How do I figure out how large of a battery pack I need? I will have 4 servos plus the receiver in an old Bipe I'm rebuilding given to me by an old friend.

What servos?

6V 1000mAh will be more than enough. 700mAh will be ok.
Look at the battery as a fuel tank the more mAh the battery has the more fuel in the tank. 6V is the norm any more

If the servos are the typical medium priced ones, then you can get by with just about any 4 or 5 cell NiMh or NiCad battery. You could also use a 2 cell LiFe but realize that will require a proper charger for the LiFe. For many years, we all got by with the 4 cell NiCads with no problems using the Futaba or Hitec standard servos. Now that many are using 2.4GHz units which are far less tolerant of microsecond voltage drops (loose lock and have to reacquire signal) many have gone to the 5 cell batteries or are using ESC's which put out a typical 5 volts to the servos. You should be able to get by fine with 700 mah batteries, in fact the 500 mah would give you several safe flights before recharging. Now, if you are using the newer digital servos, they demand more power so use the larger mah batteries for them.

Which servo you use will tell you how much power you're going to need. Some servos work on 4.8 volts only, some on 6.0 volts only and others will work on both. Digital servos generally require more amperage than analog as well. For one of my boats, I use the Futaba S9156 digital servo for the rudder. This thing works fine on a standard Futaba 4.8 volt nicad pack but will operate faster and have more torque using a 6 volt pack, as stated on the package and under the tech notes on the Tower Hobbies web site

Futaba S 148 4 of them.

Thanks all for your response and help! Doc

Those are low demand servos. You can probably get 5 safe flights out of a 700 mah pack. I'll second the suggestion to go with a 6v pack, especially if you are using Spektrum radio equipment.

Agreed, since the 148 only draws .8 milliamps when idle, the load to the battery packs is almost nil at 3.6 milliamps plus the receiver draw, 30 milliamps at rest using a Futaba R2004GF in this example. With a combined load of 33.6 milliamps at rest, you could leave the receiver on for 20+ hours before the battery pack failed, if fully charged. Obviously, when in flight, the load is greatly increased. If you were using the higher performance and/or digital type servos, all specify that they have a high current draw so a larger pack would be needed for equal flight time. One thing to be aware of is that with the 148 servos, if you use a 6 volt pack, the speed and torque are both 25% greater. The speed won't be an issue as it's only .06 seconds faster lock to lock. The higher torque would, however, allow the use of larger control surfaces or require a lower current draw for smaller ones

If he had digital servos, wouldn't that increase his requirement? Don't digital servos require much more electricity than analog?

I actually addressed that in the middle of my last post but I will point it out in the cut and paste below:
Agreed, since the 148 only draws .8 milliamps when idle, the load to the battery packs is almost nil at 3.6 milliamps plus the receiver draw, 30 milliamps at rest using a Futaba R2004GF in this example. With a combined load of 33.6 milliamps at rest, you could leave the receiver on for 20+ hours before the battery pack failed, if fully charged. Obviously, when in flight, the load is greatly increased. If you were using the higher performance and/or digital type servos, all specify that they have a high current draw so a larger pack would be needed for equal flight time. One thing to be aware of is that with the 148 servos, if you use a 6 volt pack, the speed and torque are both 25% greater. The speed won't be an issue as it's only .06 seconds faster lock to lock. The higher torque would, however, allow the use of larger control surfaces or require a lower current draw for smaller ones

Thanks for all of the responses! They have helped me and taught me what I needed to know as I learn more about this hobby of ours!

For the typical type and size airplane I use almost universally five cell Nimh 2000mah. These are double A sized cells and the packs are only slightly heavier and larger the 700 and 1000 types being suggested while offering far more flying time and or redundancy in terms of endurance.

John

That's what I went with at the suggestion of one of my club old timers. Now to mount the pack as far forward as I can. Thanks!

Excellent choice Doc. I think you will enjoy that combination in a lot of airplanes. Now since you spoke of the need to mount the battery as far forward as possible and that the airplane is an "old bipe" a type that has often suffered because of difficulties in getting the center of gravity sufficiently forward.

I would like to offer a solution that is quite simple needing only just a little effort: and that is put the battery along side or under the engine all the way forward, just behind the prop. Most designs can accommodate this inside the cowling. Its just a matter of fabricating a plywood plate on a base that can be glued and screwed to the firewall. Tie wraps can secure the battery to the plate or a little box for the battery can also be used. I have done this with many airplanes and can preclude the use of lead. In most cases the batteries actually run cooler as well.


John

There is absolutely no room in the engine compartment! The Bipe is a Sportster 40 which was built in the early 90's by a friend. There's plenty of room behind the firewall which is where the fuel tank (8 oz) and old 4.8 V batt was mounted. I will do the same and design a mounting similar to what you suggested.
While were here...The engine is an old Enya 45 model 5001. It started right up after I cleaned the grime it collected stored in my friends garage for so many years. Was running strong while i was adjusting the needle and then the nut, washer and prop came loose and flew a across my garage! I've had it running previously and failed to check the nut for tightness. Is it common for a 2 stroke to do that? I know my 4 strokes do on occasion.

In the case of a wood prop after long term storage it is quite common for them to loosen. They do loose moisture over time and do shrink a bit.

In the case of an old nylon or glass filled prop after a long term storage especially if you did not own the airplane going into storage folks do at time put props on without tightening during or after maintenance etc.

John

Futaba S148 servos are still available and do use either 4.8v or 6v.

http://www.futabarc.com/servos/analog.html

Got 'em!