View Full Version : rudder angle of deflection

03-29-2005, 03:38 PM
I was wondering what the angle of deflection for airfoil shaped (naca 63-012)articulated rudders (two) on a tractor autogiro should be. I see that fixed wing conventional gear aircraft seem to like around 30 degreese (60 included). The Acrosport I am familiar with lifts the tail at around 15 miles per hour, so it is operating in our speed range, but it does not have a true airfoil shape rudder and only has one tail. The prop balst seems to affect it more than I would imagine since the prop is about 15 feet away from the rudder. The airfoil I am interested in stalls at around 17 degrees, but as an articulated tail it is acting like a cambered airfoil. It is hard to imagine that it would need 30 degrees. It seems to be hard to get a nice looking joint over about 24 degrees. I would be gratefull for any thoughts on this. Thank you, Vance

03-29-2005, 04:50 PM
I have been told that anything more than 20 degrees is drag, if you go past 17 degrees you get airflow seperation on a rudder and it effectiveness lessens.

03-29-2005, 06:14 PM

Have you tried using JavaFoil to generate some numbers on your airfoil?


03-29-2005, 10:50 PM
Thank you David, that is kind of what I thought. but the tail draggers around here all seem to have 30 degrees. I am not understanding what hapens at very slow speeds and how a quartering wind affects it as the speed bleeds off. I would think it would lose rudder effectivness as the speed is reduced, but it would have just as much tendency to wethercock.

Thank you Mike, I haven't tried it because I am not realy sure what to call the rudders, they are realy a cambered airfoil when the rudder is deflected. I haven't had much luck with computer modeling when I don't know the questions. I will see if I can make sense out of the airfoil program.

I am leaving for Bensen days on Friday, so I will have a lot of time to think about it. I wish I had more knoledge to apply to the thought process. Texas at night is a great place to be introspective.

Generaly the Motels I stay at don't have an internet conection so I will miss the forum.

Thanks very much for the help, Vance

03-30-2005, 06:03 AM
Wow! That is quite a program. Thank you again Mike, do you know how to use it?

No dought it will take me quite a while to learn to use it. I feel like some of the instructions are written for someone on a very different level of confusion than mine. I see where it can conbine multiple airfoiles and shift shapes. It is very exciting. Thank you again, Vance

Doug Riley
03-30-2005, 07:18 AM
A rudder with both fixed fin and hinged aft section is a flapped airfoil. There's lots of info about flapped foils in Abbott and von Doenhoeff, Theory of Wing Sections, Chapter 8. Unlike single-piece foils (which stall at 12-14 degrees AOA), flapped foils work at quite high angles of deflection of the flap.

Vance, you might find that you don't need 30 degrees if your surfaces are especially large and/or placed well back. Be prepared to increase/decrease the pedal leverage ratio and max deflection, based on test results.

See you at B-Days.

03-30-2005, 07:35 AM
Thank you Doug. What does the Autogiro do with too much deflection and under what flight conditions does it so it?

I think that with a little triming I will be able to get close to 30 degrees

I look foward to seeing you again at Bensen days. I should be able to ask better questions because I am confused on a year higher level. I am gratefull that you are so generous with your knoledge.

I purchased the book on your recomendation. It has been most illuminating. Thank you again, Vance

Doug Riley
03-30-2005, 07:49 AM
If you don't need the full 30 deg., the downside to having it will be that the pedals will be too sensitive. You'll tend to overcontrol in yaw.

The gyros I've flown (no tractors yet, dang it) used more rudder during full-throttle takeoff than in any other non-aerobatic flight regime. If you can keep the machine straight on takeoff with a cross-wind adding to the engine torque effect, then you probably have enough rudder power.

One exception: Enclosed-cabin pusher tandems may need more rudder power to recover from a slip than they do on takeoff. I doubt this would be a problem with a tractor, since there's apt to be less side area ahead of the CG and a longer tail boom.

03-30-2005, 08:00 AM
Thank you Doug, that makes sense.

How much cross wind?

Should the rate be constant as related to pedal movement?

