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Old Oct 31, 2014, 05:09 AM
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psychedvike's Avatar
United States, MN, Shoreview
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Washout ???

OK here's a question I think I know the answer to, but some discussion might confirm my conceptions or point out that they are really misconceptions after all.
I have just finish putting together a gravity feed (draw bar design) hot wire cutter. I am looking forward to making some nice wing cores. I have built quite a few scratch built foamies and think this setup will expand my capabilites and open up some options.
Alright back to the question. By turning the trailing edge of the wing tip template off center I can add washout to the wing. My question is what are the advantages to doing this and what is a good number in degrees to turn up the trailing edge.
The first project I have planned is a flying wing based on plans I found here on RC Groups called the Fierce Arrow . It's a line control plane from the sixties. I plan on converting it to RC and adapting foam core wing construction. I also have built my own vacuum forming setup so I am going to play with custom canopies and cowlings for this one. I really think this one will be my best work so I don't want to add washout incorrectly and make it fly like crap. So this is my first step in educating myself on washout. Any input would be appreciated,. Here is a like to The Fierce Arrow in the line control plans thread.
http://www.rcgroups.com/forums/showt...w#post19180112
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Old Oct 31, 2014, 08:25 AM
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hallo's Avatar
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What they'll tell you is that this must be calculated for the specific design and speed if you want to make it right. I think however that if you introduce about -0,5* washout at the tip, you will probably not make anything worse. This should give you better performance and less tip turbulence in a lifting situation like in a turn or when flying slow, and also ensure that the tip wont' stall. That's very simplified I guess..

P.S. the link didn't work for me for some reason.

tk
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Old Oct 31, 2014, 08:42 AM
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This can be done geometrically or aerodynamically by the use of different airfoils.
You can start by researching zero lift lines and then the Excel spreadsheet called LiftRoll.

http://webspace.webring.com/people/a...l/COOKBOOK.htm

For flying wings perhaps Dr Panknin's formula for twist would be very helpful. I used this on a model called Tinamou that flew wonderfully. You can find the spreadsheet at Tailwind Gliders. It's called "Flying Wing Calc".
http://tailwindgliders.com/Files.html

Curtis
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Old Oct 31, 2014, 09:44 AM
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Great information. I will have to follow up tonight while passing out candy. Thanks gentlemen.
Here's the link for the plan http://www.rcgroups.com/forums/showt...288656&page=13
Not sure where the first link came from.
Thanks again tk and Curtis
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Old Oct 31, 2014, 12:18 PM
B for Bruce
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Well, the foam is flexible enough that you don't need the cores to have the washout. The core will flex enough that it can be easily twisted and the washout set by adding the sheeting or other skin to the core.

But it makes total sense to cut the washout into the core so that the BEDS left have the correct washout as an aid in sheeting the cores and getting the washout angle accurate and identical for each side.

For an RC conversion of the Fierce Arrow I'd stick to a dead flat wing. As I recall that design the airfoil is a constant physical thickness from root to tip. And even if it isn't the low aspect ratio, thick airfoil and intended aerobatic style of flying for such a model says not to use any washout that will cause different characteristics for inside and outside looping or upright to inverted flying.
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Old Nov 01, 2014, 07:13 PM
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I was thinking that it would be a good idea to reduce tip stall. But I am concerned that it will make a big impact on aerobatic performance.
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Old Nov 01, 2014, 07:52 PM
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Perhaps it would be best not to complicate the design if it's not necessary. Nice project!

tk
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Old Nov 01, 2014, 08:33 PM
B for Bruce
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Quote:
Originally Posted by psychedvike View Post
I was thinking that it would be a good idea to reduce tip stall. But I am concerned that it will make a big impact on aerobatic performance.
If this is a model intended to spend nearly as much time upside down as right side up then it sure will. You want to use other options for delaying the tip stall in that case.

The BEST option is to work hard to minimize the weight of the model. It's pretty easy to add stall strips to force an earlier stall on a light model. But it's really tough to significantly delay a stall on a heavier model.

If it's not a scale model then don't make the wing highly tapered. Note that most of the designs around with reputations for nasty slow speed manners have radically tapered wings.

All else being equal and within a consistent family of airfoils the thicker and more rounded nose radius airfoil will have a little higher stall angle than the thinner and sharper version. And example would be a wing with a NACA 0012 on the root and a 0014 or 0015 at the tip. Such a wing should provide some resistance to tip stalling.

Part of avoiding tip stalling is to fly the model appropriately. For example when flying a slow approach at just a little over the stall speed don't make large aileron inputs. Use only a little and wait for the slower reaction. Any airfoil near the stall can be pushed beyond the stall if the aileron hanging off the rear is deflected down by a good amount. It raises the section's angle of attack too much and can end up over the stall point. Hence the reason why a snap roll is induced with proverse aileron input along with the rudder and elevator. It ensures a sharp stall on the wing you want to drop and drag back the rest of the model.
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Old Nov 02, 2014, 09:47 AM
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Control line designs having washout ?
I once had a control line flying wing much like the one you are considering powered by a O&R 23.
it went round n round but would not fly like my Chief or Brave-or Stuntwagons etc..
Washout is a patch- a compromise -and IF the design is right, not needed or desireable.
IF for example the model always flew exactly straight on into the air- it would stall evenly
tip to tip
On good aerobatic designs - it is often the case that a slight yaw has to be induced to get ONE wing panel to stall and start a spin.
On other types the stability is marginal and one wing panel will stall when not intended to do so.
Heavier Piper Cubs are notorious for this.
a bit of washout as a patch, can be helpful but it always reduces overall effective lift.
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Old Nov 02, 2014, 03:07 PM
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Vienna
Joined Oct 2004
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Quote:
By turning the trailing edge of the wing tip template off center I can add washout to the wing. My question is what are the advantages to doing this and what is a good number in degrees to turn up the trailing edge.

The only correct answer to this is, it depends
If, for example, you have a rectangular planform, you dont need any washout at all. On a rectangular planform you can even convert a halfsymmetric profile into a fully symmetric one at the wingtip, thus reducing induced drag, and still have the best stall caracteristcs.

The more taper a wing has, the more interesting gets washout. Reason is, the more taper the less the Reynoldsnumber is on the wingtip, compared to the rest of the wing.
The less the Reynoldsnumber, the less angle of attack a certain profile can handle, resulting in nasty habits such as tip stall.

If and how much washout, or a change in the profile, is needed, can be calculated with software like this: http://www.hanleyinnovations.com/3dfoil.html
or http://www.flz-vortex.de/flz_vortex.html

regards
Andi
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Old Nov 05, 2014, 06:36 AM
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Much to consider. Thank you for the links. I will take a look at them.
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Old Nov 06, 2014, 06:35 PM
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Vienna
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Both work very well, the Vortex demo - which is functional - is free.
The Software from Hanley is somewhat more professional and gives you also detailed data about drag etc. etc.
With both you can calculate also angle of incidence an center of gravity.

regards
Andi
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