Variable Incidence Horizontal Stabilisor

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Well-Known Member
Oct 20, 2003
Northern NSW Australia
Oh how I hate not having my comp back up (yet).
I've got all these great ideas to try out and nothing to draw them with bar the good old Mk1 pencil, and no scanner to post them with.
Anyway, I'll just have to be descriptive.
I was thinking about an advanced method of trimming which even though it could add a bit of weight, could very well provide a superior method of trimming an aircraft over a wider range of mission parameters.
The idea stemmed from experimentation soon to be undertaken my Mr Langford of KRnet fame on the optimum angle of incidence for the horizontal stabilisor to give a neutral trim state at cruise.
He is basically making his horizontal adjustable on the ground, to be fixed once he has an incidence he is happy with.
I thought,why not take this one step further, and build a permanant pivot point in at the spar, and using an electrically driven threaded rod, similar to a feed screw on a lathe, to make the horizontals' incidence variable?
Could this provide a much grater range of trim? It may well be much more efficient than a traditional trim tab.
Sure, if it works it'll be almost twice the weight of a conventional electric trim, but for the versatility and possible greater effectiveness, why not?


Well-Known Member
Mar 2, 2003
Western Washington
Actually quite a few airplanes use this method of trimming rather than the more standard elevator trim tab. The most common example is the Super Cub but quite a few others use this method, including most commercial aircraft.

The use af a variable incidence horizontal results in more trim authority, which in turn results in a wider allowable CG range, especially at the forward end of things.

The drawback is added system complexity and possibly less mounting stiffness. The latter is especially important since it can be a source of flutter. The jack screw can also be the weak link in the system and so it has to be a focus of good design and mounting, as well as maintenance (as the Alaska Airlines airplane found out several years ago).


Well-Known Member
Dec 26, 2003
Taree Airport Australia
As ORION said, the 'adjustable stab' is actually fairly common and, as mentioned , can give a wider trimming range.
Another bonus is that through-out the trimming range, the stick/yoke will tend to stay in the same place (like R/C gear) as opposed to moving to a new position as you retrim the elevator.
But the downside is still the weight and complexity.
Also, even if built strong enough for air loads, you might find that if you accidentally push you KR gently into the back of the hanger, you could rip off the whole tail :eek:
instead of just crunching the elevator. :wail: :mad: