Twin ic throttle mixing

[Jon H]

Posted by Charles Smitheman on 15/05/2019 12:38:12:

One reason for bothering with a twin i.c. is surely the challenge of being ridiculously complicated.

i think i have been doing this too long as i consider an ic sport twin to be no more complicated than a trainer. Its either that or the fact that models like my P39 have up to 5 servos, 2 sequencers, 2 air tanks, a spool valve and miriad of pipes etc just for the retracts. I have even taken to chopping up my cowlings into several bits as apparently one isnt good enough.

Dont worry, ill show my self out.

[Nigel R]

I'm only planning on the usual 4 channels for my sport effort with a Y lead for the throttle servos and to spend a little time getting everything as mechanically identical as I can.

Jon your P39 retracts sound more complex than most entire models in my fleet!

[Martin Harris - Moderator]

Posted by Chris Walby on 15/05/2019 13:34:37:

I have tried throttle mixing on a small electric twin as it has no rudder servo, from memory it produced some very non scale yaw/pitch effect which was most unpleasant and fly. Not needed as it was a land launch so no benefit to taxiing either!

One of your clubmates did the same on his hand launched Mosquito and was quite happy with the results.

Reading some of the posts above does raise some thoughts though. Mention was made of the effects of over complication and its just possible that I might have suffered from just that with my throttle mixes. It could explain an unfortunate incident...

A couple of weeks ago on a very gusty day with a fearsome crosswind, I decided to exercise my superior skills with my Slim Twin and impress my clubmates with my mastery of the air. In hindsight I might have wondered why the Red Kites, who regularly watch our pathetic attempts at aviation with a look of avian disdain, had decided to view my antics from the safety of their roosts in the nearby woods...

I took the precaution of lining up diagonally across the runway into wind, opened the throttle and the model leapt into the air immediately - no real surprise in the strong wind as it is off in a few lengths even with no wind. What was a surprise was the immediate roll to port which failed to respond to aileron. The result was an undignified arrival on one wingtip and a cartwheel.

I put it down to the effects of curlover from a nearby hedge and mishandling at low airspeed but on thinking some more, I'm wondering if I might have left it in No.2 start position. That would have resulted in a yaw to the left as both engines would continue to run with the No.1 (port) engine only idling which combined with an initial roll due to turbulence fits my recollection of panic the flight.

I don't recall finding the switch in that position after picking up the pieces or having suspicions that the port engine was not contributing its fair share during the couple of rather fraught seconds that the flight lasted but it would fit the scenario and perhaps act as a reinforcement of the warnings that with complication comes the greater possibility of making a mistake. I still like the ease of handling during starting (and the positive effects of asymmetric mixes) and will continue to use the various mixes but it highlights the extra awareness that you need.

[Jon H]

Posted by Nigel R on 15/05/2019 15:48:34:

Jon your P39 retracts sound more complex than most entire models in my fleet!

It was all in the name of keeping the engine cool. I needed the u/c doors to function to stop the air going the wrong way. Strangely the cooling and doors have been totally reliable when they were the thing i was most worried about!

Martin, its another reason all of my models are the same and the throttle is the throttle. I have one powerplant per model, even if the powerplant has 4 individual engines it is one powerplant. Switches are all in the same place and i use the same procedure for all of them. In some cases i even flick to high/low rate for takeoff or landing even on models that have no other rate set up. Its just to keep the habit and prevent me forgetting something if i have not flown the model for a while. Its something i recommend to all new warbird pilots. Unless you can find the gear and flap switches blindfolded leave it on the ground. You dont want to be fumbling around looking for switches on the first climbout.

[cymaz]

Try setting the throttles with one servo for this...

[gillyg1]

That looks nice cymaz, what is it? Home design or plan/kit, and what size engines, / wingspan? Would be interested.

[gillyg1]

By the way chaps, what are we using for all the connections between wing and fus, as in so many servo leads or multi point connectors, and I'm not going sbus before anyone suggests it.

[Peter Miller]

Posted by cymaz on 15/05/2019 16:57:37:

Try setting the throttles with one servo for this...

Not a problem!

This model had four engines all worked from one control. They were four G-Mark 12 twin cylinder engines. They worked a treat. Admittedly it was control ine so they all worked from on bellcrank but on servo would have done the same job

[cymaz]

Posted by gillyg1 on 15/05/2019 17:36:55:

That looks nice cymaz, what is it? Home design or plan/kit, and what size engines, / wingspan? Would be interested.

