Flying with two motors

[SPP SciO]

Beginner question here - when is it appropriate to fly with two motors, and what's the advantage?

Here's the full context, any comments or advice are welcome of course -

My students have been flying the Laser Cut Planes model, the Neopolitan - I know it's not a top performer but I've learned that the event is won by flying and not building, and these models are inexpensive and easy to build, so we have a few, and they're constantly being repaired after crashing into light fixtures etc. (I wish there was a better place to test but alas!)

They've typically been using 0.125" rubber and winding to about 1 in/oz according to our torquemeter, and definitely getting better as they go higher - but they've wound up breaking several rubber bands, and anything past 1.5 just seems like it's putting too much stress on the wire, it feels dangerous. They've tried some 0.130 we ordered from FF (along with their torquemeter) and that seemed to give better results, but they're still not breaking 45 seconds, and always landing with many turns left. A cleaner flying space would be better, but I also feel that if they helicopter flew straight, it would suffer far fewer truncated flights.

After encouraging them to watch more videos, they saw a JH Aero video where he uses two motors, no torquemeter, just a binder clip with a paperclip, and gets an incredible flight that didn't appear straight at all, but had so much power it kept climbing. This of course had instant appeal; their helicopters flew all around like that, and if they didn't have to bother with a delicate torquemeter, so much the better. They tried the double motor approach, and surprisingly it seemed to work; they had much more power and the helicopter kept climbing. Unfortunately the second attempt wound up snapping the motor stick, and another helicopter was similarly damaged later; it seems obvious that the Neopolitan isn't designed to handle that much rubber. Still, I was surprised that I had never heard of people doing this before, and I'm also not sure what the advantage is over just using thicker rubber.

From what I saw, the model in the JH Aero video had a much shorter and sturdier motor stick, with much larger rotors; would it be advisable at all to have them start with a thicker piece of balsa as a motor stick and try to custom make a helicopter from existing parts? Or, should we leave the dual-motor idea on the shelf for now and practice winding safely to 1.8-2 on the torquemeter?

Thanks for reading!

[coachchuckaahs]

Coach:

It would help to have specifics on the rubber and flight info. What linear density is your rubber (g/in)? What mass of rubber, and/or loop length? How many winds, how much stretch, etc.

The effect of doubling up the motors is the same as having a much wider motor. Either works. In both cases, effectively 1/4" wide motor (or two 1/8" motors) is likely way overkill, and it is not a surprise that this broke the motorstick. Even with doubling up, you want to know and record the effective linear density, and be able to adjust that. You could start with trying to double up 3/32" motors (for an effective 3/16"), but that is still likely overkill. Most helis in this weight and size range will need linear densities 0.060-0.095 g/in, depending on the pitch and diameter of your rotors. This is a very wide range. Doubling the 1/8" motor will put you in the 0.120g/in range. Keep in mind as well that the motors vary in thickness up to 15%, even within a batch. While the cut width may be consistent, the thickness is not controlled and varies in manufacture, and thus the 15% variation in linear density.

Typically a motor in the 1/8" width class will be able to wind to 2.0-2.5 oz-in, but you will want to back off to a launch torque around 1.0-1.3 oz in. Winding to full then backing off will get far more turns than simply winding up to launch torque

The super simple wire torque meters, with 0.015" or 0.020" wire sensors, will not hold up to 2+ oz-in torque. They probably start to deform after about 3/4 turn, or perhaps 0.7=0.8 oz-in. While you can wind without a torque meter, you will not be able to have repeatable flights. You can, however, with turns count, get to about 80% of max turns, which is better than nothing! Using a torque meter will allow you to get closer to 95% of max turns, or more.

You can certainly build a stronger motor stick, and adding trussing like the FFM kit will help you do so without too much mass, but the trusses can interfere with student handling.

The LCP heli is a great simple kit and will get you in the air with more than a lawn dart. John builds a great kit that is almost guaranteed to fly and is simple to build. It will place "middle of pack", above all the "oh well" teams, but below the top tier. This will be a huge step forward for most teams. If you want the next level, the FFM and J&H kits are both excellent this year. However, teh FFM kit chose to stay with a 4-blade rotor, giving up the bonus. IMHO, that will be a limiting factor this year, though it does get excellent times. You can also custom build using the fixtures by AeroMartin on Thingyverse. Keep in mind that a custom build requires you to source all of your parts, but can be very rewarding (or equally frustrating).

Coach Chuck