Slingshot

[Wayne]

@OribaSJ88 (Ollie) had brought a pretty sweet 3D printed plane he had designed to the 60" races in Banos. Rafael feel in love with it and Ollie was nice enough to share the print files with him. About a week ago Rafael took his up to Tic point and had a good time flying it, but without flaps I guess it was a little hard to land there. (It is good to have flaps at Tic.) So Rafael added some flaps and gave me the prototype and we took them out to Tic on Sunday where we were rewarded with great sunny weather and white caps on the water and of course the locals pilots Dave, Bruce and Will. Great to see the guys again, and just a really nice time. Here are a few pictures:

Even Tic Point is a little dry this time of year.

Rafael with his aileron version. The green one above has flaps.

The Slingshot is a riot! It is so much fun to fly. It rolls very fast and tracks very nicely. She gets a good lick of speed, but also slows down well. Just a well behaved model that is darn easy to build. Not too big, not to small. Ollie you hit a homerun with your design.

We passed the models around and let all of the locals give it a try. It was great to see them all messing around instantly and flying it like they have owned it for years. That is great for a couple of planes that had total time of less than 30 minutes on them combined. LOL

The flapped version needs some more investigation. Getting some odd handling from the flaps that I have not encountered before. Deployment of flaps at full speed is not practical as the blooning will take the model vertical instantly. Oh well, not all planes like that. But even feeding in a little flap makes a lot of ballooning even at lower speeds. OK, just feed in some down elevator until the plane settles in, then you can fly around like a normal flapped model with normalish elevator compensation mix. But every time you hit the flaps, be ready for it to pop up for a second or three.

First couple of landings were spot on, could helicopter down and even backed into the slope impressing the locals.

Then I started trying to tune the plane in a bit more, I was a little nose heavy, so shifted some lead backward a bit and played with more landings. The wind had also picked up, and I found myself with full down elevator when the flaps were fully deployed. fully is about 85 degrees. It has a lot of flap. Just as the plane hits ground effect, you no longer need all of that down elevator. This resulted in some hard pancake landings. She took it without a complaint.

More on this later - got to run...

 

[OribaSJ88]

Hi Wayne,

Thanks for the kind words. The original design is a little more than two years old and my second attempt at designing a 3D-printed slope soarer/racer. The version that Rafael printed is a refined version of the original where I tried to improve the printability and allow it to better work within the limits of my older, bed-slinger printer. I've tried slightly larger versions and, while they fly well enough, have never captured the fun and the maneuverability of the original. The added mass of the larger designs didn't improve the durability either.

It flies surprisingly well for a sailplane whose wingspan is just slightly over 1 meter. It flies at Coyote Hills once the wind is blowing between 10 and 12 mph. When trimmed properly, I have never detected any odd flight characteristics. It retains speed fairly well and turns tightly with a little snap flap. The roll rate can be insane depending on how much aileron travel you're using. At Coyote at least, I've never felt the need to have flaps for landing. Spoilerons seem to do the job pretty well. I prefer the KISS principle on such a small design. That said, your mileage may vary.

Some of the benign flight characteristics are no doubt due to the wing plan form, but the airfoil deserves a fair amount of credit. I borrowed the JX-GT airfoil, an "all-round" design, from a German F3F/F3B effort. They were kind enough to publish the results of their work. Below is a link to the airfoil I used. They also published an entire family of airfoils for F3F/F3B, symmetrical, flying wing, high wind and low wind conditions. It's quite the compendium! At a later date, I plan to try some of their "faster" airfoils.

https://github.com/jxjo/Airfoils/tree/main/JX-GT

Ollie

 

[Wayne]

Ollie - You are 100% on all accounts. Bigger makes them more fragile, and she is just the right size IMHO. Easy to print and build, economical and darn fun to fly. Thanks for sharing the airfoil, was going to ask you what it was.

Rafael will post up a video that shows the landing with flaps at Tic Point, it was pretty darn easy for the first few landings when the lift "sporty, but challenges popped up when the lift got a little higher.. Will discuss that in a bit as I found it very interesting.

Rafael played with flaperons and I talked him into trying spoilerons, and he managed good landings with both options, but dedicated flaps are your friend at Tick Point for the little landing area., especially when the white caps start forming on the ocean. We are going to use a little smaller servo on the next builds, save a bit of weight and cost.

Quick discussion about the bard door flaps on this model. They worked great at "sporty" lift, but later in the day we ran into an interesting issue with the heavier lift. I found myself using full down elevator to come in on approach. I've never run into this before. I suspect that the flaps were all but blocking out the elevator. As the plane got within ground effect, then all of that down elevator suddenly took hold, I think in ground effect the dirty air from the flap had cleaned up and allowed the elevator a full blast of normal airflow.

