hello,how does aileron diffrential affect rolls?

By adding some yaw during a roll, it moves the roll axis. If your model rolls on the thrust line, don't mess with it. If it's off, your model will roll like someone flipping a flapjack. You might be able to see this as a yaw in the half reverse cuban eight...the model will point off course after the half roll.

Too much information: Raising or lowering ailerons can have both a differential and dihedral effect. This is good to know if you have roll coupling with rudder and you don't want to cut your wing to put in a small dihedral change.

On aircraft without aileron differential, when one aileron goes "up" the other aileron goes "down" the same amount.
On aircraft with aileron diff, when one aileron goes "up" the other aileron goes "down" a lesser distance.

If the down-going aileron creates more drag than the up-going aileron, the greater drag of the down going aileron will cause adverse yaw. In extreme cases, the adverse yaw will cause the plane (model or fullsize) to bank one way and actually turn (yaw) the other way.

This adverse yaw causes rolls to look sorta like spirals---to prevent the spiral roll, we adjust aileron movement so that the down-giong aileron moves less than the up-going aileron. Easy to do with a computer radio--if you are using a servo for each aileron and have them plugged in to the right slots. Much less easy if you have to change throws mechanically.

Different planes require different amounts of differential--some require none; usually takes a few flights to get it just right.

Cheers,

HI All,

A quick test for this is to fly into a 45 degree upline and perform high-rate aileron rolls to one direction. You will quickly see if the plane arcs off one direction or the other - or maybe rolls on a string (lucky). Repeat this in the opposite direction. I think continous rolls are needed versus 1 roll to really see the tendancy.

After adjusting the differenctial so that the plane will do continous aileron rolls on a 45 deg upline, the normal rolling elements are now perfomed easier. Prior to fixing this, it was a tad more difficult to maintain baseline and heading during rolls. After this, it is easier (4/8 opposite slow, & 3/4 opp slow rolls for example).

Jim W.

hi guys,thanks a bunch,u guys gave some great info man

I'd like an aerodynamic explanation of the 45 degree line test with rolls. The way I visualize it is that the aileron deflection creates some unwanted yawing and pitching moments on the plane in addition to rolling moment. After completing single non-corrected roll, you should end up in the same attitude as you started (minus gravity effect) no matter what the differential is (barrel roll). When you do 1/2 roll only, you should see the unwanted displacement and heading change.

What happens when you steepen the 45 test line to 90 degrees ? The same aerodynamic principles apply, but all of a sudden, you can't define "opposite direction". Yet most planes will veer off somewhere.

Paul

I use the half roll for just the reason you mention, Paul.
I don't think the angle of the test line is that important, either. I find 90 degrees is a little harder to see. You have to enter coming directly toward or away from yourself to clearly see the 1/2 roll. Flying the pattern box, we are simply more experienced with 45 degree turnarounds.

I was playing w/ Differential today. In my top hinged aileron plane, suggestion in the instructions was to have opposite of normal differential

So per JW above I rolled left continuously on a 45 upline, the plane heading tended toward the right. Repeat going downwind w/ same result.
Did the same rolling right continuously on a 45 degree upline, the plane heading tended toward the left.

So I reduced the down throw of both ailerons, repeat above until the heading change went away, so now if I repeat above the heading change in left rolls isnt noticable. I still have some heading change on the right hand rolls but had a good last flight so I put up for the day.

Does this sound correct?

FWIW the plane does seem easier to handle now, but wind was calming down as my trimming was taking place.

Brian,

I have not noticed different differential values per roll direction..I suppose its possible but I would bet your control throws on each aileron is not the same.

When I work hard to get the travels exactly the same as accurate as I can....by the way I think I'm well within 1/10th of a degree using the method I have. I actually now stick pointers on both wings at the same time. We found last year that the surfaces can have slight differences at different points in the stick travel. This stuff gets solved with minor little tweaks to mechanical setup.

When travels are the same Then differential is identical left and right. Or at least this is my experience with lots of different models over the last few years.

One note as servo pots wear the differential values change...and they can change dramatically. I keep on top of mine and check the servos about every 30 flights and I regularly make some minor tweaks like 1-2% on the Travel volume to keep the servo traveling identically as the pots wear. At about 100 flights the pots reach my threshold for replacement. So if you have fresh servos in the model then start the trimming of this function. Then it will be set. AS you change out servos to fresh ones later on the differential value will be the same.

