MoBetta' Profiles

Larry. In the Pathfinder thread you mentioned the use of 1/2' wood for a profile fuse. It seems like this idea is getting us closer to the concept of the "box" fuselage. On the Mo'Best I built, and most good profiles for that matter, the nose construction is dependent on 1/8" ply, unless I'm missing some new developement, to take the structural and vibrational loads. Using 1/2" wood for the aft perimeter framing and for the nose construction holds out the possibility of using 1/64" ply for the whole fuse front to back. The combination of a D. Moon style super-stout motor crutch extending to the wing, 1/2" balsa filler back past the TE, a front balsa tripler laminated on with CF, and the thin ply covering sounds like a possible winner. Having a one piece ply skin for the fuse would eliminate the step at the rear of the fuse improving looks and easing finishing. If the nose was well engineered it seems that it could handle the lower vibration levels of modern motors and could result in a fuse that would give a built up version a run for it's money in terms of weight and torsional resistance. One downside would be the inability to have a tank recess but with greater movement towards clunk tanks this might not be much of a problem anymore. How far off base am I here? Apologies in advance if this is already what you are doing.---Mark

Are we reaching a point here where building a profile is almost as much work as a full built up fuselage? Now, unless you are building it for a profile event... Remember, you don't need a cowl on a built up fuselage, and they do give a performance advantage with a twistless fuselage. (Check the "Das Ugly Stunter" design.)

>Are we reaching a point here
>where building a profile is
>almost as much work as
>a full built up fuselage?

Probably so but a profile, even a work-intensive advanced one, still has a few advantages over built-up fuses. Visible & easily changeable fuel plumbing; resistance to ham-fisted handling, ease of building straight; and visible & easily tweakable controls. All easier for an aspiring PA enthusiast to deal with at the lower end of the learning curve.---Mark

Mark,

You are right, the nose strength is more affected by the quality of the plywood doublers than the difference between a 1/2" or 3/8" sheet.

And I am using a motor mount crutch assembly, made up of 1/2" x 3/8" hard maple engine mount stock. I'm convinced it will work beautifully with the common modern low-vibration motors.

You mentioned using the 1/64" ply to plank the entire length of the fuselage, and this definitely makes sense. The reason I haven't been doing so (with a kit version in mind) is that the cost of the longer pieces. The full length 1/64" plywood fuse side planking requires a total sheet size of 39" x 7-1/2", while planking only from the aft edge back uses a sheet only 27" x 7".

The full length planking sheeting would definitely help a stress riser at the aft end of the nose doubler, but this area is over the high point of the wing and with the wing in place, it isn't really much of a weak point (in my opinion).

As you mentioned, the 1/8" plywood doubler is important, and one thing I have always stressed is to use quality SIG aircraft birch plywood, instead of Lite Ply. But 1/8" birch plywood is heavier, of course. What I have found is that 3/32" birch plywood (5-ply) is a bit lighter, and plenty strong. The 3-ply 3/32" plywood is not quite as strong, but is slightly lighter still. I'd recommend the 5-ply stuff.

Turning the 1/2" x 3/8" motor mounts 90 degrees, for a 3/8" nose sheet width is 1/8" narrower in the lateral dimension and therefore less rigid (laterally) than 1/2" wide. There is a small benefit for the engine mount holes (when using the universal engine mounting plate only), for distributing its load with a 1/8" wider bolt pattern on the UEMP. I'm not sure that is significant, but is convenient.

And using a 3/8" wide nose sheet with 3/32" wide doublers means the engine cutout slot isn't really quite deep enough for the average engine any more. So I ended up adding another laminate of 3/32" wood on the outboard doubler side, just a little U-shaped piece around the engine cutout area. This gets the widths and depths back to what we normally see with 1/8" nose doublers and 1/2" nose sheet.

OK, seeing as how we all understand the value of laminations for strength and stiffness, imagine this sandwich on the nose: (1) 3/8" hard maple engine mount and balsa nose sheet in center, (2) 1/64" plywood planking sheets, full fuselage length, on either side of that, (3) 3/32" 5-ply aircraft birch doublers, nose to wing high point, on either side of that, (4) 3/32" 5-ply outboard U-shaped tripler around the engine cutout, (5) 3/8" balsa inboard tripler (same as inboard doubler, to high point of wing). That adds up a a nose which is 1-1/16" at its widest point around the engine cutout, counting the inboard 3/8" balsa tripler. That compares to 1" for the more conventional one (as on original Mo'Best plans) with 1/2" wide nose sheet, 1/8" doublers, and a 1/4" balsa inboard tripler sheet.

Now, take out the full length 1/64" plywood side planks on and you are back to a 1-1/32" widest point nose. So you can see that the finished widths are comparable to standard practice. But I believe it is lighter and at least as strong and rigid.

Use a U-shaped 1/8" aluminum universal engine mounting plate, and you have a pretty substantial structure! I have a lot of confidence that this will work out just fine, even with the hole I cut out behind the engine to recess my metal fuel tanks.

Since it is the lack of rigidity of the profile fuselage versus a built up fuselage which bring about the complaints about profile engine run, stab twisting, etc., once you solve these problems you should have an excellent airplane.

Now, in case you hadn't noticed, the amount of effort to construct this "built up" profile fuselage just escalated, so we might start to wonder why we didn't just build the full fuselage anyway.. I dunno, maybe you just want a ship to win the P40 event.

L.

