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Throttle to rudder mix


Tim Kearsley
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I have a model - electric powered - which has no side-thrust built in to the motor mount.  I've flown the model a few times now and the lack of side-thrust is evident.  It's not easy to add side-thrust without pushing the spinner back-plate off-centre.  I'm thinking that mixing a little right rudder with throttle could improve things.  Any thoughts?

 

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Not picking holes, in your idea Tim, but this is a big no from me, and why.

The rudder input would always be there, even through loops.

Even if you switch it off, then that is yet another switch too many for me.

Rudder for me includes use for a flatter landing approach etc etc.

Find a way to alter thrust another way.

 

Unless, you make the rudder input that is required when you need it.

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Give it a go.

 

It may not be the total solution as the rudder mix will have different effects at different airspeeds but you may well find that it is beneficial.

 

I'd suggest starting with say 5% and adjust up & down to taste. If nervous about it put it on a separate switch so that you can knock it off should it become unruly. On the other hand if feeling bolder and your radio permits you can put it on a slider so you can adjust the percentage mix readily until you it suits, then lock it in.

 

I regularly tweak all sorts of low percentage mixes on my models to tune their responses. The purists may not like it but it suits me. 

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Tim, presumably you can still control the rudder independently of the throttle by using the rudder control?  Only the neutral position will vary depending upon where the throttle stick is.
If the battery voltage were to get low enough to switch off the motor, you would need to remember to close the throttle to ensure you have were to have the rudder centred to the low throttle position.  That's not a big deal though.   As John Lee says, give it a try.

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1 hour ago, Denis Watkins said:

Not picking holes, in your idea Tim, but this is a big no from me, and why.

The rudder input would always be there, even through loops.

Even if you switch it off, then that is yet another switch too many for me.

Rudder for me includes use for a flatter landing approach etc etc.

Find a way to alter thrust another way.

 

Unless, you make the rudder input that is required when you need it.

 

Agreed it is not a perfect solution, but it's not going to crash the model. @Tim Kearsley, just give it a try and see if it helps - I suspect as stated above you will be able to improve things, but not eliminate the issue completely. Depending on the model though that may be plenty good enough (i.e. fine for a trainer or sport hack, still undesirable for a precision aerobatics machine).

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I work on the basis that you have two hands and two sticks. Mixing is no problem unless one of your hands falls off. 

 

Most of my warbirds have no side thrust and its not uncommon for me to need 2/3 of my right rudder over the top of a loop. It sounds extreme, but its really not a big deal. You should use your rudder as much as your elevator and ailerons. If you arent, i recommend you start as it is very helpful in a variety of circumstances. 

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While I'm all for developing good rudder skills, I do like to trim an aerobatic model to climb vertically under full power and usually do any fine tuning with a throttle to rudder mix.  I've never found a downside to it and wouldn't bother making it switchable as it's more or less self-compensating unless you happened to be messing with the throttle on a dead stick (but why would you?).  Fly a test flight, make a mental note of the amount of rudder correction required for a full throttle vertical climb to eliminate any yaw and then programme it in before checking the accuracy.  That way you won't get any nasty surprises.

 

Any fine tuning you might feel necessary after this can be done with multi-point curves etc. if you want to be really finicky.  You can also add a throttle to elevator mix to sort out any up/downthrust issues in the same way.

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Thanks very much chaps for a diverse range of opinions!

I am guilty of not using the rudder enough and I'm actively working on rectifying that.  The model in question is a P51B, conventional balsa and ply.  I could, I suppose, shim the motor to put in some side-thrust, but the cowl and spinner backplate are all nicely lined up and, as Alex Whittaker would say, I'm a lazy savage!

I suppose it's all a matter of personal preference whether you use the Tx facilities to make life easier or whether you prefer absolute purity of control.  My favourite form of flying is thermal soaring and I've adopted the strategy of using as much mixing as I can to reduce the "burden" of flying so that I can concentrate on reading the air and finding lift.  So I use a switchable aileron-rudder mix for example.  It's where you draw the line I guess - do you use a flap-elevator mix to compensate for the effect of flap deployment?  Aileron-rudder mix?  Expo?  Or do it all with fingers and thumbs?

I'll experiment with some throttle-rudder mix and see how it goes.

