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

[Peter Jenkins]

+1 with GrahamWh. Whilst I often hear folk say you don't need expo and you can just use smaller rates that is true up to a point. If you want to fly accurately, and I mean accurately, you will need to temper the inbuilt expo as explained above with some expo to reduce sensitivity around the stick centre. I have not done the sums, but a very experienced aerobatic pilot wrote a trimming article where a 36% expo gives a linear relationship between stick and control surface position. I tried this and it works for me but I also have quite small movement on my aerobatic aircraft. A common misconception is that you need loads of movement and that aerobatic aircfraft are very twitchy - this is not true of course but encourages folk to set up far too much movement on their controls particularly for maiden flights rather than use different rates to cope with grossly out of trim aircraft on the first flight.

[Tim Flyer]

One basic thing that always needs optimising is servo geometry . Rather than flying on very low rates like 30% it’s better to move the clevis inwards and use 100% servo travel. That also improves control in that stick movement is finely translated to servo movement. Reduced rates have an almost similar effect on total control surface deflection but using full servo movement to achieve similar deflection improves control 

Edited By Tim Flyer on 24/11/2018 19:35:01

[Don Fry]

Posted by Tim Flyer on 24/11/2018 19:32:13:

One basic thing that always needs optimising is servo geometry . Rather than flying on very low rates like 30% it’s better to move the clevis inwards and use 100% servo travel. That also improves control in that stick movement is finely translated to servo movement. Reduced rates have an almost similar effect on total control surface deflection but using full servo movement to achieve similar deflection improves control

Edited By Tim Flyer on 24/11/2018 19:35:01

[onetenor]

Sort out the geometry as stated by Tim and Don and use low rates .

[Old Geezer]

Sonny, I'm a bit late arriving at this party, but for what it's worth I very happily flew a foamy AcroWot-e for a couple of seasons, set up exactly "as per" ( balance point being the later correct one, not the original dodgy one ). The recommended settings were well nigh perfect, the reduced deflections are very user-friendly especially if you get to know one another just stooging around at 1/2 - 2/3 throttle. The expo' thing is rather a matter of taste and should be just applied to the full deflection settings - initially at the proverbial 2-3 mistakes high - so if you feel uncomfortable you can safely wimp-out at the flick of the rate(s) switch(es) with lots of air underneath you. My expo' setting on full rates was 35% which smoothed out any tendency to twitchiness. I'd say the foamy A/W-e is one of the sweetest flying planes I've owned, two things to be aware of ( they're NOT criticisms ) those tapered wings can tip stall, but it's not meant to be a trainer. And if you hit a bump or hole on the patch hard, landing or taking off, you can pull the u/c off taking a chunk of foam with it - but some fibre tape and Gorilla Glue fixes it stronger than before.

[gangster]

We need to remember that once upon a time there was no such thing as computer transmitters. Basic sport/aerobatic models have not really changed. Rates were set up by choosing the appropriate hole on the horn or output arm. Differential was set up by torque rod position and people flew the models well sometimes very well. You cannot paper over the cracks of model set up with flashy gizmos. Basics are basics. Let’s also realise that the easiest model to fly is an aerobatic model set up moderately. They need not be wild untameable race horse. From that point of view the acrowot does not need expo, that comes later. Follow the wise words given by some of the posts on here about arm and horn position, turn of the flash electronics, that model will fly just as well with a basic Futaba M4 that’s 40 years old . Once you get it right you will wonder why trainers are hard to fly. The same old wives tales about low wingers also was used on tail staggers back in the day

[Peter Christy]

Gangster: Exactly right! My early mutli-channel flights were made with what is usually referred to as "reed" equipment - not even proportional! It has been likened to trying to drive a car around town with the throttle wide open and controlling the speed by switching the ignition an and off!

In truth, it wasn't quite that bad, but it certainly taught you to be very careful with your setup before you even attempted flight (no trims at all!). Yet people managed to fly advanced, low-wing, aerobatic models with a degree of precision that many club pilots would envy using today's hi-tech equipment.

Many years ago, I was fortunate to hear a talk by the then helicopter world champion. Being Japanese, his English was somewhat broken, but the message came across loud and clear: Get the mechanical set-up right first! The gizmos on the transmitter are for fine-tuning, NOT for doing the whole set-up!

--

Pete

[Ron Gray]

Wise words from PeterJ, well worth re-reading!

