Aileron Differential?

[Ultymate]

I give up

[Ultymate]

The aircraft if yawing to the right whist rolling when flying the right way up due to it's design will not suddenly start yaw the opposite way when it reaches the inverted position it's still flying in the same air.

[PatMc]

Posted by Ultymate on 05/12/2011 22:12:42:
The aircraft if yawing to the right whist rolling when flying the right way up due to it's design will not suddenly start yaw the opposite way when it reaches the inverted position it's still flying in the same air.

 

No one said the yaw direction would reverse. It will always be in the direction of the rising wing.

 

In a roll to the left differential will counteract the right yaw at first but when the aircraft is inverted the left wing will have more skyward pointing aileron movement than the right wing so the yaw will be more pronounced than it would have been without differential.

 

Without differential the yaw will be the same throughout but left rudder required to correct it during the upright phases & right during the inverted just as the opposite rudder will be needed during the opposite knife edge phases.

 

 

[Martin Harris - Moderator]

Pat - I've nearly posted a couple of times but couldn't convince myself about an explanation to something I've always taken for granted.

 

However, to try to move this on, shouldn't we consider the fact that nothing changes aerodynamically (I think we can probably ignore the inputs counteracting gravity dropping the nose) during a 360 degree roll - the "downgoing" wing doesn't change its vector when the sky is the other way up.

[PatMc]

Martin, I don't understand what you mean when you say "the "downgoing" wing doesn't change its vector when the sky is the other way up."

 

It depends how fast a roll is being executed to appear axial, also over how much sky it's covering in the time. If it's a fast roll there probably won't be any need for any KE correction & if there's no aileron diff probably little or none for that either. If it has diff you've introduced a needless problem (unless it isn't effective anyway )

If it's to be at competition speed an axial roll it will need some correction throughout but executing the roll in a downwind direction will make any deviations less obvious.

 

If it's just banging a semi scale aerobat around at fast roll rates there's certainly no need for as much, if any, KE corrections or diff.

[Biggles' Elder Brother - Moderator]

Wow guys you're getting yourselves tied in knots here. From where I sit in a sense you're both right!

 

Let's say the model is rolling to the right (as viewed from behind). One eighth of the way in the left wing has down aileron and so so is experiencing more drag - result the nose moves off axes to the left as view again from the back.

 

At the quarter (ie the knife edge) the wing with the down-aileron (that we formely called the left wing) is now the uppermost wing - its still got more drag but the effect now is to lift the nose.

 

At the halfway point, the wing with the "down" aileron is now of course on the right, and it still experiences more drag - altough its pointed upwards! Why? Well the increase in drag is not due to the aileron being "down" - the air doesn't know about up and down - but it does know about lift! The wing is inverted, its effectively producing negative lift now. Anything that increases whatever lift the wing is producing will increase the drag. There's more drag on the "down" going aileron - not because its downgoing but because its increasing the lift from that wing - making that wing work harder. There is no lift without drag - the two are inseparable. More lift (up or downward) - more drag and it doesn't matter what the sign of that lift is - positive or negative its the same. The fact that its now negative lift makes no difference at all you still get the drag on the same aileron!

 

All this means that at this halfway point the, now, righthand wing is producing more drag and so the nose is pulled to the right.

 

Into knife-edge next with the wing that has the "down" aileron underneath the plane - still it has the most drag and so the nose is pulled down.

 

So around the complete roll to the right the nose goes; left, up, right and down as the roll goes through the quarters. In other words the roll becomes barrelled. Unless of course the pilot corrects these tendencies as Pat suggests. Or, as Ulty suggests the roll is fast and yaw doesn't really have time to develop due to inertia being much larger about the yaw axis than it is about the roll axis for all planes

 

BEB

Edited By Biggles' Elder Brother - Moderator on 06/12/2011 00:23:14

[Martin Harris - Moderator]

The way I'm looking at this is that the adverse yaw is tending to introduce barrelling and counteracted by the differential. The original question that I'm trying to clarify the answer to concerns how the differential works from an inverted starting position.

