Single servo ailerons

[Keith Berriman]

Good Afternoon to all. Having aquired a kit built Acro Wot and fitting all engines servos etc and just the one for airleons I am finding when flying it is very twichy on airleons input have tweeked the servo settings and exponential etc but not resolving my problem. Some years ago I was shown how to get differential on the output by fitting a cut out circular disc to give more up than down. Am I going the right way is it more up than down ?? or the other way

[Bob Cotsford]

If you find it twitchy I don't think differential will be your answer. For reference you want more up than down if you do go with dif. Personally I'd try reducing the rates with say 70% as low rate on a switch or add maybe 30% expo, try one at a time and see which suits. Also make sure you have the expo going the right way, some systems use +ve for less sensitivity around neutral, some -ve.

[Simon Chaddock]

RC Plane Flyer

Definitely more up than down!

A down going aileron for the same deflection creates more drag than an up going one so although the plane banks it tends to yaw outwards away from the turn or in other words a good start for a spin if you are flying too slowly!

With modern light weight but powerful servos it is much simpler to use a servo for each aileron and then use the transmitter functions to control the aileron travel, end stops and differential action. Most Tx will have a differential function already available.

[Martin Harris - Moderator]

Deleted, sorry - Bob had already made the suggestion to check expo direction!

Edited By Martin Harris on 13/08/2019 14:51:31

[Jon H]

I would ask yourself the following:

Do i need to use full stick deflection on ailerons at any point during the flight?

If the answer is no, then lower the rates first. Keep reducing the rates until you get to a stage where full deflection gives you a nice rate of roll (as in you pull up to do a roll, slap the stick over and it feels nice/controllable etc) and then look at expo if you feel you need it. I really suspect you wont need it.

The more of the stick deflection you use the softer the response will feel. If you find after all this that you are at about 20% rates, measure the deflection and fit a smaller output wheel on the servo. Reset the rates back up to get yourself back to the deflection you measured before. This will also make the model feel smoother as some servos do not have fantastic resolution and move in fairly large steps (relatively speaking) and this is noticeable when you have such small movement of the servo as the error is being magnified by the geometry of your control linkage.

I also agree that you dont need differential on an acrowot.

[Bob Cotsford]

Posted by Simon Chaddock on 13/08/2019 13:55:39:

RC Plane Flyer

Definitely more up than down!

A down going aileron for the same deflection creates more drag than an up going one so although the plane banks it tends to yaw outwards away from the turn or in other words a good start for a spin if you are flying too slowly!

With modern light weight but powerful servos it is much simpler to use a servo for each aileron and then use the transmitter functions to control the aileron travel, end stops and differential action. Most Tx will have a differential function already available.

Simon, for a new build I'd maybe agree about twin servos if the wings had been cut for twin servos but this is a completed model, please don't confuse the lad now! Having owned a kit built Acrowot I'll just say that in my opinion this model doesn't need a complicated aileron setup and mine certainly didn't need differential in order to handle nicely. Thousands flew perfectly well with just a humble 148 sitting in the middle of the wing driving both ailerons. I would suggest that 1/4-3/8" (7-10mm) each way is plent of aileron movement while getting used to the model.

[alex nicol]

Apologies if this a bit obvious

Before anything is adjusted, have you measured the aileron deflection and compared it to the recommended deflections?

Also have you checked the balance point

Alex

[Jon H]

Posted by alex nicol on 13/08/2019 16:45:16:

Apologies if this a bit obvious

Before anything is adjusted, have you measured the aileron deflection and compared it to the recommended deflections?

Also have you checked the balance point

Alex

Alex makes a valid point about referencing the recommended throws but dont feel you have to abide by them. If you want more or less then by all means go for it. I totally ignored the recommended settings for my acrowot xl as i considered them to be excessive in the extreme.

[alex nicol]

The reason I asked as to the current aileron deflections was there is no mention in the thread. I just wanted to rule out the obvious and check the throws are currentlyset to something sensible.

Alex

[Keith Berriman]

So Alex was the nearest and John came second. After further bench checks I found that my Flash 8 TX triple rate switch was in the mid position and the exponential was set at zero instead of -35%. This had given me 5/16th deflection instead of the 1/4" recommended. Balance point within parameters

Thanks to all who took time to put ideas forward

[Former Member]

[This posting has been removed]

[Keith Berriman]

Looking at the size of the airelons on the wing I think a mini servos may struggle under load I have two midi servos in the workshop that would do the job just need to check the output specifications

[Brian Cooper]

Using just one servo on the ailerons, especially on a model like the Acrowot, is very "old school".

