Servo Torque guide - Sport Flying (electric)

[Richard Ashworth]

Hi all.

A colleagues plane recently hit the ground at the end of an ‘unable to stop it’ spin.

All components work independently afterwards but the fuz and battery were total write offs. The motor was still running when it hit which seems to rule out Signal loss, as a failsafe check had been done pre flight. The flying control positions on recovery were left aileron up, right down, rudder hard left, elevator full up. We cannot say how much of this was movement on impact.

The aircraft was a 7lb low wing sport aircraft 5S powered and being thrown about in a brisk sport mode, in no way 3D. Both ailerons, elevator and rudder had Hitec HS322 servos on the powered through the receiver at 5v via a SBEC.

My questions (at last) are, at 3kg/cm were the servos, mainly the rudder servo, man enough for the job? Could an ‘overload’ on a servo (rudder) have occurred in the spin and prevented an exit by overpowering it , but not be apparent afterwards? Is there a general rule of thumb re flying mode, model weight, servo power?

I have not been in rc long but have used HS325s in 4lb planes and HS5485s in 6lb planes.

Advice welcome.

[Martin Harris - Moderator]

First off, the full up elevator might be a clue to why it failed to recover! Counter-intuitively, the aileron input may also have been adding to the problem as out-spin aileron (especially with inset rather than strip ailerons) can effectively stall the dropped wing tip.  Which direction was it spinning in?

Correct spin recovery is normally: centralise ailerons and reduce throttle to idle if appropriate, full opposite rudder, short pause, stick forward until the spin stops, centralise rudder and ease out of the dive. In practice, most models will stop spinning as soon as you remove the spin inputs.

It is possible to get some models into flat spins which are difficult - maybe even impossible - to recover from.  One such was a pattern ship I mentioned in the lost model thread yesterday and the cure was to enlarge the rudder - the servo remained unchanged...

While in a spin, there are fairly light forces on the control surfaces - the model is largely stalled - so underpowered servos are unlikely to be at fault.  Rudder servos do often need to be the most powerful but this shows up most in knife edge flight at higher airspeeds. To be frank, most models have far more servo capability than they actually need.

Edited By Martin Harris on 28/07/2019 18:58:16

[Richard Ashworth]

Thanks Martin.

My colleague is an ex light plane CFI and 737 captain and his first actions were as suggested. The problem is that nothing he did worked and we are working through why he couldn’t get a recovery. We are aware that the surfaces were more or less as he put them to, intentionally, at a safe height, to put the plane into the spin, left hand direction.

I was trying to get discussion focussed on were the servos man enough without a discussion on the plane he was flying but for information it was a Ripmax ARTF Acro Wot, rip!

Edited By Richard Ashworth on 28/07/2019 19:08:34

Edited By Richard Ashworth on 28/07/2019 19:09:24

[Denis Watkins]

You do reach a point where gravity exceeds any servo Richard

Standard servos are man enough for a 7lb airframe am certain

7lb deadweight, pointing at the ground, without " Lift " can become unrecoverable

Even with an inch or two of rudder surface

[Martin Harris - Moderator]

I've not heard of similar problems with the ARTF AcroWot but the design is pretty well proven. Was it being flown with a rearward C of G perhaps?

Countless Acrowots have been flown on Futaba S148s which are only a little more powerful so I wouldn't have thought those Hitecs would cause such behaviour.

Perhaps it's worth reiterating that the angle of attack of the inner wing (and in some cases, both wings) is above the stall if the model remains in autorotation so airspeed remains low - therefore forces are unlikely to build up to the point where the servos were overloaded.  Was the spin noticeably nose down or did it flatten out somewhat? Pulsing the throttle can sometimes help to break out of autorotation in this case but doesn't always work...

My errant pattern ship was always a handful to recover from spins but when one went flat (for whatever reason) I was unable to stop it before its rather fortuitous encounter with a thick maize crop.

Edited By Martin Harris on 28/07/2019 19:32:27

[Nigel R]

More than man enough for a sport model of that size. As already said the forces in the controls are not that great.

Even assuming the rudder servo was stalled the current demand on a 322 is around 1.5a (I believe - worth checking in hitec website). N kit enough to challenge a functioning SBEC.

As you say if the SBEC failed the signal to the motor would have been lost and it would not have been running.

Without further info I would have to guess at pilot error...

[John Lee]

If the servos held in the pro-spin controls they must by definition have been be powerful enough. If they were not the control surfaces would have blown back.

