Throttle curve characteristics fixed wing IC

[Graham Bowers]

To set context, I've attended a few Introduction to Aerobatics sessions at Buckminster  and it seems the time has come to enter the murky world of Flight Modes.

 

I chatted this through with a mentor yesterday however on checking my notes, see that we discussed Spin, Aero and Landing. And within that, some aspects of throttle curves for electric powered aircraft. However not IC powered models.

 

So I'm wondering if they weren't included because there's nothing to discuss for an IC powered model and / or I forgot to ask, as my head was pretty full by then 😉

 

I'm flying an OS55AX in a 5.5 lb Balsacraft Xtreme and a radiomaster transmitter running OpenTx. I'm comfortable with programming curves on the the Tx, however am wondering what characteristics a throttle curve may have if it was to be helpful for the Clubman level precision aerobatics manoeuvres I'm attempting to master? I have  a throttle stick centre position vibration indicator set up and that's at a suitable cruise speed.

[Peter Jenkins]

Hi Graham

 

With IC engines, you tend to get a lot of power generated early on in the movement of the throttle.  What you want to arrange then is a curve that reduces the movement of the throttle arm to begin with and then accelerates it towards the end.  The difference between a throttle barrel open to 85% and 100% is not much more power.  What you want to aim for is as linear a delivery of power, let's call it thrust as we can measure that.  If you have a spring balance you can use that to measure the pull or thrust of the engine.  Measure it at full power and then set your half power with the throttle stick at half open and adjust your curve point at half power to give half the thrust.  If you have the function EXP on the curve then that will give you a nice curve joining the idle point to the max power point.  You can add further curve points to make fine adjustments as required.

 

If you correlate throttle stick position with spring balance readings you should aim to have 1/4 thrust at 1/4 open stick position and so on.  After that, it's a case of fly it and see whether you feel comfortable with the setup and if you think that you'd like more power coming in earlier just move the curve points to achieve that aim.

 

Ideally for aerobatics, you should aim to fly at half power for your datum speed.  But for smaller less powerful aircraft you might want this to be between 65-70% power.  That leaves you with 30-35% excess power to maintain the speed in vertical manoeuvres.  That does mean that you will not be able to fly as large a loop as you would like as the speed will decay.  As the speed reduces the aircraft will turn more tightly and so you will have to reduce the backpressure on the stick to maintain a round loop.  The advantage of using 50%, or even less, for datum speed gives you the excess power to fly large vertical manoeuvres while maintaining the airspeed you had in S/L flight.  I upped the power in a Wots Wot from an OS61FX to an OS 91FX, same mounting holes and lighter, to give me that option.

 

Oh, I would set a switch to give a slightly higher idle speed when flying to ensure the engine will not stop in a spin or stall turn and then drop back to your low idle for landing.

 

I hope that helps.  If not, ask away!

 

Peter

[Graham Bowers]

Thanks Peter, that helps a lot.

 

I do have a spring balance so know what I'll be doing first thing tomorrow at the field.

 

The OS is a brand new engine, new to the model and is quite a lot more powerful than the Irvine 53 (black carb) I'm used to** so I'm making the necessary adaptations to flying it.  Loops can be a LOT bigger with the OS so I'm looking forward to enjoying it.

 

I like the high idle suggestion and will include idle settings in the flight modes. I did dead-stick yesterday on a stall turn so resorted to fiddling with the throttle trim as an interim measure.

 

**The poor Irvine needs new crankshaft bearings and will be re-deployed when repaired.

 

Cheers

 

Graham

[Peter Jenkins]

Graham

 

I had an Irvine 53 with the black carb but the Mk 2 with the silver carb was much more powerful.  Coupled with a short pipe, called a throttle pipe, it has given my Wot 4 unlimited vertical performance without affecting its excellent throttle response.  Once you get used to the extra power of the OS55 try it with a throttle pipe - try Westons for one - and you will be amazed at the extra power it gives.

 

Peter

[Graham Bowers]

Thanks again Peter

 

Pipes are a topic I had on my list of things to do to explore. Interesting that throttle response wasn't affected as I had in my head that tuned pipes** work by reflecting a negative pressure wave from the exhaust exit such that it arrives at the right time at the exhaust port to assist with the scavenge process and so was of use over a limited rev range. I'm not questioning your comment by the way, I just like to understand things as fully as I can, and reconcile information as needed. **I did note the use of the term "Throttle Pipe", by the way, and assuming that's a marketing term.

 

I note Westons state:

 

"THROTTLE PIPES ARE ONLY SUITABLE FOR USE ON AIRCRAFT WHERE PROPHANGING IS REQUIRED!"

