Electric Wright Stuff B

[iflycheeseburger]

Thank you very much, coach Chuck. I’ll try your suggestions. It’s really enjoyable to see all your comments here and see your ‘super power’ to control the airplane.

One more silly question about motors: from what I learned here, lower RPM motor with larger propeller is more powerful, right?
If the motor and propeller is strong enough to push airplane to the roof height, do I still need more powerful motors or replacing propeller to improve flight time? Thank you!

[coachchuckaahs]

If you have a lower resistance motor, say a 2.1-ohm motor, it will be a higher RPM motor and may have too much additional climb. The lower RPM motor (3.3-ohm or so) will give sufficient power while being more efficient.

If you have a lower resistance, but higher RPM motor and have too much climb, you can reduce prop size, reducing load on the motor, and thus reducing power. However, doing so may also reduce the efficiency of the transfer form shaft power to plane motion. A larger prop does better here.

Increasing prop size IN ORDER TO SLOW THE MOTOR, will result in MORE power, but less efficiency, to a point. Load it up more and the power and efficiency will drop.

So it is all about matching prop size, motor, and plane.

So, to address your question, lower RPM with larger prop is more efficient, not more power. However, improved efficiency in the conversion from shaft to plane motion may result in more net power to movement. But, no, the reason to go to a higher resistance, lower RPM motor is efficiency, not power.

Our initial motor was 6.5 ohms, very low RPM. While very efficient (we were getting 1:17 at 18 feet), we could not climb to the rafters. Dropping to 3.3 ohm (nothing found in between), we easily made the rafters, and had to reduce prop to control climb. At 2.1-ohm we were unable to get sufficient time (too inefficient prop).

Coach Chuck

[iflycheeseburger]

It’s very clear now to optimize the performance of motor, propeller and airplane. Thank you very much, coach Chuck.

[cctl29]

Hi Coach Chuck,

Thanks for answering my question on quick climb vs slow climb. I just noticed that the motor on the slow climb plane was accidentally installed with a slight "down thrust". I wonder if that's the reason for the slow climb, which worked out well in our case because we were flying in a low ceiling (17') site. Now I have a question on the ratio of time spent climbing and cruising - you mentioned that for performance, our goal was for the plane to spend more time cruising than climbing. Is there an approximate ratio we should target?

The stalling we noticed was during the last circle of the flight, if under power, when the plane dropped (no observation of obvious nose-dive or tail slide) about 15 cm or so and cruised a little more before landing. I will take a video next time we get a chance to fly.

Thanks! Your help is always greatly appreciated.

[coachchuckaahs]

There is no rule of thumb. For these EWS planes, the initial peak power runs out quickly, so we see substantial climb in the first lap (say 16' for a floor launch in a 24-foot gym), and typically max altitude around the end of the second circle. This is probably only about 20 seconds into the flight. Then 2-3 laps of near-level flight followed by 3-5 laps of letdown.

On a rubber-powered plane we might expect a more controlled climb taking as much as 1/3 of the time

Coach Chuck

[iflycheeseburger]

I have a question about the total distance from from wing to the tail. To adjust CG, we can move capacitor (limited by wire length) and the front wing by sliding wing post. In J&H kit, the wire length is short. So I moved front wing closer to tail, thus have a really long nose. The airplane can still fly well, but a big portion of front part are useless. I wonder what’s the advantage to have longer distance between front and rear wing. Could I cut the and remove part of the weight? The overall lengthiness J&H is longer than freedom flight. Thanks

[coachchuckaahs]

CB:

Since you indicated that you ran out of adjustment with the capacitor, I assume you are trying to move the CG rearward.

However, you also moved the wing rearward. THIS HAS THE OPPOSITE EFFECT!. You need to set the CG RELATIVE TO THE WING. So if the plane is acting nose-heavy, you will want to move the wing Forward, not back, or move some dead weight (Capacitor) back.

Generally, a longer tail moment will tend to make the plane more efficient. We have run some VERY long tail moments in the past. Several key AMA classes have a maximum overall length in order to limit the tail moment.

Coach Chuck

[iflycheeseburger]

Thank you very much, coach Chuck. I will move the wing back and change capacitor location.

[cctl29]

I am very curious to know how much everybody's airplane weighs. One of ours weighs 10.00 g and the other weighs 10.18 because of the heavier power system. Both are overweight. For the second one, the power system weighs more than the airplane by about 0.4g. How can we reduce the weight of the power system? Right now we installed the capacitor with its legs forward. I suppose we can reduce some wire weight (0.2g maybe) if we turn it 180 degree with legs aft.

[cctl29]

We flew this afternoon. With the day getting warmer, we could hear the humming of the AC and cool air coming from the vents. We were told that there was no way to turn it off. We could barely practice because the AC just threw the plane in distress. Our flight path was much more predictable earlier when AC was quieter.

Just wondering how a tournament site regulates the air currents, or is it regulated at all? Besides moving CG forward, anything else can be done to make the airplane more stable without losing too much performance? How to read and adjust for air currents in a site?