Fat Boy "D" conversion

[Fore Check]

So I finally picked up a Fat Boy at Wal-Mart yesterday for that nice $8.88 price. I put it together with a D engine mount, and accordingly trimmed the tabs on those TTW mounted fins.

However, I'm not at all satisfied with how "flimsy" they feel. Perhaps I trimmed the tabs too much? I don't think this is the case.

I was just wondering if anyone else had a similar problem. I think I'm going to have to fillet the fins with fix-it epoxy clay just to be satisfactory (this is not desireable at all, because it will mess with my plans on the finish just a tad.)

And sorry - no pics until it's finished! Still need to seal the balsa and paint. Construction took all of about 45 min, though - including the building and installing the motor mount (I use thick CA for everything.)

 

[vjp]

Forecheck -

I have built and flown a Fat Boy with four C6-7's (the equivalent of 2 D12's) using the stock fins just fine. I think you'll be fine.

 

[vjp]

Oh, and I forgot - that was using only yellow wood glue, nothing else (except I did epoxy-coat the inside of the BT for heat protection).

 

[Fore Check]

Perhaps I should clarify.

I'm not *at all* worried about the strength of the balsa - I'm worried about the attachment joint. You can "rock" or "wiggle" the fins side to side quite a bit (you know, getting them in and out of that nice 180 degrees from each other alignment)

 

[slim_t]

Yeah, they are pretty flimsy, but they seem to do ok.
I built mine stock and have flown it many times. I've gotten a couple chips on the fins from landing on gravel, but other than that they're alright. I do think they should be thicker though.
I know a few people have laminated theirs with Avery label paper (sticker paper). I think that's what I'm going to do as well.

Tim

 

[Steward]

Your right ...if they aren't touching the inner walls of the mount...they will not be as strong as they should...there is not enough support under the fins at the outer wall.
I just finished one yesterday and was concerned about the same thing..although I didn't take off as much...very careful step!!!
I suggest if you haven't glued them yet, is to go buy some stock balsa and re-cut your fins...I've had to do that before on other rockets... Good luck...
btw...it was your suggestion the other day to go to wally mart and buy a Fat BOY...mine has turned out well...and yellow...pictures real soon....

 

[vjp]

Ah, I see what you mean. If you think you may have trimmed off too much from the tabs, and they're not glued to the MMT, there's probably too much flexibility. If you've already glued them in, you could cut through the glue joints and re-cut fins with longer tabs, but you might want to try cutting some small holes in the side of the rocket, and filling the MMT area with expanding foam.

 

[Fore Check]

Originally posted by vjp
Ah, I see what you mean. If you think you may have trimmed off too much from the tabs, and they're not glued to the MMT, there's probably too much flexibility. If you've already glued them in, you could cut through the glue joints and re-cut fins with longer tabs, but you might want to try cutting some small holes in the side of the rocket, and filling the MMT area with expanding foam.

I took care of the problem. In my D conversion, my centering rings are made of 1/8" plywood (stock item from BMS.) Anyway, I drilled a 1/4" hole in three places in the rear centering ring and reached up in there and slathered the fin/mm joint with 5 min epoxy. Kinda tedios to do with a little dowel, but everything seems much better now.

It wasn't that I trimmed them too short, Btw. What I did was give it a "tulanko tail", so I had to wrap the mm tube with radiator hose tape to keep hold of the engine hook. I don't think the CA adhered to this tape very well (think heavy duty black duct tape.) Of course, epoxy seems to stick to almost anything...........

And Steward: I've got my yellow for the fins ready to go! I'll post pics after I'm done to reveal the "other color." May be next weekend before I get finished....

 

[Fore Check]

Just to be sure: RockSim thinks it needs about 1 oz of nose weight to get a Barrowman margin of about 0.5 (which should be fine according to Galejs) with a "D" engine. Does this sound right to all of you?

 

[Johnnie]

The only mods I did for my D conversion Fatboy, was the larger motor mount and triming of the fins to fit the new larger mount. That D motor put the fatboy up there just fine with no nose weight added...Hung that puppy in a tree on it's fourth and final flight (insert sound of taps playing)

 

[vjp]

Originally posted by Fore Check
Just to be sure: RockSim thinks it needs about 1 oz of nose weight to get a Barrowman margin of about 0.5 (which should be fine according to Galejs) with a "D" engine. Does this sound right to all of you?

Sounds plenty to me - 1 oz. is more than the difference between a "C" and a "D", and it's placed farther forward of the CP than the motor is aft of it.

I've always felt that the "calibers of stability" axiom should be replaced with a "% of rocket length" axiom, anyway.

 

[astrowolf67]

I put 2.5 oz in the nose of my 24mm Fat Boy. With the epoxy added, it came to 3oz of weight. She flies great on D,E, and F motors, especially the F21 econojets.

Now the Fat Boy I finished last week, and flew this past Saturday...has 4oz of BB's, plus the epoxy, plus a 1/8 bulkhead for a total of about 6 oz of nose weight. She flew beautifully on a G35 Econojet It just has a small zipper, that will fix with some CA, and I've already figured out how to prevent it next time.

 

[Rob Fisher]

24mm fat boy I scratch built this morning. No sense in going out and buying another fat boy when I had everthing I needed lying around! just had to pick up some basswood and the nose cone. no thru-the wall fins on this fat boy!

 

[Rob Fisher]

some decals that I picked up for the D fat boy. thinking about going with purple on the BT.

 

[Johnnie]

Can't go wrong with a Purple PJ...love the flames as well.

 

[Austin]

Make sure you guys post some pics...I wanna see these conversions up close and personal.

Carl

 

[Steward]

I was planning on just a solid yellow.....
but...man those flames are wicked...!!!

and keep in mind...imitation is the sincerest form of flattery...!!!

 

[Chr$]

Here's my 24mm fat boy. I call it Fat Buoy.

I used 1/8 plywood rings with two nutserts for HP style motor rentetion.

I fly it with the milk bottle (nose cone) full on an F24.

I may fly it next on one of those new ellis mt G25 24mm su motors.

It flies on a D-12 with the bottle empty.

I places a ton of yellow glue on the motor tube before inserting the fins. Takes attention to detail to get them trimmed just right. They are strengthened with epoxy fillets at the BT transition.