skeletonizing profile fuselage

I am currently building a Brodak Pathfinder II and I am thinking about skeletonizing the rear part of the fuselage. Neither the plans nor instruction book mention this as an option. Earlier I built a 1/2A pathfinder and this was shown as an option which I did use. What are your thoughts on this subject. Will this weaken the fuselage too much on the much larger more powerful airplane, or not?

I'm assuming that you would cross-grain plank the rear of the fuse after cutting it out. This will make for a noticeably stiffer fuse but is considerable work for a design that might well not need it.---Mark

All,

I would not cross grain plank the aft fuselage of a profile. I have seen many of Tom's warp from doing this for no apparent reason. He has had good success with 1/64 plywood planking however. It's a little heavier, but much more rigid than balsa.

I no longer recommend the cross grain balsa planking for the aft portion of the fuselage. Tom Morris was using this technique for his profile Cavaliers and noticed that sometimes they had a tendency to twist and warp. In spite of the cross graining being symmetrical, funny things happen to balsa sometimes when dope is applied.

Subsequently we have switched to planking with 1/64" plywood. The interior can be filled with white foam or diagonal formers. Although the plywood is slightly expensive, nothing beats this technique for strength and stiffness in a profile.

As was pointed out, however, the framed aft fuselage is a labor-intensive technique and may not be worthwhile for all ships.

L.

I've seen the balsa planked fuse twist slightly on my Mo'Best fuse over the years but it sure is stiff and light. Next time I'll try foam & ply.

LAST EDITED ON May-22-02 AT 05:15 PM (CST)

Tom Morris displayed a 1/64" plywood planked fuselage with internal balsa truss and NO foam at VSC-14. It was extremely strong and torque resistant without the foam.

I would build the kit pretty stock, but for a scratch-built, I would saw strips and build up the fuselage like Larry C's or Tom M's. That way you can place the grain going the right direction, use a bit harder wood where you need it, etc. I'm hoping Larry will spill the beans on how to build a straight fuselage with the curved 1/64" plywood sides. That should be super stiff. Steve

Larry,

I read your great series on building a better profile.

The reason I want to frame up my fuselage is primarily to save weight. Is it practical to cut out the fuselage, truss it up
and silkspan it? will it just not be stiff and strong enough? You are without doubt an expert on these types of airplanes, and I really value your opinion.

Don Jones

Getting a little weight savings behind the CG might be a good idea as the Pathfinder tends maybe more than other designs toward tail heaviness. I have done two, a kit and a semi scratch. Both needed the stock OS muffler with an OS 40FP SSW rework to achieve a CG that I am comfortable with. The CG seemed about right when I slipped an OS 46LA with tongue muffler on.
This is a bigger airplane than most 40 size profiles(around 620 squares), has a thick airfoil and large stab and elevator and will need a really stout 40 to pull it at any reasonable weight. The LA 46, though not fully broken in and a little quirky, seemed like it might be made for the airplane. It is also an exceptional flyer with enough power, very solid, no surprises, and the trike gear makes for easy take offs and landings.

Better than the Cardinal profile? Maybe. Probably a matter of individual preference.

Shelby,
Hope I can help a little here. I designed the pathfinder that you are building. If you can make it stiffer by all means do it. But if it is just for the weight then don`t. Because you`ll have to add tail weight anyway. So just leave it for the strength and cover it with silk span or carbon. Most of the one`s we had here it Salt Lake City,needed tail weight added.

Gordie

PS Hope this helped.

LAST EDITED ON May-22-02 AT 09:01 PM (CST)

Don and Steve,

I'm afraid the BBP series needs a further upgrade. The latest cleanup is on the PAMPA site now, but I might need to add a chapter called "Don't Do That After All!", and "New Ideas", or something..

Although cutting out the interior part, adding formers, and planking it will probably work, it will not be optimum. I've seen such a method used on stabs. But the problem is that the grain of the wood ends up poorly oriented, on at least one side. It's a bit cheesy..

