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-   -   NACA 6 Series airfoil (http://www.rcuniverse.com/forum/aerodynamics-76/4538584-naca-6-series-airfoil.html)

martinoto 07-24-2006 11:34 AM

NACA 6 Series airfoil
 
Hi there...

I am working on my low wing, sporty design, symmetrical airfoil. I will opt for a rather thick airfoil, because I don't want too much speed. My first option was NACA 0016 or 0018. But I am still on the drawing board, so I have plenty of time to questioning things...

I wonder why there are not that many planes with symmetrical NACA Series 6 type of airfoils, like 63015...

When I went to the field yesterday with those airfoils sketches, all of them guys raised their eyebrows, and some simply said that it ain't gonna work at all. NACA Series 6 seems not to be popular, but... is there any specific drawback to the concept that makes me stick to the conventional symmetrical NACA's??? Any experience to share? What should I (basically) expect from it if I use it?

Thanks a lot.

Tall Paul 07-24-2006 12:02 PM

RE: NACA 6 Series airfoil
 
The 6- series airfoils are intended for large airplanes.. full scales, flying fast.
They're too complicated to build for the very low speeds (relatively) that models use.
The 4-digit series shapes are perfectly fine.

dolanosa 07-24-2006 12:24 PM

RE: NACA 6 Series airfoil
 
Awesome question that baffles a bunch of people here...what airfoil to use and what's the difference between airfoils. This is going back to college days so bear with me if I forgot something and all that.

The history of airfoils before NACA was basically to try one airfoil and if that didn't work, make some changes and then try the new one without any real standard. NACA, basically decided to make a standard geometric description of the airfoils using a formula. Basically, the engineer would only input certain variables and then the formula would spit out the shape. The 4 series needs the height of the camber, the location, and the thickness of the airfoil. Other series needs other inputs, of course. In any case, NACA made a system to create airfoils easily.

Then we wanted to go faster and fly higher and needed a lower drag airfoil than any of the 4 series can make, enter the 6 series. (Purists: yep the 16 series as well but I'll stick to the 4 and 6 for this session). A couple of things made this foil so much better than the 4 series. One, it was "optimized" for laminar flow. What this means is that as the air goes over and under the foil, it takes a much longer time to become turbulent. Basically, the air sticks to the foil longer. Second, it has something called a "drag bucket". A bunch of speed freaks here know what this is. Basically, it's an area where, at a certain range of lift coefficient, the drag remains low, compared to similar 4 series airfoils. The problem is, once you get out of the drag bucket, the drag goes up a lot. In fact, in some occasions, you may want a 4 series airfoil versus the 6 series because at least on the 4 series, drag is relatively smooth. You can see this quite clearly on a drag polar view.

So basically, you still need to know what you will use the wing for. I've heard of the 6 series being used in F3A (ex 63012) and racing 67xxx. Of course, the higher second number is the location for the maximum thickness, which may decrease drag but make a smaller drag bucket.


Hope this helps.

Butch

Jimmbbo 07-24-2006 03:17 PM

RE: NACA 6 Series airfoil
 
Quote:

ORIGINAL: dolanosa

Awesome question that baffles a bunch of people here...what airfoil to use and what's the difference between airfoils. This is going back to college days so bear with me if I forgot something and all that.

The history of airfoils before NACA was basically to try one airfoil and if that didn't work, make some changes and then try the new one without any real standard. NACA, basically decided to make a standard geometric description of the airfoils using a formula. Basically, the engineer would only input certain variables and then the formula would spit out the shape. The 4 series needs the height of the camber, the location, and the thickness of the airfoil. Other series needs other inputs, of course. In any case, NACA made a system to create airfoils easily.

Then we wanted to go faster and fly higher and needed a lower drag airfoil than any of the 4 series can make, enter the 6 series. (Purists: yep the 16 series as well but I'll stick to the 4 and 6 for this session). A couple of things made this foil so much better than the 4 series. One, it was "optimized" for laminar flow. What this means is that as the air goes over and under the foil, it takes a much longer time to become turbulent. Basically, the air sticks to the foil longer. Second, it has something called a "drag bucket". A bunch of speed freaks here know what this is. Basically, it's an area where, at a certain range of lift coefficient, the drag remains low, compared to similar 4 series airfoils. The problem is, once you get out of the drag bucket, the drag goes up a lot. In fact, in some occasions, you may want a 4 series airfoil versus the 6 series because at least on the 4 series, drag is relatively smooth. You can see this quite clearly on a drag polar view.

