once again i return to bathe in the light of your wisdom..............

hope you can help guys. some of you may recall i'm building a large'ish (9 footer) old timer cabin model (my design) and i have chosen with your help a Gottigen 501 airfoil to lift her.

I'm trying to get a ball park idea of what angle to set the tailplane at. My current best guess is to set tyhe tailplane parallel to the airfoil's chord line (drawn from the TE to the foremost part of the LE).

My issue is this........... i know that the undercamber will act similarly to having a few degrees of down flap in terms of causing the model to pitch up a bit, clearly if she rears up violently on her first takeoff i could lose the model (or it would be at least difficult to control). so i need to work out how much, if any, positive incidence is needed.

The CG should, by my guess, end up around the 35-45% mark ( a bit further back than a standard trainer model).

Bruce Mathews had an old timer design with an undercambered wing - if you're reading this Bruce, would you kind and mind telling me you incidence setting? I'd hate to get her airborn only to find the entire tail needed cutting free and re-setting at a new, lower angle.

reagards Rick

The tailplanes angle is only related to the wing chord line. The fuselage is there to maintain this angular difference. For all 'normal' models this angular difference will prescribe the 'S&L' speed of the model. If you set the tailplane chord line at 1 or 2 degrees negative to the wing chord line you will find that, with a 30% balance, the model will be speed (pitch) stable. The model will not 'rear up' at takeoff, unless you, the pilot, have done something grossly wrong, the model is grossly overpowered, and the wing/tail diference is way too much. The balance position is chosen for the degree of pitch stability you want, the old timers with large tailplane areas can simply stand a greater latitude in balance position the more modern designs without getting into an unstable region. As for the 'flap' action of an 'undercambered' airfoil, it's not something I have ever noticed, and I have flown these sections on all sorts, from gliders to scale models of very early (1910) airplanes. There is a huge drag rise with increasing airspeed, and a tendancy to climb, but usually fuselage level, not noticeably nose up. Provided you fly the model with a reasonable degree of intelligence, the wing loading is not excessive, the balance is conservative and power is reasonable you will have very little to worry about.
Evan, WB #12.

The actual rigging angle of the wing to tail will not depend so much on the airfoil but on the balance point location. The tail doesn't care if the wing is symetrical or highly cambered. It will only see the longitudinal torque generated by the spacing between the wing's 25% chord location and the actual balance point location. There's are relatively minor roles played by the airfoil's pitching moment and the high center of drag associated with a high mounted wing that has lots of dihedra but those are easily taken into account during the first flight with a few clicks of trim.

So set the tail to have about 1 to 1.5 degrees negative angle with respect to the airfoil's chord line and run the numbers for your design through one of the CG calculators available online. Assuming you want to make this a performance glider like model you'll want to set the stability margin to 5% which is the same as suggested for sporty aerobatic models. If you want to set it higher to around 10% then you'll want to increase that stab to wing angle to -2 degrees. But the closer you get to 0% stability the more efficient a soaring model you'll have.

You can actually squeak it pretty close to 0 too because the high center of drag of a typical cabin style OT model provides a stabilizing force that isn't included in the CG calculations due to their relative simplicity and focus on only direct factors. So 5% stability isn't quite as small an amount as it seems. Countering this is the pitching moment which is trying to make the model pitch down with increases in speed. But on any model with old timer like tail moment lengths and tail sizes this is a relatively minor force. Besides, if you've flown any performance gliders that are well set up using the dive test method you've already flown models with low stability margins. It's not as "dangerous" as it sounds. It just means that the nose up response to a gain in speed or power is slower and takes up more room. But it's still easily able to manage itself in hands off flight.

Due to their unpiloted nature contest free flight models are set up with stability margins that are at zero or just barely over it. And yet they are stable enough in flight to fly by themselves. So don't be afraid to run the SM at 5% and even reduce it during flight testing.

In fact if this sounds like something you'll do I'd suggest a stab to wing angle of only -0.5 to 1 degree so the elevator doesn't end up with permanent down trim. Or if the tail is removable for transport then it can be shimmed at a later date to find tune the angle and remove any elevator offset.

One thing I did with a similar model is to make the horizontal stab ground trimable, a setup like the J3cub type of setup. But it was on a fairly large airframe in the 60 size range, where the weight for such a setup was not a big deal.

thanks guys, it seems i had it pretty much right then - setting the stab to be parallel to the wing's chord line. It sounds like it should be about right withing a degree or two. unfortunatley shimming the removable tail after flight testing would involve too much surgery - due to it being bolted with sturdy ply plates.

You'd better stop the presses. I went and checked the fuselage for my big Flamingo OT that uses the 501 airfoil. It seems it's carrying quite a bit of stab negative angle. Mind you I'd LIKE to move the balance back more as it's too stable and also I've got no way to measure and allow for the fairly generous down trim it carrys on the elevator without assembling the whole model and doing some careful measurements. But given what I'm seeing of the wing and tail mounting saddles I'd stick with the 1.5'ish negative on the stab compared to the wing unless you're planning on fudging the balance back as far as you can get away with.

Mind you if you can build in a ground adjustable stabilizer mount that will allow for some shimming you can get the best of all worlds. I'd still star with about 1.5 negative at the stab though.

The reason I don't move the balance back is because I already had to put a half lb of lead in the TAIL to get it back where it is now. And this is despite having a small .35 glow engine on the nose of an 84 inch span model. It must be the two and a half lengths of 1/8 music wire in the landing gear that's giving me the nose weight. In any event I just didn't want to add another quarter lb to the weight to move it back further like it probably should get.

I have a Bleriot XI in my 'Gallery' page. It flies at 1.5 neg @ stab with c.g at 25% chord. Good luck!