Swept fin on a straight wing ME aircraft... Why??

[superfrozo]

Quick question to settle an argument, had this posed in the crew room the other day:

"Why do some straight wing multis have a swept fin?"

Best explanation I heard someone come up with was for reasons of Vmca. That is, the swept fin will have a greater Alpha crit thereby allowing it to retain it's effectiveness to greater yaw angles (and subsequently allowing for a lower Vmca) than the same aircraft with a straight fin.

Bollocks or no?

:confused:

[ifleeplanes]

Geeze all we talk about in our crew room is which pub we are off to afterwards!

[Rainboe]

How about for aesthetical reasons?

[Gary Lager]

Might have to do with structural loading, for the same reason wings are tapered: i.e to generate a load distribution which reduces towards the tip, reducing bending moment about the base and therefore also minimising structural weight requirements.

And sweeping the LE of the fin rather than both LE and TE reduces drag, sorts out the fin loads but retains rudder efficiency - a rudder normal to the airflow produces more lift for the same area than a swept rudder.

Maybe.:confused:

[411A]

Oddly enough, when Beech redesigned the QueenAir 65 to produce the model 80, the fin was swept, and more powerful engines were added.
According to a Beech design engineer at the time, the swept fin was added....to make it look better.
No additional aerodynamic benefit was anticipated, nor found.
Nor would one expect to find one...at 190 knots or so.

[pantyripper]

Mr Frozo, it seems you have the correct answer already. Same as a swept wing which realises its CL max at a higher Alpha than that of a straight wing. As for Mr Larger, I also agree with this aswell. I hear that this has been an interview question with a certain Asian airline. We also talk of beer in our crew room!:}

[Mad (Flt) Scientist]

Sorry, the Vmca answer is in the 'bollocks' category, at least as phrased.

The aerodynamic effects of sweeping the LE of a flying surface are:
1. reduces effectiveness of the surface in terms of lift (or force) versus angle of attack (or beta or whatever is appropriate)
2. delays stalling angle for the surface
3. at high sweep will induce LE vortex which further delays stall

There is a further geometrical effect, in that the force on a swept surface is further aft relative to the root chord than it would be for a straight surface.

There are also aeroelastic effects, and Mach related effects.

Now, why would I sweep the fin of a low speed multi engine aircraft?

1. I want to retain fin effectiveness to higher sideslip angles; to prevent fin stall I may try sweeping the fin LE, hoping for some LE vortex effects. Fin fillets and dorsal fin extensions are add-on examples of trying the same thing.

2. I want to move the fin BACK, but can't because my fuselage length is constrained by e.g. Vmu. So I mount the fin as far aft as I can, then give it moderate sweep to move the fin CP even FURTHER back, gaining more moment arm

The VMCA idea doesn't work because VMCA is generally a LOW sideslip case, so you don't need to worry about fin stall. The aft motion of the fin will help a bit, but in fact a unswept surface would probably be better for VMCA, since you'll get more effect at lower angles from a straight fin, if you could put it in the same place.

Not sure if sweeping is much use structurally, overall. Taper ratio might help a bit, but generally a swept structure is heavier than a straight one, because it's harder to retain the necessary stiffness. My first guess would be that structurally it's a wash.

[Old Smokey]

Cessna openly stated back in the 50s/60s that the change from a vertical to a swept fin was for purely aesthetical reasons, without any meaningful aerodynamic contribution.

Regards,

Old Smokey

[lgw bean flicker]

Hi all,

If i can put my Aerodynamicist hat back on (its been a while),........

Increasing sweep also moves the Centre of Pressure of the Fin rearwards. ie, it can also be a way of increasing the moment arm, without modifying the design too much. Naturally, the higher sweep has a weight penalty.

Aesthetics also has an important part though (just like the nose/windows configuration do!)