please explain how stab tilt works

[Bob Chiang]

Hi all,

I had a fun indoor free flight flying session yesterday, and none of us had a definitive explanation of why stab tilt induces a turn.

Oh aerodynamic gurus, please enlighten us.

Thanks in advance,
-Bob

[raptor22]

It induces yaw by introduceing sideways downforce, and therefore makes the plane turn.

[biber]

That's almost correct raptor22 but i'd say it's not a downforce but upforce. Try out a stab tilt to the left and see what direction the model chooses. If it turns right i'm right and it provides lift with a slight component to the left because of the left tilt. I bet that it will turn right.

[raptor22]

a stab exists mainly to correct the downward moment coeffecient of the airfoil. it does this by producing downforce.

--Alex

[BMatthews]

In truth the stab actually is set up to produce the force needed to counter the effects of the wing in combination with the CG position. For most free flight models flying with very large stabs and very rearward CG's the stab is actually lifting. But it lifts less than the wing does so the net effect is to stabilize the pitching moment of the wing. For more "normal" CG placements I agree that the effect is closer to neutral and can be downforce with the more forward positionings.

In effect most free flight designs are a cross between a conventional area distribution and a tandem wing if that helps. For example indoor duration models typically have the balance point very close to the trailing edge of the wing. It's even quite common for them to have it slightly behind the trailing edge.

So for MOST free flight models tilting the stab left side down will produce a right turn as the lift component is aiming to the left thanks to the tilt.

PS: Pattern models with the CG set at the neutral point will produce a zero down or up force at the stab. But the model then needs to be flown at all times. I also have a copy of the old DOS David Fraser sailplane program and part of the data that is determined is the lift coefficient of the stabilizer. It didn't take moving the CG back very far to end up with a list of Stab lift coefficients that S'd back over the Cl=0 between + and - values.

Typically full sized aircraft use CG placements that are far more conservative than our models are for safety reasons. In that field it's far more common, to the point of universality, to find stabs with negative lift coefficients. This is the source of the commonly held belief that ALL stabilizers produce downforce. But as you can see from above it just ain't so. As long as the stab produces LESS lift moment than the wing the craft can be set to be stable in pitch. The lift moment being a product of the tail lift x the tail moment arm.

[raptor22]

I just assumed we were talking about a stable airplane, here. I don't have much FF experience, so things are different there, but in the r/c world we make most airplanes (not 3d or pattern) stable. The stabs purpose in this case is simply to have an elevator and counteract downward pitching coeffecient.

--Alex

[BMatthews]

Alex, it's not even clear cut there either. In a thread about a year ago over at RCUniverse the serious aerodynamic brains showed that even with what most folks would consider to be conventional models like Velox's and Yaks and such that the stab would actually have a positive lift for CG's pretty much anywhere behind the 25% to 30% mark depending on the tail volume coefficient. I was actually quite surprised to hear about that just as I'm sure you are but they provided the proof. But in each case the balance point has to be close to the neutral point to achieve this as I suggested above. For trainers and scale models that are set up for more pitch stability the negative lift was the norm as you would expect. A lot of folks, including me, had their preconceptions shot to rags with this topic.

[raptor22]

But those are 3d planes. I was referring more along the lines of s sport model. i know some such planes use a positive lift coeffecient. But a conventionally designed positively stable model with a downwards pitching foil will have a negative lift coeffecient in the horizontal stab.

--Alex

[BMatthews]

Yes, even with most sport models it's still positive.

It's quite a lot of material but if you're interested check out the VERY long and detailed thread over at RCUniverse about this. In particular pay close attention to the material from Ben Lanterman...

http://www.rcuniverse.com/forum/m_443640/mpage_1/key_neutral%252Cpoint/anchor/tm.htm#443640