I know over the years most pattern planes required some aileron differential to get the rolls axial. It has always been explained that the down going aileron needed more or less travel. This really doesn't make sense since when the plane is inverted the effect is opposite. If there was more travel downwards when upright then there would be less travel on the downward aileron when inverted.
If however all that's being done is adjusting the differential so that the fuse rolls straight about it's centerline we are actually compensating for the wing being displaced from the axial centerline and or variations caused by incidence angles. If this is true then a wing at zero incidence close to the axial centerline should end up with close to zero differential and a wing like on a high wing trainer would require a lot of differential.
Am I confused? Better yet, can you straighten out my thinking?
Later,,,basmntdweller

The reason most planes including full scale incorporate Aileron differential is to discourage adverse yaw.
When you think about it like that it really does not matter about wing placement. It is said to be worse with a high wing though.

__________________
The long tail moments and short wings on Pattern ships is enough to make this problem almost non-existent. I don't think Pattern planes need to utilize to much differential!

Err, I was going to post but I realized I don't know that much about it. I think it depends on the airfoil, dihedral, wing position, and if you want to fly inverted or not. On a glider, a huge amount of differential is used (like 4:1). I believe the reason is that your flying at a very high coefficient of lift and the down aileron causes a great amount of drag while the up aileron decreases drag. On an aerobatic airplane, your flying at a very low coefficient of lift and the up and down aileron have close to equal drag. If your inverted, the angle of attack is reversed compared to upright and the differential (due to aileron drag) is also reversed. However, the other effects (dihedral and wing position) are not.

OK, experts can take over and correct from here.

I think that in aerobatics it is used to make vertical rolls in uplines without desviation (yaw)

OK, I have been watching these "differential" threads for a while now, and I have to chime in. Vertical up lines is not the place to set your diff. The reason is simple. The wing is at 0 degrees aoa and is producing 0 lift. (symmetrical airfoil) Any drag (induced or parasitic) created by the ailerons will be equal if the throw is equal. If your plane is wobbling on up line rolls, then the aileron geometry (or rudder trim) is not right to begin with. The place to set diff is straight and level and cruise speed. The draw back to this....that is the ONLY attitude and speed that the diff will be right. Speeding up, or decreasing the aoa will require less diff. Slowing down or increasing the aoa will require more diff. It is a compromise to get the plane as close as it can.

Aileron diff does not reverse when you are inverted. To keep the nose pointing straight, in a full scale Pitts and Decathalon, when rolling from inverted requires the same rudder input as it does from upright. Up right...right aileron, right rudder. Inverted...right aileron, still right rudder. (pilots perspective)

Hope this helps,

Bryan

Arrrg.

"The wing is at 0 degrees aoa and is producing 0 lift."

AND

"Aileron diff does not reverse when you are inverted."

Is not consistent. It's all about the angle of attack relative to the differential ratio (up aileron/down aileron). The angle of attack upright is not the same as inverted but the differential ratio is.

I understand, and agree, with the first part, but the last part lost me. Of course you use the same rudder to counter adverse yaw when inverted but you need a lot more because the aileron differential is actually making the adverse yaw worse when inverted. Again, think about the angle of attack.

I agree 100%. There have been some discussions lately implying that adverse yaw would reverse itself when inverted. That is not the case. If it were...you would have to reverse the rudder as the plane rolled inverted. I am using my full scale aerobatic experience to back up my conclusions. All I was trying to say is that adverse yaw, (the reason for aileron diff) does not reverse inverted. There is a different amount of diff needed when inverted, but there is a different amount of diff needed in ALL other phases of flight, except where the diff was set.

Bryan

it helps loads in pointed rolls and rolling circles too, especially with an Edge.

I want to clarify what I was saying in my original post about reversed aileron differential when inverted.
If you have a plane with an amount of differential setup that at full right aileron deflection the left aileron deflects down 20 degrees and the right aileron deflects up 30 degrees. As the plane rolls inverted that right aileron is now down 30 degrees and the left is up only 20. Now viewing this plane from sky view the differential is reversed from what it was. The down aileron now has the greater throw.
I hope this clears up what i was trying to say.
Later,,,basmntdweller

I think that it is not acctually the way that the model is flying, but the Aerodynamics of the wing. The bottom of the wing probably effects the air in a way that will make more drag on the Aileron. So whenever its upside down, the bottom of the wing, (now facing up), will still have the same aerodynamic effect on the aileron than it had when proper way up. But, how does it work on a symetrical wing, where the air should be effected the same top and bottom?
I have to say so, that I am not a genious on the subject, so I could be wrong.
Robert.

robert & basmntdweller , you're arriving at the same conclusion in a different way.

Here is a mental experiment:
Suppose I set up my plane to fly inverted most of the time. It has a symmetrical airfoil and must fly at a slightly positive angle of attack, relative to earth ground, to keep the airplane aloft. I set my aileron differential to eliminate all adverse yaw.

