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Aug 05, 2015, 04:20 PM
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XF-85 Goblin

Recently I found my plans for the XF-85 goblin I had done back in 2008, and found the video. I decided I'd build a new one as the old one was lost in a move. I started with the plans as they are posted. I decided to put in my normal 24mm rocket mount just so that I could use my same plug in motor pod with a grayson microjet.

Here is the flight video of the new version, below I give more detail on what I did:
McDonnell XF-85 Goblin RC parkjet (5 min 34 sec)

I had designed this with a fully movable elevator to give sufficient pitch control, and kept them horizontal as people have had pitch control issues when trying to use the angled tail surfaces.

In order to clear the rocket motor mount with the elevators. I extended some permanently mounted stubs and mounted the movable surfaces outboard. If you do a normally mounted motor in the tail you don't need to do that. I went with dual servo ailerons since they are simple.

I considered doing a top view as well, but that would have moved my CG even further forward.

The changes I made were to add a doubler just on the bottom half of the fuse below the wing, this gives something more substantial to land on and allowed me to hide a little carbon rod stiffener for the back half(it was a bit flexy) and allowed me to hide my nose weight. I also went with a round 1/8" carbon rod A frame wing spar setup since this version was going to weigh more than my original. AUW for this version was 12 oz. The ventral stub fin was added to the design after test flights on the real plane showed some instability, on mine it was not needed and like the look better without it, so I removed it, but it turns out it helped prevent prop strikes on landing, so I added a carbon pipcicle stick to do the same thing...I had to use a high motor mount to prevent the prop from hitting the ground during flare on landing. Due to this It required a good amount of thrust line adjustment, I set the thrust line so that it goes right through the CG. I tried initially without thrust offset and it was nearly unflyable.

Control setup: I used 75% aileron and 125% elevator.

It doesn't seem to have any bad stall tendencies, just wing wobble as it gets slow. Rolls are quite fast and axial. It has one flight oddity..if you don't have sufficient airspeed in a loop or if you try to pull the loop too fast, it will just get to vertical and stop/hang there...I'm not sure if due to reduced airspeed the elevator just looses authority or if the wing is blanking the tail on a hard pull. If you keep throttle on and pull gently it does nice big loops.

It looks really neat in the air, I did some panel lines and markings and it looks pretty good. It flew really well in some high winds today.
Last edited by burkefj; Aug 08, 2015 at 12:36 AM.
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Aug 07, 2015, 03:48 PM
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I finally broke down and set the thrust line so that it would go through the CG and it is a very nice docile plane now. I need no trim adjustment for launch, flight or landing. I normally get by with no thrust angle but on this very short plane it was really required.
Aug 07, 2015, 05:56 PM
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Video now included!
Mar 15, 2016, 04:18 PM
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The Goblin is a very interesting subject to model. Looks like it flies good as well.

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