I'm not really referring to wind performance, which has been well examined over the years.  I'm more interested in what makes a stunt plane resistant to minor upsets in level flight when even light winds are being disturbed by various obstructions before they reach the circle.  I am assuming that wing loading would be the number 1 factor, followed distantly by aspect ratio, tail moments, and (I would think very distantly) tip shape and airfoil.  Does this sound in the ballpark?

swept back wings, thin wings, clean tips ,speed

4 good things if you "just" want to be less upset in changing air

Randy

Minimizing side area may help (please correct me if i am wrong here!)

Lurking the British Barton club site, I've recently got the impression there's a feeling that American style fat wings may not be the ideal thing in their wind conditions.

Steve - I'm no aero expert, but if I can join this discussion, maybe I can at least stir up the smarter guys by getting them to tell me I'm all wet. I think we need to better define bumpy ride first. I fly in a LOT of turbulence when the wind comes from the direction over the trees at my field.
By my estimation, there are a few common kinds of disturbances to level flight

1) Disturbance in roll (Wingtip rocking)
2) Disturbance in lift (going up and down like riding in an elevator, without change in attitude, fuse stays parallel to ground)
3) Disturbance in yaw (tail wags, noses in on you upwind, or downwind, etc.)
4) Disturbance in pitch (the dreaded roller coaster, with changes in attitude and hunting)

Did I miss anything?

Roll - just like a front end alignment in your car, some people like a car that pulls ever so slightly to the right. The minor correction you make to keep the car straight is entirely unnoticed. In our planes, this correction is usually in the form of tip weight and vertical C/G. I have noticed when the tip weight is almost perfect, my wings will rock either in or out board equally half the time one way or the other when hitting rough air. A little extra tip weight and they tend to stay level, or only drop the outboard wing occasionally. I also think rudder height and or side area can have an effect, as well as dihedral & tip shape. If a cross wind is hitting the side of the plane, a high rudder or dihedral can catch the wind and cause roll. Some say the other wing panel in a plane with dihedral should cancel that out… but what about the fuselage in the way? Is  the other wing panel getting equal exposure to cancel it out? I somehow think not… Anyways, lots of factors here beyond my pay grade as they say.  Things I wonder about are – Would light wing tips help to stop barbell effects in roll, or would that just make it easier for the roll to start in the first place?

2) Lift-  We tend to fly at a speed where we have enough lift with a small fraction of elevator to maintain level flight. Disturbances in lift only have the wings airfoil and stab to help hold level. The level rise and fall I call “riding in an elevator” without change in pitch or attitude  is about the best you can hope for I think. If your hand is held level without making correction and the plane “falls” or is being sucked down, or bumped up, about the only thing I can imagine combating this is having less wing area to disturb, or more weight so the forward momentum is less disturbed (think swinging a brick on a string).

3) Yaw – again, like the car front end alignment, rudder, engine offset, lead-out placement, tip weight again, and distribution in fuselage side area and proportions would seem to be the factors here. A strong bump of wind from up-wind can either push your nose in or your tail depending on aero balance. Swirling bumpy air that is rolling off a tree can be hitting it fore and aft, inboard and outboard, all at nearly the same time. I like a little engine offset for this reason, relying solely on centrifugal force and zero/zero alignment means once the nose is pointed in and air speed drops, there is nothing to put the plane back on track and tighten the lines. Zero-Zero sure does look better in ideal conditions though… like I said, everything is a compromise I think. A hair of offset in the engine and rudder would seem the safe way to go. I airfoil my rudders these days and put in just enough off set to no be sure it’s not inset, the engine seems to work well for me with up to 1 deg of offset.

4) Pitch – Probably the biggest offender in a hunt, and probably why some planes hunt worse in disturbed air than calm. Bigger & higher stabs, longer moments, lighter tails, etc I would think help here, but there are bad things at the extremes no doubt.

Add all four types disturbances together at the same time and you start to see some pretty weird things in level flight. I’ve had wings go nearly knife edge upwind, or drop two or three feet in an instant.

