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Old Apr 29, 2014, 01:00 AM
Dave Pope Manawatu NZ
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Palmerston North NZ
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Planning stage.

The south north island boys are in the planning stage for a molded high aspect 2M DS machine.

Question? input needed for the pick of airfoils
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Old Apr 29, 2014, 03:17 AM
PB of 733kph
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Christchurch, New Zealand
Joined Jul 2005
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From recent trends and performance, something DP...

But as to which etc, hopefully someone else can help there
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Old Apr 29, 2014, 06:20 AM
Arrarrar!
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Australia, NSW, Wagga Wagga
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Hey Dave, I wish I lived over there as I would love to do a collaboration with you guys. I am toying with the idea too and had been wondering about airfoils- I cam to the same conclusion as Alex. Peter wick would be worth approaching too maybe, isn't he from NZ?
Why not stick with the rk40?
Another thing that Ive read is that a lot of the new generation foils have to be made super accurately eg machined not bagged to work correctly. Depending on how you do your plugs might be a factor for choice.
Cheers
Josh
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Old Apr 29, 2014, 07:13 AM
Just Stab It In
Bangalow,Australia
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Joe wurts lives in NZ. Not Peter Wick.
Joe would be the man to talk too.
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Old Apr 29, 2014, 02:03 PM
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^ agreed
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Old Apr 29, 2014, 03:15 PM
Where is the inspiration
wdeighton's Avatar
Moira
Joined Feb 2006
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Through pedigree I would go DP. He is contactable and I think he likes it too.
But all he does is wing section. Well for me at least.

When I was looking into this Joe Mannor made a point that profiles are overrated and that the plan form is paramount, I think they are of equal importance.

I think this is a really cool thing to do and hope you can pull it off
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Old Apr 29, 2014, 03:58 PM
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Wellington, New Zealand
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Dave, if you need some help with this, i'd be happy to offer what i can...
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Old Apr 29, 2014, 04:38 PM
PB of 733kph
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Christchurch, New Zealand
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Quote:
Originally Posted by wdeighton View Post
When I was looking into this Joe Mannor made a point that profiles are overrated and that the plan form is paramount, I think they are of equal importance.
Was talking to Joe Wurts about two weeks ago and he said the opposite. Planform (as long as you pick something thats pretty standard layout) isn't as crucial as foil choice. What he also said was important was reducing surface area on the whole airframe. More area, more friction sort of thing. So lots of fancy fairings etc also add to the overall surface area, hence aren't important.

That's my take on what he said
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Old Apr 29, 2014, 05:54 PM
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Australia, QLD, Elimbah
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Go Crazy

Personally, I'd use "DP 1.87/9.07_DS"

2M, narrow chord, still need thickness for servos and 9% offers more bending stiffness.

One man's thoughts ... good luck

D.
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Old Apr 29, 2014, 08:22 PM
It flies!
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To pick a good airfoil you'll need to know what speed range you're targeting. Airfoils are like gearing in a race car - you need to gear for the speeds you expect the car to be doing. All of the newer DS specific airfoils tend to become less and less efficient as the speed drops. Most of these foils become less efficient than a traditional foil (eg RG14) somewhere between 200 -300mph. All that means is that in air that's only good for 200mph, your plane might be slow/sluggish compared to some other planes.

You also need to know the approximate CL range you're aiming for. As a rule of thumb smaller, lighter planes will operate at lower CL, bigger heavier planes will operate at a slightly higher CL.

Those older, publicly available DP foils exhibit incredible performance in certain areas but the performance can fall off dramatically when you get outside the design parameters - basically that means if your design isn't suited to the foil then it could really suffer. A lot of the very fast DS foils are like this, eg the JM26 is incredible at very high reynolds numbers and mach numbers but at lower speeds it falls right off (it was designed for the D160 afterall).

The RK40 works across a wide range of DS speeds but its performance at low CL is not so good. This means any straight line flight can cost you speed, it needs to keep turning basically. Its performance starts to drop off at very high speeds too.

A high AR 2M will be operating at fairly low reynolds numbers even at 300mph, so unless you're aiming for records a really radical foil might not work well. For example, the good old RG14 will give significantly better performance at 200mph for a small chord plane compared to most of the newer foils, but will really start to suffer as speeds rise.

If your airfoil isn't perfectly suited to the task you'll probably still have a very fast plane, but the thing to keep in mind is that you could have had a faster plane if the airfoil was the best choice possible. This could mean 300mph instead of 280mph, for example. DSing tends to hide these kinds of differences because we don't fly man on man, and the fastest speed usually goes to the guy who got the best gust, but you still want to get the highest number possible on the gun for any given flight (well most DSers do anyway).

I agree with the advice to get in touch with one of the airfoil gurus. Most of them seem to be very approachable. They can tailor an airfoil for your design.
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Old Apr 29, 2014, 08:40 PM
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Fantastic post, Sean! Truly insightful!

I think the guys will be looking for a 200-300mph 2m model???
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Old Apr 29, 2014, 08:53 PM
It flies!
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Sunshine Coast, Australia
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To be honest, I don't know if there's anything publicly available that would be ideal for those speeds. There are a lot of airfoils that would be very good at those speeds, just not ideal. You're in the transitional area where the frontside style airfoils are sarting to drop off and the radical DS foils are only starting to get on step. Some of the not-publicly-available airfoils might be suitable (hint hint).

Another thing to note is the benefits of high AR all come in the moderate-high CL range. This means a high AR 2M will need to be heavier and/or flown with really aggressive tight turns to get the most from it. Lowering the AR will make it a little more sedate (if that's what you're after) and also make it a slightly bigger plane that will be heavier for a given wing loading.
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Old Apr 29, 2014, 09:17 PM
Arrarrar!
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Australia, NSW, Wagga Wagga
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This is a great thread- I really need to learn all that stuff!
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Old Apr 29, 2014, 09:36 PM
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How about a HN1038 foil like the Scratcho uses? Proven to 278mph on a 66" airframe. Flies great, nice handling, responds well to camber and also goes well inverted
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Old Apr 29, 2014, 11:10 PM
It flies!
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Sunshine Coast, Australia
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The HN 1038 would be a good choice. It has good drag characteristics across the entire CL range you're likely to see while DSing at 200-300, at the Re you'd see on a high AR 2M.

If you guys are at all interested, I've been playing with DS airfoils for the last few years. My bagged Deepend used one of my own creations and since then I've developed a better understanding of the requirements. I'm not at all an airfoil guru in the way that DP, PW, JW etc are, but I've developed a technique of blending existing airfoils, and tweaking the camber, thickness, shape etc to get the polars I'm looking for. Attached are some pics of the polars of two airfoils I developed a while back for the 200-300 mph speed range. I should say these are not based on the D3DP foils, DP asked me to keep them private and I've kept my word in that regard.

The polars attached show the airfoil performance at 300 mph for an average 12cm chord, which is about what you'd see on a very high AR 2M. SM DS 250 01 has excellent low drag at the CL which you'd typically fly a lightish 2M, and would benefit a lot from snap flaps to improve moderate CL performance. SM DS 250 06 has a wider CL range but a little more drag at lower CL. Performance above about CL=0.5 is not relevant. All airfoils could be improved slightly in the CL=0.3-05 range by using snap flaps. You can see how the RK40 is excellent above CL=0.2 but drops away badly below that. The HN1038 is good but not as good as the two SM foils at this speed (It was designed for F3B after all).

These airfoils are untested so I understand if you guys would rather stick to something known, or ask one of the gurus. But I'd have confidence using them on one of my own designs.

Sean.
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