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Building Tips and technical articles. => Building techniques => Topic started by: Jim Morris on February 28, 2007, 10:04:44 AM
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Has anyone compared an airfoil rudder as to an offset? Im thinking of using the airfoil on my new Avanti. I kinda got away from puting much off set into any rudders.
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I have never compared them (on the same design to actually test for differences), but in my opinion a rudder with adjustable offset is better than a rudder with an airfoil. I don't like anything that influences trim as much as a rudder to be "fixed".
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Hi Mike,
I can understand that logic! I am in a quandry, though. I have heard the theories from some "top" pilots/designers that as rudder (unless taken to the extreme) affects the flight path mainly only after the engine cuts.
As to the airfoiled vis a vis "offset" (and adjustable), I have used both on several occasions................
So many choices to trimming techniques!
Bill <><
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On the last couple of planes, I haven't used any rudder offset and no adjustment. Just have to make sure that thing is on straight. But I agree with Mike. If you are going to do it at all, make it adjustable.
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Yea, now that you mention it, I fly a modified Time Machine and it has an adjustable rudder, I ended up with just very little off set, seemed to have better flight qualities. Its not too late to make the Avanti adjustable.
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Hi Bill,
I have never heard of the rudder only being effective when the power is off??? Hmmmm......
As long as there is airflow over the rudder it is effecting the flight path. If the airspeed changes the rudder will create more or less line tension/yaw. I may be wrong, but I believe it can contribute to a wiggle in the corners (along with leadout position). This is also due to the change in airspeed.
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Hi Bill,
I have never heard of the rudder only being effective when the power is off??? Hmmmm......
As long as there is airflow over the rudder it is effecting the flight path. If the airspeed changes the rudder will create more or less line tension/yaw. I may be wrong, but I believe it can contribute to a wiggle in the corners (along with leadout position). This is also due to the change in airspeed.
Hi Mike,
I remember this from over at SSW some time ago with Brett and some others talking about it. They used an example of losing the rudder and it not having much effect.
I, too, believe it HAS to have an effect, Al's rudders definitely do, but I admit I don't have th eknowledge that those guys do. I think they also alluded to Bob's Genesis (minimal "rudder").
??? ??? ???
Bill <><
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I have mentioned this before, I lost part of the rudder on the Time machine and it was not fliable in the overhead stuff, so I am 100% convinced it does have an affect. Now at 3 sec laps anything will fly.
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The effectiveness of the rudder sort of depends on a number of factors; not the least of which is, how big is the thing. I can't imagine that Bob Hunt's Genesis "rudder" has much of an effect on anything, especially with the speeds we fly. On the other hand, some designs have very large rudders and can have a great effect. There is also the issue of side area aft of the CG. More side area aft mean greater directional stability or less waggle in corners (to a point). Look at the amount of side area aft of the CG on Brett's Infinity. The thing is a barn door. I his case, I don't imagine the rudder contributes substantially to what is already there.
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I totally agree Randy, my Time machine is modified to look like a 30s air racer, not much flat side area.
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I remember this from over at SSW some time ago with Brett and some others talking about it. They used an example of losing the rudder and it not having much effect.
I, too, believe it HAS to have an effect, Al's rudders definitely do, but I admit I don't have the knowledge that those guys do. I think they also alluded to Bob's Genesis (minimal "rudder").
Bill <><
Bill: There you have it! If Brett said it on SSW it must be true.. S?P
So I must be doing something wrong. I use engine offset and rudder offset and line rake. Of coarse I do not suffer from line tension anywhere!! I have seen many top flyers almost to their knees fighting for line tension on the overheads. But not me so I guess I don't need engine,rudder,line rake. One other thing I might add is I get no wobble,hinging or flop.
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Hi Guys,
years ago I chose to not spend time on finishing and a lot more time on experimenting with design stuff. I flew slower 5.4-5.5 lap times on 64 ft center handle to center of plane line length on a classic size 600 sq"-52" span plane.
I made 3 interchangeable rudders and fin combination.
1. airfoiled fin & rudder 2. flat adjustable rudder
3. flat offset fin and rudder. ( this means that the entire fin& rudder are straight but are mounted at an offset angle on the fuselage.)
I then flew multiple days of flights with all 3 options mounted and kept a flight log on effects on line tension. I did not change anything else as I had already discovered the best leadout position for all 3 options with the earliest tests.
The results:
1. The airfoiled fin/rudder and the flat-adjustable fin/rudder set straight were
the poorest performers in providing additional line tension. I was suprised
so I took several more days to just test just the airfoil vs the straight flat-
adjustable fin/rudder and after at least 40 flights I came to the conclusion
that the airfoil fin/rudder is no more effective than a flat fin/rudder .
Paul Walker and others have used a flat stab/elevator as opposed an
airfoiled stab/elevator so it may explain my results.
2. The flat adjustable fin/rudder seemed to help line tension with 1 to 2
degrees of offset but too much offset caused yaw and hurt line tension.
3. The flat fin/rudder only needed to be offset 1 degree or a little less to
equal the results of 2 degrees of rudder offset.
The net results of my tests is I have again never bothered with the extra work of airfoiling the fin/rudder on any airplane ever again and even though
an offset fin/rudder probably produces less drag with a lower offset angle I
have chosen to use a adjustable rudder because it is adjustable.
I don't claim that my tests are any kind of absolute answer for everybody but they were objective empirical tests in which I tried minimize variables in my test proceedures to results I could use. I am now too old to take on this kind of energy consuming project but I am glad I did it because we didn't have all the analysis & information that is available today and I used what I learned in all my planes.
Till next time,
Pat Robinson
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Great info, thanks Pat
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Hi Pat,
I whole heartedly agree with your findings. In fact my last 6 airplanes have had a flat (1/8" thick) rudder with 1 to 3 degrees permanent offset. The reason for the thin rudder is that it actually flexes and in my opinion somehow is self adjusting to keep the tail of the airplane flying on a straight path. The idea of a semi flexible self adjusting rudder comes from the stone ages of RC when rubber escapements were used to power/move control surfaces (lightweight servos had not been invented yet). OK, that might seem kinda of crazy reasoning but it sure works on my fleet of Divas.
Phil
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Re No. Eleven./If its set straight its effectively opperating at Neg. Incidence.
Noncomenture : Vertical Stabiliser .
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Here is what I learned about rudders.Lets look at a high speed competition free flight with a rudder the size of a Nobler. With the rudder straight a small tab 1/4 inch wide one inch tall offset 1/16 inch will turn the plane any way you want to go under power. In the glide it has no effect. One day flying a stock Flight Streak the rudder came off during flight and the plane flew better until the motor quick and then there was no line tension. Learned these two lesson in the late 50's. I have used almost no rudder offset on controline models ever since to gain line tension under power. Some models need a rudder for the glide portion but no offset is needer if the line rake is OK. Planes with a lot off line rack need offset to get the rudder back straight into the airflow. I think miss adjusted planes need offset.