Hello, when diving down to the deck with my Superstar .40 trainer (with OS LA 40), I hear the revs pick up. I'm hoping this is simply b/c air speed is increasing. But could this be the engine leaning out? Recently, I did have a dead stick episode--it appears that the engine was too lean--lots of brown stuff around the glow plug. This was strange too--I thought that when the weather got warmer, in general, one could lean out the engine a little. So now I have the engine richer than I did in the cold weather months.

So, to recap:

1) Is the sound airspeed or lean condition? (or neither)
2) In general, what does warmer weather do to the air/fuel mix requirements? (I thought I would need to tighten down the needle a little, not the other way around)

try a larger size prop...and if your motor is fairly new run it slightly rich. they tend to lean out in the air.

1) Is the sound airspeed or lean condition? (or neither)

My 4* does this. I assumed that it was normal on a dive. If I think of any rapidly descending airplane I have heard, the engine spins up.

2) In general, what does warmer weather do to the air/fuel mix requirements? (I thought I would need to tighten down the needle a little, not the other way around)

In cooler weather, the engine dissipate heat faster, which means that you do not need as much oil to keep the engine in safe temps.

In warmer weather, the engine does not cool as quickly, which means that you need more oil in the mix to keep it cool.

Hi Paul,

To answer your two questions:

1. Yes, when you dive, then engine will pick up rpms. This is normal.

2. Temperature and pressure will have an effect on the mixture settings. When it is warm out, the air is less dense. As a result, you need to reduce the amount of fuel in the mixture by leaning out slightly (ie. turning the high speed needle 'in'). When it is cold out, the air is more dense. You will often need to richen the mixture (ie. turn the needle 'out') to compensate.

Our club does flight instruction on Tuesday nights, and I've definitely seen a trend where most of the planes are running lean on cold evenings, especially if the previous Tuesday was warm.

It is good for you to be concious of whether you are running lean or not. The majority of trainers are well over powered, so it's not necessary to lean them for maximum power. If you are not sure of the 'right' setting for the mixture, go ahead and leave it a little rich.

One quick test of this is to run the engine up and pinch the fuel line to the carb. If the rpms drop off and the engine just quits, then you are too lean. If the rpms pick up dramatically before they drop off, then you are too rich. Ideally, you want to hear a slight pick up before the rpms drop off. That should be about right.

I hope that this helps,
-Wade

Another way to check lean vs. rich on an engine is to hold the plane level. Run the throttle up to get full speed on the engine. Then raise the nose up to between 60 to 90 degrees above horizontal.

If the engine drops rpm or dies, it is too lean. Open the high speed needle 3 or 4 clicks and try agein.

On the other hand, if the engine speeds up when you do this, it is set on the rich side.

I try to set my engines so they don't change rpm when help up at this angle, then open the high speed needle a click richer.

I always operate on the principle that rich engines last a lot longer than lean engines.

Note, if you try this same test, but lower the nose of the plane, then if it slows down, it is on the rich side, if it speeds up, it is on the lean side.

Hope that this info is helpful.

THANK YOU ALL

All engines will pick up some revs when flying because there's less load on the prop, especially in a dive. This is one reason all engines should be tuned a little on the rich side. But one thing I've never seen mentioned (in fact I only thought of it just now) is that most people will dive their plane somewhat towards themselves so there's going to be a Doppler shift in the pitch which makes it seem as though the revs have increased more than they actually do.

That's neato. Now I've got to do it again, for the Doppler effect if for nothing else!

One thing that I didn't see anyone mention is the fact that you should be throttling back when doing any down lines, it's a good habit to get into. If you go into a dive at full or near full throttle, the speeds attained are usually more than most planes can handle without coming apart.

Stick Jammer makes a good point.

Throttle management should be part of a well rounded R/C pilot development.

I feel that in mode 1 the throttle and rudder being both over on the left thumb tend to get ignored.

A good practice is to do inside loops. Full power on the up-side of the loop. Then as you are cresting over, cut the throttle back 1/3 or so open, and hold this on the down side. Want to make the travel time on the up and down sides of the loop as close to the same as you can. Finally, on the pull out you can add throtle back.

Practice this and I guarantee (this guarantee + $1 will get you a cup of coffee, most places) you will be flying smooth, pretty loops.

Take care,

Some thing else is Dopler shift.

when something making a sound is comming closer (siren, prop noise) sound waves are compressed giving the impresion the sound is getting faster and when the sound is going away the sound waves are further apart giving the impresion the sound is getting slower.

the doppler shift does play a small trick on the ears.... however you will be fine as long as the engine isnt screaming at mach 3.. if it sounds like its gonna blow itself to smitherines... pull the power back abit.. my first airplane i didnt know this and it really sucks when you overspeed the motor and the prop flies off!!!! that is pretty interesting when you hear the rpm just go thru the roof.... gots ta be quick on the stick!!! never did find the prop and spinner............... something else to think about.. and yes you should be about 2 or 3 clicks rich of optimum.. ive had awesome success since that day. its amazing how steep the learning curve is when you dork up!!!! the only time i dead stick is when i forget about fuel or pull the throttle back too much.. ohh well it makes the day interesting when you are on a high speed downwind and pull the power out and blamo!!! DEAD STICK!!! HEHEHE i love this hobby

GraupnerFan,
Don't you have your radio setup so you can pull the throttle stick all the way down for good idle without stalling? Use the trim button for killing the engine, that way you won't accidentally go dead stick.