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FT Goblin Pictures & COG Question

#1
Hello there! This is my first post to the FT community... I've built a couple Tiny Trainer variations and just flew my FT Goblin yesterday for the first time. It was my first experiment with a wing of any sort and wow! was it fun to fly. Honestly, I had never even launched a wing before and this really takes off right out of your hand.

The only question I have is that it does need a good bit of elevator to stay in the air. I have the same situation with both my Tiny Trainers.. no matter how much I mess with the COG they seem to want to point down the second I let up on down elevator. Maybe I am spoiled by my Bixler which seems to float on Magic Fairy Dust -but from reading I feel like these should be more stable in the air then they are. Trim doesn't help btw.

I will keep:
1. trying to move the COG back as much as I can
2. perhaps set the elevator mids to slightly raised

Or perhaps I'm just new to airplanes and wanted to share my FT Goblin build since there aren't a ton of them out there.

 

FDS

Well-known member
#2
I just trim my elevator until it flies level at a reasonable throttle. If you are using a lot of elevator then check to make sure the hinges aren’t binding and that your rates are right, you need the full movement of the gauge. As you have identified weight may be a factor as well. Also worth looking at the neutral trim, sometimes when you glue the servo in you lose the centre position, if you have a bit of down elevator in neutral because of this then it will need more elevator input to stay level.
If you set it up with the CG as shown in the plans, with a little nose heavy, then it should fly fine. You can add incidence to the Tiny Trainer wing with a 1mm spacer atbthe leading edge to increase lift.
The TT is known for ending up too nose heavy if you are not careful with the glue levels.
What you are seeing with the minis in general vs the Bixler is a higher wing loading, which means you have to fly faster to generate lift, so if you have a lower throttle the nose will pitch down as the lift drops off.
Another thing to look at is battery weight and prop choice.
I usually run only 850 2s max or 500 3s in the TT, any bigger makes it fly much worse IMO.
Prop wise a 5x3 doesn’t produce enough low down grunt vs an APC 6x4, I run only 6x4 on my TT.
Can’t comment on the flying of the Goblin as I haven’t built one yet. Yours looks very nicely built and painted.
Welcome to the forums, people here are super helpful.
 
#3
I just trim my elevator until it flies level at a reasonable throttle. If you are using a lot of elevator then check to make sure the hinges aren’t binding and that your rates are right, you need the full movement of the gauge. As you have identified weight may be a factor as well. Also worth looking at the neutral trim, sometimes when you glue the servo in you lose the centre position, if you have a bit of down elevator in neutral because of this then it will need more elevator input to stay level.
If you set it up with the CG as shown in the plans, with a little nose heavy, then it should fly fine. You can add incidence to the Tiny Trainer wing with a 1mm spacer atbthe leading edge to increase lift.
The TT is known for ending up too nose heavy if you are not careful with the glue levels.
What you are seeing with the minis in general vs the Bixler is a higher wing loading, which means you have to fly faster to generate lift, so if you have a lower throttle the nose will pitch down as the lift drops off.
Another thing to look at is battery weight and prop choice.
I usually run only 850 2s max or 500 3s in the TT, any bigger makes it fly much worse IMO.
Prop wise a 5x3 doesn’t produce enough low down grunt vs an APC 6x4, I run only 6x4 on my TT.
Can’t comment on the flying of the Goblin as I haven’t built one yet. Yours looks very nicely built and painted.
Welcome to the forums, people here are super helpful.
Thanks for all that knowledge - much appreciated!

I have been trying to fly my TT with 850 3s batts, added a mini FPV transmitter as well so its weighted down for sure. My thinking was so long as it balances on the COG points it should fly alright but perhaps for the small wing the weight really gets to it.

20190712_181355000_iOS.jpg
 

FDS

Well-known member
#4
With the minis less weight is always better. Mine flies fully vertical on 2s, I run it on that most of the time.
How was flying the TT FPV? I have a 25Mw all in one spare that I might try.
 

mrjdstewart

Well-known member
#5
make sure you have a FB thick reflex in your elevons or you will be fighting with lot's of up elevator as you mention. the bottom paper layer of your elevon should be at, or just above the top layer of the wing. the CG is just back of the leading edge of the wing at the fuse. very atypical if you haven't flown a plank before. once you get this sorted the Goblin is a great flyer and lots of fun. my current Goblin is stupid fun and does close to 120mph! :devilish:

good luck,

me :cool:
 
#6
With the minis less weight is always better. Mine flies fully vertical on 2s, I run it on that most of the time.
How was flying the TT FPV? I have a 25Mw all in one spare that I might try.
It has been my first FPV plane experience... honestly its pretty awesome. Handles loops, rolls, split-s and most other maneuvers just fine. You do have to watch it cause its nose heavy still but definitely flyable and super-fun.

