Flat vs. Airfoil at low reynolds

[sloper steve]

Flat vs. Airfoil at low reynolds
If viscosity increases as speed decreases and air acts less like pellets at slower speeds then it could bunch up in front of a square leading edge of a flat foam wing. Could this end up helping lift when combined with other factors?

Does EPP slow the plane down vs. a slick surface?
Dimples create less drag on a golf ball and some birds have tiny hairs sticking up.
Wouldn't the roughness of the EPP mimic this?
http://www.indoindians.com/golfbcore.htm
http://www.golf.com.au/ed/physics_of_golf.php

[colbyweb]

Why don't someone build an airplane with an interchangeable wing and test it out? I may just do that myself...
-Rick

[BMatthews]

That's the best way to test it.

And remember that with the current trend to hot 3D hoverbatics a flat plate not only stalls quicker and easier (that runs true regardless of size) but also the folks flying those types of models are looking as much for a place to hang the controls from as they are a wing to support the weight of the model.

I saw a 1/3 scale Extra that looked like it either has a "thick" flat plate airfoil with just a rounded leading edge or it's a super thin airfoil. I suspect the same thing applies to them. The desire for a quick and draggy stall to pull the plane to a halt fast for the transition to hovering.

Oh, and turbulators, AKA Depron skin roughness, work really well at slow speeds for energizing the airflow and preventing early separation. But that actually DELAYS the stall and may be counter productive to 3D style flying..... just a thought.

[3DXTC]

Steve,
The air could bunch up in front of the flat edge of the flat plate airfoil and part of it find a smoother flow path. In a steady state flow condition you could have a stagnation region or point at the leading edge. Based on my typical maneuvering during flight plus a review of your flying videos, a steady state flight and airflow condition is not the norm but atypical ;)

In order for any lifting surface to generate lift, it must have some angle of attack to the relative wind. Challenge this if you have information or a theory that provides a different approach. With our thick flat plate leading edge it seems to me that the wing airflow must become very turbulent when trying to flow around the right angle at the upper edge of the leading edge and as a result produce an energized vortex airflow aft of the leading edge. Isn't this effect responsible for the "jump" in wing lift between positive lift (up) and negative lift (down) that some people find disturbing about flat plate wings as they fly near the neutral level flight lift conditions. The transition between positive and negative lift conditions is a lot smoother with an airfoil wing that has a carefully rounded leading edge.

Good comment on golf balls and birds :cool: I think you are right on. One of the things that increases drag on large aircraft is a rough wing surface finish because in promotes buildup of a thick boundary layer. The thick boundary layer also reduces control surface effectiveness. You of course have noted the vortex generator vanes on aircraft wings and fins ;) Since we fly in such a different aerodynamic region, my suspicion is that the surface roughness of the EPP helps us. Just like your comment on Birds with hairy wings. I keep thinking about the insect wing. Beetles, Bees,and Dragonflys don't have smooth wing surfaces. They have turbulators built in.

TC

[3DXTC]

B for Bruce

Hi Bruce,

A thought on your thought.
Turbulators energize the airflow but they trip the airflow and it separates at the turbulator. The energized airflow re-attaches relatively quickly. This reduces boundary layer buildup and reduces the chances of a fully stalled wing airflow condition building up. In effect we have a mini-stalled region of the airfoil just behind the turbulators. Overall, the turbulator energized airflow is a good thing for small light models.

It looks 3D flying is largely done in a post stall condition. It seems that more of a deep stall condition is used to enter into maneuvers such as hover. We probably want airflow separation near the wing leading edge and re-attachment by at least the aileron hinge line for a number of these 3D maneuvers.

TC

[BMatthews]

Yes 3D but bear in mind that the airflow once in the stalled condition is quickly corrected by the prop blast over the center section of the wing. The whole thing becomes so dynamic thanks to all the power and airblast from the prop that it's hard to analyze how the airflow works in the conventional sense.

Also we need to recognize that air does not have to be attached to the wing at the aileron on both sides for the aileron to be useable. This is shown during the Harrier flight mode with 3D models where the model is obviously in a deep stalled condition but it's still dragging along with adequite roll control thanks to the air on the forward facing wing surface.

Similarly elevators on these models are still functional despite the fact that the elevator has severe separation on the rear facing side at angles beyond about 7 to 10 degrees. Certainly by the time the elevator hits 20 the one side is totally stalled but the other is still working to re-direct airflow and provide the pitching force.

It sure is a lot easier to analyse gliders where the pilots don't partake of such complications... :D

[3DXTC]

Not sure if we are saying the same thing in different words or not. Shouldn't the flying surface react essentially the same to any source of airflow? Why should it matter whether the airflow is generated by forward velocity or by prop blast? This would of course be ignoring the spiral effect of the prop generated airflow. We sure agree on the dynamic complexity :)

In deep stall conditions don't we generate much more drag than lift? That is how Rutan solved his re-entry problem on SpaceShip One. The area within prop driven airflow may not be truly deep stalled. The other effect which you mention is the deflection of the airflow by the surface not stalled. This change in momentum force resulting from the deflection of the airflow can be significant. We have all probably put our hand out the car window and experienced this directly. In hover flight, the area able to generate this control force must be limited to the area within the prop blast. I think that is what led Sloper Steve to the all flying stabilizer.

[colbyweb]

Well, I guess I am going to design and build an airplane with an interchangeable wing so I can see for myself how the two compare. I have been building a lot planes with stick fuselages and I want to build this one that way as well. I've been thinking about this since yesterday and I can't seem to come up with a good way to mount a symetrical wing to a stick (carbon rod) fuselage. I've come up with a few ideas but none of them are very elegant. I could just build an airfoil shaped cradle that would be glued on top of the fuse using the waste from the foam block that I cut the wing from. Then I could cut a second wing but on a few inches long and split it chordwise and put that on the underside of the flat plate wing so that it could mount in the same cradle. Does anyone have any other suggestions? Has anyone else here done this before?
-Rick

[3DXTC]

Colby web,

These planes are quite light if you ar going to utilize equipment similar to that which Sloper Steve uses. A potential slightly different approach for the symmetrical airfoil would be to cut a chordwise slot half the depth of the root airfoil and glue in a strip of light plywood that would form the flat base for the airfoil mounting. If you fit the plywood prior to bonding in the carbon wing spar, the spar could pass through the ply center plate. The plate could have 1/8th x 1/4th balsa strips glued to each side of the lower edgeto provide a cradel for the fuselage tube. As an alternate, the plate could have 1/8th balsa strips glued to each side of the lower edge so that a flat surface was formed that could sit on a horizontal ply wing mount plate glued to the fuselage tube. Sloper Steve listed a web site on a previous post that had plans for the"'And Now" which used the ply wing mounting plate glued to the fuselage method. Can't seem to locate that post at the moment.

