Making changes to the printed plans

Even when you have a good set of plans, sometimes it seems like a good idea to change a few details.  Perhaps you have a favorite way of building or mounting wings, or maybe you like taildraggers so much you wouldn’t ever build a plane with a nosewheel.  Any airplane you build can be customized to your favorite methods regardless of what the plans say.  For example, consider the plans for the Drake II seaplane by legendary model airplane designer Ken Willard:

drakeplans

 

I have this plan on my workbench right now.  So far all I have done is cut out a stack of wing ribs.  Ken Willard really knew how to design a model airplane, and he particularly knew about seaplanes.  If I build the Drake II exactly as shown on the plans I’m sure it will be great.  But I have my little idiosyncrasies.  For one thing, I generally like ailerons.  There are some classic planes (Q-Tee, Whimpy, Kadet) that seem even more classic in their original configuration without them, but I’ve added ailerons to a lot of planes.  My Drake II is going to get ailerons.  When building the wing, I can assume that the aileron will be placed exactly where Ken put his trailing edge stick and will be the same size.  That means I need a quarter inch stick of balsa just in front of it, glued to the ribs, and the ribs need to be a quarter inch shorter at the aft end.

Another one of my quirks involving wings is that I like a sheeted D-tube wing leading edge.  You’ll notice that the Drake wing has sheeting only in the center section.  The outboard panels have no sheeting at all.  There’s nothing wrong with this method per se, but such a structure is susceptible to twisting.  Even Monokote can twist this kind of wing if you don’t put it on correctly.  Look at this photo of a wing, harvested from the internet:

dtubewing

 

Wow, somebody did a nice job of building that.  Anyway, the leading edge section includes a complete tube with a leading edge stick, thin sheeting on the top and bottom, and shear webs bridging the gap between the upper and lower spars.  A tube such as this cannot be twisted.  If you were to build this wing exactly the same, minus one of these four pieces, the wing could be twisted.  Try it next time you build a wing.  Twist the wing at each step, and when you add the last part, suddenly it is perfectly rigid…. which is why I usually build wings this way.  Actually, in much the same way that some planes seem perfect without ailerons, sometimes you have to build wings with pretty sticks and no sheeting.  A lot of old timers would look funny with leading edge sheeting.  But my Drake II is going to be built with a D-tube structure as are most of my airplanes.

drakeplan2

With a D-tube wing, my Drake II won’t really need a big 1/2 x 1/2 stick for the leading edge as shown on the plan.  A lot of strength will come from the top and bottom sheeting, so I will be using a 1/4 stick instead.  These various factors cause significant changes to all of the wing rib dimensions.  My ribs were 1/8″ smaller from top to bottom, 1/4″ shorter at the leading edge, and 1/4″ shorter at the trailing edge, than what is shown on the plan.

Now look at those spars on the Drake II plan.  Over the years I have seen lots and lots of plans that call for balsa spars.  I personally do not like balsa spars.  They are certainly good enough for flight loads, and I have never seen them fail in flight.  But flying is by far the least of the hazards encountered by model aircraft.  Landing, getting in and out of cars, and particularly crashing, present a far greater threat to wing spars.  My first low wing plane had balsa wing spars.  The plane was the Semper Fi from RCM, and it was a dream to fly.  I was enjoying the thrill of a snappy aircraft and becoming accustomed to the flight habits of a low winger.  On my first day out with this plane I did a lot of maneuvers that were demanding enough to prove to me that balsa spars were strong enough to handle flight loads, and then it was time to land.  One wheel got caught in just a little bit of long grass, and the plane swung around and jammed a wingtip into the grass.  Next thing I knew there was nothing but shredded garbage on one side of the plane instead of a wing.  No more balsa spars for me.  Ever since then (1991) all spars are basswood or spruce.  The increase in strength is tremendous, but the weight difference is negligible.  My rule of thumb is that 10 to 20 size planes use 3/16 spars, 40 to 60 size use 1/4, and the bigger planes use 3/8.  Tiny planes use 1/8 to 3/16, depending on the situation.

Another detail I will be changing on my Drake II is the wing tips, simply because of the D-tube.  I don’t feel like blending 1/16 balsa sheeting into the curved wing tip, so I will build the panels to end at the last full rib, then add a sheetwood tip just like the pretty wing I showed above for the D-tube detail.

This may seem like a lot of stuff to change, but it really won’t make the plane fly much differently.  The ailerons will make the plane more maneuverable, but other than that, to the casual observer it should look pretty much the same as the original.  Now consider the fuselage design:

drakeplan1

 

Notice the way the front of the hull is built, with 3/32 balsa sides and bottom.  Theoretically this should provide plenty of strength for good landings, but who makes those all the time?  Sooner or later every seaplane will smack the water hard.  With a 48″ wingspan and a .15 size engine, this plane has what it takes to put a big crack in a balsa hull, and possibly tear some pieces off.  I can only assume that the original model was intended to be covered with light glass cloth.  Normally one would expect to see a detail such as this noted on the plans, but maybe it was considered a covering detail and was noted in the original magazine article, which I do not have.  At any rate, it’s not there, and I have serious doubts about the strength of the hull as shown on the plan.  In my opinion it should have light glass cloth, or a better grade of wood on the bottom of the hull.  I’m planning to build mine with 1/16 plywood on the bottom and a 3/32 balsa doubler about an inch tall along the bottom of each fuselage side.  A lot of seaplanes have a glass/resin/paint finish on the hull to keep them watertight, but that adds a lot of weight.  I’m planning to seal mine with a couple of layers of Balsarite and finish with Monokote.

There are some other details I will probably change.  The wing will definitely be held with bolts rather than rubber bands, and the engine servo will be installed in the engine pod next to the fuel tank.  I’ll probably do something different with the rudder and elevator servo mounts as well.  When it’s all done, it will look like a Drake II, and it will fly like a Drake II, but it will by my personal airplane.  My way isn’t necessarily better than somebody else’s way.  It’s just the way I like to build.