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-   -   3s versus 4s Performance (http://www.Wattflyer.com/forums/showthread.php?t=73743)

garyp1029 05-15-2014 12:10 AM

3s versus 4s Performance
 
The Dynam Waco calls for a 4s 2200 mah battery. Since I am only interested in scale performance, how much performance would I lose in that particular plane and motor by using a 3s battery rather than the 4s? Would it be significantly underpowered using the 3s? Thanks. Gary

solentlife 05-15-2014 12:25 AM

Depends on what prop you are going to use to regain at least part of lost performance.

Most models will have specified props per LiPo size ... for example ... 10x5 for 3S and 8x4 for 4S ...

Reducing voltage pack for Scale is actually not a deal .. as that is really down to YOU and the throttle stick.

You need to check thrust and amps to arrive at a solution .. unless someone can actually quote from actual use ...

Nigel

fhhuber 05-15-2014 12:28 AM

You're dropping 25% voltage which will also drop current so expect about 35% less power available... if you make no other changes.

You could go to a longer and/or higher pitch prop and thus increase load and end up near the 4S power but at higher current... You'd have to check with a wattmeter to ensure yo aren't going above the motor and ESC ratings.

I have the Dynam Tiger Moth from the same series... and it does fine with the stock prop, motor and ESC on 3S 3000 mah with only marginal aerobatic improvement by using the recommended 4 cell pack (or up to 4S 4000 mah). It seems to do better with added weight vs the 4S 2200 mah recommended.
That's not the best predictor for the Waco...

garyp1029 05-15-2014 12:40 AM

fhhuber--I think I remember one of your posts saying you did not hook up the wing support wires on your Tiger Moth; that they were only for show, not structural support. If so, do you still feel that way? Thanks. Gary

fhhuber 05-15-2014 01:13 AM

I still fly it without the wires... with the 4S 4000 or 3S 4000 or whatever other pack I stick in it.


I did have them hooked up briefly.
The wires are totally nonfunctional (useless even if intended to be functional). If you load them with a tight loop and the stock battery you'll pop them or pull the "hard points" out of the fuselage.

Let the wings flex... its fine.

kyleservicetech 05-15-2014 01:52 AM

Quote:

Originally Posted by fhhuber (Post 948393)
If you load them with a tight loop and the stock battery you'll pop them or pull the "hard points" out of the fuselage.

Geeze, :censor: ! May as well use rubber band wires. :D :D

Looks like a nice model though and that at a very good price. Does it fly well??

fhhuber 05-15-2014 03:35 AM

Flies like a Tiger Moth... very predictable. Its a biplane trainer. Could be recommended as a beginner's first model. In some ways Tiger Moths are easier to fly than a J3 or SuperCub.

kyleservicetech 05-15-2014 03:39 AM

Quote:

Originally Posted by fhhuber (Post 948410)
Flies like a Tiger Moth... very predictable. Its a biplane trainer. Could be recommended as a beginner's first model. In some ways Tiger Moths are easier to fly than a J3 or SuperCub.

My only bipe is the tiny UMX Beast 3D. Very definitely not a trainer! But with its built in stabilizing system, I've flown it in 15 MPH winds. Power off, it glides like a lead brick. :(

fhhuber 05-15-2014 03:44 AM

The Dynam models in this series ALL glide like bricks with the recommended batteries. DOUBLE the battery weight and they glide pretty well.
Just too much drag for the weight so you have to point the nose down 45 deg with the stock batteries.

kyleservicetech 05-15-2014 03:56 AM

Quote:

Originally Posted by fhhuber (Post 948414)
The Dynam models in this series ALL glide like bricks with the recommended batteries. DOUBLE the battery weight and they glide pretty well.
Just too much drag for the weight so you have to point the nose down 45 deg with the stock batteries.


I'll have to try that next time it stops raining, and the wind is below 20 MPH around here.

Can't even fly my 2500 Watt models. :oops:

fhhuber 05-15-2014 03:59 AM

Depends on the model and how you want to fly it as to if adding weight (bigger battery) will be acceptable...
For 3D flying the weight may not be acceptable at all.

