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Old Sep 12, 2013, 04:54 AM
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Joined Aug 2013
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huge f4 phantom pusher problems

Hi all ,I have just finished my scratchbuilt f4 phantom ,it is two meters long with a 530 kva pusher prop motor on it .The plane weighs 7.5 lbs ready to fly,it is built from white foam ,construction style similar to LB Millers f4 only bigger ,my problem is trimming it , and finding the cog ,I have crashed it several times , from a hand launch it flies for about 50 meters than goes into a dive and there is nothing you can do to save it ,do I need upthrust on motor or more rearward cog if anyone has experience with this type of model I would be very thankfull for your advice .
Planecrazy.
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Old Sep 12, 2013, 07:16 PM
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Pollock Pines, CA
Joined Aug 2002
1,460 Posts
You might check with J. Morgan on the Giant Planes thread. He built several F-4's. The last one was a big one like yours. He can probably give you some advice.

Dennis


Quote:
Originally Posted by planecrazy View Post
Hi all ,I have just finished my scratchbuilt f4 phantom ,it is two meters long with a 530 kva pusher prop motor on it .The plane weighs 7.5 lbs ready to fly,it is built from white foam ,construction style similar to LB Millers f4 only bigger ,my problem is trimming it , and finding the cog ,I have crashed it several times , from a hand launch it flies for about 50 meters than goes into a dive and there is nothing you can do to save it ,do I need upthrust on motor or more rearward cog if anyone has experience with this type of model I would be very thankfull for your advice .
Planecrazy.
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Old Sep 13, 2013, 09:13 AM
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United States, VA, Herndon
Joined May 2007
899 Posts
Have you done an un-powered glide test to see if it flies straight and level?

Airfoil wing or flat plate?

Any pics of the aircraft? or video of a flight attempt?

The more details you can provide the better the advise will be.
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Old Sep 13, 2013, 08:20 PM
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hi wilmracer

The air foil is flat on the bottom & I've shaped a foil into front & trailing edges. Done glide test it flies straight & lands perfectly.
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Old Sep 13, 2013, 09:38 PM
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United States, VA, Herndon
Joined May 2007
899 Posts
If the glide test is straight and level then your cg is probably close enough. Thrust angle could be the issue... a few more questions.

It seems odd that the aircraft would travel that far (50m, 164ft) and THEN experience a sudden change in flight. Are you launching at WOT? If not, does the aircraft nose down when you add throttle?
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Old Sep 14, 2013, 02:02 AM
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Joined Aug 2013
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f 4 problems

The plane is hand launched at about a 30 degree angle at full throttle it gets to about 30 feet high than goes into a sharp dive which I can't recover from .l was thinking perhaps that down thrust is taking over after my initial launch. It would be easier with a catapult,than I could do better glide tests . I will post photos when I get this new phone sorted out .I have been following your projects for sometime , and I think they are excellent .
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