One thing I have found on the molded wings. The aileron will "rise" or "Fall" slightly as the servo moves off center before it pivots. I have yet to see a molded wing that doesn't do this. The skin hinge just doesn't pivot as clean as a hinge pin will. This will drive your differential settings wild. The reason is the differential value for a small roll rate is not the same as the differential for a higher roll rate. Also the differntial tends to become model speed dependant as well. It has to do with the way the rise or fall of the surface changes the wings shape before it pivots. Not that we are talking major changes or anything....It just is something I have noticed.

Never could get a Molded wing-Aileron combo model to roll as well as a Hinged surface. If you have one of these models take a look at the surface just as you start to deflect the aileron.

Also as the models age this attribute will become much more common. The hinged surface will get some flex in it as it moves more and more times. The result is more translation of the surface before it rotates.


Troy

The linkages etc result in identical travel at least at the end points when I start out. Im sure there is some oddness in the linkage but they are very close. I measure w/ a 1/64 machinist ruler the throw and get within 1/64".

It does seem that adding the differential as I noted made the rudder work a little less though, cant say for sure until I get in wind again. The air got smooth today here, unusual in my short time here.

That's pretty close but if we do the math on that.

1/64" on a 2.5" wide aileron is 0.35 degs. If you are measuring at the widest part of the aileron say its 3.5" long then 1/64th of an inch error is 0.25degs

Now this is of course if you are a full 1/64" off.

Sorry I'm a Quality Engineer by training and I look at these types of things in terms of how accurate I can get them.

I use a 7" pointer on the surface at this the same 1/64" error is 0.12degs. This is why I say I can get it down to about 1/10th of a degree.

Just a note for how much differential I'm using in my models. On the Smaragd I flew a really long time it was 0.5 degs difference when the ailerons were at full deflection of 10.5degs. So the up aileron went 10.5degs and the down aileron would travel 10.0 degs.

My Pinnacles are running 0.6degs of differential. My Supreme's ran 0.75 deg. And my Hydeaway was around 1 full degree of differential. Now this differential setting is really determined by lots of factors but in them is how far off the "center" line of the fuse the wing is. A lower wing model will need more differential than a more mid wing model. Not to say that a model that needs more differential is not a good model...It just means it rolls better or looks better rolling with some differential in it.


So the question becomes is 0.25 to 0.35degs of error good enough when you are talking about a total differential of 0.5 degs

I guess the answer will differ greatly depending on who is flying the model.

Check the differential at different speeds. I have noticed some funny stuff with top hinged surfaces. These need more down travel than up travel because of where the pivot point is. I have theory that these differences in travels become exagerated as speeds change. This shows up in my flying when doing the rolls at the tops of a double I or at the bottom. Sometimes the speed difference is not much but the model will not roll axially the same in each instance. I Perhaps as we drift off further away from equal travels on the ailerons this becomes more of an issue. Example if you have 5% differential it might not affect you much but go up to larger values like 15-20% it comes in spades.

I helped a guy a couple years ago in Atlanta with a 33% IMAC model. The instructions called for a full 2 degree more down aileron than up aileron. At this the model would roll pretty good. But if the speed was higher or was lower than when we did the test...the model's differential was off slightly. We did the 45 degree up test with the 1/2 rolls. Then did vertical rolls once it was close to fine tune it.

Then this guys wife made a mistake and drove over his wings while they sat in the wing bags behind the truck at the flying field. SO after the divorce and planning for the manslaughter

he built new wings out of foam and balsa. Same airfoils, same wing essentially.... and by the way they were lighter than the composite versions. The ailerons were the same..only difference was the ailerons were center hinged. The result was the model needed 1 deg less down than up of differential instead of the 2 degs more down from before and was less speed sensative with regards to differential. This was a swing of 3 degs....And the model rolled the same basically. Other than the speed sensative issue.

Just some food for thought as you improve the way your model performs.

Troy

Intersting thanks.

You know as I use a 1/64 scale Im matching the line on the scale so its closer than the assumption of the accuracy of the scale.

I guess the point of my previous question was, doing multiple rolls (or half rolls) to the right the heading goes left, so reducing the down aileron throw reduces the heading change to the left. Am I understanding that to be the solution as Jim W. explains?

(My testing seems to validate this explanation but I was just interested in hearing this confirmed.)