NACA 4415

[willib]

Abstract

Wind turbines in the field can be subjected to many and varying wind conditions, including high winds with the rotor locked or with yaw excursions. In some cases, the rotor blades may be subjected to unusually large angles of attack that possibly result in unexpected loads and deflections. To better understand loadings at unusual angles of attack, a wind tunnel test was performed.

An 18-inch constant-chord model of the NACA 4415 airfoil section was tested under two dimensional steady state conditions in the Ohio State University Aeronautical and Astronautical Research Laboratory 7x10 Subsonic Wind Tunnel. The objective of these tests was to document section lift and moment characteristics under various model and air flow conditions. Surface pressure data were acquired at -60° through +230° geometric angles of attack, at a nominal 1 million Reynolds number. Also, cases with and without leading edge grit roughness were investigated. Leading edge roughness was used to simulate blade conditions encountered on wind turbines in the field. Additionally, surface pressure data were acquired for Reynolds numbers of 1.5 and 2.0 million, with and without leading edge grit roughness, but the angle of attack was limited to a -20° to 40° range.

In general, results showed lift curve slope sensitivities to Reynolds number and roughness. The maximum lift coefficient was reduced as much as 20 % by leading edge roughness. Moment coefficient showed little sensitivity to roughness beyond 50° angle of attack, but the expected decambering effect of a thicker boundary layer with roughness did show at lower angles.

Tests were also conducted with vortex generators located at the 30% chord location on the upper surface only, at 1 and 1.5 million Reynolds numbers, with and without leading edge grit roughness. In general, with leading edge grit roughness applied, the vortex generators restored the baseline level of maximum lift coefficient but with a more sudden stall break and at a lower angle of attack than the baseline.

[willib]

Merlin found this , some of the old timers probably know it by heart.

a copy is in my files its 1.5 megs , but i'll be taking pieces of it for further examination

[willib]

[willib]

this is an airfoil primer pdf ,its only 81Kb

http://www.otherpower.com/images/scimages/2965/airfoilprimer.pdf

its also in my files

it covers stuff like lift ,drag , angle of attack, coefficient of lift , coefficient of drag and Reynolds number

[willib]

i misspelled Murlins name ,sorry

http://www.otherpower.com/images/scimages/2965/NACA_4415.pdf

[Murlin]

"i misspelled Murlins name ,sorry "

Hey NP....Man....those sure are alot of numbers!!!  

No way am I going to get stuff like that on the little experiment I am planning on doing.

But it is going to be a fun experiment none the less.....

I think I may have upset a few people questioning the whole airfoil issue...but I guess you just can't please everyone, all the time....

I never was one to blindly follow orders.....heh.....

Murlin

[DanB]

Damned... you could've carved a good set of blades by now ;-)

(just kidding)

[willib]

chord length is the length from the leading edge , on the right ,to the trailing edge of your blade

this particular airfoil section is about 7.4" long , i made them  out of the thin part of  cedar shingles.

coppied from a print out that paradigmdesign provided here

http://www.otherpower.com/images/scimages/4937/airfoil_sections.jpg

the chord line is a straight line from the leading edge to the trailing edge

.

[willib]

there are  a lot of similarities between tha NACA 4415 and the airfoils paradigmdesign provided

I can only describe what i have done , i've got a zillion pictures , going back before i became an active member here so i might as well show them if anyone wants to see them , its going to take a little while to collect them in the right sizes , more to come...

[willib]

its strange the influences other peoples designs have on you .

this is my version of doug selsams props





this is from warlock.com.au





similar yes?





warlock again

some day i would like to try five of these on a carbon fiber surf fishing rod shaft

[willib]

i've come a long way in a short time , and i am by no means an expert like these guys





i could show you how to layout a section and cut it in ten minuets with a hot wire ,i think the best thing to do for all the budding blade builders out there is to make a model of a set of blades , something you can touch , rotate ,look at from all angles , and spin around if possible , because it really helps to have that as a reference ,when you are making your blades

[willib]

since the 4415 is so similar to my airfoil i am going to use this data in my blade designs in the future.





sorry about the gap

the 4415 has a max coeficient of lift at 14 degrees of 1.47

but it goes down to .78 from 20 to 22 degrees then goes back up to 1.36 at 39.7 degrees ? can anyone explain why this is?

and given this info ,at what angle should i design my blades

do i want the max lift?

i've got some pretty nice results with an AOA of 6 degrees could i go more?

should i go more ? thoughts? opinions

[Murlin]

Hot wire/foam....

