Design and Evolution of a Sport Airplane

Mar 24, 2009 No Comments by

By Gary Droppo

 

(see photos below)

 

Over approximately 15 years, I have periodically re-visited a design I started back in the days when virtually all “sport” or “fun-fly” planes were kit-built .40 glow-engined planes, such as the famous Florio 40. My undertaking was unimaginatively called MOD-1, 2 …up to my current MOD-7.

                                                                           

While the airfoil section and main span have remained constant throughout, many of the other shapes and sizes have changed, for very good reasons. As well, for versions MOD-5 and MOD-7, I moved to high- wing location rather than low-wing mounting on all of the others. Apart from cutting the fuselage sides and side doublers to the appropriate wing location and locating the dowels, building it as low or high wing involves no changes. In fact, one could even consider cutting the fuselage sides for handling either wing location and making blank(s) to cover the unused area.

 

MOD-5 is no longer around. It flew away on me whilst checking out a new receiver and autopilot combination. I believe that the receiver failed but the autopilot worked just fine, as it was last seen heading straight and level on a course to I don’t know where. Many hours spent in the surrounding bush failed to turn up my baby.

 

MOD-7 started out this fall with the same wing that was employed on MOD-2 many years ago. On that aircraft, as well as on MOD-6, which awaits re-covering to spruce up its well-worn appearance, the wing was low mounted. It has a slight dihedral, reminiscent of the early versions. It has had several different wingtips, including tip plates, a la HOTS II. I definitely do not recommend the tip plates on this wing, as it had unfavorable high angle of attack characteristics—a tendency to snap adversely at awkward times, as in swooping around in a tight turn for a quick touch-and-go. The current light, somewhat squarish tips are great and simple to build, using pieces from the balsa scrap bin.

 

The fuselage started out about 44 inches or so. Those first aircraft were very nice, had no bad characteristics and were suitable as an advanced trainer. In fact, a friend bought MOD-3 and flew it as his second model until he plowed the field with it this year in a moment of brain fade. It was over 10-years-old at the time. I flew it shortly before that event, so I could authoritatively compare it with MOD-7.

 

As I created MOD-4, I wanted something that was more dramatic, yet easily controllable for slow, relaxed flying. To improve spin performance, and make it able to flat spin, it seemed like a shorter fuselage, plus more authoritative rudder was needed. And, as I was trying to do some proper engineering of the whole thing – not just copy what everyone else always does – a few other changes were made. Balsa most commonly comes in 36-inch sheets, so the fuselage would be that length. It still is, throughout all subsequent generations. It is 3/32 sides, with limited doubling, instead of 1/4 inch per the early versions.

 

Dihedral – who really needs it, in a small sport plane? So, no dihedral and no dihedral bracing, simpler and easier wing construction. After sheeting, I wrap a band of light fiberglass or polyester fabric over the center sheeted section, then apply thinned Weldbond glue to that area. This point about dihedral has been proven recently, as I recently build a foam wing with no dihedral for MOD-7. If anything, this is a better wing for performance.

 

One major innovation (at least I think it was my own idea) is the somewhat unusual aileron design. As I use sewn hinges on most aircraft, it is easy to experiment with different control surfaces – chop them off, re-use or re-drill holes, and sew on the new pieces. This technique also allows zero-resistance movement and no gaps (a significant factor in low-speed controllability). From MOD-1 through MOD-3 or so, I used the standard full-length ailerons, gradually moving to wider ones. I felt that roll rate could be improved, as well as having a greater flap effect with a better design, rather than just adding more, and heavier, wood to the whole aileron. Adding increasingly larger barn door ailerons eventually just adds weight and drag and creates adverse yaw, without really improving roll rate. As I always use flaperons, switchable to be coupled into the elevator (typically 40% down, 30% up), there seemed to be a natural way to make this work better.

By adding considerable width through the inboard 1/3 of the aileron, plus a smooth transition to the remaining length and a final taper toward the wingtip, you get a better lift distribution. The effective, dynamic, angle of attack is increased inboard, without pushing the tip to higher AOA and premature tip stall. When these flaperons are hanging down, during landing or when you yank the plane around hard, they are extremely effective, yet you still have full roll control. This works well in gusty conditions and can let you drop the plane into a really tight space during calm conditions. With the full coupling enabled, when you point the nose down, this plane drops like a rock, as the flaperons as now extremely effective spoilerons. Yet, as soon as you relax, and subsequently add some up elevator, she immediately stops descending and brakes for a slow, smooth landing, with the full flaperon effect. I tell people it is like having three hands–one for throttle, one for elevator and ail
erons and the other for flaps/spoilers.

 

If one wanted to, it would be trivial to cut the ailerons at the transition area and have separately actuated flaps. I don’t see it for this particular aircraft, but the idea is there. I find it to be distinctive and pleasing to the eye.

 

Oh, I almost forgot, MOD stands for My Original Design – pretty imaginative, right?

 

The Package

I offer this design to anyone who enjoys building and flying sport aircraft, experimenting with design, and doing thing in the simplest/cheapest manner. I currently use DeltaCAD, an excellent program which is available at deltacad.com on a 40-day free trial basis. This software will allow printing onto multiple sheets of paper, which can then be glued and used to make the full-size drawing.

