I need an expert!

Folks,

I'm curious. Most of our (seemingly) best stunt ships are built almost to a "formula", and that formula typically involves a narrow fuselage, without a lot of frontal area.

I know that it is a practical shape.

At the same time, I know that masters like my friend Al Rabe manage some superb semi-scale stunt ships with very much larger frontal area in their fuselages.

The third thing that comes to my mind is the fact that certain profiles seem to fly amazingly well, and people have even attributed some of that to the "dirty" nature of their fuselages, with the associated drag.

Is fuselage drag "good" for CL stunters?

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I've been looking at my "Stealth" stunter fuselage recently, which has a relatively larger frontal cross section (although it is far from classical or "clean" in its shape), and wondering what the consequences of such a fuselage design might be? (Other than possibly: Butt Ugliness? )

I already know that wild and strange designs are not likely to establish some new acme of CL Stunt performance. But I can't help but wonder about such things as aerobody type designs, which I suspect will ultimately end up the airliners of the future.

Given that I don't particularly want to shake the world, but more that I am interested in having fun and playing with something that is interesting, am I being silly even wasting my time think about such things?

Your comments and opinions are solicited. In particular, suggestions for the most efficient method of forming such a shape (think: SR71 shapes).

Thanks in advance,

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"Few men during their lifetime come anywhere near exhausting the resources dwelling within them. There are deep wells of strength that are never used." -Richard E. Byrd

With enough power to haul your weight up the loop and enough drag to slow you on the downward slope, I can only see that as a benefit, right up to landing. You'll need to make up for the drag on landing, or you'll come to a shorter than wanted stop. Same thing happens with biplanes.

Al's planes look pretty slick to me, the Bearcats last year, they were more oblong in height than scale, weren't they. Or was that just his profi one.

You're right, William, Al's stuff just looks so very clean to me.

One of the things I think I'm starting to believe is that our CL Stunters are truly at odds with conventional aircraft design. Drag seems to be a prime example - as long as we have a great power train, the drag is even beneficial. I can see how the balance of large thrust and drag vectors tend to stabilize airspeed, to our benefit. But drag seems always at odds with regular aircraft.

I've also experienced what you mentioned, even with my silly little Mo'Best profile design, which has a thick, draggy wing: if you're not careful, when the motor quits, you can have a poor landing due to loss of airspeed.

Thanks for your input.

(Now, how do I mold those concave/convex/complex surfaces I'm looking for on the fuselage "strakes" <is that the word?>).

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"What is now proved was once only imagined." -William Blake

Personally I don't see your Stealth Stunter as being so draggy. It looks pretty clean to me.

On the drag thing, I really don't know your answer. I have seen some pretty draggy looking planes fly pretty good. Matt's latest Stukas have had a chin cowl just for looks. However, to make sure the air goes through the engine and not under it, the bottom was blocked off. So there is that square but catcher looking thing in the front that would appear to be as draggy as you can get. It doesn't appear to hurt performance either under power or in the glide. I guess the rest of the plane is big enough to overcome the extra drag in the glide, and with the big prop up front....

Aw, just go ahead and build it. It'll fly.

Larry,

>>Given that I don't particularly want to shake the world, but more that I am interested in having fun and playing with something that is interesting, am I being silly even wasting my time think about such things?<<

Man, I hope not or I've wasted 15 years. Most of the planes I've designed I knew going in probably wouldn't be competitive. Or at least not on the level of Paul or Ted. But they were an enormous amount of fun to build and fly. They did some things exceedingly well and others not so well or poorly, but they were all fun. Even the current elliptical job is a lot of fun to play with.

I certainly think that draggy sorts of designs have some advantages in controlled speed in maneuvers and such. And besides, the Stealth Stunter is jut too cool not to build.

I say that's one of the coolest looking ships I have seen in a long time. Its time we got away from the "numbers" and began to have some fun again. When you get her all drawn out, don't forget to offer the plans for sale. There are more nuts out here than you think! ##### squirrels are always trying to put me away for the winter!

Bob Hunt has a "drag story" to tell;

When it was fashionable to make semi-scale stunters he designed one (I don't recall which "real plane" it was a scale of), and it stunk. After the initial disppointment he decided to put a couple of bombs under the wing, like the real plane, just to make it look a little better, since the darn thing didn't fly well.

