I've been messing around with a cheap, high-wing "trainer" that's .46 size. I got it for two reasons. I wanted a beater that I could use to break in my new OS.46AX's and I wanted to play around with an airplane that had anhedral.

This ARF seems to prove the idea that the asian ARFs are being put together by people from the toy industry who don't have a complete grasp of flying models. But that's another thread..... Bottom line is that most of the effort so far has been to get the sucker sorted out. I haven't had much time yet to see what the anhedral does. I've mostly been flying and fixing. But most things are now fixed and the last couple of times out I've been able to check out "what anhedral does".

The wing is a semi-symmetrical. It definitely isn't flat bottom. Heck, almost none of the "flat parts" of this thing are flat or straight.... chuckle And I think it really might have been "designed" to have dihedral but that's irrelevant. The wing joiner fits into the wing boxes with so much slop that you actually could set almost any anhedral/dihedral you wished, if you ignored the angle of the root ribs. So I cut a new plywood joiner that gave 2.5degrees and fit the boxes as good as possible, and ground the root ribs to match my 2.5 (instead of their ~1degree anhedral). Anyway, the wing is semi-symmetrical and it appears that, at least with my model, the airfoil varies a bit in the two sides of the wing. One side is actually flatbottomed from the spar back to the TE while the other side of the wing never quite flattens out. That side is curved just enough that I believe it had to be the result of the ribs' shape, not just lousy construction. So in other words, the way the model flies might have something to do with the different airfoils left to right.

Back to anhedral...........

First thing I wondered about was which way the model would roll when it was yawed with the rudder. I've flown a bunch of dihedral rudder/elevator gliders that get their roll from rudder yaw. And I also wanted to see what rolling an anhedral airplane with it's ailerons would do to yaw. Two different situations, even if somewhat related or at least situations that're often confused.

But I ran into something that stopped me from testing roll induced yaw first.
Rolling the plane with ailerons might be somewhat obscured by the lifting airfoils. But with equal aileron movement, my model did have an obvious yaw to the outside of the turns. This was fouled up somewhat by the model's large difference in roll rate. It will roll to the right or clockwise at the rate I expected. It is extremely slow to roll the opposite way. I've tried to make up for the different airfoils by ironing in a bit of twist. The plane still has an obvious, although less pronounced difference in rate. It is still taking some aileron trim, but that's another story. (The ailerons were already hinged.... you can guess..... I've cut one off and reattached but really need to do the other as well.) I figured I needed to sort that out before I could rely on the yaw test giving realistic results. So I redid the worst aileron and twisted the wing some and.... There was still a fair amount of opposite yaw even after the roll rates were closer.

I've setup another servo output "wheel" so I could try differential aileron and that has reduced the yaw coming from the aileron roll. I figured I needed to reduce that before I tested the real question..... what does yaw cause the airplane to do when combined with anhedral? does it cause "opposite" roll or what?

With my model, when I try to do a flat turn with the rudder, just the thing that causes rudder/elevator gliders to roll into the turns, I get....... taa daa......... somewhat of a roll into the turn. And it appears to be about the same turning left as turning right.

So...... after all that, I'd still like anyone who has some experience with anhedral to share with me what they've experienced. Truth is, I think my little bit of experience isn't trustworthy owing to the lousy test tool. I plan to build a clean wing for the sucker (wish I'd done it right away as a matter of fact) and test again, but that'll be awhile. I got a KYOSHO P40 and a Corsair and a Cap232 (just putting the radio in as we speak) in line first......

Anyone??????

A low wing with dihedral becomes a high wing with anhedral when flown inverted.

As you lower the dihedral, you can expect less yaw roll coupling and at some anhedral angle, there will be zero yaw to roll coupling...meaning that the rudder will only yaw the plane without causing any roll. This is desirable in a pattern plane, it lets you use top rudder in a knife edge without haveing to kill the roll with opposite aileron.

I removed the dihedral on one high wing trainer and loved the results, much better aileron response and much easier to fly inverted.

To begin, let’s separate the term “dihedral effect� from “actual dihedral�. Dihedral effect is simply the tendency of an airplane to roll away from a sideslip. Whether a particular wing will provide that tendency depends on several other factors besides actual (physical) dihedral. Generally a high wing with no actual dihedral will exhibit some degree of dihedral effect. The amount depends on such things as height of the rudder, amount of sweep, if any, distribution of the fuselage side area, vertical location of the cg, to name a few. If a particular airplane has some dihedral effect with a flat wing, a little anhedral can eliminate it. A little more anhedral will make the aircraft roll into a sideslip, which is not generally desirable.

