Single servo ailerons

[Brian Cooper]

Posted by Alan Gorham_ on 14/08/2019 15:38:42:

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.

= = = =

Use a single servo if you prefer to but the advantages of using two servos are manifold, and the subject is deep enough for a thorough article in the pages of a magazine rather than a quick response on a forum.

Have fun.

B.C.

[Alan Gorham_]

I absolutely would use a single servo in a simple model like an Acrowot!

I was looking forward to hearing of the advantages I could gain, because apparently there are some, but it's too complicated.

If that's the case then can I suggest that there aren't any real advantages?

[Keith Berriman]

Wow what have I done I was only asking if I was to mod my servo horn to give more up than down was that the correct way? As is said in other topics ask 6 modelers this question you get 8 options.

Thanks to all who have responded on my aileron question. At present I have found my original circular horn for futaba servos but have hirec servo fitted.

I am going to reset the settings back to plan recommendations 3/16 to 1/4 on see how we go. as said earlier some how I was at 5/16 with no expo to soften the sticks

[Jon H]

Posted by Bob Cotsford on 14/08/2019 19:37:44:

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

Cant fault people using expo in this sort of situation. I know some have hands that shake a bit too and again, cant do much more than fly around the issue.

[Jon H]

Posted by RC Plane Flyer on 15/08/2019 07:26:47:

I am going to reset the settings back to plan recommendations 3/16 to 1/4 on see how we go. as said earlier some how I was at 5/16 with no expo to soften the sticks

Just throw out the plan and do it yourself in the air. Just keep knocking 10% off the deflection until you get to a comfortable rate of roll at full deflection. I would probably then add 10% back and then see what expo, if any i needed to add.

Things like engine tuning, c/g, control rates, propeller selection and so on are all things that you just have to work out for yourself. There is no written guide or set of fixed values that will be right for everyone all of the time. You just have to experiment

[Brian Cooper]

Posted by Alan Gorham_ on 15/08/2019 07:09:37:

If that's the case then can I suggest that there aren't any real advantages?

= = = =

Errrr, no, you can't.

An Acrowot can be simple if it is powered with a lowly .46 or something similar, and there are not any high expectations of intense aerobatics. . But it can be a much higher performer with a decent .90 up front. . Then it becomes a different ball game, with better vertical performance and higher stresses (demands) on the servos and the airframe . . A single servo in this type of application is going to struggle to be equal to the game, especially if it is an analogue servo from the bargain (under ten quid) end of the scale.

If the performance has been "hopped up", and you want a really twiddly rate of roll, the aeroplane with appreciate better servos, two of them, and preferably digital. . Trust me, there IS a difference. . And this is before we delve into the batteries we use, or need, in our models.

However, we all fly differently, and we all have different expectations of our aeroplanes. This is what makes the hobby so rich and diverse.

B.C.

Edited By Brian Cooper on 15/08/2019 09:47:09

[Alan Gorham_]

Sorry again Brian, you've totally failed to convince me again. If you fit a 90 2-stroke in an Acrowot, then good luck getting it to balance to begin with!

Secondly, while I accept it may be a good idea to use more powerful servos, this can be achieved using a decent single servo. Again, there's no advantage in using two servos that cannot also be gained by using one!

I'm not having a go, and in many circumstances in other models I agree, but not in this case, ever....

[Nigel R]

Hmm.

Regarding oversized engines and servos.

With double the power (90 vs 45) our metaphorical Acrowot won't be flying that much faster (approx 25% if I remember my basic drag vs power calculation right).

Perhaps another way of stating it - the force on the controls is not massively increased.

I've used the servcalc.xls (can be googled and downloaded) spreadsheet for a while. A big assumption on my part is that is correct.

Anyhow, it shows:

For a 18" x 1.25" aileron surface, at 50mph, surface deflected 30deg, servo deflected 45deg - a fairly standard setup for a 40 class sport aerobat I would suggest - the required torque is a mere 2.5 oz-in (per aileron). For both together, a still tiny 5oz-in.

