Wing to stab, engine incidence

[Steven Kientz]

I'm sure this has been brought up before. Is mounting the engine and stab/elevator at 0 deg. to the wing optimal? This is a semi- symetrical wing mid mounted in a profile fuse. The engine is higher than the wing. The model is a Tower hobbies Fun 51 that I'm converting to C/L. It will be powered by a ST51(itailian) or a OS46 LAS. Any opinions are greatly appreciated.

Steve

[Leester]

I have a Fun 51 powered by an OS 40 FP and it has plenty of power. If your gonna use a 46 or 51 HOLD ON. As far as incidence I do believe you want 0.

[Steven Kientz]

Thanks for the reply.I assumed O degrees was right but I've seen posts about engine downthrust and positive stab incidence.
At this time my only other choice for an engine is a TT 36( RC). I've thought about a Evo 36, but haven't decided on that yet.

Steve

[Serge_Krauss]

...but I've seen posts about engine downthrust and positive stab incidence. Steve

I trust this means that you have read the two large threads "across the way." Opinions vary, even among the very top stunt designers/fliers. Do the right thing with reasonable configurations, and you should be fine.

SK.

[Steven Kientz]

I'm about as far removed from an engineer as you can be, but everthing being at 0 seems to make sense to me. Granted we fly in a 1/2 sphere in a constant skid, but sometimes humans make things more difficult by overanalyzing. Since I'm a sportflyer I'm not concerned with the perfect pattern, just whether it flies and if I can keep from tearing it up. I'm still at the point of overbuilding, worried about planes surviving multiple(daily) encounters with TERRA FIRMA.

thanks
Steve

[John Miller]

Steve, if it's easier for you to set everything up at 0-0-0, then by all means do so.  You can trim out most of the problems to an acceptable level if you do this. Since you are a sport flier, and are mostly flying for fun. you'll be fine.

Don't let all our math and constant use of big scientific sounding phrases deter you from considering the option though.

It's my experience, and remember I was big on 0-0-0 at one time, that the use of about 1 1/2 to 2 degrees of downthrust, a little bit of out thrust, (mostly to make sure it's not thrusting in towards the center.) and a slight positive stab incidence, will make the plane hunt less, and groove better. Of course I'm not that interested in sport flying, rather, it's competition for me.

It's quite simple to put a few pieces of .007 metal under the front lugs, make sure the ingine is not pointing in, and add a 1/64" plywood shim under the stab leading edge. If you do so, you can remove those shims under the engine lugs any time you wish. The left over out thrust, and positive stab incidence won't hurt a thing.

[Brett Buck]

I'm sure this has been brought up before. Is mounting the engine and stab/elevator at 0 deg. to the wing optimal? This is a semi- symetrical wing mid mounted in a profile fuse. The engine is higher than the wing. The model is a Tower hobbies Fun 51 that I'm converting to C/L. It will be powered by a ST51(itailian) or a OS46 LAS. Any opinions are greatly appreciated.

    I would suggest 0-0 is pretty safe. I am one of the principal instigators of the "positive incidence" discussions. There are some sound and relatively easy-to-understand reasons for doing it. But the amount required is *tiny* compared to the usual tolerances on construction and stability of the fuselage, and overdoing it is worse than doing nothing. I suggest starting at 0-0, and then when it comes down to final installation of the stabilizer, just remember that "leading edge higher" is better than "leading edge lower" so when you are nudging it into position, just make sure it doesn't get "leading edge down".

   Of course the asymmetrical wing section throws a monkey wrench into the works. If it was *me*, I would probably figure out where the "zero-lift-line" was from the camber and high point, and tilt the stabilizer "leading edge up" to match that angle. But that would just be a guess, and I would be prepared to cut the stab off and reposition it if it didn't work out.

   In either case, you will likely end up having to adjust the elevator position with respect to a neutral flap until it turns equally either direction.

     Brett

[Serge_Krauss]

Edit: 'guess it took me a while to write, because Brett posted while I was typing this. There was a lot of overlap, and mine was really over-kill. I'll leave it at this:

One reminder...

NO CL plane can be truly symmetrical (even 0-0-0 setups), since even built perfectly symmetrical, they encounter asymmetrical aerodynamic and ballistic loads. Flying on the surface of a sphere and varying flight and engine speeds preclude anything from symmetrical loads and perfect uniformity throughout the pattern.  The relative heights of chords and thrust line are anything but symmetrical, and account some for why 0-0-0 is a "safe" start. As you know, it's not the only start point.

SK

[john e. holliday]

All this 0-0-0 stuff is getting me as I thought it meant that you drew a line down the side view of fuselage and centered everything on it.  Will have to go back and look but, I think the Cotton Candy was that way.  If so wouldn't raising the leading edge of the stab throw things out of kilter?   I could see it when the engine, wing and stab/elevator are not on the same line.  But, it must work, look how many champions use it.  DOC Holliday

[Brett Buck]

All this 0-0-0 stuff is getting me as I thought it meant that you drew a line down the side view of fuselage and centered everything on it. 

  That's usually referred to as an "in-line" airplane.

Will have to go back and look but, I think the Cotton Candy was that way.  If so wouldn't raising the leading edge of the stab throw things out of kilter?   I could see it when the engine, wing and stab/elevator are not on the same line. 

    I am not entirely sure what the confusion is, but even with everything lined up vertically, the engine still wants to pitch the nose "up" (with respect to the airplane, not the ground). In-line airplanes are  probably worse than the usual arrangement where the engine thrust line and stab/elevator are above the wing centerline. The positive incidence and/or downthrust is there, mostly, to compensate for the engines tendency to cause an "nose up" torque.

     As mentioned above, with a symmetrical wing, the required alignment shift it very small. On my airplane the tail is tilted slightly "leading edge up" by about 1/4 of a degree, or about 1/64" over about a 4" chord. Any more and you tend to alter the inside/outside turn rate.

      Brett

[Steven Kientz]

Thanks to everyone who responded. I typically put a small amount of out thrust when mounting engines. This will be my first plane with little or no rudder offset. If I find that I need that I can cut it off and start over.

Once again, thanks everyone
Steve