Now, when I flip my plane upright, what do I find? The up aileron is 20 degrees high while the down aileron is 30 degrees low. It's exactly backward from what it should be.

So what changed? The small angle of attack required to keep the plane in the air is equal relative to earth ground but opposite relative to the airplane upright compared to inverted. It's the positive angle of attack, or lift coefficient, that causes adverse yaw and ONE of the purposes for differential aileron control.

The other purpose is wing position, dihedral, and other airframe geometry.

That's the way I learned it anyhow.

HI YA'LL
iI will stick 2 cents worth in here which could benfit the thinking.
It would do well to go back to basics for clarification.
Aerodynamics is the reaction and forces created by an aircraft
MOVING through air, those last words are the clue.
The craft can be traveling through the air in ANY direction,aerodynamics is there no matter the attitude.
Upright or inverted for example is only the pilot's perception, the
craft itself does not care what it's attitude is, could be knif edge
as far as aerodynamics is concerned.
Gravity and thrust are the only non-aerodynamic forces. Thrust
is fixed' The gravity force vector constantly changes as the craft's
attitude changes. The magnitude of the force is constant.
Bottom line? An aerodynamic reaction or force application remains the same no matter the craft's attitude.
Does this simplify thinking?
Be good,

Hal deBolt [email protected]

Of course not. That's akin to saying the pilot always pulls up to go up. What's up? Away from the ground or toward the aircraft canopy. If an aircraft is maintaining level flight upright, the lift vector is in the direction of the canopy. If it is flying inverted, the lift vector is toward the landing gear. And yes, the loads on the aircraft change dramatically when the attitude changes. The aerodynamic reactions also change. The basic lift and drag of the wing, for example, are effected by angle of attack which is, in part, a function of speed and attitude.

The only force that doesn't change is gravity x mass of craft (weight). All other forces change. That's how we change the path or speed of an object. By exerting forces upon it. Those changing forces are caused by thrust, lift, and drag.

I have to disagree. Differential should be set on up or down-lines when the wing is not producing lift.

Ill even make a bolder statement. It is not possible to have a perfectly axial roll while the wing is producing lift. Why? because a symmetrical wing must have an AOA in order to produce lift. And that AOA changes between upright and inverted flight. Sooooo, on a symmetrical wing, both ailerons should move almost identical amounts.

Aridium, for an aerobatic plane, which this thread was originally started for, I agree with you. All the other blah, blah, including my own, was just for discussions sake.

BTW, AOA, and it's effect on lift and drag, is the key.

OK, I have been avoiding this thread but I think something should be cleared up. The production of lift with an airfoil creates induced drag and its this drag that causes adverse yaw. The confusion seems to be in how and when lift is produced. Lets say for the sake of discussion that a symmetrical airfoil is at "1-degree" AOA, and we'll say the lift produced is equal to the weight of the plane allowing level flight. However with the application of aileron the camber of the airfoil is changed. The side with the aileron pointing to the wheels becomes more positive in camber generating more lift {and induced drag}, the other side becomes more negative generating less lift {and less drag}. Its the difference in the induced drag that causes the yaw. The amount of yaw varies with wing location, the center of balance {not gravity}, AOA, the size and location of the tail surfaces {as related to arm and moment} and a few others. In theory with a symmetrical airfoil with its camber center passing through the center of balance with "0" AOA will not need differential, this seldom happens. I know I'm not very good at explaining things, but I hope this helps.

Yep, and you can counteract the adverse yaw by making the up aileron deflect more than the down aileron. Whereby the up aileron is casing drag due to it's large deflection and the down aileron is not causing as much due to it's small deflection. Like I said before, this ratio is as high a 4:1 on a glider operating at very high C of L. Now, fly this plane at negative AOA (inverted) and what happens? The up aileron deflects a small amount and the down aileron deflects a large amount. The result is severe adverse yaw.

For aerobatic planes, you're going to have to use the precision trimming techniques posted on other sites.

