senrak

Hey,
I would like to see some imput on the pros and cons of using all the electronics that are offered on today's computer radios. I am amazed at the number of things that my JR XP7202 will do, but I have questions.
Do I really want to mix elevator to flaps? Does mixing rudder to aileron really make a difference? And why would I want to mix two throttle channels?
The instruction manual tells us how to do this stuff, but it never talks about why.

Are there any knowlegable people [8D] out there that know about the whys? [X(]

bigedmustafa

My Feedback: (2)

There are plenty of knowledgable folks out there. I can offer some rudimentary examples of "why" and perhaps some folks more knowledgable than I can add details.

elevator > flap mixing may be useful on some models where activating the flaps causes it to "balloon" into a nose up position. A little bit of negative elevator mixed with flaps and the plane will slow down without its nose attitude being affected.

Mixing aileron > rudder will smooth out flat turns and reduce adverse yaw. It is also handy from removing coupling tendancies from knife edge manuvers. Mixing aileron with rudder would be useful on everything from your basic trainer to your scale J3 Cub to your fully 3D Edge 540.

As for mixing two throttle channels, this is used for evening out the engines or motors on twin engine models. Considering Hobbico is still selling it's Twinstar ARF for $109, this isn't really an exotic feature. There are a number of affordable twin engine ARFs on the market, and throttle mixing would be quite handy for most all of them.

Hopefully more folks will chime in with additional useful mixes.

w8ye

My Feedback: (16)

One of the great features of a computer radio is EXPONENTIAL

Exponential makes the controls less sensitive at the neutral point

Due to the Ackerman effect of our control linkages they are most sensitive at the neutral point and this makes them hard to fly

With exponential you can make them very easy to fly.

I just run high rates all the time on my models with a lot of exponential and they are easy to fly yet will really cut up the action if you desire

senrak

Okay,
I understand the exponential effect because I program that into my setups, and the initial slow movement helps me alot with control of my touchier models. I wish I had had it on my Chipmunk. Maybe I wouldn't have put it into the ground on it's maiden flight.
But how does the Ackerman effect play a roll on an aircraft? Isn't that an issue on 4 wheelers and steering control? Linear and axial movement? I don't understand how it relates. Please explain.

w8ye

My Feedback: (16)

When a servo is centered, the linear travel of the push rod is greatest about the center position. The more you get towards 90 degree travel of the servo the less the push rod travels. At 90 degrees servo rotation there would be no travel.

This is referred to as the Ackerman effect by some modelers. Physicists or math majors may take issue with this for besides being a term in car steering situations there is also a Ackerman Function in mathematics.

senrak

Oh. Okay,

So it is a mechanical aspect thing. I was thinking aerodynamic. We have an axial movement causing a linear movement. The farther the rotating servo arm moves from the 90 degree angle with the pushrod, the slower it moves. The same would hold true at the control surface end of the pushrod, actually doubling the effect. Exponential in effect makes the control surface movement consistant, counteracting the Acherman effect, which is inherent to the design.

Correct me if I'm wrong.

Realizing this, wouldn't a linear motor be a better servo? Or, the motor attached directly to the axis of the control surface?

I think I got it. Knowing this I believe I will throw away any radio I have that doesn't have exponential capability.

w8ye

My Feedback: (16)

That sounds good to me

The first servos in the 60's did have linear travel. There were big and there was a connector tab that traveled from the middle towards each end

But they were soon replaced with the current style because they weighed less

rcgreenhornet

hey i love my big gasser boat and been flying for years well anyways i was out flying with a oldtimmer and told him that i should be getting my new team/pro with the m/5 engine so he asked what kind of raido came with it well it came with the Magnum 2/PL programmable computer radio by futaba now when i told him it was a AM radio he told me that a am radio should not be used on water or in a boat so he gave me a airtronics caliber 93827 27 band FM TXRF MODULE CX2P caliber 2 ch racing system radio with a failsafe to shut down to idle if lost single or volts and a HITEC hfs 03mm recever now i tryed both of them and i like the airtronics radio better so can you help me with wich one is better of big gasser boats thanks

dhal22

this is an airplane forum.

Bozarth

My Feedback: (15)

Don't forget about the opposite effect at the control end of the pushrods. Radial to linear at the servo, linear to radial at the control surface. Trig at both ends.

