HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Mar 21, 2011, 05:59 PM
He who dares ... repairs!
Hobster's Avatar
Joined Jun 2005
682 Posts
Discussion
Aerofoil analysis help

For several years I've had a Mini Graphite wing (MH32 airfoil) on top of a cupboard gathering dust. My plan back then was to fill it full of carbon and turn it into a DS hack. But then I got Opus MCTs and forgot about it. Recently I started playing with XFLR5 airfoil analysis software and ran some results..

What surprised me is that according to the software, the Mini Graphite's MH32 section is much lower drag than the Opus' RG14 at >300mph (Re 2.5e06 to 3.0e06 and Mach 0.4 to 0.5). Lower Cd even than Rk40 and DS19, popular sections for DS. Am I running XFLR5 too far from its design? Using the software wrong? Are there other aerofoil properties that also need to be considered?
Hobster is offline Find More Posts by Hobster
Last edited by Hobster; Mar 21, 2011 at 06:04 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Mar 23, 2011, 12:51 AM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
6,915 Posts
Hobster, I'm not sure what you did. I just ran 2 identical wings (3D analysis), one with the MH32 and one with the DS19 at 300 mph. The CL(CD) is different than your findings. The DS19 definitely has lower drag at low to moderate CL. In fact the MH32 wouldn't converge above 5* at this speed. I normalized and refined the airfoil before processing.

When you defined the polar did you use the same definition for each airfoil?
wyowindworks is offline Find More Posts by wyowindworks
Last edited by wyowindworks; Mar 23, 2011 at 01:02 AM.
Reply With Quote
Old Mar 23, 2011, 01:02 AM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
6,915 Posts
Here is the CL(CD) polar with the RK40 added. Both the DS19 and the RK40 have better performance with this wing at 300mph. This again is a 3D analysis.
wyowindworks is offline Find More Posts by wyowindworks
Last edited by wyowindworks; Mar 23, 2011 at 01:33 AM.
Reply With Quote
Old Mar 23, 2011, 01:31 AM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
6,915 Posts
Here is a 2D analysis at Re 1,000,000 and 2,000,000. Polar shows DS19, RK40, an MH32.
wyowindworks is offline Find More Posts by wyowindworks
Reply With Quote
Old Mar 23, 2011, 02:01 AM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
6,915 Posts
Hang on a second...

Oooops. I just realized that I was comparing modified airfoils.

Here is a 3D analysis of the original 3 foils on an average 2 meter wing at 300 mph. The MH32 has higher drag than both the DS19 and the RK40 at low to moderate CL.
wyowindworks is offline Find More Posts by wyowindworks
Reply With Quote
Old Mar 23, 2011, 03:45 PM
He who dares ... repairs!
Hobster's Avatar
Joined Jun 2005
682 Posts
Many thanks wyowindworks for replies. Your results look more like what I would expect.

I originally tried 'Normalise' and 'De-rotate' the foil but had little effect. This time I tried Normalise and 'Refine-globally' which appears to interpolate more points on the airfoil (so hopefully more accurate calculations)

The results are more bunched up but still show mh32 as low drag (at Re 2.0e06, Mach 0.4)

Guess I will have to read the manual!
Hobster is offline Find More Posts by Hobster
Reply With Quote
Old Mar 23, 2011, 10:45 PM
Registered User
Daemon's Avatar
Lakewood, Colorado
Joined Aug 2002
28,897 Posts
Maybe you've accidentally discovered a new airfoil masquerading as an MH32. Better post up the coordinates
before it is lost.

ian
Daemon is online now Find More Posts by Daemon
RCG Plus Member
Old Mar 24, 2011, 12:42 AM
Registered User
Sydney
Joined Apr 2009
716 Posts
Hobster I also did a quick check as a 2D comparison on XFOIL at around 300mph on an average 200mm chord which was 1.8 to 2,000,000 Re.
It supports Wyowind's comments both the DS19 and the RK40 have lower drag especially at < 0.5CL.
The MH32 is not too bad and would be easier to land perhaps although it would require lots more ballast to fly with similar L/D to the DS19 and the RK40.
Give it a try if you have it.
John
OZPAF is offline Find More Posts by OZPAF
Reply With Quote
Old Mar 24, 2011, 04:18 PM
He who dares ... repairs!
Hobster's Avatar
Joined Jun 2005
682 Posts
Quote:
Originally Posted by Daemon View Post
Maybe you've accidentally discovered a new airfoil masquerading as an MH32. Better post up the coordinates
before it is lost.

ian
Beginning to wonder, lol. Coords were downloaded from UIUC Airfoil Database.
Hobster is offline Find More Posts by Hobster
Reply With Quote
Old Mar 24, 2011, 04:28 PM
He who dares ... repairs!
Hobster's Avatar
Joined Jun 2005
682 Posts
Quote:
Originally Posted by OZPAF View Post
Hobster I also did a quick check as a 2D comparison on XFOIL at around 300mph on an average 200mm chord which was 1.8 to 2,000,000 Re.
It supports Wyowind's comments both the DS19 and the RK40 have lower drag especially at < 0.5CL.
The MH32 is not too bad and would be easier to land perhaps although it would require lots more ballast to fly with similar L/D to the DS19 and the RK40.
Give it a try if you have it.
John
Thanks John.
Your graph appears to shows that mh32 is lowest Cd between about Cl 0.4 and 0.8. According to my alpha/cl graph, that equates to AoA of about 0.5 to 4 degrees. So it might be quick if I dont yank the turns too hard. Perhaps I should just give it a try as you suggest.

