Combat Wing III for FF24

[Pieliker96]

2022's Wing
2023's Wing

Now it's time for 2024.

Proposed design revisions:
1. Bigger batteries and more efficient aero (higher AR, more streamlining) for more endurance. Target is 20 minutes.
2. Bigger overall size, more overall weight, more inertia to be less affected by collisions.
3. Make liberal use of composites for strength, ditch protruding nose for impact/crashworthiness.
4. Move to a contra-rotating setup, both props in the same slot, for more thrust and noise, possibly some engine-out capability.
5. Ditch dead weight of the lighting system.
6. Increase snag-and-drop parachute size to have a bigger effect on captured targets. Move to two tow lines for double the capture capability.
7. Get a RX with signal diversity to hopefully stop the dropout issues. Possibly change protocols.
8. Removable wings for transport.
9. Bring spare parts and stock to Flite Fest for on-site repairs.



The basic weight and balance is done, as is the planform. Projected weight is 3.4kg which with a wing area of 1088in^2 is a WCL of 5.77. The TWR should be the same if the thrust scales linearly with two motors. If contrarotation makes it more or less efficient, the TWR will be affected accordingly. I'm planning to keep the same prop from last year's setup on top, then vary the lower prop while testing for maximum static thrust.

Hoping for some good combat action this year.

 

[LitterBug]

Looking forward to the mayhem!!!! I have extra spiders coming with me this year. We may get a whole swarm.....

 

[Pieliker96]

No kill like overkill as far as structures goes. Lots of ribs, lots of spar. It'll probably put me over mass budget but I think it's worth it for the durability and crashworthiness

 

[Piotrsko]

Based on my combat, being able to spread the impact forces was more important than avoiding a spar failure, particularly when hit off center. Overkill doesn't do much for that. I like I beam spars for that reason. Not wild about the outboard spar joints but YMMV. An accidental hit in the center will probably take out all your thrust

 

[Pieliker96]

Based on my combat, being able to spread the impact forces was more important than avoiding a spar failure, particularly when hit off center. Overkill doesn't do much for that. I like I beam spars for that reason. Not wild about the outboard spar joints but YMMV. An accidental hit in the center will probably take out all your thrust

Ideally I would just do hotwire foam core with fiberglass or carbon skin though I don't currently have the manufacturing capabilities. I agree with regards to the outboard spar joints, I ended up carrying them inboard to the wing to center wing join after staring at it for a bit. The fore and aft spars are carbon fiber tubes embedded in foam, the angled spars are just foam. I also have a CF rod along the leading edge, planning to do TE stripping as well.

I've made sure the motors and such are better tied into the structure this time. Last year a hit with a 200% FT scout ejected the motor, gopro, and batteries. The motors will be attached to the fore and aft spars by 3d printed blocks, and the straps for the battery and gopro will also wrap around CF tubes that integrate with the rest of the structure. In any case, I think it should be more impact/crashworthy than last year's design

 

[Crow929]

I like that motor placement design, not sure I've seen a setup with two counterrotating props in one slot. Should sound wild!

 

[LitterBug]

If you have seen the spider in combat before, He will be flying wings this year too. Hopefully with an ejection system in case his bird get's taken out!

 

[Piotrsko]

Why wouldn't you replicate a combat zagi 5 spar using arrow shafts? Back in the day that was the toughest thing around, might still be. Tales of one being run over by a parcel truck and immediately flying afterwards. Unsure if you can do the motor as strong as your design.

 

[LitterBug]

Ohhhh MAN, If I didn't have enough to finish up before FF'24, I would LOVE to print one of these buggers!

I'd like to see some brutal impacts of 3D printed flyers for one thing. 2, this thing looks like a sweet fighter.

That's just a basic 3 channel elevon setup with no vectoring.

LB

 

[Pieliker96]

Finally making some headway. Had to figure out getting plans from Onshape for the first time - ended up exporting individual parts as STL then importing into Sketchup, then use the usual unwrapping tool to SVG to tiled PDF. Mass budget is as usual looking to be considerably over target, if I can't make up static thrust with different props I may get some lighter batteries to shave a couple hundred grams - graphenes are heavy.