 

Construction

This is a simple construction. I have used this drawing now, with appropriate modifications at the time, to construct several MOD versions. I believe it to be 100% accurate. However, I have made some slight modifications to it since MOD-7 construction, anticipating MOD-8 coming up in a few months.

 

Specifically, these are minor changes:

-          moved the wing 1/2 inch forward (helps move the C of G back more, for higher performance in snap/spin and landing flare)

-          slightly larger horizontal tail surfaces (minor improvement in landing flare) – be careful about adding too much to these, as maneuvering loads on 3/32-inch balsa can be very high.

 

I use white glue, carpenters glue or Weldbond, thinned with water, for almost all of my building. I rarely use epoxy for anything; it is messy, not significantly better than anything else and more bother to work with. The water-based glues give you time to correct any mistakes. More recently, I have taken to using polyurethane glue (one version is sold as Gorilla Glue) for a number of things, such as building with Coroplast, mounting fire-walls, sticking spars and other pieces into foam wings, etc.

 

I use dowel pushrods for rudder and elevator halves. The latter employ two pushrods, combined at the front end via soldering their metal rods and using one servo.

 

The firewall is locked into place via the fuselage doublers. Additionally, I show employing triangle bracing; however, in the future, I will simply use the polyurethane glue and water mist, as it develops its own fillet! I use Balsarite, or thinned Weldbond in all engine compartment and tank area fuel-proofing. Either will allow good adhesion to covering, unlike messy epoxy. (I really do hate that stuff!)

 

 

I have used various covering materials on the MOD’s: Solartex, Ultracote, Micafilm and finally, 21st Century Fabric. As Micafilm is increasingly scarce, I use 21st Century Fabric now exclusively. Note: they have some obvious production/quality problems with RED, as I have recently had to return several rolls due to horrible paint and flaws, finally accepting some that have discoloring underneath their included instruction label – I wanted/needed that color so badly that I accepted poorer quality. Other colors that I have used from them seem to be OK. Once in the past, 10 or more years ago, I had similar problems with their Red fabric.

 

Set-ups and flying

Set up the aircraft to use flaperons (channels 1 and 2, on my Futaba 7UAFS). Provide lots of travel on all surfaces. Note: the rudder may seem to have smaller deflection than desired, due to interference with the elevators. It is fully effective, and chopping the elevators to allow a greater rudder angle will only decrease their effectiveness and weaken them at, or at least require making the stab and elevators longer, and weaker.

 

This setup will be very twitchy, unless you use exponential! Set up the ailerons to have 40% expo. Elevator may also need some, perhaps 20% to 30%. That way, you get a very relaxing and smooth control near neutral and wicked response when you “give ’er.”

 

Remember, make sure you do something to highlight the top of the wing. When you try a high-speed roll, you’ll see why! Don’t even think about counting them, if you hold the stick over – and do it nice and high up, until you can be sure you can tell whether the recovery is normal or inverted,

 

Even if you are not used to the elevator/flap mixing, set it up anyway – use 40% to 50% 2->6 mixing. You can leave it deselected via the Tx switch. Tip: put a piece of fuel tubing on the toggle switch, so you can feel it without looking for it. This plane does not need the coupling to take-off, fly and land. However, you can switch it on any time to get used to it. I recommend doing it in the air, playing around with stalls, tight turns, spins, etc., with and without the mixing and then using it for both landing and take-off. Remember, as you have some exponential on elevator, you won’t even be aware of the mixing most of the time, as it will be very minor.

 

Future

This design is evergreen. MOD-7 now has 2 wings (only one is stuck on at a time, though!). The original balsa one with slight dihedral has previously served on 2 MOD’s as a low wing and is now a high (shoulder) wing (just cut holes for the servo leads in the bottom center area and tape or cover the old hole in the top). The newest wing looks identical, except that it has no dihedral. It is of white foam, with three inch, 1/16 balsa sheeting top and bottom from leading edge to aft of the spar, then some balsa cap strips to the trailing edge balsa strip. You would not know it from a complete balsa wing, but it is cheaper/quicker to build and weighs about the same. I use polyurethane glue to fasten the leading and trailing edges and to set the spars into their foam channels. The sheeting and cap strips are contact cemented with LePages Pres-Tite Green Contact Cement – water-based, no smell, quick drying – using a foam brush. No muss, no fuss!

 

I am toying with building a MUCH bigger MOD. There is this 1.20 Saito 4-stroke sitting over there …hmmm … and 8-foot, balloon-busting IMAA-legal sport plane?

 

Multi-MOD? How about a low wing, high wing, or biplane versions. It could be done, perhaps cutting the fuselage for wing locations on top and bottom, then create plugs to cover the unused wing location when in one of the monoplane configurations and using a plug with cabane struts to locate the top of a biplane version. Perhaps just a bit too strange?

 

If you have read this rather long write-up, you may be considering building a MOD. I make the .DXF available free. My only request would be that, should you build one, or some derivation of it, please send me a picture or two, any comments regarding errors or omissions on my drawing and an honest review of how you liked it. I may be able to field a question or two, as well.

 

Good Luck!

 


MOD-2

 


MOD-3

 


MOD-5

 

MOD-6L and MOD-6H

 

MOD-7

 

MOD-7 Maiden Flight

 

MOD-7 Maiden Flight

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