Nurflugelphobia

The drag increased not because if sweep, but because the lift was moved outboard. If the video guy had twisted the wing to compensate for sweep effects, the drag would have remained the same.

 

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).

 

I'll see what my aspect ratio is when I get home. L/D is in the same ballpark.

 

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.

 

...and, VOILA!!!! Bell Shaped Lift Distribution!

Elliptical puts too much load near the tips.

 

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?)

 

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.

 

0.979 would be e, which is relative to a straight, flat elliptical wing.
BSLD will have a lower e.

 

Don't you get something sort-of BSLD-ish by using a straight center section and only sweeping the tips back?

 

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.

 

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...

 

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.

 

I lean towards using NASA’s terminology / definitions. https://www.grc.nasa.gov/www/k-12/airplane/Talks/FlightControl.ppt


BJC

 

Yep! We're on the same page.

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

 

I personally have no fear of the concept of a flying wing, but as evidenced in the other thread I’ll probably keep my distance from some of its proponents...

 

Also Horten mentions this feature..so it must be true.