confused..need watts vs weight rule of thumb please - RC Groups
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Nov 13, 2005, 12:20 PM
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confused..need watts vs weight rule of thumb please

I'm a bit confused as to how much power I will need to fly my scratch built Vickers Vimy. Any help would be appreciated,,,
wingspan:56 in
length: 34 in
twin engined bi-plane
approx weight:30-35 oz

I was thinking of using 2-gws 350c motors/gearbox with 4 blade 9x5 props as a starting guess. I'm looking for somewhat scale (slow) speed flying, but was wondering if these were a bit too much. Could I use smaller motors? I dont want to go brushless, cant afford it! Thanks for any comments

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Nov 13, 2005, 12:42 PM
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adamg's Avatar
I'm not a scale guy, but I recall hearing something like 75 watts per pound. Brushed motors have lower efficiency, so maybe aim for 100 watts per pound.
Nov 13, 2005, 12:56 PM
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Dr Kiwi's Avatar
If you are using 3s Lipo you won't want to put more than ~8A (@ 10v = 80W) through each of those motors - so the best you can hope for is ~75-80W/lb. That should be (just) enough - slow "floater" trainers can get by on 40W/lb, but you are going to have to overcome a lot of drag on your Vickers bipe. The other concern I have is what sort of gearing (8.5:1) will 4-blade 9x5's need to keep amp draw below 8A - and then will you have enough pitch speed (26mph)?

This is what Motocalc says:

Motor: GWS EM350; 5298rpm/V; 1.4A no-load; 0.173 Ohms.
Battery: Thunder Power 920 (G2); 3 cells; 920mAh @ 3.7V; 0.012 Ohms/cell.
Speed Control: Castle Creations Pixie 20P; 0.0025 Ohms; High rate.
Drive System: Direct; 9x5 4-bladed (Pconst=1.11; Tconst=1) geared 8.5:1 (Eff=80%).
Airframe: Anything you want.
Motor Amps = 8.1
Motor Volts = 10.8
Input (W) = 87.2
Motor RPM = 47433 Whoa!
Prop RPM = 5580
Thrust (oz) = 17.4
PSpd (mph) = 26.4

8-cells will be easier on the motors: though in this example amps are getting up there.

Motor: GWS EM350; 5298rpm/V; 1.4A no-load; 0.173 Ohms.
Battery: Sanyo HR-4/5AUP; 8 cells; 1700mAh @ 1.2V; 0.006 Ohms/cell.
Speed Control: Castle Creations Pixie 20P; 0.0025 Ohms; High rate.
Drive System: Direct; 9x5 4-bladed (Pconst=1.11; Tconst=1) geared 6.6:1 (Eff=80%).
Airframe: Anything you want.
Motor Amps = 9.7
Motor Volts = 9.1
Input (W) = 88.0
Motor RPM = 36088
Prop RPM = 5468
Thrust (oz) = 16.7
PSpd (mph) = 25.9
Last edited by Dr Kiwi; Nov 13, 2005 at 01:09 PM.
Nov 13, 2005, 01:05 PM
I don't want to "Switch Now"
pmackenzie's Avatar
You might find this article by Keith Shaw on twin electrics useful. It is pre-lipo and pre-brushless, but the basics still apply.
Lots of tips on building scale models in general, again by Keith Shaw, are available here
Pat MacKenzie
Nov 13, 2005, 06:24 PM
We want... Information!
Bruce Abbott's Avatar
The full-size Vickers Vimy had a power loading of 43W/lb. With twin 350's geared 5.33:1 on 7 cells you should get a similar result (estimating 41W/lb output). Smaller motors won't do unless you can achieve significantly lower weight.

Motor: GWS EPS350C (ba); 5370rpm/V; 0.9A no-load; 0.24 Ohms.
Battery: Gold Peak GP200AFHR; 7 cells; 2000mAh @ 1.2V; 0.0044 Ohms/cell.
Speed Control: Kontronic SUN3000; 0.003 Ohms; High rate.
Drive System: Generic 9x5x4; 2 motors (parallel); 9x5 4-bladed (Pconst=1.31; Tconst=0.974) geared 5.33:1 (Eff=90%).
Airframe: Vickers Vimy 56 inch;; 35oz; 5.6oz/sq.ft; Cd=0.105; Cl=0.69; Clopt=0.75; Clmax=1.2.
Stats: 74 W/lb in; 41 W/lb out; 11mph stall; 14mph opt @ 62% (28:42); 15mph level @ 64% (27:10); 610ft/min @ 29.4; -186ft/min @ -8.6.
Conditions: Sea Level, 101.3kPa, 20C

AirSpd (mph) = 0.0
Batt Amps = 21.1
Motor Amps = 21.1
Motor Volts = 7.69
Input (W) = 162.1
Loss (W) = 72.6
MGbOut (W) = 89.5
MotGb Ef(%) = 55.2
Shaft Ef(%) = 50.5
Motor RPM = 27688
Prop RPM = 5195
Thrust (oz) = 33.1
PSpd (mph) = 24.6
Time (m:s) = 5:41

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (9.38A) falls between the motor's maximum efficiency current (4.69A) and its current at theoretical maximum output (12.7A), thus making effective use of the motor.

Possible Aerodynamic Problems:

The static pitch speed (24.6mph) is less than 2.5 times the stall speed (11.2mph), which may result in reduced performance at typical flying speeds and a low maximum speed. This situation is usually acceptable for an electric sailplane or other slow-flying model.
Pitch speed can be increased by using a higher pitched and/or smaller diameter propeller, a lower gear ratio, a higher cell count, or some combination of these methods.

