Long tail moment?

[Zerts]

The Chrysalis 2M is starting to look attractive to me for my next plane.

The only thing I don't understand is the "thermal-signalling" characteristics of a sailplane with a long tail moment (as the Chrysalis is advertised) - is there an explanation of how that works?

[lincoln]

Last summer I flew an Ava for a while, after flying gliders with relatively short tails. Because of its long tail, the Ava is steadier than many other gliders. It doesn't wiggle as much, but there are other clues to use to read lift and after getting used to it, it's pretty easy. Meanwhile it's easier to fly clean with the damping from the long tail.

The tail doesn't interfere with:
-flying faster or slower due to changes in air movement
-raising a wing tip
-vertical motion of the glider

It's just that the wiggles are gone.

[glidermang]

Zerts:

I have had the joy of building at least 4 and maybe 5 (I can't remember) 1.5M Chrysalis's (or is that Chrysali?) and they were all high quality, unlike my flying skills, which is how I came to have 4 (or 5) total. Currently I fly exclusively DLG, and they typically also have "long tail moments", compared to other classes of gliders.

What the proportions of my DLGs give me are strong visual cues about airplane response. If the tail rises or falls, I find it more visually apparent than if the tail moment were shorter. It may be just subjective, but I would be extremely surprised if there were something "aeronautical" about thermal response.

I would expect strong visual response to thermals from a Chrysalis 2m, with its light weight and low wing load. That nice, big wing and that nice, big tail should telegraph changing air very well. And, I believe that is the meaning of the advertisement on the DJ Aerotech web site.

Mr. Stackhouse of DJ Aerotech has always been open about how his products work. I suggest you go to his web site and ask him. That would be the WORD for sure. Maybe he will read your question here, and answer it for all of us.

Yours, Greg

[lincoln]

Oh yeah, I flew the hlg Chrysalis for quite a while. Nice handling and easy to thermal. Plus I had a long tailed dlg. Also easy to thermal. Maybe took a little bit of getting used to. On the dlg, I found that if I put the cg fairly far aft, I could fly it around pretty well, but I couldn't "read" it. i.e. couldn't tell when it was in lift. But if the Chrysalis 2M is like the hlg, I don't think the tail is quite long enough to get into that kind of behavior. Or maybe I just never pushed the cg that far back.

[j.m.]

When I flew one of Plastic Paul's DLG's with a far aft CG, it literally "jumped" when it hit the slightest bit of lift. I think airfoil selection (undercambered or cambered) has a big role to play in a plane's reaction to thermals.

[Zerts]

Yep its funny, my Easy Glider Pro has less wing loading than my Elektro-Uhu, and even tho the EZGP is much larger, it doesn't seem to "signal" lift like the Uhu. But that could be also due to its AUW is much more than the Uhu. The Uhu has a low aspect ratio, undercambered wing and a long tail, and the EZGP has a mostly cambered wing of much higher AR and a relatively shorter tail.

[harleym]

I guess all who are referring to "tail moment" mean to say "tail moment arm". The Smooth Genie Pro at http://www.geniebuild.com/harleys_genie.html has a longer than typical tail moment arm. See the Supplemental Smooth Genie Pro file. It greatly smooths the handling in the pitch axis which is favorable except when doing the abrupt nose over prior to the vertical zoom. However, in all other flight characteristics, handling is enhanced.

[Don Stackhouse]

As far as signalling lift better, the longer tail moment arm does two things:

1. By definition, it increases the distance between the wing and the tail, so there is more of a difference between the air the wing is flying in and the air the tail is flying in. As the plane encounters lift, the wing encounters it first, resulting in a pitch attitude disturbance.

2. The longer fuselage makes these pitch attitude disturbances easier to see.

As far as handling, the static stability and control authority are linear with both tail moment arm and tail area. However, dynamic stability (the ability to damp out oscillations) is linear with tail area, but proportional to the square of the tail moment arm.

