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Author Topic: Excalibur Rudder offset  (Read 2506 times)

Offline TroyMartin

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Excalibur Rudder offset
« on: April 26, 2017, 09:06:09 PM »
I was wondering if guys are using rudder offset on their Excaliburs??   The plans show no rudder offset but I would rather be on the safe side..

Offline Dan McEntee

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Re: Excalibur Rudder offset
« Reply #1 on: April 26, 2017, 10:30:10 PM »
   Build it as per plans. Look at all the photos posted here and in the Photo Gallery, you will almost never see rudder offset. Some like to have an airfoil shape in the vertical fin, with the flat side outboard. If you do that, Be sure to sand some phillips entry into the outboard side. I have a OTS model, built by one noted modeler for another, and it had this airfoil shape in the vertical fin. When I acquuired the model, it looked like it had hardly been flown, and after my first flight or two, I knew why. The airplane turned in on launch and the lines got quite slack . I'm getting older and can't back pedal like I used to! I looked for the cause and tried trimming things every way to Sunday, and I  finally looked right down the length of the fuselage from nose to tail. I could see all of the airfoiled side of the rubber, and none of the flat side. Sitting on the ground and having prop wash hit it, it was the same as putting in left rudder. When I got the airplane in the air, it got better, but still needed some trimming. I placed a straight edge long enough to reach the nose on the flat side of the vertical fin, and the edge laid about 1/2 inch to the right of the center line at the spinner, confirming what I thought I saw looking down the nose. I made a nice, straight cut at the base of the vertical fin and cut it off. I tack glued it to the fuselage and rechecked it with the straight edge. Now the edge against the flat side was about 1/8" to 3/16" to the left of the center of the spinner. The next test flights had a nice, straight take off. Now I can retrim the model and touch up the finish on the fuselage. The way this is supposed to work, is the airfoil side creates "lift" or low pressure on the inboard side and pulls the back of the fuselage in and the nose out, helping with line tension. This isn't anything new and has been done for years. It had a much more subtle affect than a cranked out rudder, and looks better also in my opinion. When in doubt, always follow the plans and instructions.
   Type at you later,
     Dan McEntee
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Offline TroyMartin

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Re: Excalibur Rudder offset
« Reply #2 on: April 26, 2017, 11:05:30 PM »
So, if you were to airfoil the rudder and install it where the flat side (right side) of the rudder is parallel to the right side of the fuse then this would be enough to keep tension or would it be better to shape the rudder symmetrically on both sides and then install it where the leading edge of the rudder is to the left of the center line and the rear of the rudder is to the right of the center line so leading edge of the rudder is angled to the left ??

Offline Dave_Trible

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Re: Excalibur Rudder offset
« Reply #3 on: April 27, 2017, 06:06:08 AM »
I think what Dan was basically saying is that he had ended up with a bit of rudder inset with the rudder flat not parallel to the fuselage.  The best bet is just leave the flat sheet rudder flat and mount it straight away, or just enough offset to insure it's not going the wrong way.  Some combination of minor engine offset and lead out position will give you the necessary line tension.  Older designs called out for rudder offset but adjustable leadouts and having learned more about trimming has pretty much changed that way of thinking.

Dave
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Offline Dan McEntee

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Re: Excalibur Rudder offset
« Reply #4 on: April 27, 2017, 09:50:46 PM »
    What Dave said. Leaving it flat on both sides is quicker and easier to build, and right on the center line of the fuslelage is fine. You just don't need all that rudder cranked in. It just adds drag in the long run, and upon cranking hard corners while you are learning to be smooth can cause all sorts of weird stuff to happen. I have always been a fan of the Excalibur, and gotta build me one some time.
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  Dan McEntee
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Offline Brett Buck

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Re: Excalibur Rudder offset
« Reply #5 on: April 28, 2017, 06:48:14 AM »
   What Dave said. Leaving it flat on both sides is quicker and easier to build, and right on the center line of the fuslelage is fine. You just don't need all that rudder cranked in. It just adds drag in the long run, and upon cranking hard corners while you are learning to be smooth can cause all sorts of weird stuff to happen. I have always been a fan of the Excalibur, and gotta build me one some time.

     Certainly do not airfoil the fin and make it nominally straight ahead.  I would certainly make it with an adjustable rudder, so that very small misalignments in other areas can be corrected. I would expect that the rudder would be extremely effective and the roll/yaw coupling  would also be expected to be very strong (for reasons described in the other thread*). Also, the fuselage is not too strong AND the fin is very tall with nothing on the bottom (moving the Z CP position high, which also contributes to roll/yaw coupling), so even the tiniest skew of the fuselage to the wing/tail would be very difficult to work around otherwise.

   Note that in this case, it might be dynamically helpful to put in some anhedral to reduce the coupling. But it's so ugly that even in cases where I thought it might be helpful, I didn't do it.

   Be careful to sand the LE to as large a radius as possible. That pointy airfoil could be very prone to stalling if left unshaped.

     Brett

*note that apropos of the other thread, if you subscribe to the "blanking" theory, you would expect that constant-chord wings would have less effect coupling the roll and yaw together compared to tapered wings. But in fact, constant-chord wings are vastly *more* sensitive to yaw angle.
« Last Edit: April 28, 2017, 07:47:15 AM by Brett Buck »


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