Rudder TE
01-06-2014, 11:08 PM
#2
I haven't played with one yet, but I like aerodynamics and have thought about this before so I'll give it a shot:
I believe it increases drag (to avoid building up speed in down lines), and damps the effectiveness of the rudder at neutral position (like expo), both due to the turbulent wake that is created by the blunt trailing edge.
A sharp trailing edge would theoretically let the air pass over the rudder and exit in a streamline. A blunt trailing edge would create a turbulent wake with recirculation zones (eddies and vortices really). The air passing over the trailing edge would see a low pressure zone and get sucked in, which creates the eddies. The low pressure zone behind the airplane creates drag and slows it down.
If the trailing edge were sharp, again the flow passing over the rudder would theoretically exit in a streamline. In a perfect world the plane is flying straight, the rudder is perfectly aligned, and the streamline exits perfectly axial to the airplane's path. In reality nothing is perfect. If the rudder is a half degree off to one side, the streamline exits non-axial to the flight path and thus turns the airplane. A half degree the other way turns the opposite direction. So a sharp trailing edge would be what you want for the most sensitive rudder. The blunt trailing edge works like expo in that it damps the rudder's effectiveness at neutral, so you don't get a very directional streamline exiting off the rudder. Instead you get a turbulent zone of mixing air.
I know people have done this to ailerons, too, to damp the sensitivity at neutral.
This is also the reason why you shouldn't drive with your tailgate down if you drive a pickup truck. Contrary to popular belief, the tailgate does not act as a sail (proven by the one leaf that blows around in circles while you drive across town on the highway - the air is circulating, not flowing). When you open the tailgate, you make the 'trailing edge' of the truck twice as wide as it now goes from the roof all the way down to the bed, instead of the roof down to the volume of air that was blowing around in/above the bed. So the bigger turbulent wake generates a bigger low pressure zone and creates more drag than if you leave the tailgate up.
Sorry, it's a Pet Peeve.
I believe it increases drag (to avoid building up speed in down lines), and damps the effectiveness of the rudder at neutral position (like expo), both due to the turbulent wake that is created by the blunt trailing edge.
A sharp trailing edge would theoretically let the air pass over the rudder and exit in a streamline. A blunt trailing edge would create a turbulent wake with recirculation zones (eddies and vortices really). The air passing over the trailing edge would see a low pressure zone and get sucked in, which creates the eddies. The low pressure zone behind the airplane creates drag and slows it down.
If the trailing edge were sharp, again the flow passing over the rudder would theoretically exit in a streamline. In a perfect world the plane is flying straight, the rudder is perfectly aligned, and the streamline exits perfectly axial to the airplane's path. In reality nothing is perfect. If the rudder is a half degree off to one side, the streamline exits non-axial to the flight path and thus turns the airplane. A half degree the other way turns the opposite direction. So a sharp trailing edge would be what you want for the most sensitive rudder. The blunt trailing edge works like expo in that it damps the rudder's effectiveness at neutral, so you don't get a very directional streamline exiting off the rudder. Instead you get a turbulent zone of mixing air.
I know people have done this to ailerons, too, to damp the sensitivity at neutral.
This is also the reason why you shouldn't drive with your tailgate down if you drive a pickup truck. Contrary to popular belief, the tailgate does not act as a sail (proven by the one leaf that blows around in circles while you drive across town on the highway - the air is circulating, not flowing). When you open the tailgate, you make the 'trailing edge' of the truck twice as wide as it now goes from the roof all the way down to the bed, instead of the roof down to the volume of air that was blowing around in/above the bed. So the bigger turbulent wake generates a bigger low pressure zone and creates more drag than if you leave the tailgate up.
Sorry, it's a Pet Peeve.
Last edited by Jetdesign; 01-06-2014 at 11:15 PM.
01-07-2014, 04:47 AM
#3
Hi Joe,
I'm sorry but I have to disagree with some of what you are saying.
It does add drag but it also reduces or eliminates 'dead band' at neutral.
I have found it makes the rudder trim really precise and the rudder very effective at small deflections.
I am only using 7 or 8 mm max rudder throw for everything except the stalled manoeuvres.
I am not well up on the Aero,, stuff however there is quite a good deal of discussion on this in the contra thread and some good contributions from those that are ;
Quote from that by Doug Cronkhite ;
' The flare of the rudder actually enhances the effectiveness of the surface by helping to keep flow attached when deflected. It also creates a drag rise aft of the trailing edge of the rudder, creating a stabilizing force moment. ' - post # 899
Quote from Martin Hepperle ;
' Concerning the thick trailing edges, be it just thick, an added plate or a wedge like shape there is indeed a positive effect on the efficiency of the control surface. The dead band at small deflections is usually vanishing and the flow stays attached at larger deflection angles. So this is a useful feature, but it also creates some drag. SO good for aerobatic models, not so good for speed models. ' - post # 967
Brian
I'm sorry but I have to disagree with some of what you are saying.
