SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Feb 19, 2012, 12:13 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
Discussion
Curving relative wind, how much slip is ideal in turns, yaw strings videos

Let's open a new discussion space for:

Curving relative wind, how much slip is ideal in turns, yaw string videos

Also pertains to -- automatic turn coordination (sensing slip/ skid via electronic inclinometer or wind vane, automatically correcting via rudder inputs) as discussed in this thread Robotic Assistance for Sailplanes (Cross Country Soaring) -- http://www.rcgroups.com/forums/showt...8#post20794102 -- possible unfavorable consequences of eliminating sideslip, on glider stability/ control/ handling /performance

Key questions of interest--

Can we observe a bit of slip in the yaw string, when flying efficiently? Can we observe more slip in a yaw string at the nose than in a yaw string much further aft? How much slip is ideal (efficient?)

Hang on, I'm going to re-post a couple things I posted to Cross Country Soaring, here, and then post a video link--

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Feb 19, 2012 at 12:28 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 19, 2012, 12:17 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
Background--1

PPS However, such a device [automatic turn coordinator / sideslip eliminator] might open some cans of worms aerodynamically. It might work too well. For example, if we are truly eliminating slip, then there is no point whatsoever in having dihedral, as the dihedral will never feel any sideways flow and so will never generate any roll torque. A model with flat wings will fly exactly the same as a model with generous dihedral.

Since we don't see this in actual practice, this is a very strong argument that we ARE allowing a non-trivial amount of slip in our thermalling turns. It would be very interesting to verify this simply by videoing a yaw string mounted on a little vertical wire by the nose or by the CG, viewed by a little mini video camera on the vertical fin. In the interest of experimentation, the sailplane could be flown in a steady thermal turn in smooth air with the rudder held in several different positions. A data logger could collect sink rate data and the camera would observe sideslip. Post-flight, the different (known) rudder positions could be correlated with the slip angle and the sink rate. Questions of interest would be-- can we document that some sideslip as measured by the yaw string at the CG does in fact give a better sink rate than no sideslip at all? Meanwhile an on-board inclinometer could be also be recording the forces, so that we can get an "apparent tilt angle" (i.e. ball deflection) for the ideal amount sideslip.
aeronaut999 is offline Find More Posts by aeronaut999
Reply With Quote
Old Feb 19, 2012, 12:18 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
Background--2

Yaw string video experiment
Well, I mounted a yaw string on a post near (just aft of) the CG of my 2-meter Spider, and mounted a video camera on the fin, got a beautiful 40-minute sunset flight but when I landed the camera was off- have no idea how much footage I got Bummer!

But I don't have any doubt about what the results were-- the yaw string surely streamed a little toward the outside of a constant-banked turn (showing some sideslip). I think this is going to be true of all aileron-controlled RC sailplanes with any normal rudder mix and any normal flying style (rudder usage). After all most RC pilots feel-- I believe-- that a wing geometry with lots of dihedral contributes a rolling-out torque in a constant-bank turn, causing the glider to need less outside aileron input (or in extreme cases, more inside aileron) input than would otherwise be needed. For example if the model requires zero aileron input in a constant-banked turn, and you then remove some dihedral from the wing, it will then tend to roll into the turn and will require some outside aileron input. Is this not the experience of most RC glider pilots?

This can only happen if the model is being allowed to slip a bit, so that the dihedral "feels" a sideways flow and thus creates a "downwind" roll torque-- toward the outside wing or high wing. With no slip, the dihedral creates no roll torque, and has no effect on the overall balance of roll torques acting on the model.

Allowing a bit of slip makes sense from a performance standpoint as well as a handling standpoint. Assuming that the model has some dihedral, isn't it more efficient to create some sideways flow over the wing and allow the dihedral to create some rolling-out torque, than to keep the model completely coordinated in yaw (slip) and rely solely upon deflecting the ailerons to create the rolling-out torque? (After all, some rolling-out torque is always needed to offset the rolling-in tendency that we would always see in a flat-winged model, or a model with dihedral that was not being allowed to sideslip.) (This rolling-in torque arises from the fact that the outboard wingtip is travelling faster, and covering more distance, than the inboard wingtip.)

