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Aileron differential

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Birgir09/05/2019 20:53:24
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Hi everyone

A few years ago I built me a Sig Hog Bipe, which is a 60” biplane. There are four ailerons, top and bottom connected by a rod and 2 separate servos in the lower wing.

Recently I have built up a courage to fly the model after some practice with my Mini Super trainer.

Of course there are some differences between the flying of those two models. I’ve already flown it 5 times and I’m getting a better understanding of the model’s behaviour.

One thing that troubles me is that when turning, using ailerons and elevator as I’m used to from my trainer, something unexpected happened. When applying the ailerons the model banks to the correct side but when applying a small amount of up elevator it tends to climb out of the turn and raise the nose. I’ve managed to work around this by either applying rudder in the correct direction and/or rolling the model almost 90 deg and use a considerable up elevator.

A friend at the field pointed out to me that possibly this has something to do with the down aileron on the outer wing creating a drag working against the turn.

So my question is: Does anyone have a similar experience form a similar model?

If I put in an aileron differential, how much should I use?

As is the aileron movement is about 1 inch from top to bottom, half an inch in each direction.

Any comments appreciated

B

John Duncker09/05/2019 21:09:06
77 forum posts
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Have had several bipes last being a 1/4 scale Stampe. It used 2 to 1 differential IE twice as much up aileron.

stampe 024.jpg

J D 809/05/2019 22:27:34
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With aircraft of this type aileron differential nearly always needed to counter the drag of the down going aileron, [ it has more drag than the up going one moving the same amount ] Go with John's suggestions to start.

Mixing rudder with aileron is also a common fix, about half the rudder movement with full aileron. Other option is to lead with the rudder stick manuall'y.

Martin Harris10/05/2019 00:54:15
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The common advice to lead with rudder has always intrigued me. From my gliding days where dealing with adverse yaw* is universally required (due to the long wing moment and design considerations for avoiding drag producing countermeasure) and taught from lesson one, it was always stick and rudder together to initiate a coordinated turn.

Anyone know what the theory behind leading with rudder is?

*Adverse yaw is the effect you've noted - from a glider cockpit view, application of uncoordinated aileron results in a massive yaw (and rising nose) in the opposite direction.  Most modern powered aircraft require very little rudder to balance a turn as they are intrinsically less prone to it with their shorter wings and have aerodynamic aids such as Frise ailerons or in the case of some larger aircraft, drag producing spoilers as well as any aileron differential.

Mind you, although de Havilland developed the principle and introduced the use of differential ailerons with the DH82 Tiger Moth, the one I flew last year needed some good bootfulls of rudder even with its aid so learning to coordinate turns is a skill worth developing which too many model pilots tend to ignore. A useful approximation given the lack of feel and instrumentation is to simply correct any tendency for the tail to drop by adding rudder until the model "sits" correctly in the air.

Edited By Martin Harris on 10/05/2019 01:21:09

Edited By Martin Harris on 10/05/2019 01:24:29

Simon Chaddock10/05/2019 01:06:38
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Birgir

To be strictly correct the rudder is always required to perform a true coordinated turn even with differential ailerons as the degree of differential required is effected by speed, The inertia of the plane can play a part as well the engine torque applied..

In full size you have a 'turn and slip indicator' so you can accurately judge the correct rudder and aileron combination but it is rather more difficult to do this when standing on the ground. It is usually possible to set the degree of differential that gives a good result for a moderate turn using aileron only.

A more severe turn is still likely to require appropriate use of the rudder and ailerons, first to enter the turn and then to accurately maintain it.

Manish Chandrayan10/05/2019 05:06:53
558 forum posts
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A very basic question. Do you apply the rudder first in the direction of the turn and then the aileorons or first bank and then apply rudder in opposite direction to keep the nose level?

Jon - Laser Engines10/05/2019 08:20:59
4581 forum posts
171 photos

I agree with Martin. Learning to fly coordinated turns with rudder is very important and will serve you well on many types of model. On all of my scale models, be they WWI, WWII, my stampe, a piper cub or a cessna i use the rudder as much as the elevator and ailerons. Frankly, i dont know how you can fly without a rudder!

I strongly recommend you dont mix it on the radio as there are times when you need to use aileron and rudder in opposition. Examples of this are takeoff and landing (especially in a cross wind), normal turns with models that tend to roll into turns (cubs, my nieuport, tiger moth etc) and doing large loops where you may need rudder to keep on line and ailerons opposite to prevent a roll. Even during stall turns opposite aileron can tidy things up very nicely.