[Richard Ashworth]

I think its ‘There goes another theory!” From comments, the 322 on the rudder seems to have friends who vouch for its good character.

I didn’t see the start of the spin, but am told it was a standard low speed, nose up entry at 200ft.

I saw just the last 75ft when it was pointing down at about 60deg going fairly fast in what I would call more of a power on spiral dive. At that stage I would say the ailerons were as found left up, right down but there was nothing to suggest the full up elevator or full left rudder as found on recovery.

Power had been applied during the descent when a power off recovery wasn’t happening. Everything in front of the wing was unrecognisable except the undercarriage which was still attached to a bit of the mounting plate but pristine!

[Don Fry]

In fairness, a light plane CFI and 737 pilot is no qualification here. How many hours on a model.

As others have said, a spin is not a high stress manoeuvre on the servos. But, it can get high stress on the stick twiddler.

[john stones 1 - Moderator]

Wasn't there, so can't comment on the cause, anything I've owned, stops spinning when sticks get centered, flown with numerous full size pilots, most ain't much cop, there are exceptions though.A fair few have gone from models to obtain pilots licenses. Servos sound good enough to me.

[Chris Walby]

A bit left field, but do try moving all servos to their extremes at once. I was bench testing some servos, RX and SBEC when I found if you moved all at once it would brown the RX out.

The RX would auto recover but it took it a few seconds, of course notice by it flashing (but you would not have seen that as your battery was u/s).

Best bit was the servos were under no physical load, just trying to go from end stop to end stop and it would only do it if all three were moved at once. So it would not be picked up pre-flight (unless you bang all three at once) and even worse most of the time it would not happen, just when you do some thing with all three controls at once.

Of course if he's done loads of spins with that model it could be something else.

[Martin Harris - Moderator]

A spiral dive is a different animal altogether and if performed with noticeable power can easily overstress an airframe. How rigid are the wings? The originals were veneered foam - are the ARTFs built up? There's the possibility of aileron reversal for example at high speed/large deflection caused by wing flexing where aileron inputs twist the wing such that it exerts more rolling force in the opposite direction.

I certainly wouldn't advocate continuous power to recover from a spin - this normally serves to flatten it making the recovery more difficult. The power application to recover from a flat spin needs to be a pulse, to kick air over the control surfaces.

John - you're certainly right that most models recover with relaxing the spin inputs but not all do - however, to my knowledge, the Acrowot isn't one of them and usually behave impeccably although I don't remember ever flying an ARTF example.

Edited By Martin Harris on 28/07/2019 20:37:00

[Don Fry]

I'm struggling here. Are you describing a power on spiral dive, because I am surprised to see there is much left if it was, and I would not expect to see the servos still mounted to provide control positions pre impact.

And if you saw it 75 foot up, a slow spiral dive would give you no more that 2 seconds before impact. Power off. Power on, less time. That's short time to see much.

I reckon, if it was good enough for brisk aerobatics, it was fit for purpose, or a failure occurred, or the stick twiddler failed.

[Richard Wills 2]

I have an ARTF Acrowot, it recovers from a spin by just by releasing the controls pretty much instantly

[Martin Harris - Moderator]

Any chance that the 5S lump could have shifted backwards during the enthusiastic flying prior to the spin?

[Richard Ashworth]

Thanks for all suggestions.

Don - My colleague retired from professional flying 8 years ago and has flown fixed wing radio control regularity since. Currently working towards a Fixed wing B. As a CFI he demonstrated and taught spinning so fully understands recovery method, power was a last resort and the running motor eliminates signal loss.

The earth it went into was still fairly soft and covered in long ‘rough, sheep pasture grass’. It needed a bit of work to get the spinner and motor out and there was some damage to one wing tip but otherwise ‘shaken but not stirred’.

The area I was investigating was more “was were a basic under specification of the servos, especially rudder?”

Chris - All the components seem to work individual so a bench 4 servo, simultaneous end stop to end stop test is viable.

Martin - the battery was still in the right place on the tray used to locate it and the tray, although damaged, was firmly attached to the structural support so I don’t think it moved in flight.

Thanks again to all.

[GrahamC]

How many amps was the SBEC? And how many servos was it driving?

[John Muir]

Just because the prop was turning doesn't mean the motor was running. The throttle would have been closed to enter the spin and assuming the cause of the failure was the sbec overheating and cutting out, then the throttle would have remained closed. The prop would now freewheel and would speed up as the airspeed increased in the spiral dive. The fact that everything worked fine later points to the sbec as it would work as normal once it had cooled down.

