Mini Morph Marathon Part V: Mini-Cub

Background

This is another revisit to a previous plane, and further modifications; this time to the Mini-Trainer, and a series of morphs that lead it into becoming a Mini-Cub.  I also show the progression from the Minis to larger models.

Mini-Trainer:B

 Like the Mini-Bip:B, I found that extending the wing another four inches (6x24) slowed things down and made it easier to fly.  I was concerned about the side profile, so while making changes, I leveled off the bottom of the fuselage, making it a straight taper from the nose.

The wing was cut as one piece, then after gluing the camber, I cut it in the middle a bit (not all the way through) to introcude some dihedaral (same as the Mini-Scout).  Other than the wing being longer, and the fuselage being a different contour, everything else is built the same.  Originally the wing angle of attack was the same as the original Mini-Trainer, but before I glued down the back of the wing I decided to do some experimenting.  So, I left it loose and inserted two thickness of foamboard as 'shims', and held them in place with a rubber band.  The idea was that I could easily remove them and try different wing angles.  The test flights went well, handling almost exactly like the Mini-Bip.  I ended up liking the new angle, so what you see in the pictures is the outcome after I glued them in place.

Mini-Trainter (A & 1/2)

I got to wondering if I had not given the original design a fair test, so decided to modify it and see what could be done.  Like the Mini-Bip:A, I cut off the wing tips and extended the wing four inches (24 inches total).  After a touch of adjustments on the first flight, it cruised around just like the Mini-Trainer:B.

Further Mods

My initial flights before I get the controls set up correctly tend to be a little rough.  I am finding that I toss it into the air and the moment it doesn't go straight I (over) react and often nose it into the ground.  Since the skewers tend to take the brunt of the force, the holes quickly get soft and the motor angles start straying off.  To take some of the alignment burden off of the skewers, I extended the top plate back by 1 1/2 inches, so the power pod rests against it for most of its length.  I then took about an inch of foamboard and glued it to the inside of the fuselage along the bottom of the pod.  Just a little foamboard takes some of the stress off, and keeps the alignment much straighter.

There is definite evidence of some hard landings on a muddy day, but with this light of plane there is little damage.

I have not built it yet, but I am looking at a third modification to the fuselage.  The Mini-Trainer:B has a thin nose section (1 1/4 inch), and after a hard landing it ripped at the corner when the 'window' rises up.  The third design gives a bit more foam to this area, which whould take the stress better.  

The relationship of wing, tail and motor are the same as the other trainers, with some cosmetic re-design.  The following is how I go through the design process.

First I lay out a line in the middle of the paper that represents the motor thrust line (where the top of the power pod rests).  The left edge of the page represents the front of the nose.  All measurements are made relative to these two lines.  I then mark 17 1/2 inches back for the back end of the fuselage (note, this is 1 inch longer than the Mini-Scout).

The next line is 3/4 inches back, and represents the 'firewall' where the power pod is located.

I then put in two lines, one 1 3/4 inches above and parallel to the thrust line, and another 1 3/4 below and parallel to the thrust line.  This represents the top of the wing, and the bottom of the motor area.  The wing line determines the angle of attack relative to the motor (down angle on the motor), the bottom line is mostly cosmetic; determined by wanting a balanced looks and a stronger nose.  The top of the wing chord is located 4 1/2 inches back along the top line.

You then have a decision to make.  For my trainers I have a different attack angle for the tail than the wing, so the tail is flat to the thrust line; this adds a bit of 'dihedral' that helps the pitch.  This is the design you find in the Mini-Scout or the FoamCub.  Most aerobatic planes have the wing and tail in line, so the tail would rest in line with the wing (ie. see the Telemaster for this configuration).  For this plane, I choose to put the tail on the thrust line, so I measure 2 inches from the back and down the thickness of the foamboard - this is where the tail will be mounted.  Now draw a line connecting the the top of the wing chord to the front edge of the tail.  If you are going with the Telemaster style, draw the line from the top of the wing chord to the far end of the thrust line.

