HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jun 18, 2014, 01:57 PM
Registered User
FlyRob's Avatar
United States, CA, Orange County
Joined Jun 2014
8 Posts
Discussion
How did the F3B/F Airfoils evolve in the last 10/15 years ?

Hi,

What has been the evolution of the airfoils used for F3B/F in the last 10/15 years and what is the current trend ?

From what I read here and there, I understood that the airfoils evolved to privilege high speed at low lift.
Is it true ? If yes, why low lift; is that a way to lower the drag drastically ? And why do the F3B/F planes can carry more ballast than ever before ?

How do the pros develop their airfoils, do they "play" with X-foil to change the characteristics here and there and see if the polars get better or do they use some kind of programs to do an automatic optimization ?


I'm new to RCgroups and did search on this forum (and Modeling Science forum) about this specific subject but didn't find much, pardon me if I missed THE thread and if my question is a repeat.

FlyRob
Smooth winds to all
FlyRob is offline Find More Posts by FlyRob
Reply With Quote
Sign up now
to remove ads between posts
Old Jun 18, 2014, 04:34 PM
El picudo del agave
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,692 Posts
The trend toward low lift airfoils and high ballast capacities are both largely the result of snap-flap mixing. These models spend the vast majority of their flight at very low CL, and a small portion (turns and launches) at very high CL. Snap-flaps allow the design to be optimized toward the low-lift regime yet still have excellent high-lift performance.

I interchange lift and CL above but the difference is important. Generally lift = weight but CL varies with 1/airspeed^2. Hence you can see why a fast plane must operate at very low CL. Airfoil design is largely a game of shifting the CL at which peak airfoil efficiency (L/D) is achieved. Most designs try to align that peak with the most common operating condition (e.g. cruise) but that comes at a cost to the structure, handling, and off-design performance.

Automatic airfoil optimization is an evolving science. Most F3X designers just do it manually by starting with a Drela, Heppler, or Selig airfoil that is known to perform well in the application and using Xfoil or similar to make small adjustments to the camber, thickness, and high point to shift that efficiency peak toward their particular speed/weight/size and/or narrow or widen the efficiency peak to improve speed or handling. Then that airfoil is usually tweaked throughout the span to address Reynolds number and handling concerns typically thru reduced thickness and camber toward the tip.

I believe evolution is toward thinner foils which offer lower drag for obvious reasons but have structural, handling, and max CL issues. These issues are slowly being mitigated by clever planform design, better Re tailoring, more accurate molding, and better structures.
vespa is online now Find More Posts by vespa
Reply With Quote
Old Jun 19, 2014, 01:04 PM
Phil.T-tailer
Phil.Taylor's Avatar
Devon, UK
Joined Jun 2003
4,262 Posts
Quote:
Originally Posted by FlyRob View Post
From what I read here and there, I understood that the airfoils evolved to privilege high speed at low lift.
Is it true ? If yes, why low lift; is that a way to lower the drag drastically ? And why do the F3B/F planes can carry more ballast than ever before ?

FlyRob
Smooth winds to all
Because some folks realised that the F3B speed task was where you could still gain an advantage - with clever design - and full ballast. Speed task = (very) low lift. So - airfoils are now optimised for the speed and distance tasks, rather than duration. As Vespa says - add in a bit of camber flap - and a good pilot & thermal help - and you're still good for the duration task too. For F3F - the same combination also works too - you need speed, lots of ballast when the wind blows, and turning ability assisted with snap-flap.

Hope that helps - without getting too technical?

Actually - the trend towards F3B speed started a much longer time ago with the RG15 type ships - but modern airfoil software has added science to the quest

Phil.
Phil.Taylor is offline Find More Posts by Phil.Taylor
Reply With Quote
Old Jun 19, 2014, 02:59 PM
Registered User
FlyRob's Avatar
United States, CA, Orange County
Joined Jun 2014
8 Posts
Quote:
Originally Posted by Phil.Taylor View Post
Actually - the trend towards F3B speed started a much longer time ago with the RG15 type ships - but modern airfoil software has added science to the quest
Phil.
Thanks for the answers.
Phil, about what you said, there is an interesting read here:
http://users.cybercity.dk/~cws19202/...tory_class.htm

I'd like to read a discussion like that about the last airfoils used in F3B. It would be so interesting to know how those airfoils are developed to get good speed, good launch with flap, etc...
FlyRob is offline Find More Posts by FlyRob
Reply With Quote
Old Jun 19, 2014, 04:07 PM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
5,581 Posts
pffffft. Aerofoils are intelligently designed..........
satinet is offline Find More Posts by satinet
Reply With Quote
Old Jun 19, 2014, 05:37 PM
Self confessed Aeroholic
Larrikin's Avatar
Australia, QLD, Elimbah
Joined Jan 2005
1,665 Posts
Have we really come that far in profile design or is it all about marketing??

