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Old Feb 23, 2013, 02:18 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
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Anhedral, just a thought.

It's either too cold, too wet, or too windy to fly, so was sitting here earlier today looking out the window watching some of the local birds flying.

Most were seagulls, plus a few crows and jackdaws. The one common thing seemed to be that when gliding, (not flapping), most had their wings in an anhedral configuration. (Gull wing ?, not always, but more of an arc curving down from the body ).

If it's good enough for birds, and with the now increase in available flight stabilizers for models, could gliders/power models, get any advantage from anhedral ?

It seems like it works for birds, but then they have a built in stabilizer, ( a brain), probably modelers don't have the ability to fly an unstable model by eye. But with a modern AS3X, Eagle Tree, OrangeRX 3-Axis, Arduino etc etc........ ?

Any advantage ?
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Old Feb 23, 2013, 03:49 PM
A witty saying proves nothing.
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There is an advantage to particular forms of cambered spans. Gulls use hyper-elliptic cambered span, which can have a sizable aero advantage, but it is quite sensitive to the washout profile along the span:

http://ntrs.nasa.gov/archive/nasa/ca...2006130622.pdf

I doubt that it would be very hard to fly even without electronic roll stabilization. The divergence period in roll of a long wing span glider type airplane, which is where the advantage might be significant, is quite slow, and I think most decent pilots could cope with it. The airplane might not be much fun to thermal without stabilization, because of the attention required.

http://arxiv.org/ftp/arxiv/papers/0804/0804.4752.pdf

The problem is taking off and landing. I guess you could hand launch, and land inverted?

Kevin
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Old Feb 24, 2013, 07:46 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,007 Posts
Thanks for the reply Kevin.

I should have checked first, a search has come up with a thread from 2008 that also covers some of the subject -

Anhedral wings?

Seagulls just seem to be so well 'designed' , it's fascinating to just sit a watch them fly when I can't.

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Old Feb 24, 2013, 08:26 AM
Texas Buzzard
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McAllen,Texas
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A quick thought

Seagulls have a builtin autopilot. Consider lateral stability.
If anhedral was good wouldn;t factory-built light planes have it?
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Old Feb 24, 2013, 09:55 AM
Jim C Patrick
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Shenandoah County
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Quote:
Originally Posted by Texas Buzzard View Post
. . . If anhedral was good wouldn;t factory-built light planes have it?
Not really. 'Real' airplane design is extremely conservative, building nothing that isn't proven and proven again. If a modification doesn't work, people's lives may be lost, and the company liable for enormous $ damages in addition to FAA fines and sanctions.

With an RC model the risk is cost of materials and construction time. RC models are also more tolerant of 'characteristics' unacceptable to passenger aircraft, like oscillations, 12G turns, recovery from mid-airs, or dork landings.

Although I don't have the time or inclination to experiment on it, seagulls' wings have anhedral but also have sweep. Since sweep creates the equivalent of dihedral, it should be possible to have an anhedral with sweep that is neutral or stable in flight.
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Old Feb 24, 2013, 10:25 AM
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Have a look at the Antonov-225 cargo plane...
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Old Feb 24, 2013, 11:19 AM
A witty saying proves nothing.
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Anhedral for handling reasons, to counteract the dihedral effect of sweep, is a different thing than using it to get the wing tip vortices further apart.

You can get some of the induced drag reduction benefits with upward curved dihedral wings too. I suppose that was the thinking behind the Radian and the Hobie. Even multiple polyhedral breaks likely achieve some of the induced drag gains, although the increased form drag from the sharp breaks may well negate it.

http://ntrs.nasa.gov/archive/nasa/ca...1964006060.pdf

In practice I think the washout profile is critical, and the drag would be quite sensitive to yaw angle without active control. Plus you end up with a huge dihedral effect that may be as bad for aircraft handling as the anhedral. At least you can land upright though!

Yes, seagulls are quite beautiful in the air. I get to watch them soar the ridge in front of my house a lot. I have thought about an hyper-elliptic anhedral model, but the landing would require wings that could fold upwards or something. Too much complexity for the gain.

Kevin
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Old Feb 24, 2013, 11:50 AM
Ascended Master
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Tried it, didn't like it.
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Old Feb 24, 2013, 01:34 PM
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In certain parts of the envelope, the Harrier will do the same (left rudder gives right turn). Didn't like it either.

Kevin, wouldn't the tips of an hyper-elliptic anhedral be a good place for wheels?
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Old Feb 24, 2013, 05:13 PM
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Hmm, actually, no. The Rutan Quickie is such a handful to control during the roll that some were modded to have a more conventional gear. Unless you land with the wings perfectly level, the first wheel to touch the ground will drag you in a ground loop.
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Old Feb 24, 2013, 09:33 PM
Jim C Patrick
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Shenandoah County
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I know some of the flying wings are swept (some of the sweep is from taper) with a bit of anhedral. Just asking, but does anyone think that anhedral make the plane inherently unstable if there is strong sweep also?
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Old Feb 24, 2013, 10:50 PM
A witty saying proves nothing.
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The dihedral effect from sweep is proportional to wing Cl. At low Cl (fast flight) the dihedral effect from the sweep will be very small. This leaves the anhedral effect only at high speeds, which may mean roll instability and yaw reversal. The handling will change with wing Cl.