My last flight I dialed back the crow so it had less down flap (maybe 45 degrees) and less aileron up, I also increased the elevator throw some. The wind was still strong, and the flight was more normal this time around, but I screwed up the last foot of decent and hit the ground with too much energy and busted a leading edge. Easy fix. All together I'm amazed how many less than ideal landings she took with no damage. A "snap" was heard one two of them, but no damage could be found, may have been the elevator pushrod slapping the side. Anyhow, I'd like to test some more with these flaps, they are a little abnormal in size. LOL Super effective at lower lift. Makes for a nice flap trainer. I think a shorter chord for the flaps would be the easy fix. More testing just for the fun of it.

Really enjoyed the heck out of your model. She is a ton of fun. Think we will have a perfect little battery for it soon. I shoved a 2S LIFE into mine, but not a happy fit. But fed the voltage needed for the servos.

Also, I had my printed MR60V from the 60" race and there is no comparison, your Slingshot is a much better model. More fun, costs less to build, and FAR easier to build.

Think the Slingshot would be a great candidate for a one design race series.

 

[OribaSJ88]

Ollie - You are 100% on all accounts. Bigger makes them more fragile, and she is just the right size IMHO. Easy to print and build, economical and darn fun to fly. Thanks for sharing the airfoil, was going to ask you what it was.

Rafael will post up a video that shows the landing with flaps at Tic Point, it was pretty darn easy for the first few landings when the lift "sporty, but challenges popped up when the lift got a little higher.. Will discuss that in a bit as I found it very interesting.

Rafael played with flaperons and I talked him into trying spoilerons, and he managed good landings with both options, but dedicated flaps are your friend at Tick Point for the little landing area., especially when the white caps start forming on the ocean. We are going to use a little smaller servo on the next builds, save a bit of weight and cost.

Quick discussion about the bard door flaps on this model. They worked great at "sporty" lift, but later in the day we ran into an interesting issue with the heavier lift. I found myself using full down elevator to come in on approach. I've never run into this before. I suspect that the flaps were all but blocking out the elevator. As the plane got within ground effect, then all of that down elevator suddenly took hold, I think in ground effect the dirty air from the flap had cleaned up and allowed the elevator a full blast of normal airflow.

My last flight I dialed back the crow so it had less down flap (maybe 45 degrees) and less aileron up, I also increased the elevator throw some. The wind was still strong, and the flight was more normal this time around, but I screwed up the last foot of decent and hit the ground with too much energy and busted a leading edge. Easy fix. All together I'm amazed how many less than ideal landings she took with no damage. A "snap" was heard one two of them, but no damage could be found, may have been the elevator pushrod slapping the side. Anyhow, I'd like to test some more with these flaps, they are a little abnormal in size. LOL Super effective at lower lift. Makes for a nice flap trainer. I think a shorter chord for the flaps would be the easy fix. More testing just for the fun of it.

Really enjoyed the heck out of your model. She is a ton of fun. Think we will have a perfect little battery for it soon. I shoved a 2S LIFE into mine, but not a happy fit. But fed the voltage needed for the servos.

Also, I had my printed MR60V from the 60" race and there is no comparison, your Slingshot is a much better model. More fun, costs less to build, and FAR easier to build.

Think the Slingshot would be a great candidate for a one design race series.

Hi Wayne,

I appreciate the vote of confidence in proposing the Sling Shot as an ODR candidate. As I warned Rafael before I gave him the files, the design is undocumented and was really only intended for one person: me. I have a lot of respect for people like Waynsie (Soarkraft) and Geode, designers who put in the time and effort to provide a product that can be printed and assembled by the masses. It's no small amount of work. Providing the files to Rafael and allowing your team at Aloft to be a couple is all well and good, but more than that just isn't in the cards. I ask that the files and printed copies don't go beyond your team.

I had suggested that Rafael use the Sling Shot design as "inspiration" and to design his own, hopefully better, design. A few obvious change would be to replace the boom with a carbon rod (it would be way stronger and perhaps a similar weight), design it around cheaper servos that aren't embedded in the wing and there are others mods as well. There's nothing secret about the airfoil or the planform or the basic design. The biggest problem I had when I started is that I didn't have a design methodology that worked well--one that would come off my printer with minimal fuss and would stand up to a few hard landings. While the design is far from perfect, it's WAY better than the first design I printed. I think it would be a reasonable starting point for an ODR racer.

Rafael, you seem to be proficient at modding STL geometry files. Designing from scratch is big step up, but hardly insurmountable. Fusion 360 is amazing but can be intimidating because of the amount of complex, non-uniform geometry in our designs. I can help you where/when I can and I'm sure there are others who'll lend a guiding hand.

As for the difficulties you saw when you deployed the flaps, I have seen what I thought was a "blanking" effect of the elevator if I applied too much spoileron. It would porpoise. Dialing back on the throw would fix it. Flaps may present a similar issues but I have no experience with them. The elevator is placed lower than I'd prefer, but it makes the whole design SOOO much simpler and stronger.

OIlie

 

[RafaelAvila]

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[Wayne]

As for the difficulties you saw when you deployed the flaps, I have seen what I thought was a "blanking" effect of the elevator if I applied too much spoileron. It would porpoise. Dialing back on the throw would fix it. Flaps may present a similar issues but I have no experience with them. The elevator is placed lower than I'd prefer, but it makes the whole design SOOO much simpler and stronger.