Should the rudders move the same amount or should they have what would be called Ackerman in a car?

Thank you, Vance

Doug Riley
03-30-2005, 08:16 AM
Vance: I imagine you've forgotten many times more about land vehicles than I'll ever know. I'm guessing that "Ackerman" is some asymmetrical link that introduces extra toe-in on the wheel that's on the outside of the turn?

If so, I wouldn't bother with it on rudders. Air is soft and slushy (unlike pavement), so scrubbing your "tires" isn't much of an issue. In any event, rudder doesn't function in turns on gyros in the same way that swivelling the wheels does on cars. The rudder is a sideways wing, whose purpose is to create sideways lift. The aircraft isn't necessarily "going around a corner" when (or after) you apply rudder; more likely it's travelling in a straight line and you want to change the angle at which the nose meets the oncoming air. No need for Ackerman there.

Every rudder mechanism I've ever seen uses a simple horn at each end with a cable or push-pull link of some sort. Simple and adequate.

03-30-2005, 08:33 AM
Thank you Doug,

Yes that is exactly what Ackerman is.

I was still wondering about how much crosswind adding to the torque effect the rudder should be able to deal with?

Thank you, As always you have given me a great deal of information to digest, Vance

04-03-2005, 05:02 PM

Please continue to post your findings on this thread. You're helping me along my way by asking the same questions I am.



I just finished reading Abbott/Von Doenhoff and I'm not sure how to apply the discussed airfoil flaps to a rudder. It doesn't seem to me that the geometry accurately matches that of a rudder. Could you please explain in more detail how Chapter 8 relates to rudder design and what type of airfoil a rudder could be considered (leading edge flap, slotted flap, etc)?

Thank you both,

04-03-2005, 08:03 PM

I will get out ahead of Doug and tell you a rudder is a basic trailing-edge flap, not slotted, split, Fowler, slotted-Fowler, or Zap. A split flap should come out to much the same numbers because at high alphas your flow is not really that attached on the "top" side of the airfoil, so the effect that matters is the effect on the "underside" of the "wing," which in contemplating a rudder is the side to which the rudder is swung out (the side on which the pedal is depressed, and in which the aircraft will turn).

As a rule of thumb you can draw a line from the leading edge of the tailfin to the traling edge of the fully-displaced rudder at MAC, and that's your real effective angle of deflection of the surface. It's a rule of thumb with all the crudity that implies. And this rule of thumb actually dates from the 1930s when split flaps were common.

For figuring MAC at different planforms, there are some rules earlier in the book (forget where; MAC is Mean Aerodynamic Chord). But thanks to Dover's bargain reprints, we can all have that excellent book for much less than an engineering text usually costs!



04-04-2005, 05:29 AM
Thank you Kevin and Justin, Every bit of information helps.

Justin, I am going to see Doug at Bensen days so I was going to discuss this further there. I am not sure how to include you in the conversation. An Aeronautical engineer friend of mine said that his way of explaining the stall of a rudder is that have the angle of deflection is like halve the angle of attack. In other words with a thirty degree deflection it is like an airfoil seeing a fifteen degree angle of attack. That is one confusing explaination for why most biplanes have 30 degreese of rudder, sixty degrees included. Thank you, Vance

04-04-2005, 07:00 AM
In a slip the relitive wind is at an angle to the long axis - the rudder needs more travel than the stall angle alone - it would be hard to stall a rudder with 30 degrees or less of deflection.

Doug Riley
04-04-2005, 08:49 AM
I don't have much to add, except to observe that

1. Even when stalled, the rudder-fin unit continues to produce a righting moment in a slip and some yawing moment when deflected.

2. Extremely low-aspect airfoils (like the typical stubby gyro fin-rudder) have such a soft stall that they barely can be said to stall at all. Their "stall" is more of a gradual sag and reversal of the lift curve as the critical AOA is passed.

A friend and I tested a HS with a 2:1 span-to-chord ratio a few years ago on an instrumented boom. We found that, yes, there is no stall "break" at these low aspect ratios -- the lift just starts to decrease at some AOA around
14 degrees.