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Old Feb 04, 2005, 03:58 PM
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Questions concerning motor and battery combo

I am just finishing up building my 1st scratch built electric and I have never flown electric before.

The wing span is 28", wing area = 210 in^2, high wing, very similar lines to an ugly stick.

I bought a GWS 2.6:1 gearbox with a 400 motor and the flying weight minus the battery is 16 oz.

Now, I went to the hobby shop to buy a battery(preferably Nimh) and the woman said that the current 400 motor/gearbox combo would need a large Nimh battery, which would probably be too heavy.

She said that I should probably use a 350 motor, with a 5.3:1 gearbox, and a 8.4 V 1050mAh Nimh battery.

Any suggestions as to what I should do?

Will the speed 400 draw too much current?

Anyone who flys electric a lot your input would be appreciated.

Erik
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Old Feb 04, 2005, 07:40 PM
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East Anglia, UK
Joined Sep 2002
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at 28" you have a heavy old plane.

I think you need 3s LIPO in that to keep weight down. Or 8x KAN 1050 or GP1100.

with a 2.6 box, that will fly on an7x5 (3s LIPO) or 8x6 (8xNiMh)

If its too heavy to fly, you will need a long can 480 in there.

How on earth did you get a 38" plane to 16 oz less batteries?? My latest 30" plane weighs in at 7oz....

The wing loading is high enough that direct drve maybe needed - classic would be 400 Gunther prop and 7-8 cells.
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Old Feb 04, 2005, 10:13 PM
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Re:weight

It is my 1st scratch built, and my 2nd plane I have ever built. The 1st plane that I had was IC Also, I wasn't sure about all of the forces that would be placed on the plane as it flew, therefore I over built the structure.
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Old Feb 04, 2005, 10:49 PM
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3s LIPO is definitely the way to go, but I would use a slightly larger prop for more 'oomph' (eg. GWS HD-8060).

MotOpinion - Erikhaha Ugly Stick
Sea Level, 101.3kPa, 20C

Motor: GWS EPS400C stock (ba); 2760rpm/V; 0.8A no-load; 0.234 Ohms.
Battery: E-Tec ET-1200HP (ba); 3 cells; 1200mAh @ 3.7V; 0.04 Ohms/cell.
Speed Control: Castle Creations Pixie 20P; 0.0025 Ohms; High rate.
Drive System: GWS HD-8060 2.67:1 (ba); 8x6 (Pconst=1.6; Tconst=1) geared 2.67:1 (Eff=100%).
Airframe: Erikhaha Ugly Stick; 210sq.in; 17.7oz; 12.1oz/sq.ft; Cd=0.071; Cl=0.5; Clopt=0.61; Clmax=1.2.
Stats: 85 W/lb in; 57 W/lb out; 16mph stall; 23mph opt @ 54% (41:03, 30C); 26mph level @ 58% (36:21, 31C); 1007ft/min @ 29.8; -276ft/min @ -7.8.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (9.35A) falls between the motor's maximum efficiency current (5.29A) and its current at theoretical maximum output (17.9A), thus making effective use of the motor.

Aerodynamic Notes:

The static pitch speed (42.6mph) is within the range of approximately 2.5 to 3 times the model's stall speed (16.5mph), which is considered ideal for good performance.

With a wing loading of 12.1oz/sq.ft, a model of this size will have flying characteristics suited to the intermediate pilot, for use in calm to moderate wind conditions.

The static thrust (15oz) to weight (17.7oz) ratio is 0.849:1, 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 (8.46oz) to weight (17.7oz) ratio is 0.478:1, 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.
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Old Feb 05, 2005, 04:56 AM
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Yes. 8x6 will just do - I use 7x4 on 2.33:1 and 8x6 on 3:1. This is between the two...

BUT you have calced that with17.17io AUW. if it's 16oz without a battery, its going to come in at more like 20oz with LIPO cells or 23oz with NiMh. I suspect stall speed witll be nearer 17-18mph.

I have a DD 400 model (that I bunged a 480 in actually) which was almost exactly that weight without batteries. Wing area is 180 squares, and when I first tried it on 8x600AE, it stalled and crashed on the frst two launches, it needed a hefty chuck to get airborne at all. It weoghted in at 22 oz, and with a 6x3 folder it hadn't got a lot of low down thrust.

Equipped with 3s LIPO the weight came down to 19.8oz, and static thrust way way up. and launching it is not so much of a chore, but it still stalls at 20mph or so.

Its also a very very hard plane to fly in the sense that it is hugely responsive to ailerons, and is out of sight if you take your eye off it for a second.

I think eriks plane WILL fly, but it may be a hard one for an intermediate plot to handle, especially at low speeds.
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