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Pusher Park Jets For all pusher jets and jet-like models.

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Old 09-21-2013, 02:15 AM   #1
tobydogs
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Default need help finding cg on a pusger jet sratch build before maiden.stu

hey gang,i just read a very complicated thread on cg location and the programs show forward wing designs and tail sections.

i built a pusher jet using scrap box balsa and the wing is from a crashed brio 10. i did this with parts in my junk drawer so really didn't spend money or that much time...just kept it simple. but she's a day or so away from maiden and i haven't a clue how to be sure she isn't tail heavy.in fact i want slightly nose heavy for the maiden.

thanks to the 2 page thread here at wattflyer i just read i am no closer to figuring the cg.... the build thread is scratch build $50 cheater, fun jet.

heres are pictures and my design was to resemble a fun jet and the battery can move so much forward and back that i won't need weight to attain cg...just need to know where to strap the battery down with Velcro and it's launch time. the picture second to last picture that shows the battery set at that point would balance the plane on the wings spars...this to me would be to far to the Te. thanks for any help,i really don't want the first toss to be it's last.stu




arrrrrg! my title didn't spell check "pusher" WHAT THE HECK WORD IS PUSGER!!


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narrow is the place to land...wide is the space to crash....choose the narrow way!
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Old 09-21-2013, 02:49 AM   #2
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Start appx 15% of the distance between LE and TE out where the servos are.

The norm for a flying wing is about 15% MAC. For that wing shape the MAC chord line will be near where the servos are.

Expect to need the elevons up a bit from neutral for when the wings were on the Brio. Maybe close to 5 deg.
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Old 09-21-2013, 04:10 AM   #3
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thanks for the quick reply fhhuber. i think theres a certain risk i'll take and get her airborn...then results sould be obviouse of coarse. i looked at the cg balance points on the funjet ultra. very close to the leading edge,maybe an inch in from the le.

i'll measure the 15% and finish the cheat build after flying saturday. winds are supposed to be 10 to 15mph,thats what they said last weekend and it was calm and beautiful!!!

right now i'm charging 5 batts for the540t edge,then 3 for the stinger. already have 6 batts for the superfly. so even if it's blowing 15+mph i'll toss the superfly and stinger.it's been a long week at work and the field is very nice to unwind there. stu

narrow is the place to land...wide is the space to crash....choose the narrow way!
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Old 09-21-2013, 04:29 AM   #4
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this was part of a program to find cg. to bad all the configuration for cg calculations were for the typical type plane...very cool,but no drawings of a pusher prop plane. it was easy to figure out the measurement sequence and then input all the information and hit the enter calculate button and get the cg point. if only it was as simple for a strange plane like mine. i figure the power from the 3000kv donsrc wicked motor on 4 cell will be more than enough to push this bird quickly thru the air....i'll just be ready to to attempt tall grass emergency landing if need be,,,lol.

narrow is the place to land...wide is the space to crash....choose the narrow way!
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Old 09-21-2013, 05:25 AM   #5
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Finding MAC of the wing won't change for the "tail-less" model.

That picture leaves out the method to determine where MAC will be. I used a "guess-timation" for this aircraft based on the taper and sweepback looking near even which will put MAC near halfway between root and tip (about where the servos are) After finding the MAC of a lot of wings you get a feel for where it will be for most wings.

Typical test flight range for a conventional model is about 30% MAC
For a flying wing 15% is more common.
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Old 09-21-2013, 05:48 AM   #6
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Just remember the golden rule:

Nose heavy flies poorly. Tail heavy flies once.

Pat Gagnon

Owner - Nico Hobbies

Your source for micro pusher jets!
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Old 09-21-2013, 06:23 AM   #7
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Hi You Might want to give the Motor about 1 to 2 degrees of down thrust, the nose of the motor will be pointed down just a tad for a pusher wing / plane. it looks like you might have some up thrust in it right now, hard to tell by the picture.


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Old 09-21-2013, 08:20 AM   #8
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Hi Stu .... that baby is going to be ultra sensitive to CoG ... narrow long span, tail-less ...

Me ? I'd put CoG at about 10% back from root LE ... knowing it's going to be nose heavy. Then work back..... but resisting the temptation to go back too far !

The Snice Mitar I built ... wow was that way out ... I put it where calcs etc. said - she just looped UP, stalled and then ploughed in ! Still to try her with more fwd setting.

