I haven't yet had a maiden flight with my trainer because of high winds today but also because one thing that still didn't seem right when I was taxiing around was the throttle control.

The throttle idles with the stick all the way down fine, but with only a couple clicks or bumps forward goes to half throttle (and alot of thrust) even though the stick is not even close to half way. At half way with the stick , the throttle seems like it's full out with tons of thrust.

Any thoughts on this or how to set the throttle control so it's running half with the stick half way instead of just a couple clicks forward? Can I use expo on my digital Futaba 7C 2.4GHz radio to accomplish this?

Thanks in advance!!

Check your throws on your servo and on the throttle linkage. Chances are you have the throttle cable (or control rod) connected too far out on the servo horn and not far enough away on the throttle linkage at the engine. Check this first before you mess around with Expo or EPA. Have someone at the field help you out too. Is was this an ARF or a kit? Was the engine mounted in the plane when you got it?
Good luck,
Curtis

cap,

one way is to use your radios end points to adjust the amount the throttle servo moves. and make adjustments to the control rod until you get the throw you want.



bassman

It was an ARF which came with engine mounts but they weren't installed. I put the OS Max 46AX engine in the plane using the supplied motor mounts.

It's not the engine mounts that need to be adjusted. It's your throttle linkage. What you ideally want is a smooth, linear opening of the carb as you advance the throttle. Use the differently spaced holes on both the servo arm and the throttle arm to come up with the right amount of throw. Adjust the length of the pushrod as needed. Once you get it as close as you can mechanically, you'll then adjust with the travel adjustments on the radio.

A lot of times these sorts of things can be explained easier in person. Whomever is helping you should be able to assist you right out at the field, and you'll have the whole thing done and behaving properly in about five minutes.

In order to get a good amount of torque out of the servo, I would recomend moving the attachment point of the throttle linkage closer to the center of the servo arm from where it is right now, while keeping the mounting location at the same point on the engine. This will decrease your throw (shorter radius) and increase force on the throttle linkage. Keep doing this until you have just enough throw (a little more than you need preferably), then limit the throw finally with EPA. Make sure you set your link lengths with your EPA set at deafult (100% in futaba world). I wouldn't recommend using EXPO on the throttle. Not sure you can on most computer radios.
best of luck,
Curtis

When you say EPA are you talking about End Points? Futaba label for End Points on the digital radio menu is E.POINT and for Exponential it's EXP. When you say EPA I assume you have Spectrum or JR and it's End Points you're talking about, correct?

Yes, they are talking about End Point Adjustments.

You DO NOT want to use Expo on the throttle. As was pointed out several times above, you need to make sure that you have smooth linear mechanical setup on the throttle.

Ken

Thanks, Ken.

I'll wait until I get some help at the field. For now the only thing I can do is move the throttle linkage on the servo end in towards the center. It's already out all the way at the carb end.
I seem to be getting very smooth linear motion now ie. when the stick is at low end the carb opening is barely open, at half stick, it's half open, and at full stick it's wide open, which is the strange thing. Maybe I'm running too lean? I have the engine running a little on the rich side according to what I was told at the field - about three and a half rotations out from all the way in on the needle. The engine is running great, alot of power and smooth at low end and high rpms as well. It just seems to have alot of pull and high rpms at only a few clicks of throttle on the stick. I don't know, maybe it's just me since I don't know what it's supposed to be like. I do know that on the club's trainer which is also a LT-40 with a 40 size engine (mine is a 46), it doesn't work that way so it doesn't seem right.

Oh well, like I said I'll ask for a few opinions at the field from the guys I trust the most.

You can adjust the throw by simply rotating the position of the servo arm.

Remove the pushrod, move the servo to idle position, remove the servo arm, and rotate it so it almost points straight back (If the idle position is back, if the idle position of the servo is forward, point the arm so it is almost straight forward)

This will give less movement during the first half of the throw and more on the last half.

Aren't those two quotes contradictory? I'm not trying to start a war, it's just I have the utmost respect for each of your opinions and I'm trying to figure out if I'm missing something here.

