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Old Apr 26, 2012, 08:26 AM
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Windsor, Canada, near Detroit
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washout at wing tips to avoid stall

as scale sailplanes have tapered wings, there is a tendency to tip stall.
do you washout the tips? or the ailerons?
thanks
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Old Apr 26, 2012, 10:09 AM
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United States, KY, Taylorsville
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For what it's worth: I am using 'aerodynamic' washout on my current build. The profile goes from Go 549 to an almost symmetrical (home made) profile at the tip. The inner panels are constant chord and the outer are symmetricall double taper. The 1926 RRG Prufling built last year has a constant chord wing with no dihedral so it was rigged via the struts to have ~5mm washout at the tips. It is a stable flier with no nasty suprises. These old birds are 'tip stall meisters'!
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Old Apr 26, 2012, 10:24 AM
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anyway

thank you for your comment. that's a more sophisticated way.
i was thinking of washout by reducing angle of attack at the tip.
my question is if some1 uses washout, whichever way they do, and you provided the answer. you do.
and the full size had some.
thanks
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Old Apr 26, 2012, 05:01 PM
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I always wonder why people think becuase it has skinny tips, it tip stalls? Nothing could be further from the truth. I had a Nimbus 4 that had tips almost 1 inch in chord, it was an absolute sweetheart as far as slow flight, never any tip stall tendancy. I wonder where this fallacy came from? Maybe a vintage model may have some built in washout to help it fly, but a modern glass ship, I would not want any washout, it could cause srtange beahavior at higher speeds.
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Old Apr 27, 2012, 05:55 AM
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Originally Posted by s2000 View Post
I always wonder why people think becuase it has skinny tips, it tip stalls? Nothing could be further from the truth. I had a Nimbus 4 that had tips almost 1 inch in chord, it was an absolute sweetheart as far as slow flight, never any tip stall tendancy. I wonder where this fallacy came from? Maybe a vintage model may have some built in washout to help it fly, but a modern glass ship, I would not want any washout, it could cause srtange beahavior at higher speeds.
Because they do!

The choices are simple. Washout, which can cause problems at high speed. (Philip Wills wrote about seeing the wing tips bend down on his Skylark as he went through a sheer layer and suddenly found himself at red line)

Or... A tip airfoil that's a bit blunter, rises more steeply and may have the high point moved forward a bit.

If your Nimbus was well behaved it was probably well designed and almost certainly had a tip airfoil designed to have a late stall characteristic or some washout or both.

Pete
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Old Apr 27, 2012, 07:35 AM
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Pete, it must be my choice of buying extremely well designed models that negate any tip stall tendancies than eh.....
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Old May 02, 2012, 10:13 PM
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Quote:
Originally Posted by ARUP View Post
For what it's worth: I am using 'aerodynamic' washout on my current build. The profile goes from Go 549 to an almost symmetrical (home made) profile at the tip. The inner panels are constant chord and the outer are symmetricall double taper. The 1926 RRG Prufling built last year has a constant chord wing with no dihedral so it was rigged via the struts to have ~5mm washout at the tips. It is a stable flier with no nasty suprises. These old birds are 'tip stall meisters'!
You lucked out. Generally going from a cambered root to a symmetrical foil tip increases a wing's tendency to tip stall. Going to a greater camber at the tip creates the aero equivalent of a less-tapered wing, which DOES reduce the tip stall tendency without having to include washout, but at the expense of slightly higher induced drag.
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Old May 04, 2012, 09:10 AM
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You know something these folks don't, then! The Sunseeker is a sophisticated design that uses Wortmann FX 76 MPA160 at root and tip. The wing has no dihedral and the tapered wing has a straight LE. The foil induces a 'washout effect' at the tip and probably moreso when (?) the wing deflects upwards under high loads. The Nimbus 4D uses 'spoilerons' at the tip to reduce lift, increase drag and help with its awful roll rate. That sailplane is optimized for straight and level 'dolphin' thermaling. The 1929 Musterle was a very successful sailplane for its day and uses a Go 535 root profile that transitions to symmetrical at the tip. The tip has 5 degrees washout. There are many examples in history to document the use of aero and other types of washout. Were the designers 'right'? I'd say they were for their times. I think our models suffer due to Re #'s at scale and also from the fact that we are not sitting in them during flight. I know I build mine with some design changes to account for that. Many models use flaps which is another way of creating a washout effect for landing. I always make my flapped wings so the flap can be reflexed somewhat during cruising flight- just like the fullscale counterpart. There are certainly arguments for and against putting some form of washout in a wing. I have a 1/4 Cassutt that has a fully symmetrical wing and the thrust, wing incidence and stab incidence are '0'. The engine has a little right thrust for 'P' factor. It is a very sweet and neutral flying airplane but you have to control it for everything- which is what I wanted! It's all good and have fun!
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Old May 06, 2012, 07:33 PM
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It is always risky to base any design exclusively on full scale practice. From both a theoretical perspective and from personal experience flying full scale sailplanes I can attest to the fact that many full scale intermediate and high performance sailplanes do not have benign stall behavior. On board you can detect when a stall approaches, but when it does many a machine will tip stall and then roll into a spin. My guess is that the full scale designers who incorporated aero washout did so for reduced drag or structural/wing moment reasons, and not to achieve well behaved flight characteristics at the extremes. Prio to the advent of winglets and other such tip treatments, the "ideal" wing did transition to a symmetrical tip because the CL at the tip is always zero. Thus, the symmetrical foil tip would have the lowest provile drag. Subsequent studies found that the reduced drag benefit of having a symmetrical tip foil or other tip treatments in most cases was realitively small, hence, the clipped tip's on such sailplanes such as the Schweizer 1-34, 1-35 and 1-36.