 

I wonder why that is? Effect of sound emissions perhaps?

 

I'll give Weston a call. You are leading me further astray 😉 For around £50 I'm sucked right in 😉

 

Graham

 

 

 

 

 

[Paul De Tourtoulon]

If you are using petrol engines, use this to get an even throttle, I have never had to bother with methanol engines, it's all in the

use of the stick.

[Graham Bowers]

15 minutes ago, Paul De Tourtoulon said:

If you are using petrol engines, use this to get an even throttle, I have never had to bother with methanol engines, it's all in the

use of the stick.

 

Thanks for that.

 

I get that use of the stick is a skill.

 

The path I'm currently exploring, because it's fun, is the precision aerobatics learning curve.  As I'm sure you know, part of that is to use Tx functionality to relieve pilot workload so one can apply more focus to the other aspects of the flight, and I find learning that interesting.

 

The world of petrol engines is one I have yet to dip a toe in.....................

[Kevin Caton]

Hi Graham

 

For a 2 stroke IC such as the OS55 I’d suggest about 25% expo on throttle - with the response increasing at higher stick settings. That is negative expo for Futaba and positive expo for most (if not all) of the other brands. This is the opposite expo to that used for electric motors which have a more linear response than IC engine carburettors. That will give a more linear response in flight.

 

I’m sure that the mentors at the ITA sessions will have already suggested that you should reduce the control throws a lot on your 3D design as this is important for precision flying.

 

I hope you enjoy the learning experience - it never ends but it’s great fun!

 

Kevin

[Graham Bowers]

4 hours ago, Kevin Caton said:

Hi Graham

 

For a 2 stroke IC such as the OS55 I’d suggest about 25% expo on throttle - with the response increasing at higher stick settings. That is negative expo for Futaba and positive expo for most (if not all) of the other brands. This is the opposite expo to that used for electric motors which have a more linear response than IC engine carburettors. That will give a more linear response in flight.

 

I’m sure that the mentors at the ITA sessions will have already suggested that you should reduce the control throws a lot on your 3D design as this is important for precision flying.

 

I hope you enjoy the learning experience - it never ends but it’s great fun!

 

Kevin

Thanks Kevin

 

The Radiomaster Tx running OpenTx firmware can produce up to a 17 point curve so based on thrust measurements and trials I'll be able to form an opinion whether an "expo" curve or something more bespoke will be most appropriate. I imagine it'll take a few iterations to get it how I like it for now, and that'll doubtless change as I gain more experience.

 

I've learned so much from the guys on the ITA's and yes, throws are comparatively minute.

 

I'm very much enjoying the learning experience. I think I can go as far as saying I'm enjoying my flying as much and possibly more than I ever have, and I've been flying on and off for just over 40 years.

 

Graham

 

PS Due to the bad influences I've been subjected to, a Mytho S 50 has been aquired 😉

[Jon H]

Be careful changing the low end part of the curve as engines are most likely to cut as they accelerate from idle power to about half throttle. If this first half of the throttle travel is slower than the second half this will make your slow run tuning less critical and you can also run a leaner slow run mix. If the throttle opens too rapidly you are forced to use a richer slow run mix to allow the engine to keep up and this will lead to rich running at part throttle which is not ideal so there is a balance to be had. 

 

Personally i never use a throttle curve and have not experienced any problems with lumpy power delivery. I do notice though when i fly an electric model how gutless they feel at low throttle vs i/c. They arent of course, its just the fact that they run down to 0 power at zero throttle rather than something like 20% power at minimum throttle for a glow. 

 

In any event, i would perfect your flying to the point where the throttle control is holding you back and then adjust that as required. 

[Peter Jenkins]

Jon

 

We are talking precision aerobstics here.  This is not your forte.  Even the top pilots in F3A use throttle curves.  Kevin Caton is one of those - he has flown in many F3A European and World Championships representing the UK.  If you don't use throttle curves its because you don't fly precision aerobatics in competition.  Please don't muddy the waters for someone who is getting one-one coaching in how to fly precision aerobatics.

 

Thanks

 

Peter

[Ron Gray]

11 minutes ago, Peter Jenkins said:

Please don't muddy the waters for someone who is getting one-one coaching in how to fly precision aerobatics.

Steady on Peter, that's a bit harsh! The point Jon makes about the fuel mixture is very real and very valid irrespective of flying aerobatics or sports models, after all, the engine doesn't know what it's hauling along. Furthermore Jon does go on to say to adjust the throttle (curve) after learning how to fly the model to start with, there's a lot to be said for that too.

