This is posted on the Ransclan.com homepage. The S-20 is looking sick! The gear legs look a bit longer than they did on the first look plane. Or maybe that's just my mind playin tricks on me. The baggage area looks HUGE now that there is fabric on the fuselage. I like it. Hopefully its a performer like the S7.

It has the S-7 wing, so other then a wider fuselage should be a similar performer. Don't know how the weights will compare yet. It sure seems to be a winner so far. It has made this long time tandem pilot think again about the advantages of side by side seating. Ain't going there mind you, just thinking about it!, I like my airplanes skinny.

Does the S-20 still have the "ultralight" style leading edge tube spars, and not use an "airplane" style main spar at 25-30% chord? The naked photo posted does not show this clearly, but the angle between the wing struts seems to show the strut heading up toward the leading edge.

At this stage of aircraft development, where the RANS product is a mature design several steps downstream from the ultralight roots of the S-4, I cannot imagine why they would still use the old tube layout. If they have realized that welded steel tube is better for a fuselage than pop riveted aluminum, then why have they not likewise matured into a C channel or I beam spar? The qualified engineers on this forum can easily correct me, but if I remember right the strength of a spar is proportional to the square of its "height" (distance between upper and lower spar caps). So a 6 inch tall spar would be much stronger than a 4 inch tube.

No offense or disrespect intended to the designer or his philosophy whatsoever. I know the RANS aircraft are exceptionally capable and well loved. But to me, it seems that the weights, speeds, power, and capabilities of these remarkable aircraft have outgrown its ultralight design roots????

This looks sweet.

Good point EZ, I assume that they continue using the round spars because they work so well for the job. No more complicated then that. Without getting into a bunch of number crunching, (that I would be totally unqualified to do) any other wing structure they and others have come up with have not offered the same strength to weight ratio. It's not like the RANS design crowd don't know what a real airplane spar looks like. That seems to be the reality of it, regardless of the engineering involved. A little of the "if it isn't broken, don't fix it" may be involved also. Frankly, the only drawback I've experienced with the round spars, having flown with them for the last 20 years, is that yeah, they LOOK like something off an ultralight, and not "real" airplane spars. It also does not have a Continental or a Lycoming. In other words more traditional airplane people can understandably be put off by that, but damn, they work, like the Rotax, too well to change just to be more marketable to real airplane types. Other then that they are light and strong and extremely well proven by lots of fleet time by now, I doubt they'll change that aspect of the design, if so it may be marketing related not for a practical reason.

I spend a lot of time looking at my rectangular crane boom, so I get the whole thing about stiffness where it's needed, that same crane boom is pretty damn wimpy when it comes to side loads, it'd be better in that regard if the boom was round. One unintended result of the round wing spars is that they hold up better when you smack a tree or other obstacle. Barring ripping the wing out at the root, and disregarding the bracing effect of the internal cross bracing (of any type of wing) a round spar is going to be much stiffer of course then an I beam type, when being abused. Another huge and practical advantage is the ease of construction and repair, this was brought home to be when I rebuilt (actually built new wings, new spars anyway, but re-used the ribs etc.) my '46 T-Craft. A bunch more parts and labor involved. The structural record of the round spar design is as proven, nearly not as long though, as other types.

I am heading out right now to test those round spars once again!

Is there a recurring problem with people ripping the wings off their Rans because the round tube spar couldn't take the load? If not, then what's the problem?

I can only think of one incident, where that S-19 shed a wing in an aerobatics show. Different plane though, I don't think they have a lift strut design and therefore probably use a different spar?

Zzz wrote:Is there a recurring problem with people ripping the wings off their Rans because the round tube spar couldn't take the load? If not, then what's the problem?

I had no intention of calling their design into question from a safety standpoint. I know that thousands of Avids and Kitfoxes, and Highlanders, and RANS, and other similar tube spar airplanes are not falling out of the sky in droves. I was not knocking any wing layout.

My observation was that the front/rear tube spars seem to be a very common choice for wing design down in the ultralight weight and speed ranges. But for some reason, straight rectangular, C, or I beam spars seem to be the choice for everything from the J-2 Cub weight and speed ranges and larger for the last 80 years, despite any disadvantages or cost increase.

So there must be some tangible advantage somewhere, for all those manufacturers to have chosen a C channel or I beam and eat the extra cost. I'm guessing that at some weight/speed/power level, the strength/stiffness advantage versus cost graphs cross over each other, and it becomes worthwhile to go to the traditional layout. The specifics of where that crossover is, is a question for one of our many resident high brain function degreed engineers to answer.