 

Whether the aircraft is inverted or indeed pointing straight down, the differential is effective because the "lift" is being used largely to provide the rolling moment and the downgoing aileron is producing more lift and consequently more induced drag than the upgoing one.

 

I believe this fits with BEB's informed explanation and Ulty's point of view.

 

Edit: Took a while posting this and the Oracle has spoken while I wascomposing it!

Edited By Martin Harris on 06/12/2011 00:12:39

[Steven Buckingham]

So to summarise. Gravity doesn't have much impact even on a slow flying aerobatic model, its all about airflow. The airflow isn't going to change much whether the model is natural or inverted (or anywhere in between). Therefore the aerodynamics of the airframe and specifically the wing section will be the defining factor. To go back to Chrisies flat wing it really doesn't care whether its inverted or not and therefore differential is irrelevant, on scale models it does become more useful to maintain a true flight path, BUT every model is going to be different and the amount of mixing required depends on the model and the pilots preference. The only way to get a true feel for what happens with different inputs is to get out on the flying field (with hand warmers and a thick jumper given the time of year) and experiment.

[Biggles' Elder Brother - Moderator]

Just had to do some small edits to my post above because I realised I was being confusing! Using "up-aileron" to mean the aileron on the "up going wing" - i.e. the one pointed down!

 

Anyway, I've tidied up the terminolgy now and I think that should be clearer!

 

One way of thinking about this is; putting the aileron down makes the wing produce more lift - ie work harder - its that that produces the extra drag - not the "downness" as such - the extra lift. When its upside down that wing is still the one that's working harder - its just that the lift is downwards now! More lift, in any direction, means more drag!

 

BEB

[Simon Chaddock]

Lets ask this question.

 

When the plane is undertaking a true axial no height loss roll during the period when its inverted how is the weight of the plane supported?

 

Even if this situation only lasts for a moment the wing must be operating as an inverted aerofoil so what was the up going aileron is now increasing the wing camber and vice versa so normal aileron differential will temporarily compound any adverse yaw.

 

However I do wonder just how truly axial any roll really is.

At high speeds and roll rates even a very small amount of 'barrel' or change in flight line can completely eliminate any lift requirement in the inverted phase.

 

 

 

[Martin Harris - Moderator]

The angle of attack of both wings is increased relative to the level flightpath by the traditional application of down elevator during this phase (and of course we use rudder at the appropriate time) but the dominant force is applied to continue the roll so the differential is still required to counteract the barreling.

 

However, I suspect you're right in asking whether any roll is ever truly axial.

Edited By Martin Harris on 06/12/2011 00:51:39

[Biggles' Elder Brother - Moderator]

No Simon - I'm afraid that isn't so. If the model is trimmed to fly level then the wing is, as you say producing lift in that configuration. If you now invert that model (perfectly axially) then that wing will work exactly the same (the air doesn't recognise up and down or right way up etc) and will still be producing lift but that lift will now be a down force.

 

I think the problem here is the sloppy terminology we all use sometimes - we say "the downgoing aileron produces drag" but this simply isn't true! Its not that the aileron in going down that increases the drag its the fact that it is making that wing produce more lift force - that's what's making the drag. (I'm using the term "lift force" to indicate that its the aerodynamic force produced by the wing - it isn't neceaasily pointed upward)

 

Think of it this way - when a model is rolling to the right then the port wing is having to produce more lift force to promote the roll. Correspondingly the starboard wing is reducing its lift to co-operate. The model rolls. That state of affairs exists all around the roll, the port wing is always producing more lift force than the starboard wing. True, when inverted that lift force is downward directed but that doesn't matter is still more lift force on the port wing. Its this increased lift force that causes the drag, this increased lift force is always on the port wing all through the roll - so its the port wing that always drags and the adverse yaw is the same (relative to the model) throughout the roll. So this means that differential aileron does help all through the roll. The yaw does not reverse.

 

BEB

[Tim Mackey]

I like the use of the term lift force BEB - it does help the explanation. Of course, it may also be true to argue then that in your expression.....

" when a model is rolling to the right then the port wing is having to produce more lift force to promote the roll. Correspondingly the starboard wing is reducing its lift to co-operate".