I created a 3d printed 2-part design that allows me to quickly take the camera off the plane for easy switching of FPV gear and/or flying LOS.

Here's the design and OnShape Edit Link
https://www.thingiverse.com/thing:3734538

I use a TX02 camera AIO, can be had for $13-18 on BangGood with a 250 tinywhoop battery which gives me around 15-20m of flight time.. Around 3 of my 850 3s packs.

Going to try some lighter packs this weekend and hope it helps with the downward pull per the helpful info in reply #2 above.
 
#7
make sure you have a FB thick reflex in your elevons or you will be fighting with lot's of up elevator as you mention. the bottom paper layer of your elevon should be at, or just above the top layer of the wing. the CG is just back of the leading edge of the wing at the fuse. very atypical if you haven't flown a plank before. once you get this sorted the Goblin is a great flyer and lots of fun. my current Goblin is stupid fun and does close to 120mph! :devilish:

good luck,

me :cool:


Thanks for your help! When you say "the bottom paper layer of your elevon should be at, or just above the top layer of the wing " is that when the elevon is neutral/1500 etc..? To re-state my question another way, should the mid-position of the elevons be set slightly upward?

I have been testing the CG at 15-20mm back of the leading edge per the "instructions". I have read several accounts where people balance it another 10mm back from that. I have been testing with a 2200 3s battery but there is not much room for adjustment with that pack. I'll send a 1400 and 850 up in it today and see if I can get

For the record we are talking about the FT Goblin CG here.
 
#8
Flying wings require a certain amount of reflex. Reflex is when the elevons are set slightly in the up position as the neutral position.

For my Goblin, the bottom of the elevon trailing edge is 2- 3mm above the top of the foam board for the wing trailing edge (near the verticle stabliizers). My Goblin also flys well with the CG nearly 10 mm aft of the CG point in the plans. On my first flight, a combination of not enough reflex and having the CG as indicated in the plans made it feel very nose heavy . After adding additional reflex and moving the CG back slightly it has very nice flying characteristics. Now I can also reduce throttle and keep adding up elevator until the Goblin sets down nearly verticle.

I have been flying with eiher a 3s 1300 mah or 4s 1300 mah, 2205 / 2300 KV, and Runcam Split FPV setup.

Good luck, it is one of my favorite planes!
 
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#9
Flying wings require a certain amount of reflex. Reflex is when the elevons are set slightly in the up position as the neutral position.

For my Goblin, the bottom of the elevon trailing edge is 2- 3mm above the top of the foam board for the wing trailing edge (near the verticle stabliizers). My Goblin also flys well with the CG nearly 10 mm aft of the CG point in the plans. On my first flight, a combination of not enough reflex and having the CG as indicated in the plans made it feel very nose heavy . After adding additional reflex and moving the CG back slightly it has very nice flying characteristics. Now I can also reduce throttle and keep adding up elevator until the Goblin sets down nearly verticle.

I have been flying with eiher a 3s 1300 mah or 4s 1300 mah, 2205 / 2300 KV, and Runcam Split FPV setup.

Good luck, it is one of my favorite planes!
Thanks for that summary, really helpful to hear what you run.... The “reflex” makes perfect sense. Excited to put it back in the air with these updates!
 
#11
About 10 more packs of 2200 & 1400 batteries through my Goblin.. As recommended I have added the reflex. With the 1400 batteries I can dial the cg in and it flies darn near perfect. I am still struggling with the 2200's (have lots of these so my preference is make the Goblin compatible. ) Can't fly slow at all and it feels like elevator authority is lacking especially on landing if I slow down on approach I can barely keep it nose up. Battery is pushed as far back as possible.

Next Up:
- I am going to look over the radio and double check my servo throws to make sure I didn't do something dumb.
- Experiment with a 4-5mm motor standoff or 3d printed extension to balance it out with the 2200
- Maybe pull some FPV equipment out to clean up the balance. Out of 40 missions 2 have been FPV.. its just too dang fun to fly LOS!

Hope you all had happy flites this weekend!
 

FDS

Well-known member
#12
The big heavy battery is what is causing the problem with the 2200. It’s too fat for your bird! Build something bigger for the big battery, enjoy flying your small wing on a small powerpack.
The reason it won’t fly slow and needs elevator input is because the wing is stalling, it can’t generate the lift required to overcome the mass at low speeds. Air molecules don’t scale down.
Glad you got it flying well on the smaller battery.
 
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CarolineTyler

Well-known member
#13
My Goblin flies with s 2650 3S in it - the penalty of Westboard foamboard :( - It does fly great though and with that battery I get around 20 minute flights of hooligan flying :)