For general sport flying, a little dihedral effect is desirable because the airplane is laterally stable and is easier to fly. However for aerobatic flight, involving sideslips (such as knife-edge) it is not desired because it requires coordination with aileron to maintain position. It is referred to as “roll coupling� in that case. The amount of dihedral effect desired or not is a matter of personal preference. Lack of it requires more attention to fly, and pretty quick thumbs, while a little dihedral provides a more relaxing and sedate airplane.

Sounds like the "tool" is a pig. My experience is not extensive with anhedral, but I've done this mod to a pair of shoulder-wing sport planes & several trainers.

The sport planes are old Florio Flyers & the anhedral eliminated the inherent positive roll couple with rudder application, without introducing adverse yaw (which you are experiencing). They will now knife-edge with very little, if any aileron juggling, plus they now don't pitch to the belly in knife-edge nearly as much as before.

The trainers include a Hobbico Avistar, a Hobbico Superstar, a Hobbico Nexstar & a Vietnamese-made something-or-other called a "Paragon" (which it most certainly is not).

Both the Superstar & Avistar also had the wings dropped into the cabin in addition to the anhedral mod, so that is probably more extreme than your configuration. They both have dramatically reduced positive roll couple, with no adverse yaw & they have nice quick near-axial rolls.

The Nexstar simply has anhedral added & it also shows much less positive roll couple, no adverse yaw and much faster, near-axial rolls.

The "Paragon" is something else altogether. It had zero dihedral in its stock configuration, but it had adverse yaw requiring as much aileron differential as was practical. It still had a bit of adverse yaw even then & it wallowed through rolls like a drunken pig. The addition of anhedral to the Paragon brought back the adverse yaw, but it did nothing to enhance the roll characteristics. There is something adrift in its geometry, but I haven't had the time to really mess with it yet. At the moment I'm not completely sure what's the root cause of the problem & I may not bother -- it's a piece of crap. Yours may be something similar.

I have flown many high and shoulder wing planes with anhedral to correct roll with rudder and have yet to find nd downside to doing it. I have literally cut several wings apart to add in anhedral and just used heavy weight fiberglass cloth and epoxy to re-join the wing panels.

Here are the rules for dihrdral effect:
Dihedral, sweep back and a high wing location will cause roll in the direction of rudder.
Anhedral, sweep forward and a low wing location will cause roll opposite to the rudder.

Knowing this, you can understand why I tend to laugh when people think that taking all the dihedral out of a low wing will make the plane fly better. Several times I have had people with flat low wings fly level and put in rudder and watched the plane roll the other way. A flat wing only works if it is non-swept and mid located.

You have to balance the design for all these factors. Generally, 3 degrees of anhedral works pretty well. Most of the time I invert the dihedral brace, but if it is not enough, I cut a new one. I fill in the gap with an old piece of TE stock. You uslally have to trim the wing saddle for the downward angle and may have to use a rat tail file on the dowel holes.

The first plane is a World Models Happy Fli with a piped .32
Second is an Easy Sport I modified with anhedral, 2" ailerons, 3" elevators and 4" rudder and .61 powered. Made a great little airplane.
Third is an old Eagle trainer with an ahhedral wing.
Fourth is another view of the Easy Sport.
Then there is a sort of MiG combat plane
Finally, a couple of shots of a Joss Stick with anhedral. The Joss flies excellent knife edge.

I have to include my Big Stick. It has an inverted dihedral brace which isn't enough. It does need more to cancel roll.

If you don't want to cut your wing apart, you can add downward end plates as shown on a Little Stick. These add anhedral effect, cancelling adverse roll. The original Little Stick is shown without plates. I added lite ply plates, hanging down 1 1/2". These were lined off in 1/4" spaces. I flew and it rolled opposite to rudder. I then trimmed off 1/4" with some Craftsman shears and flew again. At 3/4 inch, the roll was pretty much killed. That's the way they are now. I use this plane to break in smaller engines, .20-.36.

Ed, those are all low wing airplanes with the landing gear on the wrong side of the fusilage.

