Grumman F4F: Stabiliser Question

[Hugh_T]

Continuing with my research on the construction of the F4F I have noticed that the leading edge of the Horizontal stabiliser drops by just under 0.5 degrees. Effectively an Anhedral and not a dihedral as one would expect.
The main spar is set back by 1 degree and is perpendicular to the vertical plane of the fuselage. It is the ribs that set the incidence angle of 1.5 degrees. Following the Grumman drawings, it transpires that in doing so the end result is that the leading edge tends to drop outboard by 0.5 degrees as shown below.
I don't believe this was the intent and it would certainly be unusual. Before I correct the alignment I need to just be sure that my thoughts on this are correct...it may well be that it is what was intended.
I would appreciate any comments.

 

[gingerbob]

I think that, because of the 1.5 degrees positive incidence, the line of the swept leading edge "droops". If my thinking is right, then when you look at the stab in its own plane, the leading edge would be a flat "horizontal" line.

 

[XBe02Drvr]

Sounds like airfoil washout to me. Perhaps to compensate for spiral slipstream effects? Is it symmetrical port and starboard?

 

[Hugh_T]

I think that, because of the 1.5 degrees positive incidence, the line of the swept leading edge "droops". If my thinking is right, then when you look at the stab in its own plane, the leading edge would be a flat "horizontal" line.

I think you are right. The leading edge sweeps back at 18 degrees, which coupled with the arrangement of the ribs results in this drop as viewed from the front. I have double checked the setting out dimensions and all is correct.

 

[fliger747]

Very interesting. Remembering that the point of the horizontal stabilizer is to provide downforce, it appears that a non symmetrical airfoil is used and is of course upside down from the way we usually look at these things. If the same airfoil section is used throughout this would generate a little anhedral of the mean aerodynamic chord. A very small effect and I expect a result of Grumman opting for the simplest method of construction. The actual airfoil used should be available information.

 

[chuter]

Just as a matter of note: none of Grumman's gasoline combat planes used cambered (non-symmetrical) stabilizers. (And, just for fun, neither did the DC-10 - lol.) I have no idea what the cross section is, though - good luck on that.

I'll go out on a small limb (here, hold my beer) and say that of all the American WW2 production combat planes only the P-61 had a cambered stabilizer.

 

[fliger747]

I agree that this apparent anhedral would result from a slight negative angle of incidence. I am not anywhere near my reference as to what the airfoil section was and the angle of incidence. It is quite typical for stabilizers to be symmetrical as they commonly operate at low CL values. A cambered airfoil could lead to unpleasant stall characteristics. The subsonic jets I flew had trimmable stabilizers and used symmetrical airfoil sections.