Ya I made some pretty cool RC airplane wings out of cut foam with a  balsa skin, and fiberglass cloth/resin finish.

Light as a feather and strong as heck.

They came pre cut and ready to lay on the balsa skin.   But it was a TON of work to finish them.

Murlin

[rotornuts]

That transition you see in the Cl from high to low to high again is the transition from aerodynamic lift to loss of lift due to stall and back to lift due to drag.

Follow the Cd figures as well as the Cl figures to get a picture of what's really happening.

From 12 degrees to 19 degrees you go from a Cl of 1.4 up and back down to 1.4 but at 19 degrees it cost you 4 times more drag. You can see that at 20 degrees you have stalled badly and the Cd has almost doubled in one degree of angle. It's all down hill from there as the drag is increasing dramaticly. If you were in an airplane you'd be yelling mayday, mayday!

Mike

[finnsawyer]

You mention the lift coefficient at 14 degrees, but if you look at the data the lift coefficient changes slightly from ten degrees to twelve while the drag coefficient more than doubles.  In other words stall is more like at twelve degrees.  And keep in mind that for a high TSR wind mill blade only a small part of the lift contributes to output power due to the geometry and the direction of the apparent wind, while almost all of the drag acts to rob the blade of power.  And both drag and lift are proportional to blade width.  In any case this air foil profile behaves basically like any air foil near stall.  It is interesting to see the data above stall, as they don't normally give it.  I don't know how useful it would be, though.

[willib]

hi GeoM the data after stall is provided to give data for when the blades are furled or in the process of furling and not in their normal angle relative to wind direction.i think thats the reason.

the positive and negetive angles they go into in the pdf are extreme indeed , they even go upto a positive 90 degrees AOA ,with  their model foil , which was composed of a foam core with carbon fiber skin.

i also noticed the coefficient of drag pressure Cdp is minus for an AOA of 2.2 , does this mean there is no drag at 2.2 deg AOA

there is also very little drag pressure at 4.0 & 6 degrees but it triples from 6.0 to 8.3 degrees AOA

[willib]

they put pressure sensors on the airfoil , so that a negative pressure is suction ,  (lift) i think

i havnt figured what x/c means

c is the chord length

x is an " Axis parallel to airfoil reference line"

but just looking at x/c = 0.0 , i can see that at 19 deg , lift is a max , at 20 degrees it goes into stall , mayday!

and at 6 degrees there is twice as much lift as drag pressure .

VGs are vortex generators placed on the airfoil surface

k/c are  little slatterings of simulated bug debris

[finnsawyer]

I think the negative values for the drag coefficients are just artifacts.  That is, they are just a result of how the data is handled.  Physically, drag will always rob the mill of power.  You need to dig deeper to find out what the minus sign means.  It would appear from the data that four degrees would be the sweet spot for this profile, but have they accounted for all the drag effects?  This doesn't seem to follow the usual curves where the drag coefficient has a U shape as a function of attack angle.

Here's something else to consider.  For a given TSR and point on the blade the apparent wind has a certain angle (alpha) relative to the plane of the blades.  Take this angle as fixed.  If one now increases the attack angle of the blade from say four degrees to eight, the angle between the blade and the blade plane goes from alpha - 4 to alpha - 8.  It starts out small and becomes less.  The amount of the lift force that contributes to the output power goes as the sine of this angle, so it becomes less.  For instance for a TSR of seven (an alpha of 8.13 degrees) the angle goes from 4.13 to 0.13 at the tip.  In the latter case essentially no part of the lift is contributing to output power at the tip.  On the other hand drag is affected only slightly by this change.  I think people tend to overlook this issue.

