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XF-85 Goblin started... - 12/11/2007 11:38:59 AM   
petronio


 

Posts: 186
Joined: 11/16/2004
From: almeria, SPAIN
Status: offline 		Hi,
I always liked this plane so I decided to start building it as a PSS. SOme weeks ago I opened a post in this section asking how I could simulate panel lines with oracover, once I found the answer to it I started the construction. The reason why I posted it here and not in Slope Soaring is that I'm trying to do it as scale as possible (within the limits of a PSS). Obviously I had to enlarge somewhat the wingspan and lessen the elevator dihedral. Anyway, this is what I have so far: If anyone wants to know more about the design or construction have a look at my other post:

http://www.rcuniverse.com/forum/m_6605556/tm.htm



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       Post #: 1

RE: XF-85 Goblin started... - 12/11/2007 11:53:35 AM   
abufletcher



Posts: 5172
Joined: 2/13/2004
From: Zentsuji, JAPAN
Status: offline 		Wow, certainly not your run-of-the-mill, ordinary sticks and formers model!

< Message edited by abufletcher -- 12/11/2007 11:55:36 AM >

(in reply to petronio)
       Post #: 2

RE: XF-85 Goblin started... - 12/12/2007 3:41:44 AM   
John Baligrodzki



Posts: 129
Joined: 4/16/2003
From: North St.Paul, MN, USA
Status: offline 		It's great to see models lke this being built. Please keep us up to date with more pictures and info as you build this interesting model. I can see from looking at photos of the actual aircraft you will need to make a few changes to have a decent flying model. The good news is that while a some people might recognize it they will not really be able to tell you have deviated from scale as they have only seen a picture here and there.

Good luck with your project!

Regards,

John B.

(in reply to abufletcher)
       Post #: 3

RE: XF-85 Goblin started... - 12/12/2007 12:34:16 PM   
crashontakeoff



Posts: 76
Joined: 3/28/2006
From: Bayamon, AA, USA
Status: offline 		petronio,
There is a book with all of the photos and scale features of this plane. I saw it a t AA Hobbies in Warwick Rhode Island, sorry now I did not pick it up. It was very cheap as I recall, if I had bought it I would send it to you. Very Creative building process, reminds me of making balsa cowls. Now the trick would be to get a Slope Soarer B-29 and hook up in flight, that would be most impressive.

Crash

(in reply to John Baligrodzki)
       Post #: 4

RE: XF-85 Goblin started... - 12/13/2007 9:54:40 AM   
petronio


 

Posts: 186
Joined: 11/16/2004
From: almeria, SPAIN
Status: offline 		Thank you guys . Indeed it's not a very usual plane but thats one of the things I like, I'm quite bored of always seeing the same planes in the flying field (which doesn't mean I don't like them) . Well, Crashontakeoff I had been grateful to you for the rest of my life jeje, but well its ok, actually I think I have collected all the possible pictures that I could find online, there was also a nice 3-view from airwar.ru which I used to draw the plan. Unfortunately on christmas I will be away and I'wont have too much time to continue but I will have about 4 or 5 days in which I will lock myself in at the garage and try to at least glue together the foam chunks . MOre to come...

< Message edited by petronio -- 12/13/2007 9:55:08 AM >

(in reply to crashontakeoff)
       Post #: 5

RE: XF-85 Goblin started... - 12/13/2007 11:48:39 AM   
fockewulf37


 

Posts: 1227
Joined: 3/19/2002
From: kannapolis, NC, USA
Status: offline 		I want to see the Mother ship you plan to drop this from, Good luck with your build. Looking good So far....