Likewise consider the balance of yaw torques-- the outboard wingtip moves faster, and covers more distance, than the inboard wingtip, and so the outboard wingtip experiences more drag than the inboard wingtip. This creates a yawing-out torque, which tends to make the nose point some degrees toward the outside or high side of the turn, relative to the direction of the flight path at any given moment. Does it really make sense to try to eliminate this sideslip completely? Regardless of what the slip angle ends up to be, something must be creating the yawing-in torque that counterbalances the yawing-out torque from the tip drag of the outboard wingtip, bringing the net yaw torque to zero,so that the glider ends up with some constant yaw rotation velocity (zero yaw rotational acceleration.) What is the best way to create this yaw torque? If the fuselage is slender and streamlined (doesn't create lots of drag in a sideslip), then isn't it more efficient to allow the vertical fin to meet the air at a slight sideslip angle, so that its "airfoil" flies at a non-zero angle-of-attack and generates some yawing-in torque, rather than to rely solely on deflecting rudder to create the yawing-in torque, as would be the case if the turn were completely "coordinated" (zero slip as measured at some particular point--say for the sake of this example, as measured at the vertical fin)?

I guess this fin-based argument for allowing sideslip vanishes if the vertical fin is all-moving (no separate rudder)-- then you can have your cake and eat it too-- use the whole fin efficiently not just the rudder, but also keep the fuse streamlined with the airflow-- but it still might be more efficient to allow a bit of slip so that the wing's dihedral creates some rolling-out torque, so that less (or zero) outside aileron deflection is needed to maintain a constant bank angle.

Of course if the rudder is simply left centered in a constant-bank turn, then the vertical fin MUST be meeting the air at non-zero slip angle and creating a yawing-in torque. This is the only way that the glider can counterbalance the yawing-out torque from the increased tip drag of the outboard wingtip, bringing the net yaw torque to zero.

I suppose an exception might be a glider that needed lots of outside aileron input in a constant-banked turn, and was rigged with zero differential aileron travel-- then maybe the adverse yaw from the aileron deflection could create enough yawing-in torque to make the nose point toward the low side or inside of the turn, so that the whole fuselage (including the tail) was feeling a skidding flow (flow toward the low wingtip) rather than a slipping flow (flow toward the high wingtip)? But that would be a non-typical situation I think. Maybe we would see this when flying inverted-- we typically need lots of outside aileron to hold the bank angle when inverted, and if the ailerons had differential throw when upright, they'll have differential throw in the wrong direction when inverted! The only problem with this idea is, the skidding flow would interact with the inverted dihedral (anhedral) to create a rolling-out torque, removing the need for the outside aileron input. The fact that we do typically have to hold lots of outside aileron when inverted suggests that the wing is feeling a neutral (non-slipping) flow, or a flow toward the high wingtip (sideslip), presumably due mainly to the fact that the outside wingtip is travelling further, and creating more drag, than the inside wingtip, yawing the nose to point toward the outside or high side of the turn.

Finally consider that since the flight path is curved, the relative wind is also curved, but the fuselage is not curved. Just as the span of the glider is non-trivial compared to the turn radius (creating the difference in airspeed between the two tips), so too is the length of the fuselage non-trivial compared to the turn circumference. Or to put it another way, there is some significant curvature in the flight path and relative wind even along the length of the fuselage. Therefore the fuselage cannot be streamlined to the curving relative along its entire length. Let's say for the sake of argument that we decided that we did in fact want the vertical fin to be completely streamlined to the flow. Then every point forward of the fin must feel some slip (airflow toward the outside of the turn), unless we somehow make the fuselage curve like a banana to remain parallel to the curving flow along its whole length. Again this argument makes the most sense in the context of a glider with a slender streamlined fuselage-- in something shaped more like "Le Fish" or a Schweizer 2-33, we might want a point near the CG, where the fuselage has a broad flat side, to be streamlined to the flow.

