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Throttle Curves?

Do people use them on fixed-wing?

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Jonathan M17/02/2017 08:16:21
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What's the story on throttle-curves for fixed-wing, or is this really only used to tame things on helicopters?

On my 70FS powered Acrowot I've currently got a straight-line curve, i.e. no expo, but before getting too used to throttle response positions should I look at tweaking in a gentle curve, and what sort?

I've already gotten used to the slight response lag compared to electric. Do 2S engines differ in response times to FS?

Jon

Denis Watkins17/02/2017 08:25:22
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Purists would balance the throttle response with the movement of the servo arm, as when the arm travels through its arc, the push pull is non linear

Watch the throttle linkage at the end of its travel at full throttle, and you will see barely any change at that, and the lower end

Some flyers want control of this

I have never been troubled by this, but some people will help you

A 2 stroke will "throttle" up more quickly than a 4 stroke, as you move the stick up, and both motors belong in certain models, and budgets.

4 stroke will swing a larger prop, more quietly, and use far less fuel, but sound like a BSA C15 in the right circumstances.

Edited By Denis Watkins on 17/02/2017 08:26:37

Gary Manuel17/02/2017 08:58:00
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I never bothered using throttle curve on IC models until I started flying electric as well.

I now find that without throttle curve, all the response is at the bottom half of the stick movement with very little change in power over the top half. This is not just due to the throttle linkage, it's due to the way the engine / carburettor sucks in air - it needs proportionally higher amounts of air at higher revs - it's not linear!

What I like to have is about half power with the stick in the middle. I find that setting my throttle curve to give about 25% throttle at 50% stick position achieves this.

Jon Laughton17/02/2017 09:09:17
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Jon I do use throttle curve function if required - usually if the throttle linkage is complex then it may be required to counter the locus of the point where the linkage attaches to the throttle arm on the engine as it moves and/or the same at the servo to give a truly linear response...Will explain more when we speak but it is basically different from model to model, build to build!

Note that expo and throttle curve are not necessarily the same

Cheers

Jon

Brian Cooper17/02/2017 09:13:36
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The throttle curves are particularly useful with IC power as most of the power tends to be delivered over the first half of the throttle opening -- there is only a slight increase in revs for the last 50% of throttle opening.

However, it is a "power game" where if you have a dull model which is powered with a miserable little engine which has to thrash itself to death just to keep a model airborne, it doesn't really matter about having a throttle curve . . . But if you have a generously powered model - with say with a 2:1 power-to-weight ratio, (that's twice as much thrust as the model weighs) a power curve becomes highly important, especially with twiddly 3D aeroplanes. . In these cases, it is nice to set the curve so that 50% of (throttle) stick produces 50% of power, but different people will set their own preferences.

B.C.

Peter Miller17/02/2017 09:19:04
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I use Exponential on two stroke throttles as it makes the throttle response much more linear.

I regret that my Spektrum doesn't allow me to do this but my old Futaba FF8 does.

Jonathan M17/02/2017 10:17:58
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Useful, thanks.

I note the point about expo being different to throttle curves. With the latter one can enter (certainly on my DX7 G2) a series of specific points on the TX rather than just the smooth s-curves generated by punching in expo.

I don't think I need to make any major tweaks on my Acrowot. The ASP 70FS seems more than adequate to the job, neither too puny nor too rippling. Control-linkage is direct with simple geometry, and any mechanical disadvantage at either end of the travel is minimal.

Its just a question of spending a tank's worth of fuel on the ground sometime, judging thrust changes across the travel range and fiddling with the TX settings to fine-tune.

By the way, how would I know what maximum thrust the 70FS (or any other given engine) produces?

john stones 117/02/2017 13:27:21
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Never found that a two stroke throttles up better than a four stroke myself, they may sound different as they deliver the power, but the response is as good.

John

Denis Watkins17/02/2017 13:43:31
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Here we go again on the " friendly forum"

A 2 stroke motor will throttle from bottom to top quicker than a comparible 4 stroke motor

It will go from idle to its best quicker

The information is only useful if you feel a 4 stroke lags somehow

If ownership and regular use is a measure of preference

Then the statement is based on using 5 4 strokes and 3 2 strokes, rotated on outings frequently in good weather

These are just words of opinion on the friendly forum

Constant unnecessary barracking

Edited By Denis Watkins on 17/02/2017 13:45:40

john stones 117/02/2017 13:57:32
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No idea what brings that stuff Denis, i'm a friendly fella so i'll leave it be smiley

John

Simon Chaddock17/02/2017 14:00:08
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Johnathan

Also bear in mind that the response of the carburetor throttle itself does not produce a linear power response in virtually any IC engine.

We get used to recognising that the initial throttle movement will generate a rapid increase in power although with a prop the way thrust is produced tends to match the actual response of throttle/engine power.

So you have to decide what you want the Tx response curve to do.

1. Restore the carburetor movement to exactly match that of the servo.

2. Try create actual engine power that is truly proportional to the stick movement

or just learn to live with how it is! wink 2

Edited By Simon Chaddock on 17/02/2017 14:01:34

Frank Skilbeck17/02/2017 17:49:57
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3716 forum posts
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Throttle curves on a helicopter are so you can fine tune the engine speed with the collective pitch loading, ideally to keep a pretty constant head speed.

On fixed wing I find that a throttle curve is useful to give a finer setting at low throttle openings, so you get a finer response to the throttle stick as you are coming into land. Most engines I've got don't give much more power from 2/3rd open, whereas the 1st 1/3rd gives a big change in power, so making the throttle open open less as low stick settings gives a more linear power output with respect to throttle position.