I like to frame up the perimeter with a very straight (spar grade) wood, which is NOT the very softest and lightest. This wood is actually stronger per weight, and more suitable for framing. With a solid sheet fuselage, you wouldn't want to use it because it would be too heavy. But as a perimeter piece it is just fine.

Although it is probably overkill, I used a .007" carbon fiber strip laminated on the inner surface of the perimeter strips on the last couple of fuselages. Somehow I don't think it is necessary. (Nowadays I'm using a 3/8" wide fuselage core, instead of 1/2", and the perimeter material I use is 3/8" x 1/4"..)

When you are laying up a perimeter, it is very important to get the alignment correct. On the Mo'Best, I accurately cut out both 1/64" plywood sheets, and I take the trouble to accurately mark all spar locations on inner surface of the outboard sheet. Then I use CA to first tack and then thoroughly glue the perimeter and internal strips to support the stab, and internal structure of the area.

On the Mo'Best, the perimeter pieces extend to join onto the front fuselage sheet, as shown here:

[photo not recovered: 3cec4eb605523954.jpg]

My latest approach tapers (symmetrically) only the very end portion of the tail, the part behind the elevator hinge line. And I put in a soft balsa tail block in that area.

[photo not recovered: 3cec510b07e13d9e.jpg]

There is a rectangular cutout hole for the stab, which is framed, of course. And the area below the stab has a mounting block for the tail wheel.

[photo not recovered: 3cec51d508de68d7.jpg]

The area behind the tailwheel block is accessible for tail weight, using a little removeable panel. (The dowel shown is for mounting the screw which holds on the panel.)

And the open area in front of the stab can be filled with with foam, or it can have diagonal formers installed:

[photo not recovered: 3cec53140a2aad6d.jpg]

[photo not recovered: 3cec53570a639d6c.jpg]

The cool thing about this method is that the accurate 1/64" plywood planking sides maintain the alignment.

It's a simple matter to close the assembly with the inboard planking sheet. 5 minute epoxy works fine, and you can use this for the foam filling as well.

The turtledeck/canopy portion of the new Mo'Best fuselage is pieced together from soft balsa and glued on to make it look like an airplane.

Now, about Steve's interest in the experimental curved formers. These are easily layed out with a CAD program, by drawing projections and a center line reference. Then an arc through 3 reference points. I used a 1/8" convex curvature on both sides on this center line.

Anyway, once they are cut out, you can see that we can't use the clever self-aligning method where the perimeter pieces are glued to the outboard spar. Instead it has to be frame up, in a conventional manner, like one might build a free flight fuselage side. But you need to elevate the perimeter, for clearance for the installed convex shaped perimeter. This is tedious at best. After it had all been carefully aligned and built, I used a long sanding block to carefully match the top and bottom perimeter with the curvature of the formers.

When that is all done, it is fairly straightforward to glue on the 1/64" planking.

And, as you would expect, the result is extremely rigid, being slightly cupped by the curvature. The curvature is not very noticeable on the finished fuselage. One more thing, the experimental one on Robert's latest Mo'Best is a 1/2" sheet version, versus the 3/8" sheet version I'm using lately. (With the 3/8" sheet fuselage version, I turn the 1/2" x 3/8" hard maple engine mounts 90 degrees, and make up a ladder style engine crutch assembly for the nose. There are some other compensations around the engine mount area as well.)

But guess what? The FLAT sided version is stiff as iron, and is quite a bit easier to build straight, being self-aligning. So guess which way we'd probably do it in a kit?

Hope this helps explain my method. I'm working out details, so these drawings are not perfect yet.

If you try to quote me, I'll deny all of it..

L.

