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Low wingers,very twitchy,recommend expo.YES/NO

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cymaz25/11/2018 17:26:58
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For anyone who would like to read .....

Here

Martin Harris25/11/2018 17:34:55
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A thought springs to mind - are deflection rates linear with model response rates? Much of the thinking in this thread seems to assume this to a large degree. I'm not sure that this is the case and I think I recall reading that on full size aircraft rates are only linear on average up to around 15 degrees of control surface deflection. It's possible that at typical models' low Reynold Numbers this effect may not be exactly the same...

Jus' wondering...

Peter Jenkins25/11/2018 18:15:41
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Cymaz - you beat me to it. As those who follow the link will find out, CG position is the single most powerful trimming tool available to us. Setting the CG to the quoted figure is merely the starting point of trimming. There are many reasons why the "quoted" CG position may not be right for that particular airframe apart from the position given being incorrect. You can only really identify the correct CG position for that particular model when you fly it and the trimming guide gives an excellent way of finding what is that position.

The second most neglected area, IMHO, is adjusting the engine/motor side and down thrust. Again, the guide tells you how to fix these issues.

While many sport flyers may find the rest of the trimming guidance not worth the effort, failure to do so becomes an issue if you are flying an aerobatic airframe and wish to fly aerobatics to a good standard. However, all the advice applies equally to any aeroplane and if followed you will end up with a very nice flying aeroplane with no nasty vices unless they are an intrinsic part of the design and, the fact that if it is a scale aircraft, the scale factor or Reynolds number. Suffice to say that at the the Reynolds number we fly our average Club aircraft, perhaps half of the aerofoil (wing, tailplane or fin/rudder) are not contributing anything other than drag past their maximum thickness point. That's why scale models either have rather bad flying characteristics or else have a larger tailplane than scale to address this issue.

Finally, to pick up on an earlier point of matching servo throw to the desired control surface deflection, do remember to zero the sub trim and mechanically set the servo arm as close as you can get to the 90 deg to servo edge before using the sub trim to get that angle to 90 deg exactly. Winding in loads of sub trim can restrict the control movement in the direction that has been consumed by excessive sub trim.

Martin - there is a great deal of work underlying the "gearing" of full size aircraft control movements. For non powered flying controls, the designer would use mass, spring and dampers n the control circuit and tabs on the controls to give the type of feel the test pilot felt appropriate. Today, of course, most of this is done in the flight control software. For example, current 5th generation fighters are flown by the flight computer with the pilot telling the computer via his control stick position or force (some sidesticks don't move!) how much force he wants generated. Pulling maximum G is down to the computer deciding what that is for any given aircraft configuration, weight and speed the pilot merely pulls as hard as he can and trusts to the computer to keep him safe. Of course, if the pull out he wants cannot be carried out before the earth intervenes then he flies into the ground under full control at maximum alpha!

Bob Cotsford25/11/2018 18:17:19
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Linearity must be tied to the relative sizes of the servo and control surface horns. As I see it the greater the ratio of control horn:servo horn the less linear will be the response with increasing negative expo effect.

I do think that one of the biggest factors when it comes to expo is the sensitivity and dexterity of the flyer. Certainly as I've got older I find it harder to judge small stick movements eg holding a steady aileron deflection while moving the elevator during slow rolls. Hence I find expo more useful (or essential!) than I did in years gone by.

As for rates, some models benefit from reduced movements for higher speeds. Again, as you lose dexterity this becomes more of an issue.

Let non-adjustable transmitters stay where the belong, as a piece of nostalgia. You can prise my computer enhanced transmitters from my cold, dead fingerskulou

ps - -sorry if I'm repeating previous posts but I suffer from a short attention span after dinner on a Sunday.

Edited By Bob Cotsford on 25/11/2018 18:20:11

Martin Harris25/11/2018 18:24:17
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Sorry Bob, perhaps I didn't make my musings very clear. I was talking about linearity between the amount of deflection of a control surface and e.g. the rate of roll...i.e. does a 5 degree deflection giving 90 degrees of roll per second equate to 10 degrees giving 180 degrees per second.

I have no educated idea of the answer but just wonder if any differences would be relevant to the expo/no expo debate!

Edited By Martin Harris on 25/11/2018 18:33:34

cymaz25/11/2018 18:24:53
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I have been taught that to get the best “ resolution “ between the Tx stick and movable surface is , have the push rod as close to the servo spindle as possible and on the furthest hole on the horn. Then adjust throws, after each flight. Then use expo .

I have a World Model’s Spot on 50. The cg was as per manual. The thing was really hard work to fly and had an enormous work load. My mentor and I spent several weeks doing all the mechanical fine tuning, sealing the hinge lines, adjusting the throws. It flew so, so much better that I was able to concentrate on the shape and pattern of the manoeuvre rather than the inputs.

The throws were reduced to half. But , as I was moving the sticks more it was more linear to the control surface.

