A hypothetical (for now) stunt airfoil:  16% symmetrical flapless with a "shelf" at the high point (18% back), that is, the 'foil suddenly becomes 1/16" thinner on the top and 1/16" on the bottom, then continues aft normally.

What, if anything, is likely to be different in the way this thing would perform?  Could it act like a turbulator, possibly improving stall characteristics?  Or maybe tend to induce stall?  Or you couldn't tell the difference?

Alternatively, the dropoff could be curved, gradual.  

Thanks,

You always ask hard questions.  Stick it in Javafoil or Profili and see what happens.  Better yet, build a wing.  If it doesn't work, capstrip the back.

Oo, oo.  Cover it with transparent covering.  Then after you capstrip it, cover it with another layer 1/16" higher.  Use funny covering so you get Moire patterns or something. 

I doubt you would notice it. The trailing edge is the most important attribute of the airfoil in the regime we fly; it's what starts the whole ball a-rollin'. Next is the leading edge. .18 back isn't going to do much as a turbulator on typical .40 -sized ship.

There are a variables you'll need to consider to understand it's effect:

1) chordlength
2) smoothness of the wing
3) airframe vibration
4) freestream velocity (aka airspeed)
5) freestream turbulence

Would certainly be easy to flight test. Build a nice straight ship. Fly it. Add 1/16th sheeting and fly again.

Years ago, there was a big flap (sic.) over an airfoil called "Kline-Fogleman".  This had a step at the high point, but the rest of the airfoil "curve" was actually flat!

Out of curiosity, I built an all-balsa hand launch glider using that airfoil.

Didn't fly worth a darn!  No amount of trimming or adjusting would make it stable.

(nothing works better than NACA 0018)

Floyd

Chuck, would you elaborate on this? Why is "The trailing edge is the most important attribute of the airfoil... "?

Please.  It ain't what I'd have thought.  Flow is mostly separated by then, isn't it?  There may be some value in just whacking it off, leaving a 1/4" step, according to Gary James.

You always ask hard questions...

That's cuz I know how much you heart them.  OK, here's an easy one:  what is that blue and white plane that your clown-suited doppelganger is holding in your icon picture ("icon" may not be the right word, but I can't think of the right one.  If anybody knows the word...)

You could just use capstrips.

Obviously, Tim, he already forgot them and covered the airplane.  I see this kind of a posteriori theorizing all the time.

Forgotten capstrips:  yep, senile, but not quite that bad.  As for my posterior, it has been accused of strange theorizing on occasion.  

With the right materials I think you could try this out on an existing plane.  Make a 1/16" thick balsa "shoe" that fits over the wing, hold it on with some light contact adhesive (good ol' rubber cement?), and cover it with Ultracoat.  Do your flight tests with it on and with it off, and see what you think.
If you've got a wing that's sheeted, covered, waterproof, and a bit ratty, you could just mold the "shoe" straight on the wing.

Yes, covered and a bit ratty.  This is a new, unflown stunt plane made using a Russian combat ARF wing, a bit similar to Howard's engine-test plane from some time ago (testing B-17 engines??) using a combat ARF with a very long aluminum boom and stabilitor, which he said flew purdy decent stunt. This plane has a longish fuselage with stab/elev made of featherweight balsa.  The problem is that the individual who built it used a high-temp iron-on film and melted the foam LE in several places.  The wing behind the spar has some very nifty graphics that I would like to keep without having to strip the wing completely and regenerate the graphics.  Film covered foam is a bit delicate for stunt, IMO, since planes are at least expected to last much longer than combat planes.  I thought about stripping the covering forward of the spar, filling the melted areas with Superfil, and then molding a new balsa wing front using the repaired foam itself as the buck.  Hence the shelf.  Another way might be to fill the melted areas and cover the foam with silkspan (no dope obviously) and several coats of Minwax to harden the surface, then a spray a coat of Rustoleum or other fuelproof paint on the new wing front only.  No shelf this way.

("icon" may not be the right word, but I can't think of the right one.  If anybody knows the word...)

Avatar.  It was a real word, even before the movie.