To his surprise, the extra drag the two bombs created turned the ship into a fine stunter!

OK, I guess what I was really fishing for was an idea about getting the complex shape I'm looking for on the strakes. Somehow my mind tells me that they will just HAVE to be molded, and that when you mold balsa, the plug needs to be convex.

Maybe what I need are some general rules of thumb for sectioning the pieces, so that they are moldable. I've already seen that the "double convex" sort of curvatures are possible if they are gentle, and I know that the masters actually split pieces and overlap them when molding the more difficult parts, where the balsa cannot easily stretch.

The other thing I am not accustomed to is Al's method with the section pieces, which is actually a more "conventional" modeling structure, like a lot of scale builders use.

I think I can come up with some interpolation techniques for determining cross sectional shapes between these formers. But I'm really a babe in the woods on such things.

Symmetrical, diagonal ribs, not a problem, the bottom and top surfaces might use opposite angles - but then I wondered what would be the difference aerodynamically between top and botton surfaces. Would it affect airfoil over the wing? Might it turbulate the air just a little (I've wondered about diagonal ribs doing that, in general..).

As of the beginning of April, I abandoned a couple of earlier ideas - like the hinged cowl assembly for (vertical) thrust vectoring on the engine. It started to develop into a ball joint because I thought if vertical thrust vectoring was good, well, imagine getting some extra LINE tension with some lateral.. (That was back VERY early in April when I abandoned that idea..)

Oh yeah, and the golf ball dimpled surface finish, to improve airflow and counter loss of laminar airflow over the airfoil.. That and the leading edge slats.. (That would surely cost appearance points!) VERY early April.

Seriously, though, I'd like to develop a good technique for the complex cross sectional shapes. Ideas, comments, warnings appreciated.

One (brute force) idea I had was to get some foam and just try to carve it to the fuselage shape, (just a half section could work), then slice that for half templates to make the formers, and make a convex mold from that shape. I do have some of that "ball bat" balsa block around here, I've already used that stuff for regular molds, but nothing this complex.

Another issue - notice how CUTE the interface between the wing airfoil and the fuselage strakes appears. How am I going to get that to work?

One method came to mind - just make a regular fuselage and add the strakes to it - but I have a feeling I'm heading for a lot of extra weight with that approach. No, I want a single shell structure.. (laminated sheets, but one shell).

There's a couple of other loony ideas I'm kicking around here too - stuff like "plug-in" wing panels.

Can you see it coming? This thing could turn into a battleship, badly overweight, and then I'll never know if it didn't fly well because of the weirdness of the fuselage surface shape, or simply that it shouldn't have weighed 85 ounces like it did..

Certain ideas - molded fiberglass wheel pants, wing tips, and cowl, I already know I can make work. Hey! There's an answer! Mold the fuselage sides in fiberglass, with the strakes.. Right?

Experts!?

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"I have no sceptre, but I have a pen." -Voltaire