[finnsawyer]

I'd say x/c is the fraction of the distance along the span starting at the windward edge.  They must define the axis shown as zero as some physical line across the foil.  The maximum pressure at the leading edge occurs slightly below that line.  Along the top surface the pressure drops rapidly as the air flow speeds up and then drops off slowly along the span.  I suspect any air foil will show similar curves.  No real surprise here.

[willib]

http://www.alton-moore.net/wind_calculations.html

http://www.warlock.com.au/bladecalc-abstract.htm

Both of these are very good blade calculators, my favorite is warlock's because it converts meters to inches and feet for you.

Now,as an example put in warlock's calc. click on the metric button

 3 blades

 6 tsr

 .35 eff

 1 m radius blade size

 10.1 kph

now click imperial button your chord should read 2.404" long  ,at the tip. at an angle of 0.3 degrees, angle beta.

maybe i have been spoiled because i can change the angle of attack of the whole blade even after its done. Just by pivoting the blades a tad this way or that.a nice feature to be sure

What happens if you want to change the length ( radius) of the blade and get the calc to output in inches? easy , just go back and click the  metric button , put in .875 m radius , click solve , then click imperial this gives you a blade radius of 2.9 feet , if you then click solve again,  notice the length is back to 3 feet it must be a conversion problem in th program , dont know..

some things i've noticed and have some questions on:

for a given TSR , both calculators assume a AOA , let me explain

TSR - Altons - Warlock

 9 - 4.2 - 4.2

 8 - 4.7 - 4.8

 7 - 5.4 - 5.4

 6 - 6.3 - 6.3

 5 - 7.6 - 7.6

 4 - 9.4 - 9.5

at these angles for each calculator the tip ends up at 0 degrees ,when you change the airfoil from NACA 2412 to a user defined airfoil

how is it that the NACA 2412 is defined by its Cl of .85 at an alpha of 6 degrees?

another observation i've made is that for any given set of parameters , lowering the lift coefficient , increases the chord length to make up for the lower Cl .

.

[finnsawyer]

Increasing the chord length also increases the drag.  Considering that one can impose any angle at the root when installing the blades, it is quite feasible to carve the blade such that at the tip the blade has zero angle relative to the broad side of the board and still maintain the desired attack angle.  The attack angle in turn must depend on the TSR so that at the tip one gets the desired lift.  As the TSR drops the angle alpha that I defined gets larger allowing a larger attack angle.  Anyway, that's why their programs impose a certain attack angle for a specified TSR.  I would have to see all the details for a blade design to comment further.  As I pointed out in the past, carving these blades from 2 bys somethings probably does not give good performance below half radius.

[willib]

but isnt it also true that increasing the chord length also increases the lift , and if one uses a proper airfoil , say a NACA 4415 , then the gained lift outweighs the gained drag , and there is a net gain from using a larger chord

[finnsawyer]

As it happens both the lift and drag are proportional to the chord length, so both go up by the same ratio.  There is no advantage to increasing the chord length.  Besides, there is only so much power or energy available from the wind.  If the chord length is already optimal (that's what those programs are supposed to find), then increasing the chord can not gain you any more power from the wind, but will increase drag to rob more power from the wind, so you lose output.  One relative measure that some people use is the ratio of the lift coefficient to the drag coefficient Cl/Cd.  Obviously maximizing this quantity would be good.  I don't believe it tells the whole story, however, as the angle of chord axis to the blade plane (alpha - AOA) is important.  The smaller that angle the less the lift contributes to the output power while the drag basically stays the same.

Since you seem to have a deep interest in these issues, I suggest you get a book on aerodynamics, such as the one by Dan Smith.  If you're not versed in Calculus I also suggest you enroll in courses in differential calculus and integration at a local university.  You need to understand the interplay of forces, dynamics, and geometry and these get quite complicated, but the tools do exist to handle them.  The problem is that there are two forces at work and you want to maximize the usable component of the lift force while minimizing the drag force within the constraint of the available power from the wind.  Not only that, you want to keep your RPMs up to match the characteristics of your alternator.

[willib]

"As it happens both the lift and drag are proportional to the chord length, so both go up by the same ratio.  There is no advantage to increasing the chord length.  "

i respectively dissagree , if i may.

if there is no gain from increasing chord length , then how come the blade calculators do just that .

