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HomeAircraftHelicopterHelicopter Main Discussion › Rotor Blade Airfoil and other shapes that matter...
10-30-2006 12:01 AM  12 years agoPost 21
AirWolfRC

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42½ N, 83½ W

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That's what I'm refering to also, positive lift AoA's. Back to my question.

Is an asymetric more efficient or not and if so, how can I demonstrate that ?

Wolfgang

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10-30-2006 12:20 AM  12 years agoPost 22
Bad Karma

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UK

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To show that all you would have to do is get their L/D curves and compare them, you should find that the asymetrical one has a higher L/D ratio throughout most of the range of AoA than the symetrical.

So naturally the one with the better ratio wins

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10-30-2006 12:23 AM  12 years agoPost 23
AirWolfRC

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Exactly, and that's what I'm trying to prove, also by how much.

One thing I have determined is that our helis aren't even flying any where near their best L/D. More like 1º to 2º AoA.

Wolfgang

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10-30-2006 12:33 AM  12 years agoPost 24
Bad Karma

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UK

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Well thats a problem and I dont think you can really say that our helis are not opperating at their best AoA, its a rotor not a wing, different span distances will be opperating with different L/D ratios.

To get an answer, I would start adding weight to the heli while maintaining a hover (out of ground effect) so the lift being generated should be going up.

Each time I add weight to it note the pitch, and note the throttle position, I would have weight (lift) the AoA (estimated based on pitch) and throttle opening (assuming constant RPM assume that 50% throttle = 50% power etc, which should track the drag)

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10-30-2006 12:46 AM  12 years agoPost 25
AirWolfRC

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I'll settle for L/D comparisons for now.

Specifically, NACA-0014 and a 14% reflexed asymetrical.

Wolfgang

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10-30-2006 01:25 AM  12 years agoPost 26
oldfart

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Vancouver, Canada

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Due to some of the points in my original post, you will find that an airfoil that is slightly more efficient, lifting "X" amount of weight in a hover may cause undesireable pitchiness in forward flight. And that it can become more and more pronounced as forward speed is increased.

So though it may have a good L/D in the hover, other dynamic properties may make it less desireable as a rotor blade overall.

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10-30-2006 01:44 AM  12 years agoPost 27
AirWolfRC

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Which point is that, other than gyroscopic precession and the advancing blade producing more lift ?

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10-30-2006 03:12 AM  12 years agoPost 28
SilverWings

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Oklahoma City, OK

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Its all about boundary layer control, folks!
Airwolf - let me try and give you a nuts-and-bolts answer to your question:
Is an asymetric more efficient or not and if so, how can I demonstrate that ?
Airfoils, whether symmetric or not, produce lift by the Bernoulli principle. We all know that. But what's missing in this discussion is the real world price we pay in producing that lift. That is, what drag penalty is added and where does it come from?

In 2-d airfoil theory we have two primary drag contributors: 1) Form drag and 2) Friction drag (there also can be wave drag but that's relavant to transonic operation)

Lets talk about the simpler one first: form drag. Form drag is aptly named because its caused by the shape we're pushing through the air. A fat shape displaces more air than a skinny shape (like the bow of a cruise ship pushing more water to the side than the bow of a kayak). This is intuitive. Thinner airfoils offer less form drag at low AOA.

Now for the more difficult one: friction drag. This is harder to understand because the "friction" actually is caused by the fact that we're producing a boundary layer over the airfoil surface. This boundary layer is in effect a layer of air, in contact with the wing's surface, that's traveling slower than the free-stream velocity. And since air is a viscous fluid, any boundary layer present will manifest itself in the form of DRAG. The thicker the BL, the greater the drag.

Now the answer for your question: We can do a better job of managing the boundary layer over an asymetric section than we can on a symetric section. Its all about how we manage the "pressure recovery" side of the airfoil; that's the aft (behind the max thickness point roughly) part of the airfoil. This is where the top surface flow is moving from a low pressure condition to a high pressure condition at the trailing edge. This is called an "adverse pressure gradient" since fluids or gasses in nature will travel from high pressure to low pressure. On the pressure recovery side of the airfoil, we're asking the flow to go the opposite way - that is from relatively low pressure to high pressure. This, if not managed, will create big thick boundary layers. The old 0014 isn't too advanced in this area (not at all really).

Aero engineers work hard to design mission-specific airfoils that handle the pressure recovery job in the most efficient manner. In doing so, they can tailor the speed, height and shape of the boundary layer and so can reduce drag. This is where "laminar flow" came from, and its where we can do really great work on PC's these days with Eppler code based analysis.

This is why cambered sections can most always put up better L/D numbers than symmetric sections. You can see for yourself if you take a look in "Theory of Wing Sections" by Abbot and VonDoenhoff.

Makes me think about designing a rotor blade myself! I envision a really nicely tailored section, linear pressure recovery, and engineered to optimize L/D with span. Probably would be a slightly tapered planform with sheared tips. Cool, huh? I could do the aero work, but couldn't build it. I think I could beat the 0014 L/D by at least 30%. Just need someone to build it, DARN.

No Bucks, no Buck Rogers

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10-30-2006 01:53 PM  12 years agoPost 29
AirWolfRC

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The P-51 was one of the first experiments in laminar flow. But it was not the wing that allowed low drag at high speed, it was the energy recovery in the cooling system duct, the considerably lower cooling drag. This from the guy who designed it.

Yes, I have "Theory of Wing Sections", two copies actually.
I like that formula at the bottom of page 113, ±Yt= . . . . .

I'm reasonably familiar with all the details. Problem is there are too many details and no specifics that deal with my question directly.