It does add drag but it also reduces or eliminates 'dead band' at neutral.
I have found it makes the rudder trim really precise and the rudder very effective at small deflections.
I am only using 7 or 8 mm max rudder throw for everything except the stalled manoeuvres.
I am not well up on the Aero,, stuff however there is quite a good deal of discussion on this in the contra thread and some good contributions from those that are ;
Quote from that by Doug Cronkhite ;
' The flare of the rudder actually enhances the effectiveness of the surface by helping to keep flow attached when deflected. It also creates a drag rise aft of the trailing edge of the rudder, creating a stabilizing force moment. ' - post # 899
Quote from Martin Hepperle ;
' Concerning the thick trailing edges, be it just thick, an added plate or a wedge like shape there is indeed a positive effect on the efficiency of the control surface. The dead band at small deflections is usually vanishing and the flow stays attached at larger deflection angles. So this is a useful feature, but it also creates some drag. SO good for aerobatic models, not so good for speed models. ' - post # 967
Brian
Last edited by serious power; 01-07-2014 at 04:49 AM. Reason: Typo
01-11-2014, 10:08 AM
#5
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I find it hard to believe it has any significant effect on stability or rudder authority or anything alike.
Classic aerodynamics tells us that in order to change these stability characteristics of an airplane you need to change rudder area or stabilizer area or increase rudder deflection angle.
I don't think you'll find this gimmick anywhere in the real world of full scale aerodynamic
Yoav
Classic aerodynamics tells us that in order to change these stability characteristics of an airplane you need to change rudder area or stabilizer area or increase rudder deflection angle.
I don't think you'll find this gimmick anywhere in the real world of full scale aerodynamic
Yoav
01-12-2014, 03:13 AM
#7
I find it hard to believe it has any significant effect on stability or rudder authority or anything alike.
Classic aerodynamics tells us that in order to change these stability characteristics of an airplane you need to change rudder area or stabilizer area or increase rudder deflection angle.
I don't think you'll find this gimmick anywhere in the real world of full scale aerodynamic
Yoav
Classic aerodynamics tells us that in order to change these stability characteristics of an airplane you need to change rudder area or stabilizer area or increase rudder deflection angle.
I don't think you'll find this gimmick anywhere in the real world of full scale aerodynamic
Yoav
I would not worry about too much.
A lot of people have 'self limiting' believes.
I agree with ' Classic aerodynamics tells us that in order to change these stability characteristics of an airplane you can change rudder area or stabilizer area '.
Question;
What does 'classic aerodynamics' say the effect of a rudder flair would be on yaw stability. ??
I'm betting it says that both area and section play a role.
I know the following (I'm not into aero,,, ) ;
- Flat plates work.
- Tapered sections work better.
- For lift, under-cambered sections work better again but are not suitable for sport or aerobatic flight.
Is a flared fin/rudder not two under-cambered sections integrated back to back ??
I can see more of this in F3A in future.
Brian
Last edited by serious power; 01-12-2014 at 03:38 AM.
01-12-2014, 01:08 PM
#9
Senior Member
Hmm, interesting reading, I have read about the blunt trailing edge in a book about the design and aerodynamics for rotors. It stated that a blunt trailing edge is used to create a small area of low pressure just behind the trailing edge, this helps to promotes laminar airflow at high angles of attack on the aerofoil, so maybe it also increases the rudder power at large deflections? Food for thought.
Matt
Matt
01-12-2014, 02:05 PM
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http://www.youtube.com/watch?v=TqTSyFz6DJc
Start at ~4:29, the actual wind tunnel stuff is at 5:57, and notice the air bouncing up and down at the trailing edge and the stagnation zone on the trailing edge of the upper surface.
My money's on the last inch of the airfoil (wing/stab/rudder) down to the sharp trailing edge serves as a stagnation zone and an extra leverage for the airflow to shake the surface up and down so it's cut off and the thick trailing edge is the result? Could be 100%wrong though, and it would have been interesting to see if those tests could have been done with a thick trainling edge.
What about the music though?????
Start at ~4:29, the actual wind tunnel stuff is at 5:57, and notice the air bouncing up and down at the trailing edge and the stagnation zone on the trailing edge of the upper surface.
My money's on the last inch of the airfoil (wing/stab/rudder) down to the sharp trailing edge serves as a stagnation zone and an extra leverage for the airflow to shake the surface up and down so it's cut off and the thick trailing edge is the result? Could be 100%wrong though, and it would have been interesting to see if those tests could have been done with a thick trainling edge.
What about the music though?????
Last edited by bjr_93tz; 01-12-2014 at 02:10 PM.
