Electric Wright Stuff B

[jander14indoor]

A comment on weight. It is VERY hard to lighten a plane once built. Much better to build light from the start.
To do this you need a plan, called a bill of materials. Basically list every part, weight what you have for parts, add them up, and without glue you want to be below 9.5 gm, maybe 10.0 gm max. You'll get to weight with the glue and a little clay ballast. Easier to add weight than remove it.
If your list of parts adds up to more than your target, start replacing parts with lighter parts. Smaller parts, lower density balsa, etc. Balsa density is the thing you have most control over in these planes, so be very conscious of what you are using. Fit the balsa density to the strength needed for that part. Example, wing post MUST be strong, deserve denser balsa, wing ribs don't take much stress at all, can be VERY low density. Spars somewhere in between. You also need to be conscious WHERE you put the weight. Weight in the tail has to be offset with essentially dead weight in the nose. Keep your tail parts LIGHT.

A light plane not only flies MUCH better, it trims easier (flies slower so you can see what it is doing better) and (counterintuitively) breaks less. Breaks less because it flies slower, and hits things softer!

When I run my coaching clinics, this is the key first thing to building a good plane. Once it is to weight, then you find trim easier and you can start focusing on things like motor/prop selection, which is key to a winning plane.

Jeff Anderson
Livonia, MI

[iflycheeseburger]

Hi, coach Chuck,
It’s good to see you bring the topic about motor. I have looked around and You are the only one to share so deep level knowledge of motor. Thank you very much.

This changes the performance a lot, when we switch motors from two different package, J&H, and free flight. But from the package, there is no information about motor RPM. What’s the parameters could I measure to check the motor performance? I can measure Resistance, size, weight and spin time with the same voltage. With the same voltage and propeller, the spin time changes from 1.5 minutes to 2.5 minutes. What is the relationship of spin time vs RPM, the higher RPM the shorter spin time? What does higher RPM mean?
You also mentioned low/high RPM paired with small/big propeller. I am not clear on this. Is bigger propeller a better choice than smaller one to be more effective? Could I understand it as low RPM with big propeller will fly longer?
Do we have size restriction on propeller?
Thank you very much so much for leading the discussion.

[iflycheeseburger]

I have one question about flight turning. My airplane turns to the right when it starts and goes up. Then in the middle/almost top of height, it changes turning direction. I wonder why it changes turning direction and which part should be trimmed. Thank you very much

[cctl29]

We have a questions on how to trim in a low ceiling site (6m) and prepare for a high ceiling tournament site (8m). We have been flying our plane at reduced power (charge for 10sec, and let run for 7 sec), but it still consistently hits obstructions over the clear ceiling height. Could anybody share some tips on anything we can do until we find a site with higher ceiling ? Thanks!

[cctl29]

I am just curious if the point of launch makes any difference in EWS? Assume rectangular gym with marked center. Trying to flying in imaginary circles that also centered on the center of the gym. Is launch point on the circumference maximize the circling potential?

[bjt4888]

I have one question about flight turning. My airplane turns to the right when it starts and goes up. Then in the middle/almost top of height, it changes turning direction. I wonder why it changes turning direction and which part should be trimmed. Thank you very much

Ifly,

I’ll let Coach Chuck answer your motor questions. Please let me know what design you are flying and what direction you built the the airplane to turn and what turn settings and turn setting measurements you are using (ex. stabilizer tilt, rudder or tailboom offset, thrust line offset, wing twist). As for turning one direction and then the other direction during flight, this is due to trim settings. The first thing I would check is the amount of wing washin, or wing twist. During the initial high speed portion of the climb, too much left wing washin can overpower other turn settings and either cause the airplane to fly without turning, or cause it to turn in an unintended direction. Too much rudder or tailboom offset can cause this too.

Brian T

[bjt4888]

We have a questions on how to trim in a low ceiling site (6m) and prepare for a high ceiling tournament site (8m). We have been flying our plane at reduced power (charge for 10sec, and let run for 7 sec), but it still consistently hits obstructions over the clear ceiling height. Could anybody share some tips on anything we can do until we find a site with higher ceiling ? Thanks!

Cct,

If you are climbing too fast you could try a smaller diameter or lower pitch or lower blade area propeller. The team that I am coaching is testing the capacitor charge using a multi meter. This is much more accurate than timing charging and discharging. We have found that with a fast climbing airplane 1.7 volts charge might climb 22 ft and 1.72 volts charge climbs 24 ft. So, one millivolt can have a big effect in climb height. Our multimeter was only $13 from Amazon. Once you know the exact voltage charge, you can experiment with different propeller sizes to try to reduce climb rate and allow fuller charge on the capacitor. You can also begin to determine approximately how much additional climb height is produced by a small increase inn charge voltage; although this is not a linear relationship. Propellers are cheap too, we have purchased an assortment from 45mm to 65mm diameter and you can trim a larger propeller to try multiple smaller diameters.

Brian T

[coachchuckaahs]

Hi, coach Chuck,
It’s good to see you bring the topic about motor. I have looked around and You are the only one to share so deep level knowledge of motor. Thank you very much.

This changes the performance a lot, when we switch motors from two different package, J&H, and free flight. But from the package, there is no information about motor RPM. What’s the parameters could I measure to check the motor performance? I can measure Resistance, size, weight and spin time with the same voltage. With the same voltage and propeller, the spin time changes from 1.5 minutes to 2.5 minutes. What is the relationship of spin time vs RPM, the higher RPM the shorter spin time? What does higher RPM mean?
You also mentioned low/high RPM paired with small/big propeller. I am not clear on this. Is bigger propeller a better choice than smaller one to be more effective? Could I understand it as low RPM with big propeller will fly longer?
Do we have size restriction on propeller?
Thank you very much so much for leading the discussion.

