No dihedral, no ailerons...what's the result?

[Tim K]

I was out flying with some new acquaintences last night and they had a pitts special with the ailerons disabled, rudder only. The wings have no dihedral.

It didn't fly for a multitiude of reasons, but it got me thinking about this combination. As I understand it in a R/E plane, the rudder induces yaw that turns the outboard wing so the wind is hitting the bottom of the wing causing it to move up, thus providing roll control.

With no dihedral, I would think the plane would yaw with rudder inputs with no, or almost no tendancy to roll. Am I correct?

[MCarlton]

There will usually be a roll response, even if there is no dihedral, I suspect because the induced yaw increases the airspeed of the outer tip, thus creating more lift.

Depending on the model, and the angle of attack its flying at and the amount (or absence) of washout, that roll response will vary.

Any wing which is swept will have a greater roll response to rudder, due to the fact that sweep gives a dihedral effect.

Years ago, before computer radio mixing could "mix out" the roll/yaw couple, aerobatic models, in pursuit of a "pure" yaw with no roll, sometimes had anhedral, and the trimming process involved altering the dihedral angle until the yaw/roll couple was minimised.

[JetPlaneFlyer]

Yeah.. sweep works a bit like dihedral (rough rule of thumb: 5 Deg sweep = 1 deg dihedral) as does placement on the wings high on the fuselage.. However I'd suspect that roll response to rudder would be quite limited on a plane like the Pitts.

Generally if you want to fly rudder and elevator only then you need plenty of dihedral.

Steve

[vintage1]

I was out flying with some new acquaintences last night and they had a pitts special with the ailerons disabled, rudder only. The wings have no dihedral.

It didn't fly for a multitiude of reasons, but it got me thinking about this combination. As I understand it in a R/E plane, the rudder induces yaw that turns the outboard wing so the wind is hitting the bottom of the wing causing it to move up, thus providing roll control.

With no dihedral, I would think the plane would yaw with rudder inputs with no, or almost no tendancy to roll. Am I correct?

Yes, up to a point. Sweepback gives a bit of yaw-roll coupling.

However I found out the hard way what happens to a low dihedral plane with no ailerons..a full opposite rudder diving knife edge across a field followed by that Terrible Sound...

[MCarlton]

I hate that Terrible Sound...

Oddly enough, although with low dihedral there can be a bit of roll with yaw, it seems that once a bank angle is introduced, recovery is nigh on impossible.

[BMatthews]

Your friends were badly advised or terribly misguided in disabling the ailerons on any Pitts Special variant for exactly the reasons you and the rest have already related.

You're almost right on the idea that the yaw allows the airflow to hit the bottom of the now leading wing. But far more importantly is the increase in angle of attack on the leading wing and decrease in angle of attack on the trailing wing during a yaw. These changes induce a much larger change in lift on both panels than the mere air striking the underside of the leading wing would indicate. You'd need to look at the alpha (angle of attack) to lift coefficient to see that this shift in lift due to the angle of attack is far more strong than the lift that would come from the very minimal change in presentation of the lower surces due to the yaw.

[Brandano]

there's another effect that can induce a roll, that I am not sure it would affect a pitts, but it affected the German V1 flying bombs: the fuselage can blank out portion of the inner wing. The problem with this control is that the direction of the resulting roll depends on the AOA of the wing.

[vintage1]

there's another effect that can induce a roll, that I am not sure it would affect a pitts, but it affected the German V1 flying bombs: the fuselage can blank out portion of the inner wing. The problem with this control is that the direction of the resulting roll depends on the AOA of the wing.

There are other effects that can do this..I mentioned sweepback. then there is the lateral centre of area vis a vis the CG location vertically..I think with my low wing that was the killer in that as angle of bank becomes greater, the drag is low and the CG is high..and the model 'falls off its perch'. Certainly you need a lot more dihedral to fly a low wing safely without ailerons, than a shoulder or high wing.

Experiences with various models have shown that yaw->roll coupling is not just dihedral. There are a lot more issues going on, but for good results, you need to make dihedral the dominant one.


Or conversely, if you dont want yaw->roll coupling, the first thing to do is get rid of dihedral entirely, and then start tweaking other things.

