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HomeAircraftHelicopterHelicopter Main Discussion › Rotor Blade Airfoil and other shapes that matter...
10-27-2006 03:45 PM  11 years agoPost 1
SilverWings

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Oklahoma City, OK

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A little discussion got started that wandered over to airfoil usage. In answering another members question, I realized this might best be done as a new topic.

Here was the question:
Airfoil matters, but yet i fly rc airplanes with flat bottom wings and tails and all so how do i do that?
Very insightful question indeed!!

When distilling the ability of an airplane's wing to produce lift, or for a rotor blade to lift, we mainly have two things to look at:

1) Airfoil, or "cross section"
2) Planform shape (what you see from the top-down view)

When it comes to airfoil performance, there are a few things that drive its performance: 1) Camber 2) Thickness 3) Leading edge radius

When considering planform, for rectangular shapes like our rotors, the primary driving factor is called "aspect ratio", or AR for short. Wings or rotors that are relatively short in span for their area have low ARs and wings like our rotors that are long and skinny have high ARs.

Now the point: For a high AR application (our helis) the airfoil plays a much more important role than for low AR applications (like say the wing of an F15).

Can you think of a good sailplane (high AR) that runs a totally flat airfoil? Can you think of any helicopter that runs a totally flat blade? I cannot (maybe you can).

Now how about for low AR applications: Name me a model with low AR that runs totally flat cross sections. Well, how about all those indoor foamies? Sure! Low AR, and very low speed too but they do OK.
(Although very low speed aerodynamics is a really weird area that we'll have to talk about later)

Finally, flat bottom airfoils can and do perform well across the spectrum. That's because we can run camber and a generous LE radius on a flat bottom section. Many of our helis run flat bottom airfoils and fly great. The key is flat bottom. Not flat top and bottom. A totally flat airfoil will do OK at low angles of attack, but if you try and run the AOA up very much it will stall (unless its really low speed, and/or have flow induced by a prop out in front, etc. See caveat above). You'll never get to the 14-15 degrees we can extract out of our heli blades with nice LE radius.

Therefore, the blade section or "airfoil" we run on our helis is of great importance! Has anyone tried running totally flat blades? I haven't seen it tried yet. Might be a good experiment for the BladeCP.

OK, is everyone confused now?

ps: Thanks to EPoweredRC for a great question!

No Bucks, no Buck Rogers

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10-29-2006 02:13 PM  11 years agoPost 2
SSN Pru

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Taxachusetts

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Great laymans breakdown of a tough subject!


what's your profession?


pru

Stupidity can be cured. Ignorance is for life!

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10-29-2006 03:14 PM  11 years agoPost 3
SilverWings

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Oklahoma City, OK

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Thks for the feedback Pru. This is just a subject that's really interesting to me - one that I have made a profession of. So I'll answer your inquisition with a short paragraph (at the risk of blowing my own horn):

I work in two areas: I'm a copilot for United on the B777 and fly international routes (sorry Gino I don't fly to Manila). I also work as an aerospace engineer where I work projects primarily in the experimental/sport arena. But I've worked the big projects too like the F16 (performance analyst Edwards AFB), the Kestrel KL-1C (primary design of wings and tail), and worked as the aero engineer for the "Miss America" (race #11) unlimited air racing team flying the P-51D. And an untold number of smaller jobs on airplanes ranging from Formula I to unlimited aerobatic types to light jets.

But the thing I find interesting is that model aerodynamics have their own peculiarities. Its the very low Reynolds Number regime that makes model aviation, well, sorta weird (but fun!).

No Bucks, no Buck Rogers

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10-29-2006 05:57 PM  11 years agoPost 4
dkshema

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Cedar Rapids, IA

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Can you think of any helicopter that runs a totally flat blade?
DuBro Whirlybird 505.

Dave

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10-29-2006 06:06 PM  11 years agoPost 5
dustyattic

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Castle, Oklahoma

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..That (whirlybird) is very cool. I bet those are incredibly rare. I would be happier to find one of those at a yard sale than a new raptor for the same money.

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10-29-2006 06:53 PM  11 years agoPost 6
AirWolfRC

rrProfessor

42½ N, 83½ W

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SilverWings,
What makes "flat bottom", or nearly so, airfoils, for the same plan form, produce more lift than symmetric ? Since both have roughly the same maximum Cl. Is it all in the L/D (efficiency) ?

I'm trying to justify the existence of asymmetric blades for model helis.

