SkyFun III: new ideas for managing CG shift

HobbyKing SkyFun electric pusher jet turns out to be a very adaptable RCRG platform. This is my third build. With this build, I'm trying to move the motor closer to CG so as to avoid CG shift altogether during burn, or to have CG move backward during burn.

I sliced a wedge out of the aircraft's rear and mounted a 24mm tube there. I reinforced sides of the cut with 1/32" plywood both to restore some of the mechanical rigidity and to protect foam from exhaust. I covered plywood with aluminum tape for additional thermal protection. This glider has ACME launch guides on its bottom surface. One drawback of AkyFun's profile is the difficulty of attaching rail buttons on top. Canopy is too floppy and in the way. I'm curios to see how bottom guides work.

There are 3 motors I want to test with this setup: E15, F30 and G65.

Planning to fly this on Saturday at Snow Ranch.

Ari.

That looks like it'll scream on the F and G. What's the weight without motor? Are you worried about flutter with that section taken out of the TE?

Structural integrity is indeed a concern. My goal with this build is to find out how big a concern it is. I want to build a bigger version, but I want to test this idea on a smaller glider first.

Ari.

Had two excellent flights today, one on E15 and another on F30. Hit a rock on first landing with minor dent on the wing. Second landing was smooth as anything.

Very happy with how this flies. Boost was hands off from the first, trim was right and glide needs just a touch of up elevator. Looking forward to flying it on a G65.

There's video and stills that are coming from a number of people I believe. In the meantime, here are telemetry logs of altitude and signal strength from both flights.

Ari.

Here's video of the two flights, with a bonus shred at the end of a different glider (video courtesy of David Springsteen).

Ari.

[video=youtube;fihOwMpots4]https://www.youtube.com/watch?v=fihOwMpots4[/video]
www.youtube.com/watch?v=fihOwMpots4

Nice boosts. It really gets up and goes on the F30, and I can't imagine the G65. On the E15 boost, it seemed like you were out there looking for thermals! Great job all around.

Thank you!

There's some slope lift from that hill. I briefly considered using it, but decided against it on maiden flight :=)

Ari.

Yesterday's graphs are screenshots from TX software. Here are my own renderings in gnuplot. I graph them to the same scale and generate vertical speed by simple differentiation of adjacent datapoints. I believe the >1,000' fps datapoint is spurious noise. Even without it, the speeds look pretty scary. It may be worthwhile to apply some smoothing to velocity data.

Also, some photos from qquake2k's thread. Naturally, it's impossible for me to take photos while I'm flying a glider down, so I rely on the generosity of my friends. Thank you Jim!

Ari.

Nice work Ari, I am working on getting a little more out of my Kyosho Jet Vision. I think I need to go to a small F motor to get a flight long enough to sort the trim out. Unfortunately my video man has just become a Dad so cant slip of for our launches.

That worked real well! I once tried similar but using a BT80 for suport rather than ply. The BT80 did not last too long with Estes D motors. Is there any foam melt at all?


Richard

Richard--I wonder if you can elaborate on your BT80 design. Was it a duct that you put around the rocket?

Ari.

It was an earlier version of the red and white one in the attachment. The pod was not there and the area where the foil tape is was cut out and replaced with the BT80. the motor was inbedded in the wing just ahead of it. The idea at the time was to negate the Cg shift and possibly add some thrust augumentation while the BT80 added strength. Unforeseen was that the tube was too long and got roasted on the first flight. This is on a 45* delta so the length is quite long, on a flying wing like the Skyfun the length would be shorter and might actually work.


Richard

Thank you for the photos Richard.

How do they fly with these pods on?

Ari.

The current version has a number of small but worthwhile changes. The servos are underneath but operate the elevons via Golden Cable which exits the top of the wing along the fins, top drive style. The wing tips now have 4" of 45* anhederal and the most important of all is the pod sports a 8" swept back canard.

The canard really makes a big different on the boost. Without it the plane would naturally pitch forward, but with the canard it pitchs 10-15* at which point the velocity is high enough for the canard to gain authority and then flys straight as an arrow to appogee. The original purpose was to reduce the elevon reflex improving the glide which it also does. The symetrical aerofoil won't compete with any S8Ep, though it floats well for a sport machine.

She's beat up in this pic but still flying. I have 5 more core sets and will rebuild it soon. :grin:


Richard

Very cool! Thanks for the photo. Now that twin-boom design... I think I've seen it somewhere

Ari.

