Common Questions and Techniques:
What is the absolute most important thing I should remember when building a kit?
The most important thing is two things:
(1) Make sure the CG is dead-on!
(2) Do not exceed the recommended Elevator Travel throws until you’ve flown the airplane for awhile and are sure you really want more throw!
With these two items set correctly, the plane will fly beautifully.
Note: As a rule, we do not recommend Exponential, as it reduces that “instant authority” we love, plus makes it very difficult to trim/CG. “Standard” Differential is pretty much useless on a flying wing/elevons, due to the adverse yaw, etc. However, a “Reverse Differential” of 5-9% has proven to be quite effective in some setups. Experiment a little, once your plane has been trimmed normally.
Is the CG really as critical as you say in the manual?
YES! Typically, on high-performance flying wings with a straight-swept LE or TE planform, the CG is much more critical. Use a balance jig with a couple of nails or pencil points, or upside down “V” blocks, etc., and try to be within 1/64″. (“fingertip” balancing is not recommended as you can be as much as 1/8″ off of the true CG, thus less than optimum performance).
Just as a reference, 99.9% of all airplanes that we help to trim out are waaaaaay noseheavy!
We usually end up with, at the very least, a 7 gram weight in front of the fin, to bring to the stated CG. Even if it’s only 1/16″ noseheavy, and you think it’s flying great, just wait… a perfectly CG’d plane will absolutely come to life, pop the turns, fly much faster, much lighter, fly inverted effortlessly, and rarely ever hyperstall.
The extra minute or two it takes to really dial in the CG is well worth it. Then, on the slope, you can fine-tune the plane by moving around a small stick-on weight during test flights to find the “sweet spot”.
The most widely used CG on the Moth is 1-7/16″ back from the LE, with 3/32″ elevator throw each direction (3/16″ total). However some pilots swear by the “… an inch and a half CG and a 1/16″ elevator up travel” rule. Experiment!
The Starting CG on the Halfpipe is 5-3/4″ in front of the”center of the V” at the fixed trailing edge.
The CG on the M60 is 1-13/16″ back from the LE; this is pretty much dead-on.
For the Bluto56, we prefer the CG between 5.75″ and 5.85″ in front of “the back of the drag spar”.
The bigger the air, the farther back, the quicker the turns. As always, keep moving it back until it gets a little pitch squirly… then move it forward a smidgen & you ought to be right in there.
Pumping or “HyperStall”:
If you experience any pumping (or “hyperstall” as we came to name it), it is, in fact, a stall, or rather many little stalls and many immediate recoveries. When any other “non-straight-swept” airplane stalls, it usually drops a wing and spirals down…that’s the beauty of a straight / forward swept wing… they very rarely drop a tip. Rather, if off CG, allowed to stall or given way too much elevator, they simply hyper-stall and then immediately recover; (this also allows for near-vertical landings). If the lift suddenly disappears, just let the plane cruise and allow the airfoil to work… you willl gain much more lift over 50 yards than slowing the plane down with too much up elevator.
There are really only 2 causes for hyper-stall: An “off CG” and / or too much elevator input in light lift. In medium to heavy lift, and if the CG is dead-on, it’s just a matter of making sure you don’t have too much elevator or radical exponential throws. Once both CG and travels are dialed-in, you will quickly find that sweet spot where the plane snaps a turn and rockets out the other side!
Is it really necessary to have a transmitter with “End-Point” or “Travel Volume” adjustments?
Definitely, and also very necessary for most straight-swept LE or TE designs (which gives the plane such beautiful, near zero tip-stall performance). The Elevator Function becomes much more sensitive with this planform, thus usually needs to be dialed-down to about 25-40% of the total Aileron Roll throw. Any transmitter with servo “Endpoint” or “Travel Adjustment” ability will do the trick; a fixed dual-rate setting can also work.Try not to exceed the recommended amounts unless you’re sure you want more (or risk the “hyper-stall” pumping effect by giving too much up-elevator).