So basically, you still need to know what you will use the wing for. I've heard of the 6 series being used in F3A (ex 63012) and racing 67xxx. Of course, the higher second number is the location for the maximum thickness, which may decrease drag but make a smaller drag bucket.


Hope this helps.

Butch
To add to Butch's excellent analysis, laminar flow and the "drag bucket" are very sensitive to Reynolds' Number and surface imperfections.

Our models have extremely small RN's, and for such RNs, laminar flow is very difficult to establish and maintain... It simply doesn't work very well in model scales.

Also, any irregularity in the wing surface can destroy laminar flow, negating any drag benefit. Real world practice has shown that rivets, bugs on the leading edge, dings or dents in the surface can destroy any laminar flow and associated drag reduction even for FS RNs. So much for the additional R&D dollars spent in that new wing section that looked so good on paper (Ask Piper about the ho-hum performance of their Tomahawk's GAW airfoil) :D

For a long distance, straight line record-breaking attempt, designing an airplane to take advantage of a laminar flow airoil might make sense, because an airfoil could be chosen to fit the cruise lift coefficient and its drag bucket would be present for the design conditions. However, any benefit is lost during maneuvering as the lift coefficient varies from its straight flight value... Since we spend most of our flying time maneuvering, IMO such an airfoil is a solution in search of a problem... ;)

FYI, here is a link to the 1945 "bible" of airfoil design, also downloadable as a PDF document... Makes for some interesting, if not entirely applicable, reading...

Summary of Airfoil Data by Abbott, Von Doenhoff and Stivers

http://naca.larc.nasa.gov/reports/1945/naca-report-824/

Cheers!

Jim



BMatthews 07-24-2006 06:29 PM

RE: NACA 6 Series airfoil
 
For a good thick and highly stall resistant option I'd suggest the Selig 8036 or 8037. Both these have a good reputation for being tolerant of heaver wing loadings and very stall resistant with a softer stall effect with "normal" weight models.

The 6 series should be avoided for the reasons stated in the excellent posts above.

3D XR 07-24-2006 11:33 PM

RE: NACA 6 Series airfoil
 
I don't know if this will help much, but in addition to flying different models, I use Realflight G2 simulator to constantly experiment with different wing airfoils. Symetrical wings are better for inverted aerobatics, thicker wings are better for slower flight and easier stalls. The latest airfoils for 3D and pattern flying have tapered leading and trailing edges, and profiles that get smaller towards the wing tip. Weight, speed, type of flying, washout, landing qualities are other aspects to consider. Can you find a similiar factory built plane to compare flying qualities?

martinoto 07-25-2006 09:28 AM

RE: NACA 6 Series airfoil
 
Thaks to all of you, that was exactly what I was loking for.

Jimmbbo, I already have and read (sort of) the original book from Abbott, Von Doenhorf and Stivers, but I couldn't find anything there related to low RN experience on this particular airfoil. Simmons book was very usefull in this matter also...
(By the way, I flew more that 100 hours on Tomahawks, and I was very happy with the overall flying qualities, but I wouldn't try a stall lower than 10000 feet.... I think it has to do with the t-tail rather that the GA(W) airfoil thou....)

3D XR, I don't have any simulator, but now that you said that it is possible emulate some flying performance, I will definitely think of getting one.... I understand that GP Ultrasport 40 Plus kit has a 6 series airfoil, but this is the only example I had, and I never saw it flying...

BMatthews, I will take a look to those... what I want is a symmetrical airfoil, slow and predictable at the same time... I forgot to mention that it will be a constant chord wing....

Thanks again...




dolanosa 07-25-2006 12:27 PM

RE: NACA 6 Series airfoil
 
I don't know what airfoil the US40 uses but I can tell you that it flies great. The only problem, if you can call it that, is that it will float for a long time on landing. It just doesn't want to land. Also, it tucks in under knife edge. Other than that, if I still have the plans, I would build another one in a heartbeat not because it does pattern or 3D but it's a great all-around plane, much like the Sig 4-star series or even the Avistar.

I do remember some guys a couple of years back using either a 63012 or a 64012 for a pattern wing.

BMathews is right though. Selig has some great airfoils designed especially for low R numbers. I think Great Pains used him to design the wing for their Gold Edition "Jug" which flies superbly.