Here is the write up about it. Thanks for the interest

[EvilC57]

Posted by Martin Harris on 14/05/2019 15:49:13:

Starting is where I find the ability to control the engines separately has some particular advantages. I have a 3 position switch which is set to idle starboard, idle port or full control on both.

I start the port one first, check it for full throttle running and then switch control to the starboard engine, locking the port one to idle. I then start and check the starboard engine and then activate both throttles for a full power check. Having the running engine locked at idle means I can manipulate the throttle on the one I'm starting with no sudden surprises!

I can also practice engine outs with the switch selecting either engine to idle.

Having the ability to throttle back an engine can also add to the repertoire of an aerobatic twin...

I'm afraid I don't know whether your transmitter is capable of this level of sophistication.

I balance by ear - but I'm more concerned with reliable operation than getting the RPM to any particular figure and I can't remember the last time my tacho came out of my modelling box.

Edited By Martin Harris on 14/05/2019 16:00:45

I like the sound of this, has anybody ever set this method up using a DX8, and if so, how?

[Chris Walby]

DX8 gen 1, I don't think so

DX8 gen 2, I would have thought so....and you intending on using for starting or flying?

[EvilC57]

Gen 1 unfortunately, so it's a non-starter for me then (excuse the pun).

Starting primarily, as per Martin's OP. Also, as Martin says it allows you to practice engine out situations at a safe height before you are forced to when one cuts at an 'inconvenient' time.

[Sheepish]

One possible way of doing this is using the heli canned program on a programmable TX.

I did this many years ago with an old JR X-347. I had a Chris Golds Lancaster and set it up with the inner engines on one servo as the throttle on the heli set up and the outer engines as the pitch servo on the same heli program. This allowed me to have hold functions, throttle cuts and throttle curves on the pairs of engines. The Lanc never worked out that it wasn’t a heli

I am just getting to grips with Frsky now and I guess anything you can think of would be possible with that if you have the time to spend working it out.

[EvilC57]

OK thanks Sheepish. I read somewhere earlier today, that sometimes this can be achieved in Heli mode. I'll investigate...

[Sheepish]

I hope this solves your problem. I wanted to be able to simulate engine cuts and throttle inner and outer pairs separately and also be able to cut pairs of engines on the Lanc. This was all possible but it did require a bit of creative thinking and me remembering the translation between heli terms and fixed wing terms in the program.

It will be interesting to see what you end up with.

[Chris Walby]

....Depends what you want to do? And with what sort of model

With a Gen 1 you could mix the second throttle on to aux 1, then a couple of mixes on the flap switch (centre no effect, other positions throttles low).

Gain height (four mistakes high) and flick the switch, one engine will drop to idle and if you are quick with enough speed and rudder authority you should be able to hold it. If not it will spin...assuming both engines are running or you have enough height you might miss the ground.

Waste of a good model IMHO.

If you have a simulator set it up for random engine loss and see how you get on....

Only personal experience with three engine induced spins, one not recovered/write off, one more luck than anything and the other...mostly luck and a lot of height just pulled it off with a couple of good engines (I don't think I could have recovered it with one quit engine).

Having said that there are some good models that fly very well on one engine, perhaps practice with one of those?

[EvilC57]

Right; Just for the record I’ve now discovered a way to switch in independent throttle control when needed.

I asked a clever chap on our field who flies a twin engined job whether his throttle servos are on a Y-lead, or independent. He uses separate servos with a couple of mixes in his Tx, and kindly sent me the .SPM file which achieves this.

This is how it’s done using a 1st generation DX8 and 8 Channel receiver (AR8000):

Mix 1
THR > AX3
Rate: -100%
-100%
Offset: 0%
Trim: Inh
Sw: Gear 0

Mix 2
AX3 > THR
Rate: -100%
-100%
Offset: 0%
Trim: Inh
Sw: Gear 0

These two mixes mix Throttle (Ch1) to Aux 3 (Ch8), and Aux 3 back to the Throttle, and have the effect when enabled (Gear switch down), of synchronising the servo movements on both channels for normal flight. But allowing independent movement with the Gear switch up.

In use this means that with the Gear switch UP you can start one engine and get it running while using the throttle stick to control it, then leave it idling while you start the other engine, and control it from the Aux 3 knob.

When you’re ready, setting the Gear switch DOWN brings both engines under control of the throttle stick as usual.