Bob Cotsford25/11/2018 18:35:33
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Posted by Martin Harris on 25/11/2018 18:24:17:

Sorry Bob, perhaps I didn't make my musings very clear. I was talking about linearity between the rate of deflection and e.g. the rate of roll...i.e. does a 5 degree deflection giving 90 degrees of roll per second equate to 10 degrees giving 180 degrees per second.

I have no idea of the answer but just wonder if any differences would be relevant to the expo/no expo debate!

Edited By Martin Harris on 25/11/2018 18:24:48

Martin, I probably missed your point through post dinner indolence. From experience I'd say that the rate of control effectiveness reduces as movement increases, but not a lot and it would probably depend on all sorts of factors such as surface aspect ratio, trailing edge section etc..

Martin Harris25/11/2018 18:48:26
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That aligns with my (still empty!) gut feeling. I've flown a lot of models that roll quickly with small deflections but don't appear to be overcontrolled at much larger ones. These seem to be the ones that benefit from expo the most.

Perhaps I need to think about this the next time I encounter such a model to see if it relates to control surface positions (i.e. strip, inset etc.), dimensions and/or their linkages.

Edited By Martin Harris on 25/11/2018 18:59:33

Don Fry25/11/2018 19:44:47
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Posted by Martin Harris on 25/11/2018 18:24:17:

Sorry Bob, perhaps I didn't make my musings very clear. I was talking about linearity between the amount of deflection of a control surface and e.g. the rate of roll...i.e. does a 5 degree deflection giving 90 degrees of roll per second equate to 10 degrees giving 180 degrees per second.

I have no educated idea of the answer but just wonder if any differences would be relevant to the expo/no expo debate!

Edited By Martin Harris on 25/11/2018 18:33:34

If your ailerons had a 90 ° deflection, you will not have much roll rate (the thought gives me queasy feelings). A 0° deflection gives you no roll rate. Therefore, response can't be a linear response.

Geoff S25/11/2018 21:41:48
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It could be linear up to 45 deg deflection, though. No idea if that is the case but beyond 45 deg it seems that you're going down the other side of a curve.

I do try to use as little permanent trim as possible but I'm afraid computer transmitters do tend to make me lazy; I know I should return the trims to zero electrically and reset the mechanical adjustments accordingly but sometimes it ends up being one of those activities that needs one of those rare round tuits

Geoff

Martin Harris27/03/2020 14:47:32
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An interesting snippet in a video I've just been watching on the design philosophy of the FW190 - there was mechanical expo built in to the control circuits of all three axes. Watch from about 19.30 if you're interested...

Somehow, I missed the last responses to this thread - to make it clear, I was referring to movements and responses within the "normal" range and how I felt that there might be a possibility of less linearity than on full size at our low Reynold's numbers.

Peter Jenkins27/03/2020 15:27:37
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It all depends..... Factors that come into control response are not just the physical geometry but most importantly, airspeed. We don't get any feedback on control stick forces. Many WW2 fighters became extremely heavy in roll at high speed as they relied purely on pilot input. Powered flying controls then had to introduce artificial feel so that the pilot could feel how much force he was exerting. We get the same feedback from the spring in out stick units whether the aircraft is stationary or flying downhill at max speed.

Martin introduces the issue of low Reynolds numbers (Re). The issue is at Re of around 750,000 there is a transition from laminar to turbulent boundary layer. It is not an exact number but there or there abouts as there are other variables to consider. Below that threshold the boundary layer remains laminar and is quite happy to stay attached to the wing while the pressure gradient is dropping i.e. as the air travels towards the thickest part of the aerofoil. As soon as the airstream starts to slow down after that and the pressue starts to increase - called an adverse pressure gradient - the turbulent bounday layer detaches and the wing is no longer working once that happens. Above the threshold, the laminar boundary layer transitions to a turbulent boundary layer (the boundary layer gets thicker with more mixing between layers and creates more drag) which is much better behaved and generally stays attached to the wing until the trailing edge thus making all the wing, and more importantly, the control surface at the aft end of the wing chord, work as intended.

That is why small scale models are such a handful whereas a much larger model acts more like the full size.

The canalyser used on some aerobatic models (the little wing on top of the fuselage with a monoplane) influences the flown of the air over the wing by injecting more energy into the boundary layer. It does this as it is inclined at a negative angle to the airflow at that point to create, for want of a better word, a venturi effect. This causes the airflow to remain attached to the wing, at least to the width of the canalyser and provides a much cleaner airflow to the tailplane and rudder. That is why, the rudder becomes so much more effective with a canalyser.

Full size designers also try to harmonise the feel of the controls so that you don't get a very light elevator and heavy aileron feel. That's what the full size Spitfire tended to suffer from according to pilot reports. Indeed the early Spits with fabric covered ailerons lost all aileron control at high speed as the fabric ballooned outwards and destroyed the ailerons effects as well as the control force becoming very high. Designers used control tabs for both trimming and to provide either assistance or resistance to the pilot to achieve better control harmony. As far as control harmony on models is concerned, we don't feel anything but the springs in our stick units and control effectiveness can vary as we go from very slow to very fast. It is a complex area but we have managed to avoid getting into all this complexity by empirical methods - suck it and see - and that usually, but not always, sorts out the problem. It is useful though to know some of what makes the aircraft controls behave in the way they do.