>OK, I guess what I was really fishing for was an idea about
>getting the complex shape I'm looking for on the strakes.
>Somehow my mind tells me that they will just HAVE to be
>molded, and that when you mold balsa, the plug needs to be
>convex.
>
>Maybe what I need are some general rules of thumb for
>sectioning the pieces, so that they are moldable. I've
>already seen that the "double convex" sort of curvatures are
>possible if they are gentle, and I know that the masters
>actually split pieces and overlap them when molding the more
>difficult parts, where the balsa cannot easily stretch.
>
>The other thing I am not accustomed to is Al's method with
>the section pieces, which is actually a more "conventional"
>modeling structure, like a lot of scale builders use.
>
>I think I can come up with some interpolation techniques for
>determining cross sectional shapes between these formers.
>But I'm really a babe in the woods on such things.
>
>Symmetrical, diagonal ribs, not a problem, the bottom and
>top surfaces might use opposite angles - but then I wondered
>what would be the difference aerodynamically between top and
>botton surfaces. Would it affect airfoil over the wing?
>Might it turbulate the air just a little (I've wondered
>about diagonal ribs doing that, in general..).
>
>As of the beginning of April, I abandoned a couple of
>earlier ideas - like the hinged cowl assembly for (vertical)
>thrust vectoring on the engine. It started to develop into a
>ball joint because I thought if vertical thrust vectoring
>was good, well, imagine getting some extra LINE tension with
>some lateral.. (That was back VERY early in April when I
>abandoned that idea..)
>
>Oh yeah, and the golf ball dimpled surface finish, to
>improve airflow and counter loss of laminar airflow over the
>airfoil.. That and the leading edge slats.. (That would
>surely cost appearance points!) VERY early April.
>
>Seriously, though, I'd like to develop a good technique for
>the complex cross sectional shapes. Ideas, comments,
>warnings appreciated.
>
>One (brute force) idea I had was to get some foam and just
>try to carve it to the fuselage shape, (just a half section
>could work), then slice that for half templates to make the
>formers, and make a convex mold from that shape. I do have
>some of that "ball bat" balsa block around here, I've
>already used that stuff for regular molds, but nothing this
>complex.
>
>Another issue - notice how CUTE the interface between the
>wing airfoil and the fuselage strakes appears. How am I
>going to get that to work?
>
>One method came to mind - just make a regular fuselage and
>add the strakes to it - but I have a feeling I'm heading for
>a lot of extra weight with that approach. No, I want a
>single shell structure.. (laminated sheets, but one shell).
>
>There's a couple of other loony ideas I'm kicking around
>here too - stuff like "plug-in" wing panels.
>
>Can you see it coming? This thing could turn into a
>battleship, badly overweight, and then I'll never know if it
>didn't fly well because of the weirdness of the fuselage
>surface shape, or simply that it shouldn't have weighed 85
>ounces like it did..
>
>Certain ideas - molded fiberglass wheel pants, wing tips,
>and cowl, I already know I can make work. Hey! There's an
>answer! Mold the fuselage sides in fiberglass, with the
>strakes.. Right?
>
>Experts!?
>
>[photo not recovered: 3e69bb3870f12ad5.jpg]
>
>"I have no sceptre, but I have a pen." -Voltaire

Ron B.
F4Fguy

Larry:

A few points:

(1)I've been building "scale" stunters for a long time.I've never had any problems I could ascribe to the fuse cross section.You're right,and I think Al would agree his are judicially "slimmed" to cut the cross section,but not really that much.Being somewhat cantankerous,I try to keep mine to as close a scale outline as possible.My F4f is 7" wide at the wing and 9" high at the canopy.The Macchi is almost 6" wide and 8 1/2" high.My Zero is about the same as the Wildcat.

What IS important is that the power be sufficient,especially torque.This means CUBES,and tuning* to the specifics of the plane.If the airplane is fat but well shaped,the only drag increase of significance is the wetted area.If you have a low enough drag coefficient,the shape drag may even be lower than a conventional slab sided "skinny" fuse when flying at some angle to the flight path.

All our planes have more than sufficient horse power(If we only flew level at the usual 45/50 MPH,we'd only need a .15). We deliberately make them inefficient in power delivery by running low pitch/diameter ratios in order to get accelleration when speed drops due to induced drag in maneuvers,sorta like racing in second gear.I believe it's better to turn a large diameter prop slowly at a more efficient p/d and take advantage of the extra disc area to get even better accelleration.Obviously,precession can be a problem if you don't slow the revs.I've played with piches up to 8",and revs down to 7500.Heresy? Maybe,but it works for me.

*When I say tuning I'm talking new sleeves with porting designed for low speed broad band torque, new combustion chambers,optimised carburation and high CR with low or no nitro.The Moki 61 in the Macchi turns a 15-6 at 10,200 with the pipe out at 26".The same setup turns a 12-6,13-6 or 14-6 at the same speed,it just doesn't care what the load is.it will turn a 16-6 at almost the same revs ,but the needle gets very fussy,and over heat is a problem(besides I had to make the 16-6).The important thing is that the load doesn't seem to materially effect the speed,and the thing simply will not overspeed coming downhill.

(2)Molding compound curves:

This is fairly easy to do.It does take some cut and fit,but you've gotta have some seams anyway.You just have to plan them to go where they do the most good.