Does one need to reinvent the wheel?

From THIS BLOG we read ...

"From this experience, Girsberger de­signed the RG 12 and the RG 14 which he considered as the most promising airfoil for F3B. Additionally he de­signed a more conservative (my emphasis) airfoil as he had realized that a computationally over-optimized airfoil can lack perform­ance in reality. This airfoil was the RG 15 and he published the results in 1983 for the first time."

I think it's the transitioning of profiles to suit local re and the better construction materials and techniques that has made any performance improvements.

David

For further reading, The Fletcher Design Study by Frits Donker-Duyvis. A real pioneer in transitioning profiles.

"Main reason to use an airfoil based on RG14, apart from the low drag at low lift coefficients, was the positive effect expected from this airfoil together with a broad 28.5 % flap."
"At +2 degrees flap setting the pressure distribution shows much similarity with the MH32 or MH885215 (Hepperle) airfoils which promise very good distance flight performance"

Edit ; I got the images reversed but you get the idea
Larrikin is online now Find More Posts by Larrikin
RCG Plus Member
Last edited by Larrikin; Jun 19, 2014 at 06:27 PM. Reason: images reversed
Reply With Quote
Old Jun 20, 2014, 08:08 AM
F3B and F3K
RetoF3X's Avatar
United States, TX, Dallas
Joined Mar 2009
1,699 Posts
Long live Rolf Girsberger. I had the pleasure to interview him before the 2007 F3B worlds, which lead to one section on the linked blog above.

He obtained micrographs for the only in the US published Eppler code via ETH Zurich and wrote it on punchcards for the "Supercomputer" they had at work (a turbomachinery company). A week later he got a stack of paper with the results, from which one had to draw the polars from hand on mm-paper.

Pretty amazing feat to come up with such great airfoils given the cumbersome process back then.

Andreas Herrig documented the design process for the Freestyler 1 F3B/F plane in the german magazine "Aufwind" and at one point he wrote: "we could have used a modified RG 15 airfoil, but wouldn't that be boring?" Then he elaborated how they designed their airfoils.

Reto
RetoF3X is offline Find More Posts by RetoF3X
Last edited by RetoF3X; Jun 20, 2014 at 01:40 PM.
Reply With Quote
Old Jun 20, 2014, 08:38 AM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
5,581 Posts
It's a good point about ballast carrying. The planes of today are faster AND they carry more ballast (even faster).
satinet is offline Find More Posts by satinet
Reply With Quote
Old Jun 20, 2014, 01:48 PM
F3B and F3K
RetoF3X's Avatar
United States, TX, Dallas
Joined Mar 2009
1,699 Posts
Hey Satinet,

You were not flying B at the end of the nineties? The MH 32 bombers (Calypso Cobra, Tragi, Donner, Wobbegong, while not a MH32, still a crazy launching plane) back then were able to carry lots of ballast too (4kg and more). Actually you had to, else speed was no fun.

For the ability to launch heavy and high, the launch equipment has improved too. Megaline stores much more energy than say speedline or other types some 10-20 years ago. Plus older planes were more flexible than todays planes, which makes a huge difference in launch potential in wind.

Higher aspect ratios like on the Fosa or Avatar have also only recently become possible with new materials and construction*. My Fosa Lift is the best launching plane I have ever had, so the high AR seems to help a bit.

Best,
Reto

*Okay, the original Caracho by Weberschock had a pretty high AR though and that was ten years ago. But it had a one piece 3.3m wing, ouch.
RetoF3X is offline Find More Posts by RetoF3X
Reply With Quote
Old Jun 20, 2014, 02:10 PM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
5,581 Posts
Hi Reto
No I started f3b about 3 years ago (flying since 06 iirc)
It does depend on the model but I find the new models carry ballast well say compared to my tragi.