[JaRaMW]

You're almost right on the idea that the yaw allows the airflow to hit the bottom of the now leading wing. But far more importantly is the increase in angle of attack on the leading wing and decrease in angle of attack on the trailing wing during a yaw.

But "higher (lateral) AOA" and "air hitting the bottom of the wing" is the same effect.

Also, the lateral airflow around the fuselage during a side slip plays a role even without dihedral. The air flowing around the top of the fuselage locally increases AOA of the windward wing while the air flowing around the bottom locally decreases it. This causes a roll moment away from the sideward air flow for high-wing aircraft and a roll moment into it for low-wing aircraft. Thus, high-wing aircraft are better suited for side-rudder controlled rolls. For a biplane, however, the opposite effects of high and low wing should rather cancel this roll moment. Sweepback, the fuselage blanking out some air flow around the inner wing and the velocity difference due to yaw would then be the only effects favoring a slight roll into the turn.
Ah yes, and of course the sideward lift created by the fuselage itself during a slip and its center of lift relative to the CG. But I think that's a minor part.

[BMatthews]

I used to think like that too until I did some math based on a 2 degree yaw. I calculated the shift in the exposure for both surfaces and it was really minor. Then I calucated the shift in lift coefficients for both wings and it was far more influential. It's been a couple of years since I did all the math and I don't really want to do it again so you're on your own on this... :D

As for air swirling around the fuselage this will be a major issue only during a full fledged side slip. When you're talking a couple of degrees of yaw it's again a very minor issue. It also doesn't explain why a high wing model with no dihedral will act like it has some dihedral while a low wing model with no dihedral will act like it has anhedral. If you look at the lift from the yaw of the fuselage for these two examples they SHOULD react the exact opposite. Yet only in extreme examples of super deep fuselages is this POSSIBLY the case when you look at a variety of real life examples.

The effect is so pronounced that folks that for some reason want to make a high wing aerobatic model have resorted to including a little anhedral in the wing to counter this effect so they can do knife edge flying without the need for some aileron input to hold it from rolling back upright.

[JetPlaneFlyer]

But "higher (lateral) AOA" and "air hitting the bottom of the wing" is the same effect.


But lift is not produced by "air hitting the bottom of the wing"... Air 'flows' around the wing (both top and bottom) and the turning of this flow is what creates lift.
'Air hitting the bottom of the wing' sounds awfully like the false 'skipping stone lift theory' (http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html)

Steve

[JaRaMW]

But lift is not produced by "air hitting the bottom of the wing"... Air 'flows' around the wing (both top and bottom) and the turning of this flow is what creates lift.
'Air hitting the bottom of the wing' sounds awfully like the false 'skipping stone lift theory' (http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html)

Steve

Yep I know, just wanted to point out that the free stream air coming in "more from below" means higher AOA.
The longitudinal AOA does not change during a side slip and the lateral AOA becomes nonzero for a wing with dihedral (while it is zero in the absence of side slip).

BMatthews, I don't see why the fuselage swirl effect wouldn't explain the dihedral-like behavior of a high wing and the anhedral-like behavior of a low wing. Look at the airflow in a yawed turn; that's exactly what happens. I agree though that this effect may be weak and neglible in cases of low yaw angles (but I'd guess that they aren't quite that low when turning with the rudder). The v^2 lift dependence will probably eat all those effects and make even a low wing plane roll into the turn, at least slightly.

[Brandano]

I'd consider the high and low pressure areas formed around the fuselage in relation to the wing. I suspect that with even minor sideslip the fuselage will generate a low pressure area on its trailing side, that will affect a high wing as a decrease of lift and a low wing as an increase of lift. If this is true, a canard wing will feel the "high wing = dihedral" effect much less than a conventional layout, where the wing root is closer to the maximum thickness point on the fuselage.

[BMatthews]

BMatthews, I don't see why the fuselage swirl effect wouldn't explain the dihedral-like behavior of a high wing and the anhedral-like behavior of a low wing. Look at the airflow in a yawed turn; that's exactly what happens. I agree though that this effect may be weak and neglible in cases of low yaw angles (but I'd guess that they aren't quite that low when turning with the rudder). The v^2 lift dependence will probably eat all those effects and make even a low wing plane roll into the turn, at least slightly.


