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Johan
04-11-2006, 12:48 PM
If we have a chart showing the power of various popular glo motors, will it not make life easyer to sellect the appropriate electric motor?
If an outrunner is rated at 200watt, is this the maximum for that motor or is it the recommended operating power?
Johan
Cape Town.

debhicks
04-11-2006, 12:59 PM
A lot of your electrics are coming with spec sheets that will tell you the weight of the airplane it will power and the thrust.

Not sure exactly what the question was. There is a program called motor calc that will help you out with all the specifications needed to fly the bird.

Manufacturers are getting a little more consumer friendly about that infomation however. You can get a good idea by looking at some specs on e-flight. For instance their 46 electric motor will be the equivelant to a 46 glow motor hauling a 40 size crafts.

Or that is my understanding of how that works. So if that is the case and the spec sheets are readily available you can use that as a guide for other motors that aren't as forthcoming with all the information they need to share.

That was just food for thought. As the electric world is changing daily and there is a little more than a prop and motor decision to make. Oh I hope this makes a little sense. I better go. :)

Matt Kirsch
04-11-2006, 02:27 PM
Johan,

While going Horsepower-for-Horsepower, or Watts-for-Watts is certainly one way to make a conversion, it's not the best way by any stretch of the imagination. Glow engines are tremendously inefficient at putting power to the air, and you can almost always get by with much less horsepower using an electric motor because electrics can spin much larger props at much lower RPMs for much better efficiency.

The best way anyone has come up with to do electric conversions is to base the power system off the plane, and how you want to fly it. I know some people poo-poo the Watts-per-pound rule of thumb, but the fact of the matter is that there are thousands of successful conversions out there that were done using those rules. I personally only use two:

1. 100 Watts per pound for glow-like performance.
2. 150+ Watts per pound for 3D.

Once you've got your Watts, you can split it out into Volts and Amps to find the battery, motor, and ESC, or you can go by charts produced by manufacturers like Model Motors (AXi outrunners), pick your configuration off the chart based on Watts, weight of the power system, and prop clearance.

Hope this is helpful.

watt_the?!
04-11-2006, 08:04 PM
you know i like your style matt, but at risk of being different,- or just to offer an alternative method:

i do it very differently, as hundreds of tests and dyno readings on glow motors proved that manufacturers use either theoretical calcs (quite an accepted standard), guesstimates and marketing to promote their motors. also, what is peak power is very different to what power can be made continuously...very sneaky of them too....oh, and then there's tuned pipes and nitro content....la la la.

what i do is determine the prop used by the glow engine, and it's rpm. and that's it.

this can be back engineered using a program like thrust hp for instance, or you can forward work back to the rpm and prop via an online or similar e calculator.

Johan
04-11-2006, 10:08 PM
Thanks for the info guys. I think I, myself, proved your 100watt per lb today. I maidened a correx/aluminium Spad of 3 lb today with a 200watt outrunner propped down to 160watts static. It flew very nicely but at near maximum power only. So, 160watts on a 3 lb plane is barely enough in spite of my wing loading of 12 oz./square ft.
Johan.

watt_the?!
04-11-2006, 10:16 PM
Thanks for the info guys. I think I, myself, proved your 100watt per lb today. I maidened a correx/aluminium Spad of 3 lb today with a 200watt outrunner propped down to 160watts static. It flew very nicely but at near maximum power only. So, 160watts on a 3 lb plane is barely enough in spite of my wing loading of 12 oz./square ft.
Johan.

the other trick is that wattage is then converted into thrust and airspeed.

so perhaps your 160W is in fact ok.

Tim

Johan
04-11-2006, 10:52 PM
Do I understand correctly that thrust in oz. is prop pitch in inches x RPM? Does the prop dia. then have no influence on thrust other than affecting the RPM? Does the watts feature in this equasion?

watt_the?!
04-11-2006, 11:15 PM
Johan,
thats not the equation, but it is something like that...cant recall. Prop dia influences thrust, but in turn is also the biggest consumer of power (watts). This results in some cases in lower rpm.

there's a piece of software out there called thrusthp. its a free download and shows the relationship between thrust/power/diam and pitch, although it oversimplifies the pitch relationship.

but its still good.

Tim

FenceMagnet
04-12-2006, 12:23 PM
thrust is proportional to diameter, not pitch.... pitch has very little influence on static thrust...

:cool:

flyranger
04-12-2006, 12:42 PM
Also, if the plane can carry the weight, putting the wattmeter on board and taking a flight will give interesting results. I propped my Cleveland Air Racer (44in ws, AUW of 31oz) with a 10x7 giving 20.2 amps static on the bench. Flew ok, but put my Whatt Meter on board (stores the peak readings) and found my peak was only 16.9 amps. The eflite Park 480 will take 25 amps peak, rated for 20amps continuous. Repropped to an 11x7 and now is a different plane! 3-d like performance and still get 20min flights on a 3s1p 3000ma lipoly.;)

Matt Kirsch
04-12-2006, 01:56 PM
Actually, everybody's right on the diameter/pitch = thrust issue. It's just that diameter and pitch affect thrust in different ways.

For static thrust, diameter will have the most effect. Pitch also has an effect on static thrust, but if you get too much pitch, the propeller blades stall just like a wing, and thrust drops off.

In flight, the pitch begins to have a signifigant effect. As the plane nears the prop's "pitch speed," basically the RPM times pitch, converted to MPH, thrust drops off. To make the plane go faster without making the motor turn faster, you would increase the pitch so the prop maintains some thrust at higher speeds.

It's all about tuning for your needs. Do you want wicked go-fast speed, good acceleration and plenty of thrust for 3D, or something in between?

slipstick
04-12-2006, 03:05 PM
Do I understand correctly that thrust in oz. is prop pitch in inches x RPM?
No that's the formula for Pitch Speed not thrust.

The formula for thrust is a lot more complex but it's mainly influenced by RPM and Diameter. Pitch has only a fairly small effect.

Steve

rebell
04-15-2006, 08:26 PM
Johan, hoe gaan dit in die Kaap?

Here is a graph somebody posted elsewhere. I tested it on a few planes and it is accurate within the range. Powered gliders, 3D and extreme models does not fall within that range.

Johan
04-17-2006, 03:05 PM
Thanks for the graph Rebell. I find it very usefull and is reducing the span of my latest Spad wing creation acordingly. It is amazing how little power is required to get airborn and how small influence the weight of an aircraft has on its ability to fly.
Johan