[Philip Lewis 3]

6 minutes ago, Ron Gray said:

Steady on Peter, that's a bit harsh! The point Jon makes about the fuel mixture is very real and very valid irrespective of flying aerobatics or sports models, after all, the engine doesn't know what it's hauling along. Furthermore Jon does go on to say to adjust the throttle (curve) after learning how to fly the model to start with, there's a lot to be said for that too.

I tend to agree, after all you won't be flying precission anything with a dead engine and what Jon said (as an engine expert) is still a very valid caution and warning.

[Ron Gray]

I suppose all of this will become irrelevant given the way that F3a has gone, allowing the motor speed to be automatically controlled in climbs and dives to automatically maintain the speed without pilot input, quoting from Peter's earlier post on his new Anthem build - 'The D3 then will hold whatever prop speed your throttle stick has commanded for straight and level flight and during any climbing or diving even if you don’t move the throttle setting'. So the next step, to make it fair for IC pilots, is for the same sort of gismo to control the IC's speed. You may as well allow the use of gyros to make sure that the flights are nice and smooth as well!!!!!

[Philip Lewis 3]

If only it was that simple Ron, it's nothing really that new either, all that the new ESC's are doing is controling the governor setting with the throttle input, (same as a heli) rather than the voltage the motor sees, so your throttle position controls the prop speend and in a dive the the ESC reduces the power to zero as the prop windmils itself up to that set speed, and likewise it applies more power to maintain the prop speed as you point the nose up, however you still need more power to climb and therefore more prop speed to climb so still have to throtttle up and you still need (to an extent) to slow it down in a dive so you still need to throttle down, so no change there really just that it's all more subtle.  

Edited April 7, 2023 by Philip Lewis 3

[Ron Gray]

Maybe I misunderstood it, I thought that the aim was to assist in maintaining the same airspeed throughout manoeuvres?

[Jon H]

1 hour ago, Peter Jenkins said:

Jon

 

We are talking precision aerobstics here.  This is not your forte.  Even the top pilots in F3A use throttle curves.  Kevin Caton is one of those - he has flown in many F3A European and World Championships representing the UK.  If you don't use throttle curves its because you don't fly precision aerobatics in competition.  Please don't muddy the waters for someone who is getting one-one coaching in how to fly precision aerobatics.

 

Thanks

 

Peter

 

Hi Peter

 

You are talking about comp level precision aerobatics, and that is totally fine. However, Graham is talking about a clubman schedule with a balsacraft extreme, which is a fun fly model and it will not offer that level of performance no matter how you dress it up. His mythos has the capability though, and a friend has one, but they are electric so its power delivery is different anyway and so my comments about engine mixture are not relevant. 

 

We have clashed swords before when i comment on aerobatics as they are 'not my forte' and i see we are here again. While i may not fly that sort of model (i mean f3a here) or that style of flying i am not ignorant about it or its requirements. Obviously not in the granular detail of someone who does, but still, im not clueless and have friends who have flown in that sort of environment. I am also not incompetent when it comes to aerobatic flying with models like acrowot xl's etc, unless you count 3d which i have no interest in at all. 

 

To bring this back to the OP, a throttle curve on a balsacraft extreme with a bog stock OS55 aiming to fly club level aerobatics is...dare i say pointless? They are simply the wrong tools for precision comp level flying.  A nice 2m composite pattern model with some fancy YS fuel injected engine or electric contra prop at the front then, sure, throttle curve until the cows come home. But a funfly? Also i am not privy to the conversation had at buckminster or the exact details of the intro to aerobatics course, but when see 'introduction to aerobatics' as a headline title i envisage learning to loop for the first time and nothing more complex than an actual introduction to aerobatic flying. If it was the 'intro to competition f3a' lecture i would gladly walk on by as i have nothing really to add.

 

If i misunderstood the course then ok, but there seems to be a disconnect between the level of course attended by Graham as i view it, and the advice given both there and here. It would be like me talking about how to fly a 50cc ultra scale Spitfire to someone with a 3 channel parkzone foamy Spitfire. The two dont really line up. 

 

So to resolve the problem i will simply ask Graham what it is he is trying to achieve. if he is trying to be the next aerobatic champ i will take my leave and you can pontificate without me, but if not then the whole point you are making is kinda moot. 

 

 

Edited April 7, 2023 by Jon - Laser Engines

[Philip Lewis 3]

Ron, your understanding is spot on and correct, but, if you think about it then if you are cruising along in a straight line and pull to a vertical then even maintaining the same prop speed won't be enought to climp, you need more RPM, the difference is a normal ESC would maintain voltage so as the prop comes under load it would slow, the new esc will apply more voltage and maintain the prop speed but that won't be enough you still need more power (more prop speed), conversley in a dive it will start to brake as the prop through windmilling attemps to overspeed the set speed but probably not enough as you will likely still need more brake to overcome gravity which is pretty strong. 