TC

[colbyweb]

Thanks TC. I actually have an And Now, so I'm familiar with it. I'll give your ideas some thinght, thank you
-Rick

[sloper steve]

More ways to add airoils:

Rectangle strip along top and bottom of the wing at 25 to 30% chord
Wrap thin sheet of EPP around it and glue just before the elevons.
----
Cut airfoil shapes, cut them in half long ways, glue on top and bottom of the wing, glue sheet over (better off just building up a hollow wing)

Here is a 'plug and play' airofoil for the flat wing.
The idea is to plug the wing in the slot to the aileron hinge.

The high point of the airfoil is farther back than the leading edge which should help smooth the pitch by moving back the lift point.

The photo is actual size except for the span.
Slot depth is 82mm

[colbyweb]

Steve, I don't get it...

[vintage1]

I do. I like it.
He means glue some half ribs on either side of the flat plate and wrap a new sheet around it.

[colbyweb]

I do. I like it.
He means glue some half ribs on either side of the flat plate and wrap a new sheet around it.
Ok, I get it now, thanks!
That was his second idea though, the part I really didn't get was the "plug and play" wing idea, but I get it now. Hmm, I'm not sure I can easily do that with my cutter, but I'll give it some thought.
-Rick

[colbyweb]

Ok guys, I have a plan :)
Here are plans for what I intend to build. The plans show it with a symetrical wing but the flat plate can be substituted by glueing a half-airfoil saddle under the flat plate. I didn't come up with anything innovative really (DRATS!), lots of flat plate or undercambered wings are mounted this way. If you want to build it, the instructions are sparse but any experienced builder should have no problem figuring it out. I've included a cad file, a full size pdf and a tiled pdf.
-Rick

[sloper steve]

The image is an airfoil with a slot cut into it. The whole thing is the width of 1/2 wing and can slide on the front. It is just a shortcut to gluing ribs and wrapping sheet around but will be heavier since it will be filled in between the ribs.

I like the carbon-stik. It will probably be able to knife with winglets :)

[colbyweb]

I figured out the plug and play wing idea earlier. Seems heavy to me, plus how would you secure the trailing edge in such a way that it would easily removable? I want to be able to exchange wings in the field for testing.
Glad you like the stik idea, and I will probably add winglets after it's flying. I think it was you who posted a video some time ago with a similar design that had winglets and I've wanted to build something like it ever since. Thanks
-Rick

[3DXTC]

Like your wing mounting solution a lot :) It has a minimum number of parts. It sounds like you are using the "scrap" foam piece from your symmetrical wing cut for mounting the flat plate wing :cool: Approaching the problem from the symmetrical wing point of view resulted in a simpler solution than my suggestions based on looking from a flat plate wing mounting perspective :) It would appear to also facilitate keeping the wing chord line location above the fuselage tube identical and therefore eliminate a potential difference that could affect your test results :cool:

[sloper steve]

I will try to cut this one tomorrow :)

A 28" wing I tested today has the spar back a little farther from the leading edge - more than 25% - close to the hinge line. It seemed to do the flat airfoil pitch thing at a much slower speed than my other plane. The darn wind.. I'll know better in smooth flight. I'd think moving the spar forward would help it prevent the first inch or two from bending or twisting.

[colbyweb]

If anyone downloaded the plan I posted earlier you will need to go back and download them again if you intend to build from them. The tail group could not have been built according to the original plan, that has been fixed along with other changes. I have actually built the plane now from these plans so they should be in good shape :)
-Rick

[3DXTC]

Steve,

Your experience with the wing spar located further aft makes sense, at least from my perspective. The EPP is very flexible :D The discussions we have had related to energized flow around the leading edge of a flat plate could be interpreted to indicate that a wing twisting effect would be produced. That wing twisting force is probably applied closer to the leading edge on the flat plate than it would be on a conventional airfoil. In effect, your aerodynamic center is located more forward on a flat plate than on conventional airfoils. If you move the spar aft, more deflection of the forward portion of the wing can occur due to the EPP lack of stiffness. This aft relocation of the wing spar may also be somewhat like moving the hinge axis too far aft on a full flying stabilizer ;) One does not want to have the stiffest portion of the wing aft of the center of pressure. If that situation exists, the wing flexing then acts to increase the angle of attack :( This can also lead to very high loads on the wing. Bad things happen to full size aircraft if this condition occurs :eek: The forward swept wing design has this problem big time.

TC

[colbyweb]

Hey guys, I have my convertable wing airplane almost finished. I still have to make the flat plate wing for it, but the airfoiled wing is ready. I should have some pics of it later tonite. Now, I was just thinking, I do some volunteer work with the technology students at our local high school, and they have a wind tunnel. I guess I could make some comparison tests in the wind tunnel and I will be there Tuesday morning. I have never used a wind tunnel before so I don't really know what I'm getting into, but I'll give it a whack. Hehe, they also have a structural stress tester that -could- be used for stress testing wings and fuselages :)
-Rick

[3DXTC]

Your wind tunnel tests will be quite interesting :cool: .

Wind tunnels are susceptible to damage from objects picked up by the airflow. Typically wind tunnel operators are very particular about the features of the models they allow in the tunnel. They want robust, no loose parts models that can be securely mounted so that nothing can get into the fan system.

I don't know how sophisticated the High School wind tunnel is. Hope the tunnel will be made available to you. May I suggest you start at the minimum airflow speed of the tunnel and increase the speed very slowly.

In free flying conditions, the air loads on our models are relieved by the response of the models to these loads.

In a wind tunnel, the model is constrained and all the loads must be absorbed by the structure of the model and transferred to its mounting in the tunnel.

I would hate to see a test report that says "model destroyed, tunnel rebuild in progress". That report has needed to be issued more than once in the world of full scale wind tunnels.

[colbyweb]

Your wind tunnel tests will be quite interesting :cool: .

Wind tunnels are susceptible to damage from objects picked up by the airflow. Typically wind tunnel operators are very particular about the features of the models they allow in the tunnel. They want robust, no loose parts models that can be securely mounted so that nothing can get into the fan system.

I don't know how sophisticated the High School wind tunnel is. Hope the tunnel will be made available to you. May I suggest you start at the minimum airflow speed of the tunnel and increase the speed very slowly.

In free flying conditions, the air loads on our models are relieved by the response of the models to these loads.

In a wind tunnel, the model is constrained and all the loads must be absorbed by the structure of the model and transferred to its mounting in the tunnel.

I would hate to see a test report that says "model destroyed, tunnel rebuild in progress". That report has needed to be issued more than once in the world of full scale wind tunnels.

Thanks, I already knew a little about that, I knew that the model to be tested had to be able to be mounted. Also, I have access to the wind tunnel, the class instructior told me I can have access to all of their facilities, which includes the wind tunnel, 2 plotters and the structure tester I told you about. I've been using the plotter already. I plan to cut an airfoil from foam and then fiberglass it, and I think I will make two... one with a normal airfoil and one with a sharp leading edge. The guy that designed the southern cross epp plane says sharp leading edges are better, but he doesn't say why. He is putting balsa triangle stock on his leading edges to sharpen them up. I'd like to look into that more. For the flat plate wing I think I can just use a 1/4" sheet of plywood. I decided to try this sharp leading edge on this convertable model I'm working on so I will soon see if I like it.
-Rick

[colbyweb]

I talked to the tech lab instructor today, ran into her at the grocery store. She says a blue foam wing will work fine their wind tunnel, they use white styrofoam models in it all the time. I'll be testing it Tuesday morning, I have no idea what kind a data I can get from testing it, but we shall see.