A good example is the turtledeck on the Wildcat.It's one sheet of balsa from the top to the wing/fuse centerline.The radiator scoop on the little RC mustang is another example.As you can see,smaller parts usually take more cut and fit than larger parts.I assume this is because the larger part has more ability to shrink and strech as required,distributing local stresses over a wider area.

If you have good A grain stock,wetted thru with water(I cant see ANY difference using Windex or ammonia)you can form an in/out curve with little fuss.I lay up all my sheets Individually and cut and fit as a secondary operation after they're molded and dry.To force the reverse curve I simply make a foam "dowel" of the same radius as the "in" curve,and bandage it over the radius on the outside of the sheet.The bandage pulls the foam piece into the reverse curve "et voila!" a reverse in/out compound curved part.Incidentally,I use carved blue extruded 2#-4# foam for my bucks.They are easy to make,cheap and have very good durability.The Macchi is the fourth plane off the same buck.

Ron B.

P.S.:The single biggest problem with large cross sections is weight.The larger the wetted area,the more material is in it.That applies to both structure and finish.That's why I've done so much work with molding and other types of monocoque construction.Both the F4F and the Zero are graduated structures of 1/20" 6# sheet,with two lams in highly loaded areas and one thickness over most of the rest.

Certainly a magnificent example, Ron. Beautiful work.

Molding smaller fiberglass parts, with rubber molds, I found that the more complex surfaces worked much better when I used a (male) "plug" that pushed the material against the surface of the mold. However, for larger pieces.. Your technique for balsa seems reasonable.

I suspect that molded fiberglass is not the answer for my fuselage.

Anyway, I did want something "interesting" to work on!

Thanks very much for your help.

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"There comes a time in every man's life and I've had many of them." -Casey Stengel

>I suspect that molded fiberglass is not the answer for my
>fuselage.
>
>Anyway, I did want something "interesting" to work on!
>
>Thanks very much for your help.
>
>[photo not recovered: 3e69bb3870f12ad5.jpg]
>
>"There comes a time in every man's life and I've had many of
>them." -Casey Stengel

Ron B.
F4Fguy

Larry:

Actually,fiberglass is feasible,but it's a PITA.the problem is ,that,if you get it thin enough to reach your weight target,it is either:(1)too"floppy"if you use 3/4 or 1 oz cloth,or (2)too brittle if you use 1/2 or 3/4 oz.veil.It does have the huge advantage of allowing graduation of the monocoque to get not only more support in high stress areas,but also to gradually spread the stress into the shell.The first three of my Macchis had glass fuselages.They worked OK.The weight was satisfactory at about 5 oz.for the assembled shell with bulkheads(less engine mount,tank etc.The biggest problem was that you had to be extremely careful where you grabbed it to avoid poking a hole in it or wrinkling it.The last one I did was one ply of veil overall,and one or two added plys at strategic locations,i.e.the firewall,wing saddle stab/rudder saddles,etc.

Ron and Larry,

I like the idea of a new look. I've always been intrigued be the variety of our stunt models. Some go for the sleek look, others the semi-scale, some just make them big, or small. If you use it enough and optimise the model's trim it usually becomes a useable design. Sometimes even competitive.

One point about prop diameter/pitch ratio and how the most of us run engines compared to you Ron, is especially noteworthy. In full scale air racing, fixed pitch props are used in the Biplane Class. The science/art of choosing the right prop to effectively make the proper compromise between acceleration and top speed is referred to as the "Black Art". My friends used to think THE set-up was a 68x68 and turn 3400-3500 rpm to acheive the needed top speed in the one mile straight after the 3 "G" , half mile turn. Another friend has proven us to have just one way of dealing with this compromise, by his using a 68x85 with a different blade shape and airfoil, along with an obvious "torque motor" providing power at a lower rpm than we using the high rpm formula.

It seems the same in stunt. We have attached ourselves to a working formula and it is working to provide the performance we see in the winners models and flying styles. If I want to practice to perfect my flying technique I will use Ted, Dave and Brett's set-up, for instance. This way I can duplicate their winning engine run and concentrate on flying instead of re-inventing the wheel. This in no way is the only way to do it, but only one way to accomplish the problem of flying a fixed pitch prop equipped, non throttle equipped, controline model in the small hemisphere in which we choose to do our thing.

