After significant effort and help from several of you, I'm happy to describe a triplane dubbed the Sub-One (so named because it has less than 1 ounce per square foot of wing surface area). This triplane is based on parts from DWE's Lil' Skeeter monowinged plane and on some of the design features (wing spacing and incidence angles) of Rick Ruijsink's Tri-X triplane and therefore is really a credit to their clever designs. I realize that actual wing loading levels of triplanes are modified by a fudge factor because three stacked wings are only about 85% as lift-efficient as three separate wings on three separate monowing planes, but the "Sub-One*" seemed like an unnecesarily complicated name.

Design:

I began with the Lil' Skeeter from DWE (www.smallrc.com). I stripped the Skeeter down to about 20 grams all up weight by removing the balsa fuselage and side reinforcements, and by trimming the wing pylon down to a thin sliver. I replaced the fuselage with 3 mm carbon fiber rod (which I soon discovered was much thicker than needed). The resulting MiniSkeeter plane flew better than the original Lil' Skeeter but was still a bit fast for the ultimate goal (living room flight) and because I did not build it with sufficient dihedral, the effective turning radius was much too large (tighter turns caused it to dive).

After reading some of the articles in the forum and on several webpages including several good aeronautics theory pages, it became obvious that while building light was good, building low wing loading levels was more important. Other than replacing the KP00 with a lighter motor, I wasn't going to lighten the weight of the plane, so going to a multiwing design was the obvious choice. After weighing every component in hand (see my earlier thread), adding two more wings of 45 sq. in. each was predicted to add 4.77 grams of weight but 90 sq. in. of wing surface. The result that while the stripped down MiniSkeeter had a loading level of 2.3 oz/sq. ft., a triplane with three Skeeter-like wings would have a predicted loading level of about 0.9 oz/sq. ft.

I also decided to forgo landing gear (a total weight, even with balsa wheels, of more than 0.6 grams including music wire for the wheels and tail dragger). Instead, I envisioned having the plane take off from a cart, which would be left behind at lift off. All of my on-grass launches would be from my hand anyway, so this weight savings made sense.

Building:

To minimize weight, I replaced the 3 mm carbon fiber rod of the MiniSkeeter with a 0.06" (1.5 mm) CF rod from DWE. I also cut back on the amount of glue and the length of wires used; everything was designed to absolutely minimize the all up weight. I kept the KP00/U80 motor/prop because the Didel gear wasn't here yet and because I knew that the added wings would increase drag significantly, possibly requiring a pretty strong motor (as I described before, the KP00 is much stronger than is needed to fly the MiniSkeeter).

To hold the three wings rigid, I used four carbon fiber rods as can be seen in the pictures below. I tried various thicknesses and settled on the 0.04" dia. rod as giving the minimum needed strength. The wing mounting rods were glued with CA directly to the fuselage rod at the 22 degree angle suggested by Rick's Tri-X plans. I also used angles of incidence matching those in Rick's plans. Each wing was built with a large (16 degrees per half wing, 32 degrees total) of dihedral angle for ease of performing tight turns. Once all three wings were assembled on minimal 1/8" balsa pylons, I mounted them on the 0.04" CF rods. The RFFS-100 receiver was mounted under the middle wing. A 0.008" steel guitar string was wrapped around the middle with as an antenna and wing protector. I also attached two 0.02" dia. CF rods to the leading edges of the top (front-most) wing for impact dispersion. As the connection surface areas between the wings and the frame were quite small, I was initially worried that the resulting structure would not have sufficient rigidity. After completion, however, the triplane was very rigid and quite strong (all joinings were made with ultra-thick CA glue).

The cart was made in about 10 minutes from scrap balsa, 0.02" music wire, and three 1" GWS white plastic wheels. The vertical wing rests were sculpted to match the contours of the lower wing. A slit in the middle of the cart matched the width of the fuselage. You can see movies of living room cart take-offs below.