“Potting” is one of the best methods for installing your servos, especially if you will be dynamic soaring or flying with a lot of ballast. Since EPP foam has only a marginal amount of compressional strength, servos can wiggle if they are not securely mounted. A 1/16″ wiggle in a servo can equate to, at the very least, a 25% loss in elevator authority at higher speeds, thus losing the ability to snap that beautiful top DS turn. The cure to this is as follows:
Once your servo well has been cut/routed out of the foam and you are ready to install it, place the servo in a plastic sandwich baggy or small piece of a plastic grocery bag. Apply a “decent amount” of 20min epoxy (1/8″ +/-) to the bottom and inside walls of the servo cutout in the wing. Push the wrapped servo into the cavity and into its final resting position, making sure there are as few wrinkles as possible in the plastic covering, and that the epoxy is well distributed.
Scrape off any excess epoxy between the bag and the foam. Make one last check to verify that the servo is still in the correct position and flush with the airfoil, and let the epoxy cure fully. When cured, remove the servo and peel away the plastic bag. Your servo will now “snap” into place and have near zero deflection in flight!
What about the “Goop” method for covering the Moth or M60 fuselage?
We rarely use the “Goop Only” technique anymore. But, for those that still want to, the procedure is described below, as wriiten way back in 2003, while testing it:
2003: This is an alternate covering method we have been testing and seems to be working well. If you decide to use this method, and haven’t yet built your wing, you might do the fuse goop procedure first, and let your fuselage cure in a warm to hot environment while you work on building the wing. This added cure time allows the goop to set up even harder.
Prep: Once the fuselage is shaped and smooth sanded, apply some “Lightweight Spackle” over the entire fuse, to fill all of the voids. When dry, very lightly sand smooth, without pitting any of the spackle. Now, if desired, spray paint the fuse any color you like (standard Krylon works well–nothing eats EPP foam).
Goop Applicaton: Standard “Household Goop” will work, though we prefer the “Marine Goop” (green tube), since it has a UV inhibitor. The Goop must be thinned to about the consistency of chocolate syrup. The thinning agent in Goop is “Toluene”, which works best, and should be available at any industrial paint supply (Sherwin WIlliams, etc.). We have heard Xylene or Methyl Ethyl Keytone (MEK) also work, but have not done much testing with these. We usually mix ours in an empty glass “Relish Jar”, so it can be sealed between coats, and when finished, to avoid evaporation and air-curing of the mixture.
Once the Goop has been thinned, apply it to the fuselage in long, smooth strokes with a medium sized brush, until both the outside of the fuse and inside of the airfoil cutout are completely covered–try not to go over the same area too many times with the wet brush, or the Goop will begin to bead up. Coats can be applied 1/2 to 1 hour apart, depending on ambient temperature and humidity… as we do more and more of these, we are starting to prefer anywhere between 10 and 15 coats of thin goop. If possible, for added protection against nose wrinkles and stress lines, you can also let the finished fuse cure in the sun, or a hot shed, etc., for several days or more. The goop will become even harder and more resistant to damage.
The entire method above not only allows you to spray paint the fuse any color you want (prior to gooping), but is much cleaner, way stronger and far more durable than strapping tape or Ultracote. For additional strength, we also use the same method on our tips and leading edges, prior to Ultracoating.
Packing tape will work for little “girly slopes”, but we really recommend Ultracote. It has far superior tensile strength, is much stronger, and just looks better. If you plan on ballasting up for ultra-high speeds, frontside or back, definitely use Ultracote–the extra tensile strength will add much more rigidity to the inherent torsional limitations of EPP foam alone. Solartex is good, but heavier, and not quite as strong, torsionally, as Ultracote, over time. It can be great on a fuse, though.
What is the build time on the kits?
We’ve seen them go from kit to airtime in a day. We have also seen pilots spend a couple of hours a night over a week building their kit, and show up with a magazine perfect, finely trimmed masterpiece. If you absolutely have to be in the air tomorrow, and since “Goop” (which sets in an hour but needs to cure for 16-24) is a dominant glue used in the building, you may want to go through the manual first and kind of plot out drying time, etc. This way, you can be working on one part of the kit while the other is curing.
Also, given the precision of our kits, some of the work has already been done:
~ The wing cores and fuselage (Moth) have been cleaned and sanded to remove all of the wire slag
~ The balsa elevon material has been weight-matched within a few grams so you don’t end up having to add a bunch of lead to the tip of one wing when lateral balancing
~ The flight and wing compartments on the Moth & M60 fuselages have already been pre-cut.
~ Everything has been cut to the absolute best tolerance EPP allows. This makes building go very fast, since you won’t have to squish, smash, fold, spindle or mutilate, as the parts go together.
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