Yes, covered and a bit ratty.  This is a new, unflown stunt plane made using a Russian combat ARF wing, a bit similar to Howard's engine-test plane from some time ago (testing B-17 engines??) using a combat ARF with a very long aluminum boom and stabilitor, which he said flew purdy decent stunt. This plane has a longish fuselage with stab/elev made of featherweight balsa.  The problem is that the individual who built it used a high-temp iron-on film and melted the foam LE in several places.  The wing behind the spar has some very nifty graphics that I would like to keep without having to strip the wing completely and regenerate the graphics.  Film covered foam is a bit delicate for stunt, IMO, since planes are at least expected to last much longer than combat planes.  I thought about stripping the covering forward of the spar, filling the melted areas with Superfil, and then molding a new balsa wing front using the repaired foam itself as the buck.  Hence the shelf.  Another way might be to fill the melted areas and cover the foam with silkspan (no dope obviously) and several coats of Minwax to harden the surface, then a spray a coat of Rustoleum or other fuelproof paint on the new wing front only.  No shelf this way.

You mean you're not doing this as a 'sperement, but just because?  Hmm.  I'd go the silkspan (or glass cloth)/Rustoleum route.

Chuck, would you elaborate on this? Why is "The trailing edge is the most important attribute of the airfoil... "?

Thanks.

Bill

Bill,

Apologize for the tardy reply. Been a busy boy lately. I'll be at Watkins Glen this weekend and then in Norwalk Ohio the next. If you want to see REAL nitromethane powered motors drop by!
Anyway... when I was a young man flying models all my teachers told me about "Bernoulli's Principle" and how that was how a wing created lift. I ate it all up and then went on to college to become an aerospace engineer. I started to understand that Bernoulli didn't really apply to real airplane wings. Look at most modern airliners - the bottom of the wing is longer than the top - whoah! After many years I finally had the epiphany that aerodynamic isn't really about  classical fluid flows, what it really is is a special branch of thermodynamics.

Bernoulli's equation will not predict the lift of an airplane wing. I looked at it and tried to figure out why. Yes, there is a spanwise component of the flow on a real, 3D wing it can't account for but something else was missing. So I drank a few more jars of shine and pondered it and then it hit me. Bernoulli does not account for viscosity!

That's...HUGE. How big? I'll tell you. One of the things we aero guys love to talk about are Reynolds Numbers. We use them to compare the airflow of a 6 inch  chord wing to a 10 foot chord wing (or funny car body!).  We can do that because the Reynolds number is dimensionless. We know that if an airfoil section is operating at a specific Reynolds number it will behave the same regardless of the chord. The length of the section is part of the number, so if we want to use a small test section compared to the actual wing we can increase the velocity of the air or use compressed air in the wind tunnel to increase it's density, or both. This way, a small section will behave the same as a full-size transport aircraft's wing and we can get meaningful data.

(Yes, I'm old enough to have used wind tunnels and not computers.)

So why the rant about Reynolds numbers? Here's the kicker: The reynolds number represents the ratio of viscous forces to the airstream's momentum. Read that again.

And that's when it all came clear to me. The air's VISCOSITY is a main driver in how an airfoil works. So I explored it more. Where does the viscosity have the greatest effect? At the trailing edge!  Once that veil was lifted, I began to understand it all. The trailing edge is sharp, when the wing starts to move if there is an angle of attack the air gets delayed at the trailing edge due to viscosity. A vortex is created and the flow field gets distorted and bingo bango, you get the circulation and the net result is the air is shed at an angle at the trailing edge. The net change in momentum up or down through a control volume around the wing times the mass flow rate of the air is equal to the lift. It really is that simple. If you change the angle of attack the air's viscosity changes the flow field and increases or decreases lift. When you finally get a high enough angle of attack, viscosity overtakes momentum and the flow becomes turbulent.

You can search about "shed vortices" if you want to understand it more.

So, even though it's been many years since I gave up competitive flying I still build a lot of models and put in a lot of flights content in my search for that perfect pattern. And every ship I build now I find ways to make the TE sharper and sharper and they keep flying better. My best results have been to split the TE on the flaps and elevators, glue in a 1/4" wide strip of 1/64 ply and profile them to a knife-edge.

There is a classic experiment you can do to visualize this next time you have a bowl of soup. Put the spoon halfway into the soup, it's an undercambered airfoil. Now, start to move the spoon. Watch how the flow warps at the trailing edge and moves forward.