[sloper steve]

That is great Rick! Thanks!

[3DXTC]

Thats cool Rick :cool:

Glad that the worst case I presented was not reality for your efforts :)

TC

[colbyweb]

The convertible wing plane is ready for maiden. Still need to build the flat plate wing, that is a project for this evening. Here are a couple of pics, first is a closup of the wing saddle, second is the assembled plane
-Rick

[colbyweb]

Got to test fly the convertible stik this morning. Was only able to fly it with the symetrical wing as the flat wing is not finished yet but there was a short window of opportunity to go test fly so I took it. The wind was 10 to 15 mph but this thing didn't seem to care a whole lot. It blew around a little, but not too much. All in all this is now the best flying plane in my stable, I am extremely pleased :) It behaves like a much larger plane. It flies faster than the flat plate winged planes I've been flying lately and a LOT smoother. I did several high speed low passes and they were rock solid. I hovered it a little, but it was too windy for that.
I'll fly it tommorow with the flat plate wing and make a direct comparison. I will also try to make up some winglets to test out as well.
-Rick

[3DXTC]

Glad that your first flight had such good results :)

Sounds like the flat plate wing will have to be great to meet the results of your symmetrical wing flight tests ;)

[banktoturn]

Flat vs. Airfoil at low reynolds
If viscosity increases as speed decreases and air acts less like pellets at slower speeds then it could bunch up in front of a square leading edge of a flat foam wing. Could this end up helping lift when combined with other factors?

Does EPP slow the plane down vs. a slick surface?
Dimples create less drag on a golf ball and some birds have tiny hairs sticking up.
Wouldn't the roughness of the EPP mimic this?
http://www.indoindians.com/golfbcore.htm
http://www.golf.com.au/ed/physics_of_golf.php

Steve,

Viscosity doesn't increase as speed decreases, although its effects
may become more important, in some ways. Your comment about
air behaving 'like pellets' sounds like a reference to the size of air
molecules. The size of air molecules is not relevant at any Reynolds
number. The Reynolds number has nothing to do with particle size.

There is a conventional wisdom in the model aircraft community
that 'flat' airfoils are as good as 'normal' airfoils at low Reynolds
numbers. This is not really true. Thinner airfoils are better suited
to low Reynolds numbers, but that's not the same thing as a flat, blunt
plate. Many model planes have much more power and lower wing
loading than full sized planes, so what we choose for our wings doesn't
matter as much. Indeed, a model plane with a simple flat plate may
fly just fine. That doesn't mean that the flat plate is better than an
airfoil at low Reynold numbers, it just means that your model plane
flies OK with a non-optimal wing. If the simplicity is more important
than performance, the flat plate is a good choice. If you want to
optimize lift and drag, then a thin, but fairly conventional, airfoil
will be better than a flat plate with blunt leading and trailing edges.

banktoturn

[colbyweb]

I seriously doubt the flat wing will will do nearly as well as the sysmetrical, but I want to see exactly how much difference there is between the two airfoils. Honestly, the full airfoil was just as quick and easy to make as a flat plate, and it does not need any carbon, it's stiff enough on it's own. I added some fiber-reinforced strappping tape for the heck of it. I stressed the wing heavily when I flew it today, and it didn't even flex. I made it from 1lb density white beaded foam, epp probably would not be stiff enough.
-Rick

[3DXTC]

Bank to turn,

Thanks for the input on viscosity. Was confused by Steve's comment on viscosity changing as speed decreases.

Yes, the effects become more important. My suspecion is that the effects are more important than most of us expect as the speed decreases and the size of our models decreases. A couple of NRL guys refered to their miniature flyers as "swimmers". They were using high powered dynamic flow analysis to evaluate their UAV miniature flyers.

We can calculate any Reynolds Number. Under what conditions does that number loose significance?

TC

[tolladay]

Banktoturn,

I agree with parts of what you are saying, and disagree with other parts. I have to admit I'm not much of an expert on aerodynamics. What I can do is post a quote (from RC Universe) by George Hicks on why he designed his Tribute airplane around a flat-plate airfoil. As near as I can tell his Tribute is the original flat-plate foamy 3D plane. (does anyone know of an eariler one?). I believe his words speak for themselves. I will add that he is an accomplished aerodynamicist working for a major aircraft manufacturer. He knows what he is talking about. Below will explain the "flat plate" airfoil selection and why it works well.

*************************************************

"The airfoil is a 1.75% thick flat plate...the funny and perhaps surprising thing is I wouldn't want it any other way.

When you start looking for airfoil sections for aerobatic aircraft the obvious choice is something of a symmetrical nature simply because you don't bias its force producing capability in a certain direction. When you couple this with low Reynolds number aerodynamics you quickly come to the conclusion that a thin section will be the best from a handling qualities point of view simply because the nondimensional aerodynamic coefficients aren't strong functions of airspeed. (Assuming of course that the size of the airplane is fixed and the kinematic viscosity of air isn't apt to change wildly in the near future).

In other words the airplane's aerodynamics should vary in a linear fashion no matter what speed you're flying.

A good example of this sort of thing is the small balsa hand-launch gliders you can buy at any convenience store. Notice that they have a simple flat (or curved) plate for an airfoil section. Most think that this is done simply because it's cheaper and easier to manufacture but lets suppose that they outfitted their little glider with a standard NACA 4412. If you look at the 2D wind tunnel data you see that the lift curve, drag polar, and pitching moment coefficients vary wildly until you get to a Reynolds (Re) number of greater than 250,000. Actually it only starts to really look good above 3,000,000 which happens to be the Reynolds numbers that it was designed to operate in. If we measure the airspeeds that our little Wal-Mart glider flies at we calculate that it operates from a Re of 10,000-80,000. This assumes that you can throw it at 60mph and it’s trimmed to glide at a min-sink speed of roughly 10 ft/s. Notice that the 12% thick NACA 4412 would be a total disaster in this Re range since the aerodynamics vary so much. Conversely, if you look at the data for flat or curved thin plates you see that the lift curves, polars and pitching moment plots barely change from a Re of 20,000 to 250,000 making it a great choice. In comparison the Tribute operates from roughly 32,000 to 300,000 which also makes it my airfoil section of choice.

The funny thing about aerobatics, especially precision and 3D model aerobatics, is that our pilot opinion, of the airplane’s handling qualities, is governed almost completely by the linearity of the aerodynamics. When we command a certain amount of Tx stick travel we expect a proportional amount of response from the airplane. If this response occurs in a nearly linear manner (or we can setup the curve via exponential or some other mechanism to make it feel linear) the human controller can easily command the model and will often pronounce the handling as good or even exceptional depending on his background and ability.

