FTFC23: Build-ruary by Inq: Inq'd Boomerang

[Inq]

I've wanted a build challenge and as part of FTFC23, I also want a plane challenge. Not that I know anything about sailplanes... but the idea of flying on thermals captivates me. Although the build challenge starts in February, I'm guessing it's OK to do design work ahead of time since building kits from FT that are designed AND pre-cut are acceptable in the challenge.

Boomerang is a flying wing, sailplane with the following design features (at this time): I'm allowed to change my mind due to input from any of you willing to give me constructive criticism or from the inner voices that haunt me.

1. Minimize weight - As I'm spec'ing two sheets of DTFB, the skin weight will be around 200 mg. I haven't analyzed for spars yet.
2. Maximize span - I doubt bigger is necessarily better, but I want it big... with the two sheets of DTFB, my initial span is 3m (~120in).
3. Minimize power - Although the jury is still out, I'd like to see if I can do this with a 1S power system using the all-in-one micro boards with receiver, servos and brushed ESC.
4. Propeller - It will be a pusher design with folding propeller. Size TBD

That's about all at the moment... (more to come)

By all means... please share some of your wisdom.

Back to challenge...

 

[Inq]

The last couple of days before the start of the Buildruary, I've been spending most of my time on the design of Boomerang. It has a few design goals that are not pinned down very well and they are very important to the overall design.

1. To optimize the design, it is not desirable to add extra weights to balance the CG properly. I also don't want a long nose just to balance. The sweep angle is used to move the Aerodynamic Center (AC in the image above) backwards with the assumption that the motor, battery and electronics are forward enough to balance the CG in front of the AC.
2. Unfortunately, a big unknown is whether I can use a 1S motor and electronics. It makes a major difference in the weight. I've done some tests on thrust produced from two 1S motors I have.

The first one has a tiny motor (716), but is geared with a relatively large propeller (140 mm). It produces only 62.5 grams of thrust using 3.2 watts.

The second one has a larger motor (N30), but a smaller propeller (75 mm). It produces even less thrust (54.3 grams) but uses over 3 times the power at 10.4 watts.

It appears that the benefit of a large propeller turning slowly far outweighs the frictional losses of the gears. Wit this in mind, I've been experimenting making some 3D printed nylon gears. The biggest problem is making the teeth small enough to be printed. I also want to incorporate a folding blade propeller. I am hoping with 3X the available power, I can turn an 8 inch propeller and up the thrust significantly so I use these tiny 1S motors, batteries, receiver and servos.

Here is my latest attempt at an integrated folding prop hub / geared propeller as of the start of the Buildruary.

 

[danskis]

You've got a lot of conflicting design parameters which makes this a very sophisticated design challenge. Just seat of the pants - with 1S this will be an indoor model or a absolutely no wind outdoor model. You might get away with 2S. Most flying wings use some sort of vertical stabilizer. Most flying wings have reflex. This is achieved either through the airfoil design or through the elevons (really the same thing). Airfoil reflex is hard to realize with DTFB. I like the idea of having an adjustable sweep angle...3d printed I assume. I'm not sure I'd lose any sleep over this one.

 

[Inq]

Just seat of the pants - with 1S this will be an indoor model or a absolutely no wind outdoor model. You might get away with 2S.

Thank you for pointing out issues. It helps me find issues and/or solidify my thoughts when I write it down. And by no means do I think all this is going to work... I'm thinking somewhere around 50-50. But I like experimenting and failure is a learning experience.

Motor - Yeah... I'm thinking the 1S is most likely doomed. The difference in weight of 1S micro system including battery is hard to beat (< 40 grams). On the 1S motors, the benefit gain of the gear drive is also hard overlook also. If I can realize a 3X on the N30 motor, that gets me up to about 150 mg. IF I can keep the plane under 300 grams (big IF) then a 50% thrust to weight ratio sounds doable.

I don't have any familiarity with 2S systems. Do you have some recommendations for... say a good, light 2S motor and ESC?

