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Old 09-06-2014, 12:46 PM   #1
rayh
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Default power system

I am in the process of building a Bristol beaufighter and the motors are quoted as 2 14 turn 700s with 3.7:1 gear boxes as the plans are ten years old can anyone recommend more modern units and power system to match
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Old 09-06-2014, 01:53 PM   #2
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What is the wingspan and estimated all up weight?

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Old 09-06-2014, 03:09 PM   #3
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Plus diameter, pitch and blade count of the props you want to spin would be helpful.
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Old 09-06-2014, 03:29 PM   #4
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700 can motors from "way back when"... Generally 9.6v and about 50 to 70 amps with some going above 90 amps. Lets call it 600 to 700 watts desired power. (input)


Its really hard to guestimate the desired prop RPM because of the assorted windings possible. Some were wound for about 2000 kV. Some a lot more and some a lot less. I can't remember the kV expected from 14 turn.

I'd say drop in E-Flite Power 32 or rough equivalents. The old brushed motors with ferrite magnets were not as efficient as a brushless with rare earth magnets (the magnets make a huge difference)
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Old 09-06-2014, 03:30 PM   #5
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Default beaufighter

estimated AUW 16lb w/s 93inches
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Old 09-06-2014, 03:51 PM   #6
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You'll come out lighter with LiPos + brushless... by quite a bit. Half the battery weight and you'll still get double the flight duration.

But it sounds like they may have opted for a bit of a wimpy power system for that large and heavy of a model.


Found a comment about what sounds like your model:

A more modern power plant would be 2x ten cells and a pair of AXI 2820s or so..probably propped down a bit to allow less current...

he has 28 cells in all in that thing..no wonder it was heavy.
2820 is not awful far from a Power 32
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Old 09-06-2014, 04:46 PM   #7
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Default beaufighter

Thanks will look into that one
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Old 09-06-2014, 06:22 PM   #8
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Originally Posted by rayh View Post
estimated AUW 16lb w/s 93inches
16 pounds, 93 inches? A good number to start with is 100 watts per pound. That comes to around 1600 to 2000 watts. For a twin that's 1000 watts per motor.

As for me, with a model with a lot of time into it, its not a good idea to go cheap on the motors. I've seen to many twin glow powered models lose an engine, and go in a second later.

One very good line of motors that can be run at their rated power levels is the Hacker A50 line. Another good line is the Hyperion motor mfgs.

I still wonder about a 93 inch wingspan model only weighing 16 pounds. That is a very large model. My 82 inch Giant Big Stick weighs 17.5 lbs.

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Old 09-06-2014, 07:04 PM   #9
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many thanks still on a learning curve with electrics
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Old 09-06-2014, 07:48 PM   #10
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I've had a 12 ft span plane at 5 lb... Span vs weight is not the issue.

Weight to wing area determines minimum airspeed to maintain level flight.

The "rule" of 100 watts per lb is for general sport aerobatic performance. They'll usually fly fine at 30 to 40 watts per lb. That's why you don't need much more than 50% throttle for just cruising around. Some can fly on less. Many full scale "performance" aircraft have had less than 50 watt per lb (less than 0.5 : 1 power:weight) its only recently that some are approaching 100 watt/lb (1:1 power:weight) for full scale competition aerobatics.

One of the big issues of RC model twins going single-engine-out is excess power... So much power on one side that without the other to balance it you can't compensate with the rudder. Its usually best to chop power, stabilize the aircraft then slowly apply power and find where max is that you can still keep it straight with reasonable rudder and aileron. Expect it to be under 50% throttle for most RC twins.
Some can fly fine single engine full power... but those are because of the designer planning for it and adjusting the thrust line.
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Old 09-06-2014, 10:35 PM   #11
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Two of these 0.90 Gas Conversion motors Will give you all the power you need $99.99 for a Motor and ESC. These motors are made By Welgard, They are a very good Quality Motor, The Welgard 5330 series has been has been used in F3A competition aerobatics world wide with great success.

http://www.graysonhobby.com/catalog/...esc-p-328.html



Motor Stats
Wire Winds / Turns: 16
Ni-Mh Battery : 16 - 24 cells
Li-Po Battery : 5 / 8 cells
RPM per volt : 380
Max efficiency : 88%
Max efficiency current : 18 - 40A (>81%)
No Load Current / 10V : 1.5A
Max Current : 60A / 60 sec
Dimension : 50 x 65mm
Shaft : 6mm
Max Watts: 1300W
Continuous Watts: 1000W
Weight : 400g (14oz)
Recommended Model Weight : 3,000 - 6,000g (6.6 lbs to 13.25 lbs)

