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Ideas on power choice

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Old 05-04-2003, 06:09 AM
  #1
BSDGuy
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Default Ideas on power choice

I decided I would do something radical, and build a balsa kit plane. I'm brand new to the hobby, and my flying before now and up to the point of getting this bird in the air (a cub) will be various toys and trainers.

The kit was plane only, and the suggested power system was a Great Planes speed 400, gearbox and 6 cells. It is a Dare Design Cub.

The specs for the plane indicate 500+ in wing area, 24 oz weight (I believe this includes the drive recommended, radio and batt).

So I downloaded motocalc and started looking at it's analysis.

Yes, I can make a 7 cell, geared s400 drive system, but it's always a bit marginal, no matter what motor, gear, and cell count I use. To slow, too little thrust, too little batt life, too hot, etc, etc.

So, I started "upping" the power system. I changed to NiMH battries, upped the cell counts, and moved up to a speed 500, then speed 600. I tried myriad combinations of prop size and pitch, along with gear ratios, etc.

I was looking to find something with good static thrust - more than 60% of expected weight. It seems that no matter how I go up in power, I get no farther. The stall speed of the plane keeps rising as weight goes up, and I continue to run into the warning that your prop pitch speed is low - or, warnings about the motor temperature, or, too little thrust. Or, I CAN sacrafice run time.

If I gear it up, put on a smaller prop, I get a "too low current" comment out of MC, and poor thrust figures.

I moved all the way up to a high output speed 600 and many cells. Each increase in power was accompanied by an increase in weight, and the relative performance of the craft actually began falling after a certain point.

I'm unfamiliar with much of what MC says, but it appeared to me that only LiPoly cells could make a dramatic performance increase.

Also, am I actually just making bad assumptions? When MC says something is typical of a trainer, does that mean it still flies well?

Just looking for feedback on MC's reports vs real life.... Any response appreciated...
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Old 05-04-2003, 08:24 AM
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Steve Lewin
 
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Default Ideas on power choice

Aren't Cubs supposed to fly like trainers ?

While I've used Motocalc for years I've never really used the Moto-opinion bit, I just look at the numbers myself. And I always switch the motor heating feature off because it seems to me to give more accurate results. Personally I'd put a geared 6V S40 and 7 or 8 cells in it, prop it for about 10-12A and fly it. I wouldn't take too much notice of static thrust figures, the plane will spend almost all of it's life far from static. If MCalc shows a reasonable stall speed and a reasonable flying speed range it should be o.k.

Putting bigger heavier motors and batteries in a small plane rarely works well. If you really want a lot more performance then small brushless motors are the only way to go, but they're not cheap. All you will gain from Li-poly batteries is slightly lower weight. They're good for extra duration but not really for extra performance.

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Old 05-04-2003, 04:53 PM
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BSDGuy
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Default Ideas on power choice

Thanks for the input, Steve. The owner of the store that sold me the kit warned me that the recommended drive is insufficient. It flies OK, but the motor runs too hot and has really short life. That's apparently real-life experience.

Now, I'm not asking to make the plane hang off the prop or anything, and I do look at the numbers more than their opinions, my point was, that I'm not really sure what I should shoot for.

When I said the thrust was low, it said it would take moderately long runs to get off the ground. I'm perfectly willing to spend the money to get li-poly batts in a series/parallel setup to have sufficient amps to fly the plane, and the weight is WAY down. I'm really after more duration, rather than ultimate power. I considered anything less than 10 minutes WOT to be too short.

In theory, this plane has, as designed and spec'd, 6.5 oz/sq ft wing loading, which is really light. But if the speed 400 is too small of a motor to fly the plane, then I was intending to move up in size (using inexpensive motors), but moving up never really "solved" any issues. My other thought was that I was willing to use 6 cell nimh packs, even if relatively short flight, since they are c heap and swapping is quick and easy.

However, I had trouble getting enough power from 6 cells without using a relatively large motor...

What I ended up with, was using a speed 600 (graupner 1793), olympus 2.3 belt drive, 11X7 prop, 6 cell RC car NIMH pack.