Simply put “be wise…linearize”

Hope this clears up some of the confusion in the airfoil selection,

George Hicks
Team JR


*************************************************

I've seen this quote in several places, but it has been probably 2 years since this was written. Does anyone know if George has changed his mind about flat-plates, or moved on to a better airfoil?

[banktoturn]

Bank to turn,

Thanks for the input on viscosity. Was confused by Steve's comment on viscosity changing as speed decreases.

Yes, the effects become more important. My suspecion is that the effects are more important than most of us expect as the speed decreases and the size of our models decreases. A couple of NRL guys refered to their miniature flyers as "swimmers". They were using high powered dynamic flow analysis to evaluate their UAV miniature flyers.

We can calculate any Reynolds Number. Under what conditions does that number loose significance?

TC

I would say that the Reynolds number never loses its significance. Many (most) airfoils designed to operate at high Reynolds number will perform very poorly at low Reynolds number though.

banktoturn

[banktoturn]

Banktoturn,

I agree with parts of what you are saying, and disagree with other parts. I have to admit I'm not much of an expert on aerodynamics. What I can do is post a quote (from RC Universe) by George Hicks on why he designed his Tribute airplane around a flat-plate airfoil. As near as I can tell his Tribute is the original flat-plate foamy 3D plane. (does anyone know of an eariler one?). I believe his words speak for themselves. I will add that he is an accomplished aerodynamicist working for a major aircraft manufacturer. He knows what he is talking about. Below will explain the "flat plate" airfoil selection and why it works well.

*************************************************

"The airfoil is a 1.75% thick flat plate...the funny and perhaps surprising thing is I wouldn't want it any other way.

When you start looking for airfoil sections for aerobatic aircraft the obvious choice is something of a symmetrical nature simply because you don't bias its force producing capability in a certain direction. When you couple this with low Reynolds number aerodynamics you quickly come to the conclusion that a thin section will be the best from a handling qualities point of view simply because the nondimensional aerodynamic coefficients aren't strong functions of airspeed. (Assuming of course that the size of the airplane is fixed and the kinematic viscosity of air isn't apt to change wildly in the near future).

In other words the airplane's aerodynamics should vary in a linear fashion no matter what speed you're flying.

A good example of this sort of thing is the small balsa hand-launch gliders you can buy at any convenience store. Notice that they have a simple flat (or curved) plate for an airfoil section. Most think that this is done simply because it's cheaper and easier to manufacture but lets suppose that they outfitted their little glider with a standard NACA 4412. If you look at the 2D wind tunnel data you see that the lift curve, drag polar, and pitching moment coefficients vary wildly until you get to a Reynolds (Re) number of greater than 250,000. Actually it only starts to really look good above 3,000,000 which happens to be the Reynolds numbers that it was designed to operate in. If we measure the airspeeds that our little Wal-Mart glider flies at we calculate that it operates from a Re of 10,000-80,000. This assumes that you can throw it at 60mph and it’s trimmed to glide at a min-sink speed of roughly 10 ft/s. Notice that the 12% thick NACA 4412 would be a total disaster in this Re range since the aerodynamics vary so much. Conversely, if you look at the data for flat or curved thin plates you see that the lift curves, polars and pitching moment plots barely change from a Re of 20,000 to 250,000 making it a great choice. In comparison the Tribute operates from roughly 32,000 to 300,000 which also makes it my airfoil section of choice.

The funny thing about aerobatics, especially precision and 3D model aerobatics, is that our pilot opinion, of the airplane’s handling qualities, is governed almost completely by the linearity of the aerodynamics. When we command a certain amount of Tx stick travel we expect a proportional amount of response from the airplane. If this response occurs in a nearly linear manner (or we can setup the curve via exponential or some other mechanism to make it feel linear) the human controller can easily command the model and will often pronounce the handling as good or even exceptional depending on his background and ability.

Simply put “be wise…linearize”

Hope this clears up some of the confusion in the airfoil selection,

George Hicks
Team JR


*************************************************

I've seen this quote in several places, but it has been probably 2 years since this was written. Does anyone know if George has changed his mind about flat-plates, or moved on to a better airfoil?

I think I can tell you that George still has the same opinion. Note that he equates goodness with linear response, but is willing to get the linear response by programming the radio. Linear response is not the most important thing for all planes, and I don't know that a flat plate is the best choice for linear response in any case.

I have posted on this topic at some length in the past, so I will avoid getting carried away here. My main point is that there is a difference between a thin airfoil and a flat plate airfoil. Low Reynolds number definitely favors a thin airfoil, but not necessarily a flat one. In particular, the leading edge should be rounded, and the trailing edge should be tapered and fairly sharp, assuming that you value low drag and decent lift. Note that George uses the example of a 12% airfoil. It's no surprise that a 12% airfoil doesn't do well at low Reynolds numbers. If you're inclined to experiment, compare a flat plate to a 'traditional' airfoil with the same thickness, area, and weight. If you can't notice the difference, and the flat plate is more convenient, then it is probably a good choice. As a practical matter, a flat plate with the leading edge nicely rounded and the trailing edge nicely tapered is what I would call a thin, but 'traditional' airfoil.

banktoturn

[tolladay]

banktoturn,
Okay, I can follow what your saying, and I appreciate the desire for brevity. This is not as much a pet pieve of mine as a way of learning more, so I do enjoy the well thought out opinion. If a 'traditional' airfoil is significantly superior, then why is it not used in 3D planes? Is it because of easy of building? Does the high drag flat-plate better mimmic the dynamics of large scale 3D planes?

I'm also really curious to see what Rick's (colbyweb) experience is like with the wing switch.

I can see that some of the reasons why I like flat plates are starting to look more like 'faith' and less like 'science'. Has anyone done and posted some real numbers? I'd love to see some charts that compare two similarly sized airfoils, one flat-plate and the other rounded on the front and tapered at the back.

[banktoturn]

banktoturn,
Okay, I can follow what your saying, and I appreciate the desire for brevity. This is not as much a pet pieve of mine as a way of learning more, so I do enjoy the well thought out opinion. If a 'traditional' airfoil is significantly superior, then why is it not used in 3D planes? Is it because of easy of building? Does the high drag flat-plate better mimmic the dynamics of large scale 3D planes?

I'm also really curious to see what Rick's (colbyweb) experience is like with the wing switch.

I can see that some of the reasons why I like flat plates are starting to look more like 'faith' and less like 'science'. Has anyone done and posted some real numbers? I'd love to see some charts that compare two similarly sized airfoils, one flat-plate and the other rounded on the front and tapered at the back.

tolladay,

A 3D plane has so much thrust that a high-drag wing is not a problem. This kind of plane simply does not require the same things of its wing that other kinds of planes do. A flat plate probably works just fine for this application. That's different than saying that a flat plate is better at low Reynolds number than a 'traditional' airfoil.

banktoturn

[sloper steve]

I believe foilsim allows you to compare flat foil to an airfoil of any thickness.

