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Airfoil-missing losses
By amiklic1, Section Wind Posted on Tue Sep 27, 2005 at 12:07:23 PM MST
What would be the loss if no airfoil is used in blade production

I plan to make some aluminium blades for Hug's 4-foot wind genny. I'll use 3mm aluminium marine-grade sheet, just cutted and the curved as the front side of original wooden airfoil-shaped blade is.
Does anyone know what approx. losses can I except??. The side that wind hits to would be just as the original one, but the back side of the blade has no airfoil-shape.
I plan to have it laser-cutted, maybe put some profile on the back side to strenghten it, but just if I see I need that. Is 3mm too thin for that????

Airfoil-missing losses | 15 comments (15 topical)

Re: Airfoil-missing losses (3.00 / 0) (#1)
by Norm on Tue Sep 27, 2005 at 06:35:24 AM MST

As I see it ....that would be like the plastic
pipe blades...except aluminum would suffer from
metal fatigue sooner than some types of plastic.
( :>) Norm.
( :>) Norm

Sheet Aluminum Blades = Scary + other stuff (3.00 / 0) (#2)
by richhagen on Tue Sep 27, 2005 at 08:12:34 AM MST

I wouldn't do that. I don't think the thin sheet of aluminum will resist flexing enough to prevent metal fatigue. The thicker, the better to resist flexing, but I do not think that aluminum is the optimal material for making a whole blade out of. Not much elastic deformation in aluminum. I picture metal flying off at high speeds soon. It seems more dangerous to me.

If you use no air foil, you will only get drag forces from the air pushing the blades around. This will limit your blade speed to the speed of the air or less. It is the lift forces that allow for the higher rotation speeds relative to the air which an axial alternator of that design would need to produce any useful power in low winds.

Now with your curved blades you actually would have an airfoil, however with a profile similar to a NACA 4415 profile similar to what is usually made for these, the flat side would be facing the wind (upwind) and the curved side would be on the down wind side. I am not sure how to calculate your efficiency, it would depend upon the specific air foil and how it is matched to your alternator. By knowing the wind speed, the swept area, and the power output you can calculate the overall system efficiency. It would still vary with the wind speed as well. The Betz limit is the goal.

Most important is to have fun with the project. If you do that and produce something that's very useful and you're way ahead.
Rich
'A Joule saved is a Joule made'

Re: Sheet Aluminum Blades = Scary + other stuff (3.00 / 0) (#3)
by Ungrounded Lightning Rod on Tue Sep 27, 2005 at 10:27:34 AM MST

Not much elastic deformation in aluminum. I picture metal flying off at high speeds soon.

Ditto. (Boats don't flex much, so you can get away with aluminum in some places and have it last. Blades are another matter entirely.)

Not quite. The angle of attack can be chosen to get nearly any tip speed ratio you want. You can airscrew at several times the wind speed no problem. What the lift forces give you is TORQUE. Without the lift increasing your TSR loses you torque faster than it gains you speed, giving you a net loss of power once your TSR gets close to or higher than the wind speed. (So the conclusion is right even if the reason is a near miss.)

[ Parent ]

Re: Sheet Aluminum Blades = Scary + other stuff (3.00 / 0) (#5)
by Dreadstar on Sat Oct 01, 2005 at 11:34:33 PM MST

hmm seems to me they make airplane propellers out of aluminum.

Modern propellers are fabricated from high-strength , heat-treated,aluminum alloy by forging a single bar of aluminum alloy to the required shape.

Maybe you can find an aircraft propeller in the right size and trs if you wish to go with metal. Trying it from thin strips would not be a good idea.
Take out the (at) and substitute @ for replies via email
[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#4)
by Chagrin on Wed Sep 28, 2005 at 08:49:20 AM MST

www.tlgwindpower.com makes aluminum blades. Their 5' diameter rotor uses 1/8" aluminum sheet.

Re: Airfoil-missing losses (3.00 / 0) (#6)
by IntegEner on Thu Oct 13, 2005 at 08:18:15 AM MST

