[capncrunch]

I'm curious as to what anyone can post about the effects of rectangular wings vs. tapered LE/TE, double taper, triple taper (paul calls it quad taper for the mojo) etc. have on 3D stuff... I know plenty about all the rest of the engineering and also about planning out a wing for efficiency, but there are different requirements here...

-b

 

[PaulSwany]

Well... My theory is that a tapered wing allows the wing to be more stable in a knife edge. by tapering it down on the tips, the air flow comes off a little cleaner and doesn't induce as much wing rock....

That and trying to get the surface area above and below the thrust line equal and you should have a knife edging machine!!

I'll find out if I'm right as soon as I get the Mojo in the air........

 

[capncrunch]

well i guess im after sort of the difference between a profile 3d trainer and an all out uber badass leet machine.

paul do you think the burrito's reputation for hoverability is a) its wing b) its tail moments or c) its weight/other stuff... im gonna guess a, and here's my reasoning:

-a wing that is tapered gets progressively lighter as you go from the center outwards. this keeps the moment of inertia low, so that it will roll real fast. (like holding your legs in when spinning in a chair)

-a constant-chord, constant thickness wing (of the same area, and similar density) will have a longer inertial moment, so it takes more to move it around. think of a tightrope-walker's balancing pole here...

tip-stalling happens when the outer part of the wing stalls first, and it becomes bad with large amounts of taper or rear-sweep, because the lift distribution is far from linear, and so the tips can stall at much lower reynolds numbers than the inner part of the wing. since this often happens unevenly, the wing will tip from one side to the other.

one solution is washout, putting the tips at a lower angle of attack than the center, but that isnt so great on a plane that will spend half its time upside down. how else to get more lift out to the tips? fatten up the airfoil so it creates more lift. its not uncommon to see front/back tapered wings with constant thickness airfoils.

so what's wing rock? It's not tipstalling, in harrier you're basically flying the wing in the propwash, anything outside of that should be fully stalled.

back to where i mentioned a linear lift distribution. this is nice for stall properties, but its not efficient (lift makes downwash, downwash off tips makes tip vortices, aka induced drag). im not sure... I have to think about this for a bit...

-barrett

 

[stratcat]

I'm sure the extra weight from the square wing is harder to roll and do freestyle with. Like a tight rope walker with his pole. Not sure if that helps

 

[PaulSwany]

Actually... My Burrito and my Sledge both roll at about the same rate... It's just that the Sledge is a little more "loose" in the air... It's a more capable plane... More stable in knife edge, but a little less stable in hover or harrier.

I think the tapered wing allows the plane to track cleaner in knife edge because of the air flow comming off it can roll off easier as the wing gets smaller... More abrubtly with a constant cord wing.... This also makes the wing a little less stable in a harrier, because it can tip stall easier.

 

[Spaz]

a couple points, not that i'm any expert

a wing that is tapered gets progressively lighter as you go from the center outwards. this keeps the moment of inertia low, so that it will roll real fast. (like holding your legs in when spinning in a chair)

I think the gradual decrease in wing area tward the tip makes for less aerodynamic resistance in the roll, the weight is only less by the shortening of the ribs, in most cases only a few grams or oz..

My theory is that a tapered wing allows the wing to be more stable in a knife edge. by tapering it down on the tips, the air flow comes off a little cleaner and doesn't induce as much wing rock....

what also helps for double or even quad tapered wings is that when in KE the bottom wing is now sweeping forward, which puts a whole lota area infront of the CG, tapering relieves some of this CG/area shift, a strait LE would not help

tip-stalling happens when the outer part of the wing stalls first, and it becomes bad with large amounts of taper or rear-sweep, because the lift distribution is far from linear, and so the tips can stall at much lower reynolds numbers than the inner part of the wing. since this often happens unevenly, the wing will tip from one side to the other.

tip stalling is bad but in 3D we want the whole wing to stall as smoothy as possible, lift distribution is far from linear even on constant cord wings which is what i think you were saying anyway, you can actually have a tapered wing with a more uniform lift dist. even at mostly stalled high Alfa situations the wing is lifting some, see George Hicks article in 3d Modeler mag vol1, Matchlessaero calles it "Surfing" i like that comparison

back to where i mentioned a linear lift distribution. this is nice for stall properties, but its not efficient (lift makes downwash, downwash off tips makes tip vortices, aka induced drag). im not sure... I have to think about this for a bit...