The point of my question,
Of the blades available for our helis, are the reflexed asymetric more efficient than the 0014 or not ?

Is there a clearly demonstratable reason to use reflexed asymetrics for a heavy camera ship and if so, how much better are they ?

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10-30-2006 02:06 PM  12 years agoPost 30
ChristianM

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Oslo, Norway

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I read an article a while back about a couple of guy breaking the duration record for helicopter (both electric and IC category). They did a lot of testing in order to optimize and one of the things they did some testing with was blades. I don't remember the details but they used asymmetrical (nearly flat bottom if I remember correctly) and a got quite a bit longer flight time out of them which is an measure of efficiency. Now I can not tell you the improvement in terms of a percentage but that would also depend on head speed, weight, etc.

Another thing they did was to use longer tail booms to reduce the load on the TR (longer moment arm => less force) and improve the duration.

Christian

Burn fuel, be happy

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10-30-2006 02:46 PM  12 years agoPost 31
AirWolfRC

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If you happen to find a link to those efforts, that would be great.

After some more checking, comparing L/D numbers, it looks like the difference may be only about 20% in favor of asymetric.

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10-30-2006 04:00 PM  12 years agoPost 32
Bad Karma

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UK

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Well 20% (quite a reasonable number) isnt anything to sneeze at.

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10-30-2006 04:20 PM  12 years agoPost 33
AirWolfRC

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For steady state use, yes, but in model heli flight, still not that much.

Consider that we fly our pod & boom helis at a Cl of about 0.1 and best L/D is at about 0.8 We fly at about 1/4 of best EFFICIENCY.

We can improve EFFICIENCY about 300% by simply slowing down the head speed.

But if we fly at best EFFICIENCY, we give up snappy performance.
Like gas milage, do you want milage or acceleration ?

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10-30-2006 05:42 PM  12 years agoPost 34
Bad Karma

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UK

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Consider that we fly our pod & boom helis at a Cl of about 0.1 and best L/D is at about 0.8 We fly at about 1/4 of best EFFICIENCY.
That entirely depends on what part of the blade you are looking at, different distances from the centre will have different AoA's very different, even negative near the centre in some cases.

So just saying the blades have a Cl of 0.1 is a bit pointless, saying it has it in a hover, at such and such a distance, thats more useful.

If that 0.1 is near the 60-70% rotor diameter (which I'm guessing is where you have measured it) then you should find that that is its lowest value for a hover.

That is unless you are talking about the combined effective Cl, if thats what you mean never mind.

Then when the thing is moving, you are going to have one blade at a higher AoA than the other (retreating blade) so that one will have a higher Cl.

This is what I was on about earlier when saying not to treat it as a wing but remember that its a rotor.
But if we fly at best EFFICIENCY, we give up snappy performance.
So what, we are on about heavy lifting here, which to most people means things like camera lifters or scale helis in R/C or things like the Skycrane or the CH-47 in full scale, both heavy lifters but neither is needed to be snappy.

When heavy lift is the name of the game then 20% extra lift for free just by having a flat bottomed set of rotors is good, thats more than 20% more cargo that you could lift, afterall your aircraft is already covered by the lift you already have and the cargo you are carrying is also covered, so 20% could mean double or tripple the cargo.

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10-30-2006 06:02 PM  12 years agoPost 35
AirWolfRC

rrProfessor

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The only quantitative information here, I have provided (20%). Will you be helping with more pertinent information or just flooding the issue with more ancilary details ?

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10-30-2006 06:13 PM  12 years agoPost 36
Bad Karma

rrVeteran

UK

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They arnt ancillary details, if you want to see how the blade is producing lift and wether the figures that one calculates are representitive of the whole blade you have to consider these things, you cant just chuck out the parts that you dont want to do because its simpler if you do.

When you say 0.1 how does anyone know if that 0.1 is the overall effective Cl or the Cl just at the point you calculated.

If its the former then sure say 0.1, but if its the latter it means that it probably isnt actually 0.1 at all except at that one point.

Sorry but this is all pertinent information as considering all this will give a better picture of what is happening.

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10-30-2006 06:18 PM  12 years agoPost 37
AirWolfRC

rrProfessor

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It all applies, yes, but you don't seem to recognize the fact that those factors apply equally to both types of blades and as such, can be mostly washed out of the comparison that is the basis of my question.

Don't loose sight of the forrest for looking at the trees.

I brought this question up for my own curiosity and the curiosity of others. Asymetrical baldes exist, are they worth the effort ? It was not intended to be a mini course in aerodynamics.
When you say 0.1 how does anyone know if that 0.1 is the overall effective Cl or the Cl just at the point you calculated.

If its the former then sure say 0.1, but if its the latter it means that it probably isnt actually 0.1 at all except at that one point.
I welcome you to run some numbers to find out.

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10-30-2006 06:21 PM  12 years agoPost 38
Bad Karma

rrVeteran

UK

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Ok then, yes they are worth it.

As I said if it means that you can double the cargo you can lift then that is a big bonus.

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10-30-2006 06:24 PM  12 years agoPost 39
AirWolfRC

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Run some numbers. DOES it double the cargo capacity ? I don't think so.

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10-30-2006 06:25 PM  12 years agoPost 40
Bad Karma

rrVeteran

UK

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I welcome you to run some numbers to find out.
I was asking you.

Is that 0.1 just from that one point (how do I know what blades/heli etc etc you did it with)

Or did you take a range of results from the centre of the disk to the tip and get the overall average Cl from all of those?

If thats what you did then it doesnt matter, it is 0.1, but if you only took the value from one point then that Cl is only a valid number to base anything on when talking about that point only, which means you cant use that to describe the blade as a whole.

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