Full-Flying Horizontal Stab Guidelines

[Slow Riser]

Hi,
I'm trying to do some research on sizing a full-flying horizontal stab for a small airplane. Wing area is around 125 sq inches, 24" span and 5" chord.

Are there general guidelines on how best to size the stab.

Thanks...Henry

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www.zebrahobby.ca

[Mike Taylor]

Same size as the regular stabilized/elevator would be, but you only need a total of 15 degrees movement so use a very long control arm. About a third of the area goes in front of the pivot...

[Slow Riser]

Thanks Mike,
Is there a recommended stab size? I guess it also depends how far back the stab is located.

I have also read that the position of the control horn has to be put behind the pivot from a stability standpoint. I have read that its the back third of the stab. Is this correct?

Henry

--
www.zebrahobby.ca

[Mike Taylor]

The flying stab plane I have (haven't flown it in 20 years, but it still looks like it would fly) is a 1970's Cirrus glider. The control horn is an inverted aluminum 'L'. I made others like it for other planes from U-control bellcranks with one arm cut off. The pivot for the bellcrank is at 1/3 the chord, and the short end of the crank points aft. Rods run through the middle of the the pivot bearing and also through the end of the aft-facing part of the horn. The stab halves have matching tubes that the rods slide into. The long arm hangs down and has the push rod from the servo contected to it. With only 1/3 of the area in front of the pivot, there were no stability issues. Other forms of attaching the controls (like a flexible tube and cable set-up) would work as well, and they would want to connect behind the pivot. I could take some pictures of the Cirrus, but there is very little showing since it is almost all built insided the vertical stabilizer. All that shows is the pivot rod and a small arc in the tail with another rod sticking out.

[Mike Taylor]

Here's a quick and dirty sketch of the way it is set up. Not to any scale, but this is one way to do it...

[BMatthews]

For a typical sport power model 20% would be a decently safe figure assuming you've got a more or less normal tail moment. If it's really short and has a lifting airfoil then you may want to make the tail a little bigger, like say 25%.

On the other hand a fully symetrical airfoil hardly needs much of a tail at all to be stable. It's different if you want lots of elevator response though.

[Sail 'n Soar]

Originally posted by Mike Taylor
The flying stab plane I have (haven't flown it in 20 years, but it still looks like it would fly) is a 1970's Cirrus glider. The control horn is an inverted aluminum 'L'. I made others like it for other planes from U-control bellcranks with one arm cut off. The pivot for the bellcrank is at 1/3 the chord, and the short end of the crank points aft. Rods run through the middle of the the pivot bearing and also through the end of the aft-facing part of the horn. The stab halves have matching tubes that the rods slide into. The long arm hangs down and has the push rod from the servo contected to it. With only 1/3 of the area in front of the pivot, there were no stability issues. Other forms of attaching the controls (like a flexible tube and cable set-up) would work as well, and they would want to connect behind the pivot. I could take some pictures of the Cirrus, but there is very little showing since it is almost all built insided the vertical stabilizer. All that shows is the pivot rod and a small arc in the tail with another rod sticking out.

I have an even older Windfree glider with probably 35-40% of the stab area ahead of the pivot point - works great. But in general it is safer having 25% or less of the area ahead of the pivot point so that the axis is at or ahead of the aerodynamic center. On slow aircraft it probably doesn't make much difference (e.g., the above mentioned thermal gliders). But with hotter aircraft an aft pivot results in an unstable stab, which will have a tendency to pop between two neutral states - one with a positive and one with a negative angle of attack - accenting any play in the control system. A highly cambered stab will mitigate this tendency, but who flies with a highly cambered stab?

[Ollie]

The best location for the pivot axis is a line that runs through 25% of the mean aerodynamic chord. Other locations may work but put an unnecessary load on the servo and linkages if the pivot is too far aft and risk flutter if too far forward. If the pivot line is aft of the aerodynamic center of the surface the stab will blow back and any slop in the linkage will result in dead band. The further aft of the aerodynamic center the pivot is, the lower the flutter speed will be. When the pivot line is at the aerodynamic center the surface is said to be aerodynamically balanced and the force on the servo and its linkage is zero. In other words, a properly placed pivot reduces the servo torque requirement to just enough to overcome the inertia of the system as there are on aerodynamic loads on the servo and linkage.

The area of the horizontal tail should result in a tail volume coefficient between 0.3 and 0.4. Larger tail volume coefficients will work but will have unnecessary drag. The tail volume coefficient is the ratio of stab area to wing area times the tail moment arm divided by the wing's mean aerodynamic chord. The tail moment arm is measured from the aerodynamic center of the wing to the aerodynamic center of the stab.