What is always interesting when I introduce newcomers to precision aerobatics is how little control movement you actually need to fly the aircraft smoothly and accurately. They in turn are surprised at what large stick movements are needed when flying manoeuvres since we tend to use no more than 10 deg up and down on elevator. True you need more to stall the aircraft in order to spin it, but that can be accessed either by switching rates or using a lot of expo. I prefer switching rates.

Expo is what you get used to. I use 35% expo on my low rates for aerobatics and up to 65% on full rates so that I get roughly the same control movement up to 60% of stick movement. As I only access the final part of the stick travel to stall and spin the aircraft, which by that stage is travelling quite slowly, there is no great twitchiness with the larger control throws at those speeds. The rates go back to low on the vertical down line after the spin.

In simple terms, I can have full aft stick on the elevator and still get a nice diameter loop at the standard flying speed.

Martin McIntosh27/03/2020 18:44:04
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Wise words as usual Peter. I have skipped through this thread but note that an Acrowatt e was mentioned.

Long ago in my F3A days I would almost have given my right arm for expo which was unheard of then, especially on rudder.

These days when I set up a new model of whatever type I use 30% on low rates and 40% on high which gives me a wide choice on a maiden and I do not possess one which has no expo at all otherwise I may as well use £40 basic sets. In my club I sometimes hear `I don`t like expo because I have never tried it`. Why have a fancy set of gear if you do not use it properly?

I have been to the R/C Hotel quite a few times and on the last visit most models were of the foamie e type, none with expo and they frown on you adding it. Talk about twitchy! Far too much control throw as well. I am more used to models which will not spin at full deflection on low rates.

As a general guide, if your model will stall on full elevator on low then you have too much movement or the cg is too far back. Set the ailerons on low to give 3 rolls in 5sec, quite slow.

Another tip which I have suggested to people and it works is to increase the Tx spring tension to maximum which can give you much more feel to the model. Most sets these days have very light sticks which I for one do not get on with.

Peter Jenkins28/03/2020 00:10:25
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Martin, excellent point on using the capabilities of today's feature packed Transmitters.

I was used to rates and expo but when I found flight modes and stick position switches on my JR XG11 I was delighted - once I'd worked out how to use these features. I can now fly a snap roll to left, right and up or down simply by dragging the right hand stick to the four corners and by switching mode from Aero to Land disable the snap which you really don't want to engage on final approach in turbulent weather!

I forgot to mention the stick springs so thank you for adding that in. I think it was Doug Spreng (of Sprengbrook fame when he joined forces with Harry Brooks - our highest ever placed aerobatic pilot at the Kenley World Champs - joint first I think) who said "light springs sell transmitters, stiff spings win competitions". While tightening your Tx stick springs might not win you a championship place on its own it certainly helps when you try and improve your flying. I would prefer even stronger springs than those fitted to my Tx as the extra feel they impart is very helpful in gaining some of feedbacl full size pilots get when using their controls. That having been said, our control harmonisation is much easier to achieve!

Martin McIntosh28/03/2020 10:26:09
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When I flew comps. I mostly used a Prestige 6 with Orbit sticks. These were very heavy and of a completely different design to any used now. This meant that there was a certain amount of play around centre. This was used to my advantage because to fly inverted or use a little top rudder I simply leaned on the stick to take up the play.

Later I got sponsored by Simprop briefly. Their sticks were accurate and super light, no use to me whatsoever.

When I took up the hobby again after a layoff I bought JR which I still use. It took a very long time to get used to the sticks.

Peter Jenkins28/03/2020 10:53:27
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Agree! There is a similar situation when moving from flying thumbs to using a tray and flying finger and thumb. Took me 3 months to make the transition before I got back to the same level of skill with a tray. Then, however, things got much better and I doubt I'd have reached the level I am at now flying thumbs. That is, however, a personal thing as there are many top pilots who fly thumbs or finger/thumbs holding the Tx with no neck strap - some even holding the Tx behind their backs! Difficult to do wearing a neck strap!

It's what suits you best but it's worth trying things for a while to see if the effort to change your way of holding the Tx proves beneficial. A quick try on one flight is no use at all.

MattyB28/03/2020 11:36:06
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Posted by Martin Harris on 27/03/2020 14:47:32:

An interesting snippet in a video I've just been watching on the design philosophy of the FW190 - there was mechanical expo built in to the control circuits of all three axes. Watch from about 19.30 if you're interested...

Somehow, I missed the last responses to this thread - to make it clear, I was referring to movements and responses within the "normal" range and how I felt that there might be a possibility of less linearity than on full size at our low Reynold's numbers.

The last post before you was in 2018... 

wink

 

Edited By MattyB on 28/03/2020 11:38:47

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