Kurt

w8ye

My Feedback: (16)

On a sport plane it never seems to come out even because of the smaller amount of surface deflection compared with servo arm movement.

Bozarth

My Feedback: (15)

But that's because of geometry, not "Akerman's function". Good eats.

Kurt

w8ye

My Feedback: (16)

What ever you want to call it. The geometry doesn't come out in your favor.

Bozarth

My Feedback: (15)

The only thing I'm calling it is "proportional." Your reference to Akerman's function implied that control surface movement is not proportional to rotary (angular) servo movement. It is. Expo changes the proportionality in our "favor." Even better eats. [8D]


Kurt

senrak

Okay. Now that we have that kinda figured out.
What about two servos on the same channel via a wye or a spliter vs different channels?
I have had modelers tell me that if you have two rudder servos it is better to have them on individual channels. Like channel 4 and 7.
The only reason I can see for that is to be able to adjust trims separately. Is there another valid reason?

w8ye

My Feedback: (16)

None that I can think of

senrak

(Quoting Kurt)
Don't forget about the opposite effect at the control end of the pushrods. Radial to linear at the servo, linear to radial at the control surface. Trig at both ends.

I was thinking about that.

The movement of the two arms between a pushrod would always be proportional. The movement may not be equal depending on which hole on the arm is used, but neither is important in the conversation about the ackerman effect.

The important thing would be to slow down the movement around center. As w8ye stated that is were it is the greatest, and were it becomes important that it be minimal.



Dale

FlyerInOKC

My Feedback: (6)

Back in the late 60s early seventies my first radio was a Heathkit three channel radio. It came with the Heathkit version of the Kraft KPS-9 servos. These things had a arm like modern servos plus two atachment points that went back and forth in a straight line. to give you an idea of the size place two Futaba servos end to end and you would be about right on the size. They worked nice and i got an "A" in a high school electronics class for successfully building the kit and flying with it.

Bozarth

My Feedback: (15)

"...movement around center. As w8ye stated that is were it is the greatest..."

Not true. Movement around (near) center is not greater than that at the greater throws. i.e. 10% movement of stick equals 10% movement of control surface, and so on. It's a proportional result until you add expo. FlyerInOKC's old (respectfully speaking!) straight line attachment points on the servo set up would not create a linear response at the control surface but rather one with a softer center (sine curve at it's purest). Fun stuff!

Kurt

w8ye

My Feedback: (16)

Exponential sure corrects all the problems with our control systems and makes the models more fun to fly.

Charlie P.

Unless your servos are almost exactly matched and the control linkages ARE exactly matched they will end up fighting each other. Pit a pair of 80 oz-in servos against each other with even a slight imbalance and the first loser will be your battery voltage and available flight time, the battle also occurs at the control horn mount screws, the balsa around the control horn backing plate or dowel or the servo teeth if it is a beey horn & mount. The problem will go away after prolonged use when the mounting holes have elongated, but then something just won't feel right in your control response and you'll think it is a servo not returning to center properly.

If you use a single channel for two servos instead of a simple "Y" use a servo reverser that allows one to be tuned to match the other.

w8ye

My Feedback: (16)

Servo reverser $8.95

http://www.horizonhobby.com/Products...ProdID=EXRA325

Or you can use digital servos and reprogram them to match?

Bozarth

My Feedback: (15)

Agreed!!! I'm spoiled by computer radios and the remote for my tv.

Kurt

TROUBLED 61sf

My Feedback: (1)

I SAY USE WHAT WORK AND FORGET THE REST IF YOU LIST TO ALL AND WHAT YOU WANT AND LEAVE THE REST IT WILL MAKE FOR THE BEST I USE A FUTABA 9 Z IT SEEMS TO BE THE BEST FOR ME I CAN ONLY TELL YOU TO GET WITH YOUR CLUB PRO AND HAVE THEM SIT DOWN WITH YOU AND YOUR MANUL AND SET THINGS UP THE WAY YOU CAN AND ARE ABLE TO FLY WHAT WORK FOR ME MAY NOT FOR YOU ALL IN ALL HAVE FUN AND THE SKY IS THE LIMITS COMPUTER RADIOS ARE FUN BUT THEY ALSO CAN CONFUSE SOME THAT HAVENT HAD MUCH TIME WITH THEM SO FRIEND A LITTLE UP THEN TURN LEFT AND HAMMER THE GAS

senrak

What's that. Digital servos are programable?



DW