Frank Skilbeck17/02/2017 17:58:00
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As regards thrust, do a search and you can find spreadsheets and online calcs that let you workout the static thrust. For instance my TT75FS turns a 12 x 6 prop at over 9,500 rpm and this equates to approx 2.5kg static thrust

Jonathan M17/02/2017 21:01:04
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249 forum posts
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Posted by Simon Chaddock on 17/02/2017 14:00:08:

Also bear in mind that the response of the carburetor throttle itself does not produce a linear power response in virtually any IC engine.

We get used to recognising that the initial throttle movement will generate a rapid increase in power although with a prop the way thrust is produced tends to match the actual response of throttle/engine power.

So you have to decide what you want the Tx response curve to do.

1. Restore the carburetor movement to exactly match that of the servo.

2. Try create actual engine power that is truly proportional to the stick movement

or just learn to live with how it is! wink 2

Posted by Frank Skilbeck on 17/02/2017 17:49:57:

On fixed wing I find that a throttle curve is useful to give a finer setting at low throttle openings, so you get a finer response to the throttle stick as you are coming into land. Most engines I've got don't give much more power from 2/3rd open, whereas the 1st 1/3rd gives a big change in power, so making the throttle open open less at low stick settings gives a more linear power output with respect to throttle position.

Cheers for these.

Yes, Simon and Frank, that's how I understand and experience it, and a slightly finer proportionality is exactly what I'd like to achieve - especially around the lower throttle range to help with lower speed flight/manoeuvres and landings. So far I've found small stick adjustments in this zone produce rapid jumps in engine power.

Jonathan M17/02/2017 21:39:32
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249 forum posts
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Posted by Frank Skilbeck on 17/02/2017 17:58:00:

As regards thrust, do a search and you can find spreadsheets and online calcs that let you workout the static thrust. For instance my TT75FS turns a 12 x 6 prop at over 9,500 rpm and this equates to approx 2.5kg static thrust

I seem to learn something new every few hours at the moment!

This site **LINK** has just such a calculator, which shows my ASP70FS turning a 13x6 prop at 9,500 rpm max produces 3.0kg static thrust.

Posted by Brian Cooper on 17/02/2017 09:13:36:

However, it is a "power game" where if you have a dull model which is powered with a miserable little engine which has to thrash itself to death just to keep a model airborne, it doesn't really matter about having a throttle curve . . . But if you have a generously powered model - with say with a 2:1 power-to-weight ratio, (that's twice as much thrust as the model weighs) a power curve becomes highly important, especially with twiddly 3D aeroplanes. . In these cases, it is nice to set the curve so that 50% of (throttle) stick produces 50% of power, but different people will set their own preferences.

So the Acrowot weighs in at 2.8kg, thus a power-to-weight ratio of just slightly over one (1.07 to be a tad more accurate)... so neither puny nor quite 3D capable...

... but just as I like it! laugh

Jonathan M19/02/2017 19:49:54
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249 forum posts
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So, had a go fiddling with the throttle-curve this afternoon, and now have a much happier response, where 50% stick equates to 38% throttle, out of 80% max on the curve. 100% is fully open when eyeballing the carb during servo setup, but I couldn't hear or feel any increase in revs on the ground beyond 80%, so clipped the curve at this limit.

Is this, however, a false assumption - i.e. would one expect higher revs than this 80% during faster flight?

dsc_0295.jpg

Percy Verance19/02/2017 21:12:42
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Four strokes don't throttle as well as a two stroke?

May I respectfully suggest you try a Laser Denis?........

Edited By Percy Verance on 19/02/2017 21:15:27

Gary Manuel19/02/2017 21:24:54
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What you have there Jonathan is 50% stick = 38% throttle and 100% stick = 80% throttle.

That's only 2% away from a straight line (50% stick = 40% would be straight), so you haven't really achieved much - apart from maybe limiting your full throttle position to 80%.

 

What you need to do before you set a throttle curve is to:

1/ Remove your throttle curve so that it's a straight diagonal line from 0 to 100%

2/ Adjust your TRAVEL / END POINTS so that low stick = tickover and high stick = full throttle (barrel fully open - just before the servo stalls). This is your starting point.

3/ Assuming a 5 point curve, readjust your throttle curve for position 1 = 0%, 5 =100%, 3= 25% (or whatever gives the feel you want at mid position).Position 3 is the only one you should need to touch to start with. Position 2 and 4 should self adjust - if not, set them to give a smooth curve.

Edited By Gary Manuel on 19/02/2017 21:45:36

Gary Manuel19/02/2017 21:25:52
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Posted by Percy Verance on 19/02/2017 21:12:42:

Four strokes don't throttle as well as a two stroke?

May I respectfully suggest you try a Laser Denis?........

Edited By Percy Verance on 19/02/2017 21:15:27

I must have been running my 4 strokes wrong then Percy, because my Saito's throttle just fine.

Edit - misread your post Percy - yes I agree with you.

Edited By Gary Manuel on 19/02/2017 21:32:00

john stones 119/02/2017 21:46:35
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Posted by Percy Verance on 19/02/2017 21:12:42:

Four strokes don't throttle as well as a two stroke?

May I respectfully suggest you try a Laser Denis?........

Edited By Percy Verance on 19/02/2017 21:15:27

O.S n Y.S aint too shabby either wink

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