 

One could therefore say the starboard wing is not reducing its lift - its also increasing its lift force - but its inverted at the time

Sorry

[Paul Adams]

So if I am flying my model which has fully symmetrical aerofoil in level flight inverted, the lift created by the wings still needs to provide a lifting force. This will require a bit of down elevator to be dialed in which changes the angle of attack of the wing from say a degree positive with respect to the airflow (ie a degree negative with respect to the model). If I now initiate a roll one wing will have to increase its lift and the other to reduce its lift. I am in agreement with the concept that the wing which needs to increase its lift will produces more drag which will cause the yaw effect. However if you have aileron differential mixed in, and initiate the roll from inverted the lifting wing with regards to the ground will be subject to a greater control throw, hence will have more drag. So there must be something more in it than the above explanation.

 

 

I am wondering therefore if the aerofoil has a major impact on the effect and what we are doing is attempting to simplify the forces so it makes it easier for us to understand. Let me explain further, take a standard wing aerofoil like a Clark Y section. To produce lift it needs to be angled at say a couple of degrees positive incidence. If you then add ailerons and go into a roll situation, to balance to get the same drag from both wings (ie no yaw) would probably require a different amount of aileron. When inverted the angle of attack of the wing needs to be much greater so would require a different mix of differential. As we can only apply a single value of differentiation then the best we can do is experiment until we get the best compromise. Trying to put the theory behind the practice in this situation is beyond my understanding as it will be continually changing throughout the roll,

[Myron Beaumont]

My mind is now boggling . Just to add to the terminalogical confusion ,there is another factor to further complicate the issues being discussed or then again it might just make it clearer.

Surely ,a lot depends on the aerofoil section and also the siting of the thrust line compared with the vertical position of the wing compared to the vertical component of the CG (Its height above the CG vertical component ). A good example might be comparing a high wing Piper Cub (lifting type wing section) with an aerobatic machine (symetrical wing section ) and the thrust line and mid section profile of the wing all lie more or less in the same plane .You could call it the "datum line" if you so desire .In other words ,nothing will change from flying the right way up to being inverted (perfectly axial rolls?).All I know from my own experience of both types is that with a Cub I built years ago when starting in RC (bit of a pig in a turn requiring very little down aileron -if any I found out ) I had to put in opposite rudder to a turn for advantageous yawing to compensate for the drag effect on the down aileron wing until I was advised to greatly reduce the amount of down movement . Originally there was no differential mentioned on the plan (Graupner kit) by the way . This was achieved by fiddling with the geometry involved from a single mid- span aileron servo by the way in those days.

On the other hand,one of my own scratch built aerobatics was design as aforementioned ie everything on a single datum line . RESULT ---NO dfferential necessary .

[Martin Harris - Moderator]

I think you're right that the aerofoil will have an effect on the need for differential to keep things somewhere near axial but you need to read BEB's last explanation again. Forget where the ground is - it's not relevent in this context and it's the direction of roll which is significant.

Edited By Martin Harris on 06/12/2011 11:28:28

[Paul Adams]

I have read it again but it does not explain the situation of a fully symmetrical wing where there is no difference to the wing to which way up it is (assuming in both cases the elevator is adjusted for level flight). For level flight both will produce the same lift and therefore have the same angle of attack, have the same aerofoil. so to the airflow there is no difference to which way up the model is. This being the case the lift increasing aileron needs to move the same ratio in relation to the lift reducing aileron to produce the same drag from each wing half which is not dependent on which way up the model is. However with differential dialed in the ratio of movement of the lift increasing aileron in relation to the lift reducing aileron is dependent on which way up the wing is. As such it must be more involved than just drag increased yaw caused by the lift increasing wing.

[Andy Gates]

I am going to ignore the aerobatic use of aileron differential and focus in the area that I use aileron differential for and that is for slow speed handling.

 

I normally use between 30 & 50% differential on the ailerons on my machines to reduce the chances of tip stalling and to reduce the rudder requirement when flying at slow speeds and for landings.

 

This certainly makes life much easier when things are getting a little critical in terms of airspeed and lift.

