Diels F7F Build - My First Twin... - RC Groups
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Jul 12, 2017, 10:02 PM
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MKellyvich's Avatar
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Diels F7F Build - My First Twin...


Getting started on a Diels Engineering F7F Tigercat for twin rubber. Intent is to fly it at WestFAC this fall in the Double Trouble mass launch. I'd welcome any advice or suggestions, especially from those who've built and flown twins.

This looks like it should be a fairly straightforward build - laminated keels, split formers, typical built up wing and tail. I started with the nacelles - cut the parts from the printwood, pinned matching parts together and sanded to final shape, laminated the nacelle longerons and started assembling the right and left nacelles in parallel.

Cheers,

Mike
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Jul 13, 2017, 01:29 PM
Balsa Flies Better!
Hi Mike

I like the layout of this airplane, although there is a lot of frontal area- which suggests that lighter is better. (Flies slower- less drag to overcome). I've built a couple of twins- here are some thoughts-

1) The fuse is basically there to hold the tail on. It can be built really lightly- except the problem is that in an "arrival" the nose gets crunched. So you've got to keep a certain amount of beef up front.

2) I'm assuming the airplane has one piece wing, i.e. no butt joints to the fuse. The loads on the wing in between the nacelles are much higher than on a single. Beef things up- add some beef to the TE- maybe some additional bracing.

3) Plan on lots of down thrust. The effective distance between the prop and the wing is somewhat shorter than on a conventional layout, which means that you have to use more down thrust to get the same effect.

4) Make sure you have plenty of dihedral! (Also washout, but you've got a nice planform on that airplane.) You've got lots of weight outboard on these airplanes. Twins can be harder to get out of a spiral....

5) I find some system to hold the props after they've been wound helps. On my B-26- the left motor needs to be a little stronger than the right one, so it often gets a few more turns.

HTH

Sam
Jul 13, 2017, 03:17 PM
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MKellyvich's Avatar
Sam,

Thanks much! Wing is one piece, was thinking about adding a diagonal brace inside the wing between the fuse and each nacelle to help with cartwheeling loads. Spars are 1/16 x 1/8 top and bottom, which seems like it should be sufficient. Nose has a block up front and will get some light infill. Plan (and the original plane) has a fair amount of dihedral, and I was planning on building in about 1/4" washout at each tip.

Keeping in mind our earlier discussions about twins with Tom Arnold, I'm going to try to resist the urge to build for the crash in the hopes of keeping it light enough to fly.

Thanks again,

Mike
Jul 13, 2017, 06:07 PM
Balsa Flies Better!
Hi Mike

On my next twin, I wouldn't be averse to adding some sheet or just more spars between the nacelles. My poor B-26 got really beat up there.

One whacko suggestion- use magnets to hold the nose on- could be built lighter.

Sam
Jul 14, 2017, 01:12 AM
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Quote:
Originally Posted by MKellyvich

Keeping in mind our earlier discussions about twins with Tom Arnold, I'm going to try to resist the urge to build for the crash in the hopes of keeping it light enough to fly.

Thanks again,

Mike
I don't remember where I first heard it or read it but this adage has stuck with me; "Build them to fly, not to crash." I even have it written on the build list that is at my building table.
Jul 25, 2017, 04:14 PM
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MKellyvich's Avatar

Nacelle work


Slowly assembling the nacelles. I've deviated from the plan by adding some infill to the cowls. The kit wood is nice and light, but needs the jigs to keep things straight until all the stringers are in.

Mike
Jul 26, 2017, 08:09 PM
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MKellyvich's Avatar

About time to break out the sanding block...


One nacelle ready for shaping, second one coming along close behind.
Jul 28, 2017, 08:37 PM
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MKellyvich's Avatar

Sanding...


Nacelles are assembled, first cowl has been smoothed out. Also started assembling the stab in between sanding sessions.
Aug 02, 2017, 12:44 AM
Scale Builder
Subscribed! Great choice of subjects and I will look forward to seeing it at WestFAC. Onward and upward!
Aug 02, 2017, 04:49 PM
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MKellyvich's Avatar
Welcome aboard, Chad. It was starting to feel a little lonely in here...

Got the stab assembled. Working out a jig to assemble the fuselage on, ordered parts for it today. Going to start on the wings in the meantime.

Mike
Aug 04, 2017, 12:24 AM
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MKellyvich's Avatar
Sanded the infill to shape on the second nacelle, then spent this evening cutting out the wing ribs and remaining fuselage/rudder parts. Hope to build the wing and a jig for fuselage assembly this weekend.
Aug 04, 2017, 12:37 AM
Scale Builder
Quote:
Originally Posted by MKellyvich
Welcome aboard, Chad. It was starting to feel a little lonely in here...

Got the stab assembled. Working out a jig to assemble the fuselage on, ordered parts for it today. Going to start on the wings in the meantime.

Mike
Thanks for the welcome Mike. Nice, clean workmanship on the Tigercat and I am enjoying your progress. I would desperately like to find the time to build something for WestFAC myself but I've already got too many priorities and time is running short. No matter, I will enjoy it as a spectator only if circumstances dictate. I've not built a free flight model in many years but I happened to stumble on to the last WestFAC event and it really stirred up the memories. I cut my teeth on rubber powered scale and so it will always be my "first love". At any rate I will quit rambling and let you get on with it. Would enjoy meeting you at the event and putting faces to names.

Cheers!
Aug 04, 2017, 02:18 PM
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John Cole's Avatar
Mike,
I'm following along too. Tigercats are special to me.. I grew up near the Sonoma County airport, N. Calif., home of Sis-Q Flying Service in the 60's - 70's. You could hear them coming from far away, that low, distinctive rumble of the F-7's, TBM's, and PBY's on their way to some distant forest fire. We lived in the flight path, and I'd wait for what seemed an eternity till one of them would finally break over the pine trees, grabbing for speed and altitude, while fully loaded with fuel and fire retardant. A memory permanently burned into my young mind.
Great project you have going.
John
Aug 04, 2017, 08:41 PM
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MKellyvich's Avatar
Thanks John! I grew up in southern Arizona and have great memories of the fire bombers climbing out of Libby Army Airfield for drops in the Huachuca mountains. We mostly had B-17s and Privateers (this was in the '70s), later the B-17s disappeared and we saw more DC-4s. Often we could watch them fly over to the mountains and see the drops. That radial sound was guaranteed to see me flying out the back door to get a glimpse.

I don't recall any of the smaller fire-bombers operating in our area - would have been great to see.

Here's the scheme I have planned for this 'cat.

Cheers,

Mike
Aug 05, 2017, 06:39 PM
Registered User
Very cool paint scheme and a nice break from the all blue military scheme. The only juiced up military scheme I have ever seen is big green and white bands around the cowls and aft fuselage indicating instrument trainers. Nice choice as this one does not have the big ugly belly tank too.


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