But air hitting and making the fuselage "lift" on a low wing should ADD to the self correcting roll. The side area is above the wing after all. And despite typically being more or less equal area both above and below the true CG point this symptom is still so frequently found. But various designs have shown that this is not the case. For example the Top Flite Contender with no dihedral and a low wing was well known for it's adverse roll when rudder was inputed.

Similarly you would think that a high wing model with what amounts to a below the wing "keel" (the fuselage) would try to "trip" over the low slung fuselage and suffer from adverse rolling effects when yawed. But more than one designer that insisted that their high wing should be neutral in yaw has had to resort to anhedral to achieve that goal despite having a center of gravity that is close to being vertically aligned with the side area.

Your idea that the air is swirling around the fuselage is likely not really the case either except on the ground when the prop wash is all the air the fuselage is seeing. Once in flight this corkscrew like effect is largely diminished as the plane and prop reach a point where they are screwing themselves through the air and there's little if any actual swirl. Or are you refferring to some other effect?
EDIT- and here's another one....

http://www.rcgroups.com/forums/showthread.php?t=1077585

[vintage1]

My experience suggests its not the wind pressure on the models fuselage, but the relationship of the wing drag to the centre of gravity, vertically, that does the damage..

As the model sidselips, the wing is the greater drag element,and it its above the CG it turns into the yaw, if its below it turns way. Once in the bank, if rudder is released, the high wing model is now sliding sideways, and a high drag centre rights the aeroplane, a low drag one leaves it in a nasty knife edge that it doesn't want to pull out of.

[BMatthews]

Now THAT makes a lot more sense. And I'd have to concur that the wing is the major source of drag in any airplane. It's an addition of parasitic and induced drag but drag nontheless so it makes sense that it would tend to hinge around the wing.

[flyingwing12]

I was out flying with some new acquaintences last night and they had a pitts special with the ailerons disabled, rudder only. The wings have no dihedral.

It didn't fly for a multitiude of reasons, but it got me thinking about this combination. As I understand it in a R/E plane, the rudder induces yaw that turns the outboard wing so the wind is hitting the bottom of the wing causing it to move up, thus providing roll control.

With no dihedral, I would think the plane would yaw with rudder inputs with no, or almost no tendancy to roll. Am I correct?

Actually it will turn whether it has dihedral or not. like you said it will cause one wing to slice through the air faster and create more lift and it will eventually roll!!!! :cool: :cool: :cool:

[JetPlaneFlyer]

Actually it will turn whether it has dihedral or not. like you said it will cause one wing to slice through the air faster and create more lift and it will eventually roll!!!! :cool: :cool: :cool:

Not necasaarily. The increased lift due to one wing having higher airspeed than the other only occurs as the plane is in the process or yawing, ie while the plane is rotating. Once the yaw is established there is no airspeed differential. Dihedral effect is a much larger factor in the yaw/roll couple.

If you had a mid/low wing plane with no dihedral (or dihedral effect) you would already know that they generally wont turn on rudder... In fact low wing models without dihedral often roll in the oposite direction to rudder input: http://www.rcgroups.com/forums/showthread.php?t=1077585

Steve

[Texas Buzzard]

Way back when I flew the Rudder Bug with genourous dihedral and rudder only (contol) we had to make the area of the rudder small for rudder movement was very effective in causing a banked turn so much so that we just "bumped" the rudder. That is the effect of a dihedral of about 6 degrees per panal.

Later my Sweet Stick with NO diehedral would just yaw when just the rudder was applied. No bank whatsoever. It would refuse to turn with just rudder input! :eek:

Old design books tell us that 3 (three) degrees of sweep back equals one degree of dihedral. :)

I did build one quasi-delta ( it had a tiny stab on top of the fin) with 30 degrees of sweepback and absolutely no dihedral. It flew rock solid on a 0.09 PAW Diesel. It was a Delta with a cheater hor. stab. for trimming. ;)

[flyingwing12]

Not necasaarily. The increased lift due to one wing having higher airspeed than the other only occurs as the plane is in the process or yawing, ie while the plane is rotating. Once the yaw is established there is no airspeed differential. Dihedral effect is a much larger factor in the yaw/roll couple.