I'm really having to think about how I put this thing together, there's a lot of dependencies as far as which parts slide onto which and need to be installed before others. Hopefully I haven't made a catch-22 somewhere.

After seeing how big the outer elevons were in-person I did some torque calcs and determined my servo setup was marginal, so I'm going to subdivide the outer elevons and run another pair of servos. Each elevon will be Y-ed together due to 6-channel limitations so I won't have any endpoint adjust ability between the surfaces, though trim can be accomplished by bending the pushrods a tad.

 

[Pieliker96]

Making good pace. The center and right wings are pretty much there, just need to do the servo linkages, winglet, power wiring, and battery hatch. I ended up printing some angled shims for the motor mounts since I didn't quite get them square to the bottom of the wing the first time around. Thankfully the offset between the motors still allows plenty of clearance to get props on and torqued. I've also set up a radio mix to allow me to kill either engine in-flight, should I need to.

As soon as I get the power wiring finished I can start looking at thrust testing and getting an idea of what my TWR is going to be. Hopefully it'll end up around last year's, though I have my doubts. In any case the contrarotation should make it far easier to launch because of the lack of torque and left-turning tendencies.

 

[Pieliker96]

Servo covers going on, should serve to harden the control system against impact both in the air and when landing.

The power wiring is also done. 2x14 guage into a 12 for the ESC and a 14 for a crossover between the ESCs took all that both of my soldering irons had. It's times like this I wish I had three hands, or a blowtorch. In any case the wiring is complete with no burns to the floor, the plane, or me. Thrust testing is up next!

 

[L Edge]

I have a way to sync the motors/prop in the air and adjust trim in flight if the offset of motors causes yaw. In fact, if there is enough offset of the motors, you then can add differential power so you can use it for the rudder. By the way, to see if you are sync, beside hearing it, find a florescent light, take your plane and site the props with the light and add some throttle, voila, I can even stop the props from movement with my trim.

First flight test I would hold your wing vertical, hold by hand, add power and try to hover(release hand for about a tenth of a second and grab quickly) and see if it yaws. If it yaws, move your transmitter trim so it eliminates that problem. You only use 2 channels and don't even need any mixes. Interested, let me know and I will explain.

I did that for my Warthog and SR-71 with EDF's to have it fly straight from a hand launch.

If I were to try this project, I would build a thin tapered large lightweight wing basically doing what your doing(to prevent destruction) and then use lightweight fiberglass/epoxy with plenty of power to outrun hits. I would add a center rudder(called an "udder rudder). I've seen Formula 1 pylon racers that are fiberglass epoxy with missing parts of wings, etc still fly after collisions with props using rudder to fly and land.

 

[Pieliker96]

Yaw shouldn't be an issue with the contra setup since both props shoot through the centerline - they're staggered vertically. In any case I'll be ditching the contra setup for a side-by-side prop-in-slot config since thrust testing did not go well. I maxed out at 5200g static thrust which is only 1.25TWR, far too low for safe vertical takeoff.

I made a faulty assumption early on that a contra prop setup would theoretically produce at least twice the static thrust of both motors combined, the idea being that if both motors produce an equal change in velocity of the air then the momentum out the back and therefore thrust is doubled. That is true, but the problem is that the second motor in line now requires far more power to accelerate the air. If the velocity out the back of the second motor is Vi + deltaV, the power required isn't proportional to just deltaV^2, but deltaV^2 + 2*Vi*deltaV. Theoretically, a power-matched contra setup has a maximum static thrust 1.404 times that of a single prop (not considering any swirl efficiency gains).

The new center wing will put the motors side-by-side, counter-rotating, in the same slot. The span of the center wing will grow which will give me more wing area and aspect ratio, helping to compensate for my weight overshoot and improving aero efficiency. I may also truncate the control surfaces before the center and add a space for a third chute line, which will give me the option to run 1, 2, or 3 lines.