(in reply to petronio)
       Post #: 6

RE: XF-85 Goblin started... - 12/14/2007 7:52:14 PM   
DanSavage



Posts: 577
Joined: 4/17/2002
From: Trabuco Canyon, CA, USA
Status: offline
quote:

ORIGINAL: petronio
quote:

ORIGINAL: petronio
Hi,
I always liked this plane so I decided to start building it as a PSS. SOme weeks ago I opened a post in this section asking how I could simulate panel lines with oracover, once I found the answer to it I started the construction. The reason why I posted it here and not in Slope Soaring is that I'm trying to do it as scale as possible (within the limits of a PSS). Obviously I had to enlarge somewhat the wingspan and lessen the elevator dihedral. Anyway, this is what I have so far: If anyone wants to know more about the design or construction have a look at my other post:

http://www.rcuniverse.com/forum/m_6605556/tm.htm


Nice subject. I've always loved the 'Bug'.

Here's a video of another modeler who built a Bug. (XP-85) I looked at the thread linked above and it looks to me like your CG is a little aft of where it should be. When you calculated the CG, did you carry with wing planform all the way to the centerline, or just calculate for the exposed wing area?

Dan

(in reply to petronio)
       Post #: 7

RE: XF-85 Goblin started... - 12/14/2007 8:09:43 PM   
fockewulf37


 

Posts: 1227
Joined: 3/19/2002
From: kannapolis, NC, USA
Status: offline 		Great Video.. Bad outcome...

(in reply to DanSavage)
       Post #: 8

RE: XF-85 Goblin started... - 12/14/2007 9:30:21 PM   
petronio


 

Posts: 186
Joined: 11/16/2004
From: almeria, SPAIN
Status: offline 		Thanks for the video DanSavage, I had already seen it but it was funny to see it again hehe. I read some time ago the post where the video was and according to the guy he had some problems with his radio and the elevator or something like that. At least the thought that it wasn't presumably the model consoles me , Im quite conscious that because of that little tail moment one should have very little throw on the elevator. I also abandoned the original idea of using a reflexed airfoil, as if it were a flying wing and using ailevons to control it. Instead I will go for the S-3021 and have the elevator working as a whole, inserting some carbon tubes in it and moving both elevators with one servo (if its possible) and if not with two.
I used an excel program that I have to calculate the CG but I calculated it again using the usual method (attached image), according to this the CG is in fact slightly forward. In the program I have to input the data about the wing and in the second case I used the top view of the wing in the plan. byeeee

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(in reply to fockewulf37)
       Post #: 9

RE: XF-85 Goblin started... - 12/14/2007 9:59:20 PM   
DanSavage



Posts: 577
Joined: 4/17/2002
From: Trabuco Canyon, CA, USA
Status: offline
quote:

ORIGINAL: petronio

Thanks for the video DanSavage, I had already seen it but it was funny to see it again hehe. I read some time ago the post where the video was and according to the guy he had some problems with his radio and the elevator or something like that. At least the thought that it wasn't presumably the model consoles me , Im quite conscious that because of that little tail moment one should have very little throw on the elevator. I also abandoned the original idea of using a reflexed airfoil, as if it were a flying wing and using ailevons to control it. Instead I will go for the S-3021 and have the elevator working as a whole, inserting some carbon tubes in it and moving both elevators with one servo (if its possible) and if not with two.
I used an excel program that I have to calculate the CG but I calculated it again using the usual method (attached image), according to this the CG is in fact slightly forward. In the program I have to input the data about the wing and in the second case I used the top view of the wing in the plan. byeeee


You're welcome. The only problem I saw with that model was that it was massively tail-heavy. That's what caused the uncontrollable pitching up and down. When a model is that tail-heavy, the elevator doesn't respond to input so it can make it seem like there's a problem with the radio.

I don't think you'll need to make it into a flying wing. I think the tail area is sufficient to fly the model.

Your method is fine. But, if you only calculated it using the visible part of the wing, then it'll end up as tail-heavy as the model seen in the video, and probably with the same results.

To correctly calculate the CG, carry the wing leading and trailing edges all the way to the fuselage centerline.

I've got a 3-view of a Bug at home. I'll calculate the CG then post my results and you can compare what I got to your results.

Cool project.