B

Jimmbbo 07-25-2006 12:31 PM

RE: NACA 6 Series airfoil
 
Quote:

ORIGINAL: martinoto

Thaks to all of you, that was exactly what I was loking for.

Jimmbbo, I already have and read (sort of) the original book from Abbott, Von Doenhorf and Stivers, but I couldn't find anything there related to low RN experience on this particular airfoil. Simmons book was very usefull in this matter also...
(By the way, I flew more that 100 hours on Tomahawks, and I was very happy with the overall flying qualities, but I wouldn't try a stall lower than 10000 feet.... I think it has to do with the t-tail rather that the GA(W) airfoil thou....)

3D XR, I don't have any simulator, but now that you said that it is possible emulate some flying performance, I will definitely think of getting one.... I understand that GP Ultrasport 40 Plus kit has a 6 series airfoil, but this is the only example I had, and I never saw it flying...

BMatthews, I will take a look to those... what I want is a symmetrical airfoil, slow and predictable at the same time... I forgot to mention that it will be a constant chord wing....

Thanks again...

martinoto -

I agree - Abbott and Von Doenhoff's data is suggestive but not definitive for model scales... Would be great if there were a companion version for low RN flight...

The GA(W) was '70s leading edge technology (pun intended ;) ) when Piper chose it for the Tomahawk. they designed the airplane after interviewing thousands of CFIs and got most of the design right, but the GA(W) wing as built by Piper has shown unpredictable stall characteristics and been the target of a number of "fixes" (including a marginally successful stall strip Airworthiness Directive). The GA(W) plot shows sharper stall characterisics than the tried and true NACA 2412, and was used intentionally to increase pilot awareness of stall dynamics (which it arguably does).

After giving some 2000 hours of instruction in the airplane, including hundreds of stalls and spins, I can attest to the fact that stalling a Tomahawk is like a box of chocolates - you just never know what you're gonna get. And I agree that altitude is your best friend, and would add that watching the T tail during the stall is ill-advised for the weak of stomach ;) Never flew the similar Beech Skipper to see if it behaved similarly, but would like to hear from someone who has...

Cheers!

Jim



BMatthews 07-25-2006 03:12 PM

RE: NACA 6 Series airfoil
 
If you're looking at symetrical and want a predictable but delayed stall that works at model speeds look at the Eppler 472 473 and 474. They work nicely at our speeds and offer a nice speed reducing higher drag in dives thanks to the thickness and forward high point.

dolanosa 07-25-2006 05:21 PM

RE: NACA 6 Series airfoil
 
1 Attachment(s)
This is for the people who want a visual on what we're talking about. Hope this helps.

In this trial version of SNACK, I copied a clip of the drag polars for three airfoils with basically the same thickness: the NACA 0016, NACA 63016, and the Eppler 473, all at a R# of 300000.

To those who don't know about the drag bucket, the 63-016 foil shows it very well. It's when the change in drag is less compared to the relative change in lift. If you take a look at the 63-016 airfoil at a CL of .5, you notice that the drag plot changes abruptly. The higher the CL, the more flat the drag plot becomes, (drag increases a lot). Look below .5CL, the drag plot goes nearly vertical which means that the rate of change of drag is less per same change in lift.

In this case, once the airfoil breaks 3.5 degrees AoA, drag goes up quickly and overtakes the Eppler at around 6 degrees AoA. What does this mean? Well, say, you're pylon racing with identical planes and the only difference is the airfoil. In theory, during the turns, once you hit 6 degrees AoA and more, the Eppler-foiled plane will outrun your 63012-foiled plane. When you go on the straight runs, the 63012 plane will catch up and presumably overtake the Eppler plane. Of course, this is all theoretical and there are many more variables to consider but noobs should get the gist.

Hope this helps.

B





rmh 07-25-2006 05:44 PM

RE: NACA 6 Series airfoil
 
I love to look at all the numbers and airfoils
As far as to how they work on a given craft --is much like the commentary on the Piper Tomahawk.

Using a shape that someone has assigned a number to - makes some guys feel more comfortable
The actual value certianly varies
especially on models
Sometime - wander down the line at a really big model aerobatic meet -and look at the wing shapes
usually they fall into an area group (sq In) but thickness and entry design -are all over the place
now, watch the planes fly - note if any of the "airfoils" have any edge over the other
the planforms and areas and thicknesses all are reasonably close to each other but the airfoils? - there are a million of em

Tall Paul 07-25-2006 06:03 PM

RE: NACA 6 Series airfoil
 
A smooth curve from front to back, an entry radius suitable for the task, thickness the same..
That's all that's needed.
After building a lot, the shapes get to look an awful lot like the same, for almost every task.
It's mostly pilot, in models.