I have discovered that the tell tale sign that you have over done the differential is when the plane gives the feeling of the wing digging in while in a banking turn (as opposed to a yaw turn for the aerobatic pilots).

 

I also sort the differential using mechanics rather than the tx, just the way I work but it feels better to me. This is easily done by off setting the servo output horn or the control surface horn.

 

For those not to clear on the mechanical way of doing things I would recomment the RM Propo book, possibly very out of date but incredibly informative and in this case relevant.

[Steve W-O]

Posted by Andy Gates on 06/12/2011 14:09:03:

I am going to ignore the aerobatic use of aileron differential and focus in the area that I use aileron differential for and that is for slow speed handling.

 

I normally use between 30 & 50% differential on the ailerons on my machines to reduce the chances of tip stalling and to reduce the rudder requirement when flying at slow speeds and for landings.

 

This certainly makes life much easier when things are getting a little critical in terms of airspeed and lift.

I have discovered that the tell tale sign that you have over done the differential is when the plane gives the feeling of the wing digging in while in a banking turn (as opposed to a yaw turn for the aerobatic pilots).

 

I also sort the differential using mechanics rather than the tx, just the way I work but it feels better to me. This is easily done by off setting the servo output horn or the control surface horn.

 

For those not to clear on the mechanical way of doing things I would recomment the RM Propo book, possibly very out of date but incredibly informative and in this case relevant.

 

 

Like I said in my earlier post, prevention of tip stalling is the most important reason for differentioal to me, but I'm surprised it is only the two of us who think so, as it makes a big difference

[Martin Harris - Moderator]

 

Posted by Paul Adams on 06/12/2011 13:40:29:

I have read it again but it does not explain the situation of a fully symmetrical wing where there is no difference to the wing to which way up it is (assuming in both cases the elevator is adjusted for level flight). For level flight both will produce the same lift and therefore have the same angle of attack, have the same aerofoil. so to the airflow there is no difference to which way up the model is. This being the case the lift increasing aileron needs to move the same ratio in relation to the lift reducing aileron to produce the same drag from each wing half which is not dependent on which way up the model is. However with differential dialed in the ratio of movement of the lift increasing aileron in relation to the lift reducing aileron is dependent on which way up the wing is. As such it must be more involved than just drag increased yaw caused by the lift increasing wing.

For a fully symmetrical section to produce lift it has to meet the airflow at a positive angle of attack so it's not actually seen by the airflow as symmetrical when it's working. The ailerons modify the effective aerofoil profile differently depending on the rolling direction.

[Andy Gates]

Steve W-O

 

I suspect there are not so many people who explore the slower end of flight (and I don't mean prop hanging) like you and I obviously do.

[Paul Adams]

Agreed, hence why I said with the elevator adjusted in both case to give level flight, so the wing will be at the same angle of attack, its just which way up that has changed.

 

With a left or right roll of the same rotation rate, one aileron has to increase lift the other decreases lift, however in both roll directions of the same rate, the amount of movement on the aileron will be the same ie . In both cases the up going aileron will have the same aerofoil profile and the down going ailerons will have the same (but which will be different from the upgoing aileron) a different aerofoil profile. Its just which wing half each aerofoil profile is on changes.

 

So that being the case and with differential dialed in ( I have assumed 50%) how with the model right way up do you require 10 degrees of up and 5 degrees of down to produce a roll without yaw, but when inverted require 5 degrees of up and 10 degrees of down to produce the same roll without yaw.

 

 

[Andy Gates]

Oops, duplicate post - sorry

Edited By Andy Gates on 06/12/2011 15:17:17

[Martin Harris - Moderator]

Because the barrelling from the adverse yaw we're trying to control is produced by the rolling force - you have to forget which way up the model happens to be to understand this and take care of that aspect with appropriate pitch and yaw adjustments.

 

An axial role is perfectly possible with adverse yaw being produced - just more difficult to do well as you're compensating for it in addition.

[Simon Chaddock]

Andy

Its nice to see someone else actually uses 'mechanical' differential but then I am still using 'steam age' transmitters!