Hi ya'll,
Seems I should add to previous post on basics
First on aileron differential, this applies to ANY style airplane, but
need is more apparent with pattern because the desire is to
maneuver precisely, slight maneuver misalignment with a sport
style is usually not a concern, however with pattern it would cost
points.
The reason for the differential is complex. but the resultant is
simple. Down aileron simply creates more drag than up, so the
up aileron movement is increased to equalize drag.
Some examples to illustrate the basics given previously.
'twas said: aerodynamics is created by the airplane moving thru
air. it moves so regardless of it's attitude.
Consider: craft is trimmed for level flight, the wing incidence is
set relative to the line of flight, not the horizon. The gravity forc is perpendicular to the line of flight. The nose is raised for a climb,
the gravity force vector is now angled back reducing the thrust
force which slows the craft. To maintain the climb angle some
elevator is used which increases the angle of attack relative to
the line of f light. Additional lift created offsets the loss due to the slower air speed. Gravity has simply upset the previous
aerodynamics which are still created by movement.
Similarily take a powerull craft and pull it from level flight into
vertical, aerodynamics remain the same, only change is gravity
now opposes only thrust.
Another consideration: inverted flight. with a symmetrical airfoil
slight down elevator is usually required to maintain level. Know
some positive incidence to the line of flight was required upright,
in relation to the LOF when inverted this becomes negative and
must be raised to positive.
Also: a flat bottom airfoil. inverted requires considerable elevator
force. Two reasons are envolved. First is same as with symmetrical. 2nd is gravity again. Upright the airfoil lift equals gravity, inverted the lift generated adds to gravity, It is possible
for this airfoil to create lift when upside down by using a high angle of attack, again relative to the line of flight.
If you have flown a flat bottom inverted you may have noted that
the attack angle required is at or close to the stall angle. This may
be a rather low angle brcause inverted this airfoil is inefficent
Observation is the craft "mushs" or flounders
Note mysteries become more clear when we remember that
aerodynamics always relate to the line of flight no matter what
direction it might be. Plus the application of the gravity force
changes with the craft's attitude.

Hal, I'm not sure what you're trying to contibute.

I'm not sure what I'm trying to contribute any more either.

I'm done.

ilikeplanes,
I believe you are inferring that these discussions get carriied
away until they drift from the origional subject.
Could be some have something which sorta fits and it is an
opportunity to express that. You have ideas?
Experience teachs and etched in my mind is learning that any
and all answers to problems and effects can be traced to basics.
So when I see people seemingly looking for details instead of
first going to ground one they appear to be wasting effort and time.
Have always tried to be of help when and as I can, OK?
Got ideas? would be pleased to hear them, OK?

hal deBolt [email protected]


Got ideas? Would be pleased to hear them!

I simply can not agree with this statement. IMHO when the AOA is 0 this is not true. Both ailerons create equal drag on a symmetrical wing(if they move the same amount). I think everyone will agree that a symmetrical wing flying level does have an AOA. In that case your statement is true.

The original post was referring to the effect of differential when inverted. How it actually becomes reversed, which I agree with. I'm not sure why we got in to the discussion about yaw. I thought differential was used to correct how axial a plane rolls, not trying to subdue yaw. Well, except maybe on a glider which doesn't really care about rolling that much

BTW I think most of us are saying the same thing, just in different ways. I am always amazed at the wealth of knowledge everyone has (I mean that in a positive way!).

Aridium, This is where the confusion is. Its not the deflection of the aileron that creates the drag, its the change in the camber line of the airfoil. Even at "0" AOA when you add aileron you now have an AOA because the camber of one wing will be positive and the other will be negative relitive to the fuselage datum line. At this point how much the difference in lift produced {it can be to the canopy OR the wheels at "0" AOA} is dependent on the placement of the wing on the side of the fuselage. The reason for this is that the airframe does not rotate around the center of the wing but around the center of balance. In a pattern plane setting the differential at true vertical with "0" AOA will compensate for the wing offset from the balance point only, this dimension does not change upright or inverted. You will still get yaw with AOA but you don't have to compensate for the airframe, only the AOA. I hope this helps, and I know its kinda weird.

Try calculating the amount of alpha when the typical pattern model is flying in level flight. It's somewhere between 1/4 to 1/2 of a degree, a very small amount. So upright vs. inverted differences are also very small.

Differential is used more to get an aerobatic model to roll around it's longitudinal axis, which goes through the placement of the cg. The placement of the wing to the vertical cg is the most significant factor in the amount of differential needed to get the model to roll axially. The lower the wing, the more differential is needed.

Also, with the proliferation of separate servos driving each aileron, it is extremely important that the throws of each aileron are measured and adjusted accurately, within .1 degree. I've found anything less will effect the quality of the roll characteristics.

Tony, I think you said the same thing I did although I'm not sure about lower wings needing more. It has been my experiance that high wings tend to yaw more than low, but that may just be the case with the models I have flown. Also planes rotate around the center of balance, this is a point somewhere in the center of the fuselage where if you could suspend the plane it would be balanced in all 3 axis. I also belive that the angle of attack is relitive to airspeed and wing loading and is not a fixed angle. However being old with only 2 active brain cells I could be wrong ;-}

Hello Hal:

I've got a simpler answer that explains nothing at all. Some planes require differential (Edge 540), some don't (Ultimate).

Go out and fly your plane without differential. If it seems to yaw and everything else is set right, try it the next time with 15-20% diff. It will work, or it won't.

Can't figure out why biplanes never seem to need differential though. Must be too advanced a design concept.

Good night all,
Silversurfer