Stu ... tried uploading my various CoG calculators but file invalid .. email me and I'll forward on.

Here's some links :

http://fwcg.3dzone.dk/

http://wingcgcalc.bruder.com.br/en_US/?


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Old 09-21-2013, 08:50 AM   #9
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fhhuber got it spot an as far as i can see. However if you want to get a second opinion you can use a simple flying wing CG calc (which does away with tail considerations) as per one of Nigel's links

Excessively nose heavy is not always 'safer' because the plane might well 'lawn dart' strait into the dirt before you can get enough up elevator to stop it, especially for a hand launched model.

Looking at the plane I'm a bit concerned that you will be lacking directional stability. The wing has no significant sweep back, the fins are quite small and located quite close to the CG, the nose is long with quite a bit of de-stabilising side area. You might find that it's ok under power but gets out of hand when you throttle back.

The fix if it does appear to lack directional stability is to increase the fin area and sweep them back so the area is as far behind the CG as possible.
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Old 09-21-2013, 10:17 AM   #10
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I just reworked my Snice Mitar CoG as a test on the links above .....

WOW compared to the 'Licenced' CoG calcs I have - it put my CoG 25mm MORE forward than the 'safe' result .... which dare I say it - agrees with the results I had when trying to launch her.

I've now cut the fuselage top to re-arrange bulkheads to get my battery more fwd.

Glad you started this thread Stu - just shows how wrong things can be ...

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Old 09-21-2013, 03:04 PM   #11
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Rule of thumb estimations of the initial CG just give a starting point that is usually safe and further forward than required... but occasionally something strange creeps up making you need further forward.
Its pretty rare to need further forward but common to be able to end up moving the CG back more.
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Old 09-21-2013, 04:44 PM   #12
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Here is the maiden failure of the Snice Mitar...

http://www.youtube.com/watch?v=Xu0i6...ature=youtu.be

She's ready to go again............

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Old 09-21-2013, 04:57 PM   #13
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If all else fails it's very easy and quick to cut out a foam 'chuck glider' with the same plan-form shape as your RC model add the fins, reflex the TE a shade, and test glide it until you arrive at a good stable glide.

This is a good option if the shape is unusual and difficult to estimate with a calculator.
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Old 09-21-2013, 05:29 PM   #14
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Originally Posted by JetPlaneFlyer View Post
If all else fails it's very easy and quick to cut out a foam 'chuck glider' with the same plan-form shape as your RC model add the fins, reflex the TE a shade, and test glide it until you arrive at a good stable glide.

This is a good option if the shape is unusual and difficult to estimate with a calculator.

+1 I will be doing that with my F-20 too, I have used CG Calculators before, only to have the plane come out Tail Heavy Every time I use them, I am going to Try a Calculator one more time and set the CG at 25% MAC at the fuselage, I Read some where that 25% MAC is good for any plane any time any where

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Old 09-21-2013, 06:17 PM   #15
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Originally Posted by CHELLIE View Post
I am going to Try a Calculator one more time and set the CG at 25% MAC at the fuselage, I Read some where that 25% MAC is good for any plane any time any where
If you measure at the fuselage then it's not necessarily MAC, if there is any forward or rearward sweep then it will be miles out. And 25% MAC, even if it was the 'true' 25MAC position would be dangerous for tailless designs and fatal for canards.

So you need to be careful with such rules of thumb.

You cant really go wrong with the chuck glider method though, as long as you make it an accurate copy (scaled down is ok) it will work.
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Old 09-21-2013, 10:16 PM   #16
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wow! thanks for the input and I'll check that down thrust issue alsochellie.i thought down thrust for a pusher was slightly point the spinner/collet tip up like the parkjet and funjet.. this was really a "just for kicks" build and never really planned any exact science...thats pretty obvious.

this day at the field went from great to bad to worse. flew the superfly and stinger several times and then went to fly the electifly edge 540t....did a range check using the power down button on the new eclipse 7 hitec tx which i never used that button before[normally i just walk far enough and check controls]. all seemed great. took off to a porpoising beast. it wanted to dive and turn right at the same time with sticks in neutral. i fought it up to 3 mistakes high to calmly tried to trim it, and adjust my thoughts for bringing the beast down in one PC as i figure a control surface is jammed. she destroyed herself in the tall grass.

