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Old Jul 26, 2010, 06:57 AM
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Don Stackhouse's Avatar
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SD 7037 and MH32 both work fine without flaps. SD 7037 is a bit thicker than it needs to be, but not too bad in that regard, works better at low speeds. Use the MH32 if you want better high speed performance.

The Clark Y is a good full-scale airfoil. At model Reynolds numbers it's not nearly as good as a lot of folks want to believe.
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Old Jul 26, 2010, 08:43 AM
Making wood fly since 2007
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USA, MN, Rochester
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Well hello Don,

Thanks for stopping in and thanks for the airfoil insights. Have you had any time to make new progress on your skunk works project that you teased us with last fall?

Wayne
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Old Jul 26, 2010, 10:49 AM
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Elk Grove, CA
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Don:

What is your opinion of the S 3014 that Ray Hayes puts on is "Bird" series?

FC
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Old Jul 27, 2010, 06:41 AM
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Quote:
Originally Posted by Windependence View Post
.... Have you had any time to make new progress on your skunk works project that you teased us with last fall?
There are several projects, and they're coming along slowly, in between a number of other projects. Between product support work (a huge part of my overall workload, but very necessary), consulting work (had to go to Detroit a couple weeks ago, probably Seattle next week, lots of work here in between), some military UAV work, "Monarch Luthiers" - my musical instrument repair and manufacturing business (32-string harp design, two mandolin designs, special tooling for tapered-bore pennywhistles, repairing a 1688 David Techler violin with 31 cracks in the front and 5 in the back, among other things), I'm spread a little thin. I also need to finish designing and building the solar heating system for our house before the cold weather arrives. The price of propane these days is more than we can afford.

However, the two big airplane design projects are coming along, just lots of little details to work out. If it was easy, someone else would have done it already. Got several others in the works as well, but not as far along.
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Old Jul 27, 2010, 06:44 AM
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Quote:
Originally Posted by FoamCrusher View Post
Don:

What is your opinion of the S 3014 that Ray Hayes puts on is "Bird" series?

FC
Shape is not bad, but too much thickness. Drag sorta falls apart at Re's below about 150K, which equates to chords of less than about a foot.
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Old Jul 28, 2010, 03:04 AM
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Originally Posted by Bob Cook View Post
Hi Guys,

I'll get a hold of Tony the lazer guy, and ask him about doing the wing in other air foils. I tend to think he will do it but there may be some charge for that. The program he has now in his computer, is for a D box sheeting only on the wing. If your going to do something other than D box sheeting you need to tell him so he can change the program. I'll write him and let you know what he says.

Bob in Seattle
I would recommend that you stick with the fully sheeted wing, not only does this maintain the airfoil profile more accurately, but the wing is a lot stronger and more resistant to twisting.

Mike
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Old Jul 28, 2010, 07:20 AM
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An alternate to full sheeting is to stop the front sheet at the spar (or thickest part of the rib) and use rib caps from spar to TE. This works especially well if the trailing edge is built-up over the rib profile rather than a solid triangle piece.

Alan
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Old Jul 28, 2010, 07:33 AM
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In terms of torsional stiffness to avoid flutter, there is more than just the raw stiffness that matters.

The aerodynamic center of the airfoil is at about 25% aft of the leading edge. To avoid flutter, the local C/G of the structure, and the "shear center" (the axis the structure rotates around when it is twisted) both need to coincide with the aerodynamic center. This means keeping the mass as forward as possible (on full-scale helicopter rotor blades they often have mass balancing weights in the leading edge), and the stiffness as well.

Fully sheeting a wing does not add all that much additional stiffness beyond what you get by sheeting from the leading edge to the main spar, it adds a bunch of extra weight (which means more energy to control and damp out to suppress flutter), that extra weight moves the local C/G aft of the 25% point where you want it to be, and the stiffness it adds is all aft of that 25% location as well, which shifts the shear center aft.

Altogether a slight positive, combined with a bunch of significant negatives.

Cap strips stiffen and strengthen the ribs in chordwise bending (usually not needed in most models unless the chord is very long and/or the ribs are very light), but they add mass aft of the desired local C/G, and they have little effect on torsional stiffness. From a flutter standpoint you're probably better off without them.
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Old Jul 28, 2010, 08:55 AM
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Originally Posted by Don Stackhouse View Post
In terms of torsional stiffness to avoid flutter, there is more than just the raw stiffness that matters.

The aerodynamic center of the airfoil is at about 25% aft of the leading edge. To avoid flutter, the local C/G of the structure, and the "shear center" (the axis the structure rotates around when it is twisted) both need to coincide with the aerodynamic center. This means keeping the mass as forward as possible (on full-scale helicopter rotor blades they often have mass balancing weights in the leading edge), and the stiffness as well.

Fully sheeting a wing does not add all that much additional stiffness beyond what you get by sheeting from the leading edge to the main spar, it adds a bunch of extra weight (which means more energy to control and damp out to suppress flutter), that extra weight moves the local C/G aft of the 25% point where you want it to be, and the stiffness it adds is all aft of that 25% location as well, which shifts the shear center aft.