Since modern LSA airplanes have much more power and performance than a stock Cub or Ercoupe, I was just postulating that they probably have reached that crossover point, and only the inertia or "old habits" from the company's ultralight days have not caught up with this reality.

Gotcha EZ, I didn't think you were knocking what so obviously works, and in fact I too wonder where the design cross over is for the type of spar. And let's not forgot Jim Bede's design, some of them used round spars and were fast as hell. It would be interesting to ask Randy at RANS.

Got another 1.8 hrs in that round tubed machine since my earlier post, checking out my next crane job and taking some pics, plus about 15 hill landings, warning, thread drift!

Randy Schlitter is a very approachable guy. He's on Facebook and responds to IM's and email queries. If you really wanted an answer you could ask him. He's like the anti-Van... very approachable and friendly.

Case in point: I called Rans one day to ask a question during my build, the question guy was out and the parts guy didn't know the answer so they transferred me to Randy. He answer my question quickly and then spent the next 20 minutes talking to me about airplanes in general. Now we're friends on Facebook.

svanarts wrote: Now we're friends on Facebook.

Oh....... how sweet

Haha, totally joking svanarts! Hope to meet you on the lakebed next week.

This question has not been keeping me awake at night, it's just something that popped up when I looked at the naked photo of the S-20. I already pretty much knew that it was strong enough to be safe, because RANS has been around long enough, with near-universal satisfaction form their customers, and no "CH-601" structural problems. I'm glad he is a good guy, approachable, etc.

The strength per pound may be better with a C section, box or I-beam spar, but strength per dollar is better with a tubular spar. The assembly process is also a bit easier with a tube spar. That is why the Grumman Cheetah and Tiger aircraft used tube spars. Any shape can be designed to work, but standard tube shapes don't require any special fabrication and can readily be assembled with the lightening holes in the ribs. I think it is as simple as that.

highroad wrote:

svanarts wrote: Now we're friends on Facebook.

Oh....... how sweet

Haha, totally joking svanarts! Hope to meet you on the lakebed next week.

Jealous?

I hope to be there.

I'm interested in the S-20 so I took a fair amount of pictures at Oshkosh. The following links to a slideshow

https://picasaweb.google.com/1077619411 ... 6159808034

rjb wrote:I'm interested in the S-20 so I took a fair amount of pictures at Oshkosh. The following links to a slideshow

https://picasaweb.google.com/1077619411 ... 6159808034

Those headrests on the seats look interesting.

Rans S20 tubular spar photo.




The tube spar design in spite of being "ultralight" in it's origins works very, very well. It's light, strong and simple to build. Avid first, then Kitfox copied it and finally Just did too, just to name a few "real" airplanes, besides Rans, that use them. There is also a strengthening insert (Kitfox uses an extruded aluminum "I" beam) inside the tube to add to the vertical load capacity.

Looks like Rans abandoned the 10" wheels, too.

Speaking of tubular spars....the trailing edges of the flaps and ailerons are tubing also, (good picture of that above) and the blunt edges seem to be just right for low drag. KITPLANES Barnaby Wainfain had a good article on trailing edges and drag a while back, and he clearly stated that unless a sharp TE is done correctly it is more drag then a rounded LE. And, doing it correctly is tough to do structurally, actually very delicate and prone to damage. Done incorrectly, the airflow can't follow the sharp edge, it can a blunt or rounded edge. The illustrations he used showed what he meant:a very tough shape to brake form without cracking and other problems. He went on to say that a rounded LE can approximate the lowest possible drag of the ideal but hard to form shape, and be much easier to fabricate and less prone to damage.

RANS maybe lucked out or had it all figured out, but the TE of the wing is actually pretty low drag while at the same time being butt simple

I've heard this before, and when I fabricated aft fairings for my A-frame gear legs, I made sure to not make them pointy, but blunt. Easier and less drag works for me.

I mentioned Jim Bede had some designs using tubular spars, it may have been Steve Wittman... or maybe both?

Av8r3400 wrote:There is also a strengthening insert (Kitfox uses an extruded aluminum "I" beam) inside the tube to add to the vertical load capacity

Just uses this I-beam as well,if I remember right, a 3 foot section of I-beam centered on the strut attach points.

They need to finish this project up! I think Steve Henry beat Ran's time on this with his last Highlander build in in his garage! They knocked that thing out quick.

I like everything but the spring gear. I think that Rans is working on a more rugged gear for the S-7 as we speak here. Perhaps it will end up on the S-20 as well.

I like it.

EB