If you double the tail area, you get twice the static stability, control authority and dynamic stability.

However, if you double the tail moment arm, you get twice the static stability and control authority, but FOUR TIMES the dynamic stability.

In my experience, dynamic stability is a bigger factor in handling quality than static stability.

However, there are downsides to having too much tail mometn arm. Getting proper balance between spiral stability and dutch roll resistance becomes trickier, aeroelasticity issues (such as flutter, and bending of the tail boom when you move a control surface) become more of an issue, and the maneuverability of the plane can become a problem. Although sufficient chord and deflection in the control surfaces can offset this, a plane with too much tail moment arm can end up with the nimbleness of a school bus if the designer doesn't do their homework.

As always, there are good things, but also too much of a good thing. If you do it right, you can get a really sweet handling airplane. Go overboard and you can get something that's tough to build and an unresponsive truck to fly.

[Zerts]

Thanks Don, very excellent explanation.

[harleym]

Don's learned comments are enlightening and most welcome. There is always a practical limit to a variation.

The way the wood & glassed-over Smooth Genie Pro fuse is constructed (with CF laminate sandwiched between ply doublers in critical areas) works well in practice. Making it 5" longer than the regular big Genie fuse was purely arbitrary and pleasing to my eye.

Strengthwise, I don't think it would have been practical to do something that slim in fiberglass, even with CF reinforcement in the layup.

[Charged]

Well it seems it is the right time to ask about differencies in two tail types . What do you think about a tail where the stab is ahead of the fin like Supra and the X tail like Xplorer differencies. Supra like tail fin has a longer tail moment in the yaw axis of the wing . But which is more efficient than other .?

[harleym]

My own experience with the "dual tail moment arm" is zero, so I leave that up to the threorists. Beauty is said to be in the eye of the beholder. The conventional tail pleases mine and can be sturdily mounted on what I think of as a "real" fuselage. Apparently pod and boom designs look good to a lot of fellows but border on being offensive to my eye.

I guess it's akin to some guys preferring brunettes rather than blondes. I recall meeting a brunette who took my eye. We set up a date for the weekend. When I picked her up, she'd dyed her hair blonde, wrongly assuming the "gentlemen prefer blondes" saying was true. The blonde image turned me off. I lost all interest.

[j.m.]

I think the pod and boom fuse design is only sexy when it has the right wing, tail, and pod shape to it. Otherwise, it can indeed get pretty ugly.

But the main reason folke use it is because it has unmatched weight to strength ratios.

And it's less dragy.

[schrederman]

Quote> But the main reason folke use it is because it has unmatched weight to strength ratios.... And it's less dragy.

I am not so sure of this, after reading about some of the instances of stabs blowing off in competitions. I'm not sure about the draggy part, either. All of that V-mount hanging out there is a lot of parasitic drag. That's the part that increases with the square of airspeed. I've not read anywhere where this is proven to have less drag than a normal fin-stab junction.

And... pod/boom fuses are in fact... UGLY... to me, at least...

Jack

[lincoln]

I fly an "ugly" Mantis, so I don't take that beauty contest stuff all that seriously. I think part of the idea with the two different arms is that you can get rid of some intersections between the tail surfaces. If damping is the concern, the biggest worry there is the fin. The wing has a MUCH larger moment of inertial in yaw than it does in roll. so putting the fin further back makes sense.

It's not clear to me that one has to use the funny stab mount, although it does hold the stab up there without a whole lot of intersection area, even if the area that there is is kind of messy. I suspect that this has been worked out carefully. Particularly since if you examine something like the Daedelus airplane, you will see a v mount or similar. Or so I remember. BTW, Helmut Lelke was using this kind of stab mount, made of metal, back in the '80s. And I don't think he ever blew it up, despite some very hard launching.

I watched a lot of stabs on platforms NOT blow up yesterday, despite extremely vigorous launches.