The LA-Heli Rotorshape

Sometime last year I was looking at the Robot Birds web site and I came across their web page for the Rotorshape from  LA-heli.  I just had to have one!

Dry buildThis is the body of the Rotorshape.  It's just two tear-drop shaped sides, a platform for the battery, a mount for the motor and undercarriage and a mount for the two rotor control servos.



Extra strength joinsThe parts are glued together with CA, but I decided to add some extra reinforcements in the form of some balsa strips to increase the gluing area. 


Partly builtI didn't take many pictures during the build, but here it is with the motor installed, the main boom and rotor mast fitted (both carbon tubes) and the head assembled thought the blades had not yet been covered.
Broken boomImfact I didn't take any more pictures until after it had flown a few times.   The maiden was carried out on a day when the wind was really a bit too strong, but it flew well from it's first launch.  The small wheels mean that on a grass field the only practical way to get it flying is with a hand launch.

It took quite a few flights to get used to its flying characteristics.  Although you fly it like a conventional fixed wing model (with alierons and elevator on the right stick), the way it reacts to control inputs takes a while to learn.  I can best describe it as being "radio guided" rather than "radio controlled".  There is quite a delay between the control input and the model reacting as it "wobbles around the sky".   Since there very little to actualy see in the sky it is very easy to get disorientated and not know in which direction it is actually pointing.  Consequently I crashed it several times as a result of it getting a bit to far away and then  rolling it the wrong way.  Quite remarkably these crashes often resulted in no damage at all, but once the rotor mast shapped and later the main boom snapped as shown on the right.
Inserted rodThe repair was quite simple.  The boom was snapped off and a short section of aluminium tube glued (with epoxy) inside the carbon tube.  The fit was quite close so I put some epoxy into the ends of the carbon tube so that it would be spread along the aluminium as it was inserted.


fixedI spread a thin film of epoxy onto the rod before putting it together and the excess neatly joined the carbon tube ends.


RudderAfter I had been flying the Rotorshape for a few weeks, I decided to add a rudder to improve the control.  Although it flew well without a rudder, it is now much easier to control in turns.   The rudder was made from a piece of "pizza base", the control horn was cut out of an old vending machine card and epoxied into a slot cut in the rudder.  The  hinge is  nothing more than a strip of PVC tape.   There was space in the fuzelage  just infront of the rotor mast to mount a small 5g servo.


Rudder ServoThe long contro rod had to be repositioned as at first the rotor blades sometimes hit the rod at ripped the horn out of the rudder. 

Hand LaunchThe small wheels and narrow width of the undercarriage mean that landings must be directly into wind otherwise the model tends to topple over.  The blades are held in place with small sacrificial balsa pegs, which break when the blade tips hit the ground and save the blades from damage.  The small wheels and lack of a steerable tail wheel also mean the land launching is the only viable means of take off.   Even in a slight breeze the rotors can be spun up to almost flying speed by holdiing the model nose up, then only a few paces at walking speed will be needed to get the rotors up to full speed.   Mine only needs half throttle and a gentle "push into the air" to launch.


inflight1inflight2
inflight3Click on this image to get to a video of my Rotorshape flying.

Rudder Modifications

In the pictures above you can clearly see the rudder control rod above the main boom.  In this position is is rather vulnerable to rotor strikes if the model tips over when landing.  The  shock to the servo when this happens has so far stripped the gears on three servos.  I've also had to replace the rudder once and re-glue the control horn twice.

So, I decided to alter things so that the rudder control rod now lies underneath the main boom which will hopefully protect it from rotor strikes.


The load on the servo is quite small so I chose to simply attach the servo to the main boom with "hot glue".  Although this has moved the servo back further behind the CofG it's still close ehough not to have made any noticable difference to the trim.

The rudder control horn needed to be enlarged as it needs to clear the back end of the main boom which is immediatley ahead of the front edge of the rudder surface below the  horizontal  stabiliser.   As before the control surface was cut out from a "pizza base", the control horn is a piece from an old magantic strip payment card and it is held in the rudder with some epoxy.  The hinge is made from "blenderm" tape.
I've added a small skid to the bottom of the boom to stop the bottom of the rudder touching the ground.  Looking at the picture I think I still need to trim off the bottom edge of the rudder a bit more.

Bigger Wheels

One evening in very calm winds I tried a "touch and go" for the first time.  It wasn't really planned it just happened.  After a landing the rotor was still spinning and I just opened up the throttle again and off it went :)   Next time round I kept it moving on the ground, opened it up and off it went again :)   So, having mastered touch and goes,  a few days later I decided to try a take-off from a standing start.  The wind was about 10mph and with it sitting on the ground with the rotor tilted all the way back it was spinning up quite fast so I was sure I could get it to fly off the ground. The biggest problem was that to start with the small wheels were getting stuck in the uneven grass, but once the rotor started to produce some lift it could start to move forward without nosing over.  With the  rudder I was able to keep it pointing directly into the breeze, and once moving forward the rotor quickly spun up to flying speed and it lifted off smoothly without rolling either way.  The cure to the problem caused by the small wheels was obvious... fit some bigger wheels !
Big Wheels
On my way home from work yesterday I had dropped into my LHS to get some glues and as I looked at the racks of "stuff" I spotted some foam wheels that looked about the right size....

Here you can see the old and new wheels for comparison.  The old ones were 35mm dia. and the new ones are 50mm dia.  I used acouple of short pieces of carbon rod to  make up adapters to match the wire diameter to the (larger) hole in the wheels.

If you look very closely you can see the tail boom has aquired an outer sleeve over the spot where I snapped it a few weeks ago !
Prop clearanceWith the original wheels and an 8" prop there was very little ground clearance.  You can see the tip of the prop has a green stain from all the grass it has been cutting while going touch and goes !

The bigger wheels have several advantages:

They run much better on a grass surface and don't get stuck between the grass.

The prop ground clearance is increased to the point were the prop doesn't strike the ground when the tail lifts during a take-off run.

The model sits more "nose up" on the ground which makes it easier to spin up the rotor in a breeze and makes a take-off run shorter.

Click the picture below for a video.

ROG


The improvements were immediatley obvious when I first  opened the throttle with the model sitting on the grass.  Without hesitation it started to move forward and I found it could easily be steered with the rudder (even without a steerable tail wheel).   At first there wasn't enough breeze to spin up the rotor  but I patiently waited for a thermal to come through.  A quick spin with my "digital pre-rotator"  (*),  and with the elevator stick all the way back, it set off across the grass.  It was surprised how quickly the rotor spun up to flying speed but I managed to release the elevator stick soon enough to prevent a premature lift-off.  It rose slowly into the air at which point a bit more throttle put it into a steady climb :)  
(*) My Finger !

Throttle to Aileron mix

When I tried a "dead stick" landing the other week I found the model rolled quite sharply to the right when I closed the throttle.  I carefully noted how much left stick I needed (about 40%) to keep it straight and how much throttle it needed to fly straight and level (50%).   I then set  up two of the programmable mixes on my Futaba  FF7 to mix  some throttle into the aileron.  I set the  mix's "offset" to 50%, so at cruise  throttle the mix would make no difference to the ailerons.  I had to guess on the percentages for the mix but set them at 20% as a first guess.

The first thing I tried in flight was to switch on the mix when flying straight and level to check that the offst point was correct.  As I hoped this made no difference to the trim.
Next I tried slowly reducing the throttle to check the mix percentage was correct.  However 20% was too much as the model now rolled slightly to the left.   After a couple of adjustments I've settled on 15%.   Now I can perform "dead stick" landings without problems.