Altogether a slight positive, combined with a bunch of significant negatives.

Cap strips stiffen and strengthen the ribs in chordwise bending (usually not needed in most models unless the chord is very long and/or the ribs are very light), but they add mass aft of the desired local C/G, and they have little effect on torsional stiffness. From a flutter standpoint you're probably better off without them.
I wouldn't necessarily disagree with what you are saying and accept your points, but personally, I would still go for the fully sheeted wing, it is a lot stronger for those inevitable mis-haps when you hit the ground with a wing tip and I hate to see the covering dipping between the ribs.

Mike
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Old Jul 28, 2010, 09:17 AM
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There are ways to deal with covering sag, such as what we do on the Chrysalis series wings, but they require a lot of engineering work and specialized techniques.

However, even without those corrections and compensations, according to Martin Hepperle's analysis the beneficial effects at low Reynolds numbers of the covering sag may pretty much cancel out the negatives, and in at least some cases (Re < 100K) may provide a net benefit:

http://www.mh-aerotools.de/airfoils/ribs.htm

The other thing to bear in mind is that the actual amount of sag aft of the main spar tends to be pretty minimal.
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Old Jul 28, 2010, 10:26 AM
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Originally Posted by alstrahm View Post
So basically I guess I am saying the airfoil best to use is how you are going to build it, I am leaning toward the gull wing RES version so the stock airfoil would work for me.
Al,

I was going back over the thread and noticed your comment above. It is my understanding that if you build the gull wing version that you will need to use ailerons with it. Gull wing planes, as a whole, require ailerons as the wing shape does not behave like a true dihedral wing in terms of turning control. I hope this does not dissuade you from going in on the build as the Raven can be built with a traditional poly wing.

Wayne
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Old Jul 28, 2010, 11:49 AM
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Options

One of the things I mentioned very early in this thread is that there are multiple options when it comes to the wings of the Raven. I have little doubt that there will be a number of us that venture off of the plan when it comes to airfoil choice. Well here is another option.

One of the wing options is to go with a foam core balsa sheeted wing. I have located some foam cores that would fit the bill nicely for the Raven. They are made from Hi-Load 60 foam and come as a 120" wing. The airfoil is a SD 7032 which is designed for full trailing edge setup and the cost is $75. The wing is originally designed for the Genie Easy LTS (another plane I considered) and I believe it can very easily be adapted to the Raven 3M.

These cores, along with Supra cores, are available from Stealth Plane Works. Here is a link to their website should any of us want to pursue that avenue.

http://www.stealthplaneworks.com/CorePricing.aspx

Wayne

Edit: Although narrower at the root, this wing is almost identical in overall area to the original Raven wing area. This wing can also be bagged for those of you so inclined. Bagging instructions are available on the Genie website: http://augiemckibben.tripod.com/site...y_lts_file.pdf
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Old Jul 28, 2010, 07:20 PM
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Seattle
Joined Jan 2003
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Hi Guys,

All the fundamental design stuff is very interesting indeed. My wing sheeting skills are next to none. It will be a big struggle for me, just to do the D box area. Good thing were gonna have all Winter to do it.

Bob in Seattle
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Old Jul 29, 2010, 01:42 AM
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granada don's Avatar
Granada Hills Ca.
Joined Nov 2009
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Hi Wayne
If i did not have my new woody in the work's i would be in your Raven build too. I will be looking each day to see & learn more building stuff that is current. Looks like your group will have quite a few Ravens being build at one time , i was wondering how many planes have been built at the same time on a forum build log ? The woody will be starting about the same time as your group get's going will be fun to watch and learn. Cool stuff.

Don
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Old Jul 29, 2010, 10:06 AM
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Hi Don,

Thanks for watching. I have no doubt that there have been many group builds here on RCGroups. The biggest one that I am aware of took place over in the electric soaring thread a couple of years ago. If I am not mistaken close to 20 people participated in that one when they built a Chrysalis 2M glider. It is very possible that there were more than that building but not everyone posted on their builds.

I think one of the most interesting aspects of this build, for me anyway, will be to see how people actually build it. Unlike newer ARF's or molded ships, this plane has limitless options on construction and finish. I liken the build to a classic car restoration. Some of the builders will build it exactly like the plan. They will have their classic design and that will be great. Others will want to hot rod it a bit with different airfoils and such, I always like to looks and sound of hot rods. Still others may completely customize it, changing wing construction, adding fiberglass and carbon to it so it can be launched to the moon, making mods that result in a plane that resembles the original in basic appearance only. Again I would love to see it.

All of these ideas will help us all become better builders and better repairers. They will help all of us better understand how one change affects the rest of the whole. I hope you enjoy your planned build this fall. Just remember that if, in a few years, you want to build a Raven 3M this thread will be here for you to review and learn from.

Wayne
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