Were to put my stab in my two meter F3A pattern plane , I have seen some designs that put it between the thrust line and wing center line and some that put it in line with the wing center line can someone please help with this problem thanks

assumptions--plane needs to fly a straight, unmixed knife edge; stab trails the wing and is actually in the plane's tail.
depends on several factors: wing needs to have correct dihedral and placement vertically such that pitching isn't confounded by rolling upon rudder application. Downthrust--just enough to keep a straight power-on vertical up line (pitching to canopy or belly). Fuse shape--large, bulbous fuse topside and flat bottom side in general don't require a subfin on rudder and often require a dorsal on top.

What you should be trying to do is to raise center of pressure on the fin-rudder surfaces. Lower stab than thrustline helps with this type fuse because it exposes more fin-rudder above the stab. Increasing fin span would help (also increases rudder span and less below and more above stab). Often an aerodynamic balance on rudder helps the bulbous topside fuses.

For this fuse type the tail should fly sl. high in S&L. That requires either positive inc. in stab (about 1/2 deg is enough usually)

If your fuse was more balanced laterally in terms of area distribution, then rudder-fin area could be more evenly distributed about the stab. This fuse type often could use some subfin in tail.

There's alot to it and it's hard to get it exact the first time without cutting and regluing. Something you may wantto play with is small chuck gliders built to your correct scale planform, in which you change the relationships of the surfaces. There's alot more of course, but this will get you going.

MattK

thanks matt for your help, I have the plans for a gator r/c typhoon 2000, and the stab is in line with the wing and that seemed kinda different for some reason. i plan on building the model to fly pattern contest again, its been a number of years ago, but the 2meter rule was not in place in pattern. I think it well fly fine, if not its just balsa Thanks again for your help.

You didn't mention if your engine is inline with the wing.

A lot of pattern a/c are set up with 0-0-0. Engine-wing-stab.

That layout idea starts with the thought that you want everything the same inverted as upright. And of course the thought goes on from there to other attitudes. But the thought then thinks about the downwash from the wing and wanting to place the stab equally to that when upright or inverted. Well, the downwash from the wing is affected by the propwash. If the prop thrust is above or below, then lining up the stab to the wings downwash gets to be a lot more complex. So the whole exercise actually starts with where the engine is relative to the c/l of the wing.

I think that when the thrust line is biased in one direction, the stab can be biased in the same direction if you wish to have it equally effective right side up to upside down. How much to displace it is a real problem.

Horizontal tail in line with wing helps inverted flight trim. If the horizontal is right in line with the wing then it sees the same downwash from the wing during upright or inverted flight. The downwash is always helping the nose up. It decreases the elevator trim difference between right side up and inverted slightly.

Multiflyer

In reply to darock, I have moved the engine vertically +/- 1/4" from some nominal location and the stab +/- 3/4" vertically, and neither of those moves changed the knife edge trim much.

When moving the engine a larger distance than 1/4", the move was confounded by inadequate/incorrect dihedral which then produced additional unwanted effects in roll and pitch upon rudder application. It didn't matter whether the model was in S&L of knife flight. As far as the stab is concerned, 3/4" isn't enough vertical movement. However, any more movement than that causes other undesired effects in overall flying quality of the model, and wasn't pursued (tail wobble in roll for example)

After getting the engine at the correct location such that no roll couple was effected, 2 things helped the pitch to landing gear problem for a given fuse design (a bulbous top fuse with essentially a flat bottom): 1) adding a dorsal fin about 12"Lx1 1/2"Deep in front of the fin at its base. 2) an increase in rudder span above the stab (added an aerodynamic balance). Both of these moves essentially raise the CP of the vertical tail, such that the vertical down moment when rudder is applied is greater than before the changes.

To boot, the balance on top of the rudder increased rudder effectiveness such that snapping maneuvers improved (promoted quicker inboard wing panel stall and recovery).


MattK