The Tiger Moth can hover with the stock battery... it can't when using the 4S 4000 mah.

JetPlaneFlyer 05-15-2014 06:44 AM

Voltage has a huge impact on power output. if you keep the same prop then power changes to (approximately) the cube of input voltage. So 3/4 ^ 4 = 0.42 (42%)

So expect to lose over half of the original 4s power. This would mean that unless the model originally had a huge surplus of power you will find yourself very badly under-powered, maybe you wont even get off the ground!

You could get back some of the power by using a bigger prop but the amp limit of the motor and perhaps ground clearance of the prop tips will limit what you can do there.

hayofstacks 05-15-2014 08:39 AM

Exactly.

a 12x8 on 3 cells pulled 350 watts on my ultra stick. 4 cells was right in the 750-800 watt range.

you need a watt meter if you play with cell count.

CHELLIE 05-15-2014 10:51 AM

With a 3 cell lipo like JPF said, it might not even get off the Ground, that plane has a low 650KV motor, and you need the 4 cell voltage for decent motor power, also being a bi plane, it has a lot of drag to it, if you want scale performance, just cut back on the throttle, but use the 4 cell lipo.

http://www.graysonhobby.com/catalog/...np-p-1899.html


Motor Size: 3720 (650kv) outrunner brushless motors (installed)

solentlife 05-15-2014 11:44 AM

Quote:

Originally Posted by CHELLIE (Post 948439)
With a 3 cell lipo like JPF said, it might not even get off the Ground, that plane has a low 650KV motor, and you need the 4 cell voltage for decent motor power, also being a bi plane, it has a lot of drag to it, if you want scale performance, just cut back on the throttle, but use the 4 cell lipo.

http://www.graysonhobby.com/catalog/...np-p-1899.html


Motor Size: 3720 (650kv) outrunner brushless motors (installed)

Yep .... as I said earlier :

Quote:

Reducing voltage pack for Scale is actually not a deal .. as that is really down to YOU and the throttle stick.
I generally regard any motor that is less than 1000kv - to be a high voltage unit ... unless specially designed model. Putting 3S on a low KV motor is like choosing 1st or 2nd gear in a car.

Nigel

fhhuber 05-15-2014 03:58 PM

Well there's the factors involved in converting watts into useful power at the prop too... So the end result is not quite a squared relationship between voltage and useful thrust. You drop 25% voltage and you get about 35% less thrust, not about 55% less thrust. (maybe the same max static thrust if you can still hit the RPM where the prop stalls)

On a draggy design the higher end power is mostly wasted. There's very little difference in top speed of my Tiger Moth using 3S vs 4S. I only see the difference in vertical.... such as attempting a hover.

Compare with my Dynam P-51. Dynam ships these with 3S 2200 mah. There is a significant speed difference dropping in a 4S. The P-51, by design, has significantly lower drag. (and again no prop change is needed...) Performance goes up in every way by using 4S 3000 mah to 4S 4000 mah. It can carry a 4S 5000 mah but that starts to be more weight than it likes to carry.

Exact results will vary with the airplane.

solentlife 05-15-2014 04:28 PM

Absolutely agree ..

I usually regard a well powered average sunday sport model to be :

Up to 35% throttle - too low to sustain flight, but enough to help when landing

35 - 65% - controlled descent

65 - 75% - Cruise range

75 - 85% - Upper cruise range

Over 85% is usually burning amps for very little gain unless it's a speed demon or vertical climber !

It's not that far of Glow actually as well ... last 15% of throttle is all smoke and noise usually.

My Edge 540 is designed for 1600 - 1800 4S and goes like stink ... Put a 3S 2200 in her and she's only a touch slower but more tractable.
She does react to most throttle range though ... useful is about 40 - 90%

My Ultimate Bipe though .. is not suitable to go down in pack ... she's 3S 1800 - 2200 .. not advised to go up to 4S, and certainly not down to 2S. Throttle range is useful at 40 - 80% with very little gain over the 80% except in vertical.

Draggy planes appreciate diameter ... speedies appreciate pitch ..

