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Jul 11, 2017, 09:33 PM
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aeronaut999's Avatar
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Hatching a theory-- re sweep, ground effect, roll torque generated by sideslip

This pertains to some things I've been musing over for some years now-- some of you know what they are--

For those who want the r.c. model connection, I have indeed used an rc model in exploring some of the related ideas--

Anyway here's the specific question

Imagine a swept wing, at zero bank angle, in ground effect. Imagine a sideways (left to right) airflow component (sideslip) due to a temporary sideways gust, deflected rudder, whatever.

In general sweep contributes a dihedral-like response to sideslip, i.e. contributes a roll torque in the "downwind" direction, assuming we're flying at a positive angle-of-attack.

I have some understanding that ground effect reduces downwash through impinging on the associated vortices (is that fair to say?)

Would it be fair to say that since the vortex activity is greatest near the wingtips, ground effect increases the effectiveness of the wingtips more than it increases the effectiveness of the rest of the wing? In other words, in terms of lift distribution (not drag distribution), ground effect could be modeled as an increase in wing area, or an increase in angle-of-attack, of the outboard parts of the wings in relation to the inboard parts of the wings?

If the wing has washout, could we say that the effect of ground effect on lift distribution is similar to the effect of a decrease in washout?

Bear in mind that the dihedral-like roll torque contributed by sweep during a sideslip is highly dependent on lift coefficient, or we could say, on angle-of-attack.

Basically the idea I'm getting around to is the idea that in ground effect, during a wings-level sideslip, the dihedral-like effect of sweep "sees" a larger "effective span" of the wing than it does out of ground effect, and so more roll torque is generated in ground effect than out of ground effect.

Does that seem likely or am I looking at the problem the wrong way?

Meanwhile, if the wing also has anhedral, the roll torque created by anhedral during a wings-level sideslip is not very dependent (if it all) on what the lift coefficient or angle-of-attack of the wing was before the sideslip began. (E.g. we still get a roll torque in the expected direction even if we were flying at the zero-lift-angle-of-attack, in a ballistic arc, or for that matter even if the initial angle-of-attack was negative, such as during sustained inverted flight.) And by the same token it seems the roll torque contributed by anhedral during a sideslip will be much less affected by ground effect than will be the roll torque contributed by sweep.

Does that make sense?

If so, ground effect would tend to shift the "effective dihedral" (defined as the combined effects of sweep and anhedral, in terms of roll torque generated by sideslip) of such an aircraft in the more positive or less negative direction, at least in the wings-level case. (I don't want to get into the effect of what happens when one wingtip is lower in ground effect than the other, at least right now.)

None of this is meant to suggest that a wings-level sideslip is necessarily a steady-state situation--I'm just looking at the instant of time after the sideways flow begins and before the aircraft starts to roll and/or turn, and exploring what factors are affecting how strong the roll torque is and in which direction it acts.


PS haven't we talked in the past about roll torque created by the shift in the position of the wingtip vortices during a sideslip, even with a simple flat (no dihedral) rectangular wing? (Maybe we should add "mid-wing" to that list, i.e. neither high-wing nor low-wing.) Let's see, which way did that roll torque go? And would that effect also be reduced by ground effect? Thinking it through quickly it seems that that effect ought to create a dihedral-like, "downwind" roll torque, and it seems that yes it ought to be reduced by ground effect.

I guess I'm imagining that in the case of a swept-wing aircraft, the effect I'm proposing above might be more important than the simple vortex-displacement effect we see with the rectangular wing, in terms of any positive or negative shift in "effective dihedral" that is specifically due to entering ground effect. So that in the case of the swept-wing aircraft with some anhedral, the net shift in "effective dihedral" we see as we enter ground effect is in the more positive or less negative direction. Might this be more likely to be true if the swept wing is highly tapered, rather than nearly constant-chord? I know taper also complicates the effect I'm proposing as well but... just kind of thinking out loud here at this point--

For starters maybe set aside the vortex-displacement effect as a separate issue and just consider the question of whether it is legitimate to imagine that ground effect tends to reduce the intensity of the wingtip vortices, and whether it is legitimate to imagine that this increases the "effective span" of the wing as seen in terms of the lever-arm that is at play in terms of the dihedral-like roll torque exerted by sweep in the presence of a sideways flow. And whether it is legitimate to visualize that the lever-arm or "effective span" at play in terms of the roll torque exerted in the presence of a sideways flow by any actual dihedral or anhedral that may be present, is NOT affected (or is not affected as strongly) by ground effect.

Ultimately I know we can't break all these things down into separate components and instead we need to model the entire flow field around the wing but still it seems some insight might be gained into considering the problem in this way.

