Tuning For Flight

[Mike Stevens]

I am starting to prep for i/c flight...I think i have read the engines lean out in flight so tune in the ground for max revs and then richen up a bit...?

[Don Fry]

What model, what engine. What past experience.

[Mike Stevens]

Wot 4, OS 46AX.....fly electric but limited flight experience with ic....

[Former Member]

[This posting has been removed]

[Denis Watkins]

11 x 6 Mike, for starters, if its a new motor and run it in in the air.

On the ground, model tethered firmly, get the motor warmed up and tune for max revs, then anticlockwise a couple of clicks.

At this point, if new, the motor will reluctantly tickover at landing revs, but will soon run in enough to idle around 2000 rpm.

If run in, just get on with it and fly.

If new, check all nuts and bolts again after landing, as the warm up can soon loosen bits

[Former Member]

[This posting has been removed]

[Mike Stevens]

Engines are run in and i have flown it before on buddy box and not using a starter - chicken stick and a couple of flicks and it starts. Using Prosynth though a few years old now so I need to get some fresh fuel. Understand about weather - cool damp day denser air and more power - just looking to avoid dead stick if i can ...lol Also have an Acrowot, same set up - dragged both models out of the loft recently after building about four years ago and getting them flight ready again.

 

Edited By Mike Stevens on 09/10/2017 07:32:09

Edited By Mike Stevens on 09/10/2017 07:33:40

[Denis Watkins]

Your sorted Mike, a run in OS46AX is very reliable, and if all is well, good glow plug, screws tight, engine warm and tuned to run

Then any fear of a dead stick would be a tank/ plumbing/ pressure leak

Good fuel, good motor, with a good glow will not dead stick, just fuel pressure/ flow, could stop a motor

Tuning can be a few clicks out either way, but with good plumbing, the motor will still run.

The motor should start on idle and run if low and high needle are close to being in tune.

You should not have to throttle higher to start.

Now its cooler outdoors, once started, alternate revs with the throttle stick, rather than leave on tickover too long,

Get the motor warm

[Cuban8]

Sorry, double post.

Edited By Cuban8 on 09/10/2017 08:19:22

[Cuban8]

Posted by Cuban8 on 09/10/2017 08:06:53:

Chances are your Wot4 / OS46AX combination is going to have a much better performance than your electric models. Take it easy on the first few flights so it doesn't get away from you and you'll be fine. I'd guess the 46AX is good for around 1.2 H.P, so that's a very rough equivalent to a tad below 1000W in an electric set up.

If you can get a club mate to show you how to run-in and set the engine it'll be so much easier. It's really not difficult once you've watched and listened, but an incorrect needle setting by an inexperienced operator can cause a lot of damage to an engine without you realising it. Just to confuse matters, ABC and ringed engines require different techniques when running in.

 

 

 

Edited By Cuban8 on 09/10/2017 08:27:05

[Mike Stevens]

Thanks all - yes all run in and idle fine once warmed up - just learning a new technique and conscious I have a heavy bit of kit in the air with a big metal lump on the front.......

[Rich too]

I know Jon at Laser disagrees, but the age old method of tuning and then lifting the model into a vertical position with a WOT always worked for me. It tends to lean the motor slightly so if held vertical it maintains max revs it should be good to go ....

[cymaz]

There is no need to use an electric starter on your 2 strokes. There is a simple method for starting them, once you have the hang of it it's easy. It also avoids bending the crank and flooding it. Good luck with you flights

Here is my short video if you are interested.....

[Jon H]

Posted by Rich too on 10/10/2017 05:37:41:

I know Jon at Laser disagrees, but the age old method of tuning and then lifting the model into a vertical position with a WOT always worked for me. It tends to lean the motor slightly so if held vertical it maintains max revs it should be good to go ....

Yeaa, the old nose up test. Unless prop hanging its a bit of a waste of time im afraid.

In general, Engines go rich in flight as they unload, so tune for max rpm and just get on with it. If the engine overheats it needs better cooling not a richer mixture.

cymaz, nice work on the back flick. I find it works on almost everything as long as you can give it a good prime

Edited By Jon Harper - Laser Engines on 10/10/2017 08:49:53

[Cuban8]

Trying to get my head around this...............tune for max rpm on the deck....................prop unloads in the air...............increased rpm gives higer  flow of air through carb but needle setting effectively limits flow of fuel to that what was set on the ground.....................more air same fuel.................mixture now lean?????

Jon's my engine guru so I'm clearly in the dark over something here.

Edited By Cuban8 on 10/10/2017 10:15:52

[Mr.B.]

Higher flow through the carb = high pressure difference = carb sucks fuel harder.

[Denis Watkins]

The RC car racing boys can teach us a thing or too here

They have venturi inserts, that reduce the diameter of the carb inlet

And this increases power?

[Jon H]

revs are one thing, load is another. RPM rises because load decreases and reduced load usually requires a leaner mix. Drop an inch on prop dia and what do you get? more rpm on the engine and leaner mix needed on the needle.