The part I’m having trouble envisioning is what would be the ideal? Probably like everything else we do, we will find that what works “best” will be some kind of compromise where the plane can handle some turbulence yet still turn the kind of clean corner you want, groove like you want,  track in the rounds like you want, give the line tension you want, etc etc.

That should give the smart guys plenty of ammo. I’m sure I’ve butchered at least a half dozen aerodynamic terms as I typed this in about 5 minutes flat, gotten at least that much theory wrong as well.
Should keep them busy for days. 

EricV
 

I'm enjoying this thread. I've a few pet ideas myself, that I'll share.

Dihedral is not likely to hurt as long as it isn't too extreme. Mild amounts of dihedral seem to be more self righting than upsetting when side gusts are encountered. I believe that when such a plane is hit with a side gust when at the upwind quadrent, tipping force is opposed to a degree, by centrifugal/centripetal forces. When a tipping gust is encountered while the plane is in the down wind quadrent, We still have the same forces acting, as well as a possible wind induced slight banking towards the outside of the circle. This should help maintain level flight, and increase line tension slightly.

Wings with extreme leading edge taper seem to fly better in turbulence, and the wind. The Australian design, "Firecracker", is the wing plan style I'm thinking of for illustration. The greater leading edge sweep acts like additional dihedral without the tips being raised to possibly catch some of the wind. Dihedral seems to have a stabilizing effect on an aircrafts flight.

One thing that I haven't thrown into this post, is "circular flight path".

Induced, or actual dihedral, in calm air, should be trying to add a slight rolling force towards the center of the circle. The combined effects from tip weight, leadout exit, rudder effectiveness, side loading, fore and aft, incidences in the thrust line, and control surfaces, all combine to keep our plane relatively stable, mostly countering these forces.


Wind or turbulence, adds additional upsetting forces that, unless we have a wide margin of static stability designed in, causes our model to switch back and forth between these forces as one or more of the trimming elements is overpowered. In a few feet of flight, the upsetting forces have regained control of one or more elements, and must now cope with a different set of problems, for the next few feet of flight. This continues as we fly around the circle.

It seems that designing a wider margin of stability would be the goal for designing a good wind flier.

We could make the plane nose heavy, that would help slightly, but we want the plane to perform the pattern as well. We could lower the aspect ratio to somewhere between 1.5-1, and 4.5-1. This limits our turning abilitysome, but if we make the area larger so the wing is more lightly loaded, that will help. We can also consider getting rid of the flaps so we remove one of the unsettling trim elements designed into our modern stunters. We might consider stab and elevators at larger than the common 25% seen on our modern designs.

With the better control of power by modern engines and E-motors, thinner wings would, and are being considered by top pilots. 14-16% seem to be where most of these designers are currently at. Philes "Skinny Diva" doesn't refer the the width of the fuselage.

Perhaps one of the planforms to consider would be one with a root airfoil chord of perhaps 14 or so, inches, and a root thickness of 18% to 20% tapering down to a tip chord of 6.5 to 8 inches and an airfoil thickness of 8%-10%. The span would fall within the 50"-55" range.

Such a wing may work very well because the tips should stall first in any maneuvering, either on purpose, or by other forces, like turbulence. The center section should remain in control, and the size, shape, and loading, without flaps, should reduce drag and tip turbulence in level flight.

Other than the wing sweep, an d highly tapered wing planform, I just designed a version of "Wild Bill's" Doodle Bug.

You know, he may have been onto something.

EATHER , not Ether.. Im not sure you want to sniff him either..   

I think general level flight stablity is a key factor also. Look at the Nobler as a design. Certainly for level flight its rock solid, and also takes some more effort to get it to turn compared to New'er designs. There is a corrolation between level flight stablity and sensitivty to pitching motions relative to stable flight in turbulance.

I remember Brian Ether here in Australia saying something like the highly tapered wing planform was selected for windy weather flying and it succeeds because it places the centre of pressure of each wing closer to the fuselage, or "under its armpits."

Wind gusts that influence and shift this centre then have less leverage to act upon the model with, hence the stability of the design.
Also noted was his use of a similarly tapered tailplane.