Aerodynamic Notes:

With a wing loading of 5.59oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (33.1oz) to weight (35oz) ratio is 0.945:1, 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 (15.3oz) to weight (35oz) ratio is 0.436:1, 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.
Nov 14, 2005, 06:54 AM
jrb's Avatar
From the FAQ ( ): :

Q. What is the watts/pound rule?
A. Jim Bourke
Basically, the rule states that 50 or 60 watts per pound (110 watts/kg) is needed to produce good sport flying characteristics. Some airplanes, such as gliders, can use a smaller ratio (30 watts/lb or 66 watts/kg), while others, like pylon racers, may need a much higher ratio (80 watts/lb or 176 watts/kg.). This ratio is normally computed using watts "in". There is an implicit assumption that the motor being used is at least 70% efficient. cheap "can" type motors, or motors that are being operated beyond their specified current rating, will produce a misleading ratio.
Nov 14, 2005, 09:05 AM
Light and floaty does it
Work in Progress's Avatar
Very slow lightweight "floater" models, like this, can manage on a lot less than 50 watts/lb, especially if (like this) it is very draggy and therefore pretty much a one-speed aeroplane. But an excess of power can often be useful in a pinch. I think Bruce has it summed up in his Motocalc model above.
Nov 14, 2005, 11:00 AM
You win again, gravity!
Muxje's Avatar
Well, the rule of thumb still leaves a lot to interpretation. What is "good"? What is "sport flying"? I'm building a heavy'ish scale fighter at about 7,5lb all up weight, and I've had advice ranging from 400W to 1000+W.

Looks like the motocalc gives better advice though.
Nov 14, 2005, 11:17 AM
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vintage1's Avatar
If you delve through the maths of this you will find that a clean slow flyer can operate on about 25W/lb even allowing for can motor inefficiency.

Experience tells me that two 350 engines will be fine for that sort of weight.

Use 2s LIPO and prop for 6-7A apiece - should give about 100W total input, which is fine for a pound and a half of model.

Beware of too low pitch speed though - you MUST have enoiugh of it or you will have a nightmare model to fly.

Motocalc will sort it all out and is pretty close on can motors.
Nov 14, 2005, 01:24 PM
Plane dodger
KillerAir's Avatar
I agree with Vintage here. Recently I have been flying a high wing trainer ( SA SQuiRT 400) AUW is 16oz. Powered by a 3s pack of tp 1320 prolites and a Little Screamers motor propped with a gws 8x6HD I get 38-39 minutes of relaxed flying. That equates to 1.9A current average and roughly 20w/lb. Total power is ~80watts/lb. One thing that you have to consider as part of the equation is wing loading. Lightly loaded plane obviously require less power to fly.
Dec 12, 2005, 01:53 PM
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peterangus's Avatar

Would you like to hear from someone who has actually flown a slow electric biplane?

At 6 oz/sq ft you will need only about half the power-to weight recommended by Shaw/Kopski.

The first flight attempt with my 0-400 was a disaster--because of wing flexing caused by excessive power [and speed]. Most models are safe with lots of power, but you must be cautious with delicate models, especially if the wings have low torsional stiffness.

The three shown below have a wing loading about twice yours. I don't fly them in winds over 6 mph.

For my suggestions on power/weight relative to wing loading see
Last edited by peterangus; Dec 12, 2005 at 02:06 PM.
Dec 12, 2005, 02:03 PM
Personally, I like 200 watts / lb, but then, am trying to do 140 to 160 mph
Dec 12, 2005, 08:20 PM
Registered User
vintage1's Avatar
most of my slow stuff operates at 70W/lb or so, but it doesn't need to.

I get extremely high rates of climb, and actually end up pooitling around at about half throttle.

IF your weights are as indicated, the only model I had with that sort of flying speed and wing loading, was Peter Rakes IPS powered Sperry monoplane. (Alas destroyed by 'friendly fire' after I could see no other way of getting it out of the tree it had been up for 6 weeks). That was about 7oz AUW and flew on about 18W on a 2sLIPO pack at not brilliant efficiency ( I use 3s LIPO now on those motors)

So about 40W/lb input. Probably 20W/lb output

Now, you want to know what 40W/lb on a model that stalls around 11mph is like.

Well, its calm weather. It can even, if you prop it right, handle a 10mph breeze...I could hold the model hovering, or open the throttle to its max speed of about 24mph, and trot upwind...BUT if there was ANY turbulence at all, I was in deep doggy-do. It simply could not overcome any sink at all. ROC was at best about 250fpm. Or 4 feet per second. Many a time I had it struggling to get out of a rolling mass of air, caught on the downside.

I tried two props - one was a larger fine pitched prop, the other a coarser pitch smaller diameter. The larger one was fine in low wind, but would never take the model upwind at all. I used to fly on nearly WOT the whole flight.
The smaller one with a coarser pitch - that Motocalc admitted was within acceptable pitch range - had a little less climb rate- not a lot in it. But a huge amount more top speed.

I can't emphasise this enough. DO trust motocalc's pitch speed comments, and try to get to 2.5x stall speed. I am concerned that heavily geared large 4 blades will simply NOT be enough to get this model to fly. You may need to go to smaller than scale diameter props and drop the gear ratio a bit.

I think if it were me I'd probably go for 2x 350 A geared and a pair of 6x5 props fettled together to make a 4 blader. on a 2s LIPO 2200 pack.

I think 9" props is heading for disaster. I reckon to keep current in bounds you will need a D box on the 350s. and according to motocalc, you will be lucky to get 19mph pitch speed.

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