It is far better to use two servos, and then dial-in expo and rates to suit. . . There seems to be a mental barrier about using more than 30% expo. . It is permitted to use more if required. . Just keep dialling it in until the correct "feel" is accomplished.

Also, by using two servos, they can be individually "tuned" to get the best out of the aeroplane.

B.C.

[Shaun Walsh]

Posted by Brian Cooper on 14/08/2019 10:59:40:

Using just one servo on the ailerons, especially on a model like the Acrowot, is very "old school".

It is far better to use two servos, and then dial-in expo and rates to suit. . . There seems to be a mental barrier about using more than 30% expo. . It is permitted to use more if required. . Just keep dialling it in until the correct "feel" is accomplished.

Also, by using two servos, they can be individually "tuned" to get the best out of the aeroplane.

B.C.

I agree about the mental barrier comment. I have a Hobbyking Hummer and have 50% expo dialed in on the ailerons and elevator, it makes it far easier to fly smoothly but I can still chuck it around like a mad thing if I want to.

[Keith Berriman]

Just in case I did not make myself clear I think this kit was built some 8+years ago with a modeller who followed the plans. So it came with the single servo option and torque rods fitted I have a feeling with the finish it was covered in balsaloc and then some heavy duty covering.  My start point on expo is -30% and then I go up or down to suit the model characteristics and my style of flying

Happy days when I get this one trimmed out 

Edited By RC Plane Flyer on 14/08/2019 11:44:51

[Martin Harris - Moderator]

Posted by RC Plane Flyer on 14/08/2019 09:21:39:

Looking at the size of the airelons on the wing I think a mini servos may struggle under load I have two midi servos in the workshop that would do the job just need to check the output specifications

There are plenty of mini servos with at least as much torque as the ubiquitous Futaba S148 which was probably used by more Acrowots than any other - using a pair of typical mini servos would effectively give around twice the power of your existing standard servo!

If you do use mini servos, you might consider specifying metal geared ones though as the teeth are not always very tough on plastic versions.

By the way, the control surfaces referred to are "ailerons" - confusion with the word "air" may lead to some of the common misspellings but it is a French word meaning fin or little wing.

Edited By Martin Harris on 14/08/2019 11:45:38

[Jon H]

Posted by Brian Cooper on 14/08/2019 10:59:40:

Using just one servo on the ailerons, especially on a model like the Acrowot, is very "old school".

It is far better to use two servos, and then dial-in expo and rates to suit. . . There seems to be a mental barrier about using more than 30% expo. . It is permitted to use more if required. . Just keep dialling it in until the correct "feel" is accomplished.

Also, by using two servos, they can be individually "tuned" to get the best out of the aeroplane.

B.C.

Its not a mental barrier, using excessive expo makes many models extremely unpleasant to fly. 30% is already excessive and i have never needed to use more than 10% on any model i have ever owned. Granted, most are sport and scale, but an acrowot is hardly a twitchy monster.

I would always fly with no expo and get the rates right first. Then, if you feel you must add some expo. If you do it the other way around you end up like a friend did with 60% expo on the elevator of his spitfire which made the model impossible to land. Knocking the rates down by about 30% and removing all the expo made it an absolute doddle to land.

Just dialling in expo is completely the wrong thing to do.

[Nigel R]

"After further bench checks I found that my Flash 8 TX triple rate switch was in the mid position and the exponential was set at zero instead of -35%. This had given me 5/16th deflection instead of the 1/4" recommended. Balance point within parameters"

Hmm. Well, report back after you've flown again.

It is worth nothing that thousands and thousands of Wots have been flown with no expo, no rates and a single central aileron servo.

My opinion - dual servos aren't the panacea they are sometimes made out to be, especially on a traditional style sport model, and differential - should it be needed - is quite easy to achieve with an offset servo horn, or on the torque rods themselves.

[Torsten Spitzner]

Posted by Jon - Laser Engines on 14/08/2019 11:52:19:

Posted by Brian Cooper on 14/08/2019 10:59:40:

Using just one servo on the ailerons, especially on a model like the Acrowot, is very "old school".