Here is some brash comments ready for review:

Under 10mph an airfoil doesn't do too much and the weight of the airfoil may out weigh the extra lift it makes.
In level flight an airfoil always has more drag since it is thicker and diverts the path of more air. A light foamie can dive straight down faster than a foamie with an airfoil.

Airfoils are impractical to make compared to flat parts therefore the cost of a kit is much higher with airfoils than flat wings.

Construction is more difficult with airfoils.
Tapered wings require two pieces of foam to make the wing. That is extra glue and also requires the fuselage to have the airfoil cutout in it. Or you can build one up and that is way more work and weight than a flat wing.

Airfoils start getting more useful at speed and in the 3D world are meant for heavier planes. For super light flippy airplanes they just weight it down and unless you love building just make it take longer for you to get flying.

Airfoils eliminate pitching problems that flat foils have but you should not have been going that fast on your flat foil you should be doing tricks at slow speed :)

If you want to mod your plane to avoid pitching problems maybe a heavier plane made for speed is a better choice.

Now given that here is a two piece airfoil made for sticking on the flat wing to test it out.
Also to further contradict everything I just said I can offer a wing for the 3D planes no problem. Each wing half will cost the same or more than the whole flat kit. It could be a hinge yourself wing so no matter where you hinge it you will have an airfoiled aileron.

The gap in front of the LE in the photo won't be there when the halves are installed.

From the test I am guessing the airfoil will help it go a little slower and will reduce pitch problems. (ok that is 3 contradicitons now I'm out)

[banktoturn]

I believe foilsim allows you to compare flat foil to an airfoil of any thickness.

Here is some brash comments ready for review:

Under 10mph an airfoil doesn't do too much and the weight of the airfoil may out weigh the extra lift it makes.
In level flight an airfoil always has more drag since it is thicker and diverts the path of more air. A light foamie can dive straight down faster than a foamie with an airfoil.

Airfoils are impractical to make compared to flat parts therefore the cost of a kit is much higher with airfoils than flat wings.

Construction is more difficult with airfoils.
Tapered wings require two pieces of foam to make the wing. That is extra glue and also requires the fuselage to have the airfoil cutout in it. Or you can build one up and that is way more work and weight than a flat wing.

Airfoils start getting more useful at speed and in the 3D world are meant for heavier planes. For super light flippy airplanes they just weight it down and unless you love building just make it take longer for you to get flying.

Airfoils eliminate pitching problems that flat foils have but you should not have been going that fast on your flat foil you should be doing tricks at slow speed :)

If you want to mod your plane to avoid pitching problems maybe a heavier plane made for speed is a better choice.

Now given that here is a two piece airfoil made for sticking on the flat wing to test it out.
Also to further contradict everything I just said I can offer a wing for the 3D planes no problem. Each wing half will cost the same or more than the whole flat kit. It could be a hinge yourself wing so no matter where you hinge it you will have an airfoiled aileron.

The gap in front of the LE in the photo won't be there when the halves are installed.

From the test I am guessing the airfoil will help it go a little slower and will reduce pitch problems. (ok that is 3 contradicitons now I'm out)

Your first point is why I distinguish between thin and flat. A flat plate should be compared to a 'traditional' airfoil of the same thickness. Having said that, a decent airfoil will probably have lower drag than a plate even if it is considerably thicker. A more complex shape is likely to be more difficult and time-consuming to make than a less complicated one. This gives you a cost/benefit decision to make, which is always a good exercise for your brain.

banktoturn

[colbyweb]

I just got back from the high school wind tunnel a little while ago. Their wind tunnel is not very usefull really, all it can do is graph the lift/drag. I have printed graphs of the results from each airfoil and I will post them as soon as I have time to get them scanned. But I can sum it up very well without posting the graphs. I graphed each wing from 0 to 21 mph. The sysmetrical airfoil (NACA 0014, 7" long) created very little drag and started creating lift at about 12mph, and dramatically jumped up at 19mph then leveled off. I actually ran the test on the sysmetrical airfoil up to 40mph but didn't graph it. There was no significant change between 19 and 40 mph. The flat plate (1/4" x 7") created -0- lift from 0 to 21 mph but offered a little less drag than the sysmetrical airfoil. The graph is in foot/lbs in .10 increments and the difference between the two airfoils at 20mph is about half a notch or .5 foot/lbs.
I have not had time to finish the flat plate wing for the convertible plane yet but I've started it. Hopefully I can finish it today and get it tested.
-Rick

[tolladay]

Rick,
Thanks for the results. Sounds kind of interesting. Did you modify the AOA while testing? From what I understand a flat plate only generates lift with some kind of positive incidence. You might retest both airfoils at some point at 2 and 4 degrees positive incidence. Also what is the width of the airfoils? Can't do the reynolds numbers with that info.

banktoturn,
It sounds like we agree that different airfoils are good for different planes and types of flying. More than once I have probably posted that a Flat plate is superior, and now I see the error of my ways. It may well be superior to an airfoil for a light 3D plane, but that is not the same as superior at low Re.

[colbyweb]

Rick,
Thanks for the results. Sounds kind of interesting. Did you modify the AOA while testing? From what I understand a flat plate only generates lift with some kind of positive incidence. You might retest both airfoils at some point at 2 and 4 degrees positive incidence. Also what is the width of the airfoils? Can't do the reynolds numbers with that info.


The airfoils were 4" wide each. Yes I played with the angle of attack, all it did was create more drag. I've decided I want to test an undercambered foam wing against a clark Y and I will be back at the high school tomorrow morning. I may play with the symetrical/flat combo again while I'm at it. I need to get a screen shot of the graph and find a way to email it to myself. I'm not sure I can do that from the computer that runs the wind tunnel, since it's a dedicated unit.
As I understand the concept of lift from what I was taught in school, lift is when a vacuum occurs above the wing surface. Increasing the angle of attack just creates drag however at speed it causes the flat surface to "plane" along, as would your hand if you hold it out of a moving car window but this isn't technically "lift".
In case no one noticed (and I forgot to point it out) there was almost no difference in performance between the two airfoils at less than 12 mph.
-Rick

[tolladay]

As I understand the concept of lift from what I was taught in school, lift is when a vacuum occurs above the wing surface. Increasing the angle of attack just creates drag however at speed it causes the flat surface to "plane" along, as would your hand if you hold it out of a moving car window but this isn't technically "lift"...
-Rick

I think I see the problems here. Lift from lower pressure above a wing and higher pressure below is the classic way that lift is taught in school. Good old Bernoulli (sp?) was the first guy to do the math for this. Unfortunately its not the whole story. You can also get lift from angle of attack (AOA, although I'm not sure that AOA is the correct technical term). This is true for an airfoil, a plank of wood, or (as you pointed out) a hand outside the car window. If the force raises the plane then its lift. Period. Nothing technical about it.

From what I understand, AOA is the only way a fully symetrical wing can produce lift. If you think about it, at zero degrees inclination the airflow is identical on both the top and the bottom of the wing. Which side is the low pressure area and which side is the high pressure area? Hard to get lift that way.

