washout, root stall and other gadgets - Page 3 - RC Groups
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Feb 26, 2013, 12:50 PM
B for Bruce
BMatthews's Avatar
Originally Posted by Oldog
......So the shape of the airfiol remains constent, just smaller.
Would this then tend to act like a constent chord?
No, it won't. It'll still react to the speed difference of a long span when flying a very slow and tight turn.

Keep in mind that you're talking about an extreme situation here. The only way the inside tip on a long span will stall in a turn is if you are flying the overall model VERY close to the stall speed already. Like it would need to be a turn radius that is only a span or three large. And that implies a high bank angle "corkscrew" like turn being done while on the verge of stalling. Not many folks fly that way. The more normal slow flying speed sort of turns are done open and flat where this sort of stall due to the speed difference from inside to outside tip speeds is very minimal.

As for the taper? Taper is taper. It doesn't act like a constant chord just because you scale the airfoil. It acts like a correctly tapered wing, which is what it is.

The reason that extremely tapered wings are bad for us model designers is that the reynolds number of the small chord at the tips falls off unless it's a big and somewhat faster flying sort of model. Light slow flying models will see the Reynolds numbers at the tips drop down quite a ways. This puts the airfoil at that point into the range of lift to drag charactaristics where the small tips can either stall before the root or develop draggy flow separation bubbles. This is why if we MUST use a higher taper ratio it's common to build in a bunch of washout so the tips are not loaded as strongly and the wing tends to all stall at the same time. But even here larger, heavier, faster flying models might not notice this effect to the same degree as a smaller, lighter, slower flying model like a glider.

The "tricks" we're talking about in this thread are NOT substitutes for proper airfoils. At best they force or delay the stall by fairly small amounts. And if not done right can produce bad effects at flying speeds other than slow, near stalling speeds.

For example, the trick of slightly more rounding the leading edge can easily backfire if it's done by a lot. The resulting blunt nose of an overdone re-shape can produce an airfoil which is nothing at all like the intended one. In most cases it should take a rather keen eye to even detect the difference in the nose radius from root to tip of such a modification. If the root airfoil has, for sake of discussion, a nose radius that makes it look like there is a 1/8 dowel in the leading edge then the nose at the tip with a tapered wing should still look like it has a 1/8 dowel in the leading edge. The smaller chord length matched with the consistent nose radius will give you all the rise in nose radius you should need.
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Feb 26, 2013, 05:27 PM
Registered User
Then "washout" is still my best friend. I think. ......(I read your post four times)

FYI...However I made a discovery on this ARF. It seems that the left wings are built by
one employee while the right by another. Resulting with a nice radius on one wing and a fairly blunt ( flat ) leading edge on the other.
Sort of "built in disaster". I have always loved the building part of the hobby, but as kits become more rare and arfs cheaper, well you get the idea.

I have always checked for incidence and washout but never paid much attention to the leading edge.
This may be why I found so many posts warning of tip stall on this particular ARF.

Just a note for ARF buyers to watch out for.
Feb 27, 2013, 02:02 AM
B for Bruce
BMatthews's Avatar
From flying models built by student modelers that had bluntish and somewhat squared off leading edge I found that it really does mess up how a wing performs. If you're able to easily see the difference such as you indicate then be prepared for the worst. It will be especially bad with one wing looking like it has a proper leading edge and the other is poorly done.

I'd suggest that it would be time well spent to carefully slit and peel away the covering on the roughly done side and shape it to match the other then patch the covering. Or if you are careful perhaps you can slit it along the bottom behind the leading edge, fold it back and tape it out of the way and then fold it around and down and re-seal it. You may need to buy some Balsarite to allow doing this.
Feb 27, 2013, 05:37 AM
An itch?. Scratch build.
eflightray's Avatar
Here's a graphic representation I just came across of potential stall speed relative to bank angle. It's for a full size aircraft but gets the idea across that when banked the aircraft's stall speed is potentially higher.

So flying slowly and then doing a banked turn the aircraft's speed should really be increased to reduce the risk of a stall.
Probably one of the major factors for the 'dreaded down wind turn, fall out the sky' problems that some flyer seem to have. They slow too much on the down wind leg, (visually relating the models ground speed, not air speed), then turn and .......woops!.

Feb 27, 2013, 03:53 PM
Registered User
Thank you E
A picture is worth a thousand words but that link isGREAT !
Mar 03, 2013, 10:21 AM
Registered User
At slow speeds keep the bank angle shallow
Mar 17, 2013, 06:06 PM
Registered User
At last done. flaps built, wing recovered with 2 degrees wash out in tip.

Need sunshine
Mar 17, 2013, 06:16 PM
Registered User
Tried to post a picture but...guess I didn't do it right
Mar 18, 2013, 12:46 PM
B for Bruce
BMatthews's Avatar
Originally Posted by Oldog
Tried to post a picture but...guess I didn't do it right
From looking at the code for your image it looks like you are trying to post it directly from your F drive. For that don't use the IMG codes. Use the "Manage Attachments" button down below the text box
Mar 18, 2013, 03:02 PM
Texas Buzzard
Texas Buzzard's Avatar
Originally Posted by eflightray
How about raising the ailerons slightly for a little reflex, that could give a similar affect to washout.

But, the real answer is don't slow it down too much. A model prone to dropping a wing tip, (tip stall), often in that dreaded 'down wind turn', just need to be flown faster and further out on the down wind turn. Once lined up and flying straight with the runway, then start to slow down, (flaps etc ? ).

The higher the wind speed, the easier it is to start relating ground speed to the model, and slow down too much on the down wind leg.
.................................................. .................................................. .........
I agree with you. That phrase, "downwind turn" got my attention.
Most fliers/posters say there is no such thing as a spin due to the down wind turn. They say the plane does not know the wind is blowing....ha ,ha.

They ignore the KineticEnergy factor. Also they ignore that wing loading is also a factor. This true of conventional as well a a Delta wing aircraft. Just last month I saw a new twin-engined scale ship drop that wing and auger in on turning on final. Pilot error contributed. Turning too tightly at too slow a speed. I also use a little neg. wingtip warp on planes weighing less than 5 pounds.
Mar 18, 2013, 03:38 PM
Registered User
ShoeDLG's Avatar
Originally Posted by Texas Buzzard
They ignore the KineticEnergy factor.
Because it's complete bunk.
Mar 20, 2013, 12:51 AM
Registered User
OK see if this works flaps down

Yeah thats better I am looking fwd to first flight . Been wet and windy a lot.
Mar 20, 2013, 12:28 PM
Registered User
with those flaps deployed you shouldn't need any additional washout on the wing. the flaps will increase the camber AND the aerodynamic decalage on the central portion of the wing, and automatically provide you with washout.
Mar 20, 2013, 08:18 PM
Registered User
I see your point about the flaps.
But while the covering was off the wing it was easy to warp in some washout and then fine tune it with heat after covering, so I do have abut 2 degrees in the tips. The photo shows "full" flaps(about 40 deg). Half flaps is about 25. I am using a Spectrum DX7S that allows two settings and a reduced servo speed.
I will try some slow approach patterns at a higher altitude and see if all works out
Mar 20, 2013, 08:37 PM
Registered User
Thanks all for the help

Will post results if I ever get out to fly it.
Last edited by Oldog; Mar 24, 2013 at 02:10 AM.

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