I like your Stealth Larry, and I like Moon's big front end ship he showed a drawing of a while back. I think they will work just fine using the trimming techniques we all know. As far as the chines and strakes, they can and will change the way the air flows around the model at variuos AOA. These are used on some full scale acro airplanes, the Pitts Special with leaf spring landing gear especially benefits from strake-like fairings around the first third of the strut leaving the fuselage, this helping to smooth the airflow around the wing root of the bottom wing (which is right behind the strut) and keep the overwing airflow attached longer during high AOA. Some monoplanes are fitted with strakes forward of the leading edge wing root, this is said to allow higher AOA than if left off. This is an imitation of the F-18-style strake which is said to allow longer attachment of laminar flow. My understanding of these is anecdotal and practical rather than scientific. I can see how a strake in front of a flying surface could help smooth airflow and one behind could help eliminate drag. A long fairing behind the wing could effect the horizontal though, so making these strakes removable will provide the benefit of being able to change the shapes and sizes of the strakes to find the optimum compromise. Making them fixed could find the model untrimmable without use of the Exacto.

Have fun men, I am still trying to get a regular ship trimmed and competitive, I leave the experimentation to you.
Chris...

I believe that was Bob Hunt's F-105 Thundercheif. I have the plans and it's a pretty cool looking plane.

Ion,

F-105 Thunderchief? I think Bob did one of those...

>Bob Hunt has a "drag story" to tell;
>
>When it was fashionable to make semi-scale stunters he
>designed one (I don't recall which "real plane" it was a
>scale of), and it stunk. After the initial disppointment he
>decided to put a couple of bombs under the wing, like the
>real plane, just to make it look a little better, since the
>darn thing didn't fly well.
>
>To his surprise, the extra drag the two bombs created turned
>the ship into a fine stunter!
>
>Ion
.

Perhaps the F-105

Absolute value of drag is not the biggest problem as we have strong engines (or models small enough). Most important is keeping the drag constant if possible because variation in drag makes variation in speed and that is wrong. Example can bring more light: say we have model with drag of fuselage 0.6 Newton, drag of wing 1.2N, drag of elevator 0.5N, drag of landing gears 0.1N and drag of lines 2.4N. (Sorry for metric units, but I think it does not matter – important are numbers and ratios). That is about average stunter. It means that drag of fuselage is only 1/8 of final drag. So it really does not mean too much and its small variation has only little impact to final result.

But there is another point of view. In corner, when flaps and elevator are deflected, the drag goes to 2x or more of wing drag and 4x of elevator drag. Other sources of drag are almost invariant to AoA of flap/elevator deflection. If that constant fraction is higher to that variable fraction, then also final variation is smaller and thus also speed is more uniform. So higher fuselage drag could be really helpful – if you have enough power.

Larry,
After looking at your cool design and your concerns. I went to the recent post with pictures of Paul Walkers new Mustang knowing it has a Saito 72 etc. Yeah, it looks a little larger than scale, but the cleanness of the lines limits the drag. Drag is not always size as much as it is shape. NOT AN EXPERT, JUST AN OPINION.....BUILD IT AND IT WILL FLY!!!!
Also I remember reading that the Stealth (SR71) had a fuseladge that was designed to create lift. Thus it was larger than normal.

Hi Lou

You have asked quite a lot and there is probably no one simple answer to any of it. It's easier to handle if you just do one question to each post.

However here are some ideas. If you are using diagonal ribs on a tapered wing, then in order to maintain a given airfoil shape, the rib shape needs the be compensated for not only the streatch in length but also the thickness due to wing taper. That compensation needs to be different, depending on the direction of the diagnal (pointing inboard or outboard). I read an artical on this somewhere, it may have been on the pampa site. So the upshot is that having the upper and lower surface ribs run in opposite diagonals on a tapered wing will require different rib shapes for the 2 surfaces to maintain a symetrical airfoil. That looks like a complex setting up proposition. There is hope however. Here is an image of part or a fully flying tailplane for an RC glider.