When one inputs a lower Coefficient of lift , the blade calculators compensate for this , by increasing chord length , for the same wind speed , RPM & Power Output of the blades.

[willib]

 my mistake i ment respectfully

[willib]

how thin is  the balsa wood ?

i'm thinking it would work great for a blade with no taper and no twist.

[SamoaPower]

The introductory text, "the defacto paper on airfoils for wind turbines", to this post is quite misleading. The referenced paper only contends to be a report on  tests to determine the effects of surface roughness and the use of vortex generators on a particular airfoil. It does not purport to be a treatise on the selection of airfoils or design of wind turbine rotors. The paper doesn't claim to be a defacto anything, it's only this post author's interpretation of something he obviously doesn't understand very well.

There are so many mistakes, mis-statements, and mis-interpretations of data in this post, I hardly know where to begin ... so, I won't. I write this simply to offer a word of caution to the new-commers, that there are many better sources of information on this topic than what is presented here.

I believe it is irresponsible to offer an authoritative paper for review and to represent it as something it doesn't purport to be.

[SmoggyTurnip]

Increasing the chord length increases the RE number.

If there was no advantage to larger chord lengths then airplanes

should have 1" chord for their wings.

.

[finnsawyer]

I thought you were advocating increasing the chord length with the same coefficient of lift.  That is different than the case you described above.  I stand by my comment, as doubling the chord length for the same lift coefficient would double the force pulling the blade around.  This doubling of force means twice as much work is done by the blade as it makes a complete revolution.  If the time of one revolution does not change, then the power provided by the blade has also doubled.  The problem is that the wind does not support this if the original chord length was optimal.  You will, in effect, have only succeeded in increasing drag.

If, on the other hand, you halve the lift coefficient by reducing the angle of attack, you do need to double the chord length to get the same lift on the blade.  Reducing the lift coefficient may also change the drag coefficient.  If the drag coefficient is reduced by more than half, the drag may also be reduced allowing a shorter chord length for optimal performance.  But by reducing the angle of attack you have also changed the geometry.  Since the angle of the apparent wind does not change anywhere along the blade for a given TSR, you would also have a greater fraction of the lift force in the direction of rotation with a smaller angle of attack allowing a further reduction in the chord length.  Does this mean we should use a zero angle of attack, since you can still get lift then?  Well, it depends on what the drag coefficient or the ratio Cl/Cd is doing since we end up with a very wide blade.  Obviously this case is much more complex than the one that I outlined above.  There is an optimal solution, but finding it is not easy.  You need the proper data to do it, but I am not sure if the data you have presented is it.  I am concerned, as I mentioned before, about the behavior of the drag coefficient.  The data seems to imply that the resistance to air flow of what is basically a fat air foil disappears at a certain angle of attack.  This is suspect.

[finnsawyer]

The dynamics of an airplane wing and a windmill blade are different.  The airplane wing is being pulled through the air.  The lift is vertical and the drag is horizontal.  Both are proportional to the area (planform) of the wing.  In order to keep the plane aloft you need a certain amount of lift.  This results in a certain amount of drag.  Doubling the wing's (chord length) will double both. This requires a larger engine to overcome the drag.  You don't get something for nothing.  The same applies for a wind mill where the engine is the wind.  You have to stay within the limits imposed by it.  It is a more complex situation than the air plane wing.  See the explanation above.

[finnsawyer]

I came to a similar conclusion from the data alone.  There is something wrong with the drag coefficient.  It appears the paper's authors are measuring or calculating the drag due to a certain effect.  I don't think it tells the whole story.  I would not use this data for a blade design.

[SamoaPower]

I guess I gave the impression that I was being critical of the paper itself. No, not all. The paper is fine for what it purports to be. What I was being critical about is the post's author misuse of it. Sorry, if I gave the wrong impression.

The drag used is the pressure drag coefficient which I believe is different than what we are used to seeing from the polar plots for an airfoil.

[willib]

No it was pretty clear to me anyways what you were getting at.

it is also clear to me that some of us are doers and some of us are talkers

anyone reading this can decide for themselves which is which