Now you can see why I was curious about testing with some kind of incidence.

As to speeds under 12 mph. The Re for your wings 780 (or is it 720?) * 4 (wing chord) * speed (in mph) gives you the Re numbers. In this case under 35,000, which I understand is a bit of a "no mans land" are far as lift goes.

banktoturn,
Care to point out where I'm wrong?

[banktoturn]

tolladay,

Gosh, I don't want to be just the guy that says everyone is wrong!

Unfortunately, my view is that the 'bottom line' on lift is simply
that the air flows around the wing ( or other lift-generating object )
in such a way that the pressure on the top is lower than that on
the bottom, and this is what gives us lift. It is true that some other
things happen to the air in the vicinity of the wing, but the lift is
a direct result of the pressure difference. If you measure the pressure
and calculate the force resulting from the variation between the
top and bottom, you will exactly determine the lift generated by
the wing. There will be no missing components caused by other
effects. The wing may have to be at an angle of attack in order
to generate this pressure difference, but that is not a separate
effect.

banktoturn

[colbyweb]

I finished up the flat plate wing for the convertible airplane. If the wind allows I will try it out tomorrow. I have flown the other wing so much now that I know it's characteristics intimately so I should be able to make a good comparison. It's funny, this thing was just a test bed and I didn't even intend to keep it after testing, but it's turned out to be the best flyer in my stable. I'm already planning to build a lighter version of it, because I did not even try to save weight on this one.
Interesting observation of the day:
After finishing up the flat plate wing complete with servo and ready to use, I decided to compare the weight against the full airfoil wing. I have added 2" of span to the symetrical wing so it is 2" longer than the flat plate. Drum roll....
full airfoil symetrical wing, complete and ready to fly with 2" extensions: 49.5 grams
blucor flat plate complete and ready to fly: 54.8 grams.

Of course the two wings are constructed differently. Both have 1 9 gram allerc servo and the same control rods and horns. The full airfoil wing has a lot of tape reinforcement including heavy fiber reinforced strapping tape. The blucor flat plate only has one 20" length of 4mm carbon tube with no glue, it is layed in a groove and taped over.
I'll be going back to the high school tomorrow but I don't think I will have time to do anymore testing. I wanted to test some other airfoils as well as run more tests on the original two test subjects. I will try to run some more tests on the original two airfoils adding a few degrees of positive incidence to make Tolladay happy.
The real world test will be much more interesting though :D
-Rick

[colbyweb]

Oh by the way, if anyone is interested I finished up the plans for the convertible wing and started a thread on it with the new plans posted there. The plans only show the full airfoil wing and includes a template of a Profili generated NACA 0014 airfoil. Best plans I've drawn to date.
http://www.rcgroups.com/forums/showthread.php?t=329685
-Rick

[tolladay]

Gosh, I don't want to be just the guy that says everyone is wrong!

Not to worry. I was just poking a little fun at ya. :D

I will try to run some more tests on the original two airfoils adding a few degrees of positive incidence to make Tolladay happy.

Now Rick the only way you could possibly make me happy would be to... Ahhh, I see you posted the plans for your excellent little plane before I could even ask for them. Too cool. Now I'm happy. Well not quite. On Friday I'll start a week long vacation in Palm Desert. THEN I'll be happy.

[colbyweb]

Now Rick the only way you could possibly make me happy would be to... Ahhh, I see you posted the plans for your excellent little plane before I could even ask for them. Too cool. Now I'm happy. Well not quite. On Friday I'll start a week long vacation in Palm Desert. THEN I'll be happy.
Glad to hear you'll be happy soon! Where is Palm Desert by the way? Is it some kind of resort?
I forgot to post the pic of the convertible plane with the new flat wing, so here it is
-Rick

[tolladay]

Where is Palm Desert by the way? Is it some kind of resort?


About a stones throw (or a couple of good drives if your into golf) from Palm Springs. Low altitude desert, in the middle of southern Calfornia. Warm dry, with lots of golf courses and vacation rental places. Think of it as the less humid version of Florida. Of course it's hot in the summer, and I meant HOT. But all in all it's a nice place to visit, and a great place for my son to play. Very kid friendly.

I've been reading about some foam jets that have been proving to be very successful, and happen to incorporate a flat wing. Now even I wouldn't normally suggest such a wing for a jet, but I think the designer fell onto something here with the design. The planes all use a high thrust 100w setup (recommended is 15 oz. thrust and a 50 mph pitch speed) which looks at first to be hot, but the flat plate must keep the plane in check because they are definately park flyers, you could easily fly one in the space of a football field. Think of it as the jet for the common flyer. They also, for obvious reasons, have fairly good high alpha capabilities, especially the Grippen. Here's some links.

F/A-18 (http://www.rcgroups.com/forums/showthread.php?t=271581)
T-38 (http://www.rcgroups.com/forums/showthread.php?t=306817)
JAS 39 Grippen (http://www.rcgroups.com/forums/showthread.php?t=308830)

[colbyweb]

I've been reading about some foam jets that have been proving to be very successful, and happen to incorporate a flat wing. Now even I wouldn't normally suggest such a wing for a jet, but I think the designer fell onto something here with the design. The planes all use a high thrust 100w setup (recommended is 15 oz. thrust and a 50 mph pitch speed) which looks at first to be hot, but the flat plate must keep the plane in check because they are definately park flyers, you could easily fly one in the space of a football field. Think of it as the jet for the common flyer. They also, for obvious reasons, have fairly good high alpha capabilities, especially the Grippen. Here's some links.

F/A-18 (http://www.rcgroups.com/forums/showthread.php?t=271581)
T-38 (http://www.rcgroups.com/forums/showthread.php?t=306817)
JAS 39 Grippen (http://www.rcgroups.com/forums/showthread.php?t=308830)
I didn't follow your links (don't have time) but I've read some of the threads about flat winged jets as well. It seems some of them are even flying them at 100+ mph. So you're right, they must have them under control. I think I'd rather have a real airfoil on mine though :)
-Rick

[colbyweb]

Finally flew with both wings today. Hardly a breeze blowing at all today, it was really a beautiful day here. My conclusion is that for all intents and purposes the flat wing was about as good as the symetrical. There was however a dramatic difference in climb/sink rates. I immediately noticed a much longer take off run for the flat wing, and when trying to glide it dropped like a rock, whereas the symetrical wing has a fairly good glide. It took more elevator useage and more throttle to climb with the flat plate. I could not tell that the flat plate was anymore pitch sensitive than the symetrical wing, even at high speeds. Someone suggested that I might see a difference when coming out of a wing stalled maneuver such as a hover, but I don't know since I really haven't gotten the hang those maneuvers yet. I am able to fly at high alpha pretty well and I diddn't see any significant difference there either.

[timocharis]

Rick,

If the airfoil gives the plane a glide, that would be good enough to make me happy. One thing I really don't like about my flat plate planes is the "rock" effect. Sometimes (especially between frantic maneuvers) I just like to let it float a bit.