It is far better to use two servos, and then dial-in expo and rates to suit. . . There seems to be a mental barrier about using more than 30% expo. . It is permitted to use more if required. . Just keep dialling it in until the correct "feel" is accomplished.

Also, by using two servos, they can be individually "tuned" to get the best out of the aeroplane.

B.C.

Its not a mental barrier, using excessive expo makes many models extremely unpleasant to fly. 30% is already excessive and i have never needed to use more than 10% on any model i have ever owned. Granted, most are sport and scale, but an acrowot is hardly a twitchy monster.

I would always fly with no expo and get the rates right first. Then, if you feel you must add some expo. If you do it the other way around you end up like a friend did with 60% expo on the elevator of his spitfire which made the model impossible to land. Knocking the rates down by about 30% and removing all the expo made it an absolute doddle to land.

Just dialling in expo is completely the wrong thing to do.

I agree 100%. Expo is a wonderful tool but I think it is used incorrectly waaaay to often.

[Alan Gorham_]

Posted by Brian Cooper on 14/08/2019 10:59:40:

Using just one servo on the ailerons, especially on a model like the Acrowot, is very "old school".

It is far better to use two servos, and then dial-in expo and rates to suit. . . There seems to be a mental barrier about using more than 30% expo. . It is permitted to use more if required. . Just keep dialling it in until the correct "feel" is accomplished.

Also, by using two servos, they can be individually "tuned" to get the best out of the aeroplane.

B.C.

Brian

Please can you give me any solid evidence of why a simple sports aerobatic model like the Acrowot we are discussing here can be improved by the use of two servos on the ailerons?

I'd genuinely be astonished if there is any genuine improvement over a well setup simple single servo but I'd love to know your reasoning.

Additionally, why does having two aileron servos make it easier to "dial-in" expo and dual rates?

I find that statement hard to accept at face value, sorry.

[Jon H]

Cant disagree with the 3 previous posts.

[Martin Harris - Moderator]

My main reason for using expo is to hide any inadvertent control movements while flying in a steady state!

I do understand the reasoning behind condemning high expo rates as it can lead to progressive runaway - especially noticeable on an elevator during the approach and round-out.

Perhaps a more complex "curve" would be the answer - expo around the centre flattening out to a straight curve as the movement increases?  Perhaps I should experiment with some multi-point output curves on my transmitter...

However, I do wonder how control movements translate into effectiveness as their deflections change. For example, is there a linear relationship between deflection and roll rate with ailerons or do the aerodynamics dictate that the effect varies at different rates as the ailerons deflect and modify the aerofoil shape?

Expo can also compensate for less than ideal control geometry so it does have its uses.

 

Edited By Martin Harris on 14/08/2019 16:35:03

[Jon H]

Not saying there arent use cases Martin, just that you use it after you have the basics right and not as a bandage to cover a poor base setup. To be honest this applies to more or less all tx programming. If the mechanical side is pants, sort that out first, then get the rates right, then polish with expo if you need it. I suspect that if models are set up this way most wont need it. I have only used it on 1 channel on two models in my current fleet of 12 airworthy. 

Without expo you get gradually reducing amounts of movement with stick deflection and this can be very helpful on elevator especially.

Your comment about aileron effectiveness is also fair enough and i have 10% negative expo (making the centre more sensitive) on my hurricane as its ailerons have a nasty mush/dead zone in the middle.

 

 

Edited By Jon - Laser Engines on 14/08/2019 16:53:25

[Malcolm Fisher]

I am still using ancient transmitters which have no expo and one which doesn't even have servo reversing capability. Doesn't seem to bother the planes I fly.

Malcolm

[Bob Cotsford]

I find that I'm using expo more often in recent years, arthritic thumbs cant sense small pressures as well as they used to so I find small control movements harder to judge than I remember. Slow rolls ain't as smooth as they used to be! One day I'll try fitting some stronger springs on aileron and elevator.

On the question of mushy neutrals, IIRC sharp trailing edges and even better squared off ones are said to give a better response for small movements and rounded off give a reduced effect. I somehow doubt whetherthe average club flyer would really notice much difference, just like the ever increasing resolutions being advertised on modern radios.

Edited By Bob Cotsford on 14/08/2019 19:38:53