 

It just makes it more subtle as it "helps" but doesn't do the complete job for you it simply (as you said), "assists".

 

It's quite a difficult concept to explain in words.

[Nigel R]

I put a throttle curve on everything. 

 

I prefer the more linear feel of the end result.

 

I've never noted a difference in reliability for IC.

 

3d or funfly types I prefer stationary hover to be with throttle stick centred.

 

Just my preferences. 

[stu knowles]

I always use a curve on the throttle to reduce the response at low throttle position, petrol and glow.  Without it the response from mid stick to full is minimal.  Using a curve makes fine adjustments of power on a landing approach much more manageable.

 

I am surprised to see people advocating against their use, one of the benefits of modern TXs surely?

[Ron Gray]

9 hours ago, Philip Lewis 3 said:

It's quite a difficult concept to explain in words.

You’ve done a really good job in explaining it, many thanks. So it’s not like cruise control in a car which maintains your speed.

[Graham Bowers]

10 hours ago, Jon - Laser Engines said:

Be careful changing the low end part of the curve as engines are most likely to cut as they accelerate from idle power to about half throttle. If this first half of the throttle travel is slower than the second half this will make your slow run tuning less critical and you can also run a leaner slow run mix. If the throttle opens too rapidly you are forced to use a richer slow run mix to allow the engine to keep up and this will lead to rich running at part throttle which is not ideal so there is a balance to be had. 

 

Personally i never use a throttle curve and have not experienced any problems with lumpy power delivery. I do notice though when i fly an electric model how gutless they feel at low throttle vs i/c. They arent of course, its just the fact that they run down to 0 power at zero throttle rather than something like 20% power at minimum throttle for a glow. 

 

In any event, i would perfect your flying to the point where the throttle control is holding you back and then adjust that as required. 

Perhaps it’d help if I spoke a bit more about context.

 

I was a pretty scruffy flier who one day woke up and decided I’d like to put a bit more structure into my flying. There are probably a dozen forks in the road I could have taken, however the one I chose was aerobatics. I’m now a slightly less scruffy flier 😉

 

I’m really not the competitive type, and it’s enough for me to realise the potential of myself and my equipment. I’m using the Xtreme because that’s the model I have. I’m liking it so have raided the gas meter for something more capable. Maybe I’ll enter a comp and maybe I won’t. If I do, it’ll be the first one ever about anything. My recreation background includes adventure sports. Rock climbing, scuba diving, that sort of thing, where you have a great day out with your mates and the most important thing is to come home safe.

 

However I have a passion for learning. I’m like a sponge, and always have been. And aerobatics opens up a whole new world of learning opportunities. And that’s why I want to explore throttle curves and everything else, for the learning. If it doesn’t actually help with aerobatics right now that’s OK.

 

A few words on the ITA’s. They are dead down to earth and it’s up to you what you want to do. There’s a short chat in a room the first time one attends, and after that it all takes place at the flying field and pretty much on a one to one basis. I’ve been working on the basic building blocks. Loops for example, aiming for big round ones that include throttle control and adjustments for wind, and several superimposed. I arrive with ideas on what I’d like to achieve and take counsel on the day about where to expend effort. Others are practicing and refining schedules. There really is something for everyone. And over lunch on Weds we chatted about the EDF parkjets we’d built! It’s absolutely not a “sell” for F3A.

 

I appreciate the technical help you and others have given. Both here and face to face at the ITA’s. It’s clear to see all are pulling on the same rope in the same direction, and if we all had identical views of the world, the world would be a poorer place, in my opinion.

 

Bottom line, for me, is it’s all about standing in a field and having fun playing with toy aeroplanes. And I love it 😉

[john stones 1 - Moderator]

ITAs ? Introduction to aerobatics ?

[Graham Bowers]

11 minutes ago, john stones 1 - Moderator said:

ITAs ? Introduction to aerobatics ?

Yes

[john stones 1 - Moderator]

I thought we encouraged people on here ?

Aerobatics is what floats my boat, did a bit at clubman level but lacked the focus that you need, also I was mostly the only one in clubman class most weeks and I lost interest.

Believe it or not but the pilots there have a wealth of knowledge, they may not make or sell engines but they ain't too shabby at using and getting the best out of em, dead bits in the curve, yep they'll encounter that and tweak the curve to suit. Doubt many of the top lads ever started out flying with expensive models, they'll have worked their way through stages pretty much as Grahams doing, all experience and far from pointless. Like others I've used throttle curves, used them on Webras and O.S 2 strokes, helis and fixed wing, no broken engines to report.