Mini Morph Marathon - Part VI: Retrospective

Background

The previous five articles of this series have shown a process by which you can take a working airplane (Mini-Scout) and modify it into many different directions - bi-planes, trainers and warbirds.  It has been a great learning experinece, and I now have five planes that all fly well, at a cost of under $15 (including servos and wheels).

The Rules:

While there are a lot of variations in design, there are several rules that need to be carefully minded:

1. Check the transmitter settings multiple time to make sure the surfaces are moving in the right direction, and with the right amount of throw (less is usually better, at least when beginning  - use a gage to set them!).  Walk through some flight manouvers (with the throttle switched off) and make sure the controls are doing what you think they should.  It is better to be surprised on the bench than in the air. 
2. Make sure it taxies straight.  If it doesn't taxi straight, it won't fly straight.  I have damaged a lot of planes by thinking I had the rudder straight, only to toss it into the air and watch it take a hard left (or right) and nose into the ground.  Adjust the rudder so the plane travels straight, even if it doesn't look straight.  Especially when building like I do, it is too easy to not get the rudder glued exactly on the center line. Do a chuck test or hop-flight.  
3. Now that the plane should be flying straight, check the glide slope and make sure the CG is correct.  Apply just enough power to start a climb, cut the power and glide into a landing. A good glide is generally an indication of good CG (but not always!!).  Finally, go for an easy flight to look for any strange behavior.  Make an easy take-off and climb to three mistakes high.  Check how 'twitchy' the controls are.  I tend to initially leave too much travel in the controls so that the plane over reacts, now is the time to find out and make adjustments.  Make sure the turns are smooth and responsive.  Check the stall characteristics.  Does it tend to climb (or turn) when throttled-up?  Work your way into the plane gradually - it pays off in how long the plane lasts, and how much you enjoy flying it.

CG (Center of Gravity) can be tricky and a bit surprising.  The best CG does not always give the best glide slope, as I found out with the Mini-Spitfire.

Building the Mini-Spitfire has been a love/hate realtionship, as I thought I had the CG set, only to have the plane fly horribly.  I started with the CG in the same location as the other planes in this series, only to have it be 'mushy' and not responsive to controls.  I tried several flights, moving the battery further and further forward (till is was almost not attached); but with limited improvement.  The plane has seen a number of hard landings, but still held together.  To stiffen the wings I glued a skewer under each wing along the fold.

In desperation I glued another piece of foamboard as far forward as possible (almost into the propeller!).  With the 500mah battery as far forward as it would go, I still could not get a good flight.  The plane kept wanting to nose up in a climb, and the controls would be unresponsive, then change drastically. I had to fight to keep the nose down, so the controls were semi-responsive. I had gotten back in the car thinking the Spit was destined for the shredder (or given to a friend's kids as a chuck glider), when I decided to try a 1000mah battery for more weight.  The CG was only one inch back from the leading edge, and it will not glide dead-stick, but it flew like a totally different plane, gentile and responsive to the controls.  I had made the mistake in thinking that a good glide was the best CG; usually, but not always.  I flew it around for about 5 minutes at 1/2 to 2/3 throttle, thoroughly enjoying the time.  I then pushed the throttle to see what it would do.  I was trying a series of tight loops, when I lost orientation and ended up nosing it into the ground, ending that flight session.  The only damage was the foamboard for the battery ripped out, and the power pod had the front pulled loose (again!).  A little hot glue and it will be flying again; definitely another 'keeper'.

Josh Bixler says that if you get the CG right, any plane will fly.  The inverse of this is that if a plane that looks like it should fly decently doesn..'t fly, change the CG (in particular, try moving the weight forward).  If (when) I rebuild the Mini-Spitfire, I will probably extend the nose another inch, giving more weight leverage, and more room to move the battery.

Future Directions:

This series gave some basic design patterns of tail, wing and motor relationship; but the modifications could go on in many directions.

• The trainer  could have the wings extended (and tapered?), and the fuselage lengthened to be more of a glider like the Simple Soarer, or add dihedral to the wing tips and try for a Mini-Radian.
• The Mini-Spitfire could take many wing modifications for different warbirds: Mini-Mustang, Mini-Corsair.  The simple wings allow for all sorts of contours without having to worry about reinforcement and complex joins. 

Yes, that is a camera velcroed to the wing.

Conclusion:

Build, Fly, Crash, Repeat

Follow the rules, and you will crash a lot less.  All of my problems have been when I have gotten these wrong (or pushed things beyond the flight envelope and my skills!).  But with this size, the crashes have minimal damage.  For $30 in electronics, and some salvagable servos and wheels, you can have a lot of fun and make a lot of planes for only a dollar or two each.  The small size is easy to work with, even in a small park (soccer field).

The goal is to have fun, both designing and flying, and these Minis are definitely a success in both of these categories.