I have a couple planes and I've noticed the same thing with both of them. I adjusted the throttle so that the idle is nice and low, the throttle cut works perfectly, and when the stick is at full travel the carb opening is all the way open .... and everything works okay .... the problem I have is that it seems that I am at full throttle just a bit past halfway on the stick. The engine will be maxed out before the opening is all the way open. Is there a way to adjust this so I can have more travel? Some kind of throttle curve or something? Should I adjust the end point to less than full open on the carb and more in line with when the engine actually hits full power? Am I crazy or is this common?

I know what you mean. About a year or so back,a bunch of us here got on this subject of going off tach readings of where the barrel is when its at full rpm vs the barrel full open. We thought about dialing in the high end needle when the rpms are at the maxx and the barrel in whatever position it is at that point short of being wide open.

Would be less air going in but nobody ever really tried it to see how the engine would react. You want too? Anyways repositioning the rod in the servo arm will take care of it.

Kevin, I find that the throttle gives me the most problems. I've found that the best sequence is to set up a servo arm that is the same length as the throttle arm. Set you servo up with 100% travel and no expo. Set both end points at 100%. Now with the linkage loose, go full throttle. Swing the throttle arm to full open and tighten things down. Now one thing first, make sure your idle stop is backed off, and from the sounds of your ability to kill the engine, it must be. Set the throttle at idle and make sure it is really idle. From here on, you need the engine running. The top end should be OK. You can tweek the end point to get full throw, making sure you are not stalling the servo. The idle should adust with the trim now. and shoot for netural trim if possible. THe kill should shut down enough to kill the engine. Just remember that the trim on throttle only effects the idle end of travel. Once you get it set up so the carb is wide open at full throttle and the closed with the kill, you should now have linear travel on the linkage. The engine will react differently, so now you can play with the expo and get the RPM curve flatter. The key is to make the throttle and servo arms equal so the travel is the same

Don

The problem described in the original response is a common misconception. The throttle servo has a linear response to the the transmitter, but if you closely examine the throttle system on the carb, you will notice that it is NOT a linear system. The circular shape of the air opening is by its very nature a non-linear system. The size of the inlet path has a very large change, percentage wise, at the beginning, as the throttle arm is moved. As it approaches fully open, the percentage change is very small. I could probably do some higher math to actually model how it changes, but suffice it to say, that its non linear. Consequently, you get the behavior that the engine seems to hit max RPM before the throttle is fully open. There is actually a small change as it gets to full throttle, but its generally beyond the capability of the average homo sapien to distinguish without some experience.

I suppose it would be possible to program in a throttle curve using a computer radio and a tachometer such that there is a linear change in RPMs as the stick moves, but frankly I think its more trouble than its worth.

Fly more, worry less.

Brad

I dialed in some expo on the throttle of a couple of my planes for this reason. Try 30-40% and see if it seems a little more linear to you.

Know why?

Very often the limiting factor on the engine is a very simple thing. It's displacement. Nothing more, nothing less.

And why then does the carb opening work so lame right at almost full to full?
Because the engine designers know a couple of things that we're not considering. Like density altitude. And the range of temperatures their customers are going to encounter. And they deal with those problems by putting more venturi opening in the carb than the engine displacement is going to need at the lowest elevations or the coldest days.

And very few of us do our tuning and max horsepower comparing while checking the density altitude or what the temperature is for the day.

They place a carb on the suckers that will give guys in hot countries and high flying fields a slightly better chance of using the fixed displacement inside that engine. And most of us who've thought there was something wrong with our throttle response at WOT didn't do the observations on a really hot day at a really high altitude. (and if we did, we'd find out that the engine didn't have enough carb opening and didn't come close to full power at WOT!!)

It makes sense .... the shape and design of the carb does in fact create its own "curve" ... less actual travel in the extremes of the throw ... I think I'll mess with the expo and see if I like it

And there is also the design consideration that bkdavy mentions.

Here are a couple of pictures made up for this same topic awhile back. They basically illustrate his point.

The red area is how much the same movement creates in the carb. Couple of degrees up from low motor in the picture showing a tiny bit of red. And the same amount from WOT.

The movement of a servo however is from a rotation to linear, and at either end of the servo movement, the fore/aft is less for the angular change. You can do a lot to reduce this "softening" by making servo arm connection long so that the servo doesn't go full rotation to get full carb movement.

So let's look just at the throttle opening at WOT. Nothing more.

What limits the engines horsepower.... The displacement and the venturi opening.
If the day is cold and the air dense enough, the venturi opening will be adequate to pass all the gas the displacement will demand. And the venturi might be too large for those conditions, but it won't matter to the full power problem. The displacement will limit the amount of gas into the engine and we'll have to adjust the needle to give the right fuel amount for the right air amount for that venturi size. And we wind up with whatever the engine wants.

But on a hot day and a location with less dense air, the venturi can be what limits the engine. The available air that can get through the venturi won't necessarily be the amount that displacement can use. We still have to adjust the needle to get the right gas density. And if the opening is large enough for that hot day or that flying site elevation, the carb just might give the flyer fits on colder days at lower elevations. Why..... because then the airflow will be lots slower because the venturi size is too great and the engine won't get well mixed air/fuel. And we'll have fits setting the needle, and the lowspeed might also be off. So the carbs don't always suit all conditions. Or each of us's conditions.

Control line competitors noticed this and would carry a set of different size venturi to important contests. R/C engine designers decided to use the carb to vary the opening. But they're stuck with limitations. And they know that most modelers won't notice. So we see full horsepower when the throttle isn't completely open. Or the other way. Depending on how well balanced the carb design is to the engine's gas flow design and the engine's displacement.

And rather than program in a throttle curve (Or if you have a non-computer radio) you can just set your servo arm up to give you less pull in the beginning and more at the end

[sm=thumbup.gif]

DUH!

MinnFlyer ... brilliant ... thats it! So simple ... I love this hobby and this forum! Can't wait to do some tinkering with it this weekend!

Brilliant but simple.

Yup, that's me

Duh ! Sometimes I cant see the forest for the trees. Thanks Minn , perfect fix for my Twist.

I'm still cracked up over "Miniflyer".... pretty funny.

No, Mini Minn

I do the 90 and 90

another thing to consider is the stiffness of the push rod.
If you're using wire type rods, sometimes it'll bow if the jacket is not long enough(during push or opening).
you can add solder to the wire to stiffen it. Or go to metal type push rods and spend a little more time to
make proper bends to minimize binding.

Sometimes the set screw binds againt the carb's barrel groove. this will give wacked throttle reponse.
Sometimes it'll stick, sometimes it won't.
This is where Magnum engines give me the most problems.

you'll notice the same problem with the rudder as you progress.
it's always stronger when the servo is pulling.

a Z bend plier is nice to have