If you had a mid/low wing plane with no dihedral (or dihedral effect) you would already know that they generally wont turn on rudder... In fact low wing models without dihedral often roll in the oposite direction to rudder input: http://www.rcgroups.com/forums/showthread.php?t=1077585

Steve

WELL im just telling you what i experienced with my j-3 cub.

it happened.

[JetPlaneFlyer]

WELL im just telling you what i experienced with my j-3 cub.

it happened.

A J-3 Cub is a high wing model (when I last checked). if you read the full thread you will see it stated a number of times that a high wing position has an effect similar to dihedral (that's why i repeatedly state 'dihedral effect' rather than simply 'dihedral').
A cabin wing position is generally considered to have a dihedral effect similar to between 3 and 8 Deg of 'real' dihedral.. More than enough to give you some rudder/roll couple.

Steve

[KenSt]

Would this same effect be present on a high wing glider such as a Waco CG-4A? I have been wrestling with the thought of including or excluding ailerons. I intend to put in 2° per side.

[JetPlaneFlyer]

Would this same effect be present on a high wing glider such as a Waco CG-4A? I have been wrestling with the thought of including or excluding ailerons. I intend to put in 2° per side.

Yes the effect would be there but i would not rely on it alone to provide adequate response for rudder only turns. 2 Deg of added dihedral 'may' be just about enough but to be sure i'd increase it to more like 5 deg per side.. or leave it scale and add ailerons.

[BMatthews]

On the Waco note the beveled up wing tips. Those angled tips will provide a little bit of dihedral effect. I still would not rely on it to provide a strong enough roll effect.

As JPF says it WILL roll thanks to being a high wing and to some small addition having that style of wing tip but if the effect is not strong enough the roll will be slow to roll into a turn and slow to roll out of a turn. When that rate of roll gets TOO slow it's just no longer possible to make effectively quick enough maneuvers to avoid an "arrival event".

I'd suggest adding a couple of degrees as well if you want to keep it to rudder/elevator.

[vintage1]

Would this same effect be present on a high wing glider such as a Waco CG-4A? I have been wrestling with the thought of including or excluding ailerons. I intend to put in 2° per side.

Id say that in calm air, that will turn and bank OK, but it may get to an irrecoverable position if steep bank is encountered.

I have, since my 'experience' decided that with altitude, I COULD have got out of it by turning INTO the bank and hauling up..

I'd say put ailerons on..you can always remove the servos and glue them solid if not needed.

[KenSt]

Thanks guys, I usually put in huge amounts of differential to prevent adverse yaw, which is almost as bad as no rudder and no ailerons like the pitts :D

[JaRaMW]

But air hitting and making the fuselage "lift" on a low wing should ADD to the self correcting roll. The side area is above the wing after all. And despite typically being more or less equal area both above and below the true CG point this symptom is still so frequently found. But various designs have shown that this is not the case. For example the Top Flite Contender with no dihedral and a low wing was well known for it's adverse roll when rudder was inputed.

Similarly you would think that a high wing model with what amounts to a below the wing "keel" (the fuselage) would try to "trip" over the low slung fuselage and suffer from adverse rolling effects when yawed. But more than one designer that insisted that their high wing should be neutral in yaw has had to resort to anhedral to achieve that goal despite having a center of gravity that is close to being vertically aligned with the side area.

Your idea that the air is swirling around the fuselage is likely not really the case either except on the ground when the prop wash is all the air the fuselage is seeing. Once in flight this corkscrew like effect is largely diminished as the plane and prop reach a point where they are screwing themselves through the air and there's little if any actual swirl. Or are you refferring to some other effect?
EDIT- and here's another one....

http://www.rcgroups.com/forums/showthread.php?t=1077585

I was referring to something else. I'll misuse the following picture to illustrate the effect:
http://www.efluids.com/efluids/bicycle/bicycle_images/flow_patterns.GIF
just look at image (A). Suppose the cylinder is the fuselage seen from behind and the air flows from left to right, representing the side-slip in the turn. Now imagine a low wing mounted to the fuselage: on the left side, the fuselage induces a downward wind component, locally decreasing the wing's AOA while on the right side, it induces an upward component, locally increasing AOA. This asymmetric lift rolls the plane further into the turn. The opposite happens for a high wing.

edit: let's rather say it tends to roll it into the turn. This is of course only one of several roll-inducing effects during a side slip and not necessarily the dominant one.