The easiest way to practice this is a mod of the way i was taught when gliding with the air cadets. fly directly away from yourself and just roll 30 degrees left and right of level flight. dont turn, just move the stick left and right and watch the model wallow about. come round again and do the same but this time add some rudder in the same direction as the ailerons. Experiment with more/less rudder under you get a nice axial roll. Once that it programmed into the brain it becomes 2nd nature and once that happens you can then start using the controls in opposition a little more. Its a whole new world after that

Peter Miller10/05/2019 08:22:47
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Posted by Manish Chandrayan on 10/05/2019 05:06:53:

A very basic question. Do you apply the rudder first in the direction of the turn and then the ailerons or first bank and then apply rudder in opposite direction to keep the nose level?

You do NOT apply rudder in the opposite direction. You apply both in the same direction and simultaneously. This is the correct coordinated turn

Many old biplanes and models thereof will be seen to hang their tails down in a banked turn. the rudder in effect lifts the tail to make the correct coordinated turn.

Just a little side note. The Taylor Monoplane plans (Full size) specify lots of up aileron " and almost no down aileron."

J D 810/05/2019 08:57:18
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I think "lead with the rudder" comes from the early days flight when rudder was very much the primary directional flight control on most types8.jpg.

My Bleriot was built with the intension of having working wing warping [ parts are there ] However it is not used as it is just fine on rudder elevator. I do not think I would like to try flying it with only wing warp control for steering.

Simon Chaddock10/05/2019 09:14:24
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Manish

Applying the rudder in opposition to the ailerons should normally be avoided, particularly at slow speed, as it a good way to enter a spin.

In simple terms all the controls are used at the same time for a turn but the degree of input is set by the need to avoid any slip or yaw and to maintain the required 'attitude' and speed. As a result the inputs, including power, may be adjusted from beginning to end of the turn.

A plane with a big relatively heavy wing, typically a sail plane, may require a substantial initial rudder input but then little or none during the turn but similarly opposite rudder and aileron to return to straight and level flight, be it full size or a model.

A 'fast jet' configuration plane on the other hand may only really need to use the rudder when at slow speed, during aerobatics or for a cross wind landing.

For practical reasons most models have sufficient natural stability and excess power that they can tolerate some slip or yaw during a turn without getting into difficulty. The classic danger point being a low or no power 'final turn' where the extra drag from unwanted yaw can, and does, lead to disaster.

Denis Watkins10/05/2019 09:17:28
3600 forum posts
166 photos

My take on " leading with the rudder " is based on the two styles of rudder.

The most common style rudder is above the horizontal stabiliser, and exerts a small twist of the wing into the turn

Therefore lead with rudder

The other style of rudder to fly is the above and below the horizontal stabiliser

And this is more prone to move the tail, wings level, if not too aggressive

So I would subscribe to earlier flight instigating lead with rudder

Martin Harris10/05/2019 14:50:13
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I'm slightly confused with some of the reasoning here. Are you concluding that leading with rudder was a throwback to early days of flight but can still be relevant? I think that with modern understanding of control systems, that advice is now rather outdated.

The business of the location of the rudder seems a bit of a red herring. The turning mechanism of the rudder works in conjunction with dihedral. The application of rudder primarily induces a side slip - there's no significant keel effect like a boat, for example to induce a turn. The aircraft therefore experiences more lift from the forward moving wing and less from the rearward one due to the increase and decrease of angle of attack. The more dihedral there is, the greater this effect, which is why a 3 channel model using rudder to turn will have exaggerated dihedral compared with a more conventional aircraft.

With a typical aerobatic model with little or no dihedral, there's very little yaw/roll couple and this is by design to aid in keeping manouevres clean.

While the centre of pressure from the control surface exerted by different rudder configurations might be slightly misaligned with the centre of lift, I can't see it having any significant effect on the roll couple - perhaps someone may be able to argue differently? I'm sure we all miss Dave Burton's expert and clear explanations at times like this.

FilmBuff10/05/2019 16:36:34
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Posted by Manish Chandrayan on 10/05/2019 05:06:53:

A very basic question. Do you apply the rudder first in the direction of the turn and then the aileorons or first bank and then apply rudder in opposite direction to keep the nose level?

It's not a good idea when your learning or flying properly to do this. However, it can be fun to do with experience. I try and emulate a control line circuit. Control the turn with rudder and use opposite ailerons to keep the wings level as the plane flies a flat circuit. Needs the right plane, and if not careful you are opening up the possibility of a spin as others have said.

On thermal gliders, when circling in lift, I find that I control the angle of the bank with ailerons and the size of the circle with rudder.  But this may just be me!

