Exponential

[Richard Wills 2]

On 01/03/2024 at 09:26, Jon - Laser Engines said:

 

The high rate is also there to help me if an aileron servo decides to die as the increased authority will be helpful if i am down to one, and vital if the servo fails when it is not in the neutral position. If it failed at full deflection on low rate, i need a higher travel from the remaining one to stand a chance of over powering it. Sure its a very unlikely scenario, but its not impossible and its just another line of defence against a loss of control. 

 

This happened to me last year with my Hangar 9 bf-109, servo failed mid roll.  Interesting getting it back down I didn't have a higher rate, I do now! thankfully was not quite at full deflection so had just enough to keep it level and rely on the rudder and got it down safe.

Edited March 2, 2024 by Richard Wills 2

[Engine Doctor]

Ask 100 people about Exponential and you will more than likely get 100 different answers . Do your own thing . Try it and if it works fine . If your uncomfortable with it then dont use it . It works for me at varying rates for the type of model . Max is usually around 20% but thats my choice . 

[ron evans]

WOOKMAN-PDB I see the tech is available but I use Spektrum rc, so would need a stand alone gizmo.

 

Yes Jon I can see the need  for sticking to procedures in different phases of the flight with big heavy warbirds, much like the full size.

My models are much simpler and flights can be more unplanned and chaotic, going from high speed to low speed and anywhere in between five or six times a flight, a lot of switching but I'll give it a go.

Thank goodness for proportional radio, my first was bang bang SC 😱

[Learner]

I'm one of Jons 5p brigade no rates, max throws and shed loads of expo. Works for me, I like the fact I can do a tight ground touching bunt with plenty of throw to get out of trouble then into smooth slow rolls without flicking multiple switches.

[Frank Day]

Im an older newish returner I find it useful as the hands arent quite as flexible and I tend to give it too much initially so a max of 30% everywhere and then adjust it down or not depending.  Also have rates on a 3 position and play around with the whole lot, all depending how it flies.

 

Durafly ME163 Komet  on 100% rates and no expo, no chance here. Watch the videos of experienced flyers getting it off the ground. I have a single channel heading hold gyro on the rudder, just remember to switch off as soon as airborne.

 

Some high wing jobbies just a couple of switchable rates

 

 

 

 

 

 

 

Edited March 2, 2024 by Frank Day

[Martin Harris - Moderator]

The Durafly Komet is likely to be a fine example of inappropriate use of expo.  A few years ago, I had a bit of a break from flying mine and the next time I got it out, seemed to have unusual difficulty getting it airborne.  Every take off run ended up in divergent yaw oscillations to the point where I either managed to get it in the air at some random point or took it home for repairs.  It got to be a bit of a club joke - although my other flying seemed unaffected, I seemed incapable of flying this model. 
 

Once in the air, it behaved perfectly with benign flying characteristics.  I went through the ATV and expo experiments with no improvement in the take off run until the day the penny dropped and I started to wonder whether I’d remembered the battery position correctly from before the break. 
 

A few neurons fired in sync and I moved the battery forward a significant amount to a position which seemed to ring a bell.  I lined it up and it tracked perfectly, easing into the air like a basic trainer. 
 

Many subsequent flights have been uneventful - crosswinds hold no terrors and mid take off run heading corrections can be made with ease.  The mass of the battery in relation to the undercarriage position seems to be the key here. 
 

So perhaps these experienced flyers in your videos might like to try a C of G adjustment rather than messing about with expo?

[Mike Blandford]

Servos typically rotate +/- 60 degrees for full stick throw. For the rotation from 0 to 10 degrees, the linear movement is about 17% of the radius while for the rotation from 50 to 60 degrees the linear movement is only 10% of the radius. This actually results in (approximate) negative exponential. So if you use some exponential you compensate the servo rotation. The rotation of the control horn on the aerodynamic surface will similarly provide some positive exponential, although the amount of rotation of the surface is generally significantly less than 60 degrees, so you still have some negative exponential in the geometry.

I just did a test on erskyTx and I need about 40% expo set so the linear output of the servo is the same for the first 10 degrees of rotation and the last 10 degrees of rotation.

 

Mike

[Frank Day]

Hi Martin, re the Komet are you using a CG point forward of the manual recommended position

[Jon H]

8 hours ago, Mike Blandford said:

Servos typically rotate +/- 60 degrees for full stick throw. For the rotation from 0 to 10 degrees, the linear movement is about 17% of the radius while for the rotation from 50 to 60 degrees the linear movement is only 10% of the radius. This actually results in (approximate) negative exponential. So if you use some exponential you compensate the servo rotation. The rotation of the control horn on the aerodynamic surface will similarly provide some positive exponential, although the amount of rotation of the surface is generally significantly less than 60 degrees, so you still have some negative exponential in the geometry.

I just did a test on erskyTx and I need about 40% expo set so the linear output of the servo is the same for the first 10 degrees of rotation and the last 10 degrees of rotation.

 

Mike

 

The problem is, you want the negative expo as it is an aid to things like a landing flare as you end up with softer sticks at greater deflection allowing greater accuracy. 

 

However, the servo rotation argument is pretty meaningless if, like many, you fit a 2 inch servo arm and reduce the servo travel to 20% to get the required deflection. Very few go back and move their control linkage closer to the servo or fit a smaller arm. 

 

20 hours ago, Learner said:

I'm one of Jons 5p brigade no rates, max throws and shed loads of expo. Works for me, I like the fact I can do a tight ground touching bunt with plenty of throw to get out of trouble then into smooth slow rolls without flicking multiple switches.

 

All of this can be achieved without the 5p approach. There is also a misconception about switch flicking as it is mostly confined to takeoff/landing for my warbirds, but hardly ever at all for sport/aerobatic models. I have them set to do all i want them to on a single rate for the most part. To be honest though switch changes are such a non event i am not sure why its such a concern. 

 

 

[Martin Harris - Moderator]

1 hour ago, Frank Day said:

Hi Martin, re the Komet are you using a CG point forward of the manual recommended position

I don’t think I’ve flown it since the autumn and I tend to disregard manual positions once I’ve flown it so I haven’t checked it with a ruler in years!  However, measuring the model with the battery where I normally have it reveals the C of G pretty much on the forward position. 
 

The point is that expo isn’t necessarily the answer to handling problems - try overdoing it on elevator and you’ll certainly discover similar effects to a rearward C of G!

 

With the battery a couple of inches behind this position it flew quite happily but was virtually impossible to keep straight long enough to get airborne…do you have a link to any of these videos?

[Learner]

8 hours ago, Jon - Laser Engines said:

 switch changes are such a non event i am not sure why its such a concern. 

 

 

Your right, switch changes are a non event  I don't need to change any, expo does it all for me so I dont need to be concerned.😉

[Nigel R]

On 03/03/2024 at 01:19, Mike Blandford said:

Servos typically rotate +/- 60 degrees for full stick throw. For the rotation from 0 to 10 degrees, the linear movement is about 17% of the radius while for the rotation from 50 to 60 degrees the linear movement is only 10% of the radius. This actually results in (approximate) negative exponential. So if you use some exponential you compensate the servo rotation. The rotation of the control horn on the aerodynamic surface will similarly provide some positive exponential, although the amount of rotation of the surface is generally significantly less than 60 degrees, so you still have some negative exponential in the geometry.

I just did a test on erskyTx and I need about 40% expo set so the linear output of the servo is the same for the first 10 degrees of rotation and the last 10 degrees of rotation.

 

Mike

 

TX control sticks also rotate around a point...

 

That said I don't know what exact effect that has on the whole chain of controls.