Does wash out add to the effective dihedral?

[Konrad]

Does wash out add to the effective dihedral of the wing?

As I look at the wing from the point of view of a rudder command I see (to my eye) that there is less effective angle of attack of the leading wing (yaw) with washed out tips.

In a side slip I see an increase in the angle of attack in the lower wing as the ship falls.

An aircraft when viewed from the rear looks to have more dihedral with washout. When viewed from the front I see no net change in the dihedral with wash out.

So which way do I move the dihedral if I add wash out, yet I want the same effective dihedral response as a non washed out wing.

Thanks,
Konrad

[JetPlaneFlyer]

I dont believe washout adds any 'effective dihedral' at all. It's a commonly held missconception as far as I'm concerned.
It does prevent tip stalls and can threfore prevent the model dropping into a spiral dive but the effect is totally different to dihedral.

[BMatthews]

It likely depends. However I know that for low winged rubber scale models one of the trimming tricks is to use a healthy dose of washout to help keep the model stable. But it's not a full on substitute for dihedral. Also it brings with it a measure of inefficiency in the wing if you try to overdo it.

[Brandano]

On those designs where the washout is added by lifting the trailing edge while keeping the leading edge straight you are effectively adding a little dihedral to the wing. But it's no "virtual dihedral" added by the wing twist, it's just that the "average" of the wing tip is higher than that of the center of the wing. It's rather minimal, though.

[JetPlaneFlyer]

The freeflight 'Ohka' model featured in the photo below my user name is probably a good illustration of why i believe there is confussion over the claimed 'dihedral effect' of washout. The model is a small scale low wing rocket power creation with close to scale dihedral. As originally built the model had no washout, it flew ok but if it slowed up it would drop a wing (tip stall) and fall into a steep spiral dive from which it had insufficient lateral stability (dihedral) to recover from.
The solution was to add washout... The washout did not aid the Ohka's recovery from a spiral (as more dihedral may have done) but it did prevent the tip stall induced wing drop, so it never got into a spiral in the first place. Some may misinterprit this as a dihedral like effect, but it's really nothing of the sort.

[vintage1]

Twasn't dihedral it needed. Twas more rudder area...it was unstable in the YAW plane.

If you take a CG calculator, and use the fuselage side area and fin and do a computation as if for pitch staibility, if the CG needs to be ahead of where it needs to be for pitch stability, you have a model that WILL go into vicious spin easily.
Plenty of full size planes had 'anti spin strakes' or bigger dorsals added after rearward CG tests found them deeply dangerous..

[JetPlaneFlyer]

Well if it really was unstable in yaw a bit of washout cured it :rolleyes:

No, it's fine in yaw, it's got virtually no nose and a big fat fuselage aft of the CG, a rearward mounted canopy that adds more side area than a barn door and twin fins... yaw is the least of it's problems.

In my experience increasing fin area can make a model more prone to spiral divergence... I've seen a freeflight model first hand that constantly dropped into spiral dives that was totally cured by snapping off part of its fin. I also know a guy (a very experience and successful US modeller) who built a rubber power Ohka with which he struggled with spiralling problems for 15 years (yes 15 years!). He had built the model with increased fin area and in an effort to sort it increased it further with no luck. After comparing his model with mine and exchanging notes he reduced the fins back to scale size... the model was cured ;) Here's a video he sent me after the adjustment: http://www.hallmanstudio.com/Baka_4.MPG

BTW... NASA agree with me: http://history.nasa.gov/SP-367/f148.htm

Steve

[vintage1]

That drawing from NASA is more complex though..too much fin AND dihedral will make the plane tighten up, but not necessarily spin.Its akin to having a too forward CG with a big tail..to get it trimmed right, you end up with a plane that power stalls on you ..you can get tail wagging..as well

Anyway I didn't want to get into an argument: just to make the point that the lateral side area calcualtins play a deep point in yaw stability, which may or may not translate into roll stability if dihedral or sweepback is evident. CF the Zagi 'death spiral' known to be cured by a forward CG OR more fin..

