apwachholz

Comment/Discussion:
When it comes to tuning 4-stroke or 2-stroke glow engines, I often see pilots holding their plane vertically upright or upside down to test if the engine performs properly (or not) in one position or the other. I was under the impression that if the engine was tuned correctly and the fuel tank was getting pressurized properly to feed the engine enough fuel (or you have a fuel pump), there was no need to point the plane up or down while the engine was running (not to mention it seems dangerous to me). I've never tuned my engines using this process but also have never had any issues. And this includes both 2-stroke and 4-stroke engines which have been in aircraft going inverted, upright, straight down, etc.

Question:
Can anyone provide some insight as to why this procedure is used or what would cause an engine to run poorly in one orientation or the other? Thanks for any insights.

MinnFlyer

My Feedback: (4)

With a non-pumped engine, it is only the vacuum of the carb that draws the fuel (slightly assisted by the muffler pressure). So when pointing straight up, the engine has to draw fuel from the tank below it instead of the tank being level with it. This is the reason you want the engine to be slightly on the rich side.

When you point the nose straight up, you should get either no change, or a slight increase in RPM - Not a decrease in RPM which means the engine is going lean in an up-line.

That said, I often see too many people doing this test wrong. The pilot will go to WOT, and before the engine ever gets to peak RPM, the assistant is pointing the nose up.

The proper way to do it it to go to WOT, wait for the engine to reach its peak RPM, LISTEN TO IT, and THEN point the nose up so you can listen for any change.

Campgems

The reason to hold a plane vertical and go to full throttle for 20 to 30 seconds is to see if it leans out. In that orentation, the engine will tend to lean out some, and if you can notice that, you need to adjust the high end slightly. If you find peak on the needle, and then go to the rich side 300 to 400 rpm, you have accomplished the same thing. I have flown mostly four strokes, and have not had a problem using that rule of thumb. I'm not sure what would be accomplished inverting the plane for a test unless the tank height was an issue. If that is the case, and adjustment in tank height would be in order if it is possible.

Don

apwachholz

@MinnFlyer (a.k.a. Mike) -

WOT? I'm a bit slow, remember that.... : )

MinnFlyer

My Feedback: (4)

WOT = Wide Open Throttle

apwachholz

@MinnFlyer
Thanks for the 'WOT' clarification.

It makes sense what you are both saying. Another question:
Is it possible for an engine, at one point in time, to perform well in the vertical position (with proper tuning) and at a later date not perform as well?

While I know there are parts of an engine that will weaken under regular use and need replacement in time, a carburetor doesn't seem like a component that gets put under a lot of pressure and could weaken. I know some of the rubber o-rings might need replacement but can a carb have as much metal fatigue as, say a piston?What makes for a good 'vertical performing' engine (2 or 4-stroke)?Can/does an engines quality determine if it runs well in the vertical position (minus any pumps)?

Thanks again...

MinnFlyer

My Feedback: (4)

Well Andrew, theoretically any one of a million things can cause an engine to run well one day and poorly the next. For example, weather can be a factor. An engine will run differently on a hot, humid day than it will on a cold, dry day.

There could be an air leak, OR consider this: If there is a pinhole in the fuel line right at the neck inside the fuel tank, It will not suck air when the plane is level, but it WILL suck air when the nose is up. Here 's a picture I did a while back. In this case, the fuel line has fallen off, but it will give you an idea of what I mean

jessiej

When I raced motorcycles we used a similar term; WFO. Ment wide Flat Open or something like that.

jess

1320Fastback

The WFO I know doesnt mean Flat

Hobbsy

My Feedback: (102)

Set it up like this, flip fuel tank upside down in test stand then flip test stand upside down.

slopoke

My Feedback: (16)

Or,
just turn the camera upside down


Sorry, couldn't help myself

Bax

My Feedback: (11)

The engine will actually perform somewhat differently from day to day, as the weather changes. Temperature, humidity, and air pressure all factor in an engine's tuning. That's because we're using alcohol-based fuels. They require less mass of air per given mass of fuel than gasoline, so small changes in the air will affect the fuel mixture more. I've seen large changes in needle settings when the weather changed from hot and humid on one day to cool and dry the very next day.

That's why you need to check the engine's needle settings at the beginning of every flying session. They are NOT "set and forget".

apwachholz

hugger-4641

My Feedback: (6)

Larger displacement engines and gassers are not as sensitive to weather changes as smaller glow engines are, so you won't see the large scale guys doing it. I quite often tune my .46 to .60 sized two stroke engines by holding them vertical, but I hold them high enough to keep the prop above my head. If the prop does come off, it usually goes mostly forward and slightly to one side. I don't tune my larger 4 strokes this way, I use my tach when tuning them. They are too large to safely and easily hold vertical, so I leave them on the ground, but they don't seem to require adjustment as often as the smaller two strokes do.