I have been using 4 of your MPS-1A motors/props to power a FREE FLIGHT scale model airliner:
A Lockheed L-188 For FAC competition. (Flying aces club)
The motors are wired Parallel through a high voltage timer (made by a friend) and the set-up is currently powered with a HOKAM 340 mah 2 cell Li-Poly pack.
The timer is usually set for a maximum of two minutes,
The model has been very successful, And I'm very happy with the quality and reliablity of your motors....... yet she is often marginal on power for the first 15-25 seconds of flight, until she slowly gains a bit of altitude.
From : Don Stackhouse
Sounds like it needs a little faster launch airspeed. What you describe sounds like an airplane that's not at its best rate of climb airspeed, probably a little too slow, and it has to struggle its way through the extra induced drag in order to accelerate to the airspeed it needs.
I am interested in trying a 3 cell 340, or perhaps a 250 mah li-Poly cell pack to give more grunt power,
and allow the model it fly in less than PERFECT weather. Can the stock MPS-1As (4 in parallel)handle this upgrade?
I don't think that either of those batteries can handle that many amps.
A single MPS-1A will run OK in short bursts (5-10 seconds) on three 250 mah cells. After about 60 seconds it will have an internal melt-down. The nylon brackets that hold the windings on the armature will melt, spraying themselves into nylon wool all over the inside of the motor casing. Next, the windings will come off of the iron core laminations and get wound around the outside of the armature, until they jam between the armature and the magnets. I always thought that the brushes would be the first to go, but it's those nylon brackets that are the Achilles' heel.
A friend of ours tried a 3-cell 1200 mah pack in one of our B-17's. It handled the short bursts OK, but started melting down motors with sustained runs.
There's a lot more to it than just the motor. Actually the prop is probably a bigger factor.
The motor pulls whatever current it can get to try to spin the prop at an RPM equal to the motor's Kv times the applied voltage. The load put on the motor by the prop at a given RPM determines what the current is, and excessive current is what fries the motor's insides.
The power absorbed by the prop is proportional to the cube of the RPM, so a 1.5 times increase in voltage (and therefore the RPM) from adding the third cell would theoretically result in more than three times the power and over twice the current. In actual practice it isn't normally quite that much, because when you start pulling that much current from the battery, the voltage drops off. The final result depends on what current and Kv-times-voltage achieves an equilibrium with the power absorbed by the prop.
You have several options. First of all, you need to figure out how much your batteries are actually good for. A 250 mah, 8C rated pack is good for a maximum of about 2 amps, and that's for short runs. For sustained runs I'd try to stay below that. Since an MPS-1A by itself pulls about 0.9 to 1.0 amp at about 5000-5100 prop RPM (when supplied from a battery that's up to the job), four of them in parallel is already badly overworking your 250 mah battery. The only reason that it isn't dying already is because the voltage depression is pulling the voltage and RPM down enough to get the watts and amps within what the battery can supply. Even so, you're probably damaging the battery.
Check the RPM at the beginning of a flight. My guess is that it will be something less than 5000, and even a small difference in RPM is a lot of watts. Now that I think about it, this also might explain why you're seeing that sluggish initial climb rate. The props are overloading the battery to the point that it gets bogged down, so that the increase in watts from the extra current is less than the decrease in watts due to the drop in voltage.
It's sort of like trying to pull your car away from a standing start in fourth gear. The motor is making massive, maybe even dangerous amounts of torque (although still not enough to keep the motor's RPM up to normal levels), but the RPM is so low that the total horsepower is low. By shifting to a lower gear, the motor is able to rev up to where it can make some serious horsepower, and the acceleration is much better.
Once the plane gains some airspeed, the props can rev up a little faster. This reduces the current, increases the battery's voltage, and the lower current also improves the motors' electrical efficiency, resulting in more watts actually getting delivered to the prop shaft as mechanical energy.
So, one option is to cut the prop diameters down until the motors can rev up to about 5000 prop RPM. The problem is that cutting the prop diameter costs you a substantial penalty in prop efficiency (and therefore thrust), especially at low airspeeds.
A possibly better option is to reduce the props' pitch, the propeller equivalent of shifting your car to a lower gear. There isn't much choice in 5" props that would fit, but there is a 7-3.5 that we're considering for a project currently in early development. You could cut four of those down to around 5" diameter.
You could also switch to a set of MPS-2B twin-motor power systems. These have better electrical efficiency, so they would deliver more power to the propshaft. However, they would also try to pull even more from your already overtaxed battery.
The best first step is probably to try to get the needs of the airplane more in line with what the battery can supply. The newest 340 Kokam cells are rated for 20C for short bursts, which equates to 6.8 amps. In my experience that's probably optimistic, but 5 amps is probably reasonable. That means a 2-cell 340 pack should be able to supply one amp each to your four motors, total of four amps, without breaking much of a sweat, which is probably a lot more than the 250 pack is doing now. This should give you a pretty substantial power increase. Since climb rate is extremely sensitive to even small power changes, particularly on planes with marginal power to begin with, that should result in a very large increase in climb rate.
To put that in perspective, imagine an airplane that needs 95% of its installed power just to sustain flight, leaving only 5% to lift its weight to a higher altitude (i.e.: "climb").
Now, let's increase the total installed power by only 10%, leaving all other parameters the same. Now we have 15% of the original power available to lift the plane's mass uphill in a climb. This modest 10% increase in installed power just TRIPLED the climb rate!
Since the max voltage from the batteries is still only 8.2 volts, your MPS-1A's will still be within safe current and RPM levels. Your poor, grossly overworked 250 pack was suffering from depression, and so it was unable to supply the motors what they needed. The latest generation 20C-rated 340 mah 2-cell pack is better able to support the motors' requirements without getting depressed and giving up.
If that still doesn't do it, then I think we're into installing twin motors with either the 5-4.3 props you've been using (the MPS-2B), or the twin motors with the bigger 6-5 props (the MPS-2A). Then of course there's the MPS-2C we're working on (but not quite available yet) with the 7-3.5 prop. With twin motors you now have the option for running safely on either two or three cells, for a huge increase in power, depending on your prop choices. For example, an MPS-1A on two cells (not overworked cells) gets about 6 watts. An MPS-2A (6-5 prop) gets about 12-15 watts on that same 2-cell battery, and up to 25-29 watts on a 3-cell pack. The MPS-2B (twin motors, 5-4.3 prop) gets about 18 watts on three cells.
Of course on all of those twin-motor systems you need a battery with enough ability to pump out the amps to feed four motor systems.
The other thing you may want to consider is (perish the thought!) nickel-based cells!
Consider, you're running four motors (ideally at 1 amp each, which is a lot more than I expect you're actually getting now) for 2 minutes. That totals to only 133 milliamp-hours for the run. For a Li-poly battery sized to fit that requirement, say about 150 mah, that's equivalent to about 27C, which is beyond what even the latest Li-poly cells can deliver. You're carrying around much bigger capacity Li-poly cells than the mission's endurance requires, just to satisfy the current requirements.
However, about seven 150 mah or so NiMH or nicad cells (GOOD ones, assuming you can still find them!) could handle that requirement nicely, for probably a similar weight. For very high current, short-run applications (like yours), the nickel-based cells can sometimes still make sense.
Don Stackhouse
DJ Aerotech
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May 17, 2012