Now up the speed by 25% or so, after fitting that big 0.90. I'm rounding up a bit here, to 65mph. The same surface now needs 4.2oz-in - for both a total of 8.4oz-in.

How much does an antique S148 have available? 33oz-in.

Our Acrowot will need to travel at 125mph before that poor old Futaba is overwhelmed.

If the .45 gets the AcroWot to 50mph, then 125mph in straight and level would take just over 6 times then power. I leave the size of that 2 stroke glow to your imagination. Yes, you might get that in a full power dive, although with an Acrowot, I'd be surprised, it is no pylon racer.

If you want faster response, yes, you could replace the assumed bargain basement standard slow old analogue servo. But, you still need only go to a singular servo.

If you really want to use two servos, go ahead, this is all for fun after all, and using two servos is certainly a thing that works.

But the argument against the single servo on the basis of speed or power simply doesn't hold any water.

[Martin Harris - Moderator]

I think it depends on how interested you are in fine tuning a model for pleasant performance. I was taught that a downgoing aileron produces more drag so adverse yaw will always be a factor in producing roll. While these effects are more pronounced with longer wings and can be countered by accurate use of rudder, use of differential to largely negate them does make for more relaxing flying.

With a dual aileron servo system, it is much easier to fine tune this correction than by drilling arbitrary offset holes in an output disc or offsetting control horns.

There is the added advantage that should you get a servo failure, you have at least a fighting chance of landing the model in one piece - having experienced an aileron servo stopping at full movement during a roll some years ago, I know it is possible to regain enough control to land given time to get "brain in gear" and compensate for the offset with elevator and rudder.

On a recent occasion I had an aileron servo fail at some point during a flight in very turbulent conditions - luckily I had a Jeti Central Box fitted which sensed the overload and disabled the channel and I was only aware of the failure as I taxied back with the tailwind deflecting the aileron fully - a clubmate experienced a similar situation and was very lucky (skillful, Dick?) to wrestle his model onto the ground with only covering damage from the heat generated by a short circuit in the servo electronics. Of course, I appreciate that having 2 servos doubles the statistical chance of failure but in all 3 cases mentioned the models survived...statistically, with a single servo, 1.5 of them would have been likely to have crashed due to lack of roll control!

[Dickw]

Posted by Martin Harris on 15/08/2019 10:24:57:

I think it depends on how interested you are in fine tuning a model for pleasant performance. ..................

There is the added advantage that should you get a servo failure, you have at least a fighting chance of landing the model in one piece -......................

...................- a clubmate experienced a similar situation and was very lucky (skillful, Dick?) to wrestle his model onto the ground with only covering damage from the heat generated by a short circuit in the servo electronics. ............... likely to have crashed due to lack of roll control!

I would obviously like to claim skill, but perhaps luck played a bigger part than I would like to admit to .

I am firmly in the "2 servo" brigade on all the above mentioned grounds, and would like to add that with 2 servos you can operate the ailerons as flaps or as spoilers, and even couple the "flaps" with elevator like in my control line days. Elevator full up with flaps full down at speed can add a bit of fun and adrenalin into your life.

Dick

Edited By Dickw on 15/08/2019 11:04:31

[Nigel R]

I think differential is a moot point on a zero camber symmetric section. And on the sort of airframes where it is useful, do we genuinely need "accurate dialling in" or just "lots up, not so much down"?

Redundancy is a thing, but, do either of you fit two elevator servos as well? Elevator is just as much a critical control, possibly more so than ailerons.

Or dual rudder servos?

Or dual receivers?

etc...

Strip ailerons like the Acrowot has, make poor flaps; passable spoilers I guess. Inset ailerons make reasonable spoilers (see: crow brakes) when used in conjunction with flaps - perhaps the best option at the end of the day... As to coupled flaperons, yes, yes you can, but more elevator movement also does just about exactly the same thing - unless you are already at 60 deg elevator full movement, on your 3d model with enormous controls.