Another way to look at the curving relative wind is to recognize that the glider is rotating as well as translating linearly. The rotation creates the difference in relative wind direction along the fuselage length. If the glider were only rotating, the difference in relative wind direction between the nose and tail would be obvious. We still have that same rotation even in a normal turn, where the rotation rate equals once per 360 degrees of turn.

Well I'll have another go in a few days. I predict that the yaw string mounted just behind the CG will deflect toward the outside of a constant-banked turn. If I add a second yaw string at the nose, it will be interesting to see if it deflects noticeably more than a yaw string mounted further back. It will also be interesting to see if the yaw strings deflect toward the outside or high side of the turn even during an inverted constant-banked turn....

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Feb 20, 2012 at 03:29 PM.
Reply With Quote
Old Feb 19, 2012, 12:20 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
Link to video of yaw sting - 2 meter Spider

Yaw string on 2-meter Spider--take one (2 min 9 sec)


First attempt. Very light lift. Camera battery died much too soon.

Future trials will include narration of rudder mix/ rudder inputs, more sustained circling, inverted circling, etc.

Note-- if the rudder mix were off, the yaw string would move mainly during rolling motions-- in this video, rudder mix is on so yaw string stays roughly centered even during rolling motions-- yaw string appears to stream slightly (a few degrees) toward the outside of constant-banked turns. This is not unexpected-- this slip is what allows the dihedral wing geometry to create a stabilizing rolling-out torque during constant-banked turns, preventing the model from winding up to a steeper bank angle as a flat-winged model (zero dihedral) will always tend to do. If there is zero slip in a constant-banked turn, dihedral will create zero roll torque and thus will have zero influence on what aileron inputs are required to hold the bank angle constant. Zero slip is not what we see in practice, nor would it be desirable from a handling / stability standpoint, and possibly not from a performance standpoint either. The cause of the slip is the fact that the outboard wingtip moves faster, and experiences more drag, than the inboard wingtip, thus tending to yaw the nose to point a few degrees outboard of the actual direction of flight path / relative wind at any given moment. We want to minimize this slip, but we probably don't want to reduce it completely, as then the dihedral geometry of our wings will have zero effect on roll handling / stability, which would not be good.

Pilot was not making rudder inputs, rudder was mixed to automatically move with aileron hence the minimal slip during rolling motions. Ailerons were fairly near neutral (thus causing minimal rudder deflection) during the constant-banked turns.
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Feb 19, 2012 at 12:31 PM.
Reply With Quote
Old Feb 19, 2012, 12:28 PM
Pompano Hill Flyers
Miami Mike's Avatar
Miami Lakes, Florida, USA
Joined Mar 2003
8,485 Posts
Very interesting! I suggest using a longer and more flexible yaw string and then trying different things, like different amounts of rudder mix, different amounts of aileron differential, direct rudder input, and so on. Keep track of what you're doing in each case so that you can label the videos. I'd also like to see what kind of camera you're using and how you mounted it.
Miami Mike is offline Find More Posts by Miami Mike
Reply With Quote
Old Feb 19, 2012, 01:55 PM
Unrepentant Paragon addict
LVsoaring's Avatar
United States, OK, Moore
Joined Jan 2006
2,640 Posts
may I ask, what kind of vid camera did you use there? Any pics of the setup? I'd love to do some vids from that vantage point, looking forward down the fuse, with the wings in view across the frame.


oops! Didn't notice Miami Mike already asked the same thing!
LVsoaring is offline Find More Posts by LVsoaring
Reply With Quote
Old Feb 20, 2012, 10:39 AM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
Photos of cameras plus yaw stings

Quote:
Originally Posted by Miami Mike View Post
Very interesting! I suggest using a longer and more flexible yaw string and then trying different things, like different amounts of rudder mix, different amounts of aileron differential, direct rudder input, and so on. Keep track of what you're doing in each case so that you can label the videos. I'd also like to see what kind of camera you're using and how you mounted it.
Second set of photos showing longer, flexible yaw strings (casette tape and teflon pipe thread tape) plus keychain video camera -- http://www.flickr.com/photos/3788966...7629402991505/

First set of photos showing shorter, yarn yaw strings plus keychain video camera--
http://www.flickr.com/photos/3788966...7629403143293/

Link to some single photos in case you are having trouble navigating the sets--
http://www.flickr.com/photos/3788966...7629402991505/
http://www.flickr.com/photos/3788966...7629402991505/

I simply taped the camera to the vertical fin, on top of a little foam pad, with Gorilla tape or shipping tape, leaving the operating buttons clear

Google "keychain video camera" and you will find many links to the camera. They are small, cheap, and sort of a pain in the neck to operate....