For my model sailplane designs I calculate the lift distribution and local lift coefficient on wings with multi-tapered panels and varying washout, often incorporating varying airfoil shapes between root and tip. Not being the hottest model sailplane pilot, my goal is to design the wing so that stall initiates far from the tips, ideally near the root, while maintaining a reasonable induced drag over my design operating envelope. You can achieve these characteristics with a wing that tapers to a symmetrical airfoil at the tip as long as the trasnsition from a cambered to the symmetrical airfoil begins fairly far out toward the tip. You have to run the numbers for each specific planform to define constitutes "fairly far out".
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Old May 07, 2012, 10:05 PM
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Actually most sailplanes do not have highly tapered wings, the taper is very low on a high aspect ratio wing. Tip stall becomes a problem when you have a low aspect ratio with lots of taper.

The main reason folks with long skinny wings get into trouble is due to lack of airspeed. Remember to keep the entire wing flying, when you make a tight turn the inside wing slows way down relative to the outside wing, increasing AoA on the inside panel... Boom you get a stall. Long skinny wings have a very real problem with torsional forces, which could lead to the tip increasing AoA, selecting airfoils for the tip that have a negative pitch moment will help in this regard.

Nothing magical about it, simply keep the entire wing flying. Maintain energy through the turns. Some pilots are better at it than others, smooth turns and plenty of energy and you will not get into a stall situation. Generally speaking. Yes, you can stall at any speed but again it is a matter of keeping the wing flying and flying smoothly.

In my book, I go with more camber at the tip instead of wash-out. There are two main camps in engineering, both work, just a matter of how you want to go about it. I choose to study the Polars and use chamber rather than wash out.

Peace, Wolf
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Old May 08, 2012, 06:17 AM
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tapered wings

even if we consider that all is relative, still, wings of sailplanes are tapered. how much it takes to call that much or little depends on each 1.
but the point here is that if a wing is tapered, and has no wash out, the tip tends to stall 1st because it is narrower that the root, and therefore the reynolds at which it flies is lower. the lower the reynolds, the sooner the stall happens.
(that's why a constant chord wing is less prone to tip stall)
the main reason why designers of sailplanes use tapered wings is to reduce drag by reducing tip vortex: the narrower the chord, the less the vortex, and not to avoid tip stall.
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Old May 08, 2012, 11:09 AM
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Wolf, you are mostly correct. The inside wing does slow down when making a tight turn. That does decrease the Re, and hence AoA at which a foil will still, but I'm not sure that it increases the AoA. Trying to roll out of the turn at whatever AoA by deflecting the inside aileron most definitely does increase the AoA and can result in the inside wing stalling. And does result in people getting into trouble.

I believe my idea of highly tapered is a little different from yours. I base mine when the local AoA is significantly outboard from the root. Flying straight ahead and slowing to below stall speed in the SGS 1-26 (AR = 10, taper ratio .5), SGS 1-34 (AR 16, taper ratio ~.4) and Blanik L-23 (AR 13.7, don't recall the taper ration) all enter a spin when stalled. None of these are what most would consider highly tapered wings, although, increasing the taper ration further will only exacerbate the problem unless significant physical washout is incorporated.

From my model design perspective, such taper ratios for simple tapered wings qualify as highly tapered. The stall characteristics are easily tamed with some combination of washout or compound tapered wings, with the inner panels tapered more slowly than the outer panels, which is the common practice on moderate to high performance sailplanes. The combination of compound tapered wings with moderate washout - aero or otherwise - results in both reduced induced drag and improved stall characteristics, a win win.

Since I cannot sense the approaching stall in the model bird while piloting from the ground, for model design I try to err on the benign stall side. That leads to non-scale airfoil and washout choices in order to preserve scale wing planforms.

Phil, also mostly true. The lower tip cord does result in a lower Re and associated AoA at stall. But the wing will shed vortices all along its span, although greatest at the tip. The ultimate result is that the lift distribution along the span will be something between one directly proportional to the local cord and an elliptical lift distribution - this is the real reason highly tapered wings are prone to tip stall in full scale practice at least. That results in the local effective AoA being higher than the physical AoA at some wing stations and less at others. The wing will tend to stall first where the local AoA is higher. In terms of Re, as long as the entire wing is flying above the local section critical Re, the local AoA will be the dominant factor. At lower Reynolds Numbers, where my models fly at least, the two factors are equally important.
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Old May 08, 2012, 12:56 PM
yyz
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You can actually visualize using XFLR5 what is trying to be described in text in this thread and tune the wing geometry and see the affects for your changes in near real-time. It's a powerful tool that will help you bridge the gap between "My gut tells me..." or "I heard somewhere that..." kind of thinking and what is happening as the angle of attack changes
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Old May 08, 2012, 01:41 PM
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Originally Posted by yyz View Post
You can actually visualize using XFLR5 what is trying to be described in text in this thread and tune the wing geometry and see the affects for your changes in near real-time. It's a powerful tool that will help you bridge the gap between "My gut tells me..." or "I heard somewhere that..." kind of thinking and what is happening as the angle of attack changes
Here, here!
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Old May 08, 2012, 01:44 PM
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I have read words like 5 deg negative AoA and downward bending wings in high speeds in this thread. No wonder they flex if the angle is negative, if the wings are not the strongest to resist it.
I use washout in some wings, but never lower than 0 degrees AoA.
A symmetrical airfoil will stall sooner than a lifting one ( at the same or less thickness), and is very often used at the tips to prevent tip-stall, and is the solution I prefer the best.

Washout also increases stability of flight-direction.
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