Larry. The thrust of your design efforts are rightly aimed at maximising the rigidity of a profile's nose. Conventional 1/8" doublers accomplish this nicely but what I'm wondering is if there is a way to satisfy this requirement while using only a single ply skin from front to back and keeping the construction relatively simple. How does this scenario sound? Use full 1/2" square maple motor mount stock extending back to the wing. Create a normal crutch with vertical 1/2" maple at the rear of the engine cutout and the front of the wing. In the middle of this crutch install a cross-grain laminated balsa filler piece as described by Doug Moon. Since this would obviate a tank cutout a clunk tank might be best for this version. Use this same lamination technique for the bottom nose filler piece. Use light balsa for the wing cutout and build the rest of the fuse as per Mo'Best convention. Sandwich the whole business with 1/64" ply with perhaps CF laminated to the inside of the ply. Finally use an unhollowed piece of light 1/4" balsa for an inboard nose tripler.
I'm guessing that the bigger maple stock and the laminated balsa nose filler pieces along with the ply covering would be pretty darn stout and no heavier than using 1/8" or even 3/32" birch ply doublers--maybe even lighter. A nose built like this ought to be not too complicated to build and strong as the dickens. Just idly blue-skying here but what say ye?---Mark

Seems to me that full length plywood or any other stiff material would transmit engine vibrations down the length of the fuselage and shake the tail off. Anyone had that problem?
This is just speculation, I haven't tried it.
George

I don't see this as any sort of realistic problem. In the latest Mo'Best design, an engine crutch is assembled from 3/8" x 1/2" hard maple engine mount material, and the engine mounts extend to and make broad contact with the front of the wing.

Between the composite structure of the nose (plywood, maple, and balsa sheets) and the wing joint, there is a large area to absorb vibration. To shake the aft fuselage, vibrations would need to be transmitted through to the flap line of the wing.

Meanwhile, the value of the 1/64" plywood for making the aft fuselage rigid is undeniable.

L.

Mark,

Interesting ideas, there are certainly multiple ways to skin a cat. The addition of .007" CF into the laminate would make it extremely strong. I considered it for the Mo'Best nose design, and it may still end up in a kit. Such laminations need to be assembled properly, with very good glue bonding.

The use of cross-grain balsa sheet between the engine mounts behind the engine (in the recessed tank area) is also very good, as per conventional practice in full fuselage designs.

I'm addressing the fact that not everyone wants to use a recessed metal fuel tank in the Mo'Best kit design by having the option to glue a cross-grained balsa filler sheet in the area and replace the (laser-cut) piece for the tank cutout hole in the outboard doubler, which provides a similar construction.

At this point I've pretty well established the new Mo'Best nose and fuselage design approaches, but I am always interested in hearing such ideas and experiences.

The outstanding quality of the Mo'Best has always been a very rigid nose, and I've not seen any vibration problems from the recessed fuel tank. These two features are responsible for the good engine stunt run capabilities, which help make the profile competitive with full fuselage designs.

L.

This sort of brings up a question for all the envelope-pushers out there in stunt land. How exactly is a profile defined? An almost completely built-up design like the Mo'Best, with its formers and skin, gets pretty close to being a "box" type fuse. I'll bet that a light frame properly braced and skinned with thin ply is as stiff in torsion as a conventional full fuse and maybe even more so. My old Mo'Best feels every bit as stiff as most full fuse ships I've done the tail wiggle test on. It's certainly not a heavy way to build either. Mine, with an LA 46 but with a minimal finish, weighs 45oz. Not exactly porky for nearly 600 squares. The aft fuse is a total of 5/8" thick with 1/2" framing and 1/16" skinning. Perhaps this should be an informal limit for the aft fuse. Wider than this, and it certainly would be possible to build wider, your flying buddies, not to mention CDs, are going to start giving you the business about violating the spirit of the "law". 5/8" may be a practical limit as well because of the fact that a tail end thicker than this gets difficult to handle rudder-wise. One thing I think we can say for sure is that with all the design work done on profiles over the last few years the performance "penalty" of a profile setup has all but disappeared.---Mark.

LAST EDITED ON May-28-02 AT 09:38 AM (CST)

I believe that a profile isn't supposed to be wider than 3/4", except at the nose (spinner and engine allowance). I recall reading that somewhere in the rules..

A profile doesn't have to be built using a single sheet, but the fuselage is supposed to be a PROFILE.. And traditionally we have seen lots of cheek cowls and triplers shaped to match a spinner. So there's nothing particularly new or exotic here.

One other thing. Unlike carrier and speed and scale, stunt makes no distinction (in terms of classes or point penalties) between profiles and built-up fuselage ships. There really isn't a "profile" class, formally defined, although I would like to see the exact rules for the "P40" (Profile .40) event, which does not seem to have a formal AMA definition.

I suppose if you started thrashing folks badly at contests with a "built-up" profile ship, they might get around to protesting or something. No danger of that happening to ME, but it might happen with a better flier. But WHAT would they protest, exactly? If they don't like your definition of a profile, they could call it a built-up ship, and it would still have beat them..

Meanwhile, I defy anyone to find anything really specific in the AMA rules for CL profile stunt definition/distinction, which could possibly affect scoring. I think it is all covered by a phrase stating that the rules and definitions defined for other CL events apply except where noted.

(If anything, profiles seemingly ALWAYS take a scoring hit when it comes to appearance points, no matter how well they are finished. It's traditional.)

More relevant, in my opinion, is the fact that when a profile starts becoming the same or more labor intensive than a built-up fuselage, why bother to build one? I'm currently in the process of building up a couple of Mo'Bests to verify all my "kit" drawings, and the very labor intensive part is cutting out all those parts. If I had accurately laser-cut parts in hand, the extra construction effort wouldn't be much of an issue.

L.