[Brian Cooper]

Posted by Nigel R on 15/08/2019 13:44:06:

Redundancy is a thing, but, do either of you fit two elevator servos as well? Elevator is just as much a critical control, possibly more so than ailerons.

Or dual rudder servos?

Or dual receivers?

etc...

= = = =

Twin elevator servos? . . Most definitely. For redundancy, this is even more essential than twin aileron servos.

Twin rudder servos also have their place but only in large models with very powerful engines where the draught from the prop is extremely strong.... strong enough to need a couple of fully grown adults to hang on to the thing.

If we mentioned dual receivers, there could be mushroom clouds appearing over the forum...

B.C.

[Dickw]

Posted by Nigel R on 15/08/2019 13:44:06:

..........................

Redundancy is a thing, but, do either of you fit two elevator servos as well? Elevator is just as much a critical control, possibly more so than ailerons.

Or dual rudder servos?

Or dual receivers?

etc...

..............................

On a recent build of a scale model kit designed for one elevator servo (split elevator with joiner), I actually fitted smaller dual elevator servos for redundancy. My personal history suggests electro-mechanical devices like servos are less reliable than pure electrical devices like receivers.

I have many models without a rudder, so feel no need for redundancy on rudder.

I don't think anyone is saying single servo setups are bad - just that dual aileron servos give more options if you want them.

Dick

[Jon H]

Just on the redundancy thing it should be quite possible to land an acrowot without ailerons..in fact its possible to land most models rudder/elevator only but it does take a little care, and it might not be the easiest but it can be done. Its something i actually practice from time to time (in calm conditions i admit) but everything i own, including my 80 inch warbirds, can be landed without ailerons if i really have to so im pretty sure an acrowot will manage.

Unless the servo fails at anything other than zero deflection, then its game over :D

Also, you can add all the servos, rx's and batteries you like to the model but you still have a number of single point failures. You have one tranny, with one battery, and one lump of meat manipulating the sticks.

As pilot error accounts for most of the crashes we suffer i would say that going crazy trying to guard against equipment failure is a bit daft. Especially when a substantial number of equipment failures boil down to inadequate maintenance and care.

At the end of the day, probably 99% of models are killed by their owners and the time and energy put into inventing double redundant whatever is probably better spent on regular inspection and maintenance.

[Bob Cotsford]

On the point of aileron differential - not really needed on a moderately short winged sports model regardless of the wing section in my opinion (RIP BEB). On longer glider style wings or WW1 and similar bipes, yes. Sport monoplane, no!

Where it does come into it's own is precision aerobatics, if a club sports model has a bit of corkscrew to the roll no-one will really notice but for competition style flying the differential will probably be needed to get a truly axial roll.

Twin servos, imho the primary benefit is to move the point of actuation away from the end of the surface which helps reduce any twisting and can help fighting flutter.

If an aileron servo fails by running to the end of it's travel on a Wots Wot you need a new airframe.

Back to the OP - sorry!

[Martin Harris - Moderator]

Posted by Jon - Laser Engines on 15/08/2019 17:01:14:

Unless the servo fails at anything other than zero deflection, then its game over :D

...which is a pretty good reason for having a second servo!

[Bob Cotsford]

Posted by Martin Harris on 15/08/2019 18:13:28:

Posted by Jon - Laser Engines on 15/08/2019 17:01:14:

Unless the servo fails at anything other than zero deflection, then its game over :D

...which is a pretty good reason for having a second servo!

Didn't help my WotsWot when it failed at less than 50' after take-off, maybe a better flyer could have saved it but I couldn't!

[Chris Freeman 3]

In the 70's and 80's servo' s were very expensive, had little power and many more components to them than todays servo's. The receivers were also nat as todays ones and extension leads and y leads were not used as these caused interference. The transmitters did not have any programming so all the instalations were manually set up, if the controls were reversed you had to use a reverse servo or change to to opposite side of the servo if possible. If you needed more movement you had to use a longer servo arm of move in on the control horn on the control surface.