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Feb 20, 2012 at 10:46 AM.
Reply With Quote
Old Feb 20, 2012, 11:10 AM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
yaw string videos with thin flexible ribbons

2 more yaw string videos with longer, more flexible ribbons-- this is the set-up shown in the "second set" of stills in the previous post immediately above--

Still looking for just the right material / length--

Yaw string on 2-meter Spider-- Second day's experiments-- Flight 1 (4 min 23 sec)

Yaw string on 2-meter Spider-- Second day's experiments-- Flight 2 (0 min 37 sec)


These yaw strings were made of dark casette tape and white teflon pipe thread tape, as opposed to the yesterday's yarn. The idea was to better show the direction of the flow-- the yarn was a bit stiff-- but the thin ribbons of tape fluttered too much in the wind-- might be ok if shorter?

Still poor conditions, very light lift. Future trials will include much more sustained circling, narration of rudder usage, etc. In this video the rudder was simply mixed to move with the ailerons, and so was near centered during the constant-bank portion of the turns.

Note the wing drop followed by over-correction immediately after launch (both times!)-- and the large yaw string excursions-- not surprisingly, the rudder mix and differential aileron settings were not adequate to deal with the adverse yaw created by these extreme rolling motions at low airspeed. Bear in mind that adverse yaw can be created by a rolling motion even if the ailerons are not deflected.

** Edit-- hang on while I check the voice recording-- I think I did make some changes to the rudder settings mid-flight in the first of these 2 videos, I'll post a bit more detail here shortly***

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Feb 20, 2012 at 11:16 AM.
Reply With Quote
Old Feb 20, 2012, 01:32 PM
Registered User
S. FL
Joined Jan 2007
854 Posts
Very neat. From what I can see it seems you could add just a touch more rudder into the mix as the yaw string(s) are pointing slightly towards the inside of the turns. It would also be fun to film the intentional effects of too much or too little rudder by manual rudder override or more/less mixing. I think I also read somewhere that Dr. Drela felt that aileron differential was less efficient than simply compensating for adverse yaw with the rudder alone. Also it is not uncommon for FS sailplane pilots to use a small bit of top rudder in the turns while thermalling, just as your model yaw string shows.
Libelle201B is offline Find More Posts by Libelle201B
Reply With Quote
Old Feb 20, 2012, 02:27 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
Quote:
Originally Posted by Libelle201B View Post
Very neat. From what I can see it seems you could add just a touch more rudder into the mix as the yaw string(s) are pointing slightly towards the inside of the turns. It would also be fun to film the intentional effects of too much or too little rudder by manual rudder override or more/less mixing. I think I also read somewhere that Dr. Drela felt that aileron differential was less efficient than simply compensating for adverse yaw with the rudder alone. Also it is not uncommon for FS sailplane pilots to use a small bit of top rudder in the turns while thermalling, just as your model yaw string shows.
I think that the video shows the (free end of the) yaw string deflecting slightly toward the outside or high side of the turn in the constant-banked turns, on average. Is that what you were meaning to convey? Yes, adding some inside rudder would center the yaw string all the way in the constant-banked portion of the turns-- but would that be the most efficient?-- perhaps not as argued in the "background" posts earlier on this thread--

