Author Topic: Profile Fuselages (Read 9075 times)

john e. holliday · « **on:** February 26, 2016, 11:47:07 AM »

Reading the posts about the profile fuselage rules and how some have tried various ways of making them stiff reminded me of the Primary Force. It has to be the stiffest profile plane I have had to that point. Still have it. Later planes I have use Mikes method of profile construction and it makes for very stiff fuselages and no curves/warps to worry about. Instead of getting 1/2 inch sheet which I have yet to find one that will stay straight, use two 1/4 inch sheets. Laminate using Gorilla glue. I don't weight it down. I use lots and lots of clamps plus my eye balls to get the sheets straight. The glue gives me more time than I need to get it all clamped up. I hang the whole mess up for at least a day and watch the glue easing out around the edges. It also finds all the doles in the grain of the wood. The glue sands very easily. No I don't use carbon or glass fiber between the sheets. I don't believe it needs it.

Now the other alternative is the construction that Don Hutcheson shows on the B-25 and has been done on other designs. It is the inner frame work that is made and then sheeted both sides.

Bootlegger · « **Reply #1 on:** February 26, 2016, 01:32:52 PM »

Doc, the method that I use and like is 3/8" balsa fuse core, then cut out behind the wing trailing edge to leading edge of stab, then triangle 3/8"3/32 bracing to ill the cut out.
Next install motor mounts, then I plank each side with 1/64" plywood attached with epoxy (24 hour cure), after this I install 1/16" plywood doubler's for the engine. Then you can dress up the fuse as you wish.

The 3/8" balsa fuse built like this doesn't twist in the maneuvers of the patter I got this idea from Tom Farmer of Dallas Texas...

Tim Wescott · « **Reply #2 on:** February 26, 2016, 01:38:53 PM »

John, putting something stiff in the center of a fuselage may do something. But to really stiffen up a structure like that you need to put the strong stuff on the outside of the structure.

Russell Shaffer · « **Reply #3 on:** February 26, 2016, 02:43:17 PM »

You gain a lot of stiffness just by gluing two pieces together. Maybe because the grain doesn't match up? Two 1/4 pieces laminated are considerably stiffer than one 1/2 chunk for whatever reason. No, not as stiff as having ply on the outside layer, but it helps and adds almost no weight. And Michaels has 1/4 in stock.

Fredvon4 · « **Reply #4 on:** February 26, 2016, 04:02:29 PM »

Most of the Profiles I have built were a kit with the fuselage pre cut

As a kid I broke many a nose off and never knew about waggle or twisting of the aft end

returning to Control line I saw the various concerns with fuse aft end waggle/twisting so initially experimented with ply and or glass outer coverings... ended up using radially wound carbon fiber strips adhered with Z-Poxy

Two layers one from front to rear using 2 inch long strips and second on a bias the other wrap direction....on one fuselage I cut out lightening holes

I never did weigh the before and after but before and after hand twisting tells me the result is MUCH stiffer in both critical waggle and twist modus

BUT I can see how Doc's method is very good also for a scratch build

Mike Haverly · « **Reply #5 on:** February 26, 2016, 07:40:41 PM »

Sheeted on both sides is much stronger, and lighter. I've attached a PDF of the Starfinder that Norm Whittle designed. This is a large profile and the fuselage is as strong as any I've had. There are two CF strips that run the length and the whole assembly is sheeted with 1/16 balsa.

It is a PDF because I haven't figured out how to save a Corel file to something I can post on this forum.

Motorman · « **Reply #6 on:** February 26, 2016, 09:28:35 PM »

The Flying Clown in my avatar has a fuselage made from 2 pieces of 1/4" poplar epoxied together then shaped. With certain propellers it can penetrate cars.

MM

Trostle · « **Reply #7 on:** February 27, 2016, 09:40:58 AM »

Ted Fancher wrote a two volume set titled How to Design a Profile Fuselage. Unfortunately, I no longer have those volumes, it is not in print, and I cannot remember anything in them.

Those volumes are really collector's items, right up there along with his book on How to Find a Great Looking Cowl Inside a Block of Balsa.

Keith

Gerald Arana · « **Reply #8 on:** February 27, 2016, 09:57:13 AM »

Quote from: Motorman on February 26, 2016, 09:28:35 PM

The Flying Clown in my avatar has a fuselage made from 2 pieces of 1/4" poplar epoxied together then shaped. With certain propellers it can penetrate cars.

MM

And just how did you find that out?