The wing has a 1/2 inch chord height, and the wing bend is two inches back from the front edge.  Hence, I draw a parallel line 1/2 inch down from the wing line (1 1/4 up from the thrust line) and measure 2 1/2 inches back from the front for the leading edge of the wing.  Draw a line connecting the top of the wing chord to the leading edge of the wing.

Finish off the top profile by drawing the 'windshield', a line from the wing leading edge down to the thrust line.  I choose to mark a point 1 inch forward of the leading edge, but you can choose any angle that is pleasing to you.

The bottom lines are mostly cosmetic, significant if you are concerned about landing gear (tail dragger, tricycle, belly lander), but not that important for msot models, especially if you hand launch and crash land.  I chose to make a point 3 3/8 inch back and 2 inches down from the thrust line for the lowest point of the fuselage.  I then draw lines connecting this point to the bottom front point of the motor, and to a point 1 inch down from the thrust line at the back end of the fuselage.

All these measurements and lines can sound confusing, but I usually measure these out directly on the foamboard and cut (rather than draw) the lines without plans.  It is really rather simple.

The last decision is where to transition from a parallel fuselage to the taper.  In the larger Cubs this is usually forward (3 inches?), but the Minis tend to be further back (6 1/2?).  If you have room for the power pod, and are not concerned with space for other electronics, it really doesn't make much difference.

If you look at this fuselage design - you just about have the classic Cub.

Set Up

Basically the same as the others:

• 2 cell 500 mah battery
•  6x4 prop 
• CG is on or just behind the wing fold
• Throws set at 12 degrees (less is better!)
• Thrust angle is in line from the top front of the fuselage along a line to back top of the fuselage.
• Motor - RTF Mini Motor X2204 v2 2300kv - House
• ESC - B12-A Ice Blue
• Receiver - Lemon 6 Chanel

Flight Characteristics

Both of these trainers (after set-up and balancing) have a nice gentle flight.  They take little control movement for cruising. I get good level flight with between 1/2 and 3/4 throttle. Like the Mini-Bip, they are not acrobatic and will loop only if starting in a power dive. Being a 2S battern, I am not surprised, this is running on very little power.  These are bank & yank, so they do not have tight control, but this is what is good with a beginner plane.

Conclusion

The Mini-Trainers are a nice progression in building your own planes, they are easy to build, low cost and have good flight characteristics.

If you are interested in taking this further, this design provides a nice gentle growth path into larger trainers; complete with airelons and/or flaperons.

36 inch Telemaster

One of my favorite practice/play planes is a slightly scaled up version of the Mini-Cub. The original design was modeled after the Telemaster.  It is a 36 inch wing (6 inch chord), with a fuselage like the Mini-Trainer:B.  The wing is a simple fold like the Mini-Trainers, though I glued in a piece of carbon rod for security. ove photo was an early version, where the wing mount and landing gear were detachable units (held on by rubber bands).  This fuselage was taken from the FT-Soarer, as well as the tail.  The plane currently has a fuselage built along the lines of the Mini-Trainer:B. However, unlike the Minis, it takes a larger flight area, and being heavier does not take as well to hard landings (like the mini).  If I were to rebuild this plane in the future, I would probably modify the fuselage bottom contour to make it look like the Cub.

Basic Metrics:

• Fuselage: 24 inches long
• Bottom of fuselage 2 1/4 below thrust line
• Front edge of wing: 5 inches back
• Height of leading edge: 1 3/4 above thrust line.  The wing rests on the top of the fuselage, on the line from this high point to the root of the tail.  Unlike the Mini, the wing is rubber-banded to the fuselage.
• Airelons should be about 1 1/4 wide and 6 inches long. 

The next size up would be the FoamCub, very similar in design and proportions.

Looking at all of these 'trainers' together, the similarities in design are evident.  The same basic ideas can be scaled in different directions, and all be good flying planes.  With a stronger moter (or 3s battery), these go beyond simple trainers toward something capable of being more challenging.

If I get some time, and decent weather, I plan to try pushing these Minis with 3s batteries and a bit more tuning.

The real conclusion is that with one sheet of foamboard, you can do a lot of exploring and modification and come up with new designs.  Start with a good design that already flies well, and start making changes to see what happens.  About the worst that can happen is you end up with a hanger full of Minies.