For a 3D plane efficincy is not really an issue, we have 2-1 Thrust to weight and very light wing loads, and Aspect Ratios of 3ish
the best wing for the ultimate 3D monster is probably a triple or quad taper wing with nice even Lift Dist and lots of drag for keeping speed down, with huge control surfaces and for the rest, a long enough tail moment for good harrier and hover but short enough for spins and tumbles, conventional airfoils do not apply very well in most cases, either.
Disclamer:
i am not an expert or even an aerodynamics student.
sorry if i came off as blunt

 

[446]

I think the fairly low aspect ratio has a lot to do with harrierability, as does constant chord. I think tapered t.e. tend to drop a wing a little more readily as the stall "unzips" along the whole wing. I think the tapered wing does k.e. better, as my edgy burrito with the same weight, engine, and tail knife edges a little less effective than my stock omp 540; but it harriers better. I think frontal area and c.g. is more important to knife edge than wing planform. I don't think there is all too much difference in roll rate between the 2 that I've noticed, but have noticed a difference in torque rolling. less squares outboard seems to torque roll sooner and faster. and I think a sack of flour would make a big biscuit... for what it's worth!

 

[capncrunch]

what also helps for double or even quad tapered wings is that when in KE the bottom wing is now sweeping forward, which puts a whole lota area infront of the CG, tapering relieves some of this CG/area shift, a strait LE would not help

I think you're on to something here.

-b

 

[Spaz]

think you're on to something here.

thanks
i forgot to mention i've been really buisy the last year making different profile planes with different wings, tapered and not, and the biggest difference i've seen is with the airfoil not the taper or lack of, but it does seem that low AR helps also. and i think OMP actualy means "good KE" is some ancent language

 

[xjet]

so what's wing rock? It's not tipstalling, in harrier you're basically flying the wing in the propwash, anything outside of that should be fully stalled.
-barrett

I'd wager that wing rock is caused by the huge vortex that appears at the tips of the wing when it's at a high angle of attack.

I experienced *real* bad wing-rock when I used to build constant-chord unswept combat wings with tip-fins.

Here's how it works...

Any wing which meets the air at an angle of attack greater than zero degrees will have an area (usually a bubble) of low pressure on the top surface and higher pressure pushing on the bottom surface.

Naturally the low and high pressure areas would love to meet somewhere for a coffee and cancel each other out -- but there's a big fat wing in the way :-(

However, where the wing ends (ie: at the tip)_the high pressure air under the wing does tend to flow up and over -- to fill part of the low pressure area on the top-surface.

The result is a large spiralling vortex.

Now imagine for a minute that one wing produces slightly more lift than the other -- for whatever reason.

That wing will rise and the other will fall.

Now it becomes much *easier* for the high pressure air under the wing to rise up and over the wingtip because it's going to naturally move up-hill towards the tip.

This means that the tip-vortex will increase in magnitude, the pressure under that wing will fall, and the low-pressure area on top of that wing will reduce in size. The net effect is that the rising wing is now producing *less* lift than it was -- so it falls down and the other wing now is now generating more lift so the plane starts to roll the other way - then the whole process starts all over again so we get a rocking back and forth as each wing alternately creates more lift then spills it.

In normal flight, the size of the tip vortex is too small to produce any perceptable degree of wing-rock.

Slow a ship down and give the wing a huge angle of attack however and the yo-yoing tip vortices will produce a very significant oscillation.

