MiL

I'm thinking about scratching a plane similar to the Laser 3D and Diabolic, i really like the idea of the full flying stab/elevator on the Diabolic and want to incorporate that into my design. I am also thinking about doing the rudder the same way.

My question is, at what point along the chord should the pivot point be? I've seen a little discussion about this before but was unable to find it again. I seem to remember talk about an aerodynamic center and possible flutter problems, so it sounds like it might be tricky. Otherwise i would probably just stick it about 1/3 of the way down the chord and assume it would be good. Would that not work?

Also, this line of thinking lead me to wonder about eliminating ailerons and just having the wing halves tilt slightly. This doesn't seem like such a good idea, but interesting nonetheless. Some of the VERY early airplanes and gliders used flexing wings to controll the roll axis, this obviously is not done anymore and i assume for good reason.

Anyway, where should i put the pivot point on my full flying stab and fin?

Mike James

I would ask Oliver Wilson ("Ollie" on RCU) about this. He often reads and comments insightfully on the "Aerodynamics" part of the forum, and definitely knows more about aerodynamics than I do. But...

The amount of control power basically comes down to camber, I think. Often, a fixed flying surface with a hinged control area gives you more power than a full flying surface, because it generates more camber, and therefore more lift.

seafury_fb11

I think it is supposed to be at the aerodynamic center of the stab, but as Mike mentioned, I'm sure Ollie can give you a better answer.

Russ.

Ollie

A wing or tail can be pivoted for control. Wings have been used this way on high speed slope soarers. Sig even kitted such a plane, the Samuri (sp?) I think. There are advantages and disadvantages. An all moving tail, either vertical or horizontal, will not have the control power of a well designed articulated tail because the articulated tail changes both camber and angle of attack. This combination will produce a higher lift coefficient than just changing the angle of attack.

The big advantage of the all moving control surface is that it requires virtually no servo power to move it when it is aerodynamically ballanced. Another advantage is that it is extremely flutter resistant when aerodynamically balanced. Such a control surface is aerodynamically balanced when the pivot axis goes through the aerodynamic center. The aerodynamic center is very near 25% of the chord from the leading edge of the mean aerodynamic chord. The mean aerodynamic chord goes through the centroid of the area of the planform. You can download a copy of Chuck Anderson's MAC calculator program from:
http://www.dchobby.com/airfoil/maccal.html
If the pivot axis is behind the aerodynamic center the surface will have a tendency to blow back and any elasticity in the linkage will show up as deadband or in an extreme case, servo overload. If the pivot axis is located in front of the aerodynamic center there will be a tendency to flutter which gets worse as the mislocation increases.

mulligan

See below image for determining center of lift... and your ideal pivot line.

- George

mulligan

Let's try that again...

Ollie

George,

Please, compare your diagram to other graphical methods and you will see the error.

A line along the 50% chord lines intersects the diagonal from upper left to lower right to define the MAC.

BTW, the 25% chord line is only the center of pressure line if the moment coefficient about 25% of the chord is zero.

mulligan

In my haste, I drew this incorrect... one last time (I Hope )

Ollie

I should have waited a little longer. I agree with the last diagram.

Sorry, George

mulligan

We're typing too fast, Ollie!!!

I agree with your comment about CP, but I'm assuming we're talking about a symmetrical airfoil here.

- George

MiL

Wow, thanks for the answers, especially taking the time to draw a diagram Mulligan.

So just to make sure i understand correctly, I would want the pivot at 25% on the MAC, right?

mulligan

Assuming you're using a symmetrical airfoil (e.g., 0009 or 0012), 25% should be close enough. If you want to know the exact %, let me know what airfoil section you're using. Otherwise you're not going to be more than +/- 1 or 2%- and if you're building skills are that good that it matters, then I'd like to consider hiring you to build my next plane

- George

MiL

Ok, thanks guys

747drvr

My Feedback: (5)

If the pivot axis is behind the aerodynamic center the surface will have a tendency to blow back and any elasticity in the linkage will show up as deadband or in an extreme case, servo overload. If the pivot axis is located in front of the aerodynamic center there will be a tendency to flutter which gets worse as the mislocation increases. [/B][/QUOTE]

Hey Ollie,

Any chance you might have this backwards ?

Ollie

747 Driver,

Try this thought experiment. Pivot the surface near the trailing edge (well behind the aerodynamic center). Leave the surface free to pivot. When the air starts to flow over the surface, it will blowback (weathervane) so that the pivot axis is fore and the former leading edge is aft. That's what I mean by "blowback." When the surface has weathervaned 180 degrees or so, the pivot axis will then be ahead of the aerodynamic center and flutter becomes likely at a high enough airspeed.

Now restrain the rotation elastically in the normal orientation (leading edge first). Any deviation from a zero lift angle of attack will strain the elastic restraint and the angle of attack will increase, limited only by the spring constant of the elastic restraint and the increase in lift and drag due to angle of attack increase. When the angle of attack approaches stall, buffeting (vortex shedding) will start. As the aft pivot axis is moved closer and closer to the aerodynamic center the "blowback" effect will diminish until it is zero when the pivot axis reaches the aerodynamic center.