Shape and size of Rotors

BeefBear · 24-01-2006, 07:12 AM

Amongst the many breeds of rotor blade we seem to have a common thread in so far as the follow a consistant chord width. And given that a significant area of the rotor disc either generates zero or even negative lift, why wouldn't a tapered rotor blade work? Also why wouldn't a tapered and twisted rotor bladework even better?

As the velocity of a blade increases as you extend from the hub so to does the lift capacity. We could keep extending the span length so long as the blade contruction can manage the flex created by differeing lift effects, but it seems more sencible to retain a smaller disc but gain more efficiency from the rotor by making the inner section work more for us than against us.

I note that some helicopter blades actually have flight tabs ( trim tabs) to assist the blades stay on a more consistant path, so why not on gyro blades?

And since we are brain storming here, why not some form of blade reflx that comes into play as the rotor rpms are increased?

Ted

BeefBear · 29-01-2006, 05:02 PM

I note that Vortech talk about their VI aluminium extruded blades as being the next best thing to god, yet their article dismisses the concept of blade twist.

The Vortech, Inc. (VI) Rotor Blades, manufactured for both helicopters and gyroplanes, incorporate a major design advance over typical rotor blades: the virtual elimination of parasitic drag—one of the primary challenges in maximizing rotor blade efficiency. Drag is the force that tends to resist a rotor blade's movement through the air, impeding the thrust of the rotor system. Thrust is the force that creates the forward momentum of a rotor system necessary for achieving lift. Lift is the force that counteracts gravity. Parasitic drag in rotor blades is caused by the wind resistance of structures that may be necessary for the assembly of the blades, but do not contribute to—and in fact detract from—lift. Structures that create parasitic drag include rivet heads and the edges of interfacing materials used in blade construction.

Many rotor blades are constructed by riveting an upper and lower skin to a leading-edge extrusion, then riveting the trailing-edge together or to another extrusion (see Fig. 2, below). The rows of rivets and the overlapping blade sections create drag and excessive weight, and are a possible cause of fatigue cracks and blade failures. The VI Rotor Blades elegantly eliminate this drag and excessive weight by creating the entire airfoil (the shape, or curvature of the blade) from a single aluminum extrusion (see Fig. 1). Additional advantages include greatly increased strength, reliability and torsional rigidity (resistance to twisting), yielding far smoother performance and greater freedom from vibration. Although there are numerous ways in which rotor blade manufacturers assemble the materials from which their blades are constructed and thereby attempt to minimize drag and weight, no other rotor blade uses so simple, elegant and efficient a design as the VI Rotor Blades.

THE VORTECH INTERNATIONAL EXTRUDED,
ALL-ALUMINUM ROTOR BLADE SYSTEM
By James Frese, Vortech International, Inc.
After several successful years of marketing the unique untwisted, constant chord series of Vortech International, Inc. (VII) extruded 6063-T6 aluminum rotor blades, it seems an ideal time to summarize our progress to date. With any new product, there are always going to be growing pains, but as I will point out, the successes of a number of projects are very encouraging. The extruded rotor blade system has many advantages: a surface free of imperfections such as rivets or seams, blade lengths easily tailored to the customer's individual requirements, availability in two airfoil selections (symmetrical NACA 0012 and asymmetrical 8H12) and a number of chord sizes ranging from 2 ¾” to 8”, and a low cost yet dependable rotor blade. 6063-T6 aluminum was the material of choice for realizing the best compromise between strength, extrudability, and cost; the thin-skinned cross section of the blade precludes the use of higher strength aluminums, such as 6061-T6, while still keeping the cost within the budget of the average homebuilder. There is, however, no free lunch and the same general philosophy applies to the use of anything as important as a rotor blade. Care must be used when looking at the finish, blade balance and the strength of the root hub connection. The blades are supplied with a “mill” finish; it is recommended that the blade surface be initially worked with Scotchbrite, or equal, to remove any imperfections and then use standard aluminum polishing techniques to provide a long-lasting, weatherproof finish. Painting or anodizing is not advised. A full length 6061-T6 aluminum nose weight (for strengthing and balancing the system) is provided as part of the blade kit (along with plastic tip and root caps and grip plates). Balancing each blade both chordwise and spanwise is VERY important. Spanwise balance must be done for each blade set or inplane vibration will result, its severity depending on the rotor RPM and degree of imbalance.
Chordwise imbalances on any rotor blade will result in higher collective and cyclic loads for helicopters and higher cyclic loads for gyros. It may also result in rotor blade flutter, this condition is particularly noticeable with helicopters at high speed. Gyros are not as susceptible because of their usually lower airspeeds, lower disk loading and fixed collective pitch resulting in high torsional blade stiffness. Blade attachment to the hub is another very important item. This is best accomplished by looking at what others have successfully done or contacting someone with experience in the design of rotor hubs. The VII blades have been successfully used in a number of flight projects that range from gyros to flying platforms. Ed Alderfer flies his Air Command gyro with a set of asymmetric airfoil 8” blades with great success. He has written an article, “Evaluation Report: The New All-Aluminum Extruded Rotor” (Rotorcraft, May 1997), that is recommended reading for those using the VII blade for gyros. In Australia, Ric Hutchins of Ric's Rotors has made a very rugged and good flying rotor system with the 8” VII blade (Rotorcraft, October 1997). Ben Showers flies a symmetrical VII rotor blade on his Skytwister (Rotorcraft, October 1995) helicopter while the Eagle-Perch counter-rotating helicopters use the 4 3/4” chord asymmetric airfoil. The PAM Group also uses the counterrotating rotor system but with the large 8” chord symmetric airfoil. These blades turn at approximately 1250 RPM and provide a combined thrust of approximately 1000 pounds.
The VII rotor blade system is accumulating a very positive record of testing and analysis. Through experimental testing the torsional twisting coefficients have been determined as well as the area moments of inertia which have been experimentally verified. These are important when determining the strength, bending moments, and dynamic characteristics of the blade. The PAM Group has also tensile-tested their rotor system (including an 8” symmetrical section of the VII blade) up to 35,000 pounds with no adverse effects to the blade section. In addition to these success stories, several fatigue analyses have been made that show, under normal conditions, a blade life in excess of a thousand hours depending on the particular application.

