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Old Apr 22, 2011, 09:41 AM
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neversommer's Avatar
Luzern/Switzerland
Joined Aug 2008
2,894 Posts
Help!
F-18 Exhaust Diameter to small?

Hello Everyone

I just needed to make a quick new thread.

My friend bought a foamie edf F-18 Airframe only kit which he rebuild and glassed.

Now inside is a 127mm Ramtec Fan and housing with a scorpion outrunner motor.

A Castle Creations 160HV Amp esc as well as 8x4S 4000 Nano Tech Batteries to create a 12s /8000mah Battery Setup, I think.

His exhaust Diameter of the twin exhaust nozzles of the F-18 is 6.5cm or 65mm in diameter wide.

The Size of the F-18 is like a good 1.7meters long with a 1.2m wingspan.

Total Weight with these batteries is 7.4 kg. Thrust is 6.5-6.7kg.

Now...the sound is very loud and when holding the jet to make a thrust test of the ground , you can't let it go otherwise it would hit the ground.

My thought was that the 65mm diameter of the twin exhaust's is too small.

We haven't got a amp reading on it . But he want's to maiden his Jet tomorrow.

Any thoughts or ideas?

The plane is from hayoe I think.

Regards

NS.
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Old Apr 22, 2011, 11:35 AM
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Joined Nov 2005
3,619 Posts
Outlet needs to be no smaller than about 75% of FSA. In this case, you'd need about 80-90mm, so yes that outlet is way too small. (Assuming the two outlets add up to 63mm). You won't get a lot of thrust from this situation, and certainly not enough static thrust at takeoff to get airborne. Just for grins, what's the inlet area? I bet that's too small as well. Sounds like the airframe is designed for a smaller fan.
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Old Apr 22, 2011, 11:43 AM
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Swarthmore, PA 19081
Joined Dec 2004
2,838 Posts
You need a bifurcated (split) airflow exhaust duct. Each side will need to be 68.75mm in diameter. This will give about 85% FSA when you add the two areas together, and should do well. You are a little under with the 65mm openings, but it may be acceptable. Doug
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