 

[Inq]

Most flying wings have reflex. This is achieved either through the airfoil design or through the elevons (really the same thing). Airfoil reflex is hard to realize with DTFB.

Reflex - I'm self taught on aerodynamics... so correct me if this isn't correct. I'm mainly using this as my reference - https://www.mh-aerotools.de/airfoils/nf_1.htm. To summarize...

If the flying wing is "Plank" (no sweep) design... yes the wing foil itself should have a positive Moment Coefficient. That way Cm tries to dive the plane and the wing twist or reflex in the elevators will try to climb... giving pitch stability cycling like tailed plane does.

I wouldn't even try to hold a foam board wing to that critical foil shape.

However, if you use a swept wing design, then pitch stability is created by the standard CG is in front of the AC and the elevators balance it just like on a tailed airplane. Difference being the moment arm is far smaller. It also doesn't put as much critical spec on the foil shape.

 

[Inq]

Most flying wings use some sort of vertical stabilizer.

Vertical Stab - The initial drawing was a two minute sketch and didn't include them. Also, I'm under the impression that a swept wing does have some yaw stability. When it yaws, the forward wing is more perpendicular to the apparent wind, has more drag and is pushed back. However the mechanism I plan to use for the tip elevons has some vertical stabilizers to cover the elevon arms. This picture shows the arm inside the vertical stab. The bare ribs are just un-lofted sections of the vertical stab.

I also plan to put small rudders that will work like Burt Rutan's Long Eze. They only move outwards... IOW yawing to the right, only the right rudder moves outward... creating yaw and drag on only the right wing to pull the wing back.

 

[Inq]

I like the idea of having an adjustable sweep angle...3d printed I assume. I'm not sure I'd lose any sleep over this one.

Wing Sweep - I don't plan to make the plane have a variable sweep angle. I meant to say...

1. I have to basically build the wing (image below shows it uses almost 100% of the DTFB)
2. design and possibly build all the paraphernalia for a sample elevon that will go on the wing tip.
3. have to lock down the weights of all motor/ESC/receiver/battery/servos, set their locations.
4. Once I do all that... only then, can I calculate how much sweep puts CG slightly forward of the AC.
5. Then I cut the wing joint to create that sweep angle.

 

[danskis]

Apologies but I have no experience with 2S drone motors except that there are plenty of them out there. Perhaps you could use 2 of your geared motors and use variable thrust for steering. All the rest sounds plausible for a summer long project. Can't wait to see what you come up with.

 

[Inq]

No apologies necessary. After you mentioned it, I started looking in earnest. You're right... there is a huge glut of them, but they seem to be mainly of the very high KV range (14,000 KV) for racing drones using the tiny propellers. I would think... that I need a high torque motor and swing a big/low speed prop. Boomerang should climb like a mountain goat at about 10 mph.

All the rest sounds plausible for a summer long project.

...but this has to be done in February (while designing two others and building four others).

 

[Inq]

I didn't get near as far today as I expected. Heck... I'm not even going to be able to update any of my other FTFC23 projects.

I'd like to say this was my first test... and I haven't even tested it yet. This is the first one that fit together. NYLON! It's way different than the ABS I'm used to... but it is SO strong. I'm really needing to down size it. I seem to have found the right size of gear teeth for the reduction gear. Don't know if the ratio is good enough for the engine or the blades are optimized yet... but it is functional.

 

[Mutley]

amazing

 

[Inq]

ok... I'm putting a fork in using the micro receiver/servos/folding prop concept... and I'm giving up on the weight being in the 300 gram range. I've order an 1806 and smaller ESC and will live with a heavier component weight. I'm now guessing the component weight will be closer to 150 grams putting the plane closer 500 grams. Still plenty light for 10 foot span flying wing. I do want to re-visit the folding prop though. Doubt it'll make it for FTFC23.

 

[danskis]

I agree, amazing 3D printing. I've gotta say that your design requirements are some of the most challenging I've seen on FT. Alot of us are just happy if what we build will fly (me included). I can't wait to see what you come up with in the future.

