middle of the width. My method of learning has been to try to ignore some of the convetional thinking and try things on my own as first hand experience is a great way to learn. If you follow the Video link at one of web sites you will see my unit in flight. Good luck.


RE: Rotor-blade CofG

It seems to me that the pitch axis location w/respect to both the center of pressure and the chord-wise c.g. is also a consideration.  I'm presently involved in trying to solve a 2/rev vibration in a semi rigid rotor system using composite blades.  As such, this forum is giving me more brain food than any other I've encountered.

RE: Rotor-blade CofG

buni44:  Your post intrigues me.  What are the symptoms that would make you want to add wt at a partcular span station and how would you determine which span sta to add how much wt?  I don't believe that I have ever heard this point discussed before.  
How would you determine the spanwise vibration modes actually occurring?

RE: Rotor-blade CofG

Model Helis generally good at 30%or better
Just add lead in a routed groove near LE and tip
Epoxy in
Spanwise vibes,,,, oh interesting but would only happen on fullsize blades as model stuff is very ridgid for its length
Model control systems are so good now they could tolerate aero,s with solid wood blades , no weights   
Full size is far and away more fussy
And so the egg heads are needed
No offence guys


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