Note that you need to keep the Aux 3 knob in the centre of it’s rotation (where it beeps as it goes through centre) for normal flight.

Note also that you also need to bind the Rx in Preset Failsafe mode rather than Hold Last Position, to ensure that the slave (Aux 3) channel also returns to closed throttle position on failsafe operation.

Using an 8 channel Rx also leaves Ch2 and Ch6 both available if you want separate aileron servos.

I’ve tried all this so far with a battery, receiver and two servos on the bench, and it all seems to work as described (I haven’t started building the target model yet).

The chap who gave me this setup did say that if he were to get a dead engine in flight, he could switch the mix out, operate the throttle and see whether the remaining engine responds of not, from that he reckoned (knowing which engine was on the primary throttle channel) he could tell which one had cut - and put the correct rudder offset in accordingly. I suspect you would be lucky to achieve this in reality! However, it seems like a useful facility for ground running.

[David Hayward.]

On 14/05/2019 at 16:10, Jon - Laser Engines said:

Mechanical setup is the most important thing. Get that right and its likely the engines will perform pretty closely anyway.

When it comes to tuning, start one engine and tune it for max power. Make sure you warm the engine up thoroughly before you tune it by holding it at 70-80% throttle for a while before winding it up. Engine tuning changes with temperature (more on this later) so dont shoot yourself in the foot by doing a 3 second run up.

Anyway once its set on both the main and slow run needles turn it off and do the same with its best mate.

Refill the tanks and start both. Bring them to half throttle and hold for a good 30 seconds before opening up.

You may now find that one engine dies, if so restart it and dont touch anything. You already tuned it for its normal running temperature so leave it alone. Just do a longer run up to make sure the engine is up to full temperature. A friend had a pair of enya 46 4 strokes in a twin and we found that we needed a solid minute of over half throttle running before the left engine would tolerate full power. If we ran it richer, it ran like a donkey in the air which is why its important to warm up the engines before you tune them.

Once tuned just get on and fly it. If possible, adjust linkages to give as close to identical power at half throttle, and its nice if the idles are close. Dont tune with a tacho, just use your ears and use the tacho as a relaity check. The most important thing is to keep them both running, and if one stops dont panic! Open up the remaining engine, control the yaw with rudder (not aileron), and do a normal circuit to land. Dont be tempted to cut and glide as twins are normally heavy and glide like bricks so you will end up short, try to open up the power and it will spin. Keep it flying fast and you will be fine.

 

Oh i forgot, use 80% throttle for takeoff and not full power as an engine cut is less likely, and refrain from slapping the throttles about. Use the throttle smoothly, counting 1, 2, 3 from idle to full power especially on a go around. Once you find a happy throttle setting just leave it alone! 

Once used to the model you can throw out some of the above, but it has to survive the maiden first so take it easy and work up to it. 

Edited By Jon - Laser Engines on 14/05/2019 15:14:51

I'm a bit confused Jon, first you say if one engine stops open up the remaining engine - equivalent to opening up the power. Then you go on to say - open up the power and it will spin. These seem contradictory statements. I guess I am reading this incorrectly. Could you explain please? 

[Jon H]

They are contradictory but correct. 

 

The key in all single engine flying in a twin is speed. Below a certain speed you do not have enough control force to overcome the power of the engine. This is why when an engine stops in the air you must throttle back (to either save the engine or minimise the thrust asymmetry while you figure things out), get into a gentle dive and slowly open up the power again. Its all about gaining/maintaining speed to stay above this minimum control speed. 

 

When you want to land however you need to slow down and this puts you below the minimum control speed so you cant open the engine back up and expect to not spin as you are too slow. There is a chance you can accelerate slowly with small amounts of power and eventually coax it back to full power, but we are talking 20 seconds plus of acceleration and fight in a straight line and its not a realistic option. 

 

Full size twins behave very similarly and they have blue line or Vmca speeds to govern single engine operation. Sadly many private pilots do not get it right when the moment arrives and the results are usually rather terminal ? 

[Martin Harris - Moderator]

Going back rather a long way in the thread, you mentioned Jon, that engines are more reliable at 80% throttle and therefore this was your takeoff setting of choice. 
 

I’ve noticed this on a couple of your threads and I’m scratching my head to follow the logic. To my simple mind, there are fewer variables at full throttle than at an arbitrary stick position around 80% and we do our power checks and high speed needle setting at full throttle. If memory serves correctly,  you advise that engines richen as they unload so they are unlikely to cut at full throttle and any enrichment must happen at lower openings too. 
 