Jerry

Serge_Krauss · « **Reply #9 on:** February 28, 2016, 10:50:32 AM »

I've built a couple fuselages similar to Gil's, and they are very bend- and torque- resistant. They are made from 1/2" core balsa, with the aft fuselage partially hollowed out and trussed, two side laminates of 1/16" balsa, and .56-oz fiberglass applied to the surface with West Systems epoxy. For minimal weight gain, one could also laminate between the core and sides with glass or CF. With slightly less volume (?), these fuselages are lighter than my previous P-Force 1/4" balsa halves covered with silkspan (the bad stuff) and butyrate. The glass apparently takes less epoxy than the carbon veil and is marginally lighter. It also was not allowed to float, as I suspect the silkspan did in butyrate. The biased cloth was a real headache, but made a noticeable difference in resistance to twisting.

MikeyPratt · « **Reply #10 on:** February 29, 2016, 08:14:35 PM »

Quote from: Russell Shaffer on February 26, 2016, 02:43:17 PM

You gain a lot of stiffness just by gluing two pieces together. Maybe because the grain doesn't match up? Two 1/4 pieces laminated are considerably stiffer than one 1/2 chunk for whatever reason. No, not as stiff as having ply on the outside layer, but it helps and adds almost no weight. And Michaels has 1/4 in stock.

Bingo!!!!! Somebody gets it!!!!!

BillP · « **Reply #11 on:** March 01, 2016, 09:06:37 AM »

Glue lines make a big difference, especially on foam with epoxy glue. I'm a sport flyer and don't remember seeing a flexing problem with kit planes but have stiffened a few super lightweight scratch built planes. 1/2" thick profiles by edge gluing two pieces (top & bottom) lengthwise. Another by drilling diagonally from the aft fuse bottom forward to the top of the fuse...then insert a K&S hollow aluminum tube, long hardwood dowels or BBQ skewers. I do it this way because it can be done after the build if I think the plane is too flexible. I've used this method on rc plane horizontal stabs to totally eliminate high speed flutter too. The hardest part is drilling (12" length) without breaking through the sides...which has happened a time or two. I made wooden handles for my long drill bits and don't use electric. My profiles are normally sanded down to 3/8" or less and slightly oval shaped after the doublers.

Tim Wescott · « **Reply #12 on:** March 01, 2016, 10:21:56 AM »

Has anyone measured how much laminating a fuselage with something in the middle improves torsional and bending stiffness?

Russell Shaffer · « **Reply #13 on:** March 01, 2016, 04:02:08 PM »

I bet a piece of 1/32 ply between two 1/4 pieces would go a long way towards curing the tendency to break at the trailing edge on low pull outs.

Bill Johnson · « **Reply #14 on:** March 01, 2016, 04:16:26 PM »

Quote from: Bootlegger on February 26, 2016, 01:32:52 PM

Doc, the method that I use and like is 3/8" balsa fuse core, then cut out behind the wing trailing edge to leading edge of stab, then triangle 3/8"3/32 bracing to ill the cut out.
Next install motor mounts, then I plank each side with 1/64" plywood attached with epoxy (24 hour cure), after this I install 1/16" plywood doubler's for the engine. Then you can dress up the fuse as you wish.

The 3/8" balsa fuse built like this doesn't twist in the maneuvers of the patter I got this idea from Tom Farmer of Dallas Texas...

I was thinking of something along these lines but using carbon fiber tubes instead. They are pretty cheap at the LHS.

Serge_Krauss · « **Reply #15 on:** March 02, 2016, 09:38:31 AM »

Quote from: Tim Wescott on March 01, 2016, 10:21:56 AM

Has anyone measured how much laminating a fuselage with something in the middle improves torsional and bending stiffness?

I have only twisted them in my hands; so no quantified results here. I can only report that the fuselage became torsionally stiffer and lighter just through cutting out the aft area and adding 1/8" balsa trusswork. When the 1/16" balsa sides were laminated on, the aft fuselage was very noticeably stiffer. Then the biased glass made it almost un-bendable without breakage in my hands. The epoxied Fiberglass was as light as a doped finish over glass or the new silkspan. 1/64 plywood doublers would be extremely stiff, but that stuff is 10 times as dense as 6 lb/ft³ balsa. That makes 1/64" ply laminations 2.5 times as heavy as 1/16" balsa ones.

I'm sure that just laminating three pieces of fuselage wood will create a noticeably stiffer fuselage than one solid piece of the same total thickness because of the varying grain directions

Again, FWIW. - SK.