truly not dumb thumbs and i was a bit angry[quietly of coarse]. flew the superfly and sting more till it was to windy.

came home to test the controls and Rx in the wreckage to see what may have caused the catastrophic results. for what ever reason i cant seem to get my 2 unused Rx optimum7 ch to bind properly. the throttle channel works fine through the esc with the red wire cut. my ubec's power up the the rx's but not the servos...i'v tried rebinding,reprogramming. switched to aux battery power.brain racking and getting no servo connection with these 2 rxs! all my other planes are still working fine like the hog,stinger,superfly..ect...ect.

best thing to do now is let everything go for a few and re investigate it latter.

unfortunately the pusherjet build requires one of the 2 rxs available. i really should start drinking beer again....lol..[jk] instead i'll go down to the cave and try again later. stu

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Old 09-21-2013, 10:30 PM   #17
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Up/down thrust... point the end of the prop shaft furthest from the CG up for up or down for down.
Works every time
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Old 09-22-2013, 09:01 AM   #18
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Originally Posted by tobydogs View Post
for what ever reason i cant seem to get my 2 unused Rx optimum7 ch to bind properly. the throttle channel works fine through the esc with the red wire cut. my ubec's power up the the rx's but not the servos...i'v tried rebinding,reprogramming. switched to aux battery power.

Stu,

Correct me if I'm wrong but the Hitex Rx has an auxiliary 'SPC' power feed direct from the battery.. yes.

If you have this SPC hooked up then it sounds like the uBEC isn't supplying any power (damaged in crash, broken wire or similar) and the Rx is powering up only on it's SPC supply from the battery.
The SPC powers the Rx but does not supply 5V to the servos, so the servos don't work when you are only on SPC power (which makes it kinda pointless IMHO)

I read of someone who powered their model up not realising that the switch on their BEC (the type of BEC with a on/off switch) was 'off'. If it had been a normal system it would have just been dead and nothing would have powered up. With the Hitec SPC system the receiver and motor powered up, throttle worked normally. He stupidly omitted his pre flight check and attempted to take off only to realise after it lifted that he had no control over the servos, only over throttle.. needless to say it didn't end well!

Steve
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Old 09-22-2013, 02:51 PM   #19
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You say you cannot get to Bind ... but as I see it - the Rx is bound but only throttle working.

So I would try Rx's with standard ESC / BEC via the Throttle channel ... see what happens. If what JPF says is true ... then that would show up the fault with ESC supplied BEC ... basically that Ubec needs to be plugged in somewhere else !

Sad about the 540 ...

I know it's silly question - but why the UBEC ... in my models - it has to be something really high folluting or 5S + to get a UBEC ...

As to Parkjets with Upthrust ... that's an old bit done to death ... all mine have slight DOWN .....

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Old 09-23-2013, 02:30 AM   #20
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lol...ditto that on the down/up thrust talk,i was a bit out of sorts after flying yesterday when i type that post. i figure a couple of washers at the motor mounts top screws will do the trick. as far as the tail fin's go...i may increase the size,i wonder why wing sweep is so importantto weather this bird will be stable flying? iv, seen foamy straight wing builds with no vertical fins fly on utube and the the guy took the winglets off and it still flew.maybe i shold add winglet on...simple enough to do. also my funjet is very close to the same shape i built. it's where i got the idea to meld miscellaneous parts laying around together.

truly I'm not one to scratch build. I'm more of a builder/rebuilder...lolheres a few pictures of my putting the edge 540 t back together. seems when you've had 2 and your wife buys a third by accident ,there will be plenty of pc's left from the wreckage to build another. i'm only begining to fix the fuse and will be cut away extr balsa to lighten the repairs. the covering is going to be so easy and trim sheets to replace the colors...she's going to look new...and i'll not buy another electrifly 540t again. at least my ajslick lasted a couple of years.

the fuse I'm repairing is the one that landed hard when the canopy blew off,got caught on the vertical and i thought it was the control surfaces that broke. i just have to wait for the new canopy as it's the only pc missing and i need it to shape the fuse formers so it fits right. all good fun.

lastly,the Rx's are missing a jumper plug that belongs in the spc spot when the ubec is plugged in the aux battery spot[ch7]on both Rx's i have . i must have pulled it out in the car when i stripped a few parts from the wreckage i didn't want damaged. not sure what happened to it but they are also on order. it was just one of those days yesterday. iused the ubec casltle creations battery eliminator because the esc was already disabled via the red wire and i read 4cells/4 servos ubec's a good idea. maybe not needed but it flew beautifly before saturday.

i'd just like to say to all you buds who affer support to guys like me...thank button has been hit for you all!!! stu


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Old 09-23-2013, 09:18 AM   #21
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Hi Stu ... glad all is sorted .. and good luck with rebuild.