Isn't Mike's technique kind of a "poor man's exponential rate"? Again, not trying to start a war... just trying to figure it all out.

Yes, it IS a poor man's exponential.

At the risk of correcting Ken, you want linear throttle RESPONSE, not movement. This is why many high-end radios give you several-point throttle adjustment.

In a perfect world, a carb that is 1/4 open will give you 1/4 power. Ditto for 1/2 and 3/4. I've yet to see an engine deliver that kind of exacting performance.

In cappaj1's situation, small inputs on the low end give big results. It could be he is starting with the servo arm at 90 degrees (at idle) which will give the MOST amount of throw for the first half of travel and much less toward the end of travel (High end)

If this is the case, rotating the servo arm so you get less travel at low end and more at high end will solve the problem.

I'm not a poor man though I have expo on my radio available on all four channels including throttle.

I'll try doing it the suggested way though with other's help at the field, and with the linkage connections close to the center of the servo and servo arm position and let you know how I make out.

Thanks Mike. He is absolutely correct. It was a typo on my part when I said that.

Ken

Thanks Mike & Ken. You guys are the best. I love the videos. Best money I've spent so far in this hobby.

Jim,
Good luck. I'm in the same boat as you (about to make the first flight with my first plane).

Exponential functions are great where they're appropriate, but probably not on the throttle. MinnFlyer's suggestion is much more akin to a multi-point mix.

Expo changes the amount of travel for different amounts of stick movements. Think of a stick having ten different positions to the right and ten to the left. A linear setup results in a servo moving one step for every step that the stick moves. Expo changes that. You can set it up so that movements closer to the center of the stick result in less movement of the servo. One step to the left on the stick might result in 1/2 step to the left on the servo. Because of the exponential nature of the function, steps further from the center of the stick result in greater amounts of movement of the servo. With a given expo curve you might hit a linear 1:1 movement only at step five to the right or left, with steps closer to the center resulting in less servo movement. Steps beyond five result in more servo movement, increasing as one gets further to the limits of the stick. At step ten on the stick, you might have something like a 1:2 movement, where when moving between the ninth or tenth step of the stick your servo moves from position eight to position ten.

In other words, close to the center of the stick the movements of the servo are less than proportional, while towards the extremes the servo movements are more than proportional. It's helpful for "softening" the sticks around the center, and often makes for smoother flying. I use expo in lieu of dual rates. I tend to think that dual rates are just accidents waiting to happen.

Expo on a throttle would not be useful.

Multi-point mixes, however, can be useful. If your engine's throttle response isn't linear, you create a mix that makes the throttle response linear by making the servo move shorter or further distances at different points along the stick's position. It's not a common thing to do.

Cap,

I think you might have mulitple issues that are making your throttle so abupt on the low end, but once you set it up so it is mechanically just about right, then you might be able to improve it even more by using expo on your throttle.

I have a Futaba T7C and do have my throttle set to +25% expo.

before expo it seemed to me that I already had about 3/4 power while the throttle stick and carb were only at 1/2. And also there was almost no noticable power difference between 3/4 throttle and full throttle. My expo setting seems to delay the carb opening at low stick positions so I have more percision about idle speeds and the expo accelerates the throttle opening at high stick positions. At least with my set-up it seems to make the stick position better correlate to the actual power output of my engine.


EDIT: I just read Mikes post above. I see where the confusion is comming from. I will get out my manual and double check but I am quite sure that on this raido the expo setting for the throttle is not based on the center point like expo on Ail, Rud, and Elevators are. The expo for the throttle is based off the end-point, so it is designed to smooth out one end while accelerating only one end.

A typical rotating barrel carb is going to be close to full power by the time it is 65% open. The remaining rotation is going to have much less effect per degree of rotation. One only needs to look at the throttle opening and understand the amount of area open. It has a built-in curve with the greatest amount of change (per degree) being near mid-travel. As you come off idle the increase in area is increasing exponentially per degree until half throttle at which point the increase per degree reduces exponentially. A servo arm also gives the most displacement (throw) at a point 90 degrees to the thrust line (mid-point).

