# Tip geometry for control surfaces with swept hingelines

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##### Well-Known Member
I'm trying to figure out how to handle the control surface tip geometry for surfaces with a slightly swept hinge line (<15°).

It seems like there are two options:
1) Cut off the surface tip perpendicular to the spar.
2) Cut off the surface tip with a conical surface formed by revolving an approximately longitudinal control line around the hinge line.
(Does that make sense? I can illustrate if needed.)

(1) is simpler, and it avoids odd geometry. The problem with (1) is that, for control surfaces that have deflection during cruise flight (elevators, elevons) they present a cut-off surface at an angle to the free stream, presumedly adding drag. Using the cone from (2), you can have the cut-off be approximately in the free stream direction.

What do existing designs do? I'm leaning towards (2) since my cruise control surface deflection can be up to 2.5°, which is actually a pretty high amount of base area presented to the wind; but I'd love some inspiration on how the somewhat strange geometry that comes from this approach is handled.

#### Aircar

##### Banned
You would probably be surprised by the local flow direction near the tip and as soon as you deflect the surface that will change greatly -- for extreme sweep angles (or sheared tips ) take a look at the English Electric Lightning . I couldn't picture your 'conical' description BTW .

##### Well-Known Member
Yep, I know that tip flow direction will not be longitudinal; but it's even more unlikely to coincidentally align with the hinge line. Think of approach (2) as "adapt to local flow conditions"; that is, design for minimum drag from the control surface end, rather than design for simplest geometry.

##### Well-Known Member
Here's some images to explain. Both images show a wing (or other surface) with two trailing edge control surfaces with swept hinge lines. On the left wing, approach (1) is used to define the control surface tips; on the right wing, approach (2) is used. In planform (the first image), everything looks fine; but remember that surfaces will have finite thickness. The perspective view shows the "cutting surfaces" that define the tips of the control surfaces. For approach (1), these are planes (that is, a simple cutoff), but for approach (2), these are cones.

#### orion

##### R.I.P.
Most work I've seen places the trim lines in line with the chord. The angled hinge lines do result in a bit of a profile being exposed to the air flow however that occurs only during full deflection and represents a relatively minor penalty.

##### Well-Known Member
Most work I've seen places the trim lines in line with the chord.
Sorry, I'm having a hard time interpreting the sentence. Terminology issue on my end. Trim lines = where the control surface is cut off? In line with the chord = longitudinally? So approach (2) above?

The angled hinge lines do result in a bit of a profile being exposed to the air flow however that occurs only during full deflection and represents a relatively minor penalty.
Even for large surfaces? I guess it's only a few square inches per degree, so part of this might just be me triggering on an "it looks weird" issue. Another issue (besides my up-to-2.5° deflection at cruise) is that I'm lifting the top surface of the control surface a little bit above the surface of the wing (we've discussed this elsewhere as a generally good idea for flow reasons, and I need it for geometry reasons as well); which means that even at zero deflection there's that weird little strip of surface sticking out.

#### orion

##### R.I.P.
Yes, approach 2. The gaps in approach one can create weird leakage flow - I haven't really seen much published on this but in general it has always been a consensus to configure control surfaces such that the root and tip lines are in line with the theoretical chord direction.