airfoil shape tailfin/rudder on early RAF fighters?

[JackG]

Specifically looking at the Siskin, but the Bristol Bulldog looks similar.  Did these designs have any airfoil shapes on the tail and rudder or were they completely flat, or maybe just flat at the very edges?  Granger's top view drawing of the Siskin does suggest a slight tear drop shape, but I'm not seeing it in photos.  Maybe I've not found the best images?

 

 

 

regards,

Jack

[rossm]

There will be some thickness to the structure, with sharp leading and trailing edges which will give a shape - whether airfoil or slab sided I don't know, sorry. Your photos suggest the latter to me.

[pigsty]

Any actual aerofoil would generate a slight yaw moment so, unless there was a problem with engine torque and swing that couldn't be fixed with rudder trim*, they'd make the fin symmetrical.  And as rossm says, the shape would be streamlined: thicker where the rudder post is, and thinner at either edge to reduce drag.

 

* it happens - hence some like the Hurricane having a slightly offset fin.  But even then it wasn't a full aerofoil.

[Dave Swindell]

4 minutes ago, pigsty said:

* it happens - hence some like the Hurricane having a slightly offset fin.  But even then it wasn't a full aerofoil.

Fairey Battle fin - both offset and aerofoil section.

 

[MrB17]

Here’s a photo albeit poor resolution, it clearly shows the construction of the fin and rudder on a stripped down Siskin, Jaguar powered.

Regardless of whether the cross section is flat or airfoil shaped, as soon as the control surface is deflected, it becomes an airfoil and increases the pressure on the under cambered side and decreases it on the cambered side, like a wing. On fabric covered aircraft you can tell which side the control surface is deflected even from the side, because the fabric will be dished in on the inside surface towards the direction of the turn, and puffed out on the other. It’s why I don’t like dished in fabric on model control surfaces, it’s inaccurate, period. Even in flight mode, one side would be puffed out. A lot has been learned about fabric effects, but not enough.

 

Jeff

[JackG]

@MrB17 Jeff, thanks for the insight about the dished fabric effect.   Omitting that detail will make it  that much simpler to scratch build a new rudder. 

 

Am I seeing correctly there are total of four hinge points (red arrows) on the rudder, all of which attach to the rod/post that pivots the whole tail assembly?

 

 

 

regards,

Jack

 

[MrB17]

1 hour ago, JackG said:

@MrB17 Jeff, thanks for the insight about the dished fabric effect.   Omitting that detail will make it  that much simpler to scratch build a new rudder. 

 

Am I seeing correctly there are total of four hinge points (red arrows) on the rudder, all of which attach to the rod/post that pivots the whole tail assembly?

 

 

 

regards,

Jack

 

I’m not a Siskin expert, however the top point makes sense, not sure about the one above the tailplane, the other two make sense. What doesn’t make sense is how did the rudder post get involved with the horizontal stabilizer arrangement. Apparently there are different fin and rudder types on Siskins some tip forward with the tailplane incidence and some tip backwards. Wow, not a simple screwjack like a Piper Cub. In looking at the real thing, there is a bit of an airfoil shape to the rudder, where you will have some scalloping of the fabric because it’s going over a curved frame, mostly visible on the aerodynamic balance forward of the rudder post.

A word of caution, Mr. Granger’s drawings are not necessarily accurate, although they look good. I always verify with photos of the real thing, either in service or correctly restored (a rarity), unless I have the factory blueprints.

 

Jeff

[John B (Sc)]

Interesting discussion.  The Siskin tail is unusually complex; I wonder why the designer didn't do a  Cub style screwjack just on the horizontal tail?  (There must have been some interesting loads on the pivot - additional twist load because of the rudder. )

 

The tailplane surfaces on most fabric covered light aeroplanes that I am aware of are essentially fairly flat plates, bevelled at front and rear.  Simple and cheaper to build. Some may be tapered - having thinner trailing edges than leading, mostly for structural reason s- the LE needs more strength for attachment points and to absorb deflection loads.

 

As was said earlier, even without a specifically aerofoil shape, they act as aerofoils under any deflection of airflow.  I can't say I have noticed much pillowing or hollowing/dishing of fabric surfaces on deflection. Admittedly that has mostly been when looking at ailerons and wings rather than tail surfaces, which I'd only look at occasionally on a glance back - they do often vibrate quite a lot in flight, even when well braced !   Wing surfaces, even  under reasonable aerobatic  loads only show minor pillowing at most, in my experience. Different fabrics  and different aircraft do show different effects though.  Depends on the  materials and the tautening process I suppose.

 

Incidentally, I don't think metal skinned light aircraft such as the Cessna 150 or Piper Cherokee have anything fancy as tailplane aerofoil sections either - it's a long time since I looked at one carefully, but I think they are also more or less rounded plate type cross-sections, for ease of manufacture.  (*Correction: the Cessna 172 does have aerofoil shaped tail feathers - and I recall the Rallye I used to fly also had aerofoil tail contours)   

Some lighter machines such as modern sailplanes may have fully aerofoil tail surfaces, for efficiency.  Easy enough to do in fibreglass moulding.

Edited December 20, 2021 by John B (Sc)
Correction & additional comment

[JackG]

Would the rudder be hinged to the outermost post of the incidence control system, or would that be too stressful? 

 

 

 

 

[Robin-42]

The Silver Wings 1/32 Siskin has the four hinges, and I did find a photo that showed the top and bottom ones, so four seems reasonable. The rudder is hinged on the adjustable post on your drawing and it appears plenty strong enough, if engineering overkill. It’s a single engine fighter, I can’t imagine the cg moving very far!