It wouldn't be a mixing issue, since the ailerons are fairly close to neutral during the constant-banked portion of the turns. My best guess is that they are deflected slightly to give a rolling-out input (stop the glider from rolling in) but I'm not 100% sure, the deflection is so small, so the resulting rudder deflection due to the rudder-aileron mix is very tiny. Since the mixing ratio seems to be working well for the rolling-in and rolling-out portion of the turns, the way to fully center the yaw string (if desired) in the constant-banked portion would be to manually add just a touch of inside rudder in the direction of the turn. I'm sure that this would take significantly more rudder stick input than is needed simply to overcome any very slight outside rudder deflection that is caused by the rudder-aileron mix, and center the rudder. Rather you would have to give enough inside rudder stick to make the rudder actually move inside of centerline. But I suggest both in terms of performance and in terms of handling, it may be better not to do this; it may be better to let the yaw string deflect just a tad toward the outside. If you added some inside rudder, you'd also need to add some more outside aileron to keep the glider balanced in roll-- do we really want to do this? Or is it better to let the glider slip a tad, so that the dihedral, not the deflected ailerons, generates most of the rolling-out torque that is always needed to keep the glider balanced in roll. Considered in more detail in the "background" posts earlier in the thread...

-- I think you meant "bottom rudder" in your last sentence-- or not? My understanding is that it is generally recommended to leave the yaw string pointing a slight bit toward the outside/ high side-- the back (free) end of the yaw string deflected toward the outside/ high side that is-- showing that the nose is point a bit toward the outside/ high side of the turn-- but that this generally still takes a touch of inside rudder in most sailplanes. Interestingly while flying a Libelle for a few hours last summer it seemed to me that I needed to relax the inside rudder all the way to nothing if I wanted to leave the yaw string deflected a little bit toward the outside / high side. Suggesting that maybe, possibly, in this particular glider the drag from the aileron deflection alone (as the pilot maintains the necessary slight outside aileron input) was enough to nearly center the yaw string, and any inside rudder would be too much and would fully center the yaw string or even deflect it toward the inside (skidding). This struck me as an unusual case, but I don't really know from direct experience-- that was the highest-performance, highest-aspect-ratio full-scale sailplane I had ever flown. (Also I need to double-check my notes before I am 100% sure I am remembering right.) My hunch is that there is something anomalous about those results-- My expectation though is that some inside rudder is generally needed in most full-scale gliders or else the string will be deflected quite far toward the outside/ high side of the turn (too much slip).

Perhaps one reason that we're not seeing very much of this slip in constant-banked turns with the Spider is that its tail moment-arm, compared to the wing span, is much longer than what we see in a full-scale sailplane, so less slip is permitted even if the pilot doesn't hold any inside rudder in the constant-banked turn.

When doing these experiments in full-scale one must be sure to consider whether friction in the controls is holding the rudder off-center (or off whatever position it would float to in the absence of friction) when the pilot takes his feet off the pedals, etc-- that is very common and can hugely skew the results, if the pilot thinks he is applying no rudder input but really control friction is holding the rudder deflected in whatever direction he last gave an input. Also it is important to check turns in both directions in case there is some asymmetry.

If I were to try to summarize where I'm going with all this--

Sailplanes (and most soaring birds!) have evolved to have dihedral. Among other things, this relieves the pilot of having to hold so much inside aileron in a constant-banked turn, but only if the glider is allowed to slip a little bit. If this style of flying were not efficient, then it seems that the top designs might have evolved to have zero dihedral, even if this made them less pleasant to fly. If you aren't going to let the glider slip a bit, then the dihedral really isn't doing anything beneficial in the constant-banked turn, in which case it would seem that a flat wing (no dihedral) might be aerodynamically more efficient. If we think of glider design as an evolutionary process driven by contests and such, then the fact that across the spectrum, from hand-launched gliders to full-scale open class ships, we generally do see some dihedral, seems to be an argument that dihedral+ sideslip is an efficient way to thermal, and that pilots are in fact flying in this manner, whether they are consciously aware of it or not.

(An alternate viewpoint is that the practical, contest-winning benefits of dihedral have more to do with resisting an uncommanded bank and turn, or at least slowing the rate of entry into an uncommanded bank and turn, resulting in lower pilot workload and improved pilot performance, than an actual aerodynamic enhancement of the glider's circling performance....)