I have seen flat-plate planes float, but usually only with a very thin (2-3mm) plate and very clean presentation. This is not possible with EPP, and your test may be (from my point of view) very important.

I wonder what the critical factor is? (Airfoil, rounded front, something else ...)


Dave North

[3DXTC]

Rick,

First, let me express my thanks and appreciation for the building, wind tunnel testing, and flight testing you performed. I really appreciate your efforts and sharing the information as you went along.

Most of my EPP experience to date indicates that the planes have more of a parachute descent profile instead of a glide profile. The light wing loading and high drag probably are two big contrubutors.

Rounding the leading edges on the EPP wing and stabilizers seems to make a minor improvement. It is so subjective, comparing one plane configuration to another.

Your wing tests and their results have convinced me to continue with the flat plate wing and just enjoy the great fun flying. In other words, don't wory about efficiency, enjoy the action. Sloper Steve's theme of keep it simple for building, flying, and repairing these planes is right on target as far as I am concerned :) .

Thanks again for bringing together all this information on the size of planes we fly.

Best regards,

Tony C

[colbyweb]

Rick,

If the airfoil gives the plane a glide, that would be good enough to make me happy. One thing I really don't like about my flat plate planes is the "rock" effect. Sometimes (especially between frantic maneuvers) I just like to let it float a bit.

I have seen flat-plate planes float, but usually only with a very thin (2-3mm) plate and very clean presentation. This is not possible with EPP, and your test may be (from my point of view) very important.

I wonder what the critical factor is? (Airfoil, rounded front, something else ...)


Dave North
The airfoiled wing definitely glides pretty well.
Somewhere in this thred I mentioned that the leading edge of the airfoil on my test airplane is NOT rounded, but pointed. I do not know how to properly describe it so I drew a picture below. The guy who designed the Southern Cross airplanes stated somewhere that such airfoils fly better. He did not say what was better about it, but I decided to try such an airfoil and see for myself. I do not know if there is really anything better about it, but it works just fine.
-Rick

[colbyweb]

Rick,

First, let me express my thanks and appreciation for the building, wind tunnel testing, and flight testing you performed. I really appreciate your efforts and sharing the information as you went along.

Most of my EPP experience to date indicates that the planes have more of a parachute descent profile instead of a glide profile. The light wing loading and high drag probably are two big contrubutors.

Rounding the leading edges on the EPP wing and stabilizers seems to make a minor improvement. It is so subjective, comparing one plane configuration to another.

Your wing tests and their results have convinced me to continue with the flat plate wing and just enjoy the great fun flying. In other words, don't wory about efficiency, enjoy the action. Sloper Steve's theme of keep it simple for building, flying, and repairing these planes is right on target as far as I am concerned :) .

Thanks again for bringing together all this information on the size of planes we fly.

Best regards,

Tony C
Your most welcome! Thank you for the kind words, it's nice to know someone appreciates it.
My testing brought me to the opposite conclusion, but if I did not have the means to make hot wired wings I would probably stick with the flat plates. For me it is so easy to make a hot wire cut wing that I don't think I'll make anymore flat plates. It's actually cheaper, easier and quicker to make a hot wired wing than a flat plate, depending on the material it is made from. For instance I am pretty sure that if the airfoil was cut from EPP it would need carbon reinforcement and then it would take about the same time, but materials would be even higher since EPP costs more than other common foams. But I use white or blue foam for my wings and at this small scale, the only reinforcing I need is tape. So no spar slots, no additional cost (and weight) of carbon, no waiting on glue to cure. I do not ever seem to break wings in crashes so I make wings from cheap white beaded foam whenever possible. It's lighter and cheaper than blue foam.
-Rick

[3DXTC]

Interesting how ones perspective or ones needs lead to selecting a specific path to follow.

Back in my balsa and foam plane days, a number of foam core wings were damaged or destroyed. Have not tried many foam and tape wing planes. Makes sense that if the plane is as light as the ones we are flying that the damage would be minimized during a crash.

There are a pair of white foam wing cores that I cut a couple of years ago just sitting in a corner of my basement. You comments have catalyzed a reconsideration of how to utilize them.

I still very much like the crash resistance of the EPP Mini 3D planes. More of them will be built and flown as I try to increase my 3D flying skill level.

By the way, are your tail surfaces airfoiled or flat plate?

[colbyweb]

Interesting how ones perspective or ones needs lead to selecting a specific path to follow.

Back in my balsa and foam plane days, a number of foam core wings were damaged or destroyed. Have not tried many foam and tape wing planes. Makes sense that if the plane is as light as the ones we are flying that the damage would be minimized during a crash.

There are a pair of white foam wing cores that I cut a couple of years ago just sitting in a corner of my basement. You comments have catalyzed a reconsideration of how to utilize them.

I still very much like the crash resistance of the EPP Mini 3D planes. More of them will be built and flown as I try to increase my 3D flying skill level.

By the way, are your tail surfaces airfoiled or flat plate?

Tail surfaces are flat. I've played around with airfoiled tail feathers and it's really hard to tell if there's any benefit. I too like the crash resistance of the epp planes. I bought a box of epp blocks from sloper_steve when he was offering them and I've been using it :) Whenever he gets more I will probably place another order.
-Rick

[colbyweb]

hey 3dxtc,
A little story about white foam durability. I built a 40" shoulder wing, cessna-like plane this past summer just to have something for some of the kids to fly. The thing was only about 12 oz ready to fly. That plane was really put throught the wringer. Once my father in law was visiting and I let him fly it. He was doing fairly well until all of a sudden he had it diving straight down. Before i could grab it, it went in full power. The fuselage was broken in half behind the wing, and there was little left of the nose. The wing popped off (it is held by rubber bands) and received no damage whatsover. I repaired the plane and it saw quite a bit more service before I retired it. I ripped up the fuselage and threw it away, but I still have the wing and it's still like new :D. And the only reinforcment on that wing is TAPE! And it is not covered in tape like a zagi, there are just a few strategically placed strips.
-Rick

[3DXTC]

Rick,
Your airfoil comments below;

Somewhere in this thred I mentioned that the leading edge of the airfoil on my test airplane is NOT rounded, but pointed. I do not know how to properly describe it so I drew a picture below. The guy who designed the Southern Cross airplanes stated somewhere that such airfoils fly better.

are very interesting.

On some full size aircraft adding a sharp edge to the leading edge is what designers do to a section of the wing root to promote separation and initiate wing root stall. The objective is to have the wing root area stall before the wing tip stalls and causes a snap roll.

Maybe you have a wing leading edge turbulator with this design.

Tony C

[colbyweb]

Rick,
Your airfoil comments below;

Somewhere in this thred I mentioned that the leading edge of the airfoil on my test airplane is NOT rounded, but pointed. I do not know how to properly describe it so I drew a picture below. The guy who designed the Southern Cross airplanes stated somewhere that such airfoils fly better.

are very interesting.

