Rudder Deflection: What is the maximum effective angle? - RC Groups
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Oct 13, 2017, 09:10 PM
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Rudder Deflection: What is the maximum effective angle?


Hi, All,

I am sure that most (many?) of you have built kits which give the rudder deflection as "all you can get".

Now that is patently nonsense. If one obtained true maximum it would be 180*, folding the rudder parallel to the centerline and achieving zero effective rudder!

So we know that the maximum effective deflection is greater than zero and less than 180. What about 90*?

Well that would place the maximum rudder area into the airstream....but I don't see where it would generate a force at right angles to the aircraft centerline. Intuitively is would simply act as an off thrust line drag brake. Is that effective as a rudder?

A 45* deflection is intuitively satisfying...it should maximize the force generated at right angles to the airstream, geometrically at least....but does it, really, or is 30* better? And why, or why not?

I am sure the 3D pilots out there either use or have tried rudder deflections greater than 45*. What have you found?

I thought this would be an easy one until I really tried to figure it out. Please note that I am talking about rudder, not elevators and ailerons....those get involved with changing the lift of the surface and are a much more difficult (for moi) analysis.

TIA for all your comments,

Larry
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Oct 13, 2017, 09:38 PM
Registered User
The maximum effective rudder deflection is the one that makes the vertical fin stall.

To determine this, you really need a wind tunnel. Without one, you could do knife edge maneuvers at a safe altitude, and keep increasing the deflection until you found the answer.

I don't think there's a simple theoretical maximum, but 30 deg is probably as much as many typical setups can handle. That said, you will still get some effectiveness out of a stalled fin/rudder, and maybe you want higher deflection for other reasons?

Watch out for rudder reversal in a tailslide. A backward driven rudder jamming the elevators can ruin your whole day.
Oct 13, 2017, 11:18 PM
B for Bruce
BMatthews's Avatar
But even when stalled it can be shown that some airfoils not only continue to develop lift but that the amount of lift rises with further increases in the angle of attack.

Besides, it can likely be shown that even at some more moderate angles that the rudder is often carrying a big "stalled" separation bubble from the hinge line back.

Keep in mind too that it's not JUST the rudder. The rudder along with the fixed fin forms a variable camber and variably angle of attack flying surface combination. So technically even with the rudder at 90 degrees the "airfoil" the surfaces form is only at an angle of incidence of some smaller angle depending on the relative chord of each.

If you want a real extreme check out the super throw of the stabilator in this video and the resulting pivoting style of "loops" that occurs for the fist time at 1:20 and then a few more times further in. Clearly the stabilator is still "flying" even at that radical angle.


Now if we're talking about a model sailplane looking around for lift where we don't want to generate any more drag than we must then that would be a different story. In this case we would not want to push the rudder to where the air cannot flow over the hinge line and re-attach smoothly to the rudder. In such a case we're likely looking at something in the order of 10 to 15 degrees. But I've got nothing to point to that shows or otherwise indicates what we can get away with before the rudder produces a larger size separation bubble.

Hardcore Ultrabatics / VTPR with Le Fish (4 min 32 sec)


I do know that when I deflect wing flaps that up to around 40 degrees that they work relatively linearly. But past 45 there's very much a diminishing return of effect to angle And really all I get from the second 45 is a little more drag on the glider.
Oct 14, 2017, 10:35 AM
Registered User
richard hanson's Avatar
Max rudder?
How long is a piece of string.
It is strictly a case by case thing
A full flying rudder requires less deflection than a hinged one
On little foam stuff , 45 degrees is typical
On a large model of a sport design much less may half thatmay be all you want
Oct 14, 2017, 11:20 AM
Registered User
Quote:
Originally Posted by BMatthews
But even when stalled it can be shown that some airfoils not only continue to develop lift but that the amount of lift rises with further increases in the angle of attack.

Besides, it can likely be shown that even at some more moderate angles that the rudder is often carrying a big "stalled" separation bubble from the hinge line back.

Keep in mind too that it's not JUST the rudder. The rudder along with the fixed fin forms a variable camber and variably angle of attack flying surface combination. So technically even with the rudder at 90 degrees the "airfoil" the surfaces form is only at an angle of incidence of some smaller angle depending on the relative chord of each.

