Estimate flight time given RX battery size and servos
 

Iwant to estimate flight time given a size of RXbattery given a set of servos. Iwant an approximate (back of envelope) calculation.The application is for a very tame plane (not aerobatic).

Here is what Iam assuming:

1. Full size servos will draw about 250 ma of current on average (OKIknow the peak can be much higher and the at rest current draw is much lower). Is this a good working assumption for a trainer type airplane ( should Iassume less or more ?)

2. Mini servos will draw about 100 ma of current on average. Again is that a good assumption?

Thank you in advance for any pointers.

Ranga

RE: Estimate flight time given RX battery size and servos
 

everyone has different usage.....

what I suggest is fully charge the battery...
fly a full flight (assuming it is a glow plane)

then with a digital charger... recharge the battery...see how many mA you put back in to the battery.. do this a few times.. take an average...
this will then give you a good idea as to how many flights per charge.....

but I also always check battery voltage before every flight

RE: Estimate flight time given RX battery size and servos
 

This is just too hard to say...it depends on everything from the state of charge of the batteries to the winds you fly in.....
I do know that a 600 mAh battery on a full charge with 4 standard servos and a nitro engine....you can fly 3 or 4 15 to 20 minute flights

RE: Estimate flight time given RX battery size and servos
 

What size and type of battery is it? for my gasers and nitro's I use LiFe and NiMh. I have an 1100 Mah Life in one of my 40s, I can get three 12 minute flights on with the LiFe, with no worries, Probably could do more. Just start with a fully charged battery each day and you should get low on fuel before you lose battery juice. I Always take my charger to the field, and when in doubt, I charge.

RE: Estimate flight time given RX battery size and servos
 

You are as conservative as I am, Bill. I have a pair of 1100 NiMh packs in my Venus II and don't fly it more than three or four flights per charge. For the others, they currently have 1100 mah NiMh packs (one each) and I limit those too. I don't push that envelope at all.

I have a pair of new 1800mah LiFePO4 packs that are going in as replacements for the single NiMh packs but I suspect I won't push that envelope either.

CGr.

RE: Estimate flight time given RX battery size and servos
 

When I was flying a .40 size trainer I did some experimenting and found that with a 4.8v battery I used about 120mah per flight. That was just flying around in circles with no control surfaces binding using all standard servos. As always, your mileage may vary.

RE: Estimate flight time given RX battery size and servos
 

I haven't given it much thought. For the planes I have flown so far, when the motor quits, it's time to land and recharge.  I have to rethink that thought though, my next plane will have a gas motor, so I will need to get into the habit or recharging the RX and Ign packs.<div>
</div><div>I still haven't purchased the packs for the P-51. I discovered last night the RX for my Futaba will take 7.4 volts so I can use a 2S 25C 2000 mah pack for the RX.  That will save some weight and give me quite a few flights, but want to stick with 4.8 for the ignition, too many reviews showing problems with higher voltage. </div>

RE: Estimate flight time given RX battery size and servos
 

Yeah, I may be too conservative. All my NiMh packs are at least 2000 MaH, 5 cells

RE: Estimate flight time given RX battery size and servos
 

More detail :

Ihave a 1000 MaH Futaba NICD battery (yeah it is old but some seem to prefer NiCd to NiMh because of its discharge characteristics and Idid not think Ineeded a higher capacity battery).

The plane in question has 2 Full Size Futaba 3004 servos and 3 Hobbico CS-12 micro servos. Iam using a Spektrum AR600 receiver in it. The plane is rather tame (not aerobatic).

From all I am reading here, it should be safe to fly 4 10 minute flights on a full charge. (Actually my son does the flying and is always nervous about battery charge so he'll be pleased to know I did the research for him.)

Thank you all for your help!

Ranga

RE: Estimate flight time given RX battery size and servos
 

How long was your flight on the trainer? (Just as a point of reference here.)

RE: Estimate flight time given RX battery size and servos
 

There must be something about the way we did things in the Military.. we become very conservative as to how we conduct our daily business, and it rubs off in how we do such things as fly our models. I just won't take the chance that I can "get one more flight" out of a battery pack. It just isn't worth it. I'd rather take it home intact than in a plastic bag, ya know? ;)

RE: Estimate flight time given RX battery size and servos
 

If the batttery is over two years old. I personally wouldn't use it. Have you cycled it to see how "full" it comes back?

RE: Estimate flight time given RX battery size and servos
 

What is the significance of two years?From what Ihave read, a NICDbattery can be cycled about 800 to 1000 times. This number would be well above the expected lifetime (in flights) of a plane. From what Iread, the main killer of a battery is overcharging. However, if you charge the battery at a current of 1/10th of the rated capacity or less (i.e. at a current of 100 MA or less for a 1000 MA battery) there is little to no chance of overcharging even if you leave the charger connected to the battery for days. Thus it would be safe to trickle charge it using a 50 MAcharger for a couple of days.