I'm pretty sure I posted this photo previously, but here it is again, because I think you have a valid point here. Wings with elliptical lift distributions have the the most inboard a.c.'s of all commonly used planforms - somewhere around 42% (4/(3 pi)) out from the root. This is something very close to an elliptical wing, but apparently slightly different. The "Firecracker" has enough taper to bring the theoretical a.c to about 44% from the root, as compared to the 46% - 48% half-span of the average stunter. The white lines in the diagram show the taper that gives the same theoretical a.c. position as for an elliptical wing. The theoretical minimum distance out is 1/3 of the half-span, for a pointed tip.

SK

You need the right decal.

Another design that's reputed to be a decent wind flier is Delebard's "Sukhoi 26" that he won the WC with a few years ago. It has a wide root chord with a narrow tip chord. It also starts with a relativly thick root airfoil, and goes down into a thin airfoil at the tip.

The theories are numerous, as well as the exlanations, whatever it is, it seems to be pointing the way.

Tom Hampshires "Arias" is in line with much of what I believe will be a good planform for windy, turbulent, conditions.

I would say the most important property is low feedback to handle. It is very difficult to make good figures, if hand feels different line tension every 0.1s and does not know how much pull it needs to make a corner. But if the feedback is low and hand knows it will be easy, it is much simpler. It could be done either by smaller flaps, or better some trick like counterbalancers, hingeline inside the flap or the best - my tricky flaps :-)

The secon thing is good trim does not matter from which side the wind comes, it means good side area design, which does not detrim the yaw, and model which is not very sensitive to trim changes, here I vote for more sweep back as Randy says.

All combined with thick but well loaded wing (to its max wing load - not by ballast, but rather small area) and power train without too much self controllig (constant RPM certainly better than 4-2-4) will make good weapon to turbullence. At least this was way how I designed my model for wshmachines in Sebnitz and Landres and I feel turbullent air always as an advantage.

" It turns out that the possible small advantage in turbulence stability from sweep is far outweighed by the unwanted yaw-roll couple.  Tom Hampshire "

Hi Tom

NOT my findings, in the real world of actually flying them ,the sweep, wether it be LE or TE , "is" a big factor in the plane not being as upset.
Now wether that is from sweep itself or the differant chords of the tip or root rib, who knows??? I really can't see any difference in the two, If you have sweep without any chord difference you would need to have a swept LE and a TE that is swept in the same direction, I would be clueless there, and would maybe defer to people like Jack Sheeks and others who built and  flew that type wings

Regards
Randy

No, Richie is strong boy, he has lot of training, so he does not have physical troubles even in strong wind. I spoke about good model for him longer time with Fritz (daddy), and designed that one model, just for him, since that time he had several almost identical models, powered by ST60, and I think this it is key part of his success. Flying every day the almost the same model several years. He has little different trim (if I compare it to our models) - he wanted CG little bit forward. I cannot fly such models, but it is good choice for his strong hand :-)

Alexander (4th in Landres) has also regular linkage in model almost identical with MAX, just fuselage has another shapes ... also strong boy  :-P and difference is clear, Alex is typically better in strong but "laminar" wind, while my Max II is better in turbullence.

It sounds like some guys are using the same word to describe different things.

When we talk about "sweep" I expect we're talking about 1/4-chord sweep, ie taking into account the entire planform, not just one edge.

I think many people simply mean LE sweep, while others use the word sweep in isolation when they really mean to discuss hinge-line sweep. Of course in plenty of cases a model with lots of LE sweep actually has zero 1/4 chord sweep, so we probably need to be quite specific about which we mean.

I mean 1/4 chord. My older design "Next" had larger wing, we found it had too low wing load, and result was we did shorter tip chord. It does more sweep back also on LE and also on 1/4 chord since the TE was the same. Differnce was clear, Next was easy to trim (tip weight and LO position) but it was very sensitive to small differences, while Max is not so sensitive, more forgiving, but it also means that it take longer time to "see" all trimming issues and making optimal trim.