One effect of anhedral that hasn't been mentioned is that it provides increased stability at high angles of attack. If you look at most if not all modern airliners you will notice that the horizontal stabs appear to have dihedral but with up elevator they are in effect a inverted wing and actually have anhedral. In the nose high attitude for landing or take off some of the effectiveness of the vertical stab is lost due to dirty air from the fuselage so the anhedral tends to add stabilty to the tail. Also a wing with anhedral should have more stability at high angles of attack.
Regarding effective dihedral, some airplanes with high dihedral angles, such as the North American Navion, which despite having a whopping 7 degrees loses it's effective dihedral when the flaps are put down. It then has near neutral stability and will not return to level flight by itself particularly when sideslipped. The fix for this was to add interconnect cables between the ailerons and rudder. This allowed the wing to pick up by simply moving the rudder.

Actually, anhedral destabilizes straight-winged aircraft at high angles of attack. It shifts the center of pressure forward, thus having the effect of moving the C-of-G aft & increasing the nose-up trim force. Conversely, dihedral shifts the AC rearward, reducing nose-up trim forces.

Boy is this thing a dog. Yeah, it was only $40, but sometimes you don't get what you paid for. This one really hasn't been worth spit so far.

I just got back from a very pleasant day at the field. It only got over freezing after about an hour, but with most of the day having only a very slight breeze, and not a cloud in the sky, you couldn't tell that it was cold. I spent most of my flying trying to figure out why the model responded to the elevator so oddly.

It would respond to the elevator smoothly sometimes. Sometimes it'd HOOK AROUND like it'd snagged it's wing on something every once in awhile. I tried to see if there was some pattern to it. Tried it inside and outside. Even detuned the exponential in the TX for elevator. Then when I was wiping the sucker down I took a close look at the elevator. The hinges were "gone" on one side. Only one side had any connection at all. You wanna know why it wasn't patently obvious just looking at it?

This sucker ain't got one nano-ounce of balsa anywhere in it. It's all either plywood or some kind of SE Asian wood that's sorta yellow and must be part rubber, 'cause it flexes when you try to cut it with a knife. And the elevator is that yellow stuff. And it holds it's shape unless pushed or pulled. And just sitting there, the elevator looks like the hinges are good to go. How that sucker didn't flutter in flight I got only one guess. The last bit of hinge holding that one side gave up on the last landing.

I decided to put nylon hinges into the thing when I got home and that might have been a bad move too. I might have burned out my new Slot Machine cutting the slots for those hinges. The wood just sorta gummed up the blades and would bow back from the tool if I pushed it. Amazing.........

This thing has been throwing those SUDDEN TIGHT LOOPS for the last few outings. And the model has only been out about five times. So the hinges went after maybe 6-10 flights. They're just clear plastic and seem to be all different lengths.

"You get what you pay for." isn't really true when suggesting that a cheap model is only worth what you paid for it. This one hasn't been and it was only $40. (Although, I gotta admit that I've laughed my butt off a couple of times while messing with it. It actually HAS been worth $40 worth of entertainment and I've learned a bunch from it. Actually, I'm glad I got it.)

I did not know this. Isn't it also true that geometric anhedral partially inhibits spanwise flow, increasing effeciency to a point?

edit: the part I didn't know was that anhedral moves CP forward.

I am guessing that you consider anhedral to have an effect of "trapping" high pressure air beneath the wing -- air that would otherwise partially leak away spanwise, particularly as compared to a wing with dihedral. That effect would theoretically be there, but I doubt if you could even measure it with any confidence in normal flight. However, in very close ground-effect it would be real enough. The Ekranoplan uses that effect.

Yeah, I read something like that somewhere. It reduces vortex strength, leaving more energy available for lift.

You were replying to my post while I was editing it -- see the edited post

I thought I'd add a bit about aircraft stability that hasn't really been discussed.

Dihedral effect - the effect of a sideslip causing a rolling moment in the opposite direction - is stabalizing. It is very important in the design of full-scale aircraft because it allows the pilot to control roll with the rudder if the aileron controls fail. However, high lateral stability means increased dutch-roll (The aircraft rolling and yawing back and forth at a given frequency). Therefore, selecting the dihedral/anhedral angle must be done as a trade off between lateral and dutch-roll stability.

Based on the original question posted, it looks like you want to perform a completely flat turn without using aileron mixing. To do this you would have to reduce the dihedral effect of the aircraft to zero, essentially making the aircraft neutrally stable. There are many factors that contribute to the dihedral effect of the aircraft. The primary contributor is the wing, of which three parameters contribute: dihedral angle, sweep angle, and wing placement on fuselage. The vertical tail also contributes a certain amount if its aerodynamic center is above the aircraft c.g.. So, to get zero dihedral effect you could design a mid wing aircraft with no wing or h. tail dihedral, with the v. tail extending above and below the aircraft c.g..

