Holes in rudder?

I think an argument could be made for a smallish hole, appropriately shaped, in the forward sections of the rudder, permitted flow to reattach across the broad expanse of a bar door style of blade. As to improving efficiency, there also could be an argument made in as much as the rudder might "feel" bigger, though efficiency number likely wouldn't improve in a perfect fluid analyses. To the Chinese hole laden rudder, who knows, but you can bet these were seat of the paints types of modifications, not so much anything else.

Dive flaps on WW II aircraft had holes to increase drag and let enough flow past it to prevent a stall. These didn't improve efficiency and in fact dramatically decreased it, which was the point in a 60 degree plus dive, to hold aircraft below high speed stall situations and prevent air frame over stress issues. Tearing the wings off a dive bomber generally isn't acceptable. Dive brakes employed now aren't perforated. It's important to understand the difference in speed brakes, ground brakes, spoilers and dive brakes. The holes on dive brakes prevent a huge boundary layer wall from being created when deployed.

 

In addition to PAR's comment, the main reason for some aircraft air (or dive) brakes to be perforated is to avoid buffeting on either wings or tail. The buffeting is induced by large-scale vortices carried in the wake of an engaged dive brake. Large vortices carry a large amount of kinetic energy, and their frequency range can be close to one of natural frequencies of the airplane structure (in particular, of wing and tail control surfaces), which is the reason for buffeting.

By perforating the dive brake, big vortices in the wake are split into smaller ones as the airflow spills through the perforation holes. Their frequencies increase and are shifted away from the airplanes' natural frequencies.

So plate perforation is just one of ways to eliminate the aerodynamic buffeting. It is not intended to (and it does not) increase the aerodynamic efficiency of lifting bodies like wings or airfoils. A perforated plate makes a less efficient air brake (in terms of CD) than a non-perforated plate. Thus, from the point of view of airplane deceleration efficiency, it is preferable to have a non-perforated air brake - if it can be positioned in such way to not create the aerodynamic buffeting.

Similar reasoning can be made when it comes to lift. Holes perforated through a wing allow the fluid to spill from the pressure (or ventral) side of the wing blade (rudder blade in this case) towards the suction (or dorsal) side. This spillage will cause a reduction of the pressure difference between the ventral and dorsal surfaces, and hence will reduce the lift. Finally, the reduction of the lift will be bigger than the reduction of the drag, so the overall effect is a net decrease in aerodynamic efficiency of the perforated wing, compared to an equal-area non-perforated one.

All this theory could have been easily avoided by simply noting that you won't see any perforated wing around, either on airplanes or on hydrofoils. There must be a good reason for that, right?

 

Why not put an end plate on the bottom of the rudder and cut a hole above the waterline. You can step on the end plate then put your other foot in the hole.

The end plate will probably improve the effectiveness of the rudder in the bargain.

Cat boat rudders are flat on the bottom, aren't they?

 

Seems like a good idea to me, except I think end"plate" should be foil shaped winglets. I think Phil Bolger was an advocate of endplates on low aspect rudders.

 

Wasn't the idea there to actually increase drag? For dive bombers it slowed them down for more accurate bomb drop

 

Yes, dive brakes do dramatically increase drag. So much so, that the aircraft is hugely buffeted around in the process, which is why the holes where cut.

I have a few small sailor designs that have a foot step hole in the rudder head portion of the rudder. It's above the LWL of course and permits easy reboarding if you went for a swim.

Below the LWL, the hole on a barn door or other quite low aspect blade shouldn't be too bad, in regard to preformance (location dependent). Some additional turbulence related drag, but it should "reassemble" and attach, further aft on the blade, with limited detraction from the sledge hammer approach, this type of rudder uses for helm input. If an end plate was used, the blade's length probably should be decreased a bit and though Bolger did fool around with these, they have a limited range of usefulness, after which they too just become a lot of drag.

 

Holely Chinese Rudder

Chinese 'junks', as a general rule, did not have keels.

The rudder not only steered the boat, but also served as lateral resistance.