Nigel

JetPlaneFlyer 05-15-2014 07:11 PM

Quote:

Originally Posted by fhhuber (Post 948455)
Well there's the factors involved in converting watts into useful power at the prop too... So the end result is not quite a squared relationship between voltage and useful thrust. You drop 25% voltage and you get about 35% less thrust, not about 55% less thrust.

Power and thrust aren't at all the same thing, discussion up to now was refering to power reduction, not static thrust. In addition to thrust reducing by roughly the square of the voltage change, pitch speed also reduces in line with voltage change. So end result is power at the prop reduces roughly by the cube of voltage change.

The reduction in pitch speed might mean that the model has no thrust at all at it's flying speed (meaning of course that it wont fly)

fhhuber 05-15-2014 07:34 PM

He wants to know the PERFORMANCE difference... that means effective thrust from the prop.

I just looked it up... the Tiger Moth I have uses THE SAME motor and ESC as the Waco.

Pretty good bet that the Waco will fly on 3S. Very similar weight and wing area.

JetPlaneFlyer 05-15-2014 08:00 PM

Quote:

Originally Posted by fhhuber (Post 948468)
He wants to know the PERFORMANCE difference... that means effective thrust from the prop.

Well i guess we will have to agree to differ. In my view and experience performance in not only dependant on static thrust (if that's what you mean by 'effective thrust'?), but also on pitch speed. Reducing voltage (all other things being equal) has an impact on both and it's the combination of static thrust and pitch speed that have the impact on performance, not either in isolation.

Maybe it will fly, but with a 650kv motor the pitch speed will only be about 25mph or so, so it's going to be very sedate.

Anyway, the question asked was what the impact would be on performance and I think it's perfectly fair to say that with less than half the power that the impact on performance would be large. Disagree if you like.

solentlife 05-15-2014 08:07 PM

Quote:

Originally Posted by JetPlaneFlyer (Post 948471)
Well i guess we will have to agree to differ. In my view and experience performance in not only dependant on static thrust (if that's what you mean by 'effective thrust'?), but also on pitch speed. Reducing voltage (all other things being equal) has an impact on both and it's the combination of static thrust and pitch speed that have the impact on performance, not either in isolation.

Maybe it will fly, but with a 650kv motor the pitch speed will only be about 25mph or so, so it's going to be very sedate.

Anyway, the question asked was what the impact would be on performance and I think it's perfectly fair to say that with less than half the power that the impact on performance would be large. Disagree if you like.

I have to agree ... 650kv on 3S ? Even with very high pitch ... it's gonna be crap ! I would even say that 650kv on 4S is 'sedate' ....

Nigel

fhhuber 05-15-2014 08:39 PM

Well... the Tiger Moth will do inside-outside vertical 8 on 3S using that 650kV motor.

garyp1029 05-15-2014 09:14 PM

Gentlemen, I thank you all. I flew my new Electrifly Super Sportster rxr for the first time yesterday--my first low-wing electric. With the spec 3s 1300 mah battery it had almost unlimited vertical climb. I have also noticed that many non-3D planes at our field using spec batteries have "unscale" like speed and maneuvers. (Piper Cubs doing rolls!!!!) My original question was based on the thought that I really don't want a Waco or Tiger Moth which competes (speed-wise and maneuver-wise)with war birds or pattern planes. I already have a good supply of Apprentice batteries (3s 3200 mah). Thus, the idea of reducing Waco's voltage from 4s to 3s. Thanks again. Gary

thepiper92 05-15-2014 09:18 PM

The Durafly series uses 3s and only 750kV. It won't be a rocket on 3s, but the planes aren't too light either, yet they fly. Prop size would have a lot to do with it, although I am still learning myself.

solentlife 05-15-2014 10:57 PM

Quote:

Originally Posted by fhhuber (Post 948474)
Well... the Tiger Moth will do inside-outside vertical 8 on 3S using that 650kV motor.

I'm amazed ... no truly. So the motor must be producing good rpm efficiency out of the Volts x KV ? and well matched prop.

I usually consider a 3S motor to be in the 1100 and up range .. with 4S in the 750 and up ...
Only arena I would think of would be in the big stuff with large better props.

I'm always ready to learn !

Nigel


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