Sonerai II L larger rudder

[BJC]

I wonder why Paz does that

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


BJC

 

[mcrae0104]

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

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.

 

[Vigilant1]

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?

 

[mcrae0104]

Why do others calculate tail arm measured from 25% MAC?

I don't know. Using the CG makes more sense to me, but I'm guessing maybe it doesn't matter much in most cases. At any rate, the tail volume coefficients are just a first-pass tool to get in the ballpark for stability.

 

[Pops]

On the JMR I used 25% and with me and full fuel my flying CG is 25.5 %.

 

[addaon]

I wonder why Paz does that.

Moments act around the CG.

 

[mcrae0104]

Moments act around the CG.

Please see post 22. This is a question about why different sources use different definitions of tail length.

 

[addaon]

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.

 

[Elviar]

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.

 

[Pops]

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 was at about 50 %.

 

[BJC]

Always better to have more stability than not enough. But negatives in both.

Yup, there is a sweet spot for each type of airplane (aerobatic, STOL, CC, etc,) and each pilot.


BJC

 

[Pops]

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 of the CG envelope. The reason the Navion and most 4 seat airplanes has a high horizontal tail coefficient. Chart is for my JMR.