I was worried that I mounted the fuselage too low (between the lower and middle wings) when I felt how top heavy the plane was. This turns out not to be a problem in the air, though when holding the cylindrical fuselage the plane will tip over easily (all the better reason to hold it by the lowest wing pylon).

The final all-up weight with one 3.7 V LiPoly cell from Bob Selman was 24.5 grams. Wing surface area is 135 sq. in., and the weight per wing area is therefore 0.93 oz/sq. ft. The theoretical weight without any glue or wires factored in was about 23 grams, so I added 1.5 grams during the building process (though the weighing day for the completed plane had 100% humidity, while the parts were weighed dry, so the actual AUW on a dry day might be significantly lower). Total building time was two nights after work (about 9 pm till midnight).

Performance:

The first test launch (cart-based, living room) informed me that a) I put too much rightward tilt on the motor, b) the 9 degrees of downward thrust was too small, and c) the plane had much more drag, but MUCH more lift, than the MiniSkeeter. I tried in the backyard without changing anything and was amazed to see the plane fly in 3 foot radius circles to the right, slowly climbing, at a VERY slow speed (significantly less than walking speed).

I took the plane back to the bench and remounted the motor with very large (~25 degrees) of downward thrust on a hard balsa block, decreased the rightward tilt to only 2 degrees, and tried again outside. The result was, at least to this novice, amazing.

The plane is very controllable in no wind and required no rudder trim. It will fly on half throttle if you give a little down elevator. If you let the controls go at half throttle it will gently climb and stall and climb and stall. The flying speed is as low as 1-2 mile an hour (estimated), and the stalling speed is almost zero.

At full throttle it will cart-launch in about 5-8 feet from the floor and climb at more than 45 degrees. If you give it full throttle just as it is about to stall from half throttle, it will not stall but instead continue climbing. It will turn a 2-foot radius circle without difficulty, especially if you give up-elevator during the turn to bleed off speed. With the nose tipped up and 2/3 throttle given, it can almost hover in place.

It can do very tight loops (some half-baked loops are in the movies below) and behaves extremely predictably. It also can fly in weak winds, which was a real surprise (my MiniSkeeter cannot). Flying straight into a mild breeze allows the plane to hover in place and even loop (flip) in place.

Damage upon impact on grass is zero. Damage on impact on hard surfaces indooors is usually higher though, and I've had to reglue the wing supports twice.

Cart take-offs are hilarious but effective (see the movies below). The cart of course goes careening on the living room floor after the plane lifts off.

Limited living room flight is possible with this plane, but I would prefer getting to know how it handles more outside before doing a lot of living room flying since the damage potential is higher. I suspect that replacing the KP00 with a 2.5-gram lighter 6 mm pager motor will allow more comfortable living room flight as the plane will weigh 10% less. I will likely have to rebuild the fuselage to maintain a reasonable center of gravity however.

Pictures and movies are in the next post. I hope they are as fun to watch as they were to make!

Step 2: knock off 2.5 grams using a MK06-series pager motor and fly it inside the house more comfortably.

Step 3: add wireless videography to a similar plane.

-David

Here are the pictures, part 1:

The MiniSkeeter, father of the Sub-One:


Sub-One pictures before the maiden flight, on the cart:




Sub-One, sans cart:



Actuator detail on the Sub-One:

Carbon fiber wing support frame detail on the Sub-One:



Carbon fiber top wing leading edge protection on the Sub-One:


Taxiing on the wheelchair, er... cart:

Movies of living room cart take-offs:
http://evolve.harvard.edu/~davidliu/Public/tm1.mpg
http://evolve.harvard.edu/~davidliu/Public/tm2.mpg
http://evolve.harvard.edu/~davidliu/Public/tm3.mpg

Movies of outdoor flight, complete with cute snide commentary from my wife:
http://evolve.harvard.edu/~davidliu/Public/tm4.mpg
http://evolve.harvard.edu/~davidliu/Public/tm5.mpg
http://evolve.harvard.edu/~davidliu/Public/tm6.mpg