"I keep tryin' to think, but nothin' happens.." - Curly Howard

Have to agree with Gordy. I have built two Pathfinders now and with a 40 or 46 size engine the c.g. came out almost exactly as on the plans with no added weight. Lighten the rear and you will most likely need to compensate with tail weight...zero sum gain. BTW I have been getting the PF into trim with the help of Mike Pratt. Mike is really impressed with the PF and LA 46 Macs muffler combination. Mike comments last weekend after putting up a 500+ point pattern with it was "that's a darn good flying airplane that will serve you well..." That's a real compliment on the PF design, and I have to agree. Thanks Gordy!
Brian

Gordie:
The plans show a CG approx 3/8" behind the spar and this is about where it is likely to come out with an FP 40/tongue muffler. To my insensitive hand it seems to fly more to my liking with the CG at the rear edge of the spar, especially in the wind, thus the FP 40/OS factory muffler. It still turns well and flies super, again to my insensitive hand.I gain more stability-what am I losing by moving the CG forward?
BTW, a super design. Well thought out, relatively easy to build, straight forward construction. Of the two, actually three, I have built, all wings came out straight just building on a flat surface. The Cardinal profile is maybe more challenging to get straight, at least in my one attempt with that kit.

LAST EDITED ON May-24-02 AT 09:23 AM (CST)

LAST EDITED ON May-24-02 AT 00:15 AM (CST)

Jim,
You have to set the c/g for the plane. That`s where it flies the best. I like to trim the plane so that it can fly it`s very best,then adjust the handle spacing to make it easier to fly. But if it is flying good with the c/g alittle forward don`t worry about it.If your comfortable you will fly it better. Hope you enjoy the plane.

Gordie,
Since you, as the designer recommend building the Pathfinder per the instructions, that is what I will do! By the way,t
his airplane is a very nice piece of design work,and really looks great to. Did you also design the 1/2A pathfinder? I have one of those as well. That little dude really does fly well with
a Norvel .061 for power. When I was building this ship, I thought to myself, there is no way this plane is going to fly with that little engine. Boy was I wrong. I am using some .015 lines cut down to 35 ft., but I think I could probably use longer lines, of smaller diameter. What do you think?

LAST EDITED ON May-24-02 AT 00:18 AM (CST)

LAST EDITED ON May-24-02 AT 00:12 AM (CST)

Shelby,

Yes the 1/2 is a good plane. You thought it was too big Huh!
I would use .008 lines and move out to 42ft.I Think you would like it better. These little things turn so quick you might find it better to run a little nose heavy.
Hope you like the big one also.

Gordie.

Ps.Any questions I can help you with let me know.

Hi Gordie,

I am in the process of building your 1/2A Pathfinder. I have a question on engine choice.

Currently, I have a COX TD 051. Is this sufficient for this plane? If this power plant is marginal, would lightening up the fuse, flaps and the stabs according to the plan make sense? If I have to buy another engine, what would you recommend?

Line length will be based on your prior recommendation - 42' 0.008 cables. The tank would be a uniflow 1.5oz clunk tank. Wing would be covered with Monokote.

Many Thanks in Advance.

Eric Lee

Eric

I used a o51.TD and it had plenty of power. I ran a 6-3 prop and ran the engine at a lean setting. I fly at 4200 plus ft. so if you flying lower than that you should have plenty of power.Hope this info helps.

Eric,
On my 1/2A Pathfinder I made all the optional cutouts shown on the plans, and I am glad I did, because I had to add about an ounce of nose weight to get the C/G right. I used a Norvel .061 engine with A 1 1/2 oz. tank. I covered with monocoat, but I got lazy
and did not install the optional weight box or adjustable leadouts. I recommend both of these options as well as the adjustable wing tab and rudder, which I did install. My Pathfinder flys quite well as it is, but the other adjustments would be nice.
Prepare yourself for a surprise----this aint no Little Wizard. This is a real stunter!

Thanks Gordie and Don for your tips. My intention with the PF is to learn to do the pattern with it. Sound like I made the right decision.

Don, how did you ran your Norvel 061? 4-2-4, fast 4, rich 2? Just curious b/c I ordered one for my PF. I suspect my PF will come out heavy (lack of skills);therefore, I may need the extra power. Regardless, I'll try both engines.

Shelby 554
A lot of good suggestions have been posted here on this subject and I thought I would share a few of my experiences with you. I have built many profiles because they quick and easy to build and it allows me to try different design ideas and verify the results before implementing the idea into a big time stunter. Without a doubt there are two major concerns when building a profile stunter. One is making the front end strong enough to eliminate engine vibration (this vibration can give you fits with inconsistent engine runs). The second is a fuselage that allows the tail of the model to flex under load (hard corners) causing inconsistent bottoms during the pattern.