I suspect that the slower the glider's "scale speed" (say wingspans covered per unit time), the more powerful the aerodynamic effects that tend to yaw the nose out and create some slip, in the absence of inside rudder. If the scale speed is slow plus the tail moment-arm is short in comparison to the wingspan, we might expect that the pilot will need to carry some inside rudder to keep the sideslip down to a reasonably efficient value (yaw string slightly off center). If the tail moment-arm is longer and/or the scale speed is not so low-- maybe the Spider falls into this category-- we might see an optimal, slight amount of sideslip even if the rudder is simply left centered in the constant-banked turn.

I find myself disagreeing with Denker's generally excellent "See How It Flies" website ( http://www.av8n.com/how/ ) which seems to argue that a long tail arm actually tends to creates sideslip in a constant-banked turn (if the pilot makes no rudder input.) In a long-spanned aircraft like a sailplane, it seems the difference in airspeed between the inside and outside wingtips is probably the main thing that tends to yaw the nose to point toward the outside or high side of the turn, and increasing the length of the tail moment-arm should help make the tail more powerful and help minimize this slip, even if it's also true that we are putting the tail "further back in the curve" of the flow. ( See http://www.av8n.com/how/htm/yaw.html#sec-long-tail-slip )

Ok, more than enough for right now!

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Feb 20, 2012 at 03:20 PM.
Reply With Quote
Old Feb 20, 2012, 03:05 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
PS I don't want to represent most of these ideas as original to me. Google "dihedral circling performance" for some interesting links. Here's one that was near the top--

http://www.wisoar.org/Documents/Holi...Efficiency.pdf

I am an SSA member but had not read this article before-- better catch up on my back issues-- but I've encountered some of the same ideas from other sources--

However I'm not sure I agree with the contention near the bottom of the first page (p.32) that adding just enough outside aileron to prevent the glider from rolling in to a tighter circle, will typically create so much drag on the inside wing that the glider will actually skid (nose to inside, yaw string to inside) if the pilot makes no correction with the rudder. After all if the bank angle is constant both wings are creating the same amount of lift (actually that's true even when rolling, if the roll rate is constant!), so does the lowered aileron (on the slower-moving wing) really create more drag than the raised aileron (on the faster-moving wing) in the specific situation of the constant-banked turn with centered rudder or free-floating rudder? Or in other words, is outside (top) rudder actually needed to keep the yaw string centered, when the ailerons are used as required to hold the bank angle constant? That has generally not been my experience (but see note in previous post re Libelle). Food for more thought and observation.

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Feb 20, 2012 at 03:36 PM.
Reply With Quote
Old Feb 20, 2012, 03:46 PM
Registered User
S. FL
Joined Jan 2007
854 Posts
Aeronaut, I absolutely agree that most any airplane/sailplane will and does handle better with some dihedral given it's designed purpose. I think we were talking about the same thing concerning the yaw string, just in a different way. If you imagine the yaw string anchor point as the tip of an arrow, you see that the tip is pointing slightly towards the left in the left hand turn and towards the right in the right hand turn, meaning as you say more "inside" rudder. In FS that arrow point would be telling you which rudder pedal to be pushing on in order to get the yaw string exactly centered. My reference to "top" rudder is simply that you are seeing either not enough inside rudder to center the yaw string or that if you desired you could use more outside (top) rudder to induce a bit of slip in the turn, what I was referring to with some FS pilots. As far as tail moment is concerned I think it also has to include the area and design of the vertical/horizontal stab or V tail into the equation, ie longer tail moment = less and vice verse.
Happy Soaring
Libelle201B is offline Find More Posts by Libelle201B
Reply With Quote
Old Feb 20, 2012, 04:24 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
Hysteresis effects--slip depends on roll input, required roll input depends on slip?

One more thing--

There may be a sort of "hysteresis" effect as well.

Example-- full-scale aircraft with minimal vertical fin area acting at a short moment-arm -- in a constant-banked turn with yaw string centered, kick some outside rudder, nose yaws to outside, dihedral creates a roll torque toward wings-level, now you'll need less outside aileron or more inside aileron to keep the bank angle constant and this change in aileron deflection will change the yaw torque balance in a way that tends to keep the nose yawed toward the outside of the turn or even make the nose yaw further toward the outside of the turn, even as you release the rudder and let it float freely, or in the most extreme cases, even as you hold the rudder fixed in a centered position.

