Lifting Stab Myth

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Let’s first look at a highly cambered non-symmetrical wing traveling at a high cruise speed. In order to stay in level flight without climbing the wing must actually be flying at a negative incidence. Many of these airfoils actually produce lift up to –4 degrees of negative incidence. The camber of the wing overrides the “push-down” effect of the negative incidence and the wing continues to lift.

Now let’s look at a plane with a highly cambered Clark-Y that was designed with a flat stab set at the same incidence as the bottom of the main wing. The test pilot/designer takes the plane up and zooms around for a bit. He ends up adding lots of down elevator to get it to trim correctly. Why? The picture below shows it. The wing must be at a negative angle of attack to keep from producing too much lift and climbing. Since the wing is connected to the plane, the whole plane must be tilted forward. When you do that the stab sets itself at a negative incidence and provides too much down force. Down elevator is used to add camber to the stab to offset the negative incidence down force. The net force (in red) is just the right amount of down force to fly the wing at the correct (negative) angle of attack so it doesn’t climb. As you can imagine, if you have designed the main wing at a positive incidence to the stab, the problem is exacerbated and an even higher tail angle (nose-down) is necessary.

So the test pilot/designer lands, taxis up to the pits and shuts down. He then glances at the airplane and sees the deflection of the elevator. “Aha, I must have a lifting stab." he says, " I have an idea.  Instead of a flat stab with down elevator, I will make a positively cambered airfoil shape and it will eliminate the down elevator necessary to trim.” And so he does, and winds up with the picture below. Well it works, the plane flies perfectly without the down elevator.  The designer is also convinced he has created an airplane with a lifting stab and so he quickly phones Hobby Lobby to sell his idea.

If you study the picture above you see that nothing has changed. The stab is still flying at a large negative incidence which is offset by the lift producing camber. The sum of the vectors is still a net down force. One other ingredient is worthy of note: Profile Drag. The stab is at a negative incidence with a thicker section. The bottom line is that there is a thick profile encountering the relative wind. This is the same as putting a little drogue chute or kite tail in the back: an increase in drag.  It effectively moves the neutral point of the aircraft rearward and makes it more statically stable. In the full-scale modern world where drag is trying to be minimized this would be a big no-no. In the RC world where power is massive and we are not going anywhere, it doesn’t matter. Who cares about drag or efficiency when your flights consist of a series of circuits around a fixed point for about 15 minutes.

Now let’s look at a more elegant way to handle the situation. Let’s put a flat stab at a positive incidence relative to the wing. It works perfectly and provides just the right amount of down force without the profile drag. Notice, there is no camber and so the only lift (downward direction) comes from the negative incidence.

Perhaps the best method to deal with this problem  is to add a shim to the back of the wing so it’s angle of attack is reduced intrinsically. Now the fuselage flies pointed more into the relative wind and you don’t need to set the stab at a large positive incidence.

If you have gotton this far, I hope you agree with me about the so called lifting stab phenomenon.  In the history of aviation I personally cannot thing of a normal configuration plane that successfully utilized a lifting stab.  There are a few planes like the Fleet biplane (PT-1, PT-6A, etc.) designed in the early 20's that had a similar configuration as the examples above, and no doubt, the designer had the same delusion as his modern RC designer counterpart.

Finally, there are some highly specialized designs that have successfully used a lifting stab.  The most common example would be free flight models.  These models are trimmed to fly at a single airspeed.  The desire to produce a lifting stab configuration for these models stemmed from the constraints of the rules.  Wing area was limited and was only measured from the main wing.  If one could build a second wing (stab) that shared the load then the effective wing area was increased.  Many of the 1950's planes had CGs at 80% or more of the chord with large lifting stabs.  These planes were also known to do death-dives and many were marginally stable.  Later designs utilize very long tail moments with much smaller stabs.  The CGs are in the 50% of chord range. 

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