Thanks again.

 

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The rudder is a very important control and I have never understood why r/c trainers pay so little attention to it.

I would recommend all r/c flyers find their nearest 1:1 size gliding site and purchase a trial flight this will really convince anyone of the importance of the use of rudder.

What is the rudder for - it is there to control "yaw".

- a critical control on take-off and landing (not the ailerons)

- a critical control in initiating and balancing turns

- a critical control in inducing spins or exiting spins 

Mixing rudder with throttle seems to me to be an undesirable state of affairs based on my 40 years of 1:1 size power flying and gliding and even longer of r/c model flying.

If the thrust line is off then I suggest fixing the real problem rather than trying to compensate for it through the whole life of the model.

I can appreciate that many might disagree with these comments but there it is.

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A full-size glider pilot I went up with said you can always tell the Cessna pilots because they don't use rudder, so I guess from that it is more important in a glider than in anything else. Personally when teaching model pilots to fly I tell them to ignore the rudder control as it makes less work for them to do in the early stages. I have been known to only hand-over control of aileron and elevator and keep rudder and throttle under my control for simplicity.

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To clarify my comment, i mostly fly scale and no matter what you do they will never fly straight and never be in trim. This is compared to a pattern ship or general sport model. I trim them for a given speed and then fly round the rest of the issues as they present themselves.

 

I suppose the question i should asked Tim was how this lack of side thrust was impacting the model. What traits are leading you to believe its a side thrust issue? Its possible its not a side thrust issue as such and it could just be a trim issue or just the way the model is. 

 

In answer to your other points though i use elevator flap mix to trim the model the landing. You trim full size, this is no different. I use rates for additional movement on landing takeoff as often the controls loose effectiveness at low speed. No expo, no other mixes other than undercarriage sequencing. 

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I’m not sure that making direct comparisons to full size is quite as appropriate as it might seem.
 

First, we are not in the aircraft, able to see and instantaneously feel the effects of control inputs, which lead to unconscious corrections as muscle memory develops.  We don’t have the benefit of a turn and slip indicator either although at certain view angles, we can see the secondary effects of controls - that’s something that instructors could maybe be more proactive in pointing out. 

 

Secondly, things happen very much faster with a model and we don’t, as a rule, have time to re-trim with power changes - a typical climb to circuit height taking a handful of seconds.  Flight modes would be a way around these limitations but not something available to many beginners or appropriate to teaching them the basics. 

 

Neither of these factors should discourage us from trying to develop good rudder control and awareness - pitch control with power changes being a little easier and perhaps more instinctive to correct. 
 

P.S. Having seen Jon’s posting above reminds me  about flap trim changes - I don’t imagine there are many full size aircraft that aren’t re-trimmed with flap deployment which, again, isn’t nearly as practical to do with an approach started a few seconds out from touchdown instead of having plenty of time to re-trim with the benefit of tactile feedback from the stick. Typical model trim controls aren’t conducive to  use in these circumstances either, unlike in the full size world. Definitely a case where a mix can be of real benefit. 

Edited by Martin Harris - Moderator
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18 minutes ago, Jon - Laser Engines said:

I suppose the question i should asked Tim was how this lack of side thrust was impacting the model. What traits are leading you to believe its a side thrust issue? Its possible its not a side thrust issue as such and it could just be a trim issue or just the way the model is. 

Thanks Jon.  What made me think it's a side-thrust effect was that I spent some time trimming the model to fly "hands-off" at a smidgeon over half-throttle.  It needed no aileron or rudder trim but a fair bit of elevator trim.  This was due to the CG being quite forward and was later reduced when the CG was adjusted back a little.  However, after this initial trimming it was very noticeable that when pulling up into a loop (under full, or nearly full, power) the model yawed off to the left.  I ascribed this, rightly or wrongly, to a motor side-thrust effect.

 

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If your model was bent, the wing warped, it would drive you barmy the effect on it's flight characteristics and you'd sort it or bin it, trim it to cruise it wanders as throttle comes off to land, throttle changes in circuits, it drifts, verticals it goes off line, it is not just performing a loop that shows it up, it's evident on the basics also, it's annoying and easy fixed. Knowing your model goes where you put it is an aid to better flying, better control and pretty much a must for me. Why when teaching new flyers would you not teach this, you trim the model for them to aid them, not to hinder them.