Chuck

Bill,

Apologize for the tardy reply. Been a busy boy lately. I'll be at Watkins Glen this weekend and then in Norwalk Ohio the next. If you want to see REAL nitromethane powered motors drop by!
Anyway... when I was a young man flying models all my teachers told me about "Bernoulli's Principle" and how that was how a wing created lift. I ate it all up and then went on to college to become an aerospace engineer. I started to understand that Bernoulli didn't really apply to real airplane wings. Look at most modern airliners - the bottom of the wing is longer than the top - whoah! After many years I finally had the epiphany that aerodynamic isn't really about  classical fluid flows, what it really is is a special branch of thermodynamics.

Bernoulli's equation will not predict the lift of an airplane wing. I looked at it and tried to figure out why. Yes, there is a spanwise component of the flow on a real, 3D wing it can't account for but something else was missing. So I drank a few more jars of shine and pondered it and then it hit me. Bernoulli does not account for viscosity!

That's...HUGE. How big? I'll tell you. One of the things we aero guys love to talk about are Reynolds Numbers. We use them to compare the airflow of a 6 inch  chord wing to a 10 foot chord wing (or funny car body!).  We can do that because the Reynolds number is dimensionless. We know that if an airfoil section is operating at a specific Reynolds number it will behave the same regardless of the chord. The length of the section is part of the number, so if we want to use a small test section compared to the actual wing we can increase the velocity of the air or use compressed air in the wind tunnel to increase it's density, or both. This way, a small section will behave the same as a full-size transport aircraft's wing and we can get meaningful data.

(Yes, I'm old enough to have used wind tunnels and not computers.)

So why the rant about Reynolds numbers? Here's the kicker: The reynolds number represents the ratio of viscous forces to the airstream's momentum. Read that again.

And that's when it all came clear to me. The air's VISCOSITY is a main driver in how an airfoil works. So I explored it more. Where does the viscosity have the greatest effect? At the trailing edge!  Once that veil was lifted, I began to understand it all. The trailing edge is sharp, when the wing starts to move if there is an angle of attack the air gets delayed at the trailing edge due to viscosity. A vortex is created and the flow field gets distorted and bingo bango, you get the circulation and the net result is the air is shed at an angle at the trailing edge. The net change in momentum up or down through a control volume around the wing times the mass flow rate of the air is equal to the lift. It really is that simple. If you change the angle of attack the air's viscosity changes the flow field and increases or decreases lift. When you finally get a high enough angle of attack, viscosity overtakes momentum and the flow becomes turbulent.

You can search about "shed vortices" if you want to understand it more.

So, even though it's been many years since I gave up competitive flying I still build a lot of models and put in a lot of flights content in my search for that perfect pattern. And every ship I build now I find ways to make the TE sharper and sharper and they keep flying better. My best results have been to split the TE on the flaps and elevators, glue in a 1/4" wide strip of 1/64 ply and profile them to a knife-edge.

There is a classic experiment you can do to visualize this next time you have a bowl of soup. Put the spoon halfway into the soup, it's an undercambered airfoil. Now, start to move the spoon. Watch how the flow warps at the trailing edge and moves forward.

Chuck

I think I see the issue here.  Yes, the TE is a big deal, but when comparing actual flow, which has viscosity, with inviscid flow, which is a mathematical construct that's useful for some things.  Any wing we'd make would be flying in real air, which has viscosity, so different TEs wouldn't behave much differently from each other.

You can assume inviscid flow, as I remember, outside the viscous boundary layer, and use Bernoulli's equation to calculate stuff.  By Bernoulli not being correct, I think you are thinking of the usual, bogus "equal transit time" explanation of how wings work.  Yes, a 777 airfoil looks pretty much like an airfoil we've seen in books, except that it's upside down. 

...I don't know about the theory myself, but Gary James tells me that he's seen benefits from bluff TEs over sharp TEs on combat airfoils.  Gary got an encouraging note from Mark Drela saying that what Gary saw was believable and could lead to higher Clmax.

...I am fixing to try some kinda bizarre TEs for a couple of reasons.  Not as bizarre as the one on the clown's plane on the left, but moreso than most...

...Any wing we'd make would be flying in real air, which has viscosity, so different TEs wouldn't behave much differently from each other...

 

The latest issue of Brodak's Stunt World has a page of "strange airfoils".  No explanation, but they are obviously the "Kline-Fogleman" series.  When I first encountered them, back in the '70s, we all had a good laugh.  My one experiment using a K-F on a HLG was a disaster!  I gave up right away.

Floyd

I guess that's "Control Line World".  It seems I accused Brodak of being only stunt.