In the last landing in Rafael's video you can see some porpoising of the model, this was with FULL crow in "sporty" lift. This was before the lift grew.

At Aloft we have been using more and more 3D printed assemblies in our wooden kits with the approach of using wood where and when we can, but printed parts where they have a lot to offer. Your model has inspired me to think in a new direction for 3D printed models. Flip the equation, and use 3D printing for the majority of the airframe, and other materials where they make sense.

Rafael started designing a new model last night in Fusion. He is very good in CAD. (It could be argued that 3D printed models are not compatible with existing hobby shops, but the goal at Aloft is pretty simple, lets get people out their flying, and I think 3D models is one of the tools that does this.)

We shall see what comes of this.

In the end, I really like your model. Simple as it is, it is pretty darn perfect, and Rafael's mods made it work a little better for our uses. Thank you for sharing the files with us.

 

[OribaSJ88]

In the last landing in Rafael's video you can see some porpoising of the model, this was with FULL crow in "sporty" lift. This was before the lift grew.

At Aloft we have been using more and more 3D printed assemblies in our wooden kits with the approach of using wood where and when we can, but printed parts where they have a lot to offer. Your model has inspired me to think in a new direction for 3D printed models. Flip the equation, and use 3D printing for the majority of the airframe, and other materials where they make sense.

Rafael started designing a new model last night in Fusion. He is very good in CAD. (It could be argued that 3D printed models are not compatible with existing hobby shops, but the goal at Aloft is pretty simple, lets get people out their flying, and I think 3D models is one of the tools that does this.)

We shall see what comes of this.

In the end, I really like your model. Simple as it is, it is pretty darn perfect, and Rafael's mods made it work a little better for our uses. Thank you for sharing the files with us.

Wayne,

You're welcome and I'm glad you've all enjoyed the Sling Shot. It's my "go to" sailplane these days when I go to out to fly at Coyote Hills. I wish there were more of us who took advantage of the this slope, but that's a discussion for another time.

As Doc, Red and I discussed the rules proposals for the 60" class of MoM racing, 3D printed designs came up. While I print quite a few of my own designs for the slope, I think their general lack of durability is a huge issue and this would be doubly true for newbies. Think back on recent experiences. You damaged a leading edge over the weekend. I regularly have stiff weeds that will nick the leading edge or will poke hole in the surface of the wing . There's a reason why I flew the Genesis at Los Baños and am trying to get one of Doc's Sessantas--they're more durable when it counts. A 3 or 4 meter printed sailplane? I just don't see it personally and, well, it won't be strong or durable. Maybe new materials will come to our rescue. I'm a bit limited in those I can print and print almost exclusively with PETG. ASA and ABS are out of the question where I live due to the fumes.

I don't think 3D-printed sailplanes are going to put you out of business . I'm curious to see what you guys come up with. Hopefully you can improve on the Sling Shot.

Ollie

 

[Wayne]

My thoughts and Rafael's thoughts are probably going to be very different, but sometimes we can join our ideas for an even better approach to a problem. These sorts of challenges are what I really enjoy. I love that our tools offer us new options in the hobby, and it is our job to try and find new ways to put them to use. We have already seen that adding simple carbon tubes or strips to the prints offers a great improvement, that to me is just a first step in the evolution.

 

[OribaSJ88]

My thoughts and Rafael's thoughts are probably going to be very different, but sometimes we can join our ideas for an even better approach to a problem. These sorts of challenges are what I really enjoy. I love that our tools offer us new options in the hobby, and it is our job to try and find new ways to put them to use. We have already seen that adding simple carbon tubes or strips to the prints offers a great improvement, that to me is just a first step in the evolution.

Pultruded carbon fiber shapes are amazing! They're preformed structural carbon fiber elements that have a million and one uses. From my recollection, they have about 2/3 the elastic modulus of steel at a fraction of the density and with excellent strength properties. There's no need to mix epoxy, no molds and, mostly, no mess! They make 3D printed sailplanes possible. The Sling Shot wouldn't survive a hard turn or a soft landing without it. I see 3D-printed power planes built without any carbon fiber (of any kind) but I can't help but wonder how durable they are.

The phrase that sums up my take away on design for 3D printing is the following: "Just because something can be printed doesn't mean that you should or that it will work well in practice." There's still a lot of value to the KISS principle.

 

[OribaSJ88]

By the way, one of the sailplanes you have for sale indicates that it uses that same airfoil as the Sling Shot.

https://forum.alofthobbies.com/index.php?threads/new-tjirc-2-5m-wazzup-build.3837/

 

[Wayne]

Yep, and sounds like people are turning some good speeds with it.

The "Rise" from Plane Print is a 2 meter floater, and it has almost no carbon in the wing, but that form of a glider that makes some sense. It is a plane you can hand catch or land super gentle. Slope does allow us to be heavy, so 3D printing can make a go with it with a careful pilot... for a while.

I don't have any time to play with a new design right now, but have a couple of ideas I would like to try.