Movie with a tiny loop at the very end (I was worry it was coming in too hot so I purposefully stalled it at the end, which makes for soft landings:
http://evolve.harvard.edu/~davidliu/Public/tm7.mpg

Movie of my first attempted loop (somewhat successful):
http://evolve.harvard.edu/~davidliu/Public/tm8.mpg

Movies at half throttle, showing ultra-slow flight (it looks in these movies, at times, as if the plane must be suspended by a string):
http://evolve.harvard.edu/~davidliu/Public/tm9.mpg
http://evolve.harvard.edu/~davidliu/Public/tm10.mpg

David,
Very nice work. Keep it up! I was flying my stick version of my Dr1 triplane last night. I'm finding out what you are, that a hugh amount of downthrust is required. Mostly I flew my Punkin on two LiPolys as it was very very fun and performed so well.

Regards, Gordon
http://home.attbi.com/~gordonjohnson/

Gordon,

Thanks for the comments. Jean-Daniel tells me you are testing some new CF props. Can you share some of your preliminary findings (what motor, gearing, thrust, etc.)? I'm hoping that the 6 mm Didel motor will power the Sub-One but my assumption is largely dependent on how efficient their new props really are.

-David

David,
Check your PM's. Or, email me at gordonjohnson@attbi.com. I'm home tonight.

Regards, Gordon

David,
More thoughts. Check out the weights for CF tube from David Lewis http://www.homefly.com. These are the absolute lightest tubes available and are hand made by a guy in Germany who has written books on making CF tube. If you check out the weight per inch or centimeter, you will find his smallest diameter will be lighter for any given length than equivalent solid CF rod. Course, it's about $14 per rod.

Most of the successful pager motor solutions have been for planes that don't require a lot of thrust. It is an open question as to whether a 6mm pager (especially the short one) can generate enough thrust for a draggy triplane. But, there is only one way to find out.

Gordon

Thanks Gordon; I have been looking into the homefly CF tubes and will order some when I figure out exactly what I want.

I replied to your PM as well; thanks for the note!

Your Stechmucke looks simply amazing. I hope you share the story behind its conception and building; I've never seen such a combination of stablity and slow speed.

-David

Lovely work David! Very nice indeed. I'm just downloading some of those videos now.

As Aeronutz Mark always says "add nose weight and down thrust" it seems to fix most problems. Gets a bit confusing when you ask him the time though

Seriously in the videos of it flying outdoors it does look like its on the stall most of the time hence the slow flight but also a tendancy to fly funny. One thing you might consider is building your wings with less camber, you don't really need much especially when there are three of them to do the work of lifting the model and it would reduce drag.

I love the video in your lounge with the little loop and the cool landing, you look pretty pleased with it.

Keep up the good work

Graham

I wonder if it could bear the weight of the smallest gyro you can find. Would smooth out the wild stalling.

you couldn't use a Gyro with the rffs100 unless you rebuilt it. I don't think it needs it just a few manners

G

David,
I agree with Graham. A couple more thoughts (BTW, your tripe is flying better than mine). How does your tripe glide with no power? You might try that. When I did that for mine I discovered that the nose needed to be longer to get the balance correct. Also, apparently getting the angle of attack wright for triplanes is key. Did you notice on Rick's tripe that the angle of attack was highest for the bottom wing and lowest for the top wing. This makes the wing furthest back stall out first and the top wing stall last. This is better than all three wings stalling at the same time. You probably figured this out all ready.

Gordon

you know, even with the thing essentially hanging on drag (limiting forward movement) and the high aoa/hovering prop, it still looks too fast and twitchy to fly inside a house. I wonder for a 15x20 area, say, what wingloading really needs to be to be truly possible. I've seen the videos of guys flying ifo's in the office, ridiculous. a 12" plane should fly S L O W...

it also looks like the motor is geared to make the plane fly too fast, I'm positive the right pager motor geared way down would work better, but then the prop is half your wingspan...

I'd go to a bipe, less problems and the third wing is adding a lot moe drag without a lot of extra lift. diminishing returns. plus, 'tripe' sounds weird and they look funny ; )