[Texas Buzzard]

The answer is NO.

[Konrad]

On those designs where the washout is added by lifting the trailing edge while keeping the leading edge straight you are effectively adding a little dihedral to the wing. But it's no "virtual dihedral" added by the wing twist, it's just that the "average" of the wing tip is higher than that of the center of the wing. It's rather minimal, though.

Brando,
This is what I see in the second half of my question. This might add effective dihedral in a side slip (more of the bottom of the lower wing exposed to this vector {higher angle of attack in side slip on the lower wing}).

Jetplaneflier,
I see very little that would confuse a tip stall with spiral stability. Yes to keep the same level of spiral stability as the dihedral is increased one could minimize the vertical fin area.

My question is:
As the wing yaws, as a result of rudder input, will a wing with washout roll as strong to the input as a wing without washout?

To my eye a yaw induced turn will not roll as much with washout as the bottom of the wing (out at the tip) is not exposed as much to the on coming air stream. The bottom of the wing is shielded by the leading edge resulting in less of an angle of attack change from that of a non-twist wing with the same yaw input.

To my way of thinking the ship will respond to a lateral upset faster (stronger) with washout (greater induced dihedral). Yet will respond slower to yaw inputs with washout (less induced dihedral).

I welcome other points of view and explanations as I'm note fully convinced with my arguments.

Thanks for your help,
Konrad

[JetPlaneFlyer]

Konrad... lifting the tip up when you add washout is not 'effective' dihedral it's REAL dihedral (but only a very little bit)... To add pure washout one would have to twist the wing about it's aerodynamic centre so that the LE went down and the TE went up - this 'pure' washout would have no dihedral effect.

I'm not really following your LE 'shielding' the TE argument... the LE is meant to hit the airstream before the TE on any wing, that's how an airfoild is meant to work, and I cant follow how washout would change this. Remember also that the entire chord of the wing is washed out to a lower angle of attack, not just the TE.

[Konrad]

Jetplaneflyer,
So dihedral is measured at mid wing cord?
Yes leading edge hits the air flow before the trailing edge on a wing (or part) that is producing lift. If the wing tip is not producing lift (CoL zero) it is producing only drag. This happens when the trailing edge (bottom of the airfoil) does not see the air stream (Coanda effect excluded).

Konrad

[BMatthews]

Konrad, you're trying to over analyze the issue.

Washout or no the wing will react to yaw inputs the same way. The rolling effect comes from the alteration in angles of attack that the airfoils on each side see thanks to the dihedral. The leading wing seeing an apparent rise in the angle of attack and the trailing wing a reduction. This produces a shift in the lift coefficient for each side with one side gaining and the other loosing. No shading or other effects are the cause. Or at least certainly not the major cause. There's no "shielding" of the lower or upper surfaces going on in level flight with or without washout. The air will flow around both sides in a normal manner to the best of its ability. The only time that this breaks down is during extreme conditions that result in the air forming a separation bubble from the airfoil. Typically this happens on the upper surface when the wing starts operating at higher angles of attack near the stall point. It can happen on the lower surface but generall only during negative G loading like in a pushing forward of the stick to initiate a snappy dive.

I know that a lot of the texts like to show arrows of "air" hitting the lower surface of one wing and the upper surface of the other but in reality you seldom see a yaw angle of more than a couple of degrees. Hardly enough to generate that strong a side slip angle.

[Konrad]

Konrad, you're trying to over analyze the issue.


Yes I'm often guilty of this.

The thought of "shielding" is in error as the rest of the wing needs to function at a higher CoL (higher angle of attack) to compensate for the loss of lift as a result of the washout in the tips.
I conclude that there is an increase in dihedral effect as a result of washout although it is very small. For example a flat wing (ugly stick) with a 1/4 inch washout would have an added effective dihedral of 1/8 inch assuming a full span linear twist.

Thanks for everyone’s input.
Konrad