If you want a real extreme check out the super throw of the stabilator in this video and the resulting pivoting style of "loops" that occurs for the fist time at 1:20 and then a few more times further in. Clearly the stabilator is still "flying" even at that radical angle.


Now if we're talking about a model sailplane looking around for lift where we don't want to generate any more drag than we must then that would be a different story. In this case we would not want to push the rudder to where the air cannot flow over the hinge line and re-attach smoothly to the rudder. In such a case we're likely looking at something in the order of 10 to 15 degrees. But I've got nothing to point to that shows or otherwise indicates what we can get away with before the rudder produces a larger size separation bubble.

https://www.youtube.com/watch?v=_oBuqhEg0xU

I do know that when I deflect wing flaps that up to around 40 degrees that they work relatively linearly. But past 45 there's very much a diminishing return of effect to angle And really all I get from the second 45 is a little more drag on the glider.

Note how well that lightweight sailplane is handling the wind.
Oct 14, 2017, 09:11 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by EXF3BMAN
Note how well that lightweight sailplane is handling the wind.
And excellent point. And it goes to show us that there's more than one road to Rome.
Oct 15, 2017, 11:37 AM
Scott
Pylonracr's Avatar
Quote:
Originally Posted by EXF3BMAN
Note how well that lightweight sailplane is handling the wind.
Is that you Shawn????
Welcome back Meathead.

Scott
Oct 15, 2017, 11:47 AM
Registered User
Quote:
Originally Posted by pylonracr
is that you shawn????
Welcome back meathead.

Scott

shhhhhhh..............lol
Oct 15, 2017, 10:52 PM
Scott
Pylonracr's Avatar
Busted...................
Oct 23, 2017, 07:53 AM
Registered User
Quote:
Originally Posted by BMatthews
And excellent point. And it goes to show us that there's more than one road to Rome.
According to popular wisdom, all of them.
(I don't think that sailplane is THAT light, though, it is a slope aerobatics design. I love that video BTW)

[edit:]
Oh, look what I found!
Traditional vs Ultralight Builds of Le Fish Aerobatics Glider (6 min 11 sec)


And in case someone wanted to build one:
https://www.rcgroups.com/forums/show...d-a-Swiss-Fish
Last edited by Brandano; Oct 23, 2017 at 08:12 AM.
Oct 23, 2017, 12:53 PM
B for Bruce
BMatthews's Avatar
That's a great comparison video. And it looks to me like it's a slam dunk that the one in my video is one of the superlight versions just by the way it can stop in the air and start flying again. In fact the color scheme and wing cutouts along with looking a lot like the same piece of slope and trail makes me think that both our videos are from the same person. And that it's the same lightweight model. Just before he modified the stab throw to reach 90 or near to 90 for those "spin on a string" loops.

Just an incredible model design.... and some incredible piloting.
Oct 23, 2017, 07:17 PM
Registered User
ShoeDLG's Avatar
Quote:
Originally Posted by BMatthews
If you want a real extreme check out the super throw of the stabilator in this video and the resulting pivoting style of "loops" that occurs for the fist time at 1:20 and then a few more times further in. Clearly the stabilator is still "flying" even at that radical angle.
Even though the incidence of the stabilator may be very high, the ratio of pitch rate to forward speed is so high during those maneuvers that the angle of attack seen by the stabilator may not be all that high.
Oct 24, 2017, 05:44 AM
An itch?. Scratch build.
eflightray's Avatar
Here's a few close up views of control movement with a slope glider, the 'tumbling' at 1:20 on show a lot of elevator, (all flying stab) --


Fusion with chase camera (2 min 45 sec)



Ray.
Oct 24, 2017, 03:20 PM
Registered User
ShoeDLG's Avatar
Quote:
Originally Posted by eflightray
Here's a few close up views of control movement with a slope glider, the 'tumbling' at 1:20 on show a lot of elevator, (all flying stab) --
A lot of stabilator deflection, but with the accompanying high pitch rate, possibly less than stall angle of attack on the stabilator.
Oct 24, 2017, 03:59 PM
An itch?. Scratch build.
eflightray's Avatar
Back to free flight dethermalizers with pop-up tails ?


Ray.


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