BeefBear · 02-02-2006, 06:29 PM

Rotor Blade manufacturers are faced with many different questions when designing the "perfect" blade. And again I pose the question why are we moving towards longer blades when it increases the inner losss of potential. Why are we looking at creating a blade with a decreased negative lift imput value? Obviousily having longer blades increases any imbalance potential.

Has anyone ever thought about exploring the possibility of having a similar mechanism of the constant speed prop option on propellers, but applied to rotors to effect the angle of attack.

What about the potential of having a rotor blade which has a more pronounced thicknnes of depth and span of chord as an inner section ( Not just a slight taper) but more like the significant variance of a normal aircraft prop? Those manufacturers have definately worked on reducing the non lift component of their props.

Ted

Gyrodes · 03-02-2006, 12:07 AM

Ted gyro rotors are not driven they auto rotate! Have you read the books by Paul Bergan-abott on the gyro plane? If not I would suggest that you do as they will give you some understanding of what you are discussing here. Just a thought its up to you how you view this post. All the best Des

BeefBear · 03-02-2006, 05:28 PM

Des, thank you for your reply. Yes I'm very much aware that gyro rotors autorotate. What concerns me is that we seem to be chasing longer rotor lengths in an effor to improve our lift capabilities but in actual fact we seem to be ignoring the inner sector which normally delivers zero or negative lift functionality. Currently we predominately use the one simple rotor shape and yet we expect this to cover all our bases.

Des what is stoppig us from using the same dimentions as the 8H12 shape but for the inner section scale it upwards by .5 or even by 1. It seems to me that we may possibly be able to lengthen (and thicken) the chord to greater than 8 inches for the inner 1/3 of the rotor to create an greater deflection surface and also a greater increase in the upper air path to increase its pressure differential. The slower speed of the rotor at its centre to 1/3 inner sections needs to be altered some way so that we can increase the potential. Surely both factors should increase the lift potential?

Ted

russ · 03-02-2006, 07:24 PM

Ted............I'm of the view that you are getting frustrated with lack of responce to questions that you have frequently posed to us all, both here and other forums.

Your quest for answers is to be applauded, but we fly the damn things, and the real tecko side of rotors etc etc, we don't know...........it's that simple

There is only a handfull of blade makers worldwide, and with speaking one on one with a few of them over these many years..............they too are scratching their heads.

It appears that the slightest variance from the current tried and proven, has reactions that are not favourable, in one way or another. The costs incurred by some of those blade makers doing "R & D" just does not stack up with the market demands. When you can buy blades for less than 3 grand a set, coze thats the market price, wheres the profit to then reinvest into research etc.
The demand for blades is VERY small, and flyers in general can't afford big bucks. And big bucks would be demanded by makers to achieve a fair return to them.

Some years back I tested a set of blades that were out of the "normal" format, they had UNBELIEVABLE lift, flew as fast as with less throttle,........BUT..........shook like hell, the maker spent countless hours and his own money to sort these..............but gave up because he could'nt keep financing the research.

We have here in Oz, a blade maker that is looking outside the square, his recent findings are favourable, but he too scatches his head.

These things above our heads, whizing around, are not as straight forward as we all think. They are happy in their envelope, change their envelope and you introduce MANY unknowns.

Every action has a REACTION.............you .....we.......know that

If gyros were a big buck return, then some genuine, well financed research would be occuring, and all your questions would have answers.

The entire gyro community WORLD WIDE is seeking answers to exactly what you poze to us.

BeefBear · 03-02-2006, 09:08 PM

Yes russell, we owe a lot to those who try to solve this riddle. These days It should be possible to simulate rotor design and all its complexities by the use of software. ONe of the reasons I'm not keen on the longer rotor design is simply that as the blades get longer we tend to introduce abnormal behaviours into the action and reaction of the blades. Dynamic stabilty becomes uncomfortable and even uncontrollable.

It would seem that the closer we try to retain the rotor energy around its cente of momentum, then its behavour becomes more forgiving. Somewhere the other day I watched a video of a long rotor blades and the whip and shake within this blade was alarming.

Ted

BeefBear · 05-02-2006, 02:35 PM

Pete Barsden has very kindly offered to share some material with me, and I'd like to place on public record my thanks for his doing so. Does anyone have any info regarding software which is used to analysise rotor performance? Not looking for one that requires a uni degree to use.

Ted