Dan

< Message edited by DanSavage -- 12/14/2007 10:01:19 PM >

(in reply to petronio)
       Post #: 10

RE: XF-85 Goblin started... - 12/14/2007 10:41:50 PM   
Craig-RCU



Posts: 491
Joined: 1/10/2002
From: minneapolis, MN, USA
Status: online
quote:

ORIGINAL: petronio
... Im quite conscious that because of that little tail moment one should have very little throw on the elevator.


The opposite is true in my experience. When I maidened a 1/12th scale Me 163 Komet that I scratch built, I made that mistake. I set up the elevons with little elevator throw and much aileron throw. Elevator response was very weak but the roll rate was almost uncontrollable. I got it down in one piece and reversed the throw bias to favor elevator instead of aileron and I had a winner that flew great.

Fun project by the way I'll be following with great interest.

(in reply to petronio)
       Post #: 11

RE: XF-85 Goblin started... - 12/14/2007 11:28:54 PM   
DanSavage



Posts: 577
Joined: 4/17/2002
From: Trabuco Canyon, CA, USA
Status: offline
quote:

ORIGINAL: Craig-RCU


quote:

ORIGINAL: petronio
... Im quite conscious that because of that little tail moment one should have very little throw on the elevator.


The opposite is true in my experience. When I maidened a 1/12th scale Me 163 Komet that I scratch built, I made that mistake. I set up the elevons with little elevator throw and much aileron throw. Elevator response was very weak but the roll rate was almost uncontrollable. I got it down in one piece and reversed the throw bias to favor elevator instead of aileron and I had a winner that flew great.

Fun project by the way I'll be following with great interest.




I missed this earlier.

Yes, that's been my experience, too. Most jets need lots of elevator and only a little aileron.

Dan

(in reply to Craig-RCU)
       Post #: 12

RE: XF-85 Goblin started... - 12/15/2007 1:47:21 AM   
DanSavage



Posts: 577
Joined: 4/17/2002
From: Trabuco Canyon, CA, USA
Status: offline 		I took the top view from one of the pics you posted on your other thread.

Here's the CG results. The forward pink line represents 20% MAC and the aft one, 25% MAC. As you can see, there's quite a difference between mine and yours. I measured your CG on my MAC and the front of the little white box is 35% of the MAC and the back of it is 45% of the MAC.

I suspect that the guy in the video didn't include the wing area inside the fuselage when calculating his CG, which is why it flew the way it did. It's too bad because it was a beautiful model. Yours looks like it's going to be a nice one, too.

Incidentally, since you're doing this in CAD, you should be able to use it to calculate the CG on your models.

The function is called a centroid, or geometric center of a shape. I use DesignCAD 3D MAX and it has, what they call the Center of Gravity command. It's really a centroid function. The centroid happens to fall on the MAC. Drawing a line across the centroid from the leading to trailing edge defines the MAC. That's what you're seeing in the drawing I posted.

Dan

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< Message edited by DanSavage -- 12/15/2007 2:00:43 AM >

(in reply to petronio)
       Post #: 13

RE: XF-85 Goblin started... - 12/15/2007 9:47:55 AM   
petronio


 

Posts: 186
Joined: 11/16/2004
From: almeria, SPAIN
Status: offline 		Thanks for tip DanSavage. In that case I will calculate it again as you say, it's a well known fact that one prefers its model to be rather front heavy than tail heavy on a maiden . But I wonder why you have to calculate that bit too, I had never done that. That bit of the wing inside the fuselage is not sustaining at all, if it had the shape of a lifting body I could somehow understand it..?!? Anyway I will do that, it will make things a bit more difficult since as you say the model is tail heavy; there are 6 pieces of tail feathers on the very back.
Thanks again DanSavage and Craig for the tip about the throws, hmm strange one would suggest that having such a short tail moment you shouldn't put too much throw but well I'll put something more and lots of expo just in case. Thanks and byee

(in reply to DanSavage)