I’ve always taught pupils to advance the throttle steadily to full power as most takeoff accidents seem to occur during badly flown nose up initial climb outs or early rotations and the extra energy can give a little more latitude and time to resolve the situation as well as instilling a reaction to use power positively in low speed/high drag situations. Throttling back to a cruise setting at a safe height is a useful exercise to train pilots away from the use of a single power setting for the entire flight which some seem to do and it’s quite conceivable that a lower powered and draggy trainer or scale model could be dragged around an entire flight at such a setting. 
 

What have I missed here that makes a reduced power more reliable?  Vibration effects on fuel frothing is the only thing that comes to mind. 

[Jon H]

The 80% idea is not so much a set figure it's just to give the idea that you should not use full power. Info the same with all maiden flights as you never know exactly what the engine will do. 

 

The reason is that when you tune up your engine it's revving away nice and hot. By the time you dawdle out, wait for another chap to land etc the engine has cooled and may not tolerate full power for a long period. It could also be just on the edge of its tolerance and fractional leaning caused by acceleration or climb might just be enough to make it stop before it has time to warm up to the point it is back on full tune. Altough a rare occurrance, taking that little edge off the throttle guards against it and it's well worth it as an engine failure at point of rotation is mostly fatal in a twin. 

 

Its a risk/reward thing. Takeoff is the most critical phase of flight so any steps you can take to go gentle Nd err on the side of cautuon are a good idea. As you rightly point out slamming the throttle open is a bad idea and in general throttles are used like a light switch and this too is a bad idea. 

 

I suppose in general I use this 80% idea to make people think about what they are doing. Consider more factors in their flying, cover potential failures before they fly into them and, as a result, crash less. With modern regs and increasing pressures on model flying I think we should all strive to up our game and prevent as many crashes as we can as we are under the microscope in a way we never have been before. Also, I enjoy my twins a great deal and they aren't difficult to operate. They just require a little extra care and discipline that's all and these procedures are the ones I use to minimise risk and go home with nothing broken and a smile on my face

[EvilC57]

On 07/09/2019 at 20:16, EvilC57 said:

Right; Just for the record I’ve now discovered a way to switch in independent throttle control when needed.

I asked a clever chap on our field who flies a twin engined job whether his throttle servos are on a Y-lead, or independent. He uses separate servos with a couple of mixes in his Tx, and kindly sent me the .SPM file which achieves this.

This is how it’s done using a 1st generation DX8 and 8 Channel receiver (AR8000):

Mix 1
THR > AX3
Rate: -100%
-100%
Offset: 0%
Trim: Inh
Sw: Gear 0

Mix 2
AX3 > THR
Rate: -100%
-100%
Offset: 0%
Trim: Inh
Sw: Gear 0

These two mixes mix Throttle (Ch1) to Aux 3 (Ch8), and Aux 3 back to the Throttle, and have the effect when enabled (Gear switch down), of synchronising the servo movements on both channels for normal flight. But allowing independent movement with the Gear switch up.

In use this means that with the Gear switch UP you can start one engine and get it running while using the throttle stick to control it, then leave it idling while you start the other engine, and control it from the Aux 3 knob.

When you’re ready, setting the Gear switch DOWN brings both engines under control of the throttle stick as usual.

Note that you need to keep the Aux 3 knob in the centre of it’s rotation (where it beeps as it goes through centre) for normal flight.

Note also that you also need to bind the Rx in Preset Failsafe mode rather than Hold Last Position, to ensure that the slave (Aux 3) channel also returns to closed throttle position on failsafe operation.

Using an 8 channel Rx also leaves Ch2 and Ch6 both available if you want separate aileron servos.

I’ve tried all this so far with a battery, receiver and two servos on the bench, and it all seems to work as described (I haven’t started building the target model yet).

The chap who gave me this setup did say that if he were to get a dead engine in flight, he could switch the mix out, operate the throttle and see whether the remaining engine responds of not, from that he reckoned (knowing which engine was on the primary throttle channel) he could tell which one had cut - and put the correct rudder offset in accordingly. I suspect you would be lucky to achieve this in reality! However, it seems like a useful facility for ground running.