Laser cut? 3mm too thin? Someone here has some ability with aluminum and wants to use it. First, about profiles. NACA 4415 can be found on pages 490-491 of Abbot and von Doenhoff "Theory of Wing Sections" (as well as most other profiles), something that can readily be ordered from www.amazon.com or even found just by a Google lookup. How can anyone get very far without the books. Second, there is no mystery to how driving forces (sometimes labeled "lift" forces) are generated in blades - they "toss air around", that is, deflect it and since air has mass (more than that for which it is often given credit) this process generates a reaction force in the blade. Blade thickness is not required to do this and it can even be a hindrance. Boat sails are a good example and, yes, they work on the lift principle as well. So thin sheet aluminum, being sail-like, could work just fine. Third, watch the leading edge. It must be round and not sharp in order to prevent a horrible flow separation at anything other than perfect alignment with the incident airflow. Fourth, watch the lateral or "bend back" force on the blade. It can be very strong and the faster the blade rotates the greater this blade-destroying force can be. More can be said, including all the fatigue-related issues. If the writer of this note can actually bend aluminum this thick go to it, remembering to fold back the leading edge. See the website www.integener.com for some information on small wind rotators (unpowered at the moment) that use thin sheet aluminum for their blades. I am also fascinated by the reference someone gave here of www.tlgwindpower.com . The entire question of what material is best for small wind turbine blades, especially as DIY projects, needs some good shaking out.

Re: Airfoil-missing losses (3.00 / 0) (#11)
by finnsawyer on Mon Nov 28, 2005 at 09:39:46 AM MST

You seem to be contradicting yourself. On the one hand you seem to be saying the leading edge of the blade must be rounded to avoid separation of the air flow, and on the other that all that is needed is for the blade to deflect the air like a ball bouncing off of a wall. In fact, the air foil doesn't care what the air is doing. It responds to forces on it, in this case a difference in pressure between the top and bottom surfaces. This is true for a wing or for a sail. The air foil shape is chosen because it gives the greatest lift for the least drag. A board can exhibit lift, but has sizable drag. It doesn't matter whether one views the process as being kinetic based or lift based. They are the same, and I recently gave an explanation why? I won't repeat that here. You can look it up. If you have some issues with my explanation, let me know. From the stand point of anyone trying to manufacture efficient blades, I believe using the lift and drag coefficients for a given airfoil will yield the best results. I wish we could put the myth that a kinetic approach and a lift approach are fundamentally different to rest.
GeoM
[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#12)
by IntegEner on Tue Nov 29, 2005 at 09:07:07 AM MST

It is as plain as day. The blades must "shovel" the wind from one vector direction to another in producing a driving force from the wind, nothing more, nothing less. Blade thickness is not directly necessary to do this. Profiles such as the one suggested here by this individual in Europe are excellent for this purpose but are limited to a narrow range of TSR, outside of which the sharp leading edge will produce separated flow unless rounded.

Checking your posts and your files and diaries provides the insight that you have leaned more toward the electrical side rather than the windflow side of the discussions. I have leaned the other way toward the wind as it impacts the blades and, in this context, I can report that the approach that talks "turkey" like this, somewhat newer in terminology than all the aviation style of talking, is called the "Newtonian" approach (someone else said this first). It "puts the cards on the table", "lines the ducks up", "makes the case in black and white", and etc. instead of the haze surrounding all this "coefficient" talk, valid as it may be to some.

I hope others are reading this as well for it is a point that needs to be made better throughout this discussion list. Please direct me to your post that explains the "kinetic" vs. "lift" ideas and I will gladly look into it.

Knucks
www.integener.com

[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#13)
by finnsawyer on Tue Nov 29, 2005 at 10:08:03 AM MST

The blade isn't "shoveling" anything. It is a passive participant. The wind is doing all the work. This is seen most clearly when the blades are standing still and the wind starts blowing. Insofar as a change in vector direction is concerned, if you mount the blade assembly in a long tube, the direction of the air entering and leaving the tube is the same. No change in vector direction. Nothing, nada, nyet, nicht. Yet the blade assembly can still do work, which results from a change in the magnitude of the wind velocity vector but not its direction.
GeoM
[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#14)
by IntegEner on Tue Nov 29, 2005 at 04:48:20 PM MST

Speaking as the intellectual I am supposed to be but have rarely found support for myself on this basis, here is a favorite quote:

Richard P. Feynman, the noted science researcher and Nobel Prize winner (1965 - quantum electrodynamics): "The behavior of fluids is in many respects very unexpected and interesting".

It serves no purpose to debate about some of these details when the wind plays tricks on even the best of us in its "unexpected" ways. Those who have been contributing to the discussions found on this website about the electrical details, the Faraday's Laws and all the rest of it, would be surprised to find that a very great level of detail of a similarly logical and compelling nature exists also within the framework of the interactions between the wind and each inch of the lengths along the blades in observing momentum and mass conservation.

The short answer to the objection you raise is that the walls of the tube just simply redirect all the air deflected by the blades back into a straight path again. This also is why wind tunnels have seen little application in wind energy work - unless the facility is very large the walls interfere with the wind turbine's true operation out in the open.

