Rotor pitch vs. rate of climb vs. flight time duration

[RayBab]

Good morning,

Getting ready for this year’s helicopter competition and improving flight times. Assuming we have a good, light weight build, our question is about the effect of rotor pitch and effect on flight time. We built a Tornado last year and tried carefully to set the recommended pitch. It climbed briskly to the ceiling (whether a 10’ or 20’ ceiling) indicating lots of thrust AND MAYBE TOO MUCH THRUST.

1. Does excess thrust (more thrust than needed to get to the ceiling) use up motor energy too quickly?

2. If we wish to experiment with rotor angles, how do we “un-glue” and re-glue the rotor shaft?

3. An increased rotor angle will slow down rotation, but does that necessarily increase flight times?

Thanks for your thoughts

Ray

[coachchuckaahs]

Ray:

That is a key area of optimization, matching your pitch angle to your rubber.

There is not much to be done about the climb rate. The motor is at the highest torque for a short portion of the flight. It is more important to match the pitch and rubber late in the flight. If too much pitch at that point, you will leave many turns on the rubber, which will substantially subtract from your time.

In order to explore pitch angle, you will want to optimize the rubber cross section (linear density, g/in) for each pitch angle setting. Optimum will likely be when there are few turns remaining at landing (and may change a bit with ceiling height, but is best determined with the stopwatch.

So, how do you reduce the wasted energy at the beginning of the flight but not waste winds at the end? The rubber has a characteristic curve of torque while unwinding, and it will always be at a higher torque at the beginning of the flight. Power is torque times RPM, so for a given motor, the torque is set, you need to reduce RPM, by increasing pitch angle. So, ideally you would actively change pitch during the flight.

In the past the rules have forbidden variable pitch mechanisms, but a passive method such as flaring (the rotor or the main spar flexes to a higher pitch at higher load) have been utilized. Check the current rules for any restrictions on flaring or mechanisms.

Barring such a complication, the bottom line is to optimize your pitch/density combination for the later portion of the flight.

Coach Chuck

[RayBab]

Hi Coach Chuck,

Thanks for the great response. With one year of experience under our belt, we will now experiment more. Seeing some of the winning times for the Nationals, it appears we can do better. We really appreciate reading all of the coachs' responses...and for all the time that you and the other coaches put in.

Thanks
Ray

[coachchuckaahs]

For the most part, with a given design, it is a matter of matching the rubber to the rotor, and improved winding skills. Even at the Team Trials for F1D this year I saw top flyers improving winding skills with coaching from other top flyers. That is your low-hanging fruit!.

But, moving onward, there is some gain to be had by maximizing rotor swept area. The available kits may leave a little room to improve in that area. I believe, from reports (I was not there in person), that the extraordinary times were with custom designs that maximized the rotor size. There are always tradeoffs, and a kit is a safe way to go (and can win in many/most States), but it is clear that the entire build space has not been as explored as the Flight years.

Good luck, and keep asking questions when you are curious about things.

Coach Chuck