 

[Inq]

I agree, amazing 3D printing. I've gotta say that your design requirements are some of the most challenging I've seen on FT. Alot of us are just happy if what we build will fly (me included). I can't wait to see what you come up with in the future.

My Mom said I was born as an engineer/gear-head... (obviously, I'm the Michelin Man with my Papaw studying the latest car rag) Point being... #1 I like the engineering above all else... #2 completing the build is difficult for me and the only thing that gets to me to finish is when I have to prove to myself (or someone else) I'm right. But I'll also publish my failures (and have already here) and a distant #3 is actually flying it. Again... proof of concept is paramount... getting it back on the ground in one piece is nearly irrelevant (I may be on the way home already to design something else).

 

[Inq]

Boomerang Progress...

Have dropped the pen on the wing design. The gap between the two colored sections of the wing is used for more reinforcement of the spar. After a successful build of the wing, there is about two square inches of the full DTFB that is thrown away.

Built the port wing. Using one sheet of DTFB. The spar in this test article is completely and only foamboard reinforced with three layers for the spar. I'll probably do some stress testing while trying not to break it. If I think it can support an AUW of ~ 500 grams, I'll use it. If not, I'll build another with reinforcement. This candidate wing weighs 112 grams for its 60" span.

 

[Inq]

BTW...
Used the InqCutter on it... cause there is no way I could cut 60" of a double bevel without it!

Also used the InqRouter to loosen up the curving out toward the wingtip as the curve is far tighter. The jury is still out whether I think this thing has some benefit.

 

[Mutley]

Boomerang Progress...

Have dropped the pen on the wing design. The gap between the two colored sections of the wing is used for more reinforcement of the spar. After a successful build of the wing, there is about two square inches of the full DTFB that is thrown away.
View attachment 234815

Built the port wing. Using one sheet of DTFB. The spar in this test article is completely and only foamboard reinforced with three layers for the spar. I'll probably do some stress testing while trying not to break it. If I think it can support an AUW of ~ 500 grams, I'll use it. If not, I'll build another with reinforcement. This candidate wing weighs 112 grams for its 60" span.
View attachment 234816
View attachment 234817

Aspect ratio making me drool over here. Drool as in: "That is so friggin cool, I bet that thing glides forever given the lift to drag ratio is going to be superb"

 

[Inq]

Aspect ratio making me drool over here

I guess I should have calculated it at some point... I still am designing wing tips that will add a little span and sweep angle can't be calculated until I get a handle on all the weights and get a CG. At the moment, I'm WAG'ing a sweep angle of 15 degrees. If that holds, the aspect ratio will be around 43.

 

[danskis]

As you probably know the more sweep the more fore aft stability and the less sensitive the CG. Flying wings have a very narrow CG range as it is.

 

[Inq]

As you probably know the more sweep the more fore aft stability and the less sensitive the CG. Flying wings have a very narrow CG range as it is.

"If you're not on the edge, you're just taking up room."

Yes... and I'm even going to ragged edge it! Probably more of a stupid judgement call based on my piloting skills . In the beginning, I really wanted to minimize weight... no tail, no, fuselage, minimum motor. I gave up on the minimum 1S motor and nearly doubled the weight to use full size gear. With motor pushing, it either requires a nose with battery out front or sweep to pull the AC back behind the CG. I chose the latter mainly because of that and get the little bit of benefit of directional and pitch stability as a bonus. That is why I haven't nailed down the sweep angle. I have to design the wing tip elevons that I also want to experiment with. The more they weigh, the more it'll bring the CG back with sweep angle. IOW... If they weigh more, less sweep angle is needed to balance the CG. The initial 15 degree sweep assumes they weigh nothing. So chances are, the sweep angle may be less... more aerodynamically efficient, yet with less stability.

But... I have broken down and dug out something that I've had for a long time, but has never been on an airplane. Just hope it can be configured for elevons. Haven't dug up the manual yet.