Iam not sure what Ihave read matches the experience of the readers of this forum.

Ranga

RE: Estimate flight time given RX battery size and servos
 

batteries go bad if just left sitting... so as mentioned.. if it is a few years old.. I wouldn't use it...

unless you have the means to cycle it to truly get an idea...

RE: Estimate flight time given RX battery size and servos
 

For the cost of a replacement battery, especially considering the new LiFePO4 packs give you much more bang for the buck.. more power per ounce of weight.. it's just prudent to replace batttery packs that are older... rather than replacing the plane. Lose a battery in flight can really ruin your whole day.

CGr.

RE: Estimate flight time given RX battery size and servos
 

Back of the envelope calculations:

Your current loading assumptions (250ma and 100ma) are close to what I've seen with a wattmeter in my system and exercising a servo on the ground. Assuming we ignore extreme conditions (such as servos stalling and/or prolonged 3D flying), if you could simultaneously move all the servos, you'd have an 800ma amp draw. Divide your battery's 1000maH capacity by 800ma, and (in theory), your battery would last 1.25 hours. The real load on your system is probably closer to 400mah (assuming you're doing some mild aerobatics), so theoretical battery life would be about 2.5hours (1000/400=2.5).

But as has been said above, batteries, especially as they get older and have been cycled numerous times, begin to loose capacity. So you do need to be somewhat aware of the age of the pack, and the number of times it's been cycled. If you have a smart charger that will discharge/charge your pack, every couple of months it's a good idea to cycle the pack and check the maH delivered at a 0.5amp load. If you're only seeing 800maH delivered, it may be time to change the pack.

I've flew a H9 Arrow trainer with 4 standard analog servos four or five times in a day, averaging 10-15 minutes per flight on a 700maH pack and never saw a problem. That pack is now five years old, and I'm noticing that it doesn't stay fully charged the way it used to over winter storage. So it will get relegated to shop use, and something new put in the plane.

RE: Estimate flight time given RX battery size and servos
 

Just use a digital voltmeter to monitor the condition of your airborne pack. It's best if you test with a small resistor to load the pack to about 300mA while testing. That is about 16 ohms (4 cell pack), and it needs to be at least a 5 Watt rating.

With Nicads you have plenty of power at 1.15 volts per cell, which mean that a 4 cell pack is fine down to 4.6 volts. The real edge however is down around 1.07 Volts and that is where things start to go bad in a hurry.

NiCad batteries have been around for over 50 years now. The early ones were very temperamental, but the last 30 years they gave very good and reliable service as long as they are not abused. Vibration is the worst enemy of nicad cells.

With that capacity of cell and a general sport plane (40 sized or smaller), I would expect close to two hours of operation would be possible, but without testing you will never know.

You can buy a cheap DVM and a resistor for the price of a couple six packs (I used that price in honor of one of my old electronics professors that would hold up a microchip and relate it's cost in pitchers of beer).

RE: Estimate flight time given RX battery size and servos
 

You can also charge the battery after 3 or 4 or 5 flights (depending on your courage) and keep things safe.

Pushing things to the point of exhaustion is not necessarily in the best interest of neither the hobby nor the the owner of the plane, not to mention the potential victim that could be hurt by something falling out of the sky that weighs about 8 pounds traveling at about 50 miles per hour.

Why push it? What's the point? It can't be just to get another flight because with today's fast chargers, we can be back in the air in less than an hour. So, bring two planes and charge one while flying the other, or sit down with your buds and have a conversation about RC flying... or whatever tickles your fancy.

The point is to keep things safe. We say this over and over again. A Sanyo 4 cell 700 ma pack costs a whopping $11.99 - http://www3.towerhobbies.com/cgi-bin...&I=LXFD90&P=ML as an example.

So, buy two. Check the voltage (personally, I use a voltwatch2) before and after each flight. If it's questionable, charge the battery... fly a different plane or pull the battery pack out and replace it with a fully charged pack and charge the one that you pulled out.

CGr.

RE: Estimate flight time given RX battery size and servos
 

You are quite right. Battery is the cheapest part of the plane so it makes sense not to take a risk. I'll check the capacity of my battery with a power resistor and voltmeter and will discard if there is any doubt at all. Incidentally, my level of courage has never exceeded 4 flights. I was just trying to check if Ishould be a bit less courageous and Ithink I am OK (based on all the responses her).

Again, thank you all for your very helpful responses.

