sledman

My Feedback: (2)

I have a spare wing set for my 4 Star 60, and have heard of people cutting one or two sections off the end of the wings to make a sportier, faster, and more aerobatic plane. Anybody have experience with this? I think it would be like a milder version of a Something Extra if I did this. Comments anybody?

southern_touch99

I have heard about this as well, and from what I hear you need to totally flatten the wing. As you well know most 4 stars have a little diheadral. This should be taken out and perhaps shortening the wing. I also know that you are supposed to shift the CG farther back with this set up and the four star will do one heck of a flat spin.

FHHuber

don't remove all the dihedral.. If you do you end up with adverse roll coupling with rudder. (rolls right with left rudder) It needs at least 60% of the stock dihedral angle.

Built stock, except for reducing dihedral to 75% of what is shown in the planes, powered with a .91 4-stroke (using a 14X6 APC prop)and then putting retracts in the wing, the 4*60 flies like its on rails. [img]i/expressions/face-icon-small-wink.gif[/img] Its a great Pattern training aircraft. Only clip the wings if you plan on using it for Fun-Fly competition.

southern_touch99

will the previous poster please expain the coupling more. I always take all of the diheadral out on all of my airplanes and never have a problem with it....for instance sig senior no diheadral & super enlarged surfaces will knife edge like there is no tommarrow, as well as sustain hover for about 15 sec. or so. And as far as retracts go well that depends on if you want a pattern ship or a fun fly. Personally I love the fun fly stuff so...

sledman

My Feedback: (2)

I've got a Tai Ji pattern plane already, and I've got a fun fly I'm assembling right now, a Modeltech (Global) Magic Extra 300. The wing is done and mounted, the tail feathers are done, tank assembled, gear assembled, and I'll start hinging control surfaces tonight. The extra wing for the Four Star will probably stay that way....an extra in case of damage. I may sell off the Four Star and replace it with a World Models Super Sports Senior. It looks like it might be a larger, nicer, more advanced version of the Four Star. Any experience with that one?

FHHuber

Sig Kadet Sr... the wing is high mounted, you can actually give it a little anhedral (upside down dihedral) and still have positive roll coupling to the rudder.

Roll coupling is a function of fuselage shape, rudder shape (and placement compared to vertical CG) wing location and dihedral/anhedral angle. (plus any wash-out/wash-in... a bunch of stuff...)

Always comparing what happens with rudder input, and ignoring factors not included in the individual item:

Lets also always assume (big assumption...) the CG is on the fuselage "center" reference line... if the engine is at 0 offset... the crankshaft being extended would go through the CG.

If the rudder is completely above that line, it inherantly give some adverse roll coupling (with high wing planes, the CG is usually above that line, low wing planes a little below) Most planes the rudder does give this adverse coupling factor, but its usually small.

Now, assuming no dihedral, if the wing is on the above line (mid-wing) it tends to not need any dihedral to counter adverse roll If the wing is above that line, it tends to automaticlygive a bit of positive roll coupling. If it is below that line it tends to give adverse roll coupling.

Dihedral always gives a positive roll coupling effect. Anhedral gives a negative roll coupling effect. Note that each effect value changes with airspeed...

When the aircraft is designed... you add up all the positive and negative roll coupling factors. If the total is zero.. the plane simply kicks the tail to one side with rudder input (usually achieved only with mid-wing or biplane designs) At some given airspeed the design can fly knife-edge with no aileron correction. [img]i/expressions/face-icon-small-smile.gif[/img] If the effects total out positive, then the plane tends to roll the same way the rudder is deflected.

Every design is a mix of compromises... and usually the designers will ensure a positive rudder-roll coupling.

LouW

As usual, FHHuber has got it right. What some modelers call roll coupling, engineers call lateral stability. It is simply the tendency for the airplane to roll away from a sideslip. It is most commonly achieved by building some physical dihedral into the wing, although it can also be accomplished by other means, such as sweep (or placement of the center of lateral area as FHHuber described). In general a little dihedral effect makes an airplane easier and more pleasant to fly. The problem comes in knife-edge flight.

In knife-edge flight, the airplane is in a continuous rather extreme sideslip, and if it has appreciable “dihedral effect” it wants to roll back towards wings level. This roll must be resisted with the ailerons. Knife-edge in this case can be flown but it takes a little coordination between rudder and aileron. In most other flight conditions a little “dihedral effect” is desirable. If the wings are drooped such that the airplane tends to roll into a sideslip (negative “dihedral effect”) the airplane will require constant attention on the sticks and if it is very much can be a real ***** to control. Knife-edge would be more difficult also, as it would now try to roll inverted and would require coordination of rudder and ailerons, but in an unusual way.

If you notice, a lot of pattern airplanes use a little sweep to achieve a little dihedral effect which has the advantage of acting the same when the airplane is upside down as well as right side up. The goal is to enable knife-edge flight with a minimum of coupling, and enough stability for smoothness in other flight regimes. Regarding 3D or fun fly machines, almost anything goes. They are not so much flown as thrown around the sky with an excess of power which can overcome a lot of adverse stuff.