On some full size aircraft adding a sharp edge to the leading edge is what designers do to a section of the wing root to promote separation and initiate wing root stall. The objective is to have the wing root area stall before the wing tip stalls and causes a snap roll.

Maybe you have a wing leading edge turbulator with this design.

Tony C
Hmm I dunno, it certainly flies good, I can tell you that :d
Sounds like the airfoil design you're talking about would only be in or near the root area of the wing? I may have to try that.
I'm really having a lot fun with this thing, it's like flying a much larger plane.
-Rick

[sloper steve]

Oh my-- the stagger foil completely smoothed out pitch!
It worked so good I didn't even try the two curved pieces to attach on the top and bottom of the wing.

How does it look? ummmm....
Why do you think it works?
Well maybe cutting them at angles would help.
Better postitioning? Chord? It is easy to test :)

They are 1" chord and distanced about 1" from the leading edge.

It has been written at our low speeds laminar flow has about the same drag as turbulent flow.

[Foamaholic]

Wow. You like it better than without? Will you use it on all your flat planes now? I'll have to try it on the Mini.

[sloper steve]

If it looked better I might use it for speedy flying.
All it is held on by is 3 pieces of strapping tape and air can get under the gaps.
That air gap may help or hurt I don't know :)

I'm focused on learing rolling circles in small areas type flying and like the pitch reaction at slow speeds with the flat foil. At real slow speeds it had less pitch response with the airfoil because it loses the quick reaction (too smooth)

If you fly 3D outdoors and go over 20mph then I'd say an airfoil or something that is similar is good to have unless you don't mind getting used to the bad pitch effects of flat foils at speed.

Flat wings for speed is bad
(airfoil mellows pitch)

Flat wings for slow under 6oz 3D planes is good
(airfoil weighs more, slows pitch and who needs that at 2mph?)

[jrb]

How does it work?

Since the flow is subsonic the air already knows that your “stagger” is there as it passes over the leading edge; so the air flows nicely up and over the stagger – it doesn’t just go vertical when it hits the dam.

So your stagger simulates a smooth contoured airfoil between it and the leading. This will be the case only if the stagger isn’t to thick/high – so you got it right!

After the stagger, the flow doesn’t jump right back to the flat plat either; instead it too flows towards the trailing edge as if there were a smooth contoured airfoil.

Again, if the stagger thickness was dramatic it would work quite as well; again you got it right!

This was a very clever experiment; I can imagine a tapered stagger made of depron which does what you did but also adds a bit of spar to the flat plate wing. The tapering would descries it effect towards the tip, or not.

[Andy W]

Think about how spoilers work..

[jrb]

Spoilers are a perfect example of the stagger being too tall!

[sawicki]

How does it work?


In another thread, there is experimentation on folding a flap on the bottom to get similar improvements. Quick & Dirty Poly Plane (http://www.rcgroups.com/forums/showpost.php?p=3368021&postcount=31) Looks very similar. Any thoughts as to which is more effective? A strip on the bottom or a strip on the top?

Felix

[sloper steve]

For the poly plane I'd think on top would help lift (longer distance over top of wing) and on the bottom it makes the airfoil a little more symmetrical- at least the first inch!

Just tried a tapered strip only on the top at 1" wide on the tip and 1.5" wide at the root.
It helped the pitch issue a lot!

Trip Strip #1
1" wide strip 1" back from leading edge on both top and bottom.
Results: great pitch softening
Wing tested on has 8 inch root and 6 inch tip chord.

Trip Strip #2
1" wide strip 1" back from leading edge on both top and bottom.
45 degree angle cuts to smoote edges
Results: not tested

Trip Strip #3
1" wide strip 0" back from leading edge on both top and bottom.
Results: not tested

Trip Strip #4
1" wide strip 1" back from leading edge on both top and bottom.
45 degree angle cuts to smoote edges
Results: not tested

Tapered version:
1.5" wide at root and 1" at tip worked good on top of wing only.
Did not test on bottom also.

[sloper steve]

We know EPP or depron may be mounted vertical in a wing and act as a spar.
Typically the wing is hollow and air doesn't hit the vertical spar.

The 'trip strip', 'stagger foil' or whatever its name may be has now come out of its shell and shown it is functional on the outside.

We now have a vertical section around 1" thick passing through the wing.
(3 * 8mm = 24mm) top strip, wing, bottom strip
Flat carbon on top and bottom may be enough without an inside spar.

Creating spars out of EPP using I beam techniques may work well.

Example:
Splice the 1 inch strip into halves or thirds and epoxy back together.
Epoxy will create vertical spar strips (like flat carbon rod perpendicular to the wing).
One of these on each wing should reduce flex a lot.
If Epoxy cracks I know Liquid Nails would work but it is the resitance to compression and stretching that gives the strength. According to NHP epoxy 5 minue epoxy cracks and 30 minute has flex.

Depron:
Use 6 mil depron slices (8.46mm wide) and glue together 3 of them to get a 1 inch wide spar. This spar will have 3 vertical epoxy strips and 6 vertical depron skin strips. Anyone have the depron to try a sample spar?

[Gene Bond]

Great discussion, can't believe I missed it for this long.

I've used the 'stagger strips' a few times, and noticed the same tendancy, to reduce pitch sensitivity at higher speed. The added benefit of adding strength with little weight gain (and low cost) help as well.

It also begs to wonder if just gluing a c/f tube to the surface top and bottom, at 25-30% chord might do the same thing? Talk about a stiffener! The opposed spars work well, as we've proven time and again.

On the LE pointy vs blunt:
Pointy LE's promote quicker stalls. Good for 3D, bad for slow (relaxing) flying. I've fixed tip-stally planes by putting a flat strip on the LE :) calms them right down!

Steve noted the sym-covers on the flat wing calmed the aileron response down too much for slow, jerky 3D... Same thing, I think... With the flat wing, stall is almost continuous. Add an airfoil, and you have to work too hard to stall it first :)

Now guys are adding drag inducers to the TE's for indoor pattern... a whole 'nuther variable to contend with!

[Gene Bond]

Oops, forgot to subscribe...


And, I realized another contradiction :( blunt/pointy... my brain aches!

[TWrecks]

Here is an interesting if very technical paper on Low Re airfoil design. They don't claim to have found the secret sauce though. Pointy leading edge is said to be better. Also, moving max thickness back is good. Some airfoils from Dr. Drela for low Re (DAE31 etc) show these attibutes as well as high camber. In general the NACA stuff is meant for much higher Re numbers.

I read somewhere that using a "proper" airfoil results in much lower drag at small control surface deflections than a flat plate.

Tim

http://www.google.com/url?sa=t&ct=res&cd=2&url=http%3A%2F%2Fwww.iag.uni-stuttgart.de%2Flaminarwindkanal%2Fpdf-dateien%2Flowre00.pdf&ei=HakRRqKaI42UgATrzOi2Aw&usg=__z2J_VcPMu3Ru1Gib2CMe1b3TaGI=&sig2=b4JhJgA2yHRdzBqMcbdWJw