canadagoose

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Hello, i've been trying to draw up plans for my new project but with my lack of building experience regarding twin engined models, i have a question about mounting the engine nacelles to the wing. My idea is to make the bottom half the main structural part that would be bolted to ply mounting plates in the wing between 2 ply ribs with either (2) 1/4 nylon bolts or with smaller steel bolts. That way the whole nacelle could be removed if needed, then the top half could be a non structural unit either made of wood or fiberglass to just act as a cover that can be removed for access to the fuel tank and lines. Does this sound ok if i were mounting 90 4 strokes or should the nacelle be integrally built into the wing. Which way do people usually do it?
The top nacelle in the plan pic shows the upper portion removed that would just act as a cover
The other pic is my project plane.

William Robison

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Terry:

Might be just perspective, but the tail moment on your project plane looks small, pitch control might be a lot easier if you lengthen the after body a bit.

You can build the nacelle mountings as you suggested, but with the hard points in the wing and nacelle for mounting the structure weight is usually greater than with a more evenly distributed mounting stress.

I started building my Tiggerkitty this way, ended with a glued assembly but some of the hard structure is still there, made the plane a lot heavier than needed.

Bill.

canadagoose

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Bill, do you mean just glueing the nacelle to the wing sheeting? and maybe reinforcing with fiberglass? on my single engined planes i usually reinforce the firewall with glass cloth on the inside, i thought a removable nacelle might be better in case of repairs and make covering easier.
It might be just the perspertive the tail is actually fairly long, the cg is at 28% measuring from the tip of the spinners on the engines back to the cg location on the airfoil.

William Robison

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Terry:

Correct. The nacelle sides are just glued to the surface of the wing.

As an example, the Duellist, a very successful twin design of Dave Platt, has ply doublers on the sides of the nacelles to distribute the load, and they are glued only to the surface of the wing. The nacelle sides are aligned with wing ribs, but there is no reinforcement going through the wing skin.

John Marien's Twin-Air planes are similar, but since John has cut the upper surface of the wing out for tank clearance making a shorter nacelle, there is a ply frame near the back of the nacelle that extends through the wing, it's glued to the spar and the sides of the nacelle.

In the case of a ply firewall glued to a ply doubler a strip of glass cloth adds to the strength, but in the case of the nacelle being glued to the balsa skin, a well fitted joint is more than strong enough, and since any glass reinforcement would be against balsa wood, it would add weight without making the joint any stronger. If the joint isn't fitted as well as you'd like use Sig "Epoxolite" filler, using this you can strengthen the joint and make the fillet at the same time. A warning on Epoxolite - wet your finger and do all the shaping/smoothing before it hardens, it's almost impossible to sand. Otherwise excellent stuff.

The hardest thing about scratch building wing mounted nacelles is keeping the weight down, 90% of the things you think needed for strength aren't necessary, they just add weight.

Bill.

canadagoose

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Bill, thanks for the info on how others are done, maybe that's the way i'll do it too. I'll just make sure that the center section of wing out to the nacelles is built strong (fully sheeted with sheer webs and spruce spars) to resist twisting.

William Robison

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Terry:

Go [link=http://www.rcuniverse.com/forum/m_1912953/anchors_2084345/mpage_1/anchor/tm.htm#2084345]here[/link] for a twin-Air build thread, post 19 has a picture of the nacelle.

Bill.

jaka

Hello!

Yes
First making the whole nacelle in glass fiber and then gluing the nacelles directly to the balsa sheeted wing is what I think is the best thing too do ......very light and still very strong.
I just used Ca glue when I mounted my glass fiber nacelles to my Marutaka DC-3 wing (the wing in that area was first covered with 25g glass fiber and resin).
Regards!
Jan K
Sweden

William Robison

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Jan:

Looks good. But I would have used a stronger clue than CA. Personal preference.

Bill.

canadagoose

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jaka, very nice looking aluminum finish on your plane. Yes that would be a good idea covering the area of wing with a light glass cloth that the nacelle would be glue to.