[cymaz]

Posted by gangster on 25/11/2018 09:58:00:

We need to remember that once upon a time there was no such thing as computer transmitters. Basic sport/aerobatic models have not really changed. Rates were set up by choosing the appropriate hole on the horn or output arm. Differential was set up by torque rod position and people flew the models well sometimes very well. You cannot paper over the cracks of model set up with flashy gizmos. Basics are basics. Let’s also realise that the easiest model to fly is an aerobatic model set up moderately. They need not be wild untameable race horse. From that point of view the acrowot does not need expo, that comes later. Follow the wise words given by some of the posts on here about arm and horn position, turn of the flash electronics, that model will fly just as well with a basic Futaba M4 that’s 40 years old . Once you get it right you will wonder why trainers are hard to fly. The same old wives tales about low wingers also was used on tail staggers back in the day

Exactly what my aerobatic teacher tells me.....

[Jonathan M]

Just checked the settings on my ARTF 70FS-powered Acrowot. Having balanced the model corectly (with a bit of tail-weight and a tad on one wing-top) and set up the basic throws on the model exactly as per the instructions, my MID-RATE settings (with EXPO set to 18% for all three surfaces) are:

• AILE 55%
• ELEV 60%
• RUDD 125%

These rates were tweaked from my previous similar settings by a very experienced instructor, but only to ensure overall harmony between the controls (he just reduced the aileron rates slightly). I was originally on 15% expo but found this too twitchy; 20% didn't feel right, so 18% is a good compromise. He also strongly advised that I get rid of the high and low rates (which were previously +/- 10% on aileron and elevator, etc) and just fly on the settings above. I've done this ever since, to the progressive improvement in the smoothness of my flying and confidence. KISS.

Jon

PS For comparative purposes, I find I need lower expo on my gliders (i.e. just roughly cancelling out intrinsic negative mechanical expo):

• Middle Phase slope-soarer: 15% on aileron and elevator, 10% on rudder.
• Elf mini-DLG: 10% on rudder and elevator
• Blaster 2 DLG: 10% on all surfaces (however rates here do differ across different control-surfaces, but only according to different wing-camber 'Flight Mode' settings, i.e. launch, cruise, penetrate, float, land)

[Geoff S]

I use a lot of expo on aileron and elevator so there's a dead(ish) band at the centre (mode 2). I can only assume others are far more sensitive than I am if they can really detect the difference between 18 and 20% expo. I don't see how simply using lower rates does the same job because, whilst low rates will help in normal flight it also reduces the maximum throw as well and so makes aerobatics more difficult. Expo tames the small movements whilst not limiting the maxima. I usually have about 25%. It suits me but obviously it's down to the individual.

I've just set up a throttle curve on my electric Wot4 Mk 2. I haven't actually flown with it yet but the curve is roughly exponential (just 5 set points). The motor set up I have is quite powerful (700 watts for a 5.25 lb all-up weight) and I was finding adjusting rate of descent on throttle on finals difficult because a touch of throttle caused the plane to zoom unless great care was exercised. I've effectively tamed the low speed throttle response. I hope it works

Geoff

[SONNY MONKS]

I had a club member maiden my foam-e yesterday,only needed little trimming,i then flew it,and i must admit,it was my first flight with expo at 20%,with the rates recommended in the manual,and it seems much easier to fly,than with no expo at all,i even performed some loops on my second flight,a great model,im impressed.

[Tony Jones]

So far everyone has focused on the model. But, (IMHO) the individual is equally or more important. I am a bit clumsy with the sticks around the centre position, tending to apply too much deflection initially. Expo helps smooth that out. On the other hand, a fellow club flyer who is an aerobatics expert is extremely precise with his stick movements and finds that expo dulls the model's response. Each to his own.

And when will we get away from the myth that high wing models are more stable than low wingers due to 'pendulum stability'? Probably never; it's been around for so many years. Certainly true when static, but no so once the model is airborne and subject to aerodynamic forces. Don't take my word for it: go read the literature. I suggest starting with Alisdair Sutherland.

[Tony Jones]

Posted by Geoff Sleath on 25/11/2018 13:08:47:I've effectively tamed the low speed throttle response. I hope it works

Geoff

Geoff, zooming when power is applied indicates inadequate down thrust. Get that right and you'll find no need for a throttle curve.

[cymaz]

Posted by Tony Jones on 25/11/2018 17:17:15:

And when will we get away from the myth that high wing models are more stable than low wingers due to 'pendulum stability'? Probably never; it's been around for so many years. Certainly true when static, but no so once the model is airborne and subject to aerodynamic forces. Don't take my word for it: go read the literature. I suggest starting with Alisdair Sutherland.

A Seagull Spacewalker is just a stable as a trainer. Good thick wing nice blunt leading edge. Won’t tip stall for no man.

[cymaz]

For anyone who would like to read .....

Here

[Martin Harris - Moderator]

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 Jenkins]

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 Cotsford]

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 fingers

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 Harris - Moderator]

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

[cymaz]

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 Cotsford]

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 Harris - Moderator]

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 Fry]

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 S]

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