Wow. A busy day on the forums!

You are getting it on motors. Just because two motors look the same does not mean they fly the same.

We initially rated our motors by putting a small, low-pitch prop on it and then using an optical tachometer to read the RPM under little to no load. However, this method requires an expensive optical tach. Some motors have a published no-load RPM. On these, motors around 50k-60k RPM are for racing drones, and are way to fast for EWS. I looked for motors below 40k RPM no-load.

Alternatively, if you measure the resistance of the motor with a good voltmeter, you can determine whether it is high speed or high torque. First touch the probes together and get a reading so you know the resistance of your probes and zero of your meter. Then touch the probes (firmly) to the motor leads (without the capacitor in the circuit, just open motor leads). Subtract the lead resistance to get the motor resistance. We have found that form most gyms, a 3-ohm or higher value is best. We have tested 2.1 ohm, and 1.0 ohm, and have had issues with too much power and limited duration. We have flown with one motor at over 6 ohms, and while very efficient, we could not develop enough power to climb more than 18 feet, and that with a shoulder launch standing up.

The 3.3-ohm motor, such as Didel, or the efficient setup from J&H, seems to be a good starting point for most gym situations.

Generally, a big (diameter) prop on a higher resistance motor will give greater efficiency in converting shaft power to flight power. The bigger prop works on a larger "column" of air at a slower rate, which is more efficient. The tiny prop on a fast motor wastes a lot of energy thrashing the air, and has limited thrust late in flight where your time is really made.

That said, just throwing a larger prop and loading down a motor is not the answer. A fast motor loaded down heavily will turn slower, but at a high current draw. According to https://www.didel.com/microkit/moteurs/Motors.html, a typical motor has peak POWER at 50% of the no-load RPM, but peak EFFICIENCY at 83% of no-load RPM. While indoor R/C enthusiasts want peak power, we want peak efficiency (conversion of electrical energy into shaft energy.). We also have to consider the prop efficiency (conversion of shaft energy into flight energy).

Ideally you run a setup that is close to the 83% of the no-load RPM IN FLIGHT. Remember, it will unload compared to static. Loading up a high-speed motor may get you to peak power (50% RPM), but will impact efficiency dramatically.

So, higher resistance motor (3 ohms or more) has a lower no-load RPM, and thus will turn a bigger prop at the 83% sweet spot for efficiency. IF you then lack power to climb, you can add power by slightly increasing the load, either adding pitch (twisting the blades) or adding diameter. Another possibility is to add prop blade chord to the TE of the prop with scotch tape, small amounts at a time.

Not all props are created equal. Just because you have a 65mm prop, does not mean it has the same pitch as another brand prop. More importantly, many cheap props have poor airfoil shapes.

On our 3.3-ohm Didel motor, we ended up with a very high quality 55mm prop from Micron Wings (Australia). However, we were able to get similar results with a 65mm prop form Amazon, by cutting the diameter down to about 57mm a little at a time. We had different props for different gyms to get the right amount of climb.

Coach Chuck

[coachchuckaahs]

I am just curious if the point of launch makes any difference in EWS? Assume rectangular gym with marked center. Trying to flying in imaginary circles that also centered on the center of the gym. Is launch point on the circumference maximize the circling potential?

Yes. Through practice, you should know the circle size your plane flies. Visualize that circle, in regards to obstacles an drift, and launch on the circumference of the circle, tangent to it.

We typically will launch from the gym halfcourt line, in the direction of one of the baskets. Be sure to launch the right direction for your turn directions.

Coach Chuck

[coachchuckaahs]

We have a questions on how to trim in a low ceiling site (6m) and prepare for a high ceiling tournament site (8m). We have been flying our plane at reduced power (charge for 10sec, and let run for 7 sec), but it still consistently hits obstructions over the clear ceiling height. Could anybody share some tips on anything we can do until we find a site with higher ceiling ? Thanks!

This is a very good question, and key to success in indoor flying.

In WS, we use partial rubber motors, with a stick to simulate the other half, and that scales very well. A half-motor flight will be half the altitude gain, and half the time, as a similar width full motor.

Unfortunately, there is no similar analog that we have found yet for EWS. We have been fortunate to be able to test in the same size gym as States. For Regionals, we tested in our 25' gym, but flew in a 30-foot gym. For this, we tested using a "floor launch" (underhand very low) and found settings the would just touch the rafters at full charge. Then at Regionals we did a high overhand launch, which was about 5 feet higher than the floor launch. This got us to 29 feet at Regionals, and the time was very proportional to the added height.

It is important to get all the height possible, as the times will be directly related to this, and the ceiling correction factors for virtual contests are based on gym height, not plane height. It is also important to fly with a full tank of gas, and adjust the motor/prop setup to limit climb. Starting with less than full charge leaves time on the ground. But this approach uses a lot of testing to get the right combination.

Obviously if you have more than a 5-foot difference from test to competition, this method does not help much.

Keep in mind as you reduce charge in the capacitor, you are reducing peak voltage. This also reduces peak current. the POWER (ability to climb) will change with the square of the voltage, and that is why it is so sensitive and Coach Brian noted. This is also why you MUST have a means to accurately repeat the time (and minimize it) between starting the motor and launching the plane. Just a second or so of extra run time can reduce your climb by several feet.

One thing you may do, if you have plenty of test time, is develop plots of time (and altitude) vs charge voltage for your plane/motor/prop setup. Do this with several setups (different props), at least one of which will allow you full charge in your test gym. Co-plot all of this data and fit curves to the times and altitudes. If you are able to find a character of the fit curve (i.e., power law, exponential, etc.), then you may be able to use such fits to extrapolate the height and time form low charge to full charge. But this will require substantial data.

Coach Chuck