Jimmbbo 07-25-2006 06:14 PM

RE: NACA 6 Series airfoil
 
Quote:

ORIGINAL: Tall Paul

A smooth curve from front to back, an entry radius suitable for the task, thickness the same..
That's all that's needed.
After building a lot, the shapes get to look an awful lot like the same, for almost every task.
It's mostly pilot, in models.
My kinda design...

Yep... in model aviation, there's no substitue for a pilot skill except lotsa power :D

dolanosa 07-25-2006 09:14 PM

RE: NACA 6 Series airfoil
 

Quote:

ORIGINAL: dick Hanson

Using a shape that someone has assigned a number to - makes some guys feel more comfortable
The actual value certianly varies
especially on models
Yeah, I feel comfortable with numbers. I know these are'nt human-carrying aircraft but the numbers still mean a lot. UAV designers don't just pick a number out of the air when lives are at stake. Yes, these are just toys...no matter what. You made a business making toys so numbers don't matter as long as the planes fly. You can go by experience but where did your airfoils come from? Did you invent them? I really don't think so. There's definitely a place for experience only...and that's only if you're not comfortable with numbers.

Bye

rmh 07-25-2006 10:29 PM

RE: NACA 6 Series airfoil
 
I started with Buster Brown and Tige - and graduated to Florshiem Imperial
nice curves
One of my fond memories is from one of the wind tunnel designers at The old Convair factory .
he asked "where do these airfoils really come from".
Seems most were from a guy who had some really nice ships curves---

Jimmbbo 07-26-2006 12:21 AM

RE: NACA 6 Series airfoil
 
Quote:

ORIGINAL: dolanosa


Quote:

ORIGINAL: dick Hanson

Using a shape that someone has assigned a number to - makes some guys feel more comfortable
The actual value certianly varies
especially on models
Yeah, I feel comfortable with numbers. I know these are'nt human-carrying aircraft but the numbers still mean a lot. UAV designers don't just pick a number out of the air when lives are at stake. Yes, these are just toys...no matter what. You made a business making toys so numbers don't matter as long as the planes fly. You can go by experience but where did your airfoils come from? Did you invent them? I really don't think so. There's definitely a place for experience only...and that's only if you're not comfortable with numbers.

Bye

IMO the biggest advantage of using a numberd airfoil section is the ability to generate the coordiantes for making a rib, especially if the wing is tapered ;) Other than that, models have such insanely huge power to weight ratios that engine power far overshadows differences in wing section wing selection, as Dick mentioned in post #11

Cheers!

Jim

da Rock 07-26-2006 07:34 AM

RE: NACA 6 Series airfoil
 
One of the best laboratories for symmetrical airfoils that ever existed is control line combat .

Every "test airfoil" was flown in an environment where it's speed was very easily and very accurately measured. And most airfoils were directly compared to another, over and over and over. And their speed, maxCL, and induced drags were judged easily in direct comparisons. And we even got a feel for the low speed stall characteristics (when we deadsticked to a landing) every flight (every flight that ended with a whole airplane, that is).

And one thing that stood out...........

Almost everybody who flew much wound up drawing their own airfoils. And most of them found out that there wasn't much to it, nor could you get much out of it. If two different airfoils looked somewhat alike, they'd fly about the same. Building a smooth wing did way more than having the max thickness somewhere else than 25-30%

And btw, most of us can't build a wing that will accurately place a hightech airfoil on the model. In the size range we're working, even if we started with the most accurate foam blank our sheeting balsa is going to vary in thickness and we're going to wind up sanding some areas more than others and the sheeting isn't going to bond in some places and our trailing edges are NEVER going to have the right thickness and.... and.....

rmh 07-26-2006 08:33 AM

RE: NACA 6 Series airfoil
 
when all is said and done -- the single most provable difference?
absolutely twist free structure.
At least that is my finding

I saw differences - subtle but the real improvements came from shaping the wing and aileron for twist free results

martinoto 07-26-2006 09:38 AM

RE: NACA 6 Series airfoil
 
I DO believe in importance of the airfoils in RC models, mostly because of what I saw and flew and heard all this years.