This structure is extremely rigid in torsion which is I guess what you are after. It is also extremely easy to build. It is built as a slab, that has constant thickness. the ribs are just balsa strips and the joints are slotted half through with a warding file, just the right thickness. On this TP, the square at the root is 1 inch and each rib is spaced 1/16 of an inch further apart than it's inboard neibour. That allows greater strength at the root and saves weight at the tip where srtength is not so critical. The whole slab is glued with CA so is quick to build, then all joints are filleted with PVA.

The whole slab is then shaped into the desired taper and airfoil by planeing and sanding. This same process can be done on a whole wing, just rough plane to shape, then set up on a flat board with rib templets spaced out from each end. Use a length of T section as a sanding block. Glue sanding paper to the T section but at the ends where the rib templets are, reverse the paper so you maintain the thickness over the T section but don't sand the templets.

As far as airflow is concerned, there can be some advantages in that the diagonal ribs can act as turbulators, provide some stabilising of the boundary layer transition and achieve greater maximum lift before stall. Compared to a sheeted wing, there will be a small drag penalty
but compared to an open panneled wing with squared ribs, you don't get such an ugley bump at the wing spar.

As far as structure, weight and fillets go, I feel that much of the weight of an airframe comes from the glue, so if you can make good close fitting jounts and keep a simple structure, it tends to be light.

It can be lighter and easier to form a full rounded fuselage from thicker light balsa placed in an octagonal x section and sanded to a oval shape than to try and plank. Note if you just plank the the sides, top and botom, then you can sand the glue surfaces for the diagonal sheets and get a very tight joint that minimises glue. I glue a sheet of sanding paper to a glass sheet for this kind of sanding.

For larger fillets, I cut triangular balsa stock across the grain with the grain across the wider face. These add structural strength to the fillet and can accomodate some bending around the curve of the wing/fuselage joint. Then sand in the fillet radius but be carefull not to undercut the fillet.

The only thing that makes this work is using very light balsa. The extra thickness is then not a problem in either strength or weight.
There are always other opinions and someone with a better idea or method, maybe there is something here for you, maybe not.

Ken,

Your X-ribs are lovely, I just admire such things from a structural standpoint.

Even using the old StuntRib program, however, I found that they involve a considerable amount of effort.

On my Special Effects ship, I initially wanted the X-ribs, an I think I had 11 rib stations. So I started setting up for 22 different ribs, as you said, the taper of the wing affects the rib shapes. Anyway, not undoable, StuntRib easily calculates such.

Then I looked at the ship and realized I wanted the outboard wing panel an inch shorter, and that implies another 22 (different) ribs! With diagonal ribs, different length panels affect them. You can take the same set of parallel ribs and apply them to a longer or shorter panel, but not so with diagonals! So I decided against using them..

I'd like to thank everyone who provided great advice on how I should tackle this complex fuselage shape. I'll be posting some drawings and probably some more questions as I go along. Next step I will try to make some good scale drawings of the shapes, so I can make up some bucks for molding balsa.

Best regards,

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"Conformity is the jailer of freedom and the enemy of growth." -John F. Kennedy

>Ken,
>
>Your X-ribs are lovely, I just admire such things from a
>structural standpoint.
>
>Even using the old StuntRib program, however, I found that
>they involve a considerable amount of effort.
>

Thanks Larry but I think you are making this more complicated than it is. It's really quite simple. The whole structure is made as a slab (no shaped ribs at all). It's much like a foam wing slab or blank. The shaping is done by roughing with a plane, then sanding down to just 2 end templates with a long T section sanding block. Same principal as foam wing cutting. I think that most of your wing is close enough to a straight taper that you can get away with this.

I think your idea to use split ribs on different diagonals is a really good one simpler than the X structure above and probably just as effective. However shapeing after construction will save you a lot of complications and effort in generating and making all those rib shapes. That's really the point. Whether you use the X or a diagonal split rib structure is not.

Larry,

I'm putting yhr final touches on my third Steve Buso designed Super Kestrel. One thing that I love about this plane is it's great overhead line tension -- it really pulls in the overhead manuvers. If you think about it, when a ship is overhead it's really flying on the fuselage. The Super Kestrel fuselage is quite wide, just a little shy of three inches. I've always thought that this wide fuselage, acting as an airfoil, was responsible for the great overhead performance. I would say make the fuselage wide and make the top view look like an airfoil.

Yours in Stunt,

Noel Drindak