Yep, your right. I call it the 'Titan Blaster'. :eyeroll: For those who don't reconize the airframe its a bashed Air Hogs Titan $15 @ walmart/Target/etc. Rocket or electric though I don't know that the thrust line is correct as it still has pitch up under electric power but pitch down with a D11 in back. https://youtu.be/_1SIDksxIVQ
Defininetly need some expo for that one. oint: :bangpan: :sigh:


Richard

Sad video :=( It looks like you clipped the wingspan on that one?

Ari.

Actually I quite enjoyed the episode and it gave me a challenge to work out why. Its easy to repair in fact that is the repaired plane in the previous picture. Just waiting for the season to start up here. Yes it is clipped and flys much better because of it. Those original tips are not only very draggy but have washin rather than washout, or is it washout/washin? Any way they go the wrong way. I didn't believe it would fly and glide better but it does. Ditto for my tractor prop version.

The plane immediatly behind the Titan Blaster is a Proteck Vortex, very similar to the Skyfun in layout. The fuse goes all the way to the rear and may require special treatment but is also electric/rocket though I've not flown it on rocket yet. It would be a canidate for your slot treatment or the augmentor tube. [if I can work out a quick switch method] I'd like to see how the Skyfun holds up to the G motor.


Richard

I was wondering about washin--the tips indeed go the "wrong" way, with higher angle of attack towards the tip. I may cut off the tips on mine.

I'm sure SkyFun can survive a smaller, slower G like G12 or G25 or G38. I sort of painted myself into a corner with a 24mm MMT, which means the slowest G that can go in is G65, which is almost a full G.

Ari.

Lots of video when you do.


Richard

SkyFun III flew at Moffett today on F30. Photos courtesy of Rick Baldridge.

I dumb-thumbed it from 2 feet when it was flying towards me--pushed right instead of left. Embarrassing bruise on the nose but expect to repair by tomorrow.

Ari.

God I wish we had your weather. Except for the ding at the end looks like a nice flight. In the second picture the exhaust plume looks to be askew, is there a little extra soot on the port side? Any heat damage to the foam yet?


Richard

1/64" plywood and aluminum tape seem to protect the foam so far. I'm able to clean most of the soot off after flights, but the farther tip has burn marks that persist. It's possible I need a wider "divergence angle" on my trailing edge cutout.

It may be hard to see in the photo with so much reflection on the tape. How reflective the tape is after 3 flights may give you an idea of how well this sandwich stands to to exhaust.

Ari.

I think its working very well. When my delta goes up it eventually melts some of the area between the fins. Theres aluminum foil there but I guess its too conductive even with air flowing over it. A sheet of 1/64 or even card stock should put an end to that issue. Thanks for the info.


Richard

Nose job complete. I'm happy I kept the nose that I'd cut off the ducted-rocket SkyFun--it fits right back on this one.

Ari.

Wow great patch job.


Richard

Had two excellent flights today, one on CTI F30 white longburn and another on AT F30FJ (a baby F).

Graphs are in meters, first flight apogee is 789', second 469'.

This is now 5 flights on this airframe.

Ari.

Good stuff!

Question: From another thread I believe your using FrSky telemetry to get the data, is that the original vario, the low precision or the high precision sensor? I have the first two but will acquire the latter if needed. Been wanting to shove them into my rockets for a while now.


Richard
ps: I love the CTI F30.

I'm using the normal-precision one: https://www.alofthobbies.com/variometer.html

One test that I'm curious about is flying this combo in a straight rocket and graphing RSSI vs. altitude. I'm very curious to see how far up I can control a glider with this off-the-shelf system.

Also, I'm really waiting for their GPS finally to come out. RX+alt+GPS may make the most comfortable tracker for me,and quite a cheap one by rocketry standards.

Ari.

Thanks. The issue I have is that the DHT-U logs in 1 second intervals, I don't know about the Taranis [i'm awaiting the Horus] but your graphs are very nice and tight. One second is a lot of sky to miss in a rocket.

"how far up" would be very good to know! I've flown a 2 meter glider at over 1km and thats using a circular polarized antenna, you could likely get over 2km. [6500"] I don't do HPR anymore so I'll be waiting on your results. Please post!

The GPS sensor once the bugs are out and its available is outside my buget anyway. RSSI will be the rocket locator of choice for me. It astounds me that rocketeers are still spending large amounts to get ham licences and expensive trackers when even a cheap HK Tx and FrSky module will do the job and then sooo much more.

I hope you get around to the G motor soon. The fields up here are't usable for another month or two.


Richard