 

[Crow929]

Yaw shouldn't be an issue with the contra setup since both props shoot through the centerline - they're staggered vertically. In any case I'll be ditching the contra setup for a side-by-side prop-in-slot config since thrust testing did not go well. I maxed out at 5200g static thrust which is only 1.25TWR, far too low for safe vertical takeoff.

I made a faulty assumption early on that a contra prop setup would theoretically produce at least twice the static thrust of both motors combined, the idea being that if both motors produce an equal change in velocity of the air then the momentum out the back and therefore thrust is doubled. That is true, but the problem is that the second motor in line now requires far more power to accelerate the air. If the velocity out the back of the second motor is Vi + deltaV, the power required isn't proportional to just deltaV^2, but deltaV^2 + 2*Vi*deltaV. Theoretically, a power-matched contra setup has a maximum static thrust 1.404 times that of a single prop (not considering any swirl efficiency gains).

The new center wing will put the motors side-by-side, counter-rotating, in the same slot. The span of the center wing will grow which will give me more wing area and aspect ratio, helping to compensate for my weight overshoot and improving aero efficiency. I may also truncate the control surfaces before the center and add a space for a third chute line, which will give me the option to run 1, 2, or 3 lines.

Could you increase the pitch of the second prop? I'm no engineer, but I always thought since the air was already moved by the front prop, the second one wouldn't really be doing much. Almost free spinning in air moving as fast as it is trying to move.
This is something I've thought of and figure I'd ask someone who is clearly more educated on the subject such as yourself.

 

[Pieliker96]

Could you increase the pitch of the second prop? I'm no engineer, but I always thought since the air was already moved by the front prop, the second one wouldn't really be doing much. Almost free spinning in air moving as fast as it is trying to move.
This is something I've thought of and figure I'd ask someone who is clearly more educated on the subject such as yourself.

Yes, I tested with higher pitched props in the rear for this reason. I should've put my results in the previous post, but here they are:

If you can match the angle of attack and airspeed (more or less RPM, minus swirl effects) between the two props, then they should produce the same amount of thrust. The problem with increasing the pitch of the rear prop to keep an angle of attack capable of producing enough thrust is that the angle of the blade chord relative to the prop disk is higher. This results in less of the blade lift being converted to useful forwards thrust, but most importantly a portion of that blade lift acting in the direction of drag, requiring more power to get to the same airspeed/rpm.

 

[Crow929]

Yes, I tested with higher pitched props in the rear for this reason. I should've put my results in the previous post, but here they are:

View attachment 243733

If you can match the angle of attack and airspeed (more or less RPM, minus swirl effects) between the two props, then they should produce the same amount of thrust. The problem with increasing the pitch of the rear prop to keep an angle of attack capable of producing enough thrust is that the angle of the blade chord relative to the prop disk is higher. This results in less of the blade lift being converted to useful forwards thrust, but most importantly a portion of that blade lift acting in the direction of drag, requiring more power to get to the same airspeed/rpm.

It would be interesting to see the current draw from each motor as well, to see how much "work" the two motors are doing in comparison to each other.
Another question is, did it sound intimidating? 🤣

 

[Pieliker96]

It would be interesting to see the current draw from each motor as well, to see how much "work" the two motors are doing in comparison to each other.
Another question is, did it sound intimidating? 🤣

It sounded awesome. Super loud and the motors went in and out of harmonics as they spooled up because I was using two different ESCs. For the higher pitched props the full throttle RPM ended up lower on the rear motor which produced some real nice resonance

 

[Pieliker96]

Got the second wing completed and the wingtips on, here's what it would've looked like with the center section

Working on a widened center section now

 

[Pieliker96]

New center section coming along nicely. It's going to be a beast, span is now a titch over 7 feet. Motors are rotating inwards to help engine-out cases by minimizing p-factor and using the torque to hold up the dead wing. Ideally there'll be enough vertical area to sustain flight in a banked sideslip with a dead engine, we'll find out in flight testing. I should be able to get to thrust testing tomorrow to verify performance with the new setup.