I'm still confused why the strange power set-up on your Rcvrs .... can't really see the point ... maybe if I knew what SPC was abbreviated for ? I know my old JR gear had a DSC position - that was Direct Servo Control ... so you could plug in a cable between Tx - Rx and check controls without radiating RF (days of frequency control on FM).

Did you check out my Discharger I finally put together ? Really pleased with it ...

The Snice Mitar scratchbuild is re-balanced and ready to maiden again - IF the weather improves.

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Old 09-23-2013, 12:06 PM   #22
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Nigel,

SPC = Supplemental Power Connection

It's a connection that goes directly between the flight battery and the receiver bypassing the BEC. It's there as a backup supply to prevent 'Brown out' in the event that there is a dip in the 5v supply from the BEC.
This is all well and good but the 'problem' is that the SPC only powers the receiver and not the servos. So if the BEC goes out on an e-power model you have control of throttle but nothing else.
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Old 09-23-2013, 01:46 PM   #23
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OK ... understood ....

Does the battery have to be cut back on voltage at all or is just straight battery .. as in a 4S at over 16V ?

Seems one of those ideas that if it was that good - everyone would have it ...

Nigel

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Old 09-23-2013, 11:36 PM   #24
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hi Nigel,the spc is as jpflyer described. i don't use it but am using the aux battery[6v]in my larger birds and the motor and esc i'm using in the edge 540t came from the uproar which also used the 6v battery. the edge didn't need the extra weight so i went with the castle creation battery eliminator. since the red wire was already cut on the esc i wanted to give the cc a try.
I've also read that many esc with bec's are not reliable from 4cell's and up.so since the edge is on 4cellss i figured it useful. the thing that i didn't see missing on my rx was a jumper plug that belongs in the spc spot so the aux battery spot[7] wasn't powering the servoes. these rx's are all new to me do to switching every planes old assan rxs out.

i have a call out to hitec to see if they think the transmitter should be checked cause i still don't know what caused the strong down/and right turn after launch...what aileron position if it were stuck would cause this dilemma. i also need to order 4 new servo's and another thread talking servo has me thinking savox or maybe a better hitec servo than the hs 82 mgs.

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Old 09-24-2013, 10:37 AM   #25
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Stu ... I tend to stay with about 4 regular servos ...

Small less than 5gr :

http://www.hobbyking.com/hobbyking/s...cro_Servo.html

Great for in wings .. flaps etc.

Under 9gr :

http://www.hobbyking.com/hobbyking/s...5kg_07sec.html

Brilliant little servo for in-wings and those models where the 9gr is just that bit oversize to fit ...

9gr :

http://www.hobbyking.com/hobbyking/s...cro_Servo.html

The TG9e is my standard servo and I like price and performance ! Breaks ? replace !

http://www.hobbyking.com/hobbyking/s...cro_Servo.html

If I cannot get the TG9e .. then I get the HXT ...

If above are out of stock ... then I go for :

listed as 10gr ... it's only a tiny bit more longer than the 9gr ...

http://www.hobbyking.com/hobbyking/s...4kg_0_09s.html

Over 10gr up to 20gr :

http://www.hobbyking.com/hobbyking/s...Search=hk15148

And my biggest at 43gr ...

http://www.hobbyking.com/hobbyking/s...10sec_43g.html

The above serve me well ... and haven't seen any reason to pay more .. at the prices of the smaller stuff - they make good box fillers when shipping weight slack !
I used Towerpro SG and MG before - but got sick of the gears stripping .. the replacement gear sets often didn't fit ... so I junked them when they broke ... found the TG9e as a better cheaper replacement. Cheap enough that if it strips ... bin it and buy new.

You'll note that all quote good speed at around 0.1 sec .. and that seems to be born out in use.

I have a box of 1980's HIGH quality Futaba and JR servos ... boy are they S...L.....O......W !! I find it hard to believe I used to fly pattern / scale etc. with them !!

Nigel

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