Kevin
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Old Feb 28, 2013, 01:03 PM
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anhedral

Hang gliders have enough anhedral to give "negative effective dihedral" over most of the flight envelope, over all but the lowest part of the airspeed range. This is not for performance, but rather for handling (to minimize the unfavorable roll torque that would otherwise be generated by adverse yaw.)

Note that the wing is swept. Therefore the "effective dihedral" must become progressively less positive or more negative as angle-of-attack is decreased (i.e. as the airspeed is increased). Having neutral, or constant, "effective dihedral" over the entire flight envelope is not an option.

One would expect terrible spiral instability as a result of the "negative effective dihedral" noted above, but this is relieved in large measure by substantial washout. Also by roll damping, since a diving spiral involves a continual inward rolling motion or roll rate. Especially at higher sink rates (higher airspeeds).

At high airspeeds (low angles-of-attack) the tips are negatively lifting, due to washout, and thus the tips are contributing a rolling-out torque rather than a rolling-in torque, in turning flight.

Due to the combined effects of washout and roll damping, a strong rolling-in control input is needed to hold a constant bank angle in high-speed flight.

At low airspeeds (e.g. thermalling) a small rolling-out control input is often needed to hold a constant bank angle.

Thus the observed control requirements in turning flight, and the way that the required control inputs vary with airspeed or angle-of-attack, are actually opposite from what one would predict based purely on the relationship between airspeed/angle-of-attack and "effective dihedral", for a swept anhedral wing.

Adding a motor changes the relationship between bank angle and roll damping-- a climbing turn actually involves a continual rolling-out motion rather than a rolling-in motion, so roll damping tends to drive an increase in bank angle-- so the aircraft is much more spirally unstable in a powered climb than an unpowered descent, for any given airspeed.

With a single flexible hang strap, the pilot's weight acts near the CG of the wing and so we don't have a "pylon effect" or "pendulum effect", assuming that the pilot is free to swing side to side and front to back, applying zero muscle force or constant muscle force, rather locking himself in a fixed position by applying whatever amount of muscle force is required to do so. The dynamics are much the same as if the pilot were located at the CG of the wing rather than suspended below.

I know that wasn't quite the question you asked but...

Steve
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Old Feb 28, 2013, 01:15 PM
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birds

I've often wondered at the gull-like wing shape of certain birds (gulls, ospreys). With dihedral inboard and anhedral outboard, one would expect a net anhedral effect: the outboard panels are further from the CG and thus contribute more roll torque per unit area than the inboard panels.

On the other hand, on all birds, the wing is mounted high on the body and this contributes a dihedral-like effect.

Also, the gull shape has the effect of placing most of the wing area above the CG. This will contribute some dihedral-like "pendulum" effect in a sideslip--the surface area above the cg will contribute a rolling-out torque. I would think, though, that as long as the wing isn't up on a high pylon, this "pendulum" effect will be much less important than the fact that we have anhedral in the outboard wing panels which are far from the CG and generate lots of rolling-in torque in a sideslip.

So, what is the advantage of the gull-shaped wing?

Perhaps something in my analysis is flawed, and the gull-shaped wing actually gives close to neutral "effective dihedral". It's very hard to see how this could be the case though.

Note that land soaring birds that are NOT optimized for the windy marine environment generally soar with dihedral (Turkey vulture, Golden eagle). The gull-shaped wing appears to be most common among marine birds (gulls, osprey.)

I suspect that the advantages of the gull-shaped wing may be related to control more than to performance... ?

By the way, here is an example of an aircraft with a wing with an absurd amount of anhedral, as an accident of the nature of the structure, yet the wing is very high up above the CG on a ridiculously tall pylon, so we end up with positive "effective dihedral" and good hands-off roll stability (tendency to return to wings-level).

http://www.google.com/imgres?imgurl=...9QEwAQ&dur=905

(In reference to the previous post, note that with multiple lines of fixed length, the pilot is now fixed in position relative to the wing, not free to swing from side to side. In other words the wing is in fact now mounted on a fixed pylon high above the CG of the aircraft)
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Old Feb 28, 2013, 02:44 PM
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Seagulls and albatrosses don't fly high up while at sea, they mostly skim the waves. Apparently the albatross even uses dynamic soaring manoeuvres using the waves as if they were land features, to maintain speed with minimal energy expenditure. I think that their wing shape is more optimized for thicker air. Also, both of these have to take off from the water, so I think that the wing shape helps there as well, keeping the wings farther away from the surface on their take off run.
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