Workable theory?

[Peter Miller]

None of my models will continue spinning on e the sticks are released. The spin stops instantly. And some of my models will spin incredibly fast.

Of course I always locate the CG at 25% chord.

[Don Fry]

To answer the question, I reckon the 322 servo is fine for the rudder of that aircraft. I was working from an assumption it's the same as a Futaba 3001, but having looked it up, the 322 has more torque,( IRO 20%), and I would be happy with a 3001.

Now this basic airframe design goes back into the mists of time, a standard hack for hooligans, and always with standard servos. The 322 is an analogue servo. One of their merits is they are not good at overloading power systems.

Still a bit puzzled about what you saw. If it looked like a spiral dive, that's a different thing to recover from. Nothing is stalled.

[Bob Cotsford]

In fairness - there were odd reports of individual examples of the kit built AW being a bit prone to tip stalling. Mine was always a complete pussy cat but others claims have to be accepted.

However, we are now talking about a different beast just having a similar outline. The ARTF is much more lightly built with a built up wing and from what I remember of the early reviews it could come out quite badly tail heavy. The ones I've seen balanced fine with 46 two strokes up front so there seems some variation. All examples whether kit or ARTF have been fine on 'standard' eg ~3kg servos.

What does come to mind is my experience with a Joker and a King Altair when flown with a slightly too rearward C of G. Most of the time they flew beautifully, flicks and spins were always controllable until the one occasion they weren't. Both went into terminal spins continuing multiple turns after all corrective actions were tried. The Joker was terminal, the KA survived to fly many other days with no more terminal spins. I still don't know just what caused these but I suspect disorientation on the part of the pilot and a failure to recognise when recovery actions actually reversed the spins instead of stopping them.

Nowadays I'd simply review the telemetry logs to see whether I'd lost signal or not, and I always set failsafe to low throttle and neutral on all channels. Position hold could be a model killer!

[Martin Harris - Moderator]

Like Don, I'm a bit worried about the spiral dive aspect. If it was really a spiral dive with power on then you should consider the airframe twisting effects leading to "aileron reversal" I mentioned earlier.

I think you have inferred that he was attempting to recover from the spin by applying some continuous power which doesn't really equate with a spin recovery taught by an experienced instructor. Was he teaching aerobatics or just the standard syllabus though? For many years it's concentrated on the incipient spin recovery after a number of accidents during spin training and student pilots don't experience full spins so experienced aviator he undoubtedly is, perhaps the application of power and failure to recognise a spiral dive developing out of a spin (something emphasised during my time as a gliding instructor) may have been the cause?

Edited By Martin Harris on 29/07/2019 14:06:56

[Richard Ashworth]

Hi again

Power on was I understand a last resort. I only saw the last 75ft of the 200ft descent so I don’t know about the previous 125ft.

The electric setup was Quantum 55 motor, 12x8 electric prop, 5S 4000 battery, Overlander 80amp SBEC ESC, 4x Hitec HS322 servos.

The fateful flight was brisk, loops, rolls, inverted and about 4 minutes in duration when the event occurred.

For the last 3-4 months 3 turn spins have been a fairly regular practice feature of his flights on a number of models including at least two on the Acrowot two days earlier so it ‘should’ have been business as usual. It was a bit breezy so the Acrowot which is possibly his biggest model and looked totally stable during the rest of the flight was flown.

[Piers Bowlan]

I have only had one model truly spin in and that was when the signal wire of the rudder servo failed, causing full left rudder to be commanded. (it was a rudder/elevator model). The model span pretty flat and up elevator movement just made it spin flatter and faster with a slower rate of decent (as you would expect). The opposite was also true but either way recovery wasn't going to happen without being able to either centre, or apply opposite rudder, to stop the rotation. In the end the model hit the deck in a 45 degree nose down attitude causing extensive damage to the lightweight fuselage. I had it repaired and flying again the following week!

In years past I have entered one or two terminal spiral dives with the model simply burying its nose deep in the ground. RIP!

If you had to dig your Acrowot motor out of the ground I humbly suggest it was in a spiral dive, even if the entry manoeuvre was a spin.

my 2p worth!

PS; I concur - I don't think the choice of servos was the issue.

Edited By Piers Bowlan on 29/07/2019 19:55:23

[Bruce Collinson]

Richard,

Was Steve D there for the post- mortem? He has a knack, often helpful, sometimes demoralising when he proves pilot error, of isolating the defective component, assuming there was one. I assume it was Dave B’s?

BTC