Airflow through the carb will increase for sure, but as its the same air flow that is pulling fuel through the needle fuel flow will increase. Higher intake velocity, lower pressure through the venturi, greater pressure differential between the fuel tank and intake so greater effective fuel pressure resulting in more flow. If it didnt we would need to open the needle with the barrel as we change throttle positions. I know the slow run needle plays a part, but if you take 1/2 throttle or more, or the old air bleed carbs, fuel flow increases as the barrel opens. In the case of air bleeds, there comes a point where the throttle is closed so far that the above system collapses as the restriction in the carb is so tight that effective fuel pressure is so high it would drown. That is where the air bleed comes in to keep the suction where it needs to be. Slow run needles work the other way and meter the fuel.

Ram air will lean the mix as it increases pressure in the carb, but this is usually offset by the load reduction making things rich again.

As with all things there are ranges where they work and where they dont. If you take piped engines or engines that unload massively once in the air, these will not behave as above because the rpm change is so huge and pipes have an effect on the induction cycle etc etc. For general running though, on normal props doing normal things the nose up test is a bit redundant with modern engines. I have not done one in 15 years as i think its both pointless and dangerous. trying to nose up test a warbird weighing 23lbs with a 360v at full chat is not really a good idea.

[Jon H]

Posted by Denis Watkins on 10/10/2017 10:22:19:

The RC car racing boys can teach us a thing or too here

They have venturi inserts, that reduce the diameter of the carb inlet

And this increases power?

Its complicated but it can.

In the case of Laser engines we use a small ish carburettor as this gives good fuel draw and gives a higher gas velocity into the cylinder. But, its not just the carb that influences this. Our 70 has a different cam profile on the inlet with a shorter duration but higher lift. It has the effect of giving a stronger and shorter intake period and so intake velocity increased giving more power as better acceleration. Its all about the viscosity and momentum of the air.

The conservative carb size can work against us sometimes, as at very high rpm it can limit the intake charge the engine is able to draw. Our 155 is an example of this as in very cold weather will not rev as fast on a small prop (smaller than any normally used on the engine) as it will on a hot day. The air is simply too fat and heavy to get through the smaller hole. Switch to a sensible prop and the reduced rpm gives more time for the induction cycle and rpm is then increased due to the cool dense air the engine is drawing.

[Geoff S]

Posted by Denis Watkins on 10/10/2017 10:22:19:

The RC car racing boys can teach us a thing or too here

They have venturi inserts, that reduce the diameter of the carb inlet

And this increases power?

When I was riding 350cc 4 stroke trials motor cycles what was needed was lots of torque from very low revs and smaller diameter carbs increased airflow speed and helped the engine go from almost stopped to full power very quickly to overcome an obstacle like a rock step. I assume it's the same thing for model engines (without the rock step, of course!).

Geoff

[john stones 1 - Moderator]

For me, forget the physics n the reasons why n why not, if you've over tweaked it, nose up test will show it up, put it down to one of lifes great mystery's if you like.

[Cuban8]

Many thanks Jon, very nicely explained.

[Jon H]

Posted by john stones 1 on 10/10/2017 11:34:30:

For me, forget the physics n the reasons why n why not, if you've over tweaked it, nose up test will show it up, put it down to one of lifes great mystery's if you like.

Normal service resumes John as i disagree If its too lean it wont hold max revs on the ground when level. Pulling the nose to the vertical while flat out and stood still is not representative of the condition the engine will see in flight. The natural unloading of the engine will cover off the leaning effect of gravity working against the fuel flow.

A better test would be to do a nose up test while in flight. Run in flat out, pull into a vertical climb and see what happens. If it vanishes into orbit you are ok, if it dies half way up open the needle by 1 click. If it starts to get fed up just as you run out of airspeed (ie as load increases again) then i would consider tuning perfect for anything other than prop hanging.

To be honest though, all of this is just looking for problems we dont need to be worried about. If the engine is tuned for max rpm on the ground, and held there for 15 seconds without showing signs of distress then the tuning is fine. Go fly it. If it overheats and stops in he air, then tank position and cooling are likely to be the problems and not the mixture. Then we have a whole other series of things to talk about

[john stones 1 - Moderator]

Not interested in quoting your post Jon, lifes too short. Whatever method works for people, is fine by me, god forbid everyone was the same as me and held the same opinions.

[Jon H]

Posted by john stones 1 on 10/10/2017 15:05:55:

Not interested in quoting your post Jon, lifes too short. Whatever method works for people, is fine by me, god forbid everyone was the same as me and held the same opinions.

Aww dont be like that John, i was only kidding

I appreciate that there is an accepted way to do things, i just trying to point out that its actually not the best way and the many readers of the thread can get better performance from their gear if they try something new. Glow engines have changed alot since they first appeared and a great deal of the accepted 'standard practice' is out of date. Fuel and oil being the most obvious bone of contention for many even now.

By running the engine closer to its optimum performance it will be cleaner, use less fuel, give more power, be less likely to rust and less likely to stop. Whats not to like?