Last edited by aeronaut999; Jul 12, 2017 at 02:09 PM.
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Jul 12, 2017, 02:10 AM
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Steve, my first thought was, to "what real-world application" would such an insight bring us? And then to "All Aerodynamics Is Empirical", and that brought me to water-fowl. Ducks, swans, gulls, albatrosses, etc., spend a lot of time in ground-effect over water and presumably with many millions of years air-time doing this have worked out effective procedures to develop and best use their conformable wings in "ground"-effect.

There are many videos and stills showing this and I note that the outer portion of most species wings are not only swept but anhedralled. These two "design features" are employed in their normal gliding flight to various degrees to get some combination of max L/D at one extreme or enhanced roll-control with controlled sink as one would use spoilers. I watch my birds glide often like this on approach.

So my suggestion is to very carefully view videos of water-birds glide-skimming over lakes and look for evidence including BSLD, crosswind perturbations (wind easily seen over water), etc., bearing in mind the aeroelasticity of their flight feathers.

Jul 12, 2017, 07:48 AM
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here is one of "Wisdom's" hatching offspring who will only much later try out water-skimming
Jul 12, 2017, 09:52 AM
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aeronaut999's Avatar
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off topic

Here's a link to an article with some interesting points about what happens when a wing is skimming very low over the water and one wingtip is is lower than the other--

but this is a diversion. I re-direct the readers' attention to my original post which is strictly focussed on the case of zero bank angle--

Jul 12, 2017, 10:28 AM
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note the pure gliding portions

Salton Sea Pelicans gliding over the water (0 min 38 sec)

similar with slight x-wind

12 pelicans gliding over water surface (0 min 18 sec)
Jul 12, 2017, 01:21 PM
An itch?. Scratch build.
eflightray's Avatar
This experimental model, (7.2 meters long), shows it banking when turning with rudder.

It does seem to get a little high at times, perhaps it's quite lightly loaded.

Kaien-3: fugure-8 flight test of WISES model (full length movie) (2 min 33 sec)

Jul 17, 2017, 12:24 PM
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BMatthews's Avatar
This seems like a serious case of the "it depends.... ". The question also stated that it is in relation to side winds while parked firmly on the landing gear. Although I guess it could extend to side winds while taxiing. And this is a lot different than the wing in ground effect shown in the videos above. I'm not sure I see any connection.

First off let's recall that the rolling or equivalent dihedral effect from swept wings is quite minimal. A rule of thumb I've read in a lot of places over the years is that it takes roughly 30 of wing sweep to be comparable to 5 of proper dihedral. And this also only applies if the wing is operating at some positive lift condition such as in flight. Not while on the ground with the airfoil mostly sitting at what should be an angle that is near the zero lift angle for the airfoil.

Mind you I suppose there's always a "trail dragger" or "tip dragger" option to consider in which case the wing would be at a fairly strong angle of attack to any gusts. And in such a case it would also be in a primo geometry to generate WIG lift if the wind is from any frontal angle.

And because this is wind over the ground as opposed to flying in ground effect I can't help but think that this would cause more "it depends" factors to come out. For example for planes tied down on the parking aprons at the airport in a cross wind which has the most lifting effect from the wind? A Cessna 180 with the high wing or a Piper Cherokee with the low wing?

One factor you do raise is the angle of attack while on it's wheels. And for a swept flying wing we can expect a lot of span wise twist which will reduce the angle of attack on the tips during a direct side on wind direction. And if the angle is more negative than the zero lift angle I'd even expect the wing tips to be seen to flex DOWNWARDS slightly while resting on the landing gear. So this is a strong "it depends" factor.

Anhedral would likely reduce the effect since it would deflect the side winds upwards and away from the wing and the airfoil would see any front to back flow component as being at a strongly negative angle of attack.
Jul 17, 2017, 01:58 PM
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Bruce-- re immediately above-- was that meant to address my opening post on this thread? I was speaking of flight, not standing on landing gear. And I although I didn't say so explicitly, it's simplest to consider the still-air case-- I'm speaking of ground effect, not wind gradient. Though since I specified that the wings were level at the moment of interest, wind gradient wouldn't actually come into play. Well it might affect the vortices or whatnot-- well at least consider the still-air case first before adding the complication of wind gradient. Actually, if a strong wind gradient would somehow alter the effects at play even when the wing is level above flat ground, I would be interested to think about the effects of that too, after considering the still-air case.
Last edited by aeronaut999; Jul 17, 2017 at 02:04 PM.
Jul 17, 2017, 06:34 PM
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BMatthews's Avatar
OK, I messed up. Between reading the first post to writing my reply I somehow swapped out "ground effect" for "on the ground" in my mind. So much of what I posted is now bupkuss. I suppose some of it still applies to transitional side gusts though.

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