Dec 19, 2010, 06:17 AM Registered User Joined Dec 2007 22 Posts Discussion Relative importance of airfoil at 350+ If say a K100 is doing 400mph on a particular day and we could somehow push a button and change the airfoil section to a 'regular' non-DS optimized airfoil - say one of the F3F HN airfoils - how much slower would it go? (every other variable the same as far as possible) Want to understand the relative importance of an airfoil optimised for DS at these high Re's.. are we talking 5% difference, 1% difference or 20%? The only F3F model to even approach the speeds of the dedicated DS-designed machines is the Compact Wizard - is that down to the SD2030 section - or the fact that no other F3F model has been built sufficiently strong?
 Dec 19, 2010, 10:23 AM Registered User United States, CA, Los Angeles Joined Oct 2003 1,310 Posts I think it's considerable if all else is held constant. There are certainly alot of variables but it you compare airfoils in XFOIL at likely operating points, you can see 20-40% reductions in cd. Figuring that cd amounts to roughly half the drag picture along with induced, fuselage, and interference+other drag, that nets in a 10-20% difference overall. The other thing is that the F3F airfoils are typically thinner making it hard to get them strong.
 Dec 19, 2010, 12:06 PM Here comes the wind! United States, CA, Ventura County Joined Sep 2008 3,416 Posts The fast compacts were both very strong and heavy compared to any F3F plane I have seen. The compacts are easily double the stiffness of an MCT. The extra mass and strength permits the plane fly to much faster. The only limiting structural factor in a compact appears to be the joiner. The thin airfoil won't ever be able to accept a meaty joiner, but other than that the plane is a great example of how building heavy equates to more speed.
Dec 19, 2010, 11:50 PM
Just chillin @ 403 MPH
Joined Nov 2005
5,304 Posts
Hunh??

Quote:
 Originally Posted by Aerogance The only limiting structural factor in a compact appears to be the joiner.
There has never been a failure with the wizard compact DSX joiners so I don't understand why you think there is a limiting factor there

Here is a link to a pic of the Joiner system
http://www.rcgroups.com/forums/attac...mentid=2070633

Nick

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Dec 20, 2010, 03:54 AM
Registered User
Joined Aug 2002
30,722 Posts
346mph off angle to the gun turning 2 - 2 1/2 second laps. Don't see a joiner problem there at 50+ Gs.
 346mph - Wizard wound up tight (0 min 33 sec)

The K100, TT, Wiz DSX and K130 all flew within an hour of each other on the last
WR morning at Weldon, but the wind speed was highest for the first flight of the K100 and
dropped off as time passed so we never got a totally apples to apples comparison.
My subjective impression though, is that at these speeds the Kinetic is significantly
slipperier than the Wiz, and it carries its energy through bigger laps.

ian
Dec 22, 2010, 11:09 AM
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Joined Dec 2007
22 Posts
Thanks for replies guys.

I have recently downloaded XFLR5. The attached shows one of DPs DS sections DP2,06-8,67_DS vs the Compact Wizard's SD2030 section. Both similar thicknesses.

Ran the foils at Re 1.5e06 to 2.0e06 and interestingly the Cd's arent greatly different though Cl much higher on the Wiz.

For contrast, I also ran DP section against MH32 at the same Re - and this time the Cd was about 15-20% lower for DP.

I am a newbie at aerodynamics so some of the default assumptions might not be applicable at these higher Re's - and no idea what the margin of error is for the calculations..

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Last edited by Slippery Slope; Dec 22, 2010 at 02:22 PM. Reason: spellin
 Dec 22, 2010, 06:04 PM launch low, fly high New Zealand, Hawke's Bay, Havelock North Joined Dec 2004 2,133 Posts Slip, the mach number is zero in our calcs. put in 0.5 or 0.6 for mach and see if your answers change any.