As a follow up to my earlier post, the mix I described above works well in reality. The model I alluded to above was finished earlier this year and has had 3 flights so far. I can start and leave the left engine running at a fast idle, while turning my attention to starting the right engine, before resynchronising both engines to operate together by flicking the gear switch. I found that the RH engine was prone to stopping earlier than the left when the throttle was reduced, so I was able to prevent this by modifying the Mix 1 values slightly. I also have to remember to kill the engines by turning the Aux 3 knob to its end point, rather than using the Trainer/Bind switch as I usually would.

 

[Martin Harris - Moderator]

Similar to the arrangement I set up on my Slim Twin but I suspect one downside may have lead do it’s demise. 
 

Full of overconfidence, I taxied crosswind to turn into a very stiff wind. I opened the throttles expecting the model to leap off the ground and get clear of the expected turbulence but it rolled to knife edge and arced into a none too graceful cartwheel, destroying every major component part in the process. 
 

No correction seemed to have any effect in the brief time off the ground and a little later, my self-congratulations on how I handled the nasty crosswind taxi started to change to the suspicion that I might not have restored the throttle selection switch to “both” as the downwind engine being at tickover would have assisted the taxi out and exacerbated the airborne antics. 
 

P.S. On re-reading one of Jon’s posts above, this was probably a classic example of a below “blue line” speed accident although I didn’t detect an engine power problem at the time. 

Edited December 26, 2021 by Martin Harris - Moderator

[David Hayward.]

23 hours ago, Jon - Laser Engines said:

They are contradictory but correct. 

 

The key in all single engine flying in a twin is speed. Below a certain speed you do not have enough control force to overcome the power of the engine. This is why when an engine stops in the air you must throttle back (to either save the engine or minimise the thrust asymmetry while you figure things out), get into a gentle dive and slowly open up the power again. Its all about gaining/maintaining speed to stay above this minimum control speed. 

 

When you want to land however you need to slow down and this puts you below the minimum control speed so you cant open the engine back up and expect to not spin as you are too slow. There is a chance you can accelerate slowly with small amounts of power and eventually coax it back to full power, but we are talking 20 seconds plus of acceleration and fight in a straight line and its not a realistic option. 

 

Full size twins behave very similarly and they have blue line or Vmca speeds to govern single engine operation. Sadly many private pilots do not get it right when the moment arrives and the results are usually rather terminal ? 

Many thanks for responding Jon, much appreciated. 

[Chris Walby]

As a relative newbie to RC and my passion for twins (mostly ic, but electric is ok if its a twin?) has brought me to a couple of conclusions through bitter experience or following peoples advice (mostly Jon's) somas to avoid the former.

• Don't push a bad decision, if it does something odd (electric) or its not running right add altitude by trying to take off won't help!
• Have a plan B ready for when things don't work out, not just engines as U/C is even less reliable
• Practice your Plan B in part or at height, just be ready for what it might do (pitch, roll or yaw) when you are using up your 3 mistakes high buffer
• s

I have an electric Mosquito that had one motor that would 1 in 10 times contra rotate 1/4 turn before starting...found that problem when the motor lead bullet (as supplied by mfg) disconnected its self on a WOT low pass much to the use of my bicycle clips!

 

Jon is spot on with the issues with Lasers (other manufactures may do the same) about warm up, sitting in the box waiting and not fire walling the engines on take off. Two things, if it needs WOT to get off the ground then its likely to be an unpleasant flight, secondly IC twins will swing to the left so why make it worse by whipping the throttles to full when its not needed? Poor tuning and whacking the throttle on my dual ace resulted in one quitting which just reinforced a number of things to remember above!

 

If possible I just Y lead the servos when the mechanical linkage is easily accessible, once adjusted it not often needed to be fiddled with. My SG Mosquito (IC 70's) has separate channels and servo/linkage which is inside cowls which means I have a bunch of mixes set up to achieve (throttle & throttle cut) + different end points. The problem (for me) is remembering what sub trim and channel does what when at the field where it is all too easy to screw up all the port engine setting up when trying to sort out the Starboard engine tuning..don't ask how I know!

 

It not likely to be an issue when sitting in the workshop with plenty of time, but once at the field and with a number of distractions its all to easy to make a simple mistake or miss a step (like full and free movement in the right direction for control surfaces). And for these reasons it either has to be simple or idiot proof for me to employ.

 

PS I using switchable throttle control for start up how about driving the elevator to full down when not in flight mode? You shouldn't get past flight line control check or at worst it will nose over and cost you a set of props...cheaper than a full bin liner?