Nurflugelphobia

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Aerowerx

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30 deg sweep increases the drag by 20%. Funny that Al says rudder causes 20% drag increase on his videos.
It's not just the rudder. It is also that long cylindrical thingy sticking out behind the wing that keeps the tail from falling off. What is it called? Oh, yes! Fuselage!;)

And you also, again, reference that video with questionable techniques, which is based on elliptical which is questionable itself.

You can't just pick out one parameter. You have to consider the combination of all of them. For example, I currently have a design in my virtual wind tunnel that has 29.8 degree sweep, Eppler 337 airfoil, Aspect ratio 10.3, taper 0.36 and has only 2.1% loss (CL/CD=36.6).
 

Vigilant1

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Once you've swept the wing back far enough to get a lot of area behind the CG, then reduced the incidence of the tips so they won't stall first, you've effectively got a tail--two of them, on the wingtips.
 

Aerowerx

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..... 2.1% loss (CL/CD=36.6).
Actually, XFLR5 says 0.979 "efficiency". Not sure what that is measured against, but I ASSUME it is against the mythical magical elliptical shape.

And the CL/CD=36.6 beats the Wandering Albatross, from what I have read, which has a glide ratio of about 22. (Glide ratio is the same as CL/CD, isn't it?)
 

Charles_says

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The two terms for stability are Longitudinal and Lateral.

Which do you mean? Lateral, which is side-to-side?
yes, I thought Lateral stability, is what a vertical tail promotes. Sorry for the delayed response... due to work interruptions... :)
 
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Aerowerx

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yes, I thought Lateral stability, is what a vertical tail promotes. Sorry for the delayed response... due to work interruptions... :)
Ok. Just wanted to make sure we were talking about the same thing.

Yes, you are right. A vertical tail helps with lateral stability. But you can also get lateral stability (actually, it is properly called lateral damping) with a swept wing by using wing twist. If the wing is twisted enough at the tips, you can actually get induced thrust. The "induced drag" vector actually will point forward at the wing tips. This helps with the lateral stability.
 

pictsidhe

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Actually, XFLR5 says 0.979 "efficiency". Not sure what that is measured against, but I ASSUME it is against the mythical magical elliptical shape.

And the CL/CD=36.6 beats the Wandering Albatross, from what I have read, which has a glide ratio of about 22. (Glide ratio is the same as CL/CD, isn't it?)
0.979 would be e, which is relative to a straight, flat elliptical wing.
BSLD will have a lower e.
 

Aerowerx

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No, guys, lateral stability is the plane's tendency to stay upright. A vertical fin provides directional stability which is the tendency to point forward.
Then does XFLR5 have it wrong?

When you do a stability analysis and choose "lateral" and click on "Animate", the model will swing left and right (or dutch roll, depending on which mode you choose).

If you choose "longitudinal" it will do phugoids.
 

pictsidhe

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Lateral: at 90 degrees to the longitudinal axis. Depending on how you define that, it could be side to side or up and down.

I use the terms yaw and pitch...
 

Aerowerx

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From Merriam-Webster.com:

lateral

adjective
lat·er·al | \ ˈla-tə-rəl also ˈla-trəl \
Definition of lateral
(Entry 1 of 3)

1: of or relating to the side //a lateral view
2: situated on, directed toward, or coming from the side //the lateral branches of a tree
3: extending from side to side //the lateral axis of an airplane
4 phonetics : produced with passage of breath around the side of a constriction formed with the tongue \l\ is lateral

I think the usage of the terms "longitudinal" and "lateral" are confusing. Consider a phugoid. It is usually described as a longitudinal stability problem, but is an oscillation around the lateral axis. Likewise, a Dutch Roll involves an oscillation around the longitudinal that is 90 degrees out of phase with an oscillation around the vertical axis, but is considered as a lateral axis stability problem.o_O
 

Charles_says

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Ok. Just wanted to make sure we were talking about the same thing.

Yes, you are right. A vertical tail helps with lateral stability. But you can also get lateral stability (actually, it is properly called lateral damping) with a swept wing by using wing twist. If the wing is twisted enough at the tips, you can actually get induced thrust. The "induced drag" vector actually will point forward at the wing tips. This helps with the lateral stability.
Yep! We're on the same page.

mebbe I'll add more washout for increased thrust too! (heh heh!) :)
 
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