Steve Helmick · « **Reply #16 on:** March 02, 2016, 03:34:44 PM »

Quote from: Trostle on February 27, 2016, 09:40:58 AM

Ted Fancher wrote a two volume set titled How to Design a Profile Fuselage. Unfortunately, I no longer have those volumes, it is not in print, and I cannot remember anything in them.

Those volumes are really collector's items, right up there along with his book on How to Find a Great Looking Cowl Inside a Block of Balsa.

Keith

I seem to recall the How to Design a Profile Fuselage article was by the late Larry Cunningham. His last Mo'Best used 1/64" plywood skins, but the clever part was that they were arched (fuselage cross section was sorta oval).

Steve

MikeyPratt · « **Reply #17 on:** March 13, 2016, 11:19:02 AM »

Quote from: Serge_Krauss on February 28, 2016, 10:50:32 AM

I've built a couple fuselages similar to Gil's, and they are very bend- and torque- resistant. They are made from 1/2" core balsa, with the aft fuselage partially hollowed out and trussed, two side laminates of 1/16" balsa, and .56-oz fiberglass applied to the surface with West Systems epoxy. For minimal weight gain, one could also laminate between the core and sides with glass or CF. With slightly less volume (?), these fuselages are lighter than my previous P-Force 1/4" balsa halves covered with silkspan (the bad stuff) and butyrate. The glass apparently takes less epoxy than the carbon veil and is marginally lighter. It also was not allowed to float, as I suspect the silkspan did in butyrate. The biased cloth was a real headache, but made a noticeable difference in resistance to twisting.

Hi Serge,
That looks great and that should work fine. I've done something that is close to what you have done. On my new P-Force XL the cut out is bigger with bracing installed and then it sheeted with 1/16" balsa on each side set with the grain set a 45 on one side, them the grain is set at 45 in the opposite directions. The fuselage came out really light and stiff.

Later,
Mikey

Gerald Arana · « **Reply #18 on:** March 13, 2016, 07:10:50 PM »

Quote from: Steve Helmick on March 02, 2016, 03:34:44 PM

I seem to recall the How to Design a Profile Fuselage article was by the late Larry Cunningham. His last Mo'Best used 1/64" plywood skins, but the clever part was that they were arched (fuselage cross section was sorta oval). Steve

I built my P-40 Shark using 1/16" over an 1/8" frame in an oval shape. The frame was extremely flimsy until I got the sides on. But first I doped on CF veil (on the inside), installed the sides, then put CF on the outside too! Very stiff and fairly light......

Lots of stuff can be done with a little imagination.

I think next time I'll try the "toilet paper" method with 1/2 OZ Glass cloth (on the bias) and epoxy. (And its OK to say my planes are the schist!)

Jerry

Serge_Krauss · « **Reply #19 on:** March 13, 2016, 07:51:43 PM »

Thanks, Mikey. I just saw your post.

Jerry, that sounds like a VERY stiff fuselage. One caveat though, when you try the FG cloth biased, be sure to do something to stabilize the edges in two directions so that the weave can't "parallelogram." It can become an unimaginable mess quickly if allowed to follow its own course as soon as part of it is glassed. Give yourself enough overhang to work with, so that if/when this happens, you can fix it without wasting that expensive material. I liked my results, but did not at all enjoy the adventures. Ha!

SK

MikeyPratt · « **Reply #20 on:** March 22, 2016, 06:41:46 PM »

Quote from: Tim Wescott on March 01, 2016, 10:21:56 AM

Has anyone measured how much laminating a fuselage with something in the middle improves torsional and bending stiffness?

Hi Tim,
One of first Primary Force models was built by laminating .5 ounce mat between the 1/4" fuse sides, and the fuselage doublers. It was pretty stiff much more than just the 1/4" balsa sides epoxied together. I tried to quantify this by placing the fuse on the end of the table and placed a large weight on top of the fuselage doubler. Next I placed an 18" piece of maple motor mount and attached to the area of the stabilizer. Then I hung 10 ounces of weight onto maple mount and measured the amount of deflection (and or twist) to the fuse.

The first fuse was without the c/f mat was deflected at slightly less the 5/8". The fuse with the c/f mat was deflected 3/16". It was stiffer but not really sure that is the last word. After that I covered the outside of the fuse with .2 ounce c/f mat and nitrate dope. Then again, it was stiffer than before but I was unwilling to do that test again, not wanting to damage the new fuse.

Later,
Mikey