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Old Aug 22, 2010, 09:37 PM
Question gravity.
KMKAZE's Avatar
Kane County, Il
Joined Sep 2000
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Lift and the fully symmetrical airfoil...

I've doing an overhaul on my scratchbuilt sport/seaplane, extending it's wingspan a little to help reduce the wing loading and improve it's low speed handling.

I might only get limited results considering that the airfoil is completely symmetrical. Fully symmetrical airfoils of course produce no lift unless they are at a positive angle of attack, and the wing was set up with 0 degrees incidence. The plane was designed for speed and maneuverability- which I definately got plenty of, but then problem becomes landing this sucker. Flaperons are a must for landings.

So while I'm overhauling the plane, I'm installing ailerons with a thicker chord for more positive control, and I just realized this should boost the effectiveness when I drop the flaperons for landing.

If the added wingspan and increased, slightly increased aspect ratio, and the thicker chord ailerons fail to make the low speed handling any better, I'm also going to add some wingtip plates. Are there any other methods out there for improving low speed handling that I am missing?

I just hope it doesn't come to building a second wing with a flatter bottom airfoil to tame it's low speed handling while set up with floats.
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Old Aug 23, 2010, 05:41 AM
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Uncambered airfoils produce no lift unless at a positive AOA, but this is no different than a cambered airfoil at its neutral angle of lift... the main difference is that an uncambered airfoil will produce no significant pitching moment, and that it will stall at a smaller AOA when compared with the same airfoil with a camber applied to it. By stalling at a higher AOA, a cambered airfoil can produce the same lift at a slower speed, but I consider the concept that "a cambered airfoil produces more lift" incorrect. The lift produced by a wing is always the same: the weight of the plane if flying level at 1 G. More if climbing or pulling G, less if descending or going under 1 G. As far as the handling is concerned, probably you will get a greater benefit on low speed handling by adding stall strips on the inboard portion of the wing or by adding washout to it (that however will come to bite in inverted flight). If you need the absolute slow speed plane, then you are looking for a strongly cambered wing with fixed slats, spoilers instead of ailerons and a large tail to counteract the pitching moment all that camber gives you.
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Old Aug 23, 2010, 02:12 PM
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South Wales U.K.
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A friend of mine always used to say, "You've got to add more lightness".
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Old Aug 23, 2010, 03:46 PM
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I agree. "fun fly" models use fully symmetrical wings, but can slow down to a hover almost without entering a stall regime. Partly because of the airflow from the prop hitting the wing, but mostly because they pair a large surface area with the lightest possible build
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Old Aug 23, 2010, 08:29 PM
Question gravity.
KMKAZE's Avatar
Kane County, Il
Joined Sep 2000
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Quote:
Originally Posted by Brandano View Post
I agree. "fun fly" models use fully symmetrical wings, but can slow down to a hover almost without entering a stall regime. Partly because of the airflow from the prop hitting the wing, but mostly because they pair a large surface area with the lightest possible build
That's true, but my plane isn't designed for 3D. It was designed to be extremely maneuverable and very fast, kind of like a fighter... but not for 3D.
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Old Aug 24, 2010, 12:51 AM
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Leading edge slats can give the ability to fly at higher angle of attack without stalling, so could allow the model to slow up some more. If you put the slats only on the outboard part of the wing then they should prevent tip stall which could be what you are looking for when you say 'improved handling'

Slats have downsides through, unless they are retractable, which is complex, then they add drag and compromise inverted performance. Good inverted is something i assume you want from the model due to the use of a symmetrical airfoil in the first place.

Stall strips on the leading edges may help prevent tip stall but only by prematurely stalling the inner part of the wing which increases stall speed, the opposite of what you are trying to acheive. Washout is good but also doesn’t usually decrease stall speed, it just makes the stall more predictable when it happens also as noted earlier washout also seriously compromises inverted performance.

The best change you could make is as mentioned earlier... Make the model lighter. Easier said than done i know but there really is no substitute for lightness. 'Lightness' is the one thing that can improve virtually every aspect of the models flying performance with no significant 'downside'.

Steve
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Old Aug 24, 2010, 02:28 AM
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What about turbulators/vortex generators?
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Old Aug 24, 2010, 04:15 PM
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The 'Wack, BC, Canada
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Quote:
Originally Posted by JaRaMW View Post
What about turbulators/vortex generators?
They can work up to a degree IF the wing is suffering from early flow separation. And they may delay the stall a little since a "stall" is just a radical surface flow separation that doesn't ever reattach.

But not all model wings have an early flow separation. It's a "try them and see" sort of deal. If they help then you "fixed" an early flow separation issue you had. If they don't help then there was nothing wrong to fix in the first place.

On the own turbulators, vortex generators or whatever you want to make them look like and call them all do the same job. But if that job isn't needed because the condition didn't exist they won't give you something on top of that.
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Old Aug 25, 2010, 12:15 PM
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Really, adding lightness is the most practical way.
Flaperons would be next, but they generate tip stalls when flying at very speeds and high angles. And odd handling, when the plane goes the opposite direction of a command, as it can with full flaperon deflection.
A semi-symmetrical airfoil would be a reasonable change, if really good inverted flight is not really needed. More lift.. i.e. slower flight at a reasonable pitch angle.
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Old Aug 31, 2010, 04:28 PM
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Joined Jun 2000
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A symmetrical airfoil can carry SOME load, but a well chosen wing section can lift more.

For instance, the B-17 was built with a 15% full symmetrical wing section, reportedly for speed. In retrospect, this might be considered a mistake, and the B-17 might have had a slightly better range and payload with a different wing section.
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