Many people now use multiple servo's and the servos are of a better quality and power and the radio's make it far easier to sep up the aircraft. I have had very few servo failues in my aircraft and most could be traced back to vibration, poor set up or stiff controls. I often move a control surface via the pushrod and wonder how the poor servo copes with having to do this. Power source can also contribute to failures so on my larger electrics that use more than 4 cells or 5 servos then I will have a seperate power source for the radio and not the ESC / BEC

[Jon H]

Posted by Bob Cotsford on 15/08/2019 18:35:53:

Posted by Martin Harris on 15/08/2019 18:13:28:

Posted by Jon - Laser Engines on 15/08/2019 17:01:14:

Unless the servo fails at anything other than zero deflection, then its game over :D

...which is a pretty good reason for having a second servo!

Didn't help my WotsWot when it failed at less than 50' after take-off, maybe a better flyer could have saved it but I couldn't!

That was the point i was trying to make. Its not a criticism of you Bob, but even with a spare aileron servo in place the model still crashed. Depending on the situation it might have been recoverable, in which case the accident falls into the 99% pilot error category, but even if it was the shock of something actually going wrong means that even very good pilots may freeze and/or make bad choices when presented with an expected problem.

Also, just to really go for it what would have happened if the model only used the one good servo? more servos means more points of failure and lower reliability.

I am playing devils advocate here a little. Per surface servos on ailerons are common and so much easier than a 2 foot torque rod out to the ailerons at the end of a 40 inch wing panel. This is especially true when there is a flap in the way and the whole installation would be far more secure. There is a theoretical safety margin should one servo fail, but failure is more likely due to increased complexity.

To get back to the op however, using twin servos will not help him one little bit.

[Martin Harris - Moderator]

Surely Bob was saying that one (presumably) aileron servo failed at 50' and the model crashed due to the failure being the root cause? While steering on rudder is a possibility and a useful skill to practice (saved a model that for complex but inexcusable reasons I flew with reversed ailerons - but then it was a Nova trainer with plenty of dihedral) most models will have insufficient yaw/roll coupling to compensate for any meaningful aileron deflection due to a failed servo.

Had Bob been flying with a single servo, it would have been half as likely to have failed on that flight (arguably slightly more than half due to it having been worked harder over its life in that model) but that failure which must have locked the aileron in a deflected position would have been almost inevitably fatal to the model in the single servo scenario. Had Bob been slightly luckier, he might have had a little more time to orient with the instantaneous trim changes experienced and got his model under control.

I've quoted 3 examples from my direct experience where models have been saved by the second servo continuing to function after a primary control surface failure - "crash investigation" is often unable to conclude whether a servo failure was the cause or result of a crash but these incidents were obviously diagnosable. On these results, 2 servos are statistically safer although of course the sample size is ridiculously insufficient for proper conclusions to be drawn.

We have no chance of eliminating all failures or pilot errors leading to the loss of our models (if Airbus or Boeing can't make failure or pilot error proof aircraft what chance do we have?) but easily built in redundancy (especially where it can coincidentally improve efficiency and handling) seems to be a win/win situation.

Actually, my statistical analysis has improved in support of dual servos as I've just recalled another example with my fairly large Mahers Pacer where slightly odd handling was traced to an intermittent fault in an aileron servo which was found after landing for investigation. It failed completely during testing!

Edited By Martin Harris on 16/08/2019 10:35:12

[Nigel R]

re Single / Dual servo failures;

It is not a straightforward "half as likely" proposition.

The big boys do fault tree analysis and put a probability on each outcome. I'm going to do a contrived example.