Wolfgang

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10-29-2006 07:33 PM  11 years agoPost 7
rroback

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Irvine (UCI), Ca

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The only way to justify the existence of non flat bottom blades would be fairly simple, go fly upside down! I think we need to go even simplier in the explanation of lift and explain that there are basically two types of lift, lift caused by the shape of the airfoil ( thanks bernoulli) and lift because of the angle of attack, which is just the relative angle of the wing to the airstream. When you stick your hand out the window of a car, and angle it, it is forced up or down.. and your hand isn't a airfoil. I think most of our lift in most model helicopters is all about angle of attack. I bet we could fly fairly square blades ( perhaps rounded, to reduce drag a tad..) no problem. How can you say flat bottom and symmetric blades have similar cl's ( coefficient of lift, for those curious)? At high angle of attack, their lift produced would be similar, but at lower angles, the difference would be much more noticable, I think. too bad none of my studies involve helicopter dynamics. me= 3rd year aerospace engineering student.

Rhett..... I can't fly, but the Profi sure can.

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10-29-2006 07:44 PM  11 years agoPost 8
penguinchicken

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PDX

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At the risk of exposing myself as the tard that I am.

As I remember from A&P school.

Flat bottom airfoils can still generate lift at a 0 degree angle of attack, a symetric foil cannot. Airflow over a sym surface at zero AOA is not able to generate a relitive low pressure area above the wing.

You can always tell a pilot, but you can't tell him much!

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10-29-2006 07:46 PM  11 years agoPost 9
AirWolfRC

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42½ N, 83½ W

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It's not that easily dismissed.

There is a general perception that asymetric blades are better for handling higher loads than symetric.

Fact is that we put the blades at an Angle of Attack that is necessary to produce the required lift. True, asymetric blades produce the same amount of lift at a lower AoA than a symetric blade at the same angle. But that's simply a matter of the shape of the blade, it has camber.

I am looking for the reason why that perception exists in terms of relavent facts.

Do you have any ?

I'll offer the same hint again, is it the L/D (efficiency) ?

A foil is a foil is a foil, helicopter dynamics not with standing.

Wolfgang

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10-29-2006 08:42 PM  11 years agoPost 10
Sky 5

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Spring Hill, Tn

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Simply put Bernoulli's principle.

On a flat bottom airfoil the air above the airfoil travels further and thus speeds up, pressure drops and lift is created. Lift is created with zero angle of attack. This airfoil fly's very poorly inverted, excessive angle of attack is needed to fly inverted.

On a completely symmetrical airfoil the air above and below travels the same distance therefore the speed of the air is the same, no increase in speed and no pressure drop. No lift is created unless a positive angle of attack is created. This airfoil fly's equally well upright or inverted.

A barn door will fly if given sufficient angle of attack and relative wind. Part of the lift created is called impact lift and this is simply the relative wind hitting the bottom (flat) surface, and is most noticable in ground effect.

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10-29-2006 08:54 PM  11 years agoPost 11
hootowl

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Garnet Valley, Pa.

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Does flat and airfoil go together? In otherwords a totally flat blade has no airfoil... is that correct?

Wolves don't lose sleep over the opinions of sheep

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10-29-2006 09:08 PM  11 years agoPost 12
AirWolfRC

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42½ N, 83½ W

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No, flat and airfoil do not go together.

Yes, symetric blades are predominamtly used because they have the same characteristics up-side down and right-side up.

My question is necessarily limited to right-side up flight.

Wolfgang

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10-29-2006 10:28 PM  11 years agoPost 13
oldfart

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Vancouver, Canada

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There is also other considerations re: airfoils for rotor blades that is not of much importance to those for airplanes wings.

For all intents and purposes the root of an airplanes wing travels at the same velocity as does the tip. Not so in a rotor wing. Here the tip is traveling a LOT faster then the root. As lift/drag is directly related to an airfoil at specific AoA relative to airpeed, this makes the design of a rotors airfoil much more complex.

Also considering the fact that unlike the airplane wing, in order to roll, the AoA's of both rotors are changed drastically, while on the airplane, only the relative AoA at the aileron locations is changed.

Also considering that varying the AoA of some airfoils (specially a flat bottom one) will change the location of the CoL of the airfoil one can see that as we are ALWAYS changing the AoA of a rotor blade through maneuvers and to generate lift in just a collective requirement, one can see how it can get very complicated very quickly.

Because the rotor blade is rotating around the mainmast (rotor speed) and also around a feathering spindle (AoA change), the spanwise and chordwise balance points, relative to the moving CoL of the airfoil also become critical in how they will fly.

