Dear Friends,

It is with great interest that I follow the “electric” threads on this forum and recent remark made by Kim Doherty about testing the use of an accelerometer reminded me of my earlier idea to design a (IC) power train being capable to keep line tension constant. After all, isn’t line tension the governing force when it comes to control a stunter on the hemisphere?
 
Brushless electric motors offer fast and accurate power modulation over a broad range. Power stages being able to handle the loads are shelf items and I could imagine that the design of a, if I may say so, second generation in flight power management system is within reach, both technically and financially. Perhaps using lateral acceleration as an input parameter for a controller generating the minimum power required to keep the lines tight, could lead to a comfortably slow yet perfectly controllable airplane. Unfortunately the making of a mathematical model investigating the potential my proposal is beyond my personal level, but from what I have seen laid out on this forum before, I am positive that those being capable to contribute will quickly sharpen their pens.

Kind regards

Perhaps the term "constant" (line tension) I was using is misleading inasmuch as we do not really want the airplane to pull equally hard in level flight and on top of the circle. However, while we are used to gravity reducing line pull on top, sometimes up there tension gets a bit too light and we then loose the ability to properly deflect control surfaces. This is where a line pull dependent power regulator could help. Even if it would not re-establish 100% of the level flight pull.

On the other side, at 5.3 sec/lap my current 71 oz. (with fuel) MC-72 airplane generates 13 pounds of line tension in level flight (LineIII). I suspect that I could possibly fly slower and still have pull enough to allow full elevator&flaps deflection. IF it would speed-up to maintain that, yet to be defined, min. line tension speed all over the hemisphere.

Also, the line tension sensor system does not have to be an accelerometer. It would perhaps be simpler and less expensive to measure the actual line tension force at the bellcrank shaft by means of a pressure transducer.

Hi All,
Both AMA and FAI have something to say on the kinds of feedback permitted.
A great deal is not permitted, including gyros, airspeed transducers, etc.
The constant RPM governor is better than even I had imagined, though I strongly
suspect that Peter's idea of constant line tension/centripetal accelleration would be great!
For now, I choose to focus on the broader issue: "How can I fly with good line tension
everywhere with slower lap and maneuver times?" An adaptive RPM governor or throttle is one very practical route.
Airplane configurations that would never work within the constraints posed by wet tank location are now fair game, as well.
A careful reading of both AMA and FAI shows only a small amount of wiggle room, and I would generally opt for a single common solution.
Great discussion!
all the best,
Dean

Hi Igor,
I remember that thread! It was good reading. I thought you summed it up nicely at the end: simple is better and constant RPM works wonderfully. I would add that is true if the propellor diameter is large enough.
best regards,
Dean Pappas

>>>lower pitch and adjust rpms for speed.<<<

Looks I have to write little more ... it is not only pitch, it is POWER - or better - power to WEIGHT ratio ... means you can have high pitch prop, but available power at low rpm ( = torque) will push the thing to the same point ... it little different than IC engines ... IC engine has torque limited by volume and lower pitch means more power, but electric can easily work with props with high pich if we have enough torque

by other word, more power (does not matter if high rpm or low rpm) to weight means lower slippage = smaller speed variations = better powertrain quality

what Peter wanted to have (at least how I understand the initial question) is, LOWER POWER, but clever device which can handle slippage by variation of prop PRM ... that will save weight a money

Hi Dean,

I know what you're talking about, for Erik and I did most of those tests together, using -besides his stuff- the motor, ESC and battery from my WCh 2006 set-up.

I was really happy with last year's results though I too suspected there was some power left in the system.
This probably due to the rather sluggish reaction on the instantaneous load increase of the Kontronik ESC as compared to the CC. But that one didn't have the brake and I'm not goin' to fly without one...

Intuitively I cranked up the revs by 2% or so after the horizontal eights (my sequencer allows for that) and that did the trick in Spain.
I now think I've got to load the motor even more to allow for a more constant airspeed.

Never mind, we'll get there.

Very interesting was, when Bob and I swapped planes for 1 flight each when the contest was over, that the Genesis and Blue Pearl were very close in feel and behavior...


I too think there is no need for complex solutions. Just a proper Power/Weight ratio and a constant rpm will do the job. As good as IC or maybe even better.
What I really liked is the ability to talk to my coach while flying....

Bruno van Hoek