RE: Estimate flight time given RX battery size and servos
 

I'm not suggesting "Pushing things to the point of exhaustion". I explained that the true limit of the nicad cell is at 1.07 Volts. Too often I read of some nervous Nell that is only getting two flights on a pack because they used it until it only reads 4.8 Volts, and this myth that the pack is about exhausted continues to be propagated over threads like this. Half truths and flat out wrong information does nobody any good, so I use facts. I have been flying RC for 41 years and and have an engineering degree in electrical engineering, so I do know what I am doing concerning nicad packs.

On the other hand, while the nicad has been well developed, it is on it's last legs due to environmental concerns. I doubt we will see them at all in hobby applications 5 years hence.

As far as the Voltwatch - I don't consider them accurate enough for my use. The chip specifications just are not that good. Some like color organs, but I had enough of them back in the 70's.

RE: Estimate flight time given RX battery size and servos
 

Instead of guessing how long a battery will last and how much the servos draw, just test it?????? Sit on the couch and move your ailerons and elevator while watching TV. Start the clock when you first start moving the sticks; stop the clock when the battery dies. When your battery finally dies, divide that by your flight time, and that’s how many flights you can fly on that battery. There is already a huge % error built into this method, the fact that your control surfaces are moving nonstop, what this means is if your flight time is 10 min and your battery lasts an hour you can safely get six flights in. Realistically that battery would be fine for 12 flights due because I doubt a trainer is constantly doing rolling circles, but cut it in half to be safe and you should be good to go.

electricity can be hard to understand and trust for some, the easiest way to understand it is to see it in practice. Instead of guessing just test it, if your battery lasts an hour of constant elevator/aileron servo opperation (just move the gimbals around the corners) and you know for a fact you are not doing that in the air, then you should have plenty of juice for 6 flights at 10 min per flight

if want to be even more careful, after 10 min of running your servos, stop, test your voltage with a voltmeter, write it down, rinse and repeat. next time bring this piece of paper out to the field, take your voltage after each flight and compare the numbers

RE: Estimate flight time given RX battery size and servos
 

mranga, If you are flying with an "old" battery pack and you do not know it's capacity you are putting your plane and everyone around you when you are flying in jeopardy. Do not use a resistor and a voltmeter. You need a good somewhat expensive cycler and there are plenty out there. Measuring you battery voltage before each flight is a good practice. THE MOST IMPORTANT ONE IS KNOWING THE BATTERIES CAPACITY. Please know it before you fly again. Dan.

RE: Estimate flight time given RX battery size and servos
 

I have to warn against the idea above.
The flight loads under speed exerted upon ailerons, elevators, and the rudder greatly increase the resistence to movement. Assuming battery consumption in flight is the same as static loadings is not accurate.

I fly some pretty light designs and have some relatively tiny flight batteries.
The way I determine the mAh size I need is to start with a robust battery around 750 mAh. I fly the model wildly... consuming about as much as I can.
As quickly as possible, I check the "end of flight" voltage (this is critical because a battery can rebound and if I check the voltage 30 minutes after flight, the voltage will read much higher than the actual, post-stress voltage).
I then carefully recharge the battery and observe the amount of mAh it took to restore full capacity.

Meanwhile, I begin downsizing the battery. I drop to 600 mAh and repeat the careful monitoring of mAh consumption and voltage.

Once I determine a specific "consumption", I get a battery of slightly larger mAh capacity and begin using it. I fly shorter flights at first, always checking the health of the battery at the end of the flight.

While I understand every bit of being conservative with flight packs, I also hate putting a 6.7 ounce battey into a 2-3/4 pound model when I can safely get by with a 40 gram 350 mAh pack.

RE: Estimate flight time given RX battery size and servos
 

Well, this is tough to calculate, since you don't know what your servos will actually draw in flight. If you just assume something, your calculation will become useless. From flying full scale, I know the forces on your controls increase greatly with speed and other flight conditions.
Then I have seen batteries loosing their capacity quickly.
My 2 cents is - get a good charger, discharge and cycle the batteries regularly and monitor the capacity.
Also - a 'volt watch' in your plane can save you lots of trouble.
A 600 mah battery is ok for your trainer with 4 servos, but with more 'activity' or more servos I'd get a higher capacity battery.

RE: Estimate flight time given RX battery size and servos
 

A volt meter with a load is a good thing to have. I check my packs before each flight then usually check before each flight after that. When I get close to say 4.8 volts then I put the pack on a field charger and start the process over again. On those real lazy days I check before the first flight then on the third flight. My flights last about 10 minutes and I get about 5 flights per charge. Even if I have enough juice left in the pack I put it on the charger. Most of my servos are digital and all of my planes are stunt planes of some sort, I don't have a float around plane at the moment. I only take the trainer out when I have a student or the wind is up. My packs are all nim