I am not saying any given airfoil would give the plane a crucial change of behavior. I say the change will be significant, up to the point it will worth to do some research. However, I have to say that I saw planes with fully symmetrical airfoils doing poorly in basic aerobatics, compared with my actual sport plane with its humble semi-symmetrical wing, so there are more factors to b e consider.


I do not think it is useful to compare C/L with R/C planes. C/L doesn't have to cope with short runaways, or stepped descends, or landing speed, or tip stall. Even the wind is a different aspect on that type of flying. So, it is not surprise that airfoils by itself wont produce any significant change.

I honestly think my wings ar at least as accurate as those from the P-51's or Corsairs made in a rush in WW2, considering the technology available those days, so I believe that most of those numbers would apply to models, considering all the other factors.

So after reading all your valuables posts here, my conclusion is we shouldn't expect definitive change of flying characterisitics in changing airfoils, but we should expect some basic characteristics to apply, like stall and speed, and that what I looking for: an airfoil that,. basically, would help me in designing a plane with predictable stall and speed behaviors.

Cheers.

martinoto 07-26-2006 04:07 PM

RE: NACA 6 Series airfoil
 
Dolanosa, thanks for the graphics and the explanation, I can see your point....

A question: as per the graph, it seems that NACA 0016 has a more "linear" behavior throughout the changes of AoA.
May I conclude that this reflects a more predictable behavior??

Thanks and regards,

da Rock 07-26-2006 05:23 PM

RE: NACA 6 Series airfoil
 
Quote:

C/L doesn't have to cope with short runaways, or stepped descends, or landing speed, or tip stall. Even the wind is a different aspect on that type of flying. So, it is not surprise that airfoils by itself wont produce any significant change
Actually, C/L does deal with all those. The combat wings didn't have throttles, but you could easily land them fast or medium or slow. The fact they were always deadstick landings had nothing to do with how fast you landed them. You landed them with a fast long descent if they shut off "away" from your pit crew. You landed them with a short descent if they shut off near your pit crew. You often had to stretch a landing. As a pilot you got to FEEL how the wing was working while you were doing all that.

People who are not familiar with C/L often think it is way more simple than it is. One of it's beauties is that the pilot could also feel how the plane was flying. And that wasn't just the pull on the handle. When you flew Riley Wooten's VooDoo you could feel how tight it could loop and how much that cost you in speed. You open up the loop and you could feel how that affected the speed. And you could hook on another design and have it to compare in identical flying conditions in 2 minutes time from landing the VooDoo and launching the newer design.

You could feel if the wing was skidding or not. You often experienced "tip stalls". You'd get them at high speed and low speed. And you could fly "slow combat" and test those same airfoils at a completely different high speed range.

And I've only mentioned what? two details.

rmh 07-26-2006 09:51 PM

RE: NACA 6 Series airfoil
 
yeh -I learned control line stunt in 1949--and refused to do RC with the toggle switches as these had no real feel feedback
when analog RC came in I went for it and of course learned the " eye/hand feel that occurs with practice

da Rock 07-27-2006 09:57 AM

RE: NACA 6 Series airfoil
 
Mr Hanson,
I'm not surprised that you flew c/l, but am surprised when you started. ;) I had already guessed that you'd started back when the control lines were made out of dried brontasaurus guts. ;) You old guys did have it rough. When I started we'd discovered bronze and could pound out a set of lines in under a week.

I sometimes wish for the very accurate "laboratory" that CL provides. It sure would be nice to be able to so very accurately time my RC plane with a stopwatch for say 10 laps, or be able to directly compare one combat wing with a NACA0018 airfoil against the same planform/AR/chord/weight combat wing with a home made airfoil........ but then I think about turning around and around and around FAST!!!!!!!!!

nnnnnnnaaaaaaaahhhhhhhhhhhhhhhhhhhh

You're dead right about the practiced feel we can develop. And it's awesome to be flying a "free" bird.

Tall Paul 07-27-2006 11:07 AM

RE: NACA 6 Series airfoil
 
You don't have to fly in circles... I fly past a fixed video camera.
http://www.angelfire.com/indie/aerostuff/inflights.htm
I have the u/c flight path with r/c.. you get very dizzy very quickly! :)
The two HOB T-6s.. It was flown with the kit flat bottom wing, and a cheater NACa 2412 wing. Top speeds were the same. The 2412 wing was more manuverable.


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