And I don't think F-104 wing loadings will help the maneuvers too much

and here I speak about wing LOAD not plane WEIGHT ... yes, that load does help, just because wing lift is limited, a heavier load will limit "hopping", while lift in corners is made by flaps, and they are extendet only in corner and not in straight parts of figures ... that is just opposite to unflapped model spoken in thread about model for dead air

"Having said that though, it bumps through turbulent air just like any other plane"

Another item that will make a plane fly much better in level flight is one that is in perfect trim, You get a plane out of trim and it will bounce much more than one that is trimmed "spot on"

My Elliptical Dreadnought VECTRA was very good in winds and turbulance, and I alway felt I had an advantage when weather conditions were bad when flying it.

By the way it flew at some of the worst weather , high wind NATs we have had in a long time

Randy

Logarithmic device on flaps. Here you can see more about it ... it is old page, but shows what is going on:
http://www.netax.sk/hexoft/stunt/the_max_ii.htm

EATHER , not Ether.. Im not sure you want to sniff him either..   

Oops! Yes of course his last name is spelt Eather (too much thinking about my MVVS 49 diesel back there!)

Heavens he will never talk tome again after that.

PJ, I believe that Paul Turners Wind Wagon series also does well in the wind and it has a low aspect ratio and a double taper.

It sounds like some guys are using the same word to describe different things.

When we talk about "sweep" I expect we're talking about 1/4-chord sweep, ie taking into account the entire planform, not just one edge.

I think many people simply mean LE sweep, while others use the word sweep in isolation when they really mean to discuss hinge-line sweep. Of course in plenty of cases a model with lots of LE sweep actually has zero 1/4 chord sweep, so we probably need to be quite specific about which we mean.

The standard aeronautical definition is quarter-chord sweep.  We've been discussing two things: 1) turbulence response, which we all agree should be better the lower the ratio of tip chord to root chord, and 2) steady-wind rolling moment due to sideslip (dihedral effect), which works out to be pretty much a function of the half-chord sweep, as I remember.  The two are the equivalent of AC and DC, respectively. 

About the only thing I know of that's a function of LE sweep is compressibility effects, about which we don't have to worry.

Turbulence response may also vary with (quarter-chord) sweep, but I suspect that it would take so much sweep to make a difference that it would make something else go bad.

As for rolling moment due to sideslip (dihedral effect), the attached figure gives the lowdown for wings.   This is figure 8 from http://naca.central.cranfield.ac.uk/reports/1952/naca-report-1098.pdf .  You can look at the whole report for details (although nobody will), including the ugly stuff about fuselages and flaps.  In the figure, lower-case lambda is taper ratio and upper-case lambda, cleverly, is (quarter-chord) sweep angle.  See the first part of the report for more definitions.  This report is a nice cookbook.  It doesn't have much fancy theoretical stuff.

Malcolm and Angus Young?

I meant that turbulence response was for varying wind, and rolling moment due to sideslip was for steady wind (although it has an effect for varying wind, too).  That doesn't mean that I'm not on a highway to hell.

  I havent looked over many other wings in recent years. Any competition I have ever witnessed with some wind of any consequence, Billys GeoBolt is the least affected to my eye. A 10" root cord and 7 1/2" tip cord at 62" or is it 63" span. That is not to negate from the experience of the guy at the handle. Most average stunt models have 2" LE sweep. I think his 1/2" extra sweep and slim foil at the tip has everything to do with the smooth penetration his model gets. This year Im using that root and tip on a 60" span. My last model was 63' with smaller flap cord than he uses..in effect making it a bit more high aspect and a little bouncy so Im cutting the aspect ratio with a small sacrifice in span and wing area by almost 50 Sq". Swept tips on mine as well so.. not sacrificing anything really if I keep the weight close to where it was before at 59 oz. on the last one.

Any competition I have ever witnessed with some wind of any consequence, Billys GeoBolt is the least affected to my eye. A 10" root cord and 7 1/2" tip cord at 62" or is it 63" span.

That is the wing, I think, that David Fitz used to win the 2008 WC in extremely turbulent conditions.

City air or Country air !    