Conversely, a in constant-banked turn with yaw string centered, kick some inside rudder, nose yaws toward inside, dihedral creates roll torque toward a steeper bank, now you'll need more outside aileron or less inside aileron to keep the bank angle constant, and this change in aileron deflection will change the yaw torque balance in a way that tends to keep the nose yawed toward the inside of the turn or even make the nose yaw further toward the inside of the turn, even as you release the rudder and let it float freely, or in extreme cases (small fixed fin, short tail moment-arm) even as you hold the rudder fixed in a centered position.

Similarly I've encountered full-scale aircraft (Challenger ultralight, Schleicher Ka-6 sailplane) where if you gave a lot of rudder in one direction and then gave lots of aileron the opposite direction to prevent dihedral-induced roll, holding the aircraft at whatever constant bank angle was needed to produce a straight-line slip with no turn, then you could take your feet off the rudder and the adverse yaw from the aileron would keep you "stuck" in the slip with the yaw string and ball way off-center. So basically when the pilot stops responding to the ball/ yaw string and lets the rudder float, but continues to give aileron inputs to control bank, the plane/ pilot system enters a control law loop where it effectively becomes neutrally stable in yaw /slip, due to the marginal yaw stability with the rudder free-floating, and due also to the unfavorable adverse yaw from the ailerons and the way that this interacts with dihedral to create a roll torque that requires still more aileron input. (We wouldn't ever see this particular control law loop or hysteresis effect in aircraft with no dihedral, or more strictly speaking, an aircraft with no "effective dihedral" including the effective dihedral created by high-wing configuration, etc.)

We have to be alert for these effects before saying what aileron inputs the aircraft demands to hold any given bank angle, when the rudder is left to float freely. It might depend on the yaw/ slip angle that we have when we start the observation.

These kind of slip-based hysteresis effects can render some aircraft (e.g. Challenger ultralight) virtually uncontrollable if the pilot is ignoring the ball / yaw string and leaving his feet off the rudders and trying to control the aircraft with ailerons only-- the more aileron the pilot gives, the more aircraft adverse-yaws, and the more additional aileron input is needed to try to make the aircraft roll in the desired direction (or hold a constant bank angle), and so on and so forth. This isn't necessarily a problem-- it is simply expected that the pilot makes rudder inputs as needed to keep the yaw string / bubble centered in these aircraft, and certainly doesn't take his feet off the rudder pedals and expect the airplane to still respond normally to the ailerons.

An RC sailplane or airplane with these characteristics would be very hard to fly. Since we can't see the position of the ball/ yaw string, it's not really acceptable if the aircraft only responds to our roll inputs if we also make the right rudder inputs to keep the ball / yaw string centered. However, even simply leaving the rudder fixed and centered, as the rudder servo will automatically do when we don't push the thumb stick to the side, will give a much larger effective fin area than letting the rudder free-float. This will go a long way toward preventing these problems. For this reason-- the increased tail volume afforded by the non-free-floating rudder-- I suspect that it might often happen that a scale RC aircraft might be controllable with aileron inputs alone even if it's full-scale counterpart becomes uncontrollable if the pilot takes his feet off the rudder pedals. Mixing the rudder to move with ailerons, or manually moving the rudder along with ailerons, will improve the control response even more, obviously.

At any rate, since the rudder doesn't float freely in RC aircraft, I suspect we'll probably always have enough effective vertical tail volume that we don't see this kind of hysteresis effect brtween the roll input the aircraft requires in a constant-banked turn, and the slip angle that we see in a constant-banked turn, even if we aren't actively using the rudder along with the ailerons. Unless we are dealing with some weird model with so little yaw stability that is just barely controllable even under normal circumstances....