A single servo is straightforward:

1 it fails at neutral

2 it fails with control applied

3 it works!

in scenario 1, you can probably fly down on rudder
in scenario 2, you are going splat
in scenario 3 everything is good

Let's assign some exxagerated and made up chances of each.

A 1% chance of each type of failure. And thus 98% chance of everything working.

For dual servos there are now 9 combinations of outcome, and if we keep the same probability of individual servo failure, the chances work out thus:

1 left fails at neutral, right works - 0.98%
2 left fails with control applied, right works 0.98%
3 right fails at neutral, left works - 0.98%
4 right fails with control applied, left works - 0.98%
5 left fails at neutral, right fails at neutral - 0.01%
6 left fails at neutral, right fails applied - 0.01%
7 left fails applied, right fails at neutral - 0.01%
8 left fails applied, right fails applied - 0.01%
9 everything works! - 96.04%

and you may note, the probability of everything working is decreased a bit.

looking at those outcomes from a redundancy point of view:

scenarios 1, 3 and 5 can probably fly down on rudder; probability now 1.99%
scenarios 2, 4, 6, 7 and 8 are a bust; probability now 1.99%
scenario 9 everything is good 96.04%

To conclude:

It's now less likely that everything will work ok - although not by much.
The chance of an outcome where you can fly down on rudder is a bit better.

However. The big drawback is that the chance of an outcome where you get a catastrophic fail and can't fly down is also now twice as likely.

Now, my made up chances are not real. But they servo to show how the probability of success or failure is altered.

Still liking dual servos on the grounds of redundancy?

Edited By Nigel R on 16/08/2019 11:33:49

[Simon Chaddock]

I bet the OP is beginning to wish he had never asked.

[Nigel R]

we're definitely guilty of thread drift here!

[Martin Harris - Moderator]

Posted by Nigel R on 16/08/2019 11:32:18:

Still liking dual servos on the grounds of redundancy?

Yes.

Your logic assumes that with a deflected control you will almost certainly crash. Having experienced a detached servo connection during a roll leaving an aileron at full deflection I know that it is possible to land under control with significant aileron deflection on a typical sports model and although I regard myself as reasonably competent I am by no means the best pilot in the club. I ended up holding opposite aileron and a large elevator input to maintain level flight.

Although your logic seems to work on first sight with your exaggerated figures, the real word chances of a double failure within a time frame that doesn't allow a landing are so small as to be disregarded. Your analysis does not allow for any differentiation between a double failure at some point within the 7-10 minute window of a typical flight and a virtually instantaneous double failure.

[Nigel R]

Posted by Martin Harris on 16/08/2019 12:27:37:

Your logic assumes that with a deflected control you will almost certainly crash.

It was a stated assumption for the sake of a simple example.

The 'logic' itself makes no assumptions of anything.

I did state it was "a contrived example".

Posted by Martin Harris on 16/08/2019 12:27:37:

Having experienced a detached servo connection during a roll leaving an aileron at full deflection I know that it is possible to land under control with significant aileron deflection on a typical sports model and although I regard myself as reasonably competent I am by no means the best pilot in the club. I ended up holding opposite aileron and a large elevator input to maintain level flight.

Well that's one possibility. I didn't split 'fail deflected but manageable' and 'fail deflected, unmanageable'. Feel free to improve and extend!

Posted by Martin Harris on 16/08/2019 12:27:37:

Although your logic seems to work on first sight with your exaggerated figures, the real word chances of a double failure within a time frame that doesn't allow a landing are so small as to be disregarded. Your analysis does not allow for any differentiation between a double failure at some point within the 7-10 minute window of a typical flight and a virtually instantaneous double failure.

It's not my logic, I didn't invent it, it is just how mathematical probability works. Really the point of the example is to show that sometimes the resultant chance of an outcome is not what you might expect at first glance. i.e. the argument is not as cut and dried as "two good, one not so good".

If you wanted to turn my simplistic example into something closer to the real world - please do - add some outcomes, assign some more realistic probabilities, re-do the math and post the result.