engine tuning in hot weather

[Raptormad2]

Hi was at our field today with a friend and we noticed on our raptors they were running a bit hotter than normal due to the very hot weather, so I have read somewhere that in hot weather engines should be leaned out this don't sound right to me as if leaned out too much it would cause the engine to run hotter. cheers.

[Andy from Sandy]

Forget about what works for others. They will be in a different part of the world with different atmospheric conditions.

Just tune your engine such that on a full power climb out it just goes on and on without sagging and the exhaust note is constant all the time.

When you land and hit throttle hold if it hangs on the pipe without going to a tickover fairly quickly then just richen the idle until it does.

If it actually sounds lean before any tuning then richen up first before anything else.

[coolice]

Hey.

What engine do you have? This will help provide you with some information that can help on that engine.

Above is correct, tune your engine to its optimum and with the variables it could be running such as plug type, fuel make/ingredients etc.

Depending on what engine you have you may find you tune the top and bottom needles to get the full power and idle correct, but are then left with a lean mid range. On engines that have a mid range needle this is easy to fine tune, but on 2 needle (idle and main needles only) ypu will need to run the idle on the richer side. The OS/Align Hyper 50 is a classic example of this, the newer 55 class from the same manufacturer has the mid range needle on the carb.

Keep in mind the engines run time, could it be your bearings are getting old and need replacing?

A good test of the idle needle setting is to actually pinch the fuel pipe, if the engine is too lean it will pick up revs as the fuel supply depletes. It should maintain a steady idle and just stop.
The finger test on the rear crankcase cover is a good indication overall, you should be able to keep your finger on this plate without getting burnt. If you can't after some time flying then your mixture is lean.

[David Drew]

The finger test on the rear crankcase cover is a good indication overall, you should be able to keep your finger on this plate without getting burnt. If you can't after some time flying then your mixture is lean.

I tend to use the backplate test as I'm not that good at any other method.!

I have an OS50 in my Raptor that works well with this method, but I have an OS32 in my Bell 47 and two pals have 32s in their Raptor 30s and we can never get them that cool. To me the 32 just seems to be a hot running engine.

[coolice]

I tend to use the backplate test as I'm not that good at any other method.!

I have an OS50 in my Raptor that works well with this method, but I have an OS32 in my Bell 47 and two pals have 32s in their Raptor 30s and we can never get them that cool. To me the 32 just seems to be a hot running engine.

Morning mate.

Yes you're right about the smaller engines, good point.
I did mean to make reference to that from a recent experience of tuning an OS37 recently that did run hotter on the rear case cover than the larger engines.
It had been sometime since I'd dealt with anything smaller than a 50 so I was monitoring the rear temp and engine note closely.
.

[Raptormad2]

Thanks for the replies, the engine is in a raptor 90 which is an os max 91sx three needle carb running a no8 os plug when I checked the back plate I could hold my finger on it for about 7 seconds before it started getting hot haven't tried the pinch test yet will try it next time , and running on 20% opti,

[David Drew]

Morning mate.

Yes you're right about the smaller engines, good point.
I did mean to make reference to that from a recent experience of tuning an OS37 recently that did run hotter on the rear case cover than the larger engines.
It had been sometime since I'd dealt with anything smaller than a 50 so I was monitoring the rear temp and engine note closely.
.

I think if you are after every last bit of power for 3D (I wish!!) then engine temp is important for not 4 stroking or running too lean. Also transitions between needles are smooth.
If, like me you are generally sports flying it's does it generally run ok, not too rich, not too lean and ok power with a sensible backplate temp then the job's a goodun.

[Rotorhead]

It's true that you should lean it out in hot weather, the small change in air temperature between what we consider a cold winter and hot summer means little to an engine in terms of over heating, if you think of it in terms of engine temp, a dozen or so degrees is only a small fraction of its base temperature, also since we're on about materials science you should also consider it in terms of Kelvin, not Centigrade, so a dozen or so degrees between summer and winter is bugger all.

What is far more important is the engine mixture, when the air heats up it expands, and that means that in any given volume of air there are less air molecules, which means the mixture will be rich if you don't reduce the amount of fuel by the same proportion.

[Andy from Sandy]

To lean out an engine just because it is hot out today is too much of a generalisation. The engine needs to be tuned on its merits.

Tuning by back plate is also a bit misleading. It might be an indication but it doesn't tell the whole story. I have had an OS32 sizzling that would have had a 90 blown to bits.

[Rotorhead]

It's no generalisation, it's physics/chemistry Andy, if you have less air molecules in a charge you need to reduce the number of fuel molecules too, no matter what engine it is.

If you're running lean to begin with then you may have a problem and cook a motor, but that is more of a fundamental tuning issue, once it's tuned properly for the air pressure then adjustments should only be to match the pressure on the day, granted that's more of an art than a science, since no one is going to take a borometer to a field and calculate how much to change their needle settings by, but the principle is sound, less air in less fuel in, more air in more fuel in.