Twin engined conversion of Vario Acrobatic

This is a progress log of the twin engined conversion of the Vario Acrobatic helicopter.  This document is a work in progress, the bottom of the page will always have the latest information.

 The goals to be acheived by this twin engined conversion are as follows:

1) Redundancy of the following components: Engine mechanical, ignition system, carbueration, fuel plumbing, throttle servo, and clutch assemblies.

2) Nearly doubling of available power

3) A significantly larger front end of the heli that may aid in visibility.

Fabrication of the bottom plate was completed on 08-11-07, below are some pictures of it.  The fiberglass is 3/16" and turned out easier to cut and stiffer than I had expected.

08-13-07   The bottom plate below is complete, only 4 holes were 4mm off, miscalculation, fixed with re-drill.

08-13-07    The upper plate.

08-14-07 Assembly of most components except for throttle servos and fuel tanks is now complete, everything looks as planned. Some quick pictures below.

08-17-07 Test flight:

Video below, two climbs using about 7-8 degrees of pitch: I have the current curve topping out at about 8.5 degrees (tuned for a single), and I did not quite reach full stick travel on these brief attempts.  As can be observed in the second climb, there is plenty of power, and no bogging, I will eventually work up to 11 degrees of pitch in the top end, but keeping it very careful during testing. Some turbulence was present as there always seem to be wind vortices on that corner of the house. One third of the way into the video the governor is engaged for a couple of seconds and some destabilizing can be heard, governor operation is still not possible due to, I believe, excessive power touchiness or a slow set of servos.

video\P1011736.MOV

Some pictures of the 1st week prototype, still running single fuel tanks, forgive the mess, not normally that messy.

08-31-07

Due to poor range checking with the 72mhz system, it became necessary to run an XPS 2.4GHZ system, and to convert the downlink to 900MHZ. I have also added a rear fuel tank, one tank to each engine, no CG shift.  The new radio tray is 1/16" fiberglass, using a bicycle reflector mount to hold the rear, and a frame point to hold the front. Weight with all the gear and fueled up is now 28LBS.  Today's flight with XPS was flawless. I am still working on learning to manage the 28LB weight, there is plenty of power to overcome VRS on descents and hovers, however it becomes a little tiresome to deal with. I have programmed 11 degrees of top end pitch, but am running a rich high needle on each engine and rarely get into the upper end of stick travel for fear of breaking something. I may go with some Vario 427 reflex shape blades eventually as the Vblade 810's are running as much weight as I'd care to put on them.  A matching exhaust, vel stack, and filter for the R engine is coming in a couple of days, the digital camera gave an overheat warning during flight today as the downward facing exhaust on the R engine is not helpful.

Pictures of the setup as of 08-31-07

09-15-07  some issues are addressed:

The idler pulley seized in flight, but the belt continued path and I was able to land. I replaced it with a timing belt pulley off of a Mitsubishi Galant, from the auto parts store.

10-2007 I rebuilt (new cylinders, pistons, rings) both engines to have them equal.  The old ones were tired, getting only about 75psi of compression. Vario #427 blades are now in use. Helicopter is 29LBS now fueled up, with camera gear.

11-2007

The aircraft is performing very well, I am concluding the break in of the newly built engines, fasteners are remaining tight, nothing is cracking, nothing is vibrating visibly.  The other day several aerial photos were taken at 1/320 shutter, no blurs.

12-2007

Still performing well, I replaced all of the blade grip bearings and dampeners today.  The stock oring dampeners seem overwhelmed with the loads it seems, dampening is too progressive: i.e. there is a degree of free-play and beyond that the spindle bottoms out w/o further travel, this is with stiffening shims in place. This is a symptom I've had with all o-ring based rotoheads, I prefer larger purpose built dampeners.  Looking at a QuickUK head or XLV head, nothing urgent though.

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