For this reason, they were quite large. They also were like 'spade' rudders, in that they were mounted on a vertical shaft. What often happened, at least to replicas built by some westerners, is the shaft would break.

The diamond shaped holes near the trailing edge where probably meant to mitigate shock torsion loads, which the rudder assembly would experience, if the junk were hit by a quartering sea.

My guess is the holes mitigated the shock torsion loads far more than they reduced the rudder's efficiency.

 

do you have pictures of these diamond shaped holes in the junk rudders? I do not see how holes would reduce shock loads from quartering seas.

I can not imagine why you would have such holes. It is not likely the ancients were experts at understanding vortex flow or fluid mechanics, there must have been another reason for doing it, unless some beneficial affect was discovered by accident. I have never noticed these holes before and now I am curious about them.

 

I found this one, but this is a rather smallish rudder, and with the prop not really "traditional". in a google image search most junk rudders did not have any holes, only found two with holes, both diamond shaped and not at the trailing edge. I also appears most of the junks do indeed have full length keels and one large one even had lee boards.

I can not see how these holes would do any good, but that rudder does operate in some rather messed up flow based on the after body shape.

 

I submit that the holes are as much meant to be anything to do with sea keeping as the dragons on the side.

Perhaps steering was heavy so they cut holes in it but to me the fact that they are diamond strongly suggests they are merely decorative.

The one round hole nearest the prop is to remove the shaft hence it is cut right thru the rudder shaft.

 

I don't think the vessel shown above is very indicative of a traditional sea-going junk. I used to have a copy the Worcester (sp) book and seem to remember more holes for the given area. Anyone here still have a copy to refer to?

 

somewhere on this forum i read those holes are comunication windows for star and portside swimming fish, really! so, why not elaborate my thoughts as well. double flap drag rudders -like on divebombers- are used on ships as well i read, like flap rudders that increase lift over a single rudder. foils create drag and the degenerate case of a foil is a simple plate as the chineese ruder in discussion here. on a rudder be it aircraft or boat next to lift a key design factor is drag minimization and if my memory serves me well think it was Tom Speer that pointed out high induced drag on the high lift Handley Page seven slotted foil attached below wasnt good. STOL aircraft still use a leading edge slot (a fixed non-closing) gap behind the wing’s leading edge. Air from below the wing can accelerate through the slot towards the low pressure region above the wing, and exit from the slot moving parallel to the upper wing surface. This high-speed flow then mixes with the boundary layer attached to the upper surface and delays boundary layer separation from the upper surface. wonder over these flows and allready imagine a laddered holed bi-plane rudder. so perhaps its good i dont have a steady cfd program on my pc yet or i really would go completely out of my mind.

 

I don't quite know what to make of your post Yipster but from what little I understand of aerodynamics the curved upper wing surface is what provides the lift on an airplane. Would a plane still fly if the wings were simply tapered straight with a flat surface on each side? Like my catboat rudder.
I've also seen many power boat rudders which were simply constructed of flat steel plate with a much larger diameter(than the blade thickness) rudder post welded on - sometimes on the leading edge and sometimes set aft to balance the rudder. And some of these were on high speed boats!
I really beginning to wonder if all those hours I spent fairing the foils on my racing sailboats were worth the effort?

 

For racing sailboats, yes they were well-spent. A flat plate rudder welded on a round shaft cannot compete in hydrodynamic efficiency to a blade with airfoil-shaped sections. Ask anyone who has tried to change their rudder type from flat-plate to a more efficient one.
For example: http://www.boatdesign.net/forums/boat-design/rudder-design-31745-2.html#post365488
Cheers

 

When Warington Smyth wrote that perforated rudders on junks were efficient(circa 1900). He was referring to their ability to turn the boat, not to reduce drag. I remember reading an article in a yachting mag some 60 years ago that set out to prove that the drag caused by the holes not only helped the boat round but increased lateral resistance,
I'll leave that thought with the experts. Remember that before the modern era junks did not have streamlined under body shapes or keels.