Electric Wright Stuff B

[jander14indoor]

Air currents and control thereof is a constant battle for the flying events.
While the ES and Tournament director should, and most often do, try to get ventilation turned off, it is at the discretion/cooperation/goodwill of the host site (which may have a lot or very little direct connection to the tournament itself).
Unfortunately with modern building ventilation controls, this problem has only gotten worse. I've run tournaments in sites where the control system is in another building entirely, and very difficult to control unless coordinated well ahead of time, unlike the 'old' days where a manual switch in the facility could be accessed by polite request to the custodial staff on site.
So, what to do about it.
- As an ES, start asking about it early (weeks before tournament), follow up. If it isn't off when you arrive well before the students, try to contact someone. HOWEVER, once it comes time to start, leave the system on or off, (note, that isn't the same as blowing or not). What you don't want is to have system on and blowing for the first set of competitors, then get it cut off partway into the competition.
- As a competitor:
-- Have your coach contact the tournament director and let them know you think this is important, politely of course, they have MANY worries. Try to find out if it CAN be turned off.
-- If not, practice someplace with ventilation on regularly to learn how to deal with it (heck, you should do that anyway to prevent surprises!).
-- Prior to the tournament, think about what time you want to fly, if you can choose. As you noticed, the system popped on as conditions outside changed. Look at the weather ahead of time and try to pick a time when it isn't changing rapidly. Typically early morning, or very late in the day. A cloudy day with steady temperature, you won't have as much impact.
-- Flying. Study the room carefully, note where the vents are, think about your flight path, will it take it near blowers, can you find a spot where it won't. Consider tightening your flight circle if needed to stay away from blowers. Watch other flyers to identify bad spots. Watch other flyers for 'patterns' of movement. Is there drift from one end of space or the other, if so, make sure to launch on correct end so you have as much time to drift as possible.
-- Stability. For max flight time, we tend to trim these things with little excess stability margin. If it will be a real problem, trim for more stability, typically CG forward slightly and retrim wing and tail angle of attack for good flight. Take data to allow you to know where and how much to adjust BEFORE the competition, that's the real point of the flight log! Also, you will want a LONGER plane (more distance wing to tail). Doesn't increase stability, but better damps out disturbances (disturbed less/recovers faster).

Jeff Anderson
Livonia, MI

[cctl29]

Hi Coach Jeff,

Thank you so much for taking time and effort in replying and sharing your experience and insight. It is most helpful and deeply appreciated.

[coachchuckaahs]

Excellent writeup form Coach Jeff.

The ATX Invite in the fall was interesting in this regard. The air could not be turned off. Each gym segment (roll-up dividers were down) had a big vent pipe along each side. The air was blowing downward somewhat forcefully under these vents. However the air in the middle of the gym was very good, and surprisingly calm. Those that had a "safe" (not necessarily that small) circle size and recognized the situation made their plane circle in the middle and they flew great. Those that did not pick their launch spot carefully got knocked down 5 feet or so every lap.

"Reading the gym" is a critical skill to learn. Ideally, others fly ahead of you and you observe air motion, since you only get two attempts at official flights. At our regionals, we did not get an opportunity to see anyone fly with any real duration, and when we flew we discovered significant drift, ended up hanging the plane in a support for a basket. Second flight had to revert to a lesser plane.

Coach Chuck

[coachchuckaahs]

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.

We were able to get to minimum weight and even had about 100mg of ballast on our second plane. Our first one, after some judicious shaving, got to about 9.6g.

On the plane itself, the main motorstick could be reduced, and the tailboom extended (we used balsa for the tailboom, and it was a thinner section than the motorstick). This may not be an option if you built from a kit with a carbon tailboom. You can also sand the motorstick a little in less critical areas (AFTER trimming, sand the height down a bit behind and in front of the wing saddle). Leave enough full height that you can still adjust the wind a few mm each way. You may also be able to remove some mass from the saddle.

You can also consider removing insulation form the wires. This is heavy, and you can route one wire above and one below the MS, and tack in place with a small dot of glue.

During building, keep a careful log of the mass of each component after each step. You may be amazed how quickly glue weight builds up. Go VERY lightly on the spray adhesive for covering. Use thin CA with a capillary applicator. Weight after each gluing step to see how much was added. See the NFFS video on glue management at https://www.youtube.com/watch?v=owWmW_D6WYQ.

Coach Chuck

[cctl29]

Please pardon me for my ignorance. I think I read/heard somewhere that you can use metal tweezers to short the capacitor after the airplane landed but the propeller was still spinning due to some voltage left on the capacitor. I am just told that it is a no-no to short a capacitor. Which is the right way to do, letting the propeller run until it stops which may take a couple of extra minutes, or short the capacitor which instantly stops the propeller?