Recommended Propeller without gearbox
16 cells Ni-Mh : 18 x 10
20 cells Ni-Mh : 16 x 10
24 cells Ni-Mh : 14 x 8
7 cells Li-Po : 14 x 10

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Old 09-06-2014, 11:46 PM   #12
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thanks for the input
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Old 09-07-2014, 05:52 AM   #13
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Originally Posted by rayh View Post
thanks for the input
You might. Check out one of those computer programs such as www.motocalc.com to help with your power selection.

Motocalc has an opinions feature that provides a guide on how a particular power system will work on your model.

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Old 09-07-2014, 06:32 AM   #14
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Thanks will have a look
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Old 09-08-2014, 01:52 AM   #15
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Originally Posted by CHELLIE View Post
Two of these 0.90 Gas Conversion motors Will give you all the power you need $99.99 for a Motor and ESC. These motors are made By Welgard, They are a very good Quality Motor, The Welgard 5330 series has been has been used in F3A competition aerobatics world wide with great success.


http://www.graysonhobby.com/catalog/...esc-p-328.html
IMHO, Chellie nailed it with the above power system.

I just ran the numbers through www.motocalc.com, and got the following results from motocalc.

I don't know the wing area of your model, so I used 1400 Square inches, with a weight of 220 ounces, not including the motor/esc/battery system.

I also don't know the winding resistance of these motors, so used 0.04 Ohms. Hopefully that is fairly close.

Motocalc predicts that each motor will turn a 14X8 prop at 8200 RPM while pulling 44 Amps out of a 7S 3700 Milliampere Hour LiPo battery pack. That comes to 2100 watts for both motors, and about 110 Watts per pound of airplane.

With each motor weighing 14 ounces, using a reasonable level of 100 watts per ounce of motor weight, 1000 Watts would be reasonable for each motor.

The wing loading is 31 ounces per square foot, with a stalling speed of 26 MPH, assuming a total weight of 300 ounces.

Rate of climb with this setup is predicted to be around 1700 Feet per minute, a decent number for your airplane.

Nice thing about this power system, you can always back down on the power input by going to a 13X8 prop, or even a 12X8 prop. Or, if you want more power, you could go to a 14X8 which will put you right at the maximum power rating of these motors. I don't know how these motors handle running right at their max power rating though.

Below is the motocalc opinion on this setup.

MotOpinion - Bristol beaufighter
Sea Level, 29.92inHg, 59F

Motor: Greyson GH4130; 380rpm/V; 1.5A no-load; 0.04 Ohms.
Battery: FlightPower 3700 (20C); 7 series x 2 parallel cells; 3700mAh @ 3.7V; 0.0037 Ohms/cell.
Speed Control: Castle Creations Phoenix 80; 2 controls (separate); 0.001 Ohms; High rate.
Drive System: Beaufighter Greyson; 2 motors (parallel); 14x8 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: Bristol beaufighter; 1400sq.in; 301.2oz RTF; 31oz/sq.ft; Cd=0.054; Cl=0.47; Clopt=0.71; Clmax=1.24.
Stats: 115 W/lb in; 100 W/lb out; 26mph stall; 34mph opt @ 63% (41:49, 89F); 42mph level @ 76% (29:30, 96F); 1749ft/min @ 36; -298ft/min @ -5.7.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (39.8A) falls approximately between the motor's maximum efficiency current (30.5A) and its current at theoretical maximum output (311.1A), thus making effective use of the motor.
The voltage (24.7V) exceeds 12V. Be sure the speed control is rated for at least the number of cells specified above.

Aerodynamic Notes:

The static pitch speed (64mph) is within the range of approximately 2.5 to 3 times the model's stall speed (26mph), which is considered ideal for good performance.
With a wing loading of 31oz/sq.ft, a model of this size will have trainer-like flying characteristics. It would make an ideal trainer, for use in calm to light wind conditions.
The static thrust (299.4oz) to weight (301.2oz) ratio is 0.99:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (177.6oz) to weight (301.2oz) ratio is 0.59:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.

General Notes:

This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.


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Old 09-08-2014, 07:50 AM   #16
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Thanks for all the replies
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