In the rolling countryside , where the airs moveing from ' way off ' , a Clear ? run for miles
but rolling hills , Trees , the odd house and barn ; so it has whirls and pockets & can dump
etc. I.E. not constant or regular .but pretty much so , 60 %+ smooth . !
With the Little Phantom , 50 in about 500 in and near 50 Oz , the Std tips , a rib at 45 deg
to vertical , with all the air pressure at 18 - 20 Kt breeze; Vortice Wakes from Ea tip .
Maybe 6 to 10 ft. (Varies) were towing & / or loading the tips about, yawing with  bit of /
 occasional ' rock ' ( roll )  
pun not intended.
In the Valley up the coast , the rolling air from trees and disturbances ( the GROUND is irregular
so the wind flow is more disturbed - rolling , balling , waves ,speed/presure) the MINOR rock /
roll is more evident. (60+ % turbulent , Maybe 30% smooth , if youre placed well )
SO , ' We ' added extensions to the top of the tips from high point aft, swept out going aft .
Testing was ' does it turn even ' ? chopping to + 1/2 span at T.E. got that near enough .

The tips resembled Grenemeyers Bearcat & a few of the P-51s, with a bit tacked on , as stated .


City air ! theres was still considerable but limited rock and a bit of bounce from the more irregular air
at around the 18 mph / knot windspeed . Yaw was very much reduced. Near eliminated .
Out in the rolling countryside the  ' dragging the wingtips around ' previous to add ons had been eliminated.

BUT , when the wind got up to 25 mph  ( + & - ) the pressure interface got leverage.
The only reasonable place was centreish of the field.
The air ' break up ' is UPWIND and downwind of trees.
Its starting to noticeably shift flow say 50 Ft upwind of Large ( 60 ft + trees )
Also when the lower Branches are say 10 ft above ground, the  breeze flowing through there was creating
dangerous suction downward if one tried to wander into a bit of shelter. There wasnt any .

Proceeding with this lark , at 30 Kts, pressures were severe .
Good boots essential , Clothes blown everywhere (storm conditions)
Feels like the hair is vertical / on end . Also the occasional fast footwork is called for .

Jim Mannal's tretease on Flying F2B ( on Dave Days site ) gives some good tips on
Manouvre Entry and Positioning in these conditions ( a gentle breeze , you can still stand up ! )

The " Beaufort Scale " is far more practical for judeing strengths.
A reasonable aeroplane should be enjoyable to fly, utiliseing aforesaid tips
in strength where " Whitecaps are starting to form " .

Anything over say 18 Kts could be viewed as ' Extreme sports ' and liable to ' raise a sweat '
and require severe ? mental & some physical exertion .
This is why sports fields have changeing rooms , Also a 'warm up ' is usefull for full physical mobility
as limited movement may impede control.

The dozyiest plane to fly in calm air still handled 20 Kt winds, But the parrallell depth spars ,
Same thickness tip and root , had a near constant 5 - 10 Deg ( + ) rocking going about .
 Id thrown 200 % Vert Stab Volume with A rather bird wing section fin /rudder on for this .

Comparable to 60s ships such as  Argus , Atom , Aries etc, where the clean flowing lines
slide through the turbulant stream (of air) like a fish.
Bob Hunt was impressed with the '40-35' sized planes ability to deal with turbulance , without
exceeding the pilots ' comfort ' Zones.

Yachting with ' the old man ' Keeler ' , water 1/2 up side windows . " Not really starting to blow ,
 she's hardly moving . The waters right over the side windows when shes starting to go properly.
(20 - 25 Kt breeze ). I hope there glued in Properly !

I, too, think that the Firecracker should be good in wind, but for the opposite reason.  Sweep ain't what you want in wind, and it has less sweep than other airplanes.  As I remember from the NACA report I reference here a lot (I'm too lazy to look up the number again) rolling moment due to sideslip-- the dihedral effect-- is roughly a function of the half-chord sweep. The low ratio of tip chord to root chord ought to make it good in turbulence, too, for the reason the other folks cite.  Alan Resinger flies Firecrackers, but he builds them to fly clockwise so I can't ask him to fly one.  That's kinda mean, I think.

Randy says sweep is good in turbulence.  I don't see why, but I'll defer to his experience.  He's build a lot of good stunters.

I for one think that a smaller airplane does better in turblance and wind than the bigger ones. Besides I just like to fly in those conditions and our field has lots of turbulance so I get to practice the RUTEEN quite offten!