Throttle trim tab - never used it - why would you?

[Romeo Whisky]

I've been flying for years, (Mode 2) now using a Spektrum DX9 - electric models only and it has never occurred to me to question the purpose of the throttle trim tab - yet I have never used it.

It has recently been brought to my attention in the context of calibrating an ESC to recognise top and bottom stick settings but I can't see why it should be needed and I'm concerned that it might have unwanted side effects such as mess with the throttle-cut or failsafe.

Can anybody throw any light on the subject for me?

[Geoff S]

Other than as you say I don't use it for electric models but when I flew (occasionally fly) glow/petrol powered aircraft I used it to adjust the tickover and also to stop the engine. It's a useful feature and has no disadvantages if you don't need it.

Geoff.

[EarlyBird]

I used it on my second Riot, the first was fine, When setting the second one up and testing the motor without a propeller turned even though the throttle stick was fully back. I used the trim to stop the motor turning.

I have seen other examples of this but not very often.

Being all electric I assumed it was used for IC and had something to do with idle settings.

Cheers

Steve

[Dwain Dibley.]

Also, and more so with helis, sometimes an ESC will not arm until the trim is lowered.

D.D.

[Martin Harris - Moderator]

The OP's question really is a sign of the times!

[Keith Berriman]

On all my electric models I make sure the trim tab is in the mid point then set the esc High Low setting up points

No problems have some trim if needed

[Peter Christy]

RC manufacturers are not psychic, and have no idea what kind of model their equipment will be installed in. Throttle trim is pretty much essential on IC powered models, though not really needed on electric.

On my FrSky sets, I now routinely disable the throttle trim on electric models - even helicopters, where I have a "throttle hold" as a safety feature. However, it is still enabled on my IC models (at low end only).

I just love the flexibility of OpenTx!

--

Pete

[Richard Wills 2]

I also disable it on all electric models, on the dx9 set the trim step to zero, then calibrate the esc. If an esc will not arm unless you lower the trim, it is because you didn't calibrate it, in which case you are probably not getting 100% 'throttle' either.

[J D 8]

In the good old pre computer TX times the throttle trim tab was/is how you cut the engine and set the idle all on a mechanical ratchet.

[Doctor Chinnery]

JD8 - that's just how I like it - K.I.S.S.

[PeterF]

I have flown only electric for more than 5 years (I have been flying for 20 years so had IC experience) and as others have said, to avoid the issue with messing up the ESC end points I have disabled it, either setting the trim step to zero or setting those trim buttons to having no linked function in the trim menu on my current radio.

The funny thing is, one of our newer members who flies IC had a new model and asked me to maiden it. The tickover was fine on take off but the engine loosened up in flight and the tickover increased. It took 2 failed landing attempts and go arounds before I remembered about the throttle trims before I used the throttle kill switch to land dead stick. Needless to say, it provided a lot of ribbing back in the pits.

[Mike Blandford]

If the radio is changed to mode 1, then the physical trim switch that is your throttle trim becomes the elevator trim switch.

I only fly electric, so I configure my throttle trim to simply act as two push buttons, one of which is set to an "instant trim" function, a single press and all the flight trims are set to the current control positions.

Mike

[Matt Carlton]

I have also seen the throttle trim used in petrol models. A small push to break microswitch would be installed so that with the trim fully back, the switch would operate and break the ignition circuit.

That way, one could have a remote 'kill switch' back in the days when you'd have 6 channels at most. We got quite clever at making one function operate off another. Full down elevator to trigger a tow release was fairly common.

More common in control line than RC, having multiple linkages connected to a single control enabled all sorts of additional functions and features from a limited set of inputs.

Y- leads, servo reversers, microswitches and multiple bellcranks and mechanisms were the order of the day until recently when really small cheap servos and electric retracts and so on became readily available.

 

Edited By Matt Carlton on 16/10/2020 19:31:19

[Chris Walby]

My two reasons for using the throttle trim

1. I have a few ESC's that need full trim down before they arm, and that's where it stays for the remainder of the life of the model - could do this else where but seems more effort
2. For the ic models I use the trim frequently to adjust the tick over....This is a product of getting the engine through the noise test it has to be over propped to drag the RPM down. The by-product is that at low RPM the prop is still doing lots of work which makes landing on a short runway more of a challenge as it just does not want to stop. The simple solution is on the landing circuit to give it a few clicks down which guarantees a nice slow tick over.
   It does not effect fail safe or throttle cut on my TX's and saves meeting the hedge.

PS we are lucky enough for a club member to mow the grass very nicely short in the summer for the efd's, down side is little rolling resistance and a long roll out

Edited By Chris Walby on 16/10/2020 20:31:03

[gangster]

Bit like the act with 3 old dudes reminiscing. The young people of today would not believe you if you told them that it could take an evening if swapping arms, drilling holes, swapping horns and playing with push rod length to get the throttle throw correct . But even with computer radios it’s still necessary to be able to adjust the tick over speed depending on conditions of the day

[Martin Harris - Moderator]

Don't forget the offset splines on Futaba (at least) servo arms for fine adjustment! I bet not many recent flyers could tell you why the arms are numbered...

Edited By Martin Harris on 17/10/2020 19:39:33

[Frank Skilbeck]

On IC models I often make the throttle trim flight phase specific too, so I can have a high tickover for aerobatics, a mid speed one for normal flight and a low tickover for the landing.

[Peter Christy]

Posted by Martin Harris on 17/10/2020 19:38:39:

Don't forget the offset splines on Futaba (at least) servo arms for fine adjustment! I bet not many recent flyers could tell you why the arms are numbered...