All up weight of 40 oz (I threw in some extra weight just to play safe above what it should weigh), mc says stall of 15 mph, max speed of 32 mph, it should climb at 500 ft/min and run for about 10 min in flight at wot, 17 min at 80% throttle, with the motor at 110 deg.

This combo is cheap...

Next, is a speed 400 (graupner 3320), 2 3270 lipoly cells, 4:1 gearbox 10X6.5 prop. MC says it will take 10-11A static, 100% throttle.

This one has lesser performance, with a stall speed of only 13 mph, max speed of 25, climbs at 320 ft/min and has longer takeoff.

It, however, shouild fly for close to a half hour flown gently.

it costs quite a bit more, too.


Now, my point in all this is, I have no idea what to compare what to. I don't know what it should be, I don't know what's "good" or what's "bad".

There's nobody around here who flies much electric stuff, and I just want to be able to buy the parts, put them in, and have a model that can be flown well, without danger of quick failure of anything.
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Old 05-04-2003, 07:05 PM
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Default Speed 400 with 24oz plane

Hi BSDGuy - Yes, whatever you do with a 400 is going to be less than perfect, but if you can keep the weight down to the 24oz (and thus the wingloading under 7oz/sq.ft) then you have the ideal characteristics for a trainer. Going heavier and faster defeats the purpose! The suggestion of loading your motor to 10-12amps would, in my opinion, be a recipe for disaster as your motor would fry in short order - in my experience "basic can 400's" tend to overheat quickly and run erratically at over 8amps or so.

I don't have figures for a 6V 400 - mine are all 7.2V but here are some figures for them anyway. If you use a 3:1 gearbox, an 11x5.5 or 10x7 prop and 8-cells (600AE NiCd or 950KAN NiMH) you can get peak static thrust figures of almost 15oz at 7-8amps (a more reasonable and sustainable figure is probably about 12oz at 6amps). That would give you the "rule of thumb" figure of 1/2 the weight of the plane and it should fly okay.

If you want to spend some money now but have a long-lived and maintenance-free future, the simple solution is to go brushless. Here there are lots of options. (For comparison: a Speed 400 with MPI ACC 347 3:1 g/box, adaptor and 10x7 Aeronaut prop weighs, in total, about 130g).

The cheapest route would be to use an MP Jet 25/35-20 [it comes geared 5:1 and you could use a 10"-11" prop: total weight = 118g] which would save some weight, yet give you a sustainable 14-16oz @ 8amps (and a momentary peak of 22oz at 13amps!).

A Mini AC1215/20 (at 108g, again lighter than the 400) with a 2.33:1 or 3:1 will give you 14-16oz @ 10amps with a 9x6 and, with a 10x7, momentarily as much as 20oz @ 13amps. At around 8amps you'll still get 13oz or so.

An AXI 2808/20 (110g) saves you the weight of a gearbox and with a 10x7 can give you 13-14oz at (for it) an easily sustainable 8amps. If you go crazy with it it can deliver, momentarily, more than 20oz at 16-20amps!.

Hope this helps, Cheers, Phil
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Old 05-08-2003, 09:34 PM
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Default Ideas on power choice

Motorcalc is an indicator, not an absolute predicter. Consensus is that it rarely hits the nail on the head. Take a look at the GWS EPS-400 series motors. By all means, go with 7.2 volts if possible. This will reduce amperage needs. The Dare cub should easily fly on a 7.2V 400 motor with either 8 cell nimhs or 2S1P li-polys. The nimhs should give you 6-8min flight time. The li-polys would allow double or triple that depending on whether you use Kokams or Etecs, and the MAH chosen. Incidentally, the amps pulled in actual flight conditions is usally around 2/3rds static measurements

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Old 05-08-2003, 11:06 PM
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Default Motocalc etc

Hi depends -

I agree with you on MotoCalc as a predictor: it has its uses but I think it tends to produce figures which are better than one can obtain in the real world [as manufacturers do with their idealized or plain b.s. figures!]. In my various posts on motor performance ALL the figures quoted are mine, from my test set-up, not from Motocalc.

On second reading I understood your reference to "go with 7.2V" - initially I thought you meant batteries (i.e 6-cell) but I see you meant motor voltage [and, yes, I agree with you again].