01-12-2014, 03:54 PM
#11
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http://www.youtube.com/watch?v=JT33jK4Zhv4
Is it wrong to enjoy watching clips like the above? Looks ike they've been performing "Harriers" with EDF's for years....
Is it wrong to enjoy watching clips like the above? Looks ike they've been performing "Harriers" with EDF's for years....
01-12-2014, 04:58 PM
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Hmm, interesting reading, I have read about the blunt trailing edge in a book about the design and aerodynamics for rotors. It stated that a blunt trailing edge is used to create a small area of low pressure just behind the trailing edge, this helps to promotes laminar airflow at high angles of attack on the aerofoil, so maybe it also increases the rudder power at large deflections? Food for thought.
Matt
Matt
01-12-2014, 05:37 PM
#13
I have flown planes with various types of flaring on the rudder since 2004, the only thing I can say with certainty that I notice is that it gives a similar effect as adding expo on the radio.
If it has other effects they must be very small on a pattern models as I could never say with any certainty that anything else really changes.
If it has other effects they must be very small on a pattern models as I could never say with any certainty that anything else really changes.
01-12-2014, 06:18 PM
#14
As many have experienced, when I took a well trimmed Bravo and added a Contra, the yaw stability deteriorated noticeably. I experimented with a lot of rudder shapes and added side area aft of the CG.
What I noticed...a thicker or flared TE on the rudder slightly increased yaw stability, and increased drag. The drag increase was more noticeable at high speed than in downlines. I didn't do much in the way of changing expo with the different rudder configurations....didn't seem to matter very much. I did have to increase rudder throw for snaps.
I ended up using the stock rudder configuration with added side are.....for my plane and setup, this yielded the best combination of rudder authority and yaw stability and did not add any drag.
Years ago, I recall the flared rudders being used because they increased rudder effectiveness near stall speeds....essentially delaying the stall of the rudder itself. I can't say that I have ever noticed that, and I have had planes over the years with rudder TE thickness from 1mm to 8mm....and the biggest difference I notice is the thicker TE is draggier.
Regards,
What I noticed...a thicker or flared TE on the rudder slightly increased yaw stability, and increased drag. The drag increase was more noticeable at high speed than in downlines. I didn't do much in the way of changing expo with the different rudder configurations....didn't seem to matter very much. I did have to increase rudder throw for snaps.
I ended up using the stock rudder configuration with added side are.....for my plane and setup, this yielded the best combination of rudder authority and yaw stability and did not add any drag.
Years ago, I recall the flared rudders being used because they increased rudder effectiveness near stall speeds....essentially delaying the stall of the rudder itself. I can't say that I have ever noticed that, and I have had planes over the years with rudder TE thickness from 1mm to 8mm....and the biggest difference I notice is the thicker TE is draggier.
Regards,
01-17-2014, 12:29 AM
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Hi Joe,
It does add drag but it also reduces or eliminates 'dead band' at neutral.
I have found it makes the rudder trim really precise and the rudder very effective at small deflections.
I am only using 7 or 8 mm max rudder throw for everything except the stalled manoeuvres.
(...)
Quote from that by Doug Cronkhite ;
' The flare of the rudder actually enhances the effectiveness of the surface by helping to keep flow attached when deflected. It also creates a drag rise aft of the trailing edge of the rudder, creating a stabilizing force moment. ' - post # 899
Quote from Martin Hepperle ;
' Concerning the thick trailing edges, be it just thick, an added plate or a wedge like shape there is indeed a positive effect on the efficiency of the control surface. The dead band at small deflections is usually vanishing and the flow stays attached at larger deflection angles. So this is a useful feature, but it also creates some drag. SO good for aerobatic models, not so good for speed models. ' - post # 967
Brian
It does add drag but it also reduces or eliminates 'dead band' at neutral.
I have found it makes the rudder trim really precise and the rudder very effective at small deflections.
I am only using 7 or 8 mm max rudder throw for everything except the stalled manoeuvres.
(...)
Quote from that by Doug Cronkhite ;
' The flare of the rudder actually enhances the effectiveness of the surface by helping to keep flow attached when deflected. It also creates a drag rise aft of the trailing edge of the rudder, creating a stabilizing force moment. ' - post # 899
Quote from Martin Hepperle ;
' Concerning the thick trailing edges, be it just thick, an added plate or a wedge like shape there is indeed a positive effect on the efficiency of the control surface. The dead band at small deflections is usually vanishing and the flow stays attached at larger deflection angles. So this is a useful feature, but it also creates some drag. SO good for aerobatic models, not so good for speed models. ' - post # 967
Brian
Many Ultraligh aircraft uses this type of devices also in ailerons in order to keep better control characteristics at low speed.
A thick TE on any surface would do the work in any case; but a small piece of balsa/plywood will improve the performance of the surface too.