That being said maybe people are just trying more with ballast than in the past. E.g 1kg was a ballast set. Especially in f3f.

F3f has seen a lot of low times in the last few years in the uk.

Thanks for your insight Reto
satinet is offline Find More Posts by satinet
Reply With Quote
Old Jun 20, 2014, 05:33 PM
Registered User
wakumann's Avatar
Canada
Joined Jul 2003
2,587 Posts
Quote:
Originally Posted by satinet View Post
It's a good point about ballast carrying. The planes of today are faster AND they carry more ballast (even faster).


Really,
the Ellipse 1/2 was pretty fast with the RG 15 mod and could be ballasted to FAI max .
The larger fuses at this time also helped to carry more load, today it has to be Tungsten to get use of the space and sometimes they don't reach max weight .
or they flutter early ( Reto can comment

Cheers
Thomas
wakumann is offline Find More Posts by wakumann
Reply With Quote
Old Jun 20, 2014, 05:36 PM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
5,581 Posts
I don't think rg15 models are fast in contest flying any more.
satinet is offline Find More Posts by satinet
Reply With Quote
Old Jun 20, 2014, 05:52 PM
Self confessed Aeroholic
Larrikin's Avatar
Australia, QLD, Elimbah
Joined Jan 2005
1,665 Posts
Quote:
Originally Posted by satinet View Post
I don't think rg15 models are fast in contest flying any more.
Maybe ....

It would be an interesting exercise. Make a Fosa Lift or Freestyler 4 with CURRENT TECHNLOGY and MATERIALS using an RG15 an see how "slow" it really is.

D.
Larrikin is online now Find More Posts by Larrikin
RCG Plus Member
Latest blog entry: The Rubicon
Reply With Quote
Old Jun 20, 2014, 07:37 PM
launch high, go fast
luvF3b's Avatar
Australia, TAS, Devonport
Joined May 2008
155 Posts
Modern profiles do not have a concave shape on the bottom surface near the hinge line. This allows them to retain laminar flow on the bottom surface when operating close to zero lift (going fast). They are also thinner and have lower camber.

I've just run the polars for reflex flapped RG15 and a "modern profile", comparing the drag coefficients at CL = 0.02 (going fast) RG 15 0.0057, while the modern profile was 0.0047. This is a 17.5% drag reduction!

Modern construction materials have allowed thinner profiles and improved tortional stiffness. This keeps the profiles at the correct angle of attack, reducing the natural tendency of the tip to twist nose down, which creates lots of drag.

Optimization of the profiles across the wing span as wing chord changes and arranging for all profiles to hit zero lift at the same time across the span also helps with high speed performance.

I'd wager that the modern models are far quicker than those of yesteryear !

John
luvF3b is online now Find More Posts by luvF3b
Last edited by luvF3b; Jun 20, 2014 at 07:52 PM. Reason: Clarity
Reply With Quote
Old Jun 20, 2014, 09:31 PM
Self confessed Aeroholic
Larrikin's Avatar
Australia, QLD, Elimbah
Joined Jan 2005
1,665 Posts
Thanks John,

I have no doubt that there has been advances ... no question. However, I suspect that only top level pilots would have the skills to extract the improvements.

If I had a Freestyler 4 and either you, Tim, Carl or
Mike R. etc had a V-Ultra, you'd all still kick my ar$e

But I guess that's not the argument, is it?

Larrikin is online now Find More Posts by Larrikin
RCG Plus Member
Latest blog entry: The Rubicon
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion How to prep a nitro not touched in 10 years jens0771 Car Talk 1 Mar 11, 2014 12:35 PM
Discussion How Many Times Did You Crash This Year? tyhollin Electric Plane Talk 50 Jan 03, 2014 07:47 AM
Discussion Any advancements since last year in finding a lost model? TheNiceGuy FPV Talk 18 Nov 30, 2013 03:59 AM
Discussion F3B Clinic in SOCAL, Perris CA Sunday 10-13-13 djklein21 Multi Task F3X 59 Oct 18, 2013 12:26 AM
Poll How Much Profit Did Health Insurance Companies Earn Last Year? Big Foot 48 Life, The Universe, and Politics 20 Oct 28, 2009 11:17 PM