Anhedral wings?

[IloveFLIGHT!]

I can understand the benefits of dihedral in a wing, and even polyhedral. But what does anhedral (such as the F104) do for the plane? It would seem to me to make the plane very unstable? :confused: Thanks Chris :)

[Brandano]

I found a nice thread discussing the anhedral on the 104, right here on rcgroups: http://www.rcgroups.com/forums/showthread.php?t=41426
Personally I think that the T tail and the large vertical tail work almost like some "virtual dihedral" since so much more surface is projected toward the side of the roll. On most other planes the anhedral is used to counteract the dihedral effect given by the swept wings, especially if shoulder mounted. See for example the AV8B Harrier, but also the F8 Crusader or the B52. Fighter planes work better if they have neutral stability, or (in these days of FBW systems) if they are intrinsically unstable. The F104 didn't have a fly by wire system, but it probably had some stability augmentation on the roll axis.

[IloveFLIGHT!]

Thanks Brandano!

[Texas Buzzard]

I know of one model that used anhedral and was a decent flier.
It was a Bill Evans design ( glow) and it was to emulate a futuristic plane.
Hey - anhedral would be dihedral if you were flying inverted. :cool:

[BMatthews]

I know of one model that used anhedral and was a decent flier.
It was a Bill Evans design ( glow) and it was to emulate a futuristic plane.
Hey - anhedral would be dihedral if you were flying inverted. :cool:

You're not thinking of the X wing Star Wars flying wing he did that was loosely based on the one in the first movie are you?

[admodesi]

As Brandano said above, many high-wing - swept-wing aircraft have anhedral to restore some of the aileron control as this configuration creates too much stability if normal dihedral or a flat wing a used. On the Harrier and B-52, it also reduces the length of the wing tip undercarriage!

Attached is a pic of a Su-25 a made a few years ago. It only has a small amount of anhedral, but its enough to make it look good in the air, and has very benign handling characteristics. Don’t be scared of using anhedral if it is needed to make a scale aircraft look right or to fix one of the problems mentioned above.

[Texas Buzzard]

You're not thinking of the X wing Star Wars flying wing he did that was loosely based on the one in the first movie are you?
...............................................
Yes that's it. You are correct - It was an X-wing ship now that I remember. It did fly tho'.

[MCarlton]

I seem to remember someone in an old Silent Flight magazine with a PSS Klingon Bird Of Prey which flew, and that was essentially a tailess plank with "polyanhredral" so anything is possible!

[Chip01]

The anhedral is usually found on high performance jets for high MACH issues (supersonic). The wings on the F-104 were very short and very thin. The tip to fuselage was about 9 feet and the fuselage was over 50 feet long from pitot tip to T-tail. It used among other things Boundary Layer Control (BLC), leading edge flaps as well as trailing edge flaps to generate subsonic take off lift. Once cleaned up (all that stuff retracted) it was one very clean aircraft. I once heard a 104 driver say if he had 1500 pounds of fuel (about 230 gallons) at altitude (30,000 feet or so) over Dicky Goober (Kansas City, MO) he could make it to Wright Patt with fuel to spare. Unfortunately, the plane could not turn, was a point interceptor and had one of the lousiest accident records around.

The F-4 had an anhedral on the stabilator. It to used BLC, leading edge and trailing edge flaps. The later E model capped off the dreaded Boundary Layer Control and used leading edge manuevering flaps that activated as a function of angle of attack. This allowed that airplane to vastly improve its turing ability, but with all that added drag made it sort of doggy as far as Max MACH was concerned. The F-4D was a MACH 2.05 jet and the F-4E had a hard time getting to MACH 1.6 with slightly more powerful engines.

All of those airplanes had some form of stability augmentation. They also had elaborate systems to adjust the sense of feel in relation to speed. Even the old F-100 (circa 1954 airplane) had yaw dampers or a form of early stabilization system.

Cheers,

Chip

[JetPlaneFlyer]

The anhedral is usually found on high performance jets for high MACH issues (supersonic).