When designing rotor blades, all of this becomes very important to determining their in-flight dynamics....much more so then what they are mad from...wood or C/F.

This is why you will find that there are some wood blades that actually fly better then some hi-zoot looking Carbon Fiber units.

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10-29-2006 10:43 PM  11 years agoPost 14
AirWolfRC

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Yes, there's a lot going on there. The NACA0014 is mostly used for our symetrical blades because the center of pressure stays put with changes in AoA. Reflexed asymetric shapes are used for the same reason.

I guess the only way I will get my answer would be to setup an experiment with both symetric and asymetric blades and measure the efficiency to see if asymetrics are actually better for load carying. I have not been able to get a conclusion from the published airfoil data.

Wolfgang

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10-29-2006 11:25 PM  11 years agoPost 15
Bad Karma

rrVeteran

UK

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The NACA0014 is mostly used for our symetrical blades because the center of pressure stays put with changes in AoA.
Nearly stays put.

AirwolfRC, with an asymetrical section, look at its mean camber line compared to a symetrical one (straight line), any asymetrical section could be considered just a symetrical section where the mean camber line is bent just right to make the bottom flat, naturally bending it in such a way makes the top surface longer (benoulli)

They may have roughly the same Clmax as each other (only! roughly though) but the Cl curve with respect to AoA and the maximum AoA may be very different.
I guess the only way I will get my answer would be to setup an experiment with both symetric and asymetric blades and measure the efficiency to see if asymetrics are actually better for load carying. I have not been able to get a conclusion from the published airfoil data.
Done that a few years back, and yes an asymetrical one will produce more lift for the same drag, so it is better for load carrying, just look at heavy lifter planes.

The other advantage is asymetrical ones tend to have a higher stall AoA than symetricals

play about with the shape and AoA to approximate a flat section with this

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

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10-29-2006 11:35 PM  11 years agoPost 16
AirWolfRC

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42½ N, 83½ W

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They may have roughly the same Clmax as each other (only! roughly though) but the Cl curve with respect to AoA and the maximum AoA may be very different.
This is mostly true but the AoA is positioned to yield the requsite lift coeficient and the AoA spread between zero lift and max lift is still about 15º±.

So my question is still what makes asymetric better (if so) than symetric for efficient lifting ?

Wolfgang

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10-29-2006 11:46 PM  11 years agoPost 17
Bad Karma

rrVeteran

UK

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A asymetrical one is better optimised to take advantage of the bernoulli principle, as it maximises the distance over the top of the wing vs underneath compared to a symetrical one, where the flow path is only slightly increased with AoA.

With a symetrical one at say 5 degrees AoA the air over the top is accellerated because of the upwash and downwash turning the airflow, but with an asymetrical one at the same AoA it also adds in extra distance by adding more surface, which will further curve the airflow.

Try it in that sim, set the camber at zero, the AoA at 5 and the thickness at whatever you want then start playing with the camber, and look at the upwash near the LE and the downwash at the TE.

Also look under the "output" dropdown menu and select probe.

below there is a dial with two slider bars, move them to move the probe and click what you want to measure, then play with the camber, you should see that as the camber goes up the airspeed on top of the wing goes up, so therefore the lift does also.

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10-29-2006 11:48 PM  11 years agoPost 18
AirWolfRC

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42½ N, 83½ W

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I agree with that but how does that answer my question ?

All else being equal, does a symetric take more or less or the same power to generate the same lift than an asymetric ?

The fact of the P-3 Orion at about 130,000lb gross weight having a symetric wing started me asking this question.
The other advantage is asymetrical ones tend to have a higher stall AoA than symetricals
But what matters is the range of AoA, not the absolute value.

Wolfgang

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10-29-2006 11:58 PM  11 years agoPost 19
Bad Karma

rrVeteran

UK

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It should take more power since the required AoA is going to need to be higher.

The best way to compare them would be to look at the asymetrical one at 0 AoA and then match the symetrical one by lift generated, the symetrical one should generate more drag for that lift its generating since its frontal cross sectional area will be already at an elevated starting point (if matching lift)

Have you had a look at that link yet?

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10-29-2006 11:59 PM  11 years agoPost 20
Bad Karma

rrVeteran

UK

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But what matters is the range of AoA, not the absolute value.
If we are only (which is what I thought we where doing) comparing positive AoA then the absolute +AoA being higher means there is a higher range

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