Steve
aeronaut999 is offline Find More Posts by aeronaut999
Reply With Quote
Old Feb 20, 2012, 07:09 PM
Registered User
S. FL
Joined Jan 2007
854 Posts
Also related to circling (round vs oval) of an aircraft is maintaining a given airspeed accurately. If the airspeed fluctuates ie pitch changes there are now changing control inputs thereby changing all the other aerodynamic lift/drag/yaw characteristics, requiring more/less rudder, aileron and elevator. Unfortunately it is almost impossible to maintain an absolutely accurate airspeed in model aircraft given there is no ASI or horizon to use for reference.
As I am sure you know there is also something called "skidding" ie too much rudder in the turns, something far more sinister than a "slip" in the turn, and has proven quite deadly in FS if done low and slow as in the landing pattern.
Libelle201B is offline Find More Posts by Libelle201B
Last edited by Libelle201B; Feb 20, 2012 at 07:26 PM.
Reply With Quote
Old Feb 21, 2012, 09:12 AM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,165 Posts
We seem to be in agreement Libelle, I see what you mean about the point of the arrow re the yaw string. Just so people are clear, in my writing I am always talking about the direction the free end of the yaw string is moving.

I'm still a little surprised by Johnson's comment on the first page of the article linked above that some full-scale sailplanes may actually need outside rudder to keep the yaw string fully centered in a constant-banked turn, due to the inside yaw (adverse yaw) from the rolling-out aileron input that is generally needed in most turns especially non-slipping turns. I would think that the increased drag experienced by the outboard, faster-moving wing would still generally be the more powerful effect and cause the glider to still need some degree of inside rudder. Maybe that is not always true. Especially if the ailerons are rigged with zero differential throw?

More on the idea of "hysteresis" effects driven by adverse yaw from ailerons, during constant-banked turns --

Could you have a situation where you actually needed to hold inside rudder to keep the (free end of the) yaw string deflected toward the outside of the turn (slipping), and you needed to hold outside rudder to keep the (free end of the) yaw string deflected toward the inside of the turn (skidding)?

That sounds bizarre but here's how it might work-- small vertical fin acting at short moment-arm-- lots of dihedral-- no differential travel in ailerons--

--if the yaw string is deflected toward the outside of the turn (slipping), dihedral creates a rolling-out torque, inside aileron is needed to hold a constant bank angle, adverse yaw from the ailerons tends to swing the nose further to the outside, so inside rudder is needed to keep the yaw string from moving further to the outside. This would only make sense if the yawing-in torque created by the fixed vertical fin in the sideways (slipping) flow were smaller than the yawing-out torque created by the aileron deflection.

--if the yaw string is deflected toward the inside of the turn (skidding), dihedral creates a rolling-in torque, outside aileron is needed to hold a constant bank angle, adverse yaw from the ailerons tends to swing the nose further to the inside, so outside rudder is needed to keep the yaw string from moving further to the inside. This would only make sense if the yawing-out torque created by the fixed vertical fin in the sideways flow were smaller than the yawing-in torque created by the aileron deflection.

I wouldn't be too surprised if we saw something like that in the Challenger ultralight which seems to have very marginal directional (yaw) stability. I would expect most sailplanes, and especially most radio-controlled sailplanes, to have enough directional (yaw) stability that it's generally going to take a steadily increasing rudder deflection in the "normal" or expected direction to move the yaw string steadily further off to one side, even when the pilot is using the ailerons as needed to hold the bank angle constant, with some "offset" or "shift" in the zero point due to the glider's normal tendency to fly with some slip due to the increased drag generated at the outboard wingtip (when the rudder is close to neutral, yaw string points somewhat to high side or outside of turn, but combined aileron-rudder inputs continue to shift the yaw string in the expected direction as the glider continues to fly in a constant-banked turn).

Now I'll wait till I have some more flying videos to post before adding more to this thread!

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion How much wind is to much wind? Magpie Electric Heli Talk 3 Jan 16, 2012 10:14 AM
Poll How much wind is too much wind? shufflez Multirotor Talk 19 Dec 27, 2011 11:44 AM
Discussion How much wind will you fly in? rcsoar4fun Thermal 46 Sep 14, 2011 12:13 PM
Discussion How much wind is too much? evltoy Beginner Training Area (Heli-Electric) 4 Mar 14, 2011 09:51 PM
Discussion How much is too much wind? Pretorian435 Electric Plane Talk 40 Mar 09, 2011 08:55 PM