Edited By FilmBuff on 10/05/2019 16:38:02

Andrew Ray10/05/2019 17:06:22
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You roll into a turn with aileron, using elevator to maintain attitude and rudder to balance the turn so that the aircraft is neither slipping or skidding and that the ball stays in the centre or the yaw string is down the centre of the canopy. In practice all 3 controls are moved virtually at the same time.

Differential aileron helps to reduce the secondary effect of aileron which is yaw, the effect increases with increasing wing span.

Not all aircraft are the same though, it may help to counter the opposite yaw from the ailerons to lead with rudder depending on the design. I heard that some pilots flying the ASH 25 sailplane commenced a turn with opposite aileron to set up yaw into the direction they wanted to turn then quickly coordinated controls. The technique helped the glider into a thermal turn more quickly. Used (I believe) because the rudder is relatively small and ineffective.

It is not as dangerous as some make out to use crossed controls, a sideslip is used to scrub off speed or height for landing and can be taken down to a few feet off the ground in full-size terms. In fairness this is not so easy to judge in model flying and probably not necessary other than to try something different.

Geoff Peacock10/05/2019 17:33:46
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Posted by Peter Miller on 10/05/2019 08:22:47:
Posted by Manish Chandrayan on 10/05/2019 05:06:53:

Just a little side note. The Taylor Monoplane plans (Full size) specify lots of up aileron " and almost no down aileron."

I believe that on a full size Tigermoth, full movement of ailerons actually returned the down aileron back to neutral, having first gone down slightly, then returned as the input was increased.
This can be achieved by offsetting the servo horn (I think).
No doubt someone will soon agree… or not!

Martin Harris10/05/2019 18:04:57
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8478 forum posts
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Posted by Andrew Ray on 10/05/2019 17:06:22:

Not all aircraft are the same though, it may help to counter the opposite yaw from the ailerons to lead with rudder depending on the design. I heard that some pilots flying the ASH 25 sailplane commenced a turn with opposite aileron to set up yaw into the direction they wanted to turn then quickly coordinated controls. The technique helped the glider into a thermal turn more quickly. Used (I believe) because the rudder is relatively small and ineffective.

It is not as dangerous as some make out to use crossed controls, a sideslip is used to scrub off speed or height for landing and can be taken down to a few feet off the ground in full-size terms. In fairness this is not so easy to judge in model flying and probably not necessary other than to try something different.

Interesting technique. Was the theory to extract more energy from the lift by making the into lift wing work harder and stay in it fractionally longer - wasted energy producing drag from the yaw being less important than making a rapid turn?

My vintage Rhonbussard sideslipped very effectively (just as well as the non standard rather small airbrakes were not too effective) but in a fully developed sideslip, due to the fuselage blanking the tailplane, the elevator was at nearly full deflection with no sign of stalling - I'm still here to tell the tale!

Manish Chandrayan10/05/2019 18:28:18
558 forum posts
65 photos
Posted by Geoff Peacock on 10/05/2019 17:33:46:
Posted by Peter Miller on 10/05/2019 08:22:47:
Posted by Manish Chandrayan on 10/05/2019 05:06:53:

Just a little side note. The Taylor Monoplane plans (Full size) specify lots of up aileron " and almost no down aileron."

I believe that on a full size Tigermoth, full movement of ailerons actually returned the down aileron back to neutral, having first gone down slightly, then returned as the input was increased.
This can be achieved by offsetting the servo horn (I think).
No doubt someone will soon agree… or not!

Yes that's right and that's how I have set up my current quarter scale Moth. I have just 1/4" down while the up travel is limited (yes it has more available) to 2 1/2". And just like the full scale by the time the upward movement is reached the down going aileron has come back to almost neutral.

Manish Chandrayan10/05/2019 18:36:53
558 forum posts
65 photos

My question had arisen from the fact that in my smaller Tiger Moth, I have used rudder and ailerons during the turns ( same side ) and then sometimes have had to reverse the rudder input to keep the nose from sinking . That is initiate the turn with rudder ( due to dihedral secondary effect is bank) smooth out the bank with same side ailerons and if the nose starts to drop lift it up by releasing the ailerons and applying opposite rudder.

I do know that cross controls if not done properly are a surefire way to where a spin or snap

Thank you all for some interesting discussion

Martin Harris10/05/2019 22:44:01
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8478 forum posts
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I wonder if you have a rather forward C of G?

Manish Chandrayan11/05/2019 01:02:56
558 forum posts
65 photos

Yes Martin I too think so

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