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Old Dec 02, 2014, 03:50 AM
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Aerodynamic disadvantages of twin rudders.

Let's suppose that I built a single-engined pusher model which had a single boom and twin fins ... one fin at each end of the stab. The model would have a high-mounted wing, fitted with conventional inboard flaps. Somewhat like the Shadow microlight.

To save on complication, I decided to fit just one of the fins with a rudder.

Would there be any disadvantages to this?

Would it matter whether the rudder was on the RHS or the LHS?
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Old Dec 02, 2014, 05:42 AM
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Probably no disadvantage worth losing your sleep over. The one rudder will cause more drag on one side and will cause a preferential yaw toward the fin that has the rudder fitted. In general, having the rudder deflected will cause more drag, since you have to fight the weathervane effect of the other uncambered fin and have half the control surface to work with, so each control input will require a greater deflection for the same effect. But this might be compensated by the drag saved by omitting one hinge line?
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Old Dec 02, 2014, 07:07 AM
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Ya got me thinking, there.

Suppose I fit the rudder to the RHS.

Now, I demand some right yaw, to keep the model in balance, during the climb.

The combination of right rudder + the asymmetric drag should enhance that demanded yaw.

Buuuut, if I need to yaw left, the asymmetric drag would oppose the action of the rudder.


Maybe.


Anyway, probably would be difficult to detect, for the model flyer.
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Old Dec 02, 2014, 07:46 AM
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Many models fly just fine with elevator and ailerons only, and a little sideslip isn't likely to cause much trouble if the stall properties of the wing are good.
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Old Dec 02, 2014, 11:35 AM
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Anyway, probably would be difficult to detect, for the model flyer
I think you hit the nail on the head with this comment.

Some years back we saw the use of single sided elevators on old timers and sailplanes to make life easier for building. The models flew just fine and any slight roll coupling was lost within other larger effects.

I suspect that a bigger issue would be if the single sided rudder is going to provide enough yaw force. Consider that in yaw that the other fixed side will be seeing an angle of attack which forces it to generate lift that counteracts the movable side. So not only are you using half of the usual rudder size, which will give a weaker yaw, but you're also going to see the fixed side wanting to force the tail straight again.

In fact with the drag from the fixed side as it tries to straighten the tail back up fighting the drag from the displaced movable side I suspect there won't be any drag based issue. It'll still more or less balance out. At least there won't be enough of an effect to really notice.

So other than a weaker than expected yaw response I suspect you'd be fine.

From a practicality side are you really saving a lot of work though? A twin fin on a single central fuselage means a complex linkage anyway. And with the bellcranks or tail mounted servo needed to get the spanwise action to control ONE rudder it's not that much more work to match the other side up and have two rudders.
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Old Dec 02, 2014, 12:02 PM
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Yeah, I guess I'll bite-the-bullet and do it the hard way. ... twin rudders.

If I don't, I'll forever have that nagging feeling of having bodged the job.
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Old Dec 02, 2014, 01:38 PM
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If you can connect to one rudder, just link across to the other.

(From a guy with a B-25, Lancaster, A-10, all with twin rudders).
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Old Dec 07, 2014, 07:21 AM
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Not to want to spill the soup, but in my understanding most twin vertical stabilizers with rudders were used on fullsize twin-engine propeller-driven aircraft so that the yaw forces would be in the propwashes of each motor, enabling powerful yaw-compensation for potentially critically dangerous sudden one-motor-out situations [why pilots need to get twin-motor ratings].

I.e., the twin rudders were there for safety not aerodynamic efficiency per se.


[I can guess that their use on the A-10 was to help shield the motor exhaust from heat seeking surface-to-air missiles]
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Old Dec 07, 2014, 09:36 AM
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.... of course, in the case of the A-10, I think careful examination of real-world research data and findings will confirm that missile-strikes normally result in a significant aerodynamic disadvantage to the aircraft, more than outweighing the aerodynamic disadvantage of the twin vertical tails with rudders
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Old Dec 07, 2014, 07:58 PM
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The twin rudder of the A10 is indeed meant to shield the engine exhaust from the sight of IR missiles , and as a measure of redundancy. The A10 can be maintained under control with one entire half of the tail surfaces missing.
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Old Dec 08, 2014, 07:22 AM
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It is also possible that in some cases the reason for dual rudders is to fit the aircraft into existing hangars.
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Old Dec 08, 2014, 10:28 AM
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It is also possible that in some cases the reason for dual rudders is to fit the aircraft into existing hangars.
Hey, that's an aerodynamic advantage right there! Breaking off the top of the vertical tail trying to get the plane into a too-low hanger makes sideslips much more difficult later on, for ex.
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Old Dec 08, 2014, 10:57 AM
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It is also possible that in some cases the reason for dual rudders is to fit the aircraft into existing hangars.
That's actually the reason for the twin tails on the F14 and F18. Common hangars are pretty tall, but aircraft carrier hangars have an height limit.
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Old Dec 08, 2014, 01:43 PM
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interesting
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Old Dec 08, 2014, 01:55 PM
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actually just teasing: I read that there is -- or was -- a 20' clear-height limit [maybe at the elevator], the F-18 in flight with LG retracted has a height of 15 feet 4 inches (4.6 meters).

in the picture below one can easily see that with LG extended might even put it over that limit >
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