I was originally most interested in mh32 vs rg14 (9% ish) to compare against the Opus wing, so interested in any comparison against that if any one has the time

Thanks again, Steve
Hobster is offline Find More Posts by Hobster
Reply With Quote
Old Mar 24, 2011, 06:32 PM
Registered User
Sydney
Joined Apr 2009
716 Posts
Steve, these should help.Note the RG14 appears to have an edge on the DS 19 at CL's >0.3.
Girsberger certainly designed some classic foils.
The DS19 indicates however why it is so popular. A very fast and forgiving airfoil from these plots and would carry more weight.
John

PM Just discovered this on a reread.I Stumbled a bit here Steve wrt to the drag of the RG14. The RG14 has less drag(should have shown< instead of >) than the DS19 below approx 0.3CL - which may not mean much as Adam points out below.
My guess would be that the DS19 would be the better overall choice.
OZPAF is offline Find More Posts by OZPAF
Last edited by OZPAF; Mar 27, 2011 at 01:39 AM.
Reply With Quote
Old Mar 24, 2011, 07:07 PM
He who dares ... repairs!
Hobster's Avatar
Joined Jun 2005
682 Posts
Many thanks John
Hobster is offline Find More Posts by Hobster
Reply With Quote
Old Mar 24, 2011, 07:17 PM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
6,915 Posts
BTW, a 2M with 480 square inches flying 300 mph, weighing 80 ounces, and pulling 50 G, requires a CL of .326. If you hit 80G you need a CL of .521. Flying the MH32 tighter may be the faster way to fly it...if it can actually get up to 300.
wyowindworks is offline Find More Posts by wyowindworks
Reply With Quote
Old Mar 31, 2011, 05:21 PM
Registered User
Hot Air's Avatar
United Arab Emirates, Dubai
Joined Sep 2009
120 Posts
Quote:
Originally Posted by wyowindworks View Post
BTW, a 2M with 480 square inches flying 300 mph, weighing 80 ounces, and pulling 50 G, requires a CL of .326
Assuming a flight path of a circle (not quite real but a rough approximation of DS) and std density of 1.225Kg/m^3, using the above data, you would need to fly a circuit every 1.7 seconds. I don't think even aliens like Spencer L could do that.

Flying the same weight and wing area plane at 300mph in a circle at a more human lap time of say 4s would require a CL of approx 0.14. If you inherited SL's flying skills and flew 3.3s laps, a CL of approx 0.17 would be required.

The point I hope to illustrate is that, despite the tremendous 'G' forces, because of the high speed the actual CL or alpha required to generate the necessary lift force is surprisingly low. L= CL 1/2 p v^2 S. Because the value of v^2 is very large, in order to provide the required lift force the value of CL is low. Therefore, provided you can launch and land, an aerofoil with lower drag at low CL should be better.

I worked it out thus:

300mph = 134 m/s. Circuit time = 4s, therefore circumference is 134 x 4 = 536m.

radius = c/2pi so 536/2pi = 85m

Centripetal acceleration = v^2 / r so 134^2 / 85 = 211m/s^2
Force = mass x acceleration (80oz = 2.27Kg) so F = 2.27 x 211 = 479N. This is close as dammit to the lift force required to fly the plane in the assumed circle. You can work out the total lift vector required to account for the vertical component to overcome gravity but you will find the difference it makes is only about 1N so can be considered negligible during these extremely hard turns.

Rearranging the lift formula we get CL = L / .5 p v^2 S
(480sq in = 0.31m^2) so CL = 479 / .5 x 1.225 x 134^2 x .31 = 0.14

If we be more realistic and use a racetrack pattern to simulate our DS path, this does require tighter radius turns. If we approximate a racetrack using straight edges, say 30m each, we end up with a 4s circuit requiring a CL of 0.16, still a low value.

I hope this is of interest.

Best regards

Steve (another Steve, there are plenty of us in the DS world!)
Hot Air is offline Find More Posts by Hot Air
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion aerofoil programs anzacjack Flying Wings 2 Oct 18, 2010 02:39 PM
Discussion Help in choosing UAV aerofoil breezeit UAV - Unmanned Aerial Vehicles 11 Mar 03, 2010 06:11 AM
Discussion Aerofoil help breezeit Flying Wings 5 Feb 02, 2010 08:59 AM
Please help me choose and Aerofoil Lewist Modeling Science 6 Nov 23, 2005 03:32 PM
I need help choosing an aerofoil! Lewist The Builders Workshop 0 Oct 31, 2005 09:36 AM