My exact questions seem not answered in threads on similar topics or in the computer radio manual (Futaba 9CAP).
Assume the elevator's mechanical linkages throw 1.5" each way, and that 1" is desired. The clevis is at the absolute top of the elevator control horn (away from the elevator surface), and on the servo arm the rod linkage is three holes out from the servo spindle (so throw could be reduced by moving the rod linkage in two holes closer to the spindle).

1. APPROACH A: I could use the computer radio's end point adjustment to reduce throw to 1" -- assume the EPA adjustment needs to be set at 66% to do this. APPROACH B: But I could alternatively leave the EPA set at 100% and instead reduce the throw to 1" by reducing the high rate setting on the dual rates menu from 100% to, say, 66%. QUESTION: Is there any difference at all between these Approach A and Approach B?

2. If I move the servo arm linkage in as far as possible, to the arm's innermost hole (next to the servo spindle), I can get closer to the desired 1" setting. Should I do this before attempting any computer EPA/dual rates adjustments? (Putting the linkage right next to the spindle seems like it would give very little power to the control surface.)

3. Is the EPA set as a % of the regularly available servo arm movement?

4. If I use dual rates to set a high rate of 80% -- does that mean it's 80% of the EPA setting?

5. Is the regularly available servo arm movement about 40 degrees each way, that is, describing an arc of 80 degrees?


Thank you for any help you can give.

I'm assuming this is a trainer, so it will be a while till you need more than the recommended throws. I suggest leaving the dual rates alone and fixing the problem mechanically. Reduce the throw at the servo end. Do it one hole at a time until it's right. It will have plenty of power.

using the epa to reduce servo through reduces the controls resolution for fine movements. The farther the linkage is away from the spindle the greater the force on the gears of the servo (gears can throw teeth) the servo hads its maximum strength when it is near the spindle but you loose fine control of the servo. The control horn get max strength put on it on the outer hole but requires more movement to get the same angular movement. (mechanical adjustment is the prefered method of adjustment)

epa is % of standard servo movement

yes 80 % of the epa

Really what you want to do is set the Endpoint adjustment at 140 and then set the throw limit mechanically, you shouldnt need multiple rates on a trainer. But you always want you maximum throw (highes rate) set at the maximum resolution so that you take full advantage of the sensitvity of the Tx. Really well setup planes are almost always setup like this.





To affirm what has already been said, and this is a little advanced so dont let it confuse you. Save this information for after you are ready to move on to a more aerobatic plane.

IF you do use dual or triple rates, set the enpoint adjustment for that surface or pair of surfaces(ie ailerons should be the same EP for each aileron) at 140 or whatever max EP is on your Tx. Then set the highest rate in the DR/Expo screen at 140 and zero expo to start. Then make adjustments mechanically to put the high rate throw where you want it by 1st moving in on the control horn, and then out on the servo arm. Also try to make your adjustments to balance the throw mechanically if possible and only use subtrims once you cant get any closer mechanically. Mechanically imbalanced throws that are corrected electronically result in poor performance most of the time. You end up fighting weird coupling issues and all sorts of nuisance problems.

Once you get the highest rate done pick a dual or triple rate switch and assign the various rates to the same switch, and dial down your DR setting in the DR/Expo screen to get the desired throw on the lower rates. .

If possible it is desirable to move the pushrod out on the servo arm as little as possible to get the desired throw and no more. I would move in on the control horn as much as possible and still maintain good geometry before I started moving out on the servo arm. The closer the pushrod is to the centerline of the spline the greater the leverage the servo has and the less strain on the servo gear train. BUT sometimes on 3D planes where you need > 45degrees of throw it is necessary to get a really powerful servo and then use a real long servo horn. Sometimes as long as 2" to be able to get the throw you need.

Let me add a few things to the info already posted.

Yes, keep the linkage on the outermost hole on the control surface and move to the most practical innermost hole on the servo arm. Although it may seem to you that moving out on the servo arm would give more leverage, it is actually just the opposite. Always keep the servo arm as short as possible. This allows the most movement (resolution) and highest power. Lets look at resolution using some arbitrary numbers. Lets say the servo can move to 256 distinct possitions from center to each endpoint. Now when you set the end point to half travel it can only travel to 128 different positions. Set the end point to quarter travel and it can only move to 64 different positions. Set it low enuff and you might start seeing it just from position to position as you move the stick.

Depends on the radio and I have not used a 9C enuff to be sure.

Normal total movement of most servos is 60 degrees each way or 120 total. This is only achieved on some radios by going past 100% on the EPA or other throw settings to 125 or 140.

Ed M.

I agree on most of the posts above, but I think I can simplify it a bit.

1. Dual rates are fine, but always use 100% of the available servo throw for your high rates. (this may be between 120-150%, depending on your radio)

2. Use as close to a 1:1 ratio mechanically. This means if your pushrod clevis is 1/2" from your servo center, put the control surface clevis 1/2" from the bottom of the control horn. This is not a hard and fast rule, but one extreme will give great strength with low deflection, the other will give high deflection with severely less holding power. The latter can cause flutter and destroy the airplane in flight. This is why you should never use EPA to reduce high rates!!!
3. Once your high rate is mechanically set, use your dual-rate adjustment to give you your desired low-rate setting.

I believe I answered this same post in the radio forum.

For most beginners (like me), Dual Rate can be a problem. If you forget it's in High Rate, for instance, you may be in for a surprise once you are airborne, then you can't really take your eyes off the plane to find the switch (because you will be in panic mode and forget what switch does what) so you are going to attempt to land it and set the switch back to low rates, which will always raise your heart rate and blood pressure a few numbers.

Leave both low and high rates at 100%, or at the same rate.. so that accidentally switching from low to high means nothing. After you get used to everything, perhaps after you have burned up about 20 gallons of fuel , you can set your rates so that they are meaningful to you and your flying habits.

Best of luck with that. I lost one a few weeks ago exactly because of dual rates accidentally set at high, wind was obnoxious, the throttle was set to low for the landing condition (obnoxious head wind on landing), and I was in panic mode. A bad combination. Remove one item, dual rates, and perhaps I would still have that Goldberg Skylark 56.

dicksoucy,

Based on your account of this flight, your problem was not with dual rates it was with ignoring the wind and failure to perform a simple preflight of your plane and your radio. A new flyer, which I believe you say you are, should not be flying under the conditions you describe.

I strongly suggest you add a preflight plane and radio check to your standard flight procedures.

I have a preflight that I go through on every flight. It only takes a few seconds but it has saved me from problems countless times. On some occasions when I get sloppy or lazy and forget to do there were problems. Those problems were my fault, not the the complication of switches that caused the problem.

I once crashed a sailplane on a winch launch because I had the wrong profile up on my computer radio. My standard preflight would have shown me that, but I skipped it.

Was it the computer radio that caused the crash? No, it was my failure to meet my responsabilites as pilot. Full scale or model, the pilot is still the one responsible for deciding if the plane and controls are ready for flight and whether the the prevailing conditions are acceptible.

The lesson that should have come out of your incident was that you need to check your plane and your controls before every flight. You need to look at the wind and the weather before every fight. And if you fail to do it, it is your fault if the filght did not go well.

I am sure you will not like this response, but this is something you need to learn.