 

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I agree with Jonathan M here as it is not at all uncommon to need to counter torque through a loop. I have a hangar 9 pulse xt 125 and it has a bunch of right thrust on it as shipped but i still need to add rudder in large vertical manoeuvers. 

 

As mentioned before i fly a variety of scale models from WWI through 1930's biplanes, WWII fighters and even 1960's light aircraft in the form of my Piper Tomahawk. All of these need extensive rudder use even when flown in non aerobatic flight like the T'hawk.  This also is true for my aerobatic and sport models like acrowot xl, the pulse, a spacewalker, and my twin engine jobby as well. 

 

What i recommend you do is fly the model at your trimmed speed and enter a fairly steep right turn. Note the attitude of the model and do the same to he left. You might find that the nose sits high in the right turn and low in the left. You can then trim it right so turns both ways leave the model flat. This is the way i adjust my rudder trim as its often easier to see the different than in level flight. With that done, try your loop again and see if its any better. If not, or if it is but you still need some rudder then just add the rudder and fly it over. It will become 2nd nature before long and you will find that being able to independently control the direction of the model in pitch roll and yaw is a very handy thing to learn as it comes in very useful on all models when a cross wind is involved. 

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1 minute ago, john stones 1 - Moderator said:

If your model was bent, the wing warped, it would drive you barmy the effect on it's flight characteristics and you'd sort it or bin it, trim it to cruise it wanders as throttle comes off to land, throttle changes in circuits, it drifts, verticals it goes off line, it is not just performing a loop that shows it up, it's evident on the basics also, it's annoying and easy fixed. Knowing your model goes where you put it is an aid to better flying, better control and pretty much a must for me. Why when teaching new flyers would you not teach this, you trim the model for them to aid them, not to hinder them.

 

 

 

You wouldnt like my Nieuport 17 John. Its never in trim at any speed. On throttle, off throttle, turns, dives, takeoff or landing. Its always changing trim, mostly in yaw but i just ignore it. It rolls better left than right due to engine torque as well so i only roll to the left. A left turn needs right aileron, right turns need left aileron to get it round. Cross control and being out of trim is just the name of the game. Its great fun to fly because its a bit of a challenge. If it was easy, i would be bored by now.  

 

As for teaching students. I dont trim the model for them to make life easier. In fact, i often put it out of trim and let them deal with it. My job is not to make their life easy, its to teach them to fly. Some models cannot be made to fly arrow straight, they just wont do it, so pretending all models can be set up to fly like a pattern ship is counter productive in the long run. As i have mentioned before, there is no sense of achievement if its easy. Overcoming a challenge always makes you feel good. 

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 Not quite the same but I have one aircraft I did not put in enough down thrust on the motor after a rebuild when the original motor pulled its self off the nose. As the replacement was more powerful I did a beefy job on the nose and did not want to start again so I set a throttle/elevator mix in the TX that is proportional, the more power you give the more down elevator trim is fed in, power off elevator goes back to neutral Took a while to get the rate of change right but has worked well.

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Flown many a Nieuport Jon, and many other types, I think you're trying to put words in my mouth that aren't mine, I make no claims re model sort, just stating doing the basics right as I see it, makes for a more enjoyable experience, whatever the type, pretty similar to using I.C engines in my opinion, I see no great mystery myself.

Teaching ? My job is is to provide the info and the basics in a safe manner, and I do push the pupil, the route the individual takes is then their choice, last thing the club needs is a clone of me.

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Perhaps i misunderstood as it sounded like you were suggesting all models can and should be set up to be as straight as an arrow with no nasty traits when we both know that simply cant be done. I not denying that blatent issues should be sorted out, just that there comes a point where its just part of that model and you have to get on with it. 

 

Its determining if you have a handling characteristic or a problem which is not always easy.

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Most, if not all, of my scale models have no side thrust, as per full size. It is probably second nature now to add rudder on take off.

Most of my previous flying has been F3A when it would have been added as a matter of course but I believe that proportional throttle to rudder is now the preferred way by some. Not had the need to try it since the amount depends on cross winds etc and I am much less fussy now.

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