I was advised on another forum that certain planes were coupled. Would someone explain what this means?

Coupling is when one control automatically causes another control to move. For example, on some planes you might want to couple ailerons to rudder. When you move the aileron stick to the right, the rudder also goes to the right. This coupling can vary from a little bit (10%) to a whole lot (100%). The computer radios have this capability. One common form of coupling is when an aerobatic plane pitches to the belly or canopy in knife edge. In this case you want to couple the rudder to the elevator. When the rudder is deflected (either direction), you want to put automatically a little up (or down) elevator at the same time.

I thought it was when two full scale Pitts get together and make a 1/4 scale Pitts.

Is coupling something that is engineered into the plane or a result of the design?

Can you provide the context in which you heard "coupled" ?

On its own it might mean any number of things. E.g. on some planes one control affects things it really shouldn't, like the ailerons making the plane yaw as well as roll. That's one form of coupling. Deliberately connecting controls together is another form etc.

BTW a plane can't be coupled. Control effects can, either deliberately or accidentally.

Steve

If "coupled" has anything to do with tying one flight surface to another, don't worry about it. You don't want to do it.

In reply to my question as to the "right" engine for this kit he added almost I thought as an afterthought the the citobria and decathlon were coupled.

The statement doesn't make any real sense to me and there's not much point just guessing. Wouldn't it be easier to go back to "another forum" and ask whoever wrote it what they meant ?

Steve

I did --- no reply

This is how I understand control coupling: On a theoretical perfect airplane, each control would only influence one motion. The elevator would cause the nose to go up or down (pitch), the ailerons would cause side to side motion (roll), and the rudder would change the angle of the plane in relation to the direction it's moving (yaw). On a real plane, it's not that simple, I think mostly due to wing dihedral and the center of gravity. For example, by design, on a trainer with a lot of dihedral, when you push the rudder one way or the other, the plane banks (roll) as well as turns (yaw).

The normal time people worry about coupling is when you're getting into precision aerobatics. Suppose you're flying on a knife edge across the field. You would like the rudder to just control the yaw (or do you now call it pitch?), so you don't have to worry about the other controls to keep the plane on the edge.

I'm sure there's 100 times more to it, but I think that's a fairly good simple explanation.

What the guy was talking about was being "short coupled". Different concept from control coupling.

the Citabria and Decathlon are, in fact, short coupled. Most acrobatic full scale aircraft are, as would be models of them. Pattern planes are very much NOT short coupled. RC trainers are usually not short coupled.

Ok, so what does it mean?

It means that the tail moment, the distance between the wing and the horizontal tail is "short".

How short is short? I'm sure someone with formal Aero engineering can exactly define it, but if you compare pictures the kinds of airplanes I mentioned above, you can get a pretty good idea pretty quickly.

So, here's the reason it's important. Short coupled airplanes tend to be very sensitive in pitch and yaw. A longer tail usually makes a plane more stable, and more "smooth" and fly a better "groove" in the air. That's why pattern planes have such long tails, it makes them fly smoother. Combine short couping with larger control deflections, and you get fast snaprolls and very quick acting controls. This can make a plane "twitchy" or even feel "unstable".

So, if you are a beginner, you want to avoid short coupled planes, like the Citabria or Decathlon. Also, compared to a typical RC trainer, a scale Cessnea 152 would almost be "short coupled" as well. Full scale planes have structural issues models don't, so it's easier to build a long, light, but strong tail to make the plane fly smoother.

ps, that was a really "short version". Stability and control response are affected by a number of different factors. You can make an extremly short coupled plane "groove" and a plane with a very long tail moment wander all over the place, if you work at it.

Thank you. Short distance between wings and tail make for sensative controlled plane.

That's the truth!

I learned that first hand when I accidentally set the elevator throw on my SlowPoke to 1 inch travel up and down (total 2 inch travel). The maiden flight had me performing 8 foot loops using just the trim tabs! That was the scariest flight I've ever endured.