AVC, www.integener.com

[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#15)
by finnsawyer on Wed Nov 30, 2005 at 09:19:04 AM MST

Given your simplistic answer, I will take a stab at it. It is well accepted that off of the trailing edge of an airplane wing the air has a downward flowing component (your vector direction change?). As such, it can be resolved into a component perpendicular to the wing and one parallel. Now an airplane wing is not a good model for the windmill blade, as it is being dragged through the air. The primary force is drag, the secondary force is lift. In the case of a windmill blade the primary force is lift, the secondary force is drag. A better model for the windmill blade would be an airfoil stationary in a wind tunnel. If there is a change in air velocity parallel to the blade it would show up as a reduction, not an increase, due to drag. In the case of a blade assembly in a tube, then this backwash can be resolved into a component parallel to the incident wind and a component in the direction of the blade rotation, which would cause a vortex in the tube. There can be no component perpendicular to the tube walls (the blade assembly completely fills the tube). The vortex, if it exists, would quickly disappear in the tube as it encounters the tube walls. In free air the vortex would expand as it moves downstream until it disappears. The energy in the vortex would be energy lost. There may, in fact, be no vortex. As Dr. Feynman implies fluids do not always do as we expect. If that were the case, then there would be no change in the velocity vector's direction. Perhaps the "Newtonian Principle" can shed some light on this. But, as it happens, the drag coefficient accounts well for losses, while the lift coefficient accounts for the usable power from the blade assembly. This is why I recommend anyone involved in windmill blade design use the lift and drag coefficients.
GeoM
[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#7)
by Birdmanjack on Sun Nov 27, 2005 at 10:19:11 AM MST

I am using flat aluminum blades which I made myself with angled washers at 17 degrees. I used a softer aluminum to help reduce metal fatigue. The blades are about 1/4 inch thick and seem to work really good. I am running an ametek 90 volt motor and using it to heat two heating elements on a hot water tank to help my boiler. Everything is up about 32 feet in the air. Being of a machining background and now repairing power plants I see and use a lot of aluminum. Most of the blades on the high speed fans are made of it and have very few problems. I wonder how a piece of wood flying thru the air differs from a piece of metal at the speeds we are talking about. The wood is most likely heavier and would do more damage. This is my personal opinion because I have never heard of anyone getting hit with a blade of either kind. Hope you have the best of luck. The Birdman
Blades & wings amazing things:)

Re: Airfoil-missing losses (3.00 / 0) (#8)
by TomW on Sun Nov 27, 2005 at 11:06:39 AM MST

birdman;

Well I think the real stresses come from yawing and wind pushing against the blades not the rotation itself.

Both of these forces concentrate at the root. wind pressure forces are probably obvious. The yaw forces may be a bit harder to understand. Just like any spinning object there are gyroscopic forces that resist moving out of the current plane of rotation and I believe it concentrates in the root / hub area. So now you have at least 3 forces on that area the force trying to fold them back along the shaft, the force trying to jerk them off so they can fly away and the forces trying to twist them off at the hub. Modeling these is beyond my ability but it sure seems like it is a lot of forces trying to fatigue those blades twisting pushing and pulling in a never ending combination.

I have no experience with the physics involved but given a few years I am fairly sure it could beat any material to death. Just tossing that into the mix.

Good luck with them.

Cheers.

TomW

Ignarus can exsisto rememdium. Sardus est forever

[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#9)
by Birdmanjack on Sun Nov 27, 2005 at 11:26:12 AM MST

My blades do not run super fast I was looking for more power than any thing else. I probably should have put the size and lenght of the blades. Blades are about 8' from tip to tip and are roughly 6" at the base and 3" at the tip. I am geared at 8 to 1 now because I did have a problem with it trying to tear itself apart. Seems to be about right. Loads down the blades and gives me the speed to make the gen put out what I need. Probably not the perfect design but it works and does what I need. Wanted to put a picture on here but could not figure how to do it. Thanks for the comeback look forward to more.
Blades & wings amazing things:)
[ Parent ]

Re: Airfoil-missing losses (3.00 / 0) (#10)
by IntegEner on Mon Nov 28, 2005 at 07:21:25 AM MST

Since my last comment here on this thread I have purchased a set of aluminum blades from the aforementioned TLGWindpower people and am pleased with them - the 1/8" (3 mm) thickness and all. I also see where some of the comments demonstrate a lack of good direction, now understanding their quality as I now do. The leading edges can probably be rounded a little as is without much fuss or some tape added or something as help when running outside of the design TSRs or in turbulent winds. You have the correct idea - blades "shovel" the wind from one vector direction to another in accomplishing their purpose, nothing more, nothing less, and the profile thicknesses found in the aerodynamics books are for other reasons not directly related to the generation of the driving forces. When blades like this are extended to lengths of 3' or over, then it is time to start worrying about forces at the blade roots.

Knucks
www.integener.com

Airfoil-missing losses | 15 comments (15 topical)

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