I also wonder if it makes any difference between to recharge the capacitor when there is still some voltage on it and to charge it when there is none or close to 0 volt on the capacitor ?

Thanks in advance.

[coachchuckaahs]

The reason for discharging can be 2-fold: 1) The rules require fully discharged when you check in. You can fully discharge by running motor well before check-in. 2)If you are charging a set amount of time, you want a consistent starting point. Unfortunately, this would mean running motor for some time during competition, a big loss. A little residual charge probably does not impact the charge time, since initial charge is very quick.

I think I mentioned shorting to discharge. When the capacitor is barely charged, doing so should not be a huge issue. However, if this makes you uncomfortable, you could get a small resistor and use that to discharge. Keep in mind the motors we use are 1-3.3 ohms, so to discharge faster than running the motor you would need a resistor that is under 1 ohm, or just a piece of wire, or maybe, tweezers?.

If this is a concern, discharge by running the motor before check-in, and then charge with a voltmeter instead of timed charge, and thus start at any voltage since you charge up to a stopping point.

Coach Chuck

[cctl29]

Hi Coach Chuck,

Thank you very much for your response with regard to cap discharge. First of all, we used your method (metal tweezers) all along and sometimes even just to demonstrate the discharge on a time charged (a few seconds) capacitor (~ 1.7v). So far none of our capacitors are damaged - sorry I was in panic mode when an electrician say No. So I guess your method does no harm, at least in our case. I figured that letting the motor run its course will wear and tear the motor plus waste precious time.

I measured the resistance of a piece of wire (solid copper core, 20 AWG, one foot length) which is 0.2 ohm, almost same withe a pair stainless tweezers. Without a discharge pen, which is a better choice, to let the motor run (saves the capacitor), or to short the capacitor (saves time and motor)? Which part is more fragile? Thanks!

[coachchuckaahs]

Our ONLY reason for quick discharge is if we need to check in the plane and demo fully discharged, though you can do this by running motor beforehand. Early in our program we relied on timing of charge, so important to start from same point each time, but we found that unreliable compared to measured voltage of charged capacitor.

There is no real load on the motor when running out the last of the charge,. so I would think that is less damage potential. You could get a 1-ohm resistor to slowly discharge and not spin the motor. Or use a longer length of wire, thin wire, measured at 1-ohm.

Coach Chuck

[jander14indoor]

Capacitor discharge.

If you were talking to an electrician, I strongly suspect he was talking about VERY different capacitance and voltage than we've allowed in SO, more concerned about SAFETY than damage to capacitor (though some capacitor tech and some voltages may also damage the capacitor when you short it).

An important characteristic of capacitors is their ability to RAPIDLY discharge the energy.

Short story. Back in my college days (45+ years ago) the physics prof was giving a demo. He had a 1/4 farad capacitor, back then it was a HUGE thing. Like 12X12X24 inches huge. I don't remember what charge he put on it, but it was probably no more than 120V, possibly much less, but certainly more than the 3V we use. The idea was to short this capacitor in such a way as to literally blow a steel plate 10 feet into the air. Unfortunately as he went to short the capacitor he shorted it across his arm, class ended immediately as he was rushed to the hospital with no feeling in his arm! Luckily he didn't short it from one arm to the next or it WOULD have stopped his heart! Seriously, a much smaller capacitance than we use can KILL you if you charge it to high voltages. The only reason we can get away with it for SO is because the capacitors we use will burnout/short not much above their rated voltages and the the voltages are so low they don't penetrate dry skin. I'd avoid damp skin though!

As another example of how fast a capacitor can dump energy (and why shorting them can be unsafe) look up Exploding Bridgewire (EBW) Detonator in wikipedia where shorting a capacitor is used to initiate an explosion!

So, yes you can get away with shorting the capacitors we use at the voltages we use. BUT, DON'T get in the habit of shorting capacitors unless you carefully consider the capacitance and voltage. And NEVER allow your body parts to be the shorting path!

Jeff Anderson
Livonia, MI

[cctl29]

We are heading to states this weekend. I have never been to any build event at any tournament (EWS is a state only event for us), so I am wondering for the 1-hour practice flight window we are given, how are we going to practice? At a maximum, there could be 48 planes or more (EWS and WS) at the practice session. Are there any rules/courtesy we need to be aware of to avoid collision or damage to the precious airplanes (ours and others) ? Any suggestions and advices are greatly appreciated. Thanks in advance.