There are ways to reduce this phenomenon -- but that's another story

 

[Spaz]

There are ways to reduce this phenomenon -- but that's another story

well put,
tip vort's may just be another piece of the puzzle, the problem with this stuff is you can not generalize because of the complexity of the issue.
you cant say taper is bad or constant thickness whatever...

It's tough to find true causality and not just an effect from another cause.

Airfoils that stall completely when asked and are not better suited to gliders and/or control-line-stunt is a good start, though in MHO
i love this stuff

 

[capncrunch]

Naturally the low and high pressure areas would love to meet somewhere for a coffee and cancel each other out -- but there's a big fat wing in the way

OMG, when i'm an aerodynamics professor im totally going to steal that quote. May I?

There are ways to reduce this phenomenon -- but that's another story

but this is what we're after, here!

-barrett[/quote]

 

[Kenny R]

I have been trying to figure this out for a couple years. I hope to have some answers for myself in the next couple weeks.

I have one design (plane 1) that that has basically no coupling in knife edge, but it likes to wing rock in harrier. The wing has a tepered leading edge and trailing edge with the airfoil becoming thinner toward the tip.

I have another design (plane 2) that couples very badly in knife edge but is stable in a harrier (not quiet as stable as a burrito, but close). The wing has a straight leading edge, tapered trailing edgeand the front part of the airfoil has the same thickness from root to tip.

Both designs flat spin and waterfall nicely.

So here is my test...... My new plane is identical to plane 1 except for the wing. The wing has a little less taper on the leading edge and trailing edge. I maintained the same thickness on the airfoil form root to tip like on plane 2.

So what will I end up with? A plane that will knife edge and harrier very well or one that wont fly at all?

I will know in a couple weeks.

 

[TailTwister]

I saw an interesting wind tunnel demo at EAA this year. NASA had a wing panel in a mini tunnel. The wing panel has about 8 inches in length and had a MAC of maybe 2.5 or 3 inches. The tunnel was clear plastic and perhaps 2 feet long. On the wing were several of the streamers that we have all seen placed in various positions on the wing. At the trailing edge of the tip a small white string.

At zero AOA, all streamers were flat on the wing and the thread was straight back. As AOA increased, the streamers started to move, starting with the rear most ones. The string began a small spin, indicating a vortex was present. At a fairly large AOA, I'm thinking it was about 30 degrees or so, the streamers blew forward on the wing, and the string went from a circular spin to a kidney shaped patern.

I think that wing rocking is due to the vortex in part minimally, but mostly due to the reverse airflow over the wing, as it meets the oncoming air at the leading edge. A thicker and more blunt airfoil would (hypothesis only here) more directly flow the air straight down at the LE, and not allow as much interference (rocking) as if it were less controlled and moving more forward still. This must cause some air swirls in front of, above, and below the wing. Less swirls, more stability. It also made me think that this is why spoilerons help sometimes as the reverse airflow is minimized by the spoilerons being in the way. To me, the vortex appeared to be pretty much behind the wing, and would seem to have a much smaller impact.

I like the "surfing" comparison. I say it's like a boat hull on water instead of a wing through air when folks ask me about harriers.

BTW, I'm just a dumb ******* that spent way too much time playing with a toy wind tunnel at an airshow. I have no clue if any of my observations even make sense... :?

 

[Kenny R]

I have made a few decisions with my experiment(post above). Here is what I think I have learned.

The constant camber in the wing didnt seem to effect the knife edge ability but seemed to help a harrier. It does both well. The only downside that I have found to this setup Is I have lost the ability for a nice clean snap, the snap looks more like a big "swoosh". I was kinda expecting this, but I wished it would make a clean snap.

 

[Spaz]

for a cleaner snap you can do the pattern guy thing and tape 3/8" tri stock on the LE so it makes a sharper LE, outboard section of wing is all it takes, might work if snaps are important to you..