Helicopter Testing

[jander14indoor]

Many uses of torque meter.

Yes static testing of the rubber bands. Plot turns against torque, wind up and wind down. You'll see curve follows and S, rising steeply to start, staying steady for awhile, and then rising quickly again to peak where it breaks. You'll also see that it unwinds at a LOWER torque value than it winds. Its the unwind you fly on. The area under the curve is your fuel. The torque is your power to fly. You'll find that if you wind a 4 inch motor and compare it to say an 8 inch motor, the torques are the same (within error) at twice the winds on the 8 inch motor. Meaning you only really need to wind different widths of motors and can adjust the data for length by a straight ratio.

So, what do you do with that data.

First, figure out how much torque your copter needs to fly level for a given mass. Select the motor width that creates that torque for at least half to three quarters of the turns. Select a length to match that total mass. Fly & take data. Do some trials to see where the best point is for a single width/rotor combination. Within small variation, you'll be getting the most you can out of that motor/rotor combination. Try the next width/rotor combination. After a few combinations, plot all that stuff in 3-d, it should point you to a maximum. You'll get to the optimum combination faster.

Comparing copter/rotors. Build a bunch. Using the motor data, find the sweet spot for 'level' flight for each one. The one that uses the least torque for the same overall mass is the most efficient. Alternatively you could rig up a static test using an electric motor and a scale to measure lift/thrust. Measure power comsumption for various amounts of lift. Again, the best design should use the least power for the most lift.

WIth all that data come tournament time, you can now make sure you replicate that ideal motor/rotor combination and wind. Winding to torque is more consistent than winding to turns. In addition, if the ceiling height is different than you've flown before, you can control the copter height by extrapolating the data you have quickly.

Hope that provides some ideas.

Jeff Anderson
Livonia, MI

[chalker7]

Jeff explains it well above. For a slightly different perspective/wording, I wrote a relatively long post on the proper use of torque meters last year. Here is the link to that: http://scioly.org/phpBB3/viewtopic.php? ... 85#p158789
There are also sample torque curves that may help visualization in the Image Gallery.

[lucwilder42]

Thanks, that's very useful info, we usually wind by feel. Do some copters actually hover at a set distance and stay there?

[chalker7]

Thanks, that's very useful info, we usually wind by feel. Do some copters actually hover at a set distance and stay there?

I have seen it, but only a handful of times. The only way that works is if you have perfectly matched the cruise torque (the long middle region during unwinding) with the threshold torque required for the helicopter to stay aloft. I actually wouldn't recommend trying to do that, as it leaves very little margin for error.

[lucwilder42]

Ok good so since this thread is for helicopter testing, how is everybody doing?

[Orange714]

When you said that if you can't change the motor, change the rotor. How exactly do you "match" the rotor to an rubber band?
Also how long should the rubber band be, like a starting point?

Thank You!

[lucwilder42]

Instead of using length as the unit of measurement, it's sometimes easier to go by mass. So since 2 grams was the max rubber mass last year, I'd say that's a decent starting point.

As far as matching rubber and rotors, it's a matter of changing the pitch of the rotors and the mass and width of the rubber until you determine which setup yields the greatest flight times.

[mrsteven]

When you said that if you can't change the motor, change the rotor. How exactly do you "match" the rotor to an rubber band?
Also how long should the rubber band be, like a starting point?

Thank You!

I wouldn't even look at the length at all- like lucwilder said, mass is the key.
I'm going off the assumption for the moment that for a single motor helicopter about 2 g is the place to be, however further testing on my part may yield different results. Its definitely a good starting point that will yield fairly good results if any starting point could.
Generally speaking, the higher the pitch, the wider the rubber must be to create enough torque for flight and the lower the pitch the thinner. Thinner gets more rotations, but thicker makes more power of lift per cap rotation.
One isn't better than another, it depends on your own construction capabilities and the specific helicopter design, however extremes at either end will never be successful. There is a trade off, either way- i'd try to keep in the middle of the flyable pitch ranges

[chalker7]

When you said that if you can't change the motor, change the rotor. How exactly do you "match" the rotor to an rubber band?
Also how long should the rubber band be, like a starting point?

Thank You!

I wouldn't even look at the length at all- like lucwilder said, mass is the key.
I'm going off the assumption for the moment that for a single motor helicopter about 2 g is the place to be, however further testing on my part may yield different results. Its definitely a good starting point that will yield fairly good results if any starting point could.
Generally speaking, the higher the pitch, the wider the rubber must be to create enough torque for flight and the lower the pitch the thinner. Thinner gets more rotations, but thicker makes more power of lift per cap rotation.
One isn't better than another, it depends on your own construction capabilities and the specific helicopter design, however extremes at either end will never be successful. There is a trade off, either way- i'd try to keep in the middle of the flyable pitch ranges

I would definitely look at length. There is a reason we removed mass from the equation this year (beyond making it an easier event to judge.) The extra variable is an extremely interesting one that you can play around with a lot.

mrsteven is correct in that wider motors will create more torque and thinner motors will hold more winds per unit length, but that tradeoff gets more complicated when you start playing around with the length of the motor. Let's say you find out that a motor that is .125" wide provides enough torque to keep your helicopter aloft for the entire flight and is running out of turns. Obviously, if you were to somehow get more turns on the motor it should stay aloft longer. In order to do that, you have a few options:

1) Wind the original motor up more. This is risky because you should already be winding your motor close to its breaking point. If you are not currently winding your motors as much as possible (you should be breaking motors while winding with some frequency), definitely do it from now on, you are just leaving turns on the table. There is no need to backoff winds in helicopters like you might in Wright Stuff.

2) Use a thinner motor that is the same length. This motor will get more winds, have a lower torque profile (be at a lower torque at every point of unwinding) and weigh less than the wider motor. This might not work because you might not have enough torque to keep the helicopter in the air, but it also might work because you may have had much more torque than needed initially. Also, because of the slightly lower weight of the motor, the helicopter will require slightly less torque to stay in the air (but only a very small amount less.)

3) Use a motor that is the same width, but longer. This motor will get more winds, have an identical torque profile to the original motor and weight more than the initial motor. This might not work because rubber is relatively dense and if you add too much length you can actually increase your flying weight dramatically, increasing your minimum torque required to stay in the air above what the motor can provide.

If you notice, there are two things I am focusing on. The first is only changing one variable at a time (or as close to one variable as possible). This way, we can identify trends correctly. Secondly, I am focusing on torque. Long time members of these forums will recognize how much I have been promoting the use of torque meters, because they are honestly the single most important tool you can use (other than a stopwatch) to learn more about the flight patterns of helicopters. The (nearly) direct relationship between torque and thrust is extremely convenient within the context of this event and you should be able to optimize your times without too much difficulty by paying close attention to your flights, taking notes and observing the trends. If you search through my old posts, you should find a relatively detailed explanation of how torque meters work and how you can use one to determine whether the motor you're using is good for your helicopter or not.

[mrsteven]

well, as length increase, thus the mass does as well which is what i was getting at. The actual numerical length isn't as important as what it represents in terms of mass on the helicopter, but in the same way more mass = more length= more turns. Length doesn't really matter (given the same width) if you measure the mass and decide you need more turns, so more rubber, so more length. In terms of what you're measuring in the different flight times (when wound to full potential) the mass and length as proportionally related. So the longer, the more mass and thus more winds/time

So as long as you are winding to full potential (you should as Chalker said) and keeping the width variable constant, the length = mass for how the relate to changing flight times, which is what I was saying. No need to measure both as they correlate, yes?