Like everyone else, I've just been guessing on amp draw under flight conditions - I'm reassured to see that your prediction (2/3) is about what I had thought it might be.

Where does one get these Etec LiPolys? Am I to assume you feel they are better than Kokam?

Keep up the good work, Cheers, Phil.
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Old 05-09-2003, 02:35 AM
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Default Ideas on power choice

Brushless set up. MPJet 25/35/20, 8cell HR1700AUPs, 11x7e prop up to 12x6e
AUW of FW190 warbird, HL scorpio kit 27oz. Flights are very spirited climbs at 50 degrees+, times are over 9 minutes with more for security. Motor not even warm batts cool. Most cruising done at about 1/4 throttle. This will fly a cub very scale like even if you come in at 30 oz.

Another alternative is to go with a Mega 16/15/5 or 4 geared 2 and 2.5:1 10x7 to 11x7 prop. 8 cell Kan 950 deliver up to 20 Amps comfortaby and should have no problems hauling a Cub around, even if your over 30oz. These motors will deliver 150 watts no problem. 4 turn can delier over 200 with larger cells or go to a 10 pack.

Hope this helps
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Old 05-09-2003, 10:50 AM
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Default MP Jet 25/35-20

Hey KillerAir,

I am glad to hear that this motor performs well in the air. I got good test-stand results for it, but haven't had the chance to try it in flight. Using an 11x5.5 APC E and the same 8-cell 1700's I got it to produce a momentary peak static thrust figure of 22oz (626g): 13.4A/8.7V (117W) @ 6150rpm. At the more sustainable draw of 8A/9.0V (74W) it produced a very useful 16.2oz (460g) @ 5280rpm. Your use of an 11x7 or 12x6 means you might be pushing those figures even higher - but, I think its a very good thing you don't need to keep it at full throttle - I know brushless motors can take a fair amount of abuse, but the poor little thing's life span might be severely curtailed under that sort of stress!

Cheers, Phil
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Old 05-09-2003, 12:08 PM
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Dr.Kiwi, The 25/35 is a lot more efficient than is predicted. I am Only drawing 18-20A static and 150+w with a fresh pack of batts and in flight draw would be less. The are some others that are running 10 cells and 12x6e props and gettting unlimited vertical on 22-24 oz planes...ie the Park Cap. ( this is Hacker B20-15L power at half the cost.) If you move up in prop size you will notice that the draw is not that much more. Even if you are at the edge of its efficiency the chances are you are not going to be holding WOT for long, unless of course you have heavy thumbs. As for killing the motor...the only thing I can think of is throwing a magnet, but thats pretty unusual at these rpms. To me the weakest link is the MPJ gearbox...gears may eventually strip, but so far so good.

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Old 05-09-2003, 02:39 PM
  #10
Matt Kirsch
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Default Ideas on power choice

8 cells is the ONLY way to fly a 6V Speed 400 motor.

For maximum thrust on a floaty park flier like a Cub, use a 3:1 gear ratio and a 10x6 or 11x6 prop.
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Old 05-09-2003, 03:49 PM
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BSDGuy
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Default Ideas on power choice

Well, I started thinking differently, about how LIGHT I could go, and worked backward from there.

I came up with using an IPSD from GWS, running on 3 lithiums, with moderate gear and prop. I could get the weight WAY down by using 880 MaH Lipoly cells(and even using 1050 or 2000 or 2500 cells weren't bad), and in doing so, got the wing loading well below spec.

It will still climb at ~400 ft/min, it says, and run times while floating around are 20 to 40 minutes.

Mind you, this is still all hypothetical. I haven't built the model.

But, the point was, I was going to add some bracing and whatnot if I went "heavy" while building it, so I wanted something to shoot for while I built it. It generally makes sense to "go light" it seems, but not if the plane breaks in two in flight

For weight purposes, I'm cutting it back to 3 channels (saves ~1-1.5 oz) (two servos, wire, linkage, and extra building on the wing...), and going to use a little less plywood in a couple spots.

This has been fascinating reading. Hopefully what I'm soaking up on here will be meangingful when I achieve flight
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