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Sonerai II L larger rudder

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Yes, understood. I'm curious why Paz uses CG to 1/4 tail chord to define the tail arm, whereas others define it as 1/4 wing chord to 1/4 tail chord.
A guess: Pazmany is just being a bit more conservative (by basing calcs on the shortest allowable tail arm)? The CG window is typically about 25% MAC to 40% MAC. If the CG is allowed to be at 40% MAC, the plane will pivot there and it is the appropriate location for defining one end of the tail arm.
My question would be the complement of yours: Why do others calculate tail arm measured from 25% MAC?
 
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This is a question about why different sources use different definitions of tail length.
If you want to analyze from first principals, you're going to want to use CG as the basis, since otherwise the comparison is invalid. If you want to compile a database of best practices to build an empirical model from, you're going to want to use quarter chord, since you can extract that from only a three view.
 
The first Sonerai was designed for racing, so everything except horsepower was minimized, including the rudder. Many builders have added three or four inches the the rudder height, and many different shapes have been used. I added three inches of height to my rudder on my Sonerai IIL, and copied the profile of the P-40 just because I like it. It hasn't flown yet, so I can't comment on performance.
 

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Because the aircraft yaws / pitches / rolls about the CG.


BJC
Yes. Always better to have more stability than not enough. But negatives in both. Have a large amount of yaw stability will reduce your max crosswind component. Having not enough then the tail may want to wag in a little turbulence and even less stability, approaching neutral the nose of the airplane will not return to the line of flight, where you have to fly the rudder at all times . I have flown 2 homebuilt kit airplanes where if you gave it rudder in level flight and did not give it opposite rudder to bring the nose of the airplane back to the line of flight it stayed to the side . Both kitplane companies added more tail area after the first few they sold and made good flying airplanes.
The Cessna 411 was bad for waging it tail except in smooth air and had a high VMC on single engine. Even the old C-310's with the tuna wing tip fuel tanks with no baffles would wag when it's fuel in the tanks were at about 50 %.
 
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Vertical tail coefficient .020 is small-- . 045 is large. I set the JMR at .038. I like it .
Piper J-3 Cub is .022
Luscomb is .026
Navion is ,040

Horizontal tail coefficient-- .30 is small, .70 is large. I set the JMR at .508. Pitch stability is very good.
Piper Cub is .34
Luscombe is .44
Navion is .69 ( 4 seat )

Look at this picture and see how as the Horizontal tail coefficient go's down, it limits the CG envelope. The reason the Navion and most 4 seat airplanes has a high horizontal tail coefficient. Chart is for my JMR.
 

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Hi ,
I fly the Sonerai1 with the tiny rudder, I think it is enough and I like it, but you must work with your feet, like a motorglider... (it is no Cessna ;) )
I fly also with a SD1 and I think on this plane the rudder is a bit too tall...
with best regards
Juergen
 
Tim only increases the fixed vertical stab? Not the moving rudder portion?
Hmmm. You need to get friendly with TOWS chapter on High Lift Devices. Elevators and rudders are bidirectional flaps...
  • Control surfaces do not act by themselves - they amplify the forces being generated by the surfaces they are attached to;
  • Tailplanes, like all foils, have higher lift slopes when aspect ratio is increased;
  • Each increment of increased control surface chord has less increment of added lift;
  • Control forces scale with control surface area and with control surface chord - chord of the rudder counts twice, adding rudder chord could adversely impact control harmony;
  • Control surfaces can be set up to float a little or a lot. Importantly they can add to stability a lot or a little;
  • Stability is a matter of the airplane's tendency to align when tiny yaw and pitch deviations occur.
So, if you think your ratios of control forces for aileron/elevator/rudder are perfect, leave rudder alone and increase chord on the vertical stab.

If your rudder forces feel a little light compared to the other controls, add a little span to both stab and rudder.

If your rudder control forces are way light AND any yaw instability you have goes away completely when you press down on both rudder pedals, centering springs on the rudder might be enough;

If your rudder control forces are way light AND rudder pressure does not cure things, maybe a bigger rudder and centering springs will do it.

The field experience with both increased span and increased rudder chord should provide excellent guidance, while the above comments provide answers on why they work.

Billski
 
Vv = 0.05 for the Sonerai and 0.04 for a C-172 means the Sonerai should be more stable in yaw than the Cessna, right? And yet the opposite was my experience.
Tail volume coefficient has nothing to do with how far ahead of cg the prop or or forward fuselage extends ( both destabilizing), how much the aft fuselage stabilizes, and others. While it can be used for a "quick check", more thorough analysis is usually needed.
The Sonerai always fish-tails in bouncy air. Others have said the turtle deck partially blanks out the vertical stabilizer and if that's the case then the formula for Vv doesn't get at the whole picture.
Turtle deck might be in there. The engine and prop always looked "long" (proportion-wise) on the Sonerai - area forward of the cg and prop distance ahead of cg have big effects.
It is difficult to design a tail that's too large is an interesting observation. Look at Van's RVs - their vertical stabilizers are pretty big to my eye. That's what convinced me that I wasn't likely to come to grief doing this.
Yes!

Billski
 
I made my vertical fin and rudder bigger while repairing accident damage but the airplane hasn't flown yet. The height increase is seven inches over the standard 2LS tail, based on advice I got from Tim Kline, who was the first to do this as far as I know. I hope to find out this spring whether the taller fin achieves what I wanted.

Ed
2LS
The modified fin looks more appropriate, doesn't it?
 
Yes, understood. I'm curious why Paz uses CG to 1/4 tail chord to define the tail arm, whereas all others seem to define it as 1/4 wing chord to 1/4 tail chord.
Easier. Philisophically we fly the wing. Realistically we mostly fly the wing...

This is only a check that you are in the right area, not that you have optimum nailed down.

Once you go over to the idea of rotations happening (mostly) around the cg, now you have to get into where your forward and aft cg limits are. It is not difficult, and Excel or Matlab allow you to automate the calculations of neutral axes, static margins, even ability to lift the nose for flare with power off and in ground effect.

I said mostly because the CG where all the masses want to rotate. But aerodynamically the airplane wants to rotate about the neutral point in pitch and yaw axes. This can get all tangled up in your undershorts if you are not careful.

Billski
 
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