Or you could simply add wing anhedral until the net dihedral effect of the aircraft is zero. You can determine the dihedral effect through calculations (I suggest using Dr. Jan Roskam's books here) or you can do it through testing. If you put too much anhedral in, however, the aircarft will become unstable and a small sideslip will quickly turn into a huge sideslip angle. In addition to this, the rudder will cause a small rolling moment away from the turn directing if the rudder a.c. is above the aircraft c.g..

While, roll-yaw coupling may seem like a problem, it is one of the primary effects that makes aircraft controllable. All I'm saying is be careful what you ask for.

In terms of Britbrat's post about dihedral moving the aircraft aerodynamic center, that is completely false. Wing anhedral is destabilizing in the lateral mode, but has no effect on the longitudinal mode. I find that when we talk about aircraft stability, most people think of simply longitudinal stability and where the c.g. is relative to the a.c., but wing dihedral affects the lateral stability of the aircraft, which is just as important.

Varying dihedral on a given airplane does change the three-dimensional position of the aerodynamic center. This in turn causes nonlinear variation of the pitching moment coefficient with AOA. See chapter 4 of Mechanics of Flight by Phillips.

It isn't false at all -- it depends entirely upon the geometric relationship between the center of mass, and center of lift. If the center of mass & center of lift lie on the same horizontal plane in an aircraft with zero dihedral, pitch-up results in virtually no longitudinal change in that relation ship. In cases where the center of mass is vertically displaced from the center of lift & either dihedral, or anhedral are present, pitch-up will shift the center of lift axially relative to the center of mass. The longer the wingspan & the greater the dihedral, or anhedral angle, the greater the longitudinal displacement. Anhedral shifts the center of lift forward & dihedral shifts it aft.

Anhedral destabilizes aircraft in both roll & pitch.

I'm sorry, but I completely disagree. Airplane longitudinal stability is defined as: C_m_alpha < 0.

And C_m_alpha = C_L_alpha*(X_c.g. - X_a.c.)

The vertical position of the aerodynamic center has nothing (or very little) to do with it. I suggest Dr. Roskam's books on the subject. Dr. Roskam's company also sells the AAA software (Advanced Aircrat Analysis). Which is used by numerous aerospace companies. And I could show you with AAA, that anhedral does is not destabilizing longitudinally.

Of course, most of the equations used in aircraft design and analysis assume linear relationships. Whether these assumptions are valid with r/c aircraft is up for debate, but in my experience they work just fine.

Too bad you dissagree.

It is easier to understand if one takes an extreme example. Think of a wing with extreme dihedral -- the center of lift will be well above the aircraft center of mass (& probably well above the fuselage). Run a wire across between the wings at the 1/2 span location & at longitudinally above the spar (approximate location of the center of pressure). Stick a bit of tape at the center-point of the wire & tip the plane into a nose up attitude. What is happening to the center of lift, relative to the center of mass?

There is a reason I said that my statements apply for the LINEAR range. This means small angles. By taking the example to extreme dihedral angles and extreme angles of attack you have invalidated one of the many assumptions used in typical aircraft stability and control calculations. I agree with you that if you had a wing with a huge anhedral angle, it would be destabilizing longitudinally. However, for reasonable angles (<10 degrees) the dihedral/anhedral angle has VIRTUALLY no effect and can be ignored.

One of the first assumptions made when calculating the aircraft pitching moment coefficient with respect to angle of attack is that the angle of attack is small such that sin(alpha) = 0 and cos(alpha) = 1. If you work out the equations, the change in wing aerodynamic center of a wing with dihedral is:

dx_ac = y_ac*TAN(DIHEDRAL)*SIN(ALPHA)

Therefore, according to the small angle assumption, this change is negligible.

There are many higher order factors affecting stability calculations that are ignored. The important thing is to keep the primary factors in mind. Maybe I shouldn't make such bold, definitive statements, but if someone ever said "Dihedral affects longitudinal stability" in one of my design reviews I'd throw them out of my office.

If you want to include every higher order term you can in your stability calculations, go ahead. I'll take my +/-5% accuracy and move on.