First lets consider the engine vibration. By mounting the engine on its side the lateral forces created by the engine are transferred to the fuselage in a horizontal plane. These forces are not as noticeable in a built up fuselage because the forces are transmitted vertically (upright or inverted engine) to the fuselage. Our choices are simple either we built a profile with an upright engine (Mustunt) or we build a very stiff front end. Years ago I used maple engine mounts that extended to the wing cutout to help reduce the vibration and it worked to a point. It transferred the vibration to the wing and fuselage joint cracking the epoxy and the fillet after about 100 flights. I repaired the joint and added ¾’ balsa tripler (cheek cowl) to the outboard plywood doubler that was faired into the wing sheeting. Again, after 100 flights or so the joint began cracking once more. The next model (Force) I built had the maple engines mounts stop about 1” in front of the wing cutout. This time I added ½” balsa tripler to nose and this has worked perfectly for over 250 flights with no sign of a crack. In my opinion the mounts were the blame. The vibration dampening properties of balsawood are far superior to maple or plywood and the vibration was not transferred directly to the wing joint.

The next model I used the same construction technique but I added .5 carbon fiber mat that was laminated between the plywood doublers and the balsa fuselage. So far this has worked the best of all the methods tried. One other thing that I found helps is the use of a “U” shaped mounting plate between the engine and the fuselage. The ones I make are cut from a piece of .094 carbon fiber sheet available from Aerospace products.

Flexing Fuselage. Again, I have tried all kind of techniques to build a stiff profile fuselage. To me building up a profile fuselage with sheeting and formers defeats the whole purpose of why we want a profile fuselage in the first place (quick and simple to build).

Selecting the correct piece of balsawood to use on a profile has always been the number one consideration until I came up (stumbled across would be more like it) with the following method. Cut your fuselage outline from two sheets of ¼” 4 to 6 lb. balsa (much easier to find than a really stiff sheet of 6 to 8 lb. ½” balsa). Laminate the two sides together with .2 ounce carbon fiber mat and slow curing epoxy. The weight of the laminated fuselage will be about the same as a ½” sheet of 6 to 8 lb. balsa but it will much stiffer and less prone to flexing, and on the plus side it will be perfectly straight. With the addition of the plywood doublers and .5 ounce carbon fiber previously mentioned, the fuselage is as strong if not stronger than a built up profile and much faster to build.

The method I use for laminating C/F and balsa is quite easy and adds very little weight to the parts. Apply a thin coat of epoxy to the insides of the balsa parts and squeegee off as much of the epoxy as possible. Place the C/F mat on a piece of wax paper and apply the epoxy to the C/F mat and again, squeegee off as much of the epoxy as you can. The balsa parts and the mat should feel sticky from the epoxy, but they should not look wet. Place the mat onto the epoxy side of one of the previously coated parts and smooth out the C/F mat. Add the other part sandwiching the C/F mat between the two parts. Place the assembly onto a flat surface or building board and weight down until the epoxy has cured (overnight). I have used this method on all kinds of subassemblies including I-Beam spars, wing and stabilizer trailing edges, and molded parts.

See YA!

42T

I run my Norvel .061 in a rich 2 cycle. I doubt that it is capable of 4/2/4. My little engine is really cranky, but after it does start, it's a good runner. When new, these engines feel really rough and hard to turn over, but that is normal. Follow the written instructions, and the hints from Norvel's web site on starting and breaking in.
Also, if you don't already have your wing built, consider buying some longer leadouts. The ones in my plane are a bit too short to suit me. If for some reason I have to go inside the wing in the future, I will splice on a little more length.
A nice little mod. you can do any time, is to bend a spring type nose gear strut like r/c nose gears, using the same size music wire as you now have. As I fly off grass, the non spring gear tends to bend a bit on each landing and has to be restreightened often. If you indeed need to add nose weight, you will already have added some with the extra wire.