I think this subject is going a little beyond what the original intent was for, but I am curious about this subject. Bruce, I understand what you are saying about the throttle being almost fully open (probably around 90%) effective area when the barrel is rotated only roughly 65% on most engines, but what is there to say about the spray bar? I am under the impression that as the throttle rotates, the needle and seat move in relation to each other creating a larger orifice when open. Does 90% of this get accomplished by the time the barrel is rotated 65% or so?
Just curious,
Curtis

Interesting question.

I guess one could tach the engine at different "openings" and adjust the expo accordingly to achieve a linear response to input?

This would depend on the exact design of the carb. But YES, I think it would in most cases. I have no empirical proof of this however. With more air you need more fuel, this much is obvious. And then you adjust the HS needle for fine tuning. Most carbs have HS and LS adjustments for mixture but there are a few with mid-range needles/adjustments. I do not recall the brands at this time. The slope/angle of the groove (cam effect) in the barrel controls the rate of change in the needle to spraybar spacing and the mfg could tune midrange some through that.

Thanks for the responses!

We finally maidened my LT-40 today after making linkage adjustments and servo arm position changes only to the throttle control.

What we ended up doing was moving the linkage first to the innermost hole, then the middle and finally back out to the outermost hole on the servo arm and the outermost hole on the carb end. As MinnFlyer suggested moving the position of the servo arm back helped, but not as much as I had hoped.

I am still getting alot more response from the movement at the very beginning of the throw but today, I just accepted it and flew it that way. I have end points set for the throttle control but no expo at this point. I might fool around with that tomorrow though. If so I'll report my results.

The opening in the carb barrel IS proportional to the throw of the throttle linkage and arm. In other words, when the stick is at idle the barrel is barely open, when it's at 1/4 the barrel is 1/4 open, when at 1/2 the barrel is 1/2 open and so on. But the engine response isn't proportional. This is what I was trying to explain before. With only a few clicks (now) above idle, the engine begins to race too quickly. We put a tach on the prop and sure enough it showed the rpms of the engine were not proportional to the stick movement. There is a logarithmic response where the engine rpm's climb their most at the lower end of the stick movement.

I realize this is normal but on my airplane for some reason this effect seems more pronounced. That is why I thought the expo could help.

One other thing that helped on the maiden flight and subsequent flight today was setting the engine to run a little on the rich side. Tomorrow, if the weather is good and it's supposed to be, I'll be fooling with it just a little more. I'm trying to make very small adjustments and since the engine has only had four tanks of fuel through it now, run it on the rich side anyway.

BTW, you never mentioned what engine you're using. Maybe you have an air leak in the carb seat?

Also, if rotating the servo arm helped, you can always turn it some more - the more you turn it, the more noticable the change will be.

Thanks, MinnFlyer,

You had the best suggestion. I'll try rotating the servo a little more but I think I'm already at the farthest end of it's travel. Maybe not - that'd be at 9 oclock wouldn't it? And I'm probably at 8 oclock.

Oh, the engine is a OS Max 46AX. Nothing out of the ordinary I would think. The only thing is, it's a little more than needed for the LT-40.

Well, I am going to disagree with those saying not to use the expo.
Since your throttle opening is linear to throttle stick movement, I see no reason not to experiment with the expo to see if you can tune the rpm to match the throttle stick position.
While I think the method Minnflyer showed you with the servo arm positioning is a good place to start, it will only get you part way there.
I say copy your setup that has now been test flown into a new model memory and try adding the expo to the throttle on the new memory location, that way your original setup will be intact and your experiment won't cause you any grief if it does not work.
The worst that can happen is it won't work the way you want it to.
If that is the case just blow the model out of that memory location and you have lost nothing.
I am surprised that the 7c does not have an adjustable throttle curve in the ACRO programming, but my quick look at the online manual indicated it was a HELI only program.
It was years before I had a radio that had the adjustable throttle curve, I did what I could with the mechanical expo but there has always been a lot faster throttle response at the low end. When I finally programmed in that linear response I found out I did not like it at all.
Maybe it is because I have never had linear throttle response, but it did not suit my tastes so I went back to what I am used to.