Edited By Martin Harris on 17/10/2020 19:39:33

JR, too! But its a shame Hitec didn't pick up on this...!

Its always important - regardless of model types - to get the mechanical set up as close to perfect as you can get it! The computer should be for fine adjustment only, not for getting the basics right!

--

Pete

 

Edited By Peter Christy on 17/10/2020 21:30:53

[Martin Harris - Moderator]

Posted by Peter Christy on 17/10/2020 21:29:19:

Its always important - regardless of model types - to get the mechanical set up as close to perfect as you can get it! The computer should be for fine adjustment only, not for getting the basics right!

Spooky - we were having the exact same conversation at the field earlier today...I think that having been involved with model flying before adjustable travel was commonly available gives us more senior modellers an advantage in knowing how to plan and set up installations efficiently.

Edited By Martin Harris on 17/10/2020 23:45:34

[Peter Miller]

OH Yes! My first 4 channel set. McGregor. Two sticks, four trim levers and an on/off switch and a replacement servo cost a good proportion of a weeks pay. You had to know how to set the controls up and how to change the throws on the field after the first flights.

Going back even further, RCS Guidance System. A switch and a button. You haven't lived until you know what we used to have to do.

I can go back even further. ECC Telecommander. Two 45 volt hearing aid batteries, one 1 1/2 volt grid bias battery. four pencells for the actuator.

Tune the receiver with a milliameter, set the sensitivity with the milliameter. Start the engine, the relay would chatter and the escapement would chatter and all the turns come off the rubber drive to the escapement. Start all over again. I never did get a flight out of that. And it cost £10. in 1954. That was the best part of a weeks pay. To give a comparison My Raleigh Trent Sports bike (top of the line) cost £20

Now we have more gubbins on our Txs than we will ever use.

As for the throttle trim. Engines need fine trim due to weather variations and getting the tickover right and stopping the engine.

[Engine Doctor]

Posted by Peter Miller on 18/10/2020 08:20:56:

OH Yes! My first 4 channel set. McGregor. Two sticks, four trim levers and an on/off switch and a replacement servo cost a good proportion of a weeks pay. You had to know how to set the controls up and how to change the throws on the field after the first flights.

Going back even further, RCS Guidance System. A switch and a button. You haven't lived until you know what we used to have to do.

I can go back even further. ECC Telecommander. Two 45 volt hearing aid batteries, one 1 1/2 volt grid bias battery. four pencells for the actuator.

Tune the receiver with a milliameter, set the sensitivity with the milliameter. Start the engine, the relay would chatter and the escapement would chatter and all the turns come off the rubber drive to the escapement. Start all over again. I never did get a flight out of that. And it cost £10. in 1954. That was the best part of a weeks pay. To give a comparison My Raleigh Trent Sports bike (top of the line) cost £20

Now we have more gubbins on our Txs than we will ever use.

As for the throttle trim. Engines need fine trim due to weather variations and getting the tickover right and stopping the engine.

My first radio set back in 1958 was a 27 met carrier wave.The servo I had was a trunion type driven by a " Mighty Midget " motor and a galloping ghost switch box ,separate from the Tx was used to set the pulse. Throttle was an clockwork escapment .The Tx was a Tri-ang with a 10 ft plug together aerial like a tank aerial. The Rx was built by s friends dad from the RCM&E magazine , remember the small mags that cost 1 shilling and threepence ? As you say had to be tuned before every flight or sailing and hopefully stayed in tune for the duration ? The other annoying thing with them was that only one set could be used in the area , no crystals then.and if I messed about with it in the evening at home it would wipe out reception of any television near by. I never used it in aircraft as you couldn't fly them on local commons so it was used in boats on Clapham Common boat pond. Then came the Rep tritone radio with reeds that still had to be tuned before each use. Happy days

Edited By Engine Doctor on 18/10/2020 10:33:43

[Bob Cotsford]

Posted by Engine Doctor on 18/10/2020 10:32:29:The other annoying thing with them was that only one set could be used in the area , no crystals then.and if I messed about with it in the evening at home it would wipe out reception of any television near by. I never used it in aircraft as you couldn't fly them on local commons so it was used in boats on Clapham Common boat pond. Then came the Rep tritone radio with reeds that still had to be tuned before each use. Happy days

Edited By Engine Doctor on 18/10/2020 10:33:43

Aah! Memories of annoying my big sister by blipping the CW button when she was watching 'Ready, Steady Go!' on TV

[David Hall 9]

My F5J flying would be much more difficult without the throttle trim.

The trim on my more recent radios (Taranis on OpenTx) is actually a separate slider control that happens to be mixed with the stick function to provide a trim control.

As I don't use the trim on the throttle control for the motor, I have switched off that function. This gives me a slider control in a very good position on my Q7x (this Tx has no other sliders).

In my gliders, this slider is set up to adjustable the motor throttle to elevator mix when in "launch" mode (they tend to try to loop without some down elevator on high power), It is disabled, along with the throttle, in "glide" mode and in "landing" mode, it becomes the flap to elevator gain mix control (throttle stick becomes the flap control).

For me, it works so well that when I set up my X9D, I kept the same layout, even though the X9D has sliders.

[Phil Green]

Posted by Peter Miller on 18/10/2020 08:20:56:

Going back even further, RCS Guidance System. A switch and a button. You haven't lived until you know what we used to have to do.

...and can still do, if you enjoy a bit of 'real retro'...