Please Help:i Dont Wanna Give Up On Jets! - RC Groups
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Dec 06, 2005, 12:02 PM

Please Help:i Dont Wanna Give Up On Jets!

Foamy jet pilots I need your help.
I built the notorious f-18 parkjet and I love it despite the fact that is causing lots of frustartions!
I can`t get it to fly well.
I tried 3 different motos: axi 2212/26, mega 16/15/4 and bp21, all powered by 3s 1700 etec
With all three setup attempts I had problems. After many flights I found the jet affected by two main issues:
first of all, during the flight, at some random point, the plane would pull up like the cobra manouver (with no intention);
second, maybe due to the flat airfoiled wing, the plane seemed to be willing to fly only at full power. Otherwise diving like it was underpowered.

please help me figure out what's going on and what's wrong.

thank you, I really appreciate it.

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Dec 06, 2005, 12:07 PM
Registered User
RCParkflyer's Avatar
Sounds like your CG Location is off.

when you fly the plane, cut the power, if the nose lifts your CG is too far back. If the nose dips down, the CG is too far forward, just move it back.

Dec 06, 2005, 12:26 PM
Thanks Tom, I will try to check the cg again.
One question did anyone try to make an actual airfoiled wing for that plan
The few times I was able to fly it the airplane flew only with full gas .
Dec 06, 2005, 12:32 PM
bigglesjets's Avatar
Hi submarco

I dont think it's the wing profile mate, I have the f!5 and F18 and they dont have this problem.I would agree with Tom check your CG position try bringing it forward 1/2" and adjust from there.check your motor thrust line make sure its not pointed up

good luck
Dec 06, 2005, 12:48 PM
Or am I...
May also be your weight, what are you useing??
Dec 06, 2005, 12:51 PM
Al and Tom,

first of all , just to make sure, where is located your f'18 cg?

secondly yap, actually I may have some thrust line issue, beause I made a mod inorder to mount my outrunner, the thrust line shoul be neutral or what?
Dec 06, 2005, 12:52 PM
Registered User
RCParkflyer's Avatar
It really doesn't need an airfoil, just round off the leading edge. The Lift on the jet is accomplished by the angle of the wing in flight. The slower you go the higher the nose should be, so you get a "High alpha" slow flight. You should be able to fly level at half throttle. Once you nail down the CG it should be a lot easier to fly.

Here is a High Alpha Demo video from 3D Foamy.

Dec 06, 2005, 12:56 PM
I am-sad ,
my f-18 is around 16oz and all three motor, at least in static test seemed to provide more than 1=1 thrust/weight.
Dec 06, 2005, 12:58 PM
what about the motor thrust line?
Dec 06, 2005, 01:08 PM
bigglesjets's Avatar
Hi Submarco

Ive just measured my f18 and the CG is 4" from the leading edge in the center
The thrust line should be straight ,inline with the wing line or 0 degree.
I dont suppose it matters if you add slight down thrust not sure about up thrust
Cg 5.25" from rear line of wing

Dec 06, 2005, 01:11 PM
cheers mate
Dec 06, 2005, 01:18 PM
Hold on!
You said 4 inches form leading edge, in the center.
By the center did you mean at longest chord, in teh center of the fuselage?
Because if that was the case my cg was off of almost 2 inches!!!!!!!!!!!! way to forward. Jeeze, that would explain the tenbdencies to go down and not gaining height.
Dec 06, 2005, 01:34 PM
bigglesjets's Avatar
Hi Submarco

Lets get this right
5.25 inches from the wing trailing edge forward

Dec 06, 2005, 01:50 PM
I will check again
Dec 06, 2005, 02:08 PM
OK , 5.25 from Te going forward. Well, well I was off of 1.25 inches,still way forward.
Thank you very much for the clue.
I actually know what happened, when I was investiganting about teh Hornet I read some post form Steve Shumate warning pretty hard to move the cg some forward (I think it was 1/3 inch)but I must have down loaded the plans already updated with the new cg and then move it even forward thinking to follow Steve`s sugestion.

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