You're production oriented, if I read between the lines correctly, and you make common, valid, generally accepted approximations because of that. Most of the rest of us in this forum are interested in some of the higher order approximations, and a definitive statement such as, "andhedral has little no effect on lateral stability", is bound to bring counter-argument. If that's so, why bother with dihedral? Why not just make all wings flat and avoid all the bother of the structural difficulties with dihedral? Designers of model aerobatic airplanes go to great lengths to find the correct dihedral angle that results in as little roll coupling with yaw as possible. Even a small amount of coupling is annoying to an aerobatic competitor. And of course most trainers have positive dihedral because it's important to the handling qualities for a trainer. I know that yaw-roll coupling is not the same thing as lateral stability but it's related. Airplanes with neutral yaw-roll coupling are neutrally stable in roll, and so forth.

Paradigm, models are frequently configured well outside the norm for full scale aircraft, or even outside what is workable in FS aircraft without computerized control systems. They are alsocommonly flown well outside normal full scale flight envelopes. A simple standard model flight trainer has far more dihedral & far greater vertical separation between the center of pressure & the center of mass, than is the case with virtually any full scale aircraft. What isn't significant to you -- is not significant to you.

Yeah, a typical foamy or 40% aerobat might have a little trouble meeting Part 23 stability requirments! But then you don't have to fly them cross-country, with Grandma in the right seat. We can't have airplanes doing walls and waterfalls just because the pilot dropped his pen and inadvertently pulled on the yoke as he reached for it .

Just because it doesn't have a 5% static margin, or doesn't meet part 23 requirements, doesn't mean it isn't flyable as a model.

Let's see, where to start. First, about mesae's comment about me saying that "dihedral has little no [sic] effect on lateral stability." I never said that. Dihedral is the primary factor driving lateral stability. It however, has a very very small effect on longitudinal stability. My point is that aircraft designers have tools to create certain flying qualities. To create changes in lateral stability, dihedral is the primary tool. This does not really affect longitudinal stability however, because the dihedral angle is relatively small. If for some reason you go to extremely large dihedral, or anhedral angles, then the longitudinal stability could be affected. However, doing this would completely screw up the lateral stability. So much so, that it really becomes a moot point (I know it's redundant to call a forum dicussion moot).

Now, in terms of being production oriented. You're somewhat correct. I'm a modeler that also has a masters in aeronautical engineering and engineering experience. Does that mean I know more than someone with model experience? Not necessarily. But, I do know aircraft stability and control. Mesae, I think you need to reread my post about what the original question was, which is to make it possible to perform a flat turn without mixing. There is only one way to do this, and that is to reduce the the derivative c_l_beta to zero. This is what you could call yaw-roll coupling or dihedral effect. It is also known as the lateral stability parameter. If you reduce the dihedral effect to zero, then your goal is to reduce the lateral stabiltiy of the aircraft to zero. PERIOD. END OF DISCUSSION. If you disagree with that statement then you need to consult some more books.

Now, with that said, all I was saying is that one should be careful in attempting this otherwise you could get crappy flying qualities. I agree with you guys that model aircraft can be flown far outside the range of normal aircraft. At the same time, I often hear things from other r/c pilots like "Why do you waste your time with all that theoretical analysis crap. My plane flies just fine". Well the only reason your plane flies fine is because in r/c aircraft, the high power-to-weight ratio and huge control power can make up for really awful design. In other words, when you have excessive power-to-weight, an aircraft with horrible flying qualities doesn't seem to fly that different from one with good flying qualities.

The reason I said dihedral "is very important in full scale aircraft..." is because I was trying to say that having positive lateral stability in r/c aircraft isn't quite as important. With full scale aircraft being able to command sufficient roll control with the rudder is important. But in most r/c aircraft it just doesn't matter (Unless of course your trainer has no ailerons).

Just answer this: Have you ever flown an aircraft with the c.g. well behind the most aft c.g. specified (other than 3D flight)? If so, how did it fly?

Maybe I misread something; if so, please accept my apology. I wasn't arguing for dihedral affecting longitudinal stability in any important way. I did write a post that touched lightly on the vertical difference between AC and CG and thrust vector.

Based on your last post, I don't think we actually disagree about anything important.

I am not an engineer, but I am an ATP ME, CFI/II/MEI with several thousand hours professional flight time (and aerobatic competition), former air traffic controller and current FAA terminal instrument procedures specialist, 30 year modeler, and have studied a bit of theory in my off time. Don't want to give anyone a false impression about myself. If I tried, someone like you would certainly catch it quickly.

I don't disagree with your statement but you can bet that I will continue studying anyway, for my own benefit.