No... the main reason for anhedral is to add negative lateral stability.. Swept and/or high wings have a strong dihedral effect. Most aircraft, especially fighters, dont want excessive stability as it reduces control responsiveness and leads to yaw/roll coupling (dutch roll) ... So anhedral is added to achieve about neutral overall lateral stability.

There are plenty of examples of subsonic aircraft that use anhedral the BAE146 being one example, but you will find anhedral used on virtually any high wing + swept wing aircraft whether supersonic or not for the reasons explained above.

The F104 used anhedral for exactly the same reason but in this case the dihedral effect came from the big vertical stab and T tail.

Here are a few references that back up what I'm saying here:
http://en.wikipedia.org/wiki/Dihedral
http://selair.selkirk.bc.ca/aerodynamics1/Stability/Page5.html
http://www.aerospaceweb.org/question/dynamics/q0055.shtml
http://history.nasa.gov/SP-367/chapt9.htm#f145

[MCarlton]

On the subject, I have often pondered to what extent a V-tail has a dihedral effect and whether I would get a more neutrally stable V-tail model if I added a smidge of anhedral to the wings?

[MarkusN]

On the subject, I have often pondered to what extent a V-tail has a dihedral effect and whether I would get a more neutrally stable V-tail model if I added a smidge of anhedral to the wings?It does have a dihedral effect, but very small leverage because of its small span. I'd assume its influence to be negligible.

[MCarlton]

Thats what I thought.

Mind, I hate the look of anhedral unless its on a scale model, I always add a touch of dihedral to even a flat wing, otherwise it looks like a coathanger

[NX-687]

I have a tail sitter that is in the shape of a circle cut in half with elevons on the back for control, motor on front , , and rudder its called a VTO , now with right rudder, application it rolls right too ???????
So if I put in anhederal it would reduce this tendency , is that right :confused:
seems so to me :)

But when its inverted it has dihederal so , oh darn I guess you cant have it right both ways

[Brandano]

NX-687, indeed, the dihedral-anhedral effects rely on a positive AOA, which is the common state for a plane. The wing sweep will roll the plane in the direction of the rudder whether this is at a positive AOA or negative AOA, while at a negative AOA the dihedral will roll the plane in the direction opposite the rudder input. I think that your yaw/roll coupling is probably caused by a sweep effect . However it should only be noticeable in forward or backward flight, and not much in the hover.

[NX-687]

Ive a question , my model with the circular type wing with elevons on back , , what would be the effective wing sweep 45 degrees ????
That means I need 15 degrees anhedral to counter the roll with rudder problem and I assume that is 15 degrees with one side of the wing flat on the table and the other tilted up at 15 degrees
Great site for sorting my problems

[JetPlaneFlyer]

You would usually calculate wing sweep at the 25 chord line which would give you somewhere in the region of 35 deg average sweep on a semi-circular wing.

The 'dihedral like' effect of sweep varies according to Cl.. ie. it has a stong dihedral effect at high angles of attack and virtually no dihedral effect when the model is flying fast at very low AoA... So fixing the required anhedral is a compromise, a perfect ballance at all flying speeds is impossible.

You really dont want to set too much anhedral as this could give very unpleasant handling. I'd set no more than 3-5 Degrees on each panel (i.e. both wings 'drooping' at 3-5 deg).

Steve

[NX-687]

It is set at a low angle ,as you suggest, not quite sure what actual angle it is , have not measured it yet , and will test in the morning ,

[NX-687]

Its nicer to fly and throw around, its lost some of those bad mixed control tenancies and powers roughly in the direction commanded ,nice when your down real low
:)

[NX-687]

On my delta with the flat wing , no anhedral
If I put on tip rockets , in the shape of say a flat protrusion say 50 